Geologic Excursions in the Overthrust Belt and Metamorphic ... · Geologic Excursions in the Overthrust Belt and Metamorphic Cor Complexee s of the Intermountai Region n . GUIDEBOOK

Geologic Excursions in the Overthrust Belt and Metamorphic Core Complexes of the Intermountain Region

GUIDEBOOK - PART I

The Geological Society of America Rocky Mountain and Cordilleran Sections Meeting Salt Lake City, Utah May 2-4,1983

WILLIAM P. N A S H General Chairman

KLAUS D. G U R G E L Editor

GREGORY D. HARPER Field Trip Coordinator

U T A H G E O L O G I C A L A N D M I N E R A L S U R V E Y a division of Utah Department of Natural Resources and Energy

Special Studies 59 M a y 1983

Geologic Excursions in the Overthrust Belt and Metamorphic Core Complexes of the Intermountain Region

GUIDEBOOK - PART I

The Geological Society of America Rocky Mountain and Cordilleran Sections Meeting Salt Lake City, Utah May 2-4, 1983

WILLIAM P. NASH General Chairman

KLAUS D. GURGEL Editor

GREGORY D. HARPER Field Trip Coordinator

U T A H G E O L O G I C A L A N D M I N E R A L S U R V E Y a division of Utah Department of Natural Resources and Energy

Special Studies 59 M a y 1983

G E O L O G I C E X C U R S I O N S IN T H E O V E R T H R U S T B E L T A N D M E T A M O R P H I C C O R E C O M P L E X E S

Editor's Note: T h e papers conta ined in this G u i d e b o o k were sol ic i ted by the organizers o f the G S A R o c k y M o u n t a i n and C o r d i l l e r a n Sect ions and have been edi ted and g iven a c o m m o n format; however , their style and content have not been formal ly rev iewed by the U t a h G e o l o g i c a l and M i n e r a l Survey .

Contents Page

Field Trip 1 — G e o l o g y of the A l b i o n - R a f t R i v e r - G r o u s e Creek M o u n t a i n s area, northwestern U t a h and southern Idaho 1 By David M. Miller, Richard L. Armstrong, Robert R. Compton, and Victoria R. Todd

Field Trip 2 — M e s o z o i c and Early Tert iary Structure and Sed imen to logy o f the Cen t ra l Wasa tch M o u n t a i n s , U i n t a M o u n t a i n s , and U i n t a Basin 63 By Ronald L. Bruhn, M. Dane Picard, and Susan L. Beck

Field Trip 6 — Style o f M i d - T e r t i a r y E x t e n s i o n in Eas t -Cent ra l Nevada 108 By Phillip B. Gans and Elizabeth L. Miller

V

Utah Geological and Mineral Survey Special Studies 59, 1983 Guidebook Part 1 — G S A Rocky Mountain and Cordilleran Sections Meeting Salt Lake City, Utah - May 2 - 4, 1983

STYLE OF MID-TERTIARY EXTENSION IN E A S T - C E N T R A L N E V A D A

Phillip B. Gans and Elizabeth L . Miller Depar tmen t o f G e o l o g y , Stanford U n i v e r s i t y , S tanford , C A 94305

ABSTRACT T h e nor thern E g a n , Sche l l C r e e k , and Snake

ranges in east central N e v a d a are character ized by low-angle, younger-on-o lder faults and by steep, westward dips o f strata. T h e character o f the early Ol igocene unconformi ty and the ages o f syntectonic in t rus ive and vo lcan ic rocks suggest that most o f the fault ing and t i l t ing occurred in the Ol igocene . two types of " l o w ang le" faults are present: 1) O r i g i nally high-angle (60° to bedding) n o r m a l faults that rotated domino-s ty le to low angles; and 2) " F l a t " duct i le-bri t t le detachment faults that separate overlying rocks extended by high-angle n o r m a l fault ing from under ly ing duct i ly deformed rocks. C o n s p i c u ously absent are large-displacement no rma l faults that ini t ia l ly fo rmed at low angles to bedding. A p parent "bedd ing pa ra l l e l " faults are generally the result o f no rma l drag o f incompetent strata a long high-angle normal faults. T h e high-angle faults are typically " shove l - shaped" in three d imens ions and may have fanning upward splays that documen t progressive rotat ion on the planar faults.

High-angle faults in the Egan and Schel l C r e e k ranges penetrated to paleodepths o f greater than 10 k m without flattening signif icant ly; the basal detachment o f these faults is not exposed. In contrast, the duct i le-bri t t le t ransi t ion in the nor the rn Snake Range deve loped at 6 to 7 k m depth. O v e r l y i n g rocks were drastically th inned by two generations o f east-dipping high-angle no rma l faults that flattened abruptly into the Snake Range decol lement . Each generation o f upper plate fault ing was accompanied by 4 0 ° o f westward rotat ion such that first generation faults now dip gently westward, second generation faults dip gently eastward, and locally bedding is near ver t ical . L o w e r plate rocks beneath the Snake Range decol lement were drastically th inned and ex tended co-axia l ly w i t h upper plate fault ing.

T h e comparable amoun t o f ex tens ion recorded in the lower plate (350 percent) and upper plate (450 percent) together wi th the lack of stratigraphic o m i s s i o n across the Snake Range D e c o l l e m e n t suggest that the upper "p l a t e " need not have m o v e d signif icant ly w i t h respect to the lower "p la te . "

T h e no r the rn E g a n , Sche l l C r e e k , and Snake ranges l ie w i t h i n and define a 120 k m wide " b e l t " o f m i d - T e r t i a r y e x t e n s i o n . B o t h range-by-range palinspastic reconst ruct ions and the d i la t ion o f o lder M e s o z o i c structures y ie ld an average o f approx imate ly 250 percent ex tens ion across this cor r idor . Supracrustal ex tens ion in east-central N e v a d a was apparently accomodated by in-situ deep-sea ted d u c t i l e d e f o r m a t i o n and magmat i sm. " R o o t e d n o r m a l f a u l t s " a n d f a r - t r a v e l e d "ex tens iona l a l l o c h t h o n s " ( W e r n i c k e , 1982) are not impor tant geometr ic e lements . T h e presently h igh e levat ions suggest that extens ional processes mod i f i ed the ent ire l i thospher ic c o l u m n .

INTRODUCTION AND OVERVIEW M e t a m o r p h i c core complexes o f the U . S . C o r d i l l

era are thought to have e v o l v e d at least i n part du r ing the mid -Ter t i a ry and represent uplif ted areas o f p ro found crustal ex tens ion (see, for example , rev iews by C o n e y , 1979; Zoback and others , 1981; E a t o n , 1982 ; A r m s t r o n g , 1982 ) . In these complexes , gent ly folded detachment faults separate supracrustal rocks that have been th inned by n o r m a l faul t ing f rom under ly ing , variably deformed mid-c rus ta l igneous and me tamorph ic rocks. T o date, most studies have focused on the details o f the core c ompl e xe s t hemse lves 1 , such that the geologic

A n excellent collection of papers that describe the geology of many of the known metamorphic core complexes comprise G S A M e m o i r 153 by Crittenden and others (1980).

107

108 Utah Geological and Mineral Survey Special Studies59, 1983

' 2 0

Figure I. Location of the northern Snake Range ( N S R ) , Ruby Mts ( R M ) and Grouse Creek - Raft River ( G C - R R ) metamorphic core complexes with respect to the Sevier fold and thrust belt. Lower plates of core complexes show lineations developed parallel to the direction of extension. Shaded region represents highly extended region in N L Nevada and adjacent Utah. F rom Compton (1980), Snoke (1980), and King (1969).

and geometr ic re la t ionship of these apparently isolated cu lmina t ions o f me tamorph ic rocks to their su r round ing areas is s t i l l poorly unders tood. H o w e v e r , in order to fully unders tand the ex tensional processes that br ing deep-seated rocks to the surface, core complexes must be s tudied and eva l uated in the context o f the geology of adjacent areas that expose higher structural levels.

In this paper, we describe the geometr ic character of mid-Ter t ia ry ex tens ion across three ranges in east-central Nevada . T h e nor thern Egan and Sche l l C reek Ranges , together wi th the Snake Range metamorph ic core complex (F igure 1), lie w i t h i n and define a 110 k m wide corr idor that was profoundly extended in the Ol igocene . T h i s ex tens ional " b e l t " has sharp supracrustal boundar ies wi th relat ively unextended areas to the east and west.

Several unique characteristics o f this region make it particularly wel l sui ted for the study o f the k inemat ics o f Ter t iary extens ion: 1) east-central Nevada was not compl ica ted by pre-Ol igocene deformat ion except at very deep structural levels ;

2) most o f the bedrock consists o f a wel l -def ined and regional ly persistent Pa leozo ic stratigraphy that a l lows accurate de te rmina t ion o f fault offsets and palinspastic reconst ruct ions; 3) scattered remnants o f syntectonic vo lcan ic and sedimentary rocks bracket the ages of var ious extens ional episodes; and 4) variable amounts o f later uplift has exposed a broad spec t rum o f earlier Tert iary paleodepths. Th i s al lows us to examine the three-d imens iona l geometry o f n o r m a l faults and evaluate extens ional processes at both supra and mid-crus ta l levels.

W e preface our range-by-range descript ions o f T e r t i a r y e x t e n s i o n a l tectonics in east-central N e v a d a wi th a br ie f review o f earlier ideas and a discuss ion of the pre-extensional evo lu t i on of this reg ion .

R E G I O N A L S E T T I N G Introduction and Evolution of Ideas

T h e E g a n , Sche l l C r e e k , and Snake Ranges (F igure 2) are located i n the heart o f the nor thern Bas in and Range prov ince . T h e y are 10 to 25 k m

Guidebook, Pari 1 - G S A Rocky Mountain and Cordilleran Sections Meeting, 1983 109

110 Utah Geological and Mineral Survey Special Studies 59, 1983

wide , N - S t rending ranges that attain e levat ions in excess o f 3,500 m and are separated by val leys of s imi lar widths whose elevat ions average 1,800 m .

A s t r ik ing aspect o f the geology o f this region is that some ranges are compr i sed o f s tructural ly s imple h o m o c l i n e s or gentle folds that i n v o l v e upper Pa leozoic strata, whereas adjacent ranges are cut by imbricate low- and high-angle faults that typi cally place younger strata on older and expose rocks as o ld as upper P recambr ian . T h e age and tectonic significance o f these younger-on-o lder faults are the subjects o f ongo ing debate and the pr incipal focus o f this paper.

M i s c h (1960) and his students ( F r i t z , I960; A v e n t , 1961; W o o d w a r d , 1962; N e l s o n , 1966; C e b u l l , 1967; Deche r t , 1967) d id m u c h o f the early mapping in east-central N e v a d a and interpreted the y o u n g e r - o n - o l d e r faul ts as d e c o l l e m e n t s or "shearing-ofT" faults due to shor ten ing du r ing a m i d - M e s o z o i c orogeny. In contrast , o ther early workers ( Y o u n g , 1960; P layford , 1964; K e l l o g g , 1964; M o o r e s and others, 1968) conc luded that most o f the faults were Ter t ia ry n o r m a l faults and/or gravity slides. H o w e v e r , it was not un t i l A r m s t r o n g (1972) reinterpreted some o f the early mapping and h ighl ighted the geochronolog ic and geometr ic ev idence that a Ter t ia ry age for the low-angle fault ing o f east-central Nevada rece ived broader acceptance. H o s e and Danes (1973) , H i n t z e (1978) , and H o s e and W h i t e b r e a d (1981) cont inued to argue for important M e s o z o i c , albeit ex tens ional , deformat ion but most other references to this region (e.g., C o n e y , 1974; Proffett, 1977; D a v i s , 1979; W e r n i c k e , 1981) have assumed a late Tert iary age and extens ional o r ig in for the younger-on-older fault ing.

The Cordilleran Miogeocline East-central N e v a d a was the site o f relat ively c o n

t inuous she l f sedimenta t ion f rom the late P recambrian through the early Tr iass ic (Stewart and P o o l e , 1974). T h e miogeoc l ina l stratigraphy typically con sists o f an upper Precambr ian and L o w e r C a m b r i a n sect ion o f quartzite and shale, a M i d d l e C a m b r i a n to L o w e r O r d o v i c i a n l imestone sect ion, a d is t inct ive M i d d l e O r d o v i c i a n quartzi te , an Uppe r O r d o v i c i a n to M i d d l e D e v o n i a n do lomi te sect ion, and an U p p e r D e v o n i a n to L o w e r Tr iass ic sect ion characterized by variable proport ions o f carbonate, shale, and sandstone (see compi l a t i on by H o s e and B lake , 1976) (F igure 3) .

Mesozoic Shortening and Plutonism M e s o z o i c thrust faults are we l l documen ted to

the east in the Sevier orogenic belt ( A r m s t r o n g , 1968) and to the west i n western N e v a d a (Speed, 1978). I f thrust faults affected the in te rven ing reg ion , they were conf ined to deep structural levels and d i d not breach the surface (see A r m s t r o n g , 1972, and discuss ion be low) . A t supracrustal levels , m i n o r M e s o z o i c shor ten ing is indicated by gentle folds such as in the C o n f u s i o n and Butte sync l ino r i -u m s o f H o s e (1977) (F igure 2) . In contrast, at deep s tructural levels , upper P recambr ian and locally L o w e r C a m b r i a n s trata were p e n e t r a t i v e l y de fo rmed dur ing regional d y n a m o t h e r m a l , greenschist to amphibo le grade m e t a m o r p h i s m ( M i s c h , 1960; M i s c h and H a z z a r d , 1962). St ructura l ly deepest rocks are polyphase deformed and c o m m o n l y exhibi t west -dipping, axia l planar cleavages and N - S t rend ing , bedding-cleavage intersect ions (refer to R o a d L o g Stop no. 7) . M e t a m o r p h i s m and deformat ion die out qu ick ly up sect ion.

M e s o z o i c plutons have been ident i f ied in east-central N e v a d a , but the magni tude and character o f M e s o z o i c magmat i sm in this region is st i l l poorly unders tood. In the southern Snake Range , several p lutons that range i n compos i t ion f rom two-mica granite to biotite granodior i te appear to be m i d -Jurasic in age ( L e e and others, 1968, 1980). Here m e t a m o r p h i s m and deformat ion o f the country rocks are synk inemat i c wi th p lu ton emplacement . In the southern Snake Range and in the K e r n M o u n t a i n s , s trongly pe ra luminous two-mica granites are, most l i ke ly , latest Cre taceous in age (Best and other , 1974; L e e and others, 1980, in press). Other grani toid plutons i n the nor thern Egan and Snake Ranges have y ie lded a scatter o f Tert iary K - A r dates (Lee and others , 1970, 1980) , but their true ages are not certain.

M o s t o f the better dated M e s o z o i c plutons intrude M i d d l e C a m b r i a n or older rocks and have tops that are roughly concordant to o v e r l y i n g strata. If any o f these magmas ven ted , the correlat ive vo lcan ic rocks must have been eroded prior to the deposi t ion o f Ter t ia ry strata.

Early Oligocene Paleogeography Revisited The Early Tertiary unconformity - T h e best in

dicator o f pre-Tert iary supracrustal deformat ion is the character o f the Ter t ia ry unconformi ty . Y o u n g (1960) , K e l l o g g (1964) , and M o o r e s and others (1968) were all impressed by the near conformi ty of the oldest Ter t iary strata on exc lus ive ly upper Pa leozo ic rocks and suggested that pre-Tert iary structure must have been l i m i t e d to gentle folds and smal l displacement faults. These views were cham-

Guidebook, Part 1 — G S A Rocky Mountain and Cordilleran Sections Meeting, 1983 111

T Y P I C A L THICKNESSES AND L ITH0L0G1C CHARACTER OF THE FORMATIONS BASED ON PERSONAL OBSERVATIONS AND THE COMPILATION BY HoSE AND B L A K E G 9 7 6 ) . TOTAL THICKNESS IS APPROXIMATELY 1 0 , 5 0 0 M

TV

5555

; / 7 7 7 7

A R C T U R U S F O R M A T I O N ( > 600 m)

E L Y L I M E S T O N E (640 m)

C H A I N M A N S H A L E (300

J O A N A L I M E S T O N E 0 0 m)

P I L O T S H A L E uso m)

G U I L M E T T E F O R M A T I O N (510 m )

S I M O N S O N D O L O M I T E (400 m)

S E V Y D O L O M I T E (160 m)

O R D O V I C I A N - S I L U R I A N D O L O M I T E (500 m)

E U R E K A Q U A R T Z I T E (75 m)

P O G O N I P GROUP (820 m )

NOTCH P E A K L I M E S T O N E ( 6 5 ° M )

D U N D E R B E R G S H A L E (130 m)

RED TO YELLOW BASAL SANDSTONE OVERLAIN BY ALTERNATING INTERVALS OF STLTY LIMEST01E B I O C L A S T I C LIMESTONE, AND SILTSTONE-

LIGHT TAN GREY, MED. CLA S T I C INTERVALS.

TO THICK-BEDDED, CHERTY LIMESTONE; SOME S I L T Y AND BIO-

BROWN TO BLACK CLAY SHALE AND SILTSTONE; BLACK CARBONACEOUS BASE, O L I V E GREEN TO RUST SANDSTONE LENSES I N UPPER PART. LIGHT GREY, MED. BEDDED TO MASSIVE, B I O C L A S T I C LIMESTONE; MINOR CHERT. KHAKI TO YELLOW-ORANGE SILTSTONE AND SHALE; BLUE LIMESTONE BEDS NEAR BASE.

VARIA B L E PROPORTIONS OF DARK GREY SUBLITHOGRAPHIC LIMESTONE AND DARK BROWN DOLOMITE; ABUND. SOLUTION BRECCIAS I N BASAL CLIFF-FORMING UNIT.

THIN- TO MED.-BEDDED, LIGHT TO DARK BROWN, LAMINATED DOLOMITE; BASAL PART I S THICKER-BEDDED AND UNIFORMLY LIGHT BROWN.

VERY LIGHT GREY TO WHITE, THIN- TO MED.-BEDDED DOLOMICRITE; QZ-SANDY BEDS AT TOP.

LOWER PART I S MED.- TO DARK-BROWN WITH SOME STROMATOLITIC, F O S S I L I F E R O U S , AND CHERTY INTERVALS; UPPER PART I S LIGHTER BROWN AND COARSE-GRAINED (SUGARY).

WHITE, CLIFF-FORMING, F I N E - TO MED.-GRAINED, WELL SORTED ORTHOQUARTZITF,.

SLOPE-FORMING, THIN-BEDDED, S I L T Y LIMESTONE; INCLUDES INTERVALS OF, I N ASCENDING ORDER, A) FLAT-PEBBLE CONGL., B) CHERTY L S . , C) CALCAREOUS SLTST. AND SIL T Y L S . , D) B I O C L A S T I C L S . , E) BROWN S I L T S T . AND SHALE (KANOSH), AND F) BLUE-YELLOW MOTTLE) L S . ( L E H M A N ) .

CLIFF-FORMING, MEDIUM GREY, THIN-BEDDED LIMESTONE WITH ABUNDANT BROWN TO BLACK CHERT; SOME MASSIVE DOLOMITE INTERVALS.

DARK BROWN TO KHAKI CALCAREOUS SILTSTONE AND SHALE.

L I N C O L N P E A K ALTERNATING INTERVALS OF SLOPE-FORMING, PLATY, S I L T Y LIMESTONE AND CLIFF-FORMING F O R M A T I O N ( 1 2 5 ° m> MED.- TO THICK- BEDDED LIMESTONE; MINOR DOLOMITIC INTERVALS.

P O L E C A N Y O N L I M E S T O N E ( 5 2 ° M )

- P I O C H E S H A L E (130 m)

ALTERNATING LIGHT- AND DARK-GREY INTERVALS OF CLIFF-FORMING MASSIVE LIMESTONE; GIRVANELLA AND O O L I T I C BEDS ARE COMMON.

DARK GREENISH-GREY TO OLIV E S I L T Y SHALE; INCLUDES LIMESTONE BEDS NEAR TOP.

P R O S P E C T M O U N T A I N RUST-WEATHERING, UNIFORMLY THICK-BEDDED (30 to 60 cm), F I N E - TO COARSE-GRAINED Q U A R T Z I T E QUARTZITE; CONTAINS L E S S THAN 2% SHALE INTERBEDS.

(1200 m)

MCCOY C R E E K GROUP ( >2500 m)

ALTERNATING INTERVALS, 100 to 500 m THICK, OF QUARTZITE, INTERBEDDED QUARTZITE AND SILTSTONE, AND SILTSTONE AND SHALE; MINOR PEBBLE CONGLOMERATE; GENERALLY PENETRATIVELY DEFORMED AND METAMORPHOSED TO LOWER TO UPPER GREENSCHIST GRADE.

BASE NOT EXPOSED

Figure 3. Representative miogeoclinal stratigraphic section for east-central Nevada.


PI

Pu,

Pu

P R E S E N T A R E A L EXTENT OF

THE K A L A M A Z O O 'TUFF

F A U L T C O N T A C T

B O U N D A R Y B E T W E E N A R E A S OF D I F F E R E N T A G E S OF B A S E M E N T

UJ (3 z < cc LU

< 2 09

Pu

(X •

IP

C O N F U S I O N SYNCLINORIUM

< < > LU Z

I < H

Figure 4. The early Oligocene unconformity. A l l known or inferred exposures of the early Tertiary unconformity in east-centra! Nevada are compiled. Dashed lines separate areas underlain by different ages of Paleozoic basement. Balloons enclose representativ e attitudes of the basal Tertiary units and of the directly underlying Paleozoic strata. Sources of data: Brokaw 1967, Brokaw and others, 1965, 1966, 1968, 1973; Dechert, 1967; Drev.es. 1967. Fri tz . 1968; Hintze, 1980; Hose. 1977; Hose and Blake, 1976; Kellogg, 1964; Moores and others. 1968; Nelson, 1966; Playford. 1961; Sides. 1966; Young, 1960

pioned by A r m s t r o n g (1972) in his classic pre-mid-Ter t ia ry reconst ruct ion o f eastern N e v a d a , a l though he left open the possibi l i ty o f some M e s o zoic faulting in this area.

W e have re -examined many of the exposures of the Tert iary unconformi ty in east-central N e v a d a and western most U t a h and c o m p i l e d the pertinent structural and stratigraphic data (F igure 4 ) . O n l y unconformi t ies at the base o f the earliest Ol igocene volcanic rocks and pre-volcanic sedimentary sequences are inc luded in our compi l a t i on . It is very clear from this data that early Ol igocene rocks were deposited exclusively on Mis s i s s ipp ian or younger strata. Excep t ions described by N e l s o n (1959) and Decher t (1967) are actually fault contacts. In

add i t ion , the basal Ter t ia ry deposits are remarkably conformable to the under ly ing upper Pa leozo ic strata. L o c a l angular discordances are generally less than 15° and are not systematic.

Ea r ly O l igocene vo lcan ic rocks were deposi ted on strata as o ld as Mis s i s s ipp i an in only two smal l areas (F igure 4 ) . H o w e v e r , even these occurrences may be a consequence o f early Ol igocene uplift rather than M e s o z o i c deformat ion . T h e basal Ter t iary deposits in both of these areas consist o f unusual ly th ick , in t raca ldera(? ) , accumula t ions of ash-flow tuff. R e g i o n a l tumescence ( S m i t h and Bai ley , 1966) may have preceded the e rupt ion o f tuff and caused preferential e ros ion o f these near-vent areas.

T h i n intervals o f pre-volcanic conglomerate c o m -

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monly define the base o f the Tert iary sections. U n l i k e most later conglomerates in the exposed Tert iary sections, they contain only clasts o f upper Paleozoic formations ( K e l l o g g , 1960; Decher t , 1967; G r i e r , this v o l u m e ) . Th i s strongly supports the inference that at the t ime o f the early Tert iary unconformi ty , only upper Pa leozo ic rocks were exposed at the surface.

T h e early Ter t iary e ros ion surface is under la in by roughly equal areas o f Pennsy lvan ian and L o w e r P e r m i a n rocks and lesser Uppe r P e r m i a n and Tr ias sic strata (F igure 4 ) . Areas under la in by a single age strata are typically elongate in a N - S direct ion and give way laterally to areas under la in by successively older or younger basement. T h i s topology is most compat ib le wi th very gentle N - S t rending folds that had been beveled by eros ion such that Pennsy lvan ian rocks were exposed in the cores o f anticl ines and Upper P e r m i a n or Tr iass ic rocks were preserved in the cores o f syncl ines . T w o o f these M e s o z o i c ( ? ) folds are wel l exposed in the Butte and C o n f u s i o n sync l ino r iums o f H o s e (1977) (F igure 4 ) . The fold structure o f the in te rven ing area is largely obliterated by Ter t ia ry fault ing and t i l t ing but is apparent in the palinspastic reconst ruct ion we present in a later section.

Discussion - T h e amount o f M e s o z o i c supracrustal deformat ion in east-central Nevada is severely const ra ined by the early Ol igocene unconformi ty . M i o g e o c l i n a l strata had been gently folded, but only the uppermost units were exposed at the surface, and the total "strat igraphic r e l i e f ' beneath the Tert iary unconformi ty was less than 2 k m . C lea r ly , no significant M e s o z o i c faults breached the surface in this area. In contrast, the present ranges are under la in by complex ly faulted and til ted rocks that encompass every geologic sys tem from upper Precambr ian to Quaternary.

Several authors (e.g., Decher t , 1967; A r m s t r o n g , 1968, 1972; A l m e n d i n g e r and Jo rdan , 1982) have suggested that large M e s o z o i c displacements may have occurred on low-angle or bedding-paral lel " b l i n d " faults wi thout severely disrupt ing the surface. H o w e v e r , we find perfectly conformable sections that span the entire late Precambr ian to late Pa leozoic in terval and effectively rule out regional M e s o z o i c decol lements at any o f the present levels o f exposure . O n geometr ic and geochronolog-ic grounds , essentially all o f the presently exposed faults in east-central N e v a d a are the product o f post-early Ol igocene extens ional tectonics.

A direct consequence o f this paleogeographic data is that "strat igraphic d e p t h " wi th in the miogeo-

c l ine is an accurate estimate o f M e s o z o i c to early Ter t ia ry "s t ruc tura l dep th . " Severa l paleodepths der ived i n this fashion are part icularly noteworthy:

1. In the nor thern Egan Range , G a n s ( in preparat ion, see R o a d L o g Stop no. 1) describes early O l igocene rhyol i t i c d ikes that can be traced f rom subvolcan ic vents to depths o f emplacement o f approx imate ly 10 k m . H o w e v e r , even at their deepest levels the dikes are remarkably porphyr i t ic and appear to have had glassy margins . V i g o r o u s meteor ic water c i rcu la t ion together w i t h pressure quench ing a long deeply penetrating fractures apparently quenched the magmas such that "hypabyssa l " textures were produced at mid-crus ta l levels .

2. T h e Cre taceous Tungs ton ia muscovi te granite in the K e r n M o u n t a i n s (Best and others, 1974; L e e and others, 1980; in press) intrudes and metamorphoses L o w e r O r d o v i c i a n to Uppe r D e v o n i a n strata ( N e l s o n , 1959; A h l b o r n , 1977) cor responding to depths o f emplacent ranging f rom 4.5 to 2.0 k m . S i m i l a r l y , in the nor thern Snake Range , M i l l e r and others (1983) describe swarms o f muscovi te pegmatite d ikes that pervade L o w e r C a m b r i a n rocks (paleodepth o f 6 k m ) . These depths are signif icantly less than the exper imenta l ly der ived m i n i m u m depth for muscov i t e stabili ty in granit ic melts (e.g., L u t h , 1976).

3. M i l l e r and others (1983) describe uppermost P recambr i an peli t ic rocks in the nor thern Snake Range that were metamorphosed to kyani te-muscovi te -b io t i t e schists at a paleodepth o f approx i mately 8 k m . L o w e r C a m b r i a n P ioche Shale was lo cally conver ted to s taurol i te-garnet-biot i te-musco-vite schist at a paleodepth o f 6 k m . A g a i n , our reconstructed depths are m u c h shal lower than the exper imenta l ly de r ived m i n i m u m depths for these assemblages (e.g., H o l d a w a y , 1971).

It is t empt ing to i nvoke a thick M e s o z o i c and /or early Ter t ia ry sedimentary cover to exp la in the apparent sha l low depths o f these h igh pressure assemblages. H o w e v e r , it seems un l ike ly that the addi t ional requi red thicknesses o f strata c o u l d have been so neatly and un i fo rmly r e m o v e d f rom this entire area pr ior to the early Ol igocene . F o r now, we prefer to s t and by these geological ly w e l l -cons t ra ined depths, even i f they go against accepted exper imenta l w i s d o m .

Oligocene Magmatism Intermediate to s i l ic ic vo lcan ic rocks b lanketed

m u c h o f east-central N e v a d a between 38 and 30 m.y. ago (Hose and B l a k e , 1976). T h i s is the only significant v o l c a n i s m to have ever affected this area


Figure 5. Map of Nevada and Utah showing generalized distribution of 43 to 34 m.y. old igneous rocks.

and it occurred w i t h i n a broad, east-west t rending belt that extends f rom central N e v a d a to central U t a h (F igure 5). In the v ic in i ty o f the nor thern Egan , Sche l l C r e e k , and Snake Ranges , Ol igocene volcanic rocks are p redominan t ly lavas and local ly der ived tuffs that range i n c o m p o s i t i o n f rom pot a s s i u m - r i c h andes i t e to h i g h - s i l i c a rhyol i t e ( Y o u n g , 1960; Deche r t , 1967; B lake and others, 1968, 1969; G a n s , 1982) . A detai led f ie ld and petro-logic invest igat ion o f these rocks is i n progress by G a n s and G . A . M a h o o d and some p re l imina ry observations are p rov ided in the R o a d L o g Stops 1 and 5.

Int rusive and ex t rus ive relat ions descr ibed below indicate that the onset o f v o l c a n i s m precisely co inc ided wi th the onset o f ex tens iona l faul t ing in east-central N e v a d a and that m u c h o f the ex tens ion was synchronous wi th this br ie f pulse o f v o l c a n i s m . Because o f their age and tectonic sett ing, these v o l canic rocks are difficult to classify by t radi t ional schemes. T h e y fo rm a h igh-potass ium calc-alkal ic " su i t e " (Blake and H o s e , 1968) that was erupted du r ing a t ime of con t i nued subduc t ion a long the western marg in o f N o r t h A m e r i c a ( C o n e y , 1978;

Engebre t son , 1982) and are generally inc luded in compi la t ions o f " subduc t ion - re l a t ed" magmat i sm in the western U n i t e d States (Snyder and others, 1976; C o n e y and R e y n o l d s , 1977; K e i t h , 1978; E a t o n , 1982). H o w e v e r , i f east-central N e v a d a was part o f the Ol igocene cont inenta l arc, this arc was at right angles to the cont inenta l margin in N e v a d a and U t a h . C o n v e r s e l y , a l though the vo lcanic rocks were erupted in a rapidly ex tending terrane, they resemble nei ther the " b i m o d a l " basalt-rhyoli te suite thought to be characteristic o f Bas in and Range ex tens ion (Chr i s t iansen and L i p m a n , 1972), nor the a lka l ic , s i l ica-undersaturated rocks associated wi th ex tens ion in the Af r i can rift system (e.g., M a c D o n a l d , 1974).

S T R U C T U R A L E V O L U T I O N O F T H E N O R T H E R N E G A N R A N G E

Introduction M o s t o f the nor thern Egan Range was first

mapped by F r i t z (1960, 1968) and W o o d w a r d (1962) . T h e y d i v i d e d the range into a mosaic o f seven east-directed thrust sheets that they interpreted to have m o v e d eastward du r ing the M e s o z o i c . A r m s t r o n g (1972) and W e r n i c k e (1981) reinterpreted parts o f F r i t z ' s (1968) map and suggested that at least some o f the low-angle faults were Ter t ia ry n o r m a l faults. T h e structural e v o l u t i o n o f part o f the nor the rn Egan Range (F igure 6) was recently invest igated by G a n s (1982a, b, submit ted) and these f indings are synthesized below.

Geometric Relations Low-angle faults and westward tilts - T h e archi

tecture o f the E g a n Range is most apparent in cross sect ion (F igure 7) . T h i n slices o f steeply west-d ipp ing upper P recambr ian to P e r m i a n miogeoc l ina l rocks and Ol igocene vo lcan ic rocks are separated by low-angle no rma l faults that displace ove r ly ing sections eastward wi th respect to under ly ing rocks. S t eep ly w e s t - d i p p i n g , o lder -on-younger thrust faults that were mapped by F r i t z (1960) , were remapped and are clearly east-dipping no rma l faults. G e n t l y east-dipping faults typically merge downward wi th major, subhor i zon ta l faults that span the entire wid th o f the range. Ind iv idua l faults can occasionally be traced eastward up to 10 k m . T h e spacing of faults ranges f rom tens o f meters to 0.5 k m . D o w n -to-the-east d isplacements range f rom a few hund red meters to greater than 6 k m . T h e larger displacements occur where several imbricate faults merge.

B e d d i n g atti tudes are remarkably consistent. B o t h miogeoc l ina l and Ter t ia ry strata typically

Guidebook, Part ] — G S A Rocky Mountain and Cordilleran Sections Meeting, 1983 115

strike N S to N 30 E and dip 35 to 75° N W (F igu re 8) . T h e average tilt is approximate ly 50° N W about a N 15 E axis. A t t i tudes do not vary systematical ly between different fault sl ices. M o s t o f the var ia t ion occurs in less competent units near bends in faults, or because o f differential m o v e m e n t w i t h i n i n d i v idua l fault slices. Bedding- to-faul t angles are 50 to 60° on the major, th rough-going faults and range from 60 to 90° on the steeper splays. N e i t h e r no rma l nor reverse drag along faults is c o m m o n .

M a n y o f the Egan Range faults are gently c u r v e d in a concave upward fashion. T h e y dip 5 to 15° eastward on the west flank o f the range and re-emerge on the east flank where they dip 5 to 20° westward (F igure 7) . H o w e v e r , westward stratal rotations are also progressively steeper toward the east, such that bedding-to-fault angles do not decrease appreciably. T h e m a x i m u m or ig ina l curvature or " l i s t r i c i t y " that can be assigned to the no rma l faults in the E g a n Range o n the basis o f decreasing bedding-to-faul t angles wi th increasing stratigraphic depth is approx i mately 1 7 k m . (compare wi th Proffett , 1977) . D i f ferential uplift or d o m i n g , perhaps in some way related to the large in t rus ive mass o n the east f lank o f the range, is responsible for most o f the present curvature .

Three-dimensional geometry of fault surfaces -T h e rugged re l ief and near -hor izonta l faults o f the nor thern Egan Range provide an ideal set t ing to study the three-d imens iona l geometry of no rma l fault surfaces. Here , N N E t rending n o r m a l faults generally do not con t inue more than a few k i l o m e ters before they either: 1) bend abruptly into E S E -trending, " r a m p - l i k e " sides, 2) are cut by the side of an en-echelon fault, or 3) break into several strands whose c o m b i n e d offset equals that o f the or iginal fault (see F igures 6 and 7 for examples ) . T h e E S E t rending " s i d e s " dip 10 to 40° inward toward a subhor izon ta l " f l o o r " that const i tutes the ma in fault surface (compare cross sections B - B ' and Y - Y ' , F igu re 7) . T h i s geometry approximates that o f a steep-sided shove l or platter and is s imi la r to the "spoon-shapes" descr ibed by ProfTett (1977) . Bo th the corners and the edges o n the fault surfaces are surpris ingly sharp bends rather than s m o o t h , curviplanar transi t ions. T h e wid th o f the s h o v e l -shaped scoops ranges f rom 0.5 to 5 k m , and is generally less than the height (measured d o w n the m o v e ment plane) . T h e h igh ly compl ica ted map pattern o f the nor the rn Egan Range (F igure 6) is as m u c h a consequence o f the or ig ina l d i scon t inuous nature o f the no rma l faults as it is o f differential e ros ion .

Kinematic Interpretation The relation of faulting to stratal rotation -

S i m p l e geomet r ic considera t ions suggest that, as the faults m o v e d , both bedding and faults must have rotated westward in a " d o m i n o " or "deck o f cards" fashion (cf. T h o m p s o n , 1960; M o r t o n and Black , 1975) . W e r n i c k e ' s (1981) reconst ruct ion ass u m e d that strata had first been t i l ted westward and later shuff led out to the east on the present, low-angle-faults. T h i s interpretat ion requires that an un reasonably h i g h , west -d ipping h o m o c l i n e or ig inal ly lay off to the west. In contrast, A r m s t r o n g (1972) i m p l i e d that the c o m b i n e d , down-to-the-east displacements occur red whi le bedding was st i l l ho r i zontal and faults were steeply d ipping . U n t i l t i n g the Egan R a n g e cross sections results in an equally unreasonable , 6 to 10 k m h igh , east-facing scarp whose average dip is 50 to 60° .

T h u s , the low-angle faults in the Egan Range must have or ig ina ted as imbricate and en-eche lon , high-angle (50 to 6 0 ° ) , shovel -shaped, no rma l faults that mus t have rotated to low angles as they m o v e d . T h e faults penetrated to paleodepths o f 10 k m wi thou t f lat tening appreciably. H o w e v e r , at some greater depth , they must have flattened abruptly (?) into a duct i le-br i t t le t ransi t ion or detachment fault i n order to produce the attendant rotat ion. T h e paleogeography at the t ime o f no rma l fault ing was probably character ized by narrow, arcuate ridges o f bedrock separated by very s m a l l , discon t inuous basins filled wi th synorogenic debris.

Fanning-Upward Splays - Shor t splays that merge d o w n w a r d wi th relat ively planar, th rough-going n o r m a l faults are a c o m m o n geometr ic elemen t in the nor the rn Egan Range (F igure 7) . These splays typical ly fan upward such that bedding-to-fault angles increase systematical ly on s t ructurally h i g h e r and steeper splays. By geometr ic necessity, m o v e m e n t o n the splays must have been accompan ied by m o v e m e n t on the m a i n through-go ing faults.

A s imp le k inema t i c interpretat ion o f these relations is: as b locks b o u n d e d by planar faults rotate to low angles, the faults attempt to steepen themselves by cu t t ing successively deeper into the hanging-wal l (F igure 9 ) . F a n n i n g - u p w a r d splays that bound upward - th i cken ing wedges effectively chart the history o f progressive rotat ion o n a planar fault. S imi l a r splays may also deve lop at depth , in order to a l l ev i ate space p rob lems at the toes o f t i l ted b locks (F igure 9 ) . A n imp l i ca t ion o f this m o d e l is that wi th c o n t i n u e d ro ta t ion , the surface o f m o v e m e n t


Figure 6. Geologic map of part of the northern Egan Range (for Explanation, see p. 118).

Guidebook, Part 1 - G S A Rocky Mountain and Cordilleran Sections Meeting, 1983 117

Utah Geological and Mineral Survey Special Studies 59, 1983

E X P L A N A T I O N F O R F I G U R E 6

Upper Precambrian and Paleozoic Clastic and Carbonate Rocks

M D p

A R C T U R U S F O R M A T I O N sandstone, siltstone, and sllty limestone

E L Y L I M E S T O N E (650 m)

C H A I N M A N S H A L E (310 m)

J O A N A L I M E S T O N E (60 m)

P I L O T S H A L E (150 m)

G U I L M E T T E F O R M A T I O N (450 m ) limestone and dolomite

S I M 0 N S 0 N D O L O M I T E (360 m>

S E V Y D O L O M I T E (140 m)

O R D O V I C I A N - S I L U R I A N D O L O M I T E (500 m)

O e E U R E K A Q U A R T Z I T E (50 m)

O P ,

O P .

L E H M A N F O R M A T I O N (90 m) blue and yellow mottled sllty llmestono

K A N O S H S H A L E (110 m)

v. • « e E Q.<o Q. o 3 «

O P , P O G O N I P U N I T D (115 m)

bioctastlc limestone

Fed and yellow sllty limestone

O p .

O P ,

• G p m

p C a

3 P O G O N I P G R O U P ( u n d i f f e r e n t i a t e d )

P O G O N I P U N I T B (115 cherty, sllty limestone

P O G O N I P U N I T A (270 m) flat limestone pebble conglomerate

N O T C H P E A K L I M E S T O N E (590 m) equtv. to Windfall Fm. -t- Ord. Fm. A of Fritz, (960

D U N D E R B E R G S H A L E (150 m)

L I N C O L N P E A K F O R M A T I O N (1250 m) limestone and shale: equlv. to Secret Canyon Sh. Hamburg Ls. of FrttzC I960)

£e E L D O R A D O L I M E S T O N E <660 m)

Cpi P I O C H E S H A L E (210 m)

P R O S P E C T M O U N T A I N Q U A R T Z I T E (1175 m)

M C C O Y C R E E K U N I T A green to brown phyllltic siltstone

M C C O Y C R E E K U N I T B quartzite

Tertiary Igneous Rocks

Volcanic Rocks I j L a t i t e t o T r a c h y a n d e s i t e L a v a F l o w s

T l * phenocrysts of plag., rare hnbl. In a trachytic I I groundmass of plag. ± kspar ± opx ± cpx * bio.

T q l t Q u a r t z L a t i t e A s h - F l o w - T u f f

plag., bio., hnbl, and qz crystals ash matrix

devltrltied

R h y o l i t e A s h - F l o w - T u f f (35.8 m.y. -K-Ar san, bio) crystals of qz, san, plag, and bio

Hypabyssal Rocks

T i p L a t i t e P o r p h y r y

phenocrysts of plag, bio, and hnbl in a fine gralnedJ

flow-banded groundmass

R h y o l i t e P o r p h y r y 4 generations of dikes: all contain qz, san, plag, and bio In an aphanlttc groundmass: of ten argllllcally altered

Plutonic Rocks

T g r p

T a p

T M b g

G r a n i t e P o r p h y r y contains kspar megacrysts, large qz eyes In a fine grained groundmass

A p l i t e P o r p h y r y qz, feldspar, and bio phenocrysts In an aplltlc to granophyric groundmass

B i o t i t e G r a n i t e equlgranular, moderately deformed, may be Mesozoic

Faults

Contacts

5 8

Symbols

H i g h a n g l e n o r m a l f a u l t s h o w i n g d i p

L o w - a n g l e n o r m a l f a u l t s h o w i n g s t r i k e a n d d i p :

t e e t h o n h a n g i n g w a l l

F a u l t , l o c a t e d a p p r o x i m a t e l y ( w i t h i n 5 0 m )

B u r i e d f a u l t

L i t h o l o g t c < d e p o s i t i o n a l o r i n t r u s i v e ) c o n t a c t s h o w i n g d i p

C o n t a c t l o c a t e d w i t h i n 5 0 m

B u r i e d c o n t a c t

S t r i k e a n d d i p o f b e d d i n g

/ ^ 5 0 S t r i k e a n d d i p o f f l o w - b a n d i n g o r f l a t t e n i n g f a b r i c

o A g e - d a t e s a m p l e l o c a t i o n

X P r o s p e c t p i t

3 S h a f t

A d i t o r d r i f t

C a b i n


E


evolves from a planar, 60° fault to a curv ip lanar or iistric fault.

A somewhat different interpretat ion for faults at very high angles to bedding was suggested by C o l l e -ia and A n g e l i e r (1982) . They proposed that sub-vertical tension fractures develop w i t h i n major blocks bounded by 60° no rma l faults. A s the 60° (1st order) faults rotate, the tension fractures are eventual ly activated as (2nd order) n o r m a l faults and use the 1st order faults as de tachment surfaces. H o w e v e r , their mode l does not exp la in 1) the progressively steeper bedding-to-fault angles observed on the fanning upward splays of the Egan Range , or 2) why both vert ical and 60° fractures s h o u l d deve l op concurrent ly and cospatial ly, as i m p l i e d by their mode l .

The onset of extensional faulting - O u r best e v i dence for when major extens ional fault ing began in east-central N e v a d a comes f rom the nor thern Egan Range. In the v ic in i ty o f the Hun te r distr ict (F igure

6 ) , 35.8 m.y. o ld rhyol i te d ikes and smal l stocks i n trude and are clearly guided by a low-angle fault whose down-to-the-east displacement is approxi mately 1 k m ( G a n s , 1982) (see cross section F - F ' , F igu re 7) . H o w e v e r , shear zones are very c o m m o n a long the margins and w i t h i n these dikes , and coeval vo lcan ic rocks are displaced and t i l ted on geometr ica l ly identical faults (F igure 7) . In keeping wi th the regional pattern, the oldest erupt ive units rest d i sconformably on U p p e r Pennsy lvan ian or L o w e r P e r m i a n rocks - a paleosurface wi th less than 0.5 k m o f stratigraphic relief. These apparently contradictory relat ions suggest that extens ional fault ing in the nor thern Egan Range began 35.8 m.y. ago, precisely co inc ident wi th the onset o f v o l c a n i s m , and that faul t ing con t inued after vo lcan i sm had ceased.

Direction of extension - T h e m o v e m e n t d i rec t ion o n the low-angle faults, and hence, the di rect ion o f ex tens ion , was est imated by several methods: 1)

Guidebook, Par! 1 — G S A Rocky Mountain and Cordilleran Sections Meeting, 1983 121

sl ickensides a l though very rare in the nor thern Egan Range, have az imuths that range f rom N 80 E to N 60 W ; 2) i f stratal rotat ion and fault ing are anti thetic, then the N 10 to 20 E average str ike o f beds (Figure 8) impl ies m o v e m e n t in a S 70 to 80 E d i rec t ion; 3) the trends o f the sides of shovel -shaped faults range from E - W to N 60 W and probably parallel the d i rec t ion of movemen t . T a k e n together, these estimates suggest that the low-angle faults in the nor thern Egan Range are a product o f ex tens ion or iented approximately N 75 W to S 75 E .

Palinspastic Reconstruction of the northern Egan Range and Vicinity.

T h e most accurate way to calculate the strain in a complex ly faulted and t i l ted terrane is to systematically restore each fault and untilt the rocks to their previous attitude. W e palinspastically restore a s i m plified cross section of the nor thern Egan Range and southernmost Che r ry Creek Range by this method in F igure 10. T h e nor thern Egan Range is assumed to be in the hanging-wall o f a gently d ipping normal fault that separates it f rom the C h e r r y Creek Range. T h i s fault or systems o f faults is largely covered by a l l u v i u m but apparently cuts across the Egan Range north o f the cross sect ion l ine . P ro ject ing the footwall units onto the l ine o f cross section suggests that the total down-to-the-east offset on this fault(s) is approximate ly 8 k m .

O u r reconstruct ion is largely mode l independent . A l l o f the normal faults probably deve loped concurrently and their attitudes are well const ra ined at a broad spectrum o f paleodepths. Faul t slices are s imply s l id back so that stratigraphic uni ts match across the projected faults. T h e entire sect ion is then rotated back to pa leohor izonta l . T h e behavior of the low-angle faults below the present levels o f exposure is speculat ive, but it does not affect a palinspastic reconstruct ion o f the upper 10 k m o f the crust. T h e 11.5 k m increase in hor izonta l separat ion be tween two points on the M i s s i s s i p -p i a n / D e v o n i a n boundary corresponds to exactly 200 percent ex tens ion . W e r n i c k e (1981) at tempted a s imi lar reconstruct ion based on one o f F r i t z ' s (1968) cross sections, but failed to r emove t i l t ing or compensate for eros ion. H i s estimate of 16.5 k m or 300 percent ex tens ion is, in effect, a s u m of the down-to-the-east displacements on rotated no rma l faults rather than a measure o f the hor izonta l d i la t i on between two points o f k n o w n or ig inal separation.

A n y mode l for the Tert iary e v o l u t i o n o f east-central Nevada must account for abrupt boundar ies

I o%

Figure 9. Kinematic model for fanning upward splays.

between areas that exhib i t radically different structural styles and levels o f exposure . F o r example , the Butte M o u n t a i n s are immedia te ly west o f the Egan and C h e r r y C r e e k Ranges are under la in by essentially unfaul ted , f lat- lying upper Pa leozo ic rocks. In F igu re 11 we present a series o f balanced cross-


UPPER CAMBRIAN -MIDDLE ORD

MIDDLE CAMBRIAN

LOWER CAMBRIAN

S i m p l i f i e d a n d m o d i f i e d f r o m F r i t z ( 1 9 6 S )

0 1 2 3 4 5 10 K m

6 UPPER PRECAMBRIAN

S C A L E

L o w - a n g l e n o r m a l fault / H i g h - a n g l e n o r m a l faul t

Figure 10a. Geologic map of the S. Cherry Creek Range and the N . Egan Range.


W CHERRY CREEK

^RANGE

17.25 km

EGAN RANGE

1 .1

FIGURE 10(B). PALINSPASTIC RECONSTRUCTION OF THE S. CHERRY CREEK RANGE AND THE N. EGAN RANGE 5.75 km

6 v ^ y ?

Figure 10b. Palinspastic reconstruction of the S. Cherry Creek Range and the N . Egan Range.

sections f rom the Butte M o u n t a i n s to the nor thern Egan Range that chart the structural evo lu t ion o f these ranges. M a n y o f the relat ions shown at depth are entirely speculat ive, but are in accord wi th what is exposed in these and adjacent ranges. Several aspects o f the structural evo lu t i on shou ld be highl ighted:

1. Uppe r P recambr ian rocks that were at 8 to 10 k m depth prior to the early Ol igocene are presently exposed at the surface in the Egan Range , whereas they remain at mid-crus ta l levels beneath the But te M o u n t a i n s . T h i s eno rmous differential uplift is a direct consequence o f the m u c h greater amount o f extens ion toward the east.

2. Imbricate high-angle faults are shown to flatten abruptly into a subhor izon ta l , duct i le-br i t t le detachment fault, beneath w h i c h ex tens ion is accomodated by p e n e t r a t i v e s t r e t c h i n g a n d p l u t o n emplacement . T h i s inference relies heavi ly on relations in the Snake Range that we describe below.

3. Di f fe rent ia l uplift o f the nor the rn Egan Range wi th respect to the Butte M o u n t a i n s has greatly accentuated westward tilts. D o m i n o - s t y l e faults in the Egan Range probably rotated an addi t ional 10 to 20°

to their present pos i t ion and the inferred basal de tachment now dips gently west. S i m i l a r l y , the east l i m b o f the Butte s y n c l i n o r i u m has been warped upward into a m u c h steeper westward dip than is suggested by the early Ter t iary paleogeography.

T h e dura t ion o f major extens ional fault ing in the E g a n Range is not k n o w n . Reg iona l considerat ions suggest that it is largely Ol igocene . In any case, Bas in and Range faults are clearly super imposed on the o lder low-angle faults.

SYNTECTONIC VOLCANISM AND SEDIMENTATION IN EAST-CENTRAL

NEVADA

Introduction T h e nor thern Sche l l C r e e k Range offers the best

oppor tun i ty for cons t ra in ing the t i m i n g o f ex tens ion i n east-central N e v a d a . T h i c k sections o f m i d -Ter t ia ry vo lcan ic and sedimentary rocks are wel l exposed and apparently span the dura t ion o f m u c h o f the ex tens iona l faul t ing. W o r k in this area by G a n s


I 40 m.y.b.p.

1

II 35 m.y.b.p. 6.5% E X T E N S I O N V O L U M I N O U S R H Y O L I T I C TO

^ I N T E R M E D I A T E V O L C A N I C S 15.7 km

MISS. • PERMIAN (It

upper ORO. - DEV. (!)

upper ^CAMBBIA N j J^J>Rd7~(J)

middle CAMBRIAN (4)

~ 7ower CAMBRIAN (5)

upper PRECAMBRIAN (6) DUCTILE/BRITTLE V

~ £ - ^ _ - _ - _ ^ ^ ^ - = = 5 ^ ^ ^ ^ - - = - ^ ! 3 r ^ 5 2 i . T R A N S I T I O N

PRECAMBRIAN XTLN. BASEMENT

III 24.3% E X T E N S I O N

I — - 18.4 km 1

DEPOSITED EXCLUSIVELY \0N PENN.-PERM. STRATA

FEEDER DIKES GUIDED BY EARLY FAULTS

BATHOLITH-SIZEA MAGMA CHAMBERSV

IV 66% EXTENSION — 24.6 km

DOMINO-STYLE* 1 FAULTJNG AND TILTING

WITH CONTINUED MOVlEMENT. STEEPER SPLAYS CUT INTO THEi HANGING WALLS OF THE TILTED FAULT BLOCKS

V P R E S E N T I J O ^ ^ T J N S I O N 3 4 . 0 k A X I S OF BUTTE CHFRRY

B U T T E } * ' S Y N C L I N O R M J M ^ C R E E K

M O U N T A I N S

S P A C E P R O B L E M S AT T O E S OF T I L T E D B L O C K S A C C O M O D A T E D BY L O W - A N G L E S P L A Y S THAT MERGE W I T H THE D E T A C H M E N T FAULT

S T E P T O E VALLEY AT LEST 3 km OF VALLEY _ FILL INFERRED FROM

GRAVITY DATA MILLER, ET. AL.. IN PREP.)

.BASIN AND RANGE" '*tA FAULTING

PRESENT DUCTILE/BRITTLE TRANSITION 5 0

Figure 11. Structural Fvolution of the northern Fgan Range and Vicinity.

and G . M a h o o d has just begun so that here we discuss only a few ini t ia l observat ions w i th in the c o n text o f previous work in this region.

Structural Summary Y o u n g (1960) and Decher t (1967) mapped most

of the nor thern Sche l l C r e e k Range , and descr ibed a compl ica ted history o f fault ing for the area. T h e oldest faults in the range are low angle or gently west-dipping and consistently place younger strata on older. In places these faults appear to be bedding parallel whereas elsewhere they cut bedding at h igh angles. A younger system o f predominan t ly east-d ipping no rma l faults cut the older low angle faults. Bo th Paleozoic and Ter t iary strata in the Sche l l

C r e e k Range generally d ip 30 to 70° westward a l though local doma ins o f eastward tilts occur as we l l . T h e sequence and style o f faulting in the Schel l C r e e k Range is remarkably s imi lar to fault ing i n the Snake R a n g e w h i c h we describe i n detail later i n this paper.

Tertiary Stratigraphy Ter t ia ry rocks in the Sche l l C reek Range (F igure

12) consist f rom older to younger of: 1) th in , discon t inuous intervals o f pre-volcanic lacustrine l imes tone , sandstone, and conglomera te ; 2) a thick sect ion o f in termediate to s i l ic ic lava flows and tuffs; and 3) pos t -volcanic conglomerates and m o n o l i t h o l o g i c breccia b locks . T h e area immedia te -


M E T E R S

1 6 0 0 '

w o o -

1 2 0 0 -

1 0 0 0 -

3 0 0 -

6 0 0 -

MOO -

2 0 0 -

NORTH C R E E K F O R M A T I O N

G r e a t e r t h a n 500™

D i s c o n f o r m i t \ tincon f o rmi t y

T R A C H Y A N D E S I T E FLOWS

B I O T I T E L A T I T E T U F F 20 t o 50 m

T R A C H Y A N D E S I T E FLOWS 50 t o 150 m

L I T H I C T U F F

10 t o 150 m ( u n i t 5 o f Yo u n g , 1960)

H O R N B L E N D E ~ B I O T I T E D A C I T E FLOWS (200 t o 500 in) ( U n i t A o f Yo u n g , 1960)

V O L C A N I C B R E C C I A

(0 t o 150 m) ( U n i t 3 o f Young, 1960)

K A L A M A Z O O T U F F ( 50 t o 400m) ( e q u i v a l e n t t o u n i t s i and 2 o f Y o u n g , 1960)

K I N S E Y CANYON F M .

(0 t o 100 m) ) r s ! i gh t an gu 1 a r

D e b r i s f l o w and s t r e a m c h a n n e l d e p o s i t s i n c l u d e s an a i r - f a l l ( ? ) t u f f n e a r t h e b a s e and a r h y o l i t e l a v a f l o w h i g h e r up. A t t h e n o r t h e n d o f Duck C r e e k V a l l e y , composed p r e d o m i n a n t l y o f v o l c a n i c c l a s t s ; c l a s t s o f P a l e o z o i c f o r m a t i o n s become a b u n d a n t t o t h e s o u t h .

A s s o r t e d i n t e r m e d i a t e l a v a f l o w s .

A l t e r e d , s a l m o n - c o l o r e d q u a r t z l a t i t e a s h - f l o w t u f f . R a r e i i t h i c s , l a r g e b i o t i t e b o o k s

H i g h l y f l o w - b a n d e d l a v a s , p h e n o c r y s t s o f b i o t i t e - p l a g i o c l a s e - s a n d i d i n e ( ? )

E x t r e m e l y l i t h i c — r i c h t u f f , n e a r b a s e commonly c o n t a i n s 30% I i t h i c s t h a t a r e p r e d o m i n a n t l y d a c i t i c i n c o m p o s i t i o n . The t u f f c o n t a i n s phenoc r y s t s o f b i o t i t e - p l a g i o c l a s e - s a n i d i n e ( ? )

R e d d i s h - b r o w n w e a t h e r i n g , b l o c k y t o p l a t y l a v a f l o w s and domes t h a t c o n t a i n p h e n o c r y s t s o f p l a g i o c l a s e - h o r n b l e n d e - b i o t i t e ; r a r e q u a r t z e y e s mav be x e n o c r y s t s

L a h a r s , b l o c k - a n d a s h - f l o w s , r e w o r k e d a s h f l o w t u f f . i n t e r f i n g e r s w i t h f j o w s a n d domes.

Most a r e a l l v e x t e n s i v e a s h - f l o w t u f f i n e a s t -c e n t r a l N e v a d a . Compound c o o l i n g u n i t . I s z o n e d f r o m a c r y s t a i - p o o r b a s e t h a t c o n t a i n s o n l y b i o t i t e - s a n i d i n e - p l a g i o c l a s e p h e n o c r y s t s t o a c r y s t a l - r i c h t o p t h a t c o n t a i n s h o r n b l e n d e -o r t h o p y r o x e n e - c l i n o p y r o x e n e - b i o t i t e - p l a g i o c l a s e -q u a r t ^ . U p p e r m o s t e m p l a c e m e n t u n i t s r e s e m b l e a g g l u t i n a t e .

B a s a l c o n g l o m e r a t e o v e r l a i n by l a c u s t r i n e l i m e s t o n e and t u f f a c e o u s s a n d s t o n e , l o c a l l y i n c l u d e s r h y o l i t e f l o w domes.

Upper P a l e o z o i c

Figure 12. Tertiary stratigraphic section for the Schell Creek Range.

ly nor th o f D u c k Creek Va l l ey is probably a major volcanic center f rom wh ich most o f the volcanic rocks were erupted. T h e total v o l u m e o f erupted magmas is probably significantly greater than 100 cubic k i lometers .

T h e volcanic units in the Sche l l C r e e k Range are 36 to 31 m.y. o ld (B lake , 1981 personal c o m m u n i c a tion) and A n d e r s o n ( in press) reports a 27.4 m.y. old airfall tuff f rom near the base o f the younger

conglomera tes . W e are presently dating more v o l canic rocks in the range.

Timing of extensional faulting A s e lsewhere i n east-central N e v a d a , the Ter t iary

unconfo rmi ty in the Sche l l C r e e k Range suggests li t t le pre-Ter t iary deformat ion . In contrast , angular unconfo rmi t i e s w i th in the volcanic sequence and part icular ly between the volcanic rocks and the

uidebook. Part 1 — G S A Rocky Mountain and Cordilleran Sections Meeting, 1983 127

GEOLOGIC MAP OF PART OF THE NORTHERN SNAKE RANGE STANFORD GEOLOGICAL SURVEY (1981, 82)

M.E. BAXTER, R. BE AMES, J. BLACK, J. BLANK, A. BOL, A. BRANHAM. C. CARLSON, A. CARVER, A. CHEN, L. CHIN, K. CHESICK, D. CLARK, M. DEBICHE, J. EABY,

A.J. FAN, M. FITCH, P. GANS, J,D. GIBSON, W. GLOVER, T. GOODLIN, R. GREEN, S. GRIER, J. HARRIS, K. HESS, D. HRABIK, Q. KIEHN, J. KOKINOS, J. LEE,

C. MIC HELL, C. MICHELSON, E.L. MILLER, M. MILLER, K. MORRIS, K. MORRISON, N. MORTIMER, M. NEUWELD, P. PEREZ, K. PHELPS, A. PRICE, R. REIMERS, D. RODGERS,

L. ROWLES, M. SABtSKY. S. SAXENIAN, P. SCHAEFER, E. SCHERMER. N. SCOTT, J. SEYMOUR, M. STALLARD, L. STERN, D. VOSHELL, H. WHELAN, W. WHITEFORD

Y X •>-• "•• f x

high angle or e a s t - d i p p i n g normal fault bar and ball on downthrown side

low angle or gently west dipping faults

Northern Snake Range c'ecollement

EXPLANATION

Q U A R T E R N A R Y ALLUVIUM

| T c | T E R T I A R Y C O N G L O M E R A T E

| Tl | T E R T I A R Y L A C U S T R I N E D E P O S I T S

Tv T E R T I A R Y V O L C A N I C R O C K S {Includes rhyolite tuff and latite flows)

_ | T s [ T E R T I A R Y SLIDE B L O C K S

UNCONFORMITY

P E N N S Y L V A N I A N and PERMIAN (Includes Ely Limestone and Arcturus Formation)

MISSISSIPPIAN (Pilot Shale, Joana Limestone, and Chainman Shale)

| Ds | D E V O N I A N S E V Y D O L O M I T E and S I M O N S O N DOLOMITE

D E V O N I A N G U I L M E T T E F O R M A T I O N

O s O R D O V I C I A N - S I L U R I A N D O L O M I T E

L 2 L I L O W E R O R D O V I C I A N

(Includes Pogonip Group and Eureka Quartzite)

U P P E R C A M B R I A N

(Includes Dunderberg Shale and Notch Peak Formation)

C A M B R I A N L I N C O L N P E A K F O R M A T I O N C A M B R I A N P O L E C A N Y O N L I M E S T O N E

C A M B R I A N P R O S P E C T M O U N T A I N Q U A R T Z I T E and P I O C H E S H A L E

P C P R E C A M B R I A N M c C O Y C R E E K G R O U P (Quartzite units are stipled)

PLUTONIC ROCKS

[ J g | J U R A S S I C GRANITIC R O C K S

|Tmg| T E R T I A R Y ? - M E S O Z O I C ? GRANITIC R O C K S

> Tertiary slide blocks

128 U t a h Geological and Mineral Survey Special Studies 59, 1983

younger ove r ly ing conglomerates suggest that significant faulting and ti l t ing occur red dur ing the t ime span o f their depos i t ion . T h e younger cong lomer ates conta in clasts and slide blocks o f formations as o ld as M i d d l e C a m b r i a n , thus clearly post-date faulting and the format ion o f major structural relief. H o w e v e r , they are in turn t i l ted and offset by y o u n ger faults.

Stratigraphic and geochronolog ic data available so far f rom the Sche l l C r e e k Range suggest that a significant propor t ion o f the extens ional faul t ing probably occurred strictly du r ing the Ol igocene , dur ing the t ime o f v o l u m i n o u s vo l can i sm. A t the t ime o f the earliest erupt ions (36 m.y. B . P . ) , only upper Pa leozoic rocks were exposed at the surface, whereas by 30 to 27 m.y. ago, the older generat ion of faults had exposed rocks as o ld as M i d d l e C a m b r i a n . There are, however , no constraints on the upper age o f faults that cut the youngest con glomerates in the Sche l l C reek Range .

A N E X H U M E D D U C T I L E - B R I T T L E T R A N S I T I O N Z O N E IN T H E N O R T H E R N S N A K E R A N G E

Introduction O u r best evidence for what happens to ex tens ion

al fault mosaics at midcrus ta i depths comes f rom the nor thern Snake Range . Por t ions o f this range were previously mapped by N e l s o n (1959) , Hose and Blake (1976) , and H o s e (1981) . T h e most p rominent structural feature o f the range, the Snake Range deco l lement , was first descr ibed by Hazza rd and others (1953) and addi t ional invest igations and interpretat ions have been made by M i s c h (1960) , A r m s t r o n g (1972) , C o n e y (1974) , H o s e and Whi t eb read (1981) , W e r n i c k e (1981) , and R o w l e s (1982) . M i l l e r and others (1983) describe in detail the structural e v o l u t i o n o f the nor thern Snake Range based on two s u m m e r s o f mapping with the Stanford G e o l o g i c a l Survey . T h e summary below is based largely o n this work.

In the nor thern Snake Range (F igure 13), vast expanses o f duct i ly de formed upper P recambr ian and L o w e r C a m b r i a n metasedimentary rocks and granitic int rusions o f u n k n o w n age are exposed beneath the nor thern Snake Range decol lement ( N S R D ) . These rocks are structural ly concordant to the gently arched N S R D , w h i c h generally fol lows the top o f the L o w e r C a m b r i a n P ioche Shale (Figures 13 and 14). In contrast, M i d d l e C a m b r i a n to P e r m i a n and Ter t iary strata in the upper plate are broken up and tilted by imbricate n o r m a l faults that do not cut the decol lement .

A l o n g the southern flank o f the nor thern Snake Range , the N S R D plunges abruptly southward beneath the Sacramento Pass area (F igures 13 and 15). H e r e , upper P recambr ian and L o w e r C a m b r i a n in the upper plate can be traced con t inuous ly into the sou thern Snake Range where they are in the lower plate o f a deco l lement mapped by Whi t eb read (1969) . A s these two decol lements are not structurally equiva len t , the relat ions we describe below pertain on ly to the nor thern Snake Range.

Upper Plate Faulting Geometric Relations - The geometry o f upper

plate fault ing is best documen ted in the southwestern part o f the nor thern Snake Range where the upper plate is largely preserved. Exposures o f the N S R D to the nor th , east, and in a window along the N e g r o C r e e k drainage constrain its subsurface geometry and prov ide cr i t ical v iews of how upper plate faults interact wi th the decol lement . O u r descr ipt ion o f upper plate fault ing w i l l focus specifically on the N e g r o Creek area (F igure 13).

Despi te the ex t remely complex map pattern, a systematic s tructural style is evident in the upper plate. S t ruc tura l sections that " y o u n g " to the west are repeated eastward on east-dipping faults (F igures 13 and 14). O lde r , west-dipping faults w i th in these structural sections typically omi t units. T w o generat ions o f faults are even more apparent i n cross sect ion (F igure 14). T h e younger faults are spaced approximate ly 1 k m apart, d ip 10 to 20° eastward, and apparently merge wi th the N S R D . T h e older faults are more closely spaced, dip 10 to 30° westward, and are truncated downward by either the N S R D or the younger faults.

H a n g i n g - w a l l strata are displaced eastward relative to footwal l strata o n both generat ions o f faults. T h e younger faults are clearly down-to-the-east no rma l faults, whereas the older faults presently have apparent reverse offset. T h e younger faults typically juxtapose upper Pa leozoic format ions on lower Pa leozo ic format ions . T h e i r actual offsets (0.2 to 2.0 k m ) are general ly m u c h less than their stratigraphic offsets (up to 4 k m ) because they displace sequences that were previously attenuated by the older faults.

B e d d i n g atti tudes in the upper plate are ex t remely variable. M o s t strata s t r ike N 10 E to N 45 E and dip northwest , but the a m o u n t o f t i l t ing ranges f rom 0 to 90° or even ove r tu rned (F igure 16). B e d d i n g in platy or shaly format ions frequently parallels adjacent fault planes whereas more competent uni ts tend to intersect the fault planes at h igh angles.

Guidebook, Part ] — G S A Rocky Mountain and Cordilleran Sections Meeting, 1983 129

4,000-t—

- S V A K l - RANGE DECOLLEMENT

P C m c u

APROXIMATE BASE OF McCOr CREEK

P £ xln (?)

Upper plate strata: original thicknesses

Paleozoic Tertiary

pa lABCTURUS FORMATION

f LIMESTONE

M! MDp

C H A I N M A N S H A L € J O A N A L I M E S T O N E PILOT S H A L E

S IMONSEN D O L O M I T E S E V Y DOLOMITE ORDOVICIAN AMD SILURIAN D O L O M F T E . U N D I F F E R E N T I A T E D E U R E K A Q U A R T Z I T E

GEOLOGIC CROSS SECTION OF THE NORTHERN SNAKE RANGE, NEVADA

THE STANFORD GEOLOGICAL SURVEY 1981 & 1982

E. I.. MILLER AND P. B. GANS A L L U V I A L F A N

C O N G L O M E R A T E ? =L»^-»z—.-— 3 * ̂ 0 i 2m,ies

no vert i c a I e xaggera t ion

L A C U S T R I N E L I M E S T O N E Lower plate strata

V O L C A N I C R O C K S T C . O L D E R

C O N G L O M E R A T E Southern Snake Range Northern Snake Range

POGONIP G R O U P

Approximate structural thickness of

upper plate after faulting: D U N O E R B E R G S H A L E

P I O C H E S H A L E

(HENDRY'S C R E E K )

TMg - TERTIARY AND/OR MESOZOIC GRANITE

feel 12.000

Figure 14. Geologic cross section of the northern Snake Range and a summary of upper and lower plate stratigraphy.

Class ic "chaos s t ruc ture" ( N o b l e , 1941) is c o m m o n l y deve loped along closely spaced low-angle faults directly above the N S R D . B e d d i n g tilts get progressively steeper away f rom fault planes such that the steepest westward dips occur where faults are widely spaced and in the more massive l imestone units. W e emphasize that on ly these steepest dips reveal the true amount o f westward rotat ion and the or ig inal bedding-to-fault angles. A l l lesser dips are demonst rably the result o f no rma l drag o n upper plate faults.

In three d imens ions , the upper plate faults resemble faults in the Egan Range . T h e y define shove l -shaped scoops that are 1.5 to 5 k m across and have fairly planar bot toms. Bedding- to-faul t angles do

not decrease systematical ly wi th increasing stratigraphic depth on ei ther generat ion o f faults. H o w e v e r , u n l i k e the Egan Range , al l o f the faults i n the nor the rn Snake Range are b o u n d e d below by an exposed detachment fault, the N S R D .

Kinematic Interpretations - T h e fact that faults in the upper plate flatten into a subhor izon ta l de tachment plane requires that stratal rotat ion must have accompan ied upper plate faul t ing. F igu re 17 i l lustrates a sequence o f fault ing and t i l t ing events that w o u l d result i n the present bedding and fault attitudes. These s impl i f i ed sequential cross sections do not attempt to show the effect o f no rma l drag. F i r s t generat ion faults or iginated as east-dipping, high-angle (50 to 60° ) no rma l faults that subse-


GEOCHRONOLOGY

R A D I O M E T R I C S T U D I E S

B Y L E E A N D O T H E R S

U9o8, 1970, 1980 A N D I N P R E S S A , B »

K - A R

• M U S C O V I T E / W H I T E M I C A

• h I O T I T E

• H O R N B L E N D E

• 1 1 - P B Z I R C O N

LEGEND

Q U A T E R N A R Y A L L U V I U M .

T E R T I A R Y S E D I M E N T A R Y R O C K S

T E R T I A R Y V O L C A N I C R O C K S

C O M P L E X L Y F A U L T E D P A L E O Z O I C R O C K S

P R E C A M B R I A N A N D C A M B R I A N S T R A T A I N T H E

L O W E R P L A T E O F T H E N O R T H E R N S N A K E R A N G E

D E C O L L E M E N T I D U C T I L E L Y F X T E N D E D ) A N D I N T H E

S O U T H E R N S N A K E R A N G E ( N O T D U C T I L E L Y E X T E N D E D )

P O L E C A N Y O N F O R M A T I O N - C P C

€ P M

P € M C

P R O S P E C T M O U N T A I N Q U A R T Z I T E

A N D P I O C H E S H A L E

M C C O Y C R E E K G R O U P

G R A N I T I C R O C K S

•a- , I'J „ It ' 6 , ' 2 , v l 8 ^ . P O L E C A N Y O N - C A N YOUNG

rtW 307 ± 7 53.9*1 57

SNAKE CREEK- / . / C p r n , ^ 156*54*- / 'c-r WILLIAMS CANYON/ <L ^ < ~^< )4 rV4

I55 4 ± 4 m y (Rb-Sr)

SSFfo

N S R D N O R T H E R N S N A K E R A N G E D E C O L L E M E N T

S S R D S O U T H E R N S N A K E R A N G E D E C O L L E M E N T 4 Km

V E R Y L O W A N G L E N O R M A L F A U L T

N O R M A L F A U L T

Atter Hose and Blake( 1 9 7 6 ) , W h i t e b r e a d ( 1 9 6 9 ) , Miller a n d o t t i e r s , unpub l ished mapping

Figure 15. Index map of the northern and southern Snake Range, east-central Nevada, showing location of published radiometric dates.


NEGRO CREEK AREA

• P O L E S T O F O L I A T I O N S MICROFAULTS. LOWER PLATE POLES TO BEDDING, UPPER P L A T E o L I N E A T I O N S • P O L E S T O F A U L T P L A N E S ° S L I C K E N S I D E S

HENDRYS CREEK AREA

P O L E S T O F O L I A T I O N S •

Figure 16- Structural data I'rom the northern Snake Range (lower hemisphere equal area projection). (a) Alt i tude of foliation and lineation in the lower plate of the N S R D , Negro Creek area. (b) Late-stage ductile to brittle microfaults in the Prospect Mountain Quartzite, Negro Creek area. (c) Poles to bedding upper plate, Negro Creek area. (d) Attitude of level plate foliation and lineations in the Middle reaches of Hendry 's Creek. (e) Poles to closely spaced and penetrative micro-ductile normal faults in schist units along Hendry's Creek. East-dipping conjugate

set is best developed. (f) Poles to late-stage joints in Hendry's Creek.

quent ly rotated " d o m i n o - s t y l e " to low angles. Second generat ion faults also or iginated as h igh -angle faults and were super imposed on previously faulted and t i l ted strata. A s the second generat ion faults rotated to low angles, the first generat ion faults were rotated th rough hor izon ta l in to westward dips and apparent reverse offsets. N o t e that the toes o f the first generat ion faults were rotated away f rom

the N S R D whereas higher segments were rotated d o w n w a r d and are presently truncated by the deco l lement . None the l e s s , the first generat ion faults mus t have interacted wi th the same basal de tachment because they affect the entire range o f stratigraphic levels in the upper plate. La te r d o m i n g o f the N S R D probably added an addi t ional 5 to 10° o f westward tilt to the N e g r o C r e e k area.


TOTAL EXTENSION 540%

Figure I 7. Simplified geometric model showing how two generations of upper plate faults interact with the decollement (SRD) to produce the bedding and fault attitudes observed in the Snake Range.

If both generat ions of faults or iginated at the same angle wi th respect to ho r i zon ta l , then the average angular difference between them (40°) is precisely the amount o f rotat ion that occurred on the older faults alone. Once these faults had rotated to dips as low as 20° the resolved shear stress on the fault planes apparently no longer exceeded the fric-t ional resistance to m o v e m e n t , and a new generat ion of high-angle no rma l faults was developed .

T h e large bedding-to-fault angles at all stratigraphic levels in the upper plate suggests that both generations o f faults must have flattened very abruptly into the N S R D . Space problems at the toes of fault b locks were re l ieved by several types o f deformat ion: 1) brecciat ion was pervasive in the i m mediate v ic in i ty o f the N S R D ; 2) warping and fo lding c o m m o n l y "smeared o u t " less competent uni ts a long the decol lement ; 3) low-angle splays shaved off the sharp corners at the toes of major fault b locks (F igure 9) . T h e th i rd process was part icularly important du r ing m o v e m e n t on the younger , more widely spaced faults. Segments o f rotated, first gen

eration faults were apparently reactivated and served as splays at the toes o f second generat ion faults. E v i d e n c e for this type o f react ivat ion includes: 1) occas ional , small-offset , high-angle faults that appear to bo t tom into the older rotated, low-angle faults; and 2) the imperfect match o f hanging walls and footwalls on the younger faults. R e n e w e d m o v e m e n t o n older faults after they were at very low-angles (and subject to a larger reso lved no rma l stress), may exp la in why fr ict ional drag is so prevalent in the nor thern Snake Range.

T h e d i rec t ion o f ex tens ion in the upper plate was est imated to be N 55 W to S 55 E by the methods ou t l ined prev ious ly for the Egan Range . T h e precise ages of no rma l faul t ing in the Snake Range are poorly cons t ra ined , but we assume that they co in cide wi th fault ing in the Egan and Sche l l C reek Ranges . T h e best constraints come f rom Ter t ia ry rocks exposed i n Sacramento Pass descr ibed by G r i e r ( this v o l u m e ) . H e r e , a post-35 m.y . o ld lacustr ine and fanglomerate sequence was deposi ted synchronous ly wi th nearby fault ing and uplift. T h e se-

Guidebook, Pari 1 — G S A Rocky Mountain and Cordilleran Sections Meeting, 1983 133

quence was then cut by imbricate no rma l faults that merge wi th the N S R D and geometr ica l ly resemble the second generat ion o f faults in the N e g r o Creek area.

Amount of Extension in the Upper Plate - W e have est imated the amount o f ex tens ion in the upper plate for the Negro C r e e k area by three independent methods:

1. Sequent ia l ly restoring the faults a long our l ine of cross sect ion (F igure 18) y ie lds approximate ly 125 percent ex tens ion by second generat ion faults and 155 percent ex tens ion by first generat ion faults for a total o f 480 percent ex tens ion . T h i s me thod has obv ious problems because o f the i m m e n s e amount o f small-scale faul t ing, breccia t ion, and folding that has changed the shape o f the major fault slices.

2. O u r best estimate o f the average structural thickness o f the upper plate after no rma l fault ing but prior to eros ion is approximate ly 1.1 k m c o m pared to an or ig ina l stratigraphic thickness o f 6 to 7 k m . T h i s change in thickness is equivalent to about 450 percent ex tens ion .

3. C a l c u l a t i o n o f the percent ex tens ion us ing bedd ing to fault angles and the amount o f t i l t ing ( T h o m p s o n , 1960) yields 153 percent ex tens ion by both generat ions o f faults for a total o f 540 percent ex tens ion . T h i s estimate is probably somewhat excessive because it does not account for devia t ions f rom two, s imple generations o f faults (e.g., fanning upward splays) , or for internal deformat ion in fault blocks.

C o n s i d e r i n g the large uncertaint ies in all o f these estimates, we are impressed by how closely they agree. W e conclude that 450 to 500 percent is a reasonable, though perhaps conservat ive , estimate of the amoun t o f ex tens ion in the upper plate o f the nor thern Snake Range.

Lower Plate Deformation Introduction - In marked contrast to the upper

plate, rocks beneath the N S R D are relat ively flat-ly ing and not appreciably faulted. Instead, they reveal a complex history o f duct i le de format ion and magmat i sm that ended wi th a t ransi t ion into a brittle regime.

L o w e r plate rocks in the nor thern Snake Range were metamorphosed to amphibo l i t e grade and con tain s taurol i te-garnet-biot i te-muscovi te and locally k y a n i t e - m u s c o v i t e - b i o t i t e schist . M e t a m o r p h i c grade appears to increase both wi th increasing depth and to the nor th . Severa l p lutons o f biot i te ± muscovi te granite intrude the amphibo l i t e grade

rocks , but nei ther their absolute ages nor their relat ion to m e t a m o r p h i s m is k n o w n . B o t h plutons and metasedimentary rocks are in t ruded by swarms o f muscovi te-garnet bearing pegmatite d ikes (also o f u n k n o w n ageLs]) that locally compr ise more than 80 percent o f the lower plate.

Progressive (ductile to brittle) extension - S u per imposed o n all lower plate rocks is a penetrative subhor i zon ta l fo l ia t ion and l ineat ion that increases in intensi ty both eastwards and upwards towards the N S R D . T h i s deformat ion occur red at low greenschist grade and remarkably th inned all lower plate rocks. O n the east (lank of the nor thern Snake Range , stratigraphic sequences or ig inal ly about 3 k m thick have been duct i ly th inned to less than 0.5 k m (F igure 14). T h e deformat ional fabric o f lower plate rocks in the H a m p t o n C r e e k area has been descr ibed by R o w l e s (1982) .

M e s o s c o p i c structures in the lower plate record duct i le to brit t le progressive ex tens ion (F igures 16 and 19). St re tched pebbles and minera l grains i n d i cate ex tens ion in a N W - S E d i rec t ion and flattening in a ver t ica l d i rec t ion . T h e d i rec t ion of lower plate s t re tching varies f rom N 55 W on the west side o f the range to N 70 W o n the east side. Aspec t ratios o f s tretched pebbles in M c C o y C r e e k G r o u p strata o n the east flank o f the range are c o m m o n l y on the order o f 8-10: 1:0.1. Late stage brittle structures such as closely spaced mesoscopic no rma l faults, duct i le n o r m a l faults, and pervasive jo in t s , cut ear l ie r - formed duct i le features and y ie ld calculated d i rec t ions o f N W - S E - d i r e c t e d ex tens ion that are perfectly parallel to the d i rec t ion o f ex tens ion i n ferred f rom stretched pebbles (F igure 16). East-d ipp ing mesoscopic no rma l faults are somet imes preferential ly deve loped over their west -d ipping conjugate sets. In general , however , the deformational fabric o f lower plate rocks appears to have an o r t h o r h o m b i c symmet ry . T h e duct i le to brittle progressive de fo rmat ion recorded in lower plate rocks o f the Snake Range is s imi lar to extens ional fabrics descr ibed by G . H . D a v i s (1980) in many o f the A r i z o n a m e t a m o r p h i c core complexes and is markedly different f rom that typically deve loped i n rocks d u r i n g p rogress ive compress iona l deformat ion (F igure 19).

T h e age o f lower plate ex tens ion is not yet bracketed rad iomet r ica l ly , but we strongly suspect that it is Ter t ia ry and synchronous wi th upper-plate no rma l faul t ing. Indirect arguments that support this age ass ignment incude: 1) the precise parallel i s m o f lower and upper plate strain axes; 2) anomalously y o u n g Ter t ia ry K - A r mica ages f rom all rock

134

Figure 18. " C u l and paste" restoration of Snake Range upper plate faults along cross section A - A ' . Unit symbols same as in Figure 14. Heavy solid lines are second generation faults, dashed lines are early generation faults.

types in the lower plate as compared to non -stretched but equivalent s tructural levels in adjacent ranges (F igure 15); and 3) wel l documen ted m i d -Tert iary ages for identical fabrics in the nearby Raft R i v e r Range and R u b y M o u n t a i n s ( C o m p t o n and others, 1 9 7 7 ; S n o k e , 1982).

W e have estimated the amount o f ex tens ion in the lower plate by the amount o f t h inn ing o f the L o w e r C a m b r i a n Prospect M o u n t a i n Quar tz i te . L)n-deformed Prospect M o u n t a i n Quar tz i te sections in adjacent ranges are typically 1,200 m thick. O n the east flank o f the nor thern Snake Range , complete sections o f this unit are generally only 100 to 200 m thick. O n the west side o f the range the base o f the Prospect M o u n t a i n Quar t ize is not exposed. H o w e v er the reduct ion in average bedding thickness suggests that it has been th inned to approximate ly one third its or iginal thickness . A n average o f 330 percent ex tens ion across the range was de r ived by restoring the Prospect M o u n t a i n Quar tz i te to its o r ig i nal thickness whi le preserving its cross-sect ional area (F igure 15).


Tectonic Significance of the Northern Snake Range Decollement

M o d e l s for detachment fault ing in me tamorph ic core c ompl e xe s fall in two general categories: 1) models that i n v o k e a large amount o f m o v e m e n t on the de tachment faults (e.g., G . A . D a v i s and others, 1980; W e r n i c k e , 1981); and 2) models that accomodate ex tens ion in the upper plate by in-silu ex tens ion below the de tachment fault (e.g., G . H . D a v i s , 1980; R e h r i g and R e y n o l d s , 1981). T h e Snake Range , w i t h its wel l cons t ra ined structural and stratigraphic relat ions, is part icularly wel l sui ted for testing these mode l s and dec ipher ing the k inemat ics o f de tachment fault ing.

T h e restored upper plate thickness (F igure 14 and 18) and the consistent stratigraphic pos i t ion of the N S R D suggest that it or iginated at a depth o f 6 to 7 k m and was in i t ia l ly flat. A s s u m i n g that lower plate s t re tching was synchronous wi th upper plate faul t ing, then the N S R D represents an e x h u m e d mid-Ter t i a ry , duct i le-br i t t le t ransi t ion zone. Irregular zones o f duct i le de format ion and recrystal l izat ion in the lower por t ions o f upper plate fault slices suggest that, in i t i a l ly , the t ransi t ion was fairly diffuse. A s ex tens ion con t inued , the t ransi t ion was rapidly loca l ized w i t h i n a narrow (probably less than 100 m) in terval and u l t imate ly e v o l v e d into a strictly brittle fault surface.

T h e precise amount and d i rec t ion o f m o v e m e n t o f the upper plate wi th respect to the lower plate is not k n o w n as there are no offset markers . A l t h o u g h the N S R D presently juxtaposes rocks that represent radically different s tructural levels , this juxtapos i t ion can be exp la ined by two generations o f no rma l faults that th in the upper plate and by the col laps ing o f isograds in the lower plate. W e emphasize that there need not be a great amoun t o f offset on the deco l lement as the amoun t o f ex tens ion above and below is approximate ly the same! T h u s , ex tens ion by no rma l fault ing in the upper plate may have been largely accomodated in-situby penetrative stretching and magmat i sm in the lower plate.

Severa l arguments can be made for at least some m o v e m e n t o n the N S R D : 1. T h e me tamorph ic grade o f the youngest uni ts in the lower plate may locally be appreciably higher than that o f the oldest units in the upper plate. 2. T h e amoun t o f ex tens ion in the upper plate appears to be somewhat higher than the amount o f extension in the lower plate. 3. T h e overa l l assymmetry o f lower plate deformat ion (F igure 14) and the preferential deve lopment o f down-to-the-east n o r m a l faults is compat ib le


STYLES OF PROGRESSIVE CO-AXIAL DEFORMATION

CRUSTAL COMPRESSION CRUSTAL EXTENSION

' / ^ - • - t D3: SMALL SCALE KINK FOLDING — J

OF OLDER FOLIATION SURFACES THRUST FAULTING-

ABOUT STEEP AXIAL PLANES OLDER OVER YOUNGER,

HIGHER OVER LOWER GRADE A

crinkle lineations }

S2 / D2 .UPRIGHT TO OVERTURNED

TIGHT TO OPEN CO-AXIAL FOLDS

REFOLD S1 SURFACES

D 1

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ISOCLINAL TO TIGHT

RECUMBENT FOLDING ,

TRANSPOSITION OF ORIGINAL LAYERING INTO

PARALLELISM W/ AXIAL PLANE CLEAVAGE

DEVELOPMENT OF BRITTLE NORMAL FAULTS AND JOINTS

DEVELOPMENT OF LA YERING-PARALLEL FAULTS

AND DUCTILE NORMAL FAULTS

HORIZONTAL STRETCHING OF UNITS,

FEW FOLDS DEVELOPED IF LAYERING IS

SUBHORIZONTAL, ABUNDANT FOLDS IF STEEP.

PROLATE STRAIN ELLIPSOID

Figure f . Comparison of ductile to brittle progressive, deformational fabrics produced during compression and extension.

with eastward m o v e m e n t o f the upper plate wi th respect to the lower plate. 4. T h e observed strain gradient towards the decollement may indicate that lower plate deformat ion i n vo lved a componen t o f s imple shear. I f so, upper plate rocks w o u l d be increasingly a l lochthonous wi th respect to progressively deeper hor izons in the lower plate. A l t e r n a t i v e l y , the gradient in penetrative stretching may s imply reflect increasing di la t ion by plutons wi th inceasing depth.

W e r n i c k e (1981, 1982) interpreted detachment faults as very low-angle no rma l faults, or zones o f s imple shear, that may i n v o l v e the entire crust. In his m o d e l , n o r m a l faul ts i n the ove r ly ing

"ex tens iona l a l l o c h t h o n " are " r o o t e d " a long a gently d ipp ing surface to a laterally distant and m u c h deeper area in the lower plate. A l t h o u g h some m o v e m e n t may have occur red on the N S R D , there is s t rong evidence that this surface was o r ig i nally flat and d i d nor cut down-sec t ion in the direct ion o f m o v e m e n t o f the upper plate. A c r o s s the entire w i d t h o f the range, the oldest unit at the toes o f upper plate fault sl ices ( M i d d l e C a m b r i a n Po le C a n y o n L i m e s t o n e ) is i n stratigraphic cont inu i ty wi th the youngest unit i n the lower plate ( L o w e r C a m b r i a n P i o c h e Sha le ) . T h e lack o f stratigraphic (i .e . , s t ructural) o m i s s i o n across the decol lement effectively rules out large amounts o f m o v e m e n t on a


surface that or iginal ly cut down-sec t ion . It is interesting to note that a s imi lar lack o f stratigraphic omiss ion is evident in the R u b y M o u n t a i n s (Snoke and others, 1982) and in the Raft R i v e r Range ( C o m p t o n and others, 1977) and may characterize metamorphic core complex detachment faults in general.

S Y N T H E S I S A mid-Tertiary Extensional Belt

Imbricate low-angle faults and large stratal rotations are the characteristic geometr ic e lements o f the Egan , Schel l C r e e k , and Snake Ranges . In each range, they are demonstrably the consequence o f O l i g o c e n e - M i o c e n e ( ? ) ex tens ion or iented approx i mately N 65 W to S 65 E . Sharp supracrustal boundaries between ranges affected by this tecto-n i sm and relatively unfaulted ranges to the east and west delineate a north-northeast t rending " b e l t " o f mid-Ter t ia ry extens ion (F igure 20) . T h e nor thern Egan, nor thern Sche l l C r e e k , and nor the rn Snake Ranges are domina ted by down-to-the-east faults and westward d ipp ing beds, whereas ranges to the south display the opposite polarity o f faulting and t i l t ing (Figure 20) . These two structural domains are separated by a zone o f complex block faulting and ti l t ing rather than a discrete t ransform fault.

The total amount o f ex tens ion across this cor r idor can be estimated f rom the offset o f o lder structural trends. Southwest and northeast o f the mid-Ter t i a ry extensional belt the Butte and C o n f u s i o n sync l i no r i -ums are subparallel and approximately 40 k m apart (Figure 20) . W h e r e the northeast t rending belt o f extension intervenes between these older folds, they achieve a m a x i m u m separation o f 135 k m as first one, and then the other, is variably rotated, broken into segments, and " p u l l e d " out to the west. T h i s 95 k m of d i la t ion is equivalent to 250 percent extension and must be v iewed as a m i n i m u m estimate because: 1) it does not a l low for ex tens ion between the two sync l ino r iums outside the ex tens ional belt; and 2) it is averaged over the non-ex tended l imbs of the sync l ino r iums . None the less , it is c o m patible wi th our reconstruct ions o f ind iv idua l ranges wi th in the belt.

W e have palinspastically restored part o f east-central Nevada by sh r ink ing the area between the Butte and C o n f u s i o n sync l ino r iums to its or ig inal width of 40 k m (Figure 21a). W e assumed homogeneous extens ion across this cor r idor , but emphasize that, in fact, the strain is quite heterogeneous. Ear ly Ol igocene geologic contacts were obta ined f rom the Tert iary unconformi ty data (F igure 4) and indicate

a s imple pattern o f N - S t rending , gentle ant icl ines and sync l ines (F igures 21a and 21b) . T h e concave westward map pattern is a consequence o f ho ld ing the C o n f u s i o n s y n c l i n o r i u m " f i x e d " du r ing the recons t ruc t ion and may reflect differential extens ion east o f the C o n f u s i o n Range .

Othe r h ighly ex tended areas in the N o r t h Basin and Range (e.g., the R u b y M o u n t a i n s and the Raft R i v e r Range) have not been reconstructed but probably represent a s imi la r magni tude o f ex tens ion as the 250 percent we have recorded in east-central N e v a d a . These h ighly extended domains are separated by regions o f lesser ex tens ion (15 to 30 percent, Stewart , 1978). T h e total amount o f extens ion across the nor thern Bas in and Range province is probably close to or greater than the 100 percent es t imated by H a m i l t o n and M e y e r (1966) , and Prof-fett (1977) .

Discussion: Models of Extension G e o l o g i c relat ions in the E g a n , Schel l C r e e k , and

Snake Ranges help cons t ra in the k inemat ics o f crustal ex tens ion . W e present a s impl i f i ed cross-section o f east-central N e v a d a (F igure 21c) that is consistent wi th these relat ions and illustrates the key processes o f crustal ex tens ion .

1. Supracrustal ex tens ion was accompl i shed by high-angle (50 to 6 0 ° ) , shovel -shaped norma l faults that rotated to low angles as they m o v e d . F a n n i n g splays d o c u m e n t progressive rotat ion o f planar faults, and helped accomodate space problems at depth. N o r m a l drag c o m m o n l y obscures original bedding-to-faul t angles and creates the i l l u s ion o f " b e d d i n g - p a r a l l e l " fault ing. H o w e v e r , we find no evidence for large displacement no rma l faults that or iginated at low angles, even though this area has been repeatedly c i ted as a " type area" for this k i n d o f de fo rmat ion ( H i n t z e , 1978; W e r n i c k e , 1981, 1982; W e r n i c k e and B u r c h f i e l d , 1982).

2. Supracrustal high-angle faults flattened abruptly into mid-c rus ta l duct i le-br i t t le t ransi t ion zones that e v o l v e d into brit t le detachment faults. In the nor the rn Snake Range the duct i le-br i t t le t ransi t ion or ig ina ted at 6 to 7 k m depth , whereas in the Egan and Sche l l C r e e k Ranges it was deeper than 10 k m . U s i n g the Snake Range as our example , the detachmen t faults need not have appreciable offset as the amoun t o f ex tens ion above and below the Snake Range detachment fault is s imi lar . W e conclude that ex tens iona l de tachment faults can be locally deve loped , have finite boundar ies , and need not "sur face" or " r o o t . "

3. A t deeper levels , ex tens ion was accompl ished


O 10 2 0 3 0 4 0 5 0 K M ' U = J 1

Figure 20. M i d Tertiary extensional belt in east-central Nevada. Highly extended areas characterized by imbricate normal faults and steep bedding dips are shaded. Intervening unpatterned areas display little supracrustal deformation bedding attitudes are on early 1 erti-ary volcanic and sedimentary rocks but are representative of underlying Paleozoic rocks as well. Sources of data: Stewart and Carlson, 1976, and those listed for Figure 4.


B U T T E MTS

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D U C T I L E S T R E T C H I N G

E X T E N S I O N I N L O W E R C R U S T A C C O M O D A T E D B Y I N T R U S I V E R O C K S

A M D D U C T I L E S T R E T C H I N G

M E S O Z O I C ' P E N E T R A T I V E S H O R T E N I N G A N D

T H R U S T F A U L T S A T D E P T H L A R G E L Y O B L I T E R A T E D B Y

T E R T I A R Y M A G M A T I S M A N D D U C T I L E S T R E T C H I N G

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15

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M I D C A M B R I A N O L I G O C E N E P L U T O N S


by penetrative stretching and in t ru s ion , s imi la r to models proposed by R e h r i g and R e y n o l d s (1980) and Ea ton (1982). Large v o l u m e s o f syntectonic volcanic rocks in the Sche l l C r e e k Range and T e r t i ary K - A r ages in the Snake Range v i r tua l ly require mid-Ter t ia ry batholi ths at depth and suggest that extensional processes were coupled wi th and /or tr iggered by a h igh thermal f lux.

A n enigmat ic aspect o f east-central N e v a d a , and many other core complex terranes, is that these highly extended areas are also topographical ly high. T h i s cannot be expla ined by present day heat flow patterns nor by s imple isostatic r ebound due to supracrustal t h inn ing , but instead sugests that the entire l i thospheric c o l u m n beneath the ex tended areas has been modi f ied (e.g., L e P i c h o n and others, 1982). One possibi l i ty is that the unde r ly ing mantle l i thosphere was th inned by a greater factor than the ove r ly ing crust.

T h e observed stretching factor o f 250 percent and the present crustal thickness of 25 to 30 k m in east-central Nevada (Saleeby and others, 1982) requires that either the crust was incredib ly thick (85 to 100 km) prior to ex tens ion , or, more l i ke ly , large amounts o f juven i l e magma were added to the crust dur ing ex tens ion . A present-day crust composed o f up to 50 percent Ol igocene plutons (F igure 21c) is compat ible wi th our inference that ex t reme supra

crustal ex tens ion in east-central N e v a d a was genetically l i n k e d to deep-seated magmat i sm and ducti le deformat ion .

ACKNOWLEDGEMENTS W e appreciate the encouragement and financial

support g iven to us by She l l O i l and N o r a n d a M i n i n g companies and the financial support o f the Stanford U n i v e r s i t y Ea r th Sciences M c G e e F u n d in the beg inn ing stages of our work in east central N e v a d a . O u r work is presently funded by N S F G r a n t ( E A R - 8 2 - 0 6 3 9 9 ) awarded to E . L . M i l l e r and G . A . M a h o o d , w h i c h we gratefully acknowledge. G r i e r and G a n s , in add i t ion , acknowledge support o f their s tudies by T e n n e c c o O i l C o m p a n y and Stanford A M A X funds. M a n y thanks go to the members and T . A . ' s o f the 1981 and 1982 Stanford G e o l o g i c a l Survey for their en thus iasm and hard work i n the Snake Range . W e are ex t remely grateful to S o h i o O i l C o m p a n y for their recent funding o f the Stanford F i e l d C a m p . O u r unders tanding o f the geology o f east central N e v a d a has benefited f rom discussions wi th R . R . C o m p t o n , J . G a r i n g , J . H a k k i n e n , R . K . H o s e , G . A . M a h o o d , A . S n o k e , B . R o b i n s o n , G . T h o m p s o n , and especially members o f the Stanford G e o l o g i c a l Survey . F i n a l l y , we thank M e l i n a W h i t e h e a d for typing and help ing us assemble this paper, and Je f f L e e for cri t ical comments .

TERTIARY STRATIGRAPHY AND GEOLOGIC HISTORY OF T H E S A C R A M E N T O PASS AREA, NEVADA

Susan Grier Stanford U n i v e r s i t y , Stanford, C A 94305

ABSTRACT Tert iary strata in the Sacramento Pass region o f

the Snake Range , N e v a d a , accumula ted in a N W - S E t rending 0 1 i g o c e n e - M i o c e n e ( ? ) basin between two areas wi th different ex tens iona l his tor ies (see M i l l e r and G a n s , this v o l u m e ) . T h e 2 to 3 k m t h i c k s e q u e n c e u n c o n f o r m a b l y o v e r l i e s Pennsy lvan ian -Pe rmian rocks and consists o f a basal conglomerate , latite f lows, rhyol i te tuff, lacustrine l imestone , and a younger conglomerate . Paleo-current indicators and l i thotypes i n the younger con glomerate indicate a southwestern source, i m p l y i n g that the earliest fault ing occur red to the southwest

o f the basin where rocks as o ld as the La te P recambrian M c C o y C r e e k G r o u p were exposed. Large landsl ide b locks exist w i t h i n the l imestone and younger conglomera te sec t ion , also suggesting considerable s tructural re l i e f in adjacent regions.

F o u r down-to-the-east no rma l faults that are cu rved in p la in v iew repeat the Ter t iary sect ion and have t i l ted strata 30 to 50° westward. T h e faults occurred s imul taneous ly wi th or prior to final m o v e ment on the Snake Range decol lement ; however , the depos i t ion o f the Ter t iary sect ion entirely predated the exposure o f the lower plate rocks in the N o r t h e r n Snake Range .

Guidebook, Part 1 — G S A Rocky Mountain and Cordilleran Sections Meeting, 1983 141 O

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CEN

OZO

IC

TER

TIAR

Y

T c 2

CEN

OZO

IC

TER

TIAR

Y CEN

OZO

IC

TER

TIAR

Y

Tl

CEN

OZO

IC

TER

TIAR

Y CEN

OZO

IC

TER

TIAR

Y

Trt

CEN

OZO

IC

TER

TIAR

Y CEN

OZO

IC

TER

TIAR

Y

Tlf

CEN

OZO

IC

TER

TIAR

Y CEN

OZO

IC

TER

TIAR

Y

T c ,

Alluvium

Older alluvium

Glacial deposits

Younger conglomerate

Lacustrine limestone and marl

Rhyolite tuff

Latite flows

EXPLANATION

Older conglomerate

UNCONFORMITY

Arcturus Formation P a

P e

M c

Mj

o 6 N o

2 I

M D p

D g

D s i

Ely Limestone

Chainman Shale

Joana Limestone

Pilot Shale (not p r e s e n t in m H p a r e a ) — 1

Guilmette Formation

Simonson Dolomite

M Undifferentiated Mississippian rocks

j P s e j Sevy Dolomite

O O z N 2 O n i CO Ul s - J < 5 °

Notch Peak Formation

CI

C d I Dunderberg Shale

Lincoln Peak Formation

Pole Canyon Limestone

Pioche Shale

C u

C p c C m

C p i

C p m Prospect Mountain Quartzite

McCoy Creek Group

C l o

Upper Cambrian rocks

Middle Cambrian rocks

Lower Cambrian rocks

Peraluminous granite and pegmatite

Low angle normal faults with teeth on upper plate

High angle normal faults

y Gravity landslide blocks

Depositional or intrusive contacts

Undifferentiated Devonian rocks

O e

Undifferentiated Laketown Dolomite (Silurian) and Fish Haven Dolomite (Ordovician)

Eureka Quartzite

O p Pogonip Group

P Undifferentiated Paleozoic limestones

O Undifferentiated Ordovician rocks

I N T R O D U C T I O N T h e Tert iary sect ion crops out a long W e a v e r

Creek and along a broad stretch f rom just nor th o f Highway 50 to M i l l e r Bas in W a s h (F igure 1). T h i s area has been previously mapped by Hose and Blake (1976) , H o s e (1981) , and the Stanford G e o l o g i c a l Survey (1981, 1982). T h e present study was conducted to document the sedimento logica l charac

teristics o f the Tert iary sect ion, to examine the deformat iona l history o f the area, and to elucidate the s tructural evo lu t i on o f adjacent regions.

S T R A T I G R A P H Y A t the base o f the Ter t iary sect ion, unconforma-

bly o v e r l y i n g P e n n s y l v a n i a n - P e r m i a n rocks , is an older conglomera te ( T c , ) . T h i s poorly exposed con-


glomerate contains r o u n d to subround clasts de r ived exclus ive ly from uppermost Paleozoic formations (F igure 2) .

C o n f o r m a b l y ove r ly ing the older conglomerate are Tert iary volcanic rocks dated at 35 m.y . o ld by H o s e and Whi t eb read (1981) . These consist o f latite flows (Tlf) over la in by rhyol i te tuff (Trt) (Figure 2). T h e latite flows form very dark red, knobby h i l l s , are lavendar gray on fresh surfaces, and contain plagioclase, alkali feldspar, biot i te , h o r n b l e n d e , aug i t e , and s o m e qua r t z and magnetite. T h e rhyol i te ash-flow tuff is general ly dark red to green-gray and contains 5 to 10 percent phenocrysts o f quartz , a lkal i feldspar, plagioclase, bioti te, and i ron hornblende . T h e tuff is moderately to densely welded and contains abundant flattened pumice lumps , as wel l as l i thic fragments o f volcanic rocks and sandstone.

Tert iary lacustrine deposits (T l ) over l i e the v o l canic rocks and consist pr imar i ly o f a b rown to tan ledgy s lope- forming , pisol i t ic , sandy l imes tone . Interbedded wi th this l imestone is whi te , sandy tuffaceous mar l (F igure 2) . T h e l imestone is w e l l -bedded, generally w i t h planar beds vary ing from 2 c m to 3 m in thickness; wavy beds are occasional ly present. C o m m o n sedimentary features in the l imestone inc lude s i l ic i f ied s tromatol i tes , m u d cracks, s i l ic i f ied evapori tes, fenestral fabrics, and chert beds and lenses. T o w a r d the top of the sect ion, th in beds o f pebble conglomerate and sandstone are interbedded wi th l imestone , and a rhyol i te air fall tuff is exposed in trenches. T h e absence o f marine fossils and the presence o f detri tal clasts support a shal low, ephemera l , saline lake as the deposi t iona l e n v i r o n m e n t o f the l i m e s t o n e . T h e stromatol i tes , pisoli tes, m u d cracks, and s i l ic i f ied evaporites all indicate frequent dessicat ion of this lake.

Interf ingering with and ove r ly ing the lacustrine sediments is a younger conglomerate ( T c 2 ) (F igure 2). T h i s conglomerate forms red to gray hi l ls but is generally poorly exposed and concealed beneath a mantle o f younger a l l u v i u m . U n l i k e the older conglomerate , the younger conglomerate contains clasts der ived from the entire Pa leozoic sect ion (90 percent) , as wel l as f rom Precambr ian and C a m b r i a n metasedimentary rocks (1 to 5 percent) , M e s o z o i c ^ ) muscovi te-b io t i te granite (1 percent) , pera-l u m i n i o u s p e g m a t i t e (1 p e r c e n t ) , con tac t -metamorphosed schist (1 percent) , and unde r ly ing Tert iary volcanic rocks and l imestone (1 percent, lo cally 5 to 10 percent) . T h e subangular to w e l l -

rounded clasts o f the younger conglomerate are supported by a calcareous sandstone matr ix and represent two distinct mechan i sms of deposi t ion: 1) i n terbedded debris flow deposits and; 2) s tream channel deposits . T h e debris flow deposits are mat r ix suppor ted, have very poor sor t ing, and conta in boulders up to at least 2 m in diameter . A single debris flow unit may range from less than 3 to 6 m in thickness . Ra re ly , these deposits show crude channe l l i ng at the top o f the flow. T h e stream channel deposits tend to be stratified by grain size, have s u b r o u n d to r o u n d clasts, and are better sorted. S o m e beds appear planar, whi le others are lent icular wi th we l l -deve loped lag deposits. C h a n n e l size averages at 3 to 4 m long and 1 m thick in cross sect ion. T h e occasional scour features in the conglomerate inc lude groove marks and cut and fill structures. Imbr ica t ion in the younger conglomerate measured a long M i l l e r Bas in W a s h indicate a northeasterly paleocurrent d i r ec t ion , thus s ignifying a southwest source area; these conc lus ions are consistent wi th the fact that conglomera te interfingers towards the northeast wi th unde r ly ing and coeval lacustrine sediments . T h e younger conglomerate is interpreted to have been deposi ted as part o f a subareal fan c o m plex upon wh ich braided stream channe l l ing was dominan t wi th frequent debris flows.

Present in both the lacustr ine l imestones and the younger conglomerate are large landsl ide b locks , shown i n F igu re 1. These are m o n o l i t h i c or zoned po ly l i th ic , brecciated, yet coherent , masses o f Pa leozoic l imes tone , do lomi t e , and quartzite. T h e majority o f these b locks consist o f O r d o v i c i a n -S i lu r i an do lomi tes , O r d o v i c i a n E u r e k a Quar tz i te , and the O r d o v i c i a n Pogon ip G r o u p , but some blocks are composed o f strata as o ld as the C a m b r i a n Pole C a n y o n L i m e s t o n e . T h e cement o f each slide block appears to be der ived f rom the const i tuents o f the blocks themselves , for example , slide blocks o f do lomi t e have d o l o m i t i c cement , whi le those o f q u a r t z i t e c o n t a i n a s i l i c i o u s c e m e n t . T h i s characterist ic, coup led wi th the m o n o l i t h i c nature, suggests that breccia t ion and l i th i f ica t ion either predated or occur red du r ing the emplacement o f the sl ide b locks . T h e slide blocks c o u l d , in part, represent fault breccias formed a long a pre-exis t ing scarp in adjacent regions. These o l i s tos t romal sl ide b locks probably t raveled across the a l luv ia l fan c o m plex onto the soft l ime m u d in the lake. T h e Tert iary l imes tone by W i l l o w Patch Spr ing show conica l folds and convo lu te bedding beneath a large slide block o f O r d o v i c i a n E u r e k a Quar tz i te .

Guidebook, Part 1 - G S A Rocky Mountain and Cordilleran Sections Meeting, 1983 143

AGE FORMATION THICKNESS LITHOLOGY DESCRIPTION

TER

TIA

RY

Younger

Conglomerate 1500m

• oD°.or>X- 1: •

^ «. .o O o „ : <-^C. 1_... J . .

. V ^ V ! • 1 i :

R e d to g r a y , m e d i u m to p o o r l y s o r t e d c o n g l o m e r a t e w i th s u b a n g u l a r to r o u n d e d c l a s t s r a n g i n g f r o m P r e c a m b r i a n to T e r t i a r y in a g e . R e p r e s e n t s d e b r i s f l o w a n d s t r e a m c h a n n e l d e p o s i t s of an a l l u v i a l f a n c o m p l e x .

TER

TIA

RY

Lacustrine Deposits

500m

^ T V T T T T

— 1 — 1 , 1 ' 1 1 ' J 1 1

^ L r > L ^ < t ^

P : T ^ r T T 1 7 r r T 7 r

• 1 1 1 1 1 1 1 1 1 i - V -

r ^ r ^ . 1 i 1 i 1 • 1

L e d g y s l o p e - f o r m i n g , bu f f to t an , w e l l - b e d d e d ,

p i s o l i t i c l i m e s t o n e w i t h s t r o m a t o l i t e s , s i l i c i f i e d e v a p o r i t e s , c h e r t , p o l y g o n a l p u l l - a p a r t c r a c k s a n d r a r e p l a n t r e m a i n s . T u f f a c e o u s m a r l is i n t e r b e d d e d . T h i n b e d of r h y o l i t e tuff a n d s o m e p e b b l e c o n g l o m e r a t e b e d s n e a r t o p of s e c t i o n . I n t e r f i n g e r s w i th o v e r l y i n g c o n g l o m e r a t e .

TER

TIA

RY

Rhyolite Tuff 90m A > v V / V > * ' r ^

G r e e n to r e d d i s h b r o w n r h y o l i t e a s h - f l o w tuf f . C o n t a i n s 5 - 1 0 % p h e n o c r y s t s .

TER

TIA

RY

Latite Flows 0 - 1 7 0 m L- v " 1 L- \ <• ^ -1 -1 ^

, > -i - r -j 7 v

D a r k r e d to l a v e n d a r l a t i t e f l o w s . D a t e d at 3 5 M Y o l d ( H o s e a n d W h i t e b r e a d , 1 9 8 1 ) .

Older Conglomerate 40m P o o r l y s o r t e d r e d c o n g l o m e r a t e w i t h r o u n d to

s u b r o u n d c l a s t s . C o n s i s t s e n t i r e l y of u p p e r m o s t P a l e o z o i c l i m e s t o n e c l a s t s .

| PE

NN

Ely Limestone

Sill Figure 2. Stratigraphic C o l u m n .


S T R U C T U R E The stratigraphic relations w i t h i n the Ter t iary sec

tion in Sacramento Pass provide important insight into the structural history o f adjacent regions. T h e fact that the older conglomerate contains on ly upp e r m o s t P a l e o z o i c c l a s t s a n d r e s t s o n Pennsy lvan ian -Pe rmian rocks , indicates that on ly mino r structural re l ief exis ted prior to the deposition o f the pre-35 m.y. o ld conglomerate . Af te r or beginning with the per iod o f v o l c a n i s m , major faulting and uplift occurred in the adjacent areas, supplying clasts der ived from strata as o ld as P recambr ian . Th i s uplift was synchronous wi th the deposi t ion of the lacustrine sediments and of the younger conglomerate . Tec ton ic uplift is also ev idenced by the presence o f large slide b locks w i t h i n the Ter t iary sequence that range in age f rom C a m b r i a n to S i lu r ian . A s indicated by clast imbr i ca t i on , the source for the younger conglomerate was to the southwest, i m p l y i n g that the earliest fault ing was along the southwest Hank of the basin. L i tho types in the younger conglomerate also support a southern source. T h e non-fol ia ted and non- l inea ted Precambrian and C a m b r i a n metasedimentary clasts and granitic clasts in the younger conglomerate are s imilar to l i thologies presently exposed in the Southern Snake Range , whi le equivalent format ions in the lower plate in the N o r t h e r n Snake Range decol lement are l ineated and foliated (see G a n s and M i l l e r , this v o l u m e ) . Clasts o f contact metamorphosed schist are s imi la r to M c C o y Creek G r o u p schists in the contact aureole o f the Strawberry Creek pluton in the Sou the rn Snake Range . Fau l t ing to the south and west o f the Sacramento Pass Ter t iary depocenter eventual ly migrated to the flanks of the basin itself, for clasts o f Ter t iary vo lcan ic rocks and l imestone were incorporated cannibal is t ica l ly into the younger conglomerate .

F o u r down-to-the-east , arcuate, n o r m a l faults repeat the Sacramento Pass Ter t iary sequence and tilt strata 30 to 50° westward. T h e faults apparently flatten abruptly wi th depth and merge w i t h , but do not cut , the N o r t h e r n Snake Range decol lement (Figure 1). T h u s , deformat ion o f the Sacramento Pass basin occurred prior to or was coeval wi th the youngest movemen t a long the Snake Range decol lement ; however , lower plate rocks were not exposed dur ing the deposi t ion o f this entire sequence.

C O N C L U S I O N D u r i n g the Ter t iary , a saline lake f lanked by an

a l luvia l fan complex to the southwest occupied the

Sacramento Pass area. P r io r to 35 m.y. ago the topography in adjacent areas was only minor . Majo r fault ing and tectonic uplift occurred to the southwest after the per iod of Ter t iary vo lcan i sm and coeval wi th the deposi t ion o f the lacustrine sediments and the younger conglomerate . Fau l t i ng act ivi ty migra ted into the Ter t iary basin, and the resu l t ing faults that cut the Ter t iary section merge w i t h , or are cut by, the nor thern Snake Range deco l lement .

F I E L D T R I P R O A D L O G : F I R S T D A Y

Style of Mid-Tertiary Extension in East-Central Nevada

Elizabeth L . Mil ler , Phillip B. Gans, and Susan Grier

Mileage Incre- Cumu-

Description

mental 0.0

lathe 0.0

1.1 9.9

2.0 11.9

T h e roadlog starts at the junction o f Highways 50 and 93 in E l y , Nevada (Figure 1). P r o ceed nor th on Highway 93 towards M c G i l l .

T u r n left on graded dirt road immedia te ly past the C l u b 50 Cafe.

A t pavement turn left and dr ive due west across Steptoe V a l l e y .

Straight ahead is Heusser M o u n t a i n , part o f the nor thern Egan Range. A biotite granite p l u t o n (wh i t e ou tc rops in l o w e r fo reg round) intrudes upper P r e c a m b r i a n M c C o y C r e e k G r o u p (reddish outcrops and talus at the crest o f the ridge and to the r ight) . W o o d ward (1962) describes polyphase deformat ion and metam o r p h i s m of the country rocks that resemble what y o u wi l l see in the M c C o y C r e e k area tomor row.

Pavemen t ends. T u r n right o n graded dirt road. Y o u are now


S C A L E

Figure 1. Field trips stops.


headed nor th on the west Step-toe V a l l e y R o a d . O n the left is the Egan Range . T h e h igh r idgel ine o f the Sche l l C reek Range is now vis ible on the r ight , b e h i n d the D u c k C r e e k Range. M c G i l l ' s tailings ponds are i n the foreground.

5.9 17.8 Straight ahead, the p rominen t white band is one o f the most d i s t i n c t i v e Pa leozoic formations, the O r d o v i c i a n E u r e k a quartzi te . T ree covered slopes below are under la in by the P o g o n i p G r o u p , o v e r l y i n g cliffy uni ts up to the r idgel ine are the O r d o v i c i a n and S i lu r i an do lomi tes .

6.2 24.0 Steptoe R a n c h on the right.

4.6 28.6 T u r n left on road to Wate r C a n y o n : u n m a r k e d but look for m o u n d of dirt by the turnofT.

T h e l ow , ye l lowish b rown hi l l s at the m o u t h o f Water C a n y o n consist o f Pennsy l -van i an -Pe rmian strata over la in by Ol igocene vo lcan ic rocks. These units are downdropped against a large bioti te-granite p lu ton a long a major, east dipping fault (refer to Stop 3) .

1.5 30.1 C o n c r e t e p i l l a r wi th black arrow. F o l l o w the arrow by taking the right fork in the road and then cont inue straight up Wate r C a n y o n and into the p lu ton .

1.3 31.4 C o n t i n u e straight, road off to right.

0.2 31.6 Spr ing o n the right.

0.4 32.0 R o a d off to right. Park here. Introductory remarks by E .

L . M i l l e r and ove rv iew o f the geology o f the E g a n Range by P. B . G a n s . S T O P 1. O l igocene

M a g m a t i s m at M i d - C r u s t a l Dep ths .

T h e thick stack o f upper Precambr ian to P e r m i a n strata in the nor thern Egan Range was in t ruded at its base by several large composi te plutons. A t this stop, y o u w i l l examine the textural variat ions in one o f these plutons at a depth o f e m placement o f 10 k m 1 .

T h e ma in phase o f the Water C a n y o n p lu ton is an equigranu-lar biotite granite wi th K-spar megacrysts. It is coarse-grained right up to its in t rus ive contacts a n d contact metamorphoses the wall rocks. It has y ie lded a K - A r date o n bioti te o f 36.2 ± 0.7 m.y . ( A r m s t r o n g , 1970) but may be significantly older. T h e main phase is weakly de fo rmed as ev idenced by po-lygon ized and strained quartz and b roken feldspars. L o c a l l y , m y l o n i t i c shear zones are abundant .

T h e m a i n phase o f the p lu ton is in t ruded by at least two varieties o f porphyr i t ic d ikes , here informal ly named A p l i t e Porphyry and Gran i t e Po rphyry . B o t h have s imi lar m i n e r a l o g y (qz-ksp-plag-bio) but are differentiated on the basis o f texture. A p l i t e Porphyry has relat ively smal l quartz and feldspar phenocrysts in a groundmass that grades f rom aphanit ic at the margins of d ikes to aplit ic in the interiors. G r a n i t e porphyry has a fine- to coarse-grained, seriate g round-mass and is characterized by ex t remely large phenocrysts o f K- spa r (up to 10 c m ) . G r a -nophyr ic textures are c o m m o n in the groundmasses o f both types o f dikes.

T h e dikes presently dip 10 to

•The companion paper to this roadlog describes how we derive paleodepths.


40° eastward because o f westward t i l t ing . B o t h sets o f d ikes can be traced con t inuous ly westward and upward out o f the p lu ton and into the overly ing country rocks where they c o m m o n l y in t rude low-angle no rma l faults. In the H u n t e r distr ict , they are exposed at depths o f emplacement o f 1 to 3 k m , and, on compos i t iona l and textural grounds , are clearly feeders for nearby crystal-r i c h r h y o l i t e tuffs ( G a n s , 1982). Re la t ions in the H u n t e r district suggest that the onset o f ex tens ional fault ing precisely co inc ided wi th the emplacement and ven t ing o f these s i l i cic dikes . T h e d ikes and the tuffs y i e l d indis t inguishable K - A r biotite dates o f 35.8 m.y. ( G a n s , 1982).

T h u s , steep westward t i l t ing in the nor thern Egan Range has rotated an early Ol igocene magmatic system over on its side and a l lowed us to examine it f rom the or ig ina l surface d o w n to mid-crus ta l depths. A t th i s s top, the equigranular ma in phase o f the p lu ton may represent an early crysta l l ized top o f a magma chamber that was s u b s e q u e n t l y fractured and injected wi th feeder dikes f rom a deeper leve l . Perhaps m o s t s u r p r i s i n g is t h e "hypabyssa l " texture o f these mid-crus ta l dikes . T h e y are always strongly porphyr i t ic and appear to have had glassy margins. W e expla in this by: 1) p r e s s u r e q u e n c h i n g , as magmas were injected a long active fractures and vented to the surface, coup led w i t h 2) i n c u r s i o n o f m e t e o r i c water d o w n the same system o f fractures that were gu id ing the magmas.

C o n t i n u e up the ma in Wate r C a n y o n R o a d

1.0 33.0 E n d o f Wate r C a n y o n R o a d . T u r n a round and park. S T O P 2. Structural O v e r v i e w o f the N o r t h e r n Egan Range .

W e wi l l be gone f rom the vehic les for approximate ly 2]A h o u r s so br ing l u n c h and water. F r o m the end o f the road, we wi l l h ike up Wate r C a n y o n into the in ter ior o f the Egan Range . Part o f this walk w i l l be through brush so it is i m p e r a t i v e tha t we s t i ck together.

A detailed account o f the s t ruc tu ra l e v o l u t i o n o f the nor thern Egan Range is p r o v i d ed by G a n s and M i l l e r (this v o l u m e ) . T h e range is c o m posed of th in slices o f steeply west -dipping upper P recambr i an to Pe rmian rocks and O l i g o cene volcanic rocks that are separated by low-angle no rma l faul ts . Each fault displaces o v e r l y i n g sect ions eastward wi th respect to unde r ly ing sections such that younger strata always rest o n older. W e w i l l begin in L o w e r C a m b r i a n Prospect M o u n t a i n Quar tz i te in the lowest s tructural s l ice , cross a l ow-ang l e fault into U p p e r C a m b r i a n D u n d e r b e r g Shale and N o t c h Peak L i m e s t o n e in the next higher s l ice, and f inally cross another fault onto a kl ippa o f Pennsy lvan ian E l y L i m e s t o n e . F r o m here we can get a broad ove rv iew o f the i n ternal structure o f the nor thern Egan Range and discuss in detail the k inemat ics o f no rma l fault ing.

R e t u r n to the vehic les and d r i v e back out the Wate r C a n y o n R o a d .

2.6 35.6 T u r n left at m o u t h o f Wate r C a n y o n on road that curves up to a road cut.

0.2 35.8 Park on road that bears to the right. S T O P 3.

148 Utah Geological anil Mineral Survey Special Studies 59, 1983

Here we w i l l qu ick ly examine an e x h u m e d fault surface that dips 25 to 30° eastward and parallels the eastern flank o f the n o r t h e r n Egan Range . Here a th in fault s l iver o f Pogon ip G r o u p separates O l i gocene volcanic rocks f rom the lowest exposed level in the pluton - a total offset o f greater than 10 k m . T h i s is not a detachment fault! Several h igh a n g l e n o r m a l faults have merged and rotated to low angles to produce the observed jux tapos i t ion . T h i s fault appears to be warped over the p lu ton in a convex upward fashion. C o n t i n u o u s exposures of the fault to the nor th rol l over from 25 to 30° eastward dips on the flank of the range to subhor izon ta l on top.

C o n t i n u e straight on this road.

1.5 37.3 Back to ma in Steptoe Va l l ey Road . T u r n left.

O n the left, the white outcrops on the low flank o f the Egan Range consist o f s i l ic i f ied and brecciated granite - part o f the same fault zone we e x a m ined at Stop 3.

4.5 41.8 T u r n r igh t . Y o u are now headed east across Steptoe V a l l e y . Straight ahead is the Sche l l C r e e k Range. R e d d i s h outcrops are exposures o f the Ol igocene K a l a m a z o o vo lcan ic sequence ( Y o u n g , 1960).

3.5 45.3 Intersect H ighway 93; tu rn right (south) .

12.0 57.3 T u r n right at paved road tur-noff to D u c k C r e e k V a l l e y (s ign) .

2.9 60.2 P u l l over a long side o f road and park. S T O P 4. In t roduct ion to the Sche l l C r e e k Range .

B r i e f overv iew of the geology o f the Schel l C r e e k Range by P . B . G a n s .

T h i s part o f the nor thern S c h e l l C r e e k R a n g e was mapped in detail by Y o u n g (1960) . H e described a sequence o f fault ing and t i l t ing events that closely matches what we see in the Egan and Snake Ranges. Y o u n g (1960) was one o f the first workers in east-central Nevada to document that Ter t iary rocks were extens ive ly faulted and til ted and that essentially all o f that f a u l t i n g in th is area was Ter t iary . A l t h o u g h he called l o w - a n g l e , younge r -on -o lde r faults " th rus t s , " he was unsure as to their o r ig in .

T h i s stop provides a good v iew o f some typical fault geometr ies in the nor thern Sche l l C reek Range . A low-angle fault displaces Ordo -v ic i an -S i lu r i an D o l o m i t e eastward and drags E u r e k a Quar t zite in the footwall into parallel i sm wi th the fault plane. T h i s fault is then cut by a younger moderate ly east-dipping fault.

C o n t i n u e straight on the paved road into D u c k Creek V a l l e y .

O v e r v i e w o f D u c k Creek V a l l e y .

D u c k Creek Va l l ey is a peculiar basin w i t h i n a range. It is under la in by a thick section of p r e d o m i n a n t l y w e s t - d i p p i n g c o n g l o m e r a t e s c a l l e d the N o r t h C r e e k F o r m a t i o n ( Y o u n g , 1960). T h e y uncon-formably over l ie Ol igocene v o l canic rocks and Pa leozoic rocks on the eastern margin of the val ley and are in fault contact wi th C a m b r i a n rocks on the west side. Y o u n g (1960) bel i eved that these conglomerates were M i o - P l i o c e n e in age and that D u c k Creek V a l l e y was


formed by relat ively young Bas in and Range faulting. In con t ras t , A n d e r s o n (1982) documen ted that at least the base o f N o r t h Creek F o r m a t i o n is Ol igocene and proposed that D u c k Creek V a l l e y formed as a c o n s e q u e n c e o f preferential e ros ion of weak clastic sediments f rom an uplifted O l i g o cene basin.

1.1 61.3 T u r n left onto dirt road and take right fork immedia te ly after the turnoff.

1.9 63.2 R o a d off to left, cattle guard; bear right.

0.7 63.9 Outc rops of debris flows near the base of the N o r t h Creek F o r m a t i o n on the left; take left branch at fork.

1.2 65.1 Park on saddle. S T O P 5. V o l canic Stratigraphy o f the Schel l C r e e k Range.

A detailed mapping and analyt ical inves t iga t ion of the v o l canic rocks in the Schel l C reek Range is current ly in progress by G a n s and G . M a h o o d . W e hope to use the volcanic rocks to bracket the age(s) and rate(s) o f extens ional deformat ion in this area and ul t imately unders tand the interplay bet w e e n v o l c a n i s m and extens ion .

A t this stop we wi l l briefly e x a m i n e some o f these syn-tectonic vo lcanic rocks. W e wi l l start near the bot tom of the Ter t iary sect ion and hike up through a basal sequence o f lacustr ine l imes tone and tuffaceous sandstones and then the three lower units o f the K a l a mazoo volcanic sequence. T h e lowest vo lcanic unit is a fairly distal outf low sheet o f the K a l a m a z o o Tuff . It is over la in by a th in in terval o f lahars and v o l -

3.8

2.5

2.5

5.1

17.2

canic breccias wh ich in turn , is over la in by b io t i te -hornblende dacite lava flows. These units were probably all de r ived f rom a major vo lcanic center i m mediately nor th o f D u c k C r e e k V a l l e y .

R e t u r n back out to the p a v e d D u c k C r e e k V a l l e y R o a d .

68.9 T u r n left o n the D u c k C r e e k Va l l ey R o a d .

70.0 T u r n left on gravel road toward B i r d Creek (s ign) .

72.5 P u l l into the circle and park at the B i r d C r e e k C a m p g r o u n d . S T O P 6.

I f t ime permits , we wi l l qu ick ly examine some exce l lent exposures o f the N o r t h C r e e k F o r m a t i o n . T h e con glomerates here conta in predominant ly Pa leozo ic carbonate clasts whereas to the nor th the clasts are most ly der ived f rom the Ol igocene vo lcan ic rocks. A t this stop the conglomerates inc lude clasts o f O r d o v i c i a n and C a m b r i a n l imestones suggesting that major structural re l ie f had been formed by the t ime they were deposi ted. W e are presently dating a rhyol i te flow that over l ies these con glomerate beds.

75.0 R e t u r n out to the D u c k C r e e k V a l l e y R o a d and turn right.

80.1 R e t u r n out to H ighway 93 and turn left back to E l y .

97.3 A r r i v e E l y .

F I E L D T R I P R O A D L O G : S E C O N D D A Y

Description Mileage Incre- C u m u -

mental lative 0.0 0.0 Intersect ion H ighways 50 and

93 , down town E l y . D r i v e east on H ighway 50.

150

17.8 17.8 A s c e n t to C o n n o r ' s Pass , Sche l l C r e e k Range . M i o g e o c l i nal strata in the C o n n o r ' s Pass q u a d r a n g l e ( m a p p e d by Drewes , 1967) are c o m p l e x l y faulted and t i l ted but not in a consistent d i rec t ion . B o t h east and west tilts occur as this part o f the Sche l l C r e e k Range lies w i t h i n a d i f fuse boundary separating a nor thern d o m a i n o f westward t i l t ing f rom a southern terrane o f eastward t i l t ing (F igure 20) . M o s t o f the rocks we are d r i v i n g through are P e n n s y l v a n i a n - P e r m i a n l imestones .

4.0 21.8 C o n n o r ' s Pass. W h e e l e r Peak o f the southern Snake Range at 2:00 in the distance (e levat ion 13,067 ft).

2.8 24.6 Here we cross a low-angle fault (the " S c h e l l C r e e k T h r u s t " o f D r e w e s [1967]) that separates b recc i a t ed D e v o n i a n r o c k s f rom metamorphosed and c o m plexly folded midd l e C a m b r i a n silty l imestones . It is not clear whether this fault is a regionally extens ive detachment fault or is s imply a ti l ted no rma l fault. D r e w e s (1967) interpreted the S c h e l l C r e e k T h r u s t as M e s o z o i c , bu t A r m s t r o n g (1972) suggested a Tert iary age as it i n v o l v e s Ter t iary strata.

1.8 26.4 J u n c t i o n o f H ighways 93 and 50, take left fork and cont inue east on 50. M t . M o r i a h , the highest peak in nor thern Snake Range lies in the distance at 11:00.

2.0 28.4 T u r n left on N o r t h Spr ing V a l l e y R o a d . T o the west, v i ew of C l e v e C r e e k drainage in the Sche l l C r e e k Range . R e d d i s h talus slopes on right are underl a i n by C a m b r i a n Prospect


M o u n t a i n Q u a r t z i t e ; wh i t e cliffs are M i d d l e C a m b r i a n l imes tone . N o r t h o f here, the entire eastern flank o f the Sche l l C r e e k Range consists o f a west-t i l ted sect ion o f late Precambr ian and Ear ly C a m b r i a n clastic rocks. U p to 8,800 ft o f structural sect ion o f late Precambr ian M c C o y C r e e k G r o u p rocks ( M i s c h and H a z z a r d , 1962) are exposed along this flank of the range. T h e h igh crest l ine o f the range is underlain by L o w e r C a m b r i a n Prospect M o u n t a i n Quar tz i te .

20.2 48.6 T h e light bands at the base o f the first hi l ls to the left are carbonate intervals w i th in the lower part o f the M c C o y Creek G r o u p .

3.5 52.1 T u r n left on dirt road up M c C o y C r e e k (marked wi th s ign) .

2.7 54.8 Camps i t e to left; turn a round and park. S T O P 7. M e s o z o i c P e n e t r a t i v e D e f o r m a t i o n at D e e p Structura l L e v e l s .

A t this stop, we wi l l l ook at phyl l i tes and quartzite in structural ly lower exposures of the M c C o y Creek G r o u p . T h e attitude o f deformat ional fabrics at this stop is typical o f M e s o z o i c deformat ion elsewhere in east-central N e v a d a in that the p rominen t cleavage is west-d ipp ing and axial planar to rare east-vergent mesoscopic folds (F igure 2) . Here this cleavage crenulates an older (SI ) surface that is subparal lel to bedding. B o t h cleavages die rapidly up sect ion and are restricted to phy l l i t i c intervals . Sedimentary structures are very we l l preserved except near the bo t tom o f the sequence and are always upright ; folds are scarce to absent. Bedding-cleavage inter-


PIERMONT CREEK MCCOY CREEK TAFT CREEK

3 8oo° .

POLES TO BEDDING POLES TO S1 C L E A V A G E POLES TO S2 C L E A V A G E

Figure 2. Structural data (lower hemisphere equal area project) Range.

section l ineat ions are or iented n o r t h - s o u t h a n d a r e subhor izon ta l .

Ac ros s the Snake , Sche l l C r e e k , and Egan Ranges , deep structural levels (generally P re cambr ian M c C o y C r e e k G r o u p but locally as h igh as the M i d d l e C a m b r i a n ) are variably m e t a m o r p h o s e d (greenschist to amphibo l i t e grade) and polyphase deformed. T h i s deformat ion and me tamorph i sm is wel l dated in the southern Snake Range by syn- to late-k i n e m a t i c plutons that are about 160 m.y. o ld ( M i s c h , I960; L e e and others, 1968, 1980). A l t h o u g h deformat ion and m e t a m o r p h i s m is as yet undated in the other ranges, the s imi lar i ty in structural style between these three ranges suggests a s imi la r age.

A s d e s c r i b e d by M i s c h (1960) , deformat ion and metam o r p h i s m across east-central N e v a d a inc reases gradual ly down-sec t ion and is more i n tense near the margins o f Jurassic plutons. W e feel that mid-Jurass ic differential shorten ing wi th depth in the hinter-

L 0x2 F2 FOLD AXES WITH SENSE OF OVERTURNING

from the M c C o y Creek Group on the east Hank of the Schell Creek

land o f the Sevier thrust belt may be associated wi th the format ion of, and/or a deep-seated express ion o f earliest thrust m o v e m e n t to the east. T h u s the Jurassic part o f the Sevier thrust belt cou ld root at dep th here in east-central N e v a d a ; however this entire region may be a l loch thonous above younger (Cretaceous) th rus t s . A n y such thrusts, however , w o u l d have to lie at structurally deeper levels than those exposed today, most l i k e l y w i t h i n c r y s t a l l i n e basement.

R e t u r n to paved road with possible v iew stop o f Snake R a n g e , d e p e n d i n g on sun angle.

0.9 55.7 View Stop: T h e Snake Range decol lement is gently d o m e d across the nor thern Snake Range and separates variably me tamorphosed and duct i le ly ex tended C a m b r i a n Prospect M o u n t a i n Q u a r t z i t e a n d P i o c h e Sha l e f rom faulted C a m b r i a n Po le C a n y o n L i m e s tone and younger uni ts in the upper plate. F r o m this vantage


point y o u can see the nearly f la t d e c o l l e m e n t p r o j e c t benea th M o u n t M o r i a h , a kl ippe of C a m b r i a n and O r d o v i c i an ca rbona te rocks . T h e decol lement gives the nor thern Snake Range its flat-topped p h y s i o g r a p h y ; the fores ted bumps on this smoo th surface are kl ippes of upper plate rocks. A l o n g the west flank of the range, the decol lement bends abruptly down towards us; the smal l rugged hi l l s at 10:00 to 11:00 are kl ippes o f upper plate C a m b r i a n l imestone resting on lower plate Prospect M o u n t a i n Quar tz i te .

1.6 57.3 N o r t h Spr ing Va l l ey R o a d , head south (r ight) .

5.8 63.1 Just before cattle guard, take a left on good graded dirt road and head east across Spr ing V a l l e y towards the S n a k e Range.

5.1 68.2 Cross roads, turn right and dr ive south.

7.1 75.3 O n the left is the Negro C r e e k -M i l l e r Bas in region of the nor thern Snake Range, where the best exposures o f upper plate r o c k s are preserved. N e g r o C r e e k is an erosional window into the lower plate and is an ideal place to study the geometry of upper plate faulting and the interact ion o f the upper plate faults with the decol lement (refer to G a n s and M i l l e r , this v o l u m e ) .

U n f o r t u n a t e l y , roads into the M i l l e r Bas in -Negro Creek area are inacessible this t ime o f year.

T h e bare, ro l l ing hi l l s further south on the left are under la in by C a m b r i a n Prospect M o u n tain Quar tz i te . Here it is locally shattered and folded but not

penetrat ively stretched as it is e l s e w h e r e i n the nor thern Snake Range . A gently east-d ipp ing fault separates this exposure o f Prospect M o u n t a i n f rom faulted and t i l ted Paleozoic carbonate rocks (forested s lopes) . It is unclear whether this fault is a cont inua t ion of the Snake Range decol lement , or jus t an " u p p e r p la t e" normal fault. I f it is the Snake R a n g e d e c o l l e m e n t , " l ower p la te" duct i le stretching must d i e o u t to t h e w e s t . A l t e r n a t i v e l y , i f it is an upper plate fault, the N S R D must cut down-sec t ion to the west.

0.8 76.1 Highway 50 turn left (east).

3.8 79.9 Sacramento Pass. O n the r ight , W h e e l e r Peak in the southern Snake Range is near the apex o f a broad d o m e under la in by C a m b r i a n Prospect M o u n t a i n Q u a r t z i t e , P r e c a m b r i a n M c C o y C r e e k G r o u p rocks , a n d J u r a s s i c p l u t o n s (Whi t eb read , 1969).

2.2 82.1 T h e red outcrops on the left side o f h ighway are a l luv ia l fan deposits o f the Sacramento Pass Ter t iary sequence.

0.9 83.0 B o u l d e r y exposures in the T e r t i a r y sequence on both sides o f highway are slide blocks o f E u r e k a Quar tz i te and O r d o v i c i a n - S i l u r i a n do lomi te .

3.3 86.3 Left tu rn on graded dirt road and park. S T O P 8.

F r o m this vantage point , we can v iew the nor thern Snake Range deco l lement ( N S R D ) in the background , and the Sacramen to Pass Ter t ia ry sequence in the foreground. T h e N S R D is def ined by a th in , subhor i -zontal white ledge o f marble t ec ton i t e a b o v e wh ich are

Guidebook. Part I — G S A Rocky Mountain and Cordilleran Sections Meeting, 1983 153

prominent cliffs o f variably til ted Pa leozoic l imes tone . T h e lower plate in this v iew consists of the S i lve r C r e e k granite wh ich we wi l l see at the next stop. T h e N S R D dives abruptly south over the southern edge o f the p lu ton and beneath the T e r t i a r y s e c t i o n i n t h e foreground.

T h e Tert iary sequence c o n sists pr imar i ly o f a l luv ia l fan conglomerates and lacustr ine l imestone that are syntectonic wi th no rma l fault ing in the Snake Range. A detailed de- 3.8 scr ipt ion o f this sequence is p r o v i d e d by G r i e r ( t h i s 0.2 v o l u m e ) . T h e d e p o s i t i o n a l base o f the Ter t iary sect ion is exposed in the foreground; P e n n s y l v a n i a n - P e r m i a n E l y l imestone on the right is overlain roughly conformably by a 2.1 thin conglomerate fo l lowed by 35 m.y. o ld latite flows and 3.1 r h y o l i t e t u f f ( H o s e a n d Whi t eb read , 1981) w h i c h fo rm the knobby red ridge and smal l 1.5 adjacent mounds . T h e pre-35 m.y. o ld conglomerate contains only clasts o f uppermost Paleozoic format ions suggesting that 0.3 there was very little s t ructural re l ie f in this region pr ior to vo lcan i sm. T h e volcanic rocks 1.5 are in turn over la in by the ye l lowish , we l l -bedded , lacus- 0.1 trine l imestone on the right. Less resistant mar l forms the slopes between the ridges. 0.9

T h e Sacramento Pass T e r t i ary sect ion in part obscures the 0.1 re la t ionship between the nor thern and southern Snake Range s t ruc tu r a l d o m a i n s (F igure 0.1 16). A t one locality along the southwest flank o f the nor thern 0.8 Snake Range , the N S R D jux ta p o s e s u n s t r e t c h e d M c C o y 0.1 C r e e k and Prospect M o u n t a i n Quar tz i te in an upper plate posit ion against duct i le ly stretched

M i d d l e C a m b r i a n and older rocks in a lower plate pos i t ion . These upper plate units can be traced into the southern Snake Range , where they are flat-ly ing beneath W h e e l e r Peak. T h u s , i f the N S R D cont inues sou thward , it must cut to deeper stratigraphic levels and lie beneath the southern Snake Range " d e c o l l e m e n t " mapped by Whi t eb read (1969) .

C o n t i n u e east on Highway 50.

90.1 M o r i a h ' s Grea t Bas in Inn.

90.3 Junc t i on in h ighway, take left fork ( U . S . H ighway 50 East ) . Take immedia te left on good graded dirt road and head nor th .

92.4 D i p - R a n c h G O S L O W .

95.5 T u r n left on dirt road marked by a tire.

97.0 Grea t v iew o f N S R D - we w i l l h i k e up t h e r e t o m o r r o w m o r n i n g .

97.3 R o a d to r i g h t , c o n t i n u e straight.

98.8 F o r k - t a k e right.


99.8 F o r k , take left.


100.0 Ga te .

100.8 F o r k - t a k e right.

100.9 Take sharp turn to right. Just before road drops down into the drainage, turn a round and park. S T O P 9. Sou thernmos t


5.5

3.0

0.2

0.7

0.4

3.4

106.4

Exposu re o f the N S R D . B r i n g l unch and water wi th

y o u O n this h ike . W e wi l l walk down the road, turn left and walk up the canyon. A t the m o u t h of this narrow canyon , near-vert ical ly d ipp ing D e v o nian G u i l m e t t e l imes tone and do lomi te const i tute a smal l scrap o f the upper plate a long .the southern flank o f the nor th ern Snake Range. A s you walk in the canyon , y o u cross the decol lement into the ma in b iotite granite phase o f the S i lve r C reek p lu ton . Here the granite has been pervasively injected by swarms of muscov i t e pegmatite d ikes and si l ls . B o t h rock types have been variably s t r e t ched in to concordance with the deco l lement . W e w i l l veer right and look at exposures o f 1) ultra my lon i t i c p l u t o n i c r o c k s immedia te ly beneath the deco l lement , and 2) th in s l ivers o f assorted P a l e o z o i c f o r m a t i o n s o r " c h a o s " l ike structure in the upper plate.

R e t u r n to cars and head out the same way we came in .

Back at ma in dirt road - go right.

109.4 R a n c h - s low d o w n .

109.6 200 yards past ranch take a right on S i l v e r C r e e k R o a d .

110.3 Rese rvo i r turnoff to right - c o n t inue straight.

110.7 F o r k - turn left and con t inue on the ma in road.

114.1 Cat t le guard and turn off to right, cont inue straight.

A t about 3:00 y o u see w e l l -b e d d e d T e r t i a r y l a c u s t i n e l imestone that contains large s l i d e b l o c k s o f P a l e o z o i c

l imes tone . B r o w n lumps on far ridge crests are slide b locks o f Pa leozo ic do lomi te and the grey ridge is a sl ide b lock of C a m b r i a n P o l e C a n y o n L i m e s t o n e .

0.7 115.5 C a v e r n o u s outcrops on the right are more Paleozoic slide blocks .

0.3 115.8 Just beyond fence, pu l l over and park. S T O P 10. Tert iary L a c u s t r i n e L i m e s t o n e and Cong lomera t e .

H e r e we w i l l examine the top o f the lacustrine l imestone and mar l where it is interbedd e d w i t h and over l a in by fanglomerates . T h e Tert iary sect ion here is approximate ly 1.8 k m th ick ( G r i e r , this v o l u m e ) and is repeated by at l ea s t 3 d o w n - t o - t h e - e a s t , arcuate, no rma l faults wh ich have rotated the sections 30° to 50° westward. T h e presence of e n o r m o u s slide blocks o f rocks as o ld as M i d d l e C a m b r i a n and clasts as o ld as Precambr ian i n dicate that this sequence was deposi ted dur ing active faulting and the format ion of considerable s tructural relief. Clasts of l i n e a t e d and foliated rocks such as those present in the lower plate o f the N S R D are conspicuous ly absent in the Ter t ia ry sequence here and elsewhere.

Back to cars and cont inue up the road as we dr ive west up M i l l e r Bas in wash, note the e n o r m o u s thickness o f a l luv ia l fan deposits , and also the fact that the dip o f beds remains ap¬p r o x i m a t e l y c o n s t a n t up sect ion.

1.7 117.5 S m a l l road turn off to left, pul l ove r and park. S T O P 11. Sl ide B l o c k s and M o n o l i t h o l o g i c Brecc ia .


W e have just crossed a major N E - t r e n d i n g arcuate normal fault that downdrops a l luv ia l fan deposits against the lacustrine l imes tone; the throw on this fault is about 1.6 k m . W e wi l l look at slide blocks and m o n o l i t h o l o g i c b r ecc i a o f Paleozoic do lomi te and O r d o v i cian E u r e k a quartzite. They rest on a l luv ia l fan conglomerate and are over la in by lacustrine l imestone .

Back to cars and cont inue along road through lacustrine l i m e s t o n e a n d m o r e dark b rown do lomi te slide blocks.

0.5 118.0 Intersection. T u r n left and go back out to highway.

3.1 121.1 Highway 5 0 - t u r n left to Baker .

0.2 121.3 S T O P 12 ( i f t ime permits) -Roads ide Rest .

H e r e , brecciated P recambr i an or C a m b r i a n (?) quartzite lies beneath steeply d ipp ing T e r t i a r y depos i t s along an almost Hat fault wh ich apparently surfaces to the nor th where it dips about 30° (F igure 13). It is unclear how this fault relates geometr ica l ly to the Snake Range decol lement .

6.8 128.1 M o r i a h ' s Grea t Bas in Inn , take right fork to Baker (5 mi les ) .

5.3 133.4 D i n n e r and spend the night in Baker .

F I E L D T R I P R O A D L O G : T H I R D D A Y

Mileage Description Incre- Cumu-mental lative

0.0 0.0 Beg inn ing in down town Baker , take paved road (488) to L e h m a n C a v e s N a t i o n a l M o n u m e n t .

4.9 4.9 T u r n right on W h e e l e r Peak R o a d .

3.3 8.2 S T O P 13. V i e w Stop o f the N o r t h e r n Snake Range.

F r o m this point we have a good view nor thward towards the Snake Range decol lement and the faulted Ter t iary sect ion in Sacramento Pass. B e h i n d y o u , about 3,000 ft o f Prospect M o u n t a i n Quar tz i te is exposed on the face of W h e e l e r Peak. H e r e this impress ive ly thick sequence of quartzite consists o f beds about 1 to 3 ft thick. R e m e m b e r what these rocks look l ike as this afternoon we w i l l walk through a sect ion of Prospect M o u n t a i n Quar tz i te that has paper-thin beds and is only a few hundred feet thick.

3.5 11.7 Serene ove r look , turn around, v iew o f mighty W h e e l e r Peak.

G o back down 488 to Baker.

12.0 23.7 A t Baker , turn left on 487 and dr ive nor th .

5.1 28.8 100 yards before Highways 50 and 487 merge, turn right on cross path, cross 50 and head nor th on graded Snake Va l l ey R o a d .

2.1 30.9 R a n c h - slow up.

3.1 34.0 T u r n left at tire.

1.8 35.8 C o w p o n d on left, take road to right. Spectacular view o f the N S R D as we dr ive into our next stop. M a r b l e mylon i te beneath the N S R D forms a conspicuous ledge halfway up the moun ta in . T i l t e d and faulted Pa leozo ic strata above the marble tectonite ledge are clearly truncated by the N S R D . The dark rocks beneath the marble my lon i t e are biotite granite o f the S i lve r Creek p lu ton .

1.5 37.3 Sharp right turn.


0.6 37.9 Cat t le pond , Y in road, bear left. A s we dr ive into O l d M a n C a n y o n , the low hi l l s on ei ther side o f the drainage consist o f u p p e r p l a t e D e v o n i a n to Pennsy lvan ian carbonates. T h e decol lement dips beneath us.

1.6 39.5 Park. S T O P 14. Snake Range Deco l l emen t in R o c k C a n y o n .

T h i s is the type locali ty o f the S R D as descr ibed by M i s c h (1960) . W e wi l l h ike through the deformed S i lve r Creek plu ton up to the S R D . T h e S i lve r C r e e k p lu ton is variably deformed and ch lo r i t i zed ; it ranges in compos i t i on from hornblende diori te to biotite granite, and contains occasional aplite d ikes (also deformed) . T h e p lu ton intrudes M i d d l e C a m b r i a n (?) marbles and calc-si l icate rocks. Towards the deco l lement , both the p lu ton a n d p e n d a n t s b e c o m e m y l o n i t i z e d . Y o u can actually put your finger on the decol lement surface at this stop, but it is a fairly subtle fault; Po le C a n y o n L i m e s t o n e above the decol lement is recrystal l ized and brecciated whereas the same l imestone beneath it is duct i ly deformed.

O n the way d o w n from the deco l lement , we w i l l look at a large pendant o f intricately folded calc-sil icate and l imestone that has "escaped" duct i le stretching.

Perhaps by the t ime you read this guide book, we w i l l have a U - P b date for the S i lve r C r e e k p lu ton . So far, it has y ie lded a K - A r biotite age o f 25.5 ± 1.3 , a K - A r white mica age o f 31.1 ± 1.7 (Lee and others , 1970) and a hornblende age of 215 m.y . ( R . K . H o s e , personal c o m m u n i c a t i o n ) .

Back to cars and return to main Snake V a l l e y R o a d .

9.1 48.6 T u r n left (north) on Snake Va l l ey R o a d . A s we dr ive nor th along the east flank of the Snake Range, the rugged b rown hi l ls you see in the foreg round are upper plate k l ippen o f m id -Pa l eozo i c do lomi te and O r d o v i c i a n Eu reka Quar tz i te . B e h i n d the k l ippen are light grey-ye l low, smooth slopes under la in by stretched lower plate Prospect M o u n t a i n Quartz i te . T h e N S R D projects above the s m o o t h hi l ls and beneath the vegetated cliffs o f upper plate in the background.

1.0 49.6 W e l c o m e to U t a h .

1.2 50.8 M a i n road veers to right, take left fork for 100 yards and then bear left into Ha tch Rock Quar ry .

3.0 53.8 T a k e left fork. C a v e r n o u s , b r ecc i a t ed Ordov ic i a -S i lu r i an do lomi te in the upper plate forms the ridge l ine to the south . T h e S R D is defined by a thin ledge at 10:00. T h e light-co lored slopes below the thin ledge and straight ahead are stretched, lower plate Prospect M o u n t a i n Q u a r t z i t e . T h e decol lement dips about 15° towards us so that it barely clips the top o f t h e h i l l at 2:00 where there is a little k l ippe of D e v o n i a n G u i l m e t t e L i m e s t o n e , and projects under brecciated C a m b r i a n l imestone to our right.

0.1 53.9 Take right fork.

0.3 54.2 Take left fork.

0.5 54.7 Cat t le gate. T h e cliffs to the left and in front o f us expose impress ively th inned Prospect M o u n t a i n Quartz i te (uppermost quartzite un i t ) . Quartz i te and schist units beneath the


Prospect M o u n t a i n belong to the Precambr ian M c C o y Creek G r o u p . T h e d is t inc t ive steel blue-grey schist beneath the Prospect M o u n t a i n is the Osceola A r g i l l i t e , the upper-most uni t of" the M c C o y Creek G r o u p .

0.1 54.8 C r e e k c r o s s i n g - H e n d r y ' s C reek . If road condi t ions are bad we wi l l stop here and walk.

0.4 55.1 Bog. Park cars here and we w i l l hike about '/: mi le up the road to the first pine trees on right, then up the side o f the canyon to the high ridge. S T O P 15. L o w e r Plate D u c t i l e E x t e n s ion , and D i scus s ion of the Nature and significance o f the Snake Range D e c o l l e m e n t .

Exposures of metasedimentary rocks in the deeply incised c a n y o n s a l o n g the eastern flank of the Snake Range best exhib i t the duct i le t h inn ing of lower plate strata. Here , Precambr ian M c C o y C r e e k G r o u p quartzi te , schist, and over ly ing C a m b r i a n Prospect M o u n t a i n and P ioche Shale are i n v o l v e d in a progressive duct i le to brittle e x t e n s i o n a l deformat ion (F igures 17 and 20) . T h e section we wi l l h ike through up the canyon wall is now less than .5 k m thick, but represents an or iginal thickness of about 3 k m (F igure 15), corresponding to more than 500 percent ex tens ion . St ra in is parallel to and increases towards the deco l lement , strain axes are or iented N 60 to 70 W ( X ) , Z subvert ical and perpendicular to the foliat ion and Y sub-hor izon ta l . Stretched pebbles in the M c C o y Creek G r o u p c o m m o n l y have aspect ratios o f 10:1:. 1. Beds in the Prospect M o u n t a i n Quar tz i te or ig inal ly 1 to 3 ft thick are now only a few inches th ick , and the 4,000

ft thick Quartz i te is in places only several hundred ft thick. D u c t i l e s tretching was fol lowed by coaxial brittle extens ion that successively formed: 1) s m a l l - s c a l e d u c t i l e n o r m a l faults and spaced "ex tens iona l c leavage" , 2) micro and mesoscopic conjugate brittle normal faults, and 3) subvert ical jo ints perpendicular to the earlier d i rect ion o f ducti le ex tens ion .

T h i s progressive extensional deformat ion affected amph ibolite grade rocks, but apparently occurred at lower greenschist grade condi t ions , as exh ib i ted by the synkinemat ic growth of retrograde chlori te and white m i c a . O l d e r m e t a m o r p h i c minera ls such as biotite and muscovi te have been mechan i cally rotated into paral le l ism wi th the new stretching fabric, b u t q u a r t z has b e h a v e d d u c t i l e l y . W h e r e the older me tamorph ic fabric was steep wi th respect to the new strain axes, it was k i n k e d about sub-hor izonta l axial planes. Older s y n k i n e m a t i c m e t a m o r p h i c f a b r i c s are p r e s e r v e d i n ro ta ted , bent, b roken , and twisted porphyroblasts .

F r o m the top o f t h e ridge between H e n d r y ' s and H a m p t o n C r e e k drainages there is an excel lent view of the Snake Range decol lement and o f the lower plate units. T h e v iew to the east towards the C o n f u s i o n Range is approximately parallel wi th the new C O C O R P l ine and an opportune locat ion for discussing the subsurface extent o f the Snake Range decol lement .

R e t u r n to cars and go back to Snake Va l l ey Road .

T u r n right and go back to Highway 50.

H ighway 50, go east back to D e l t a and Salt L a k e C i t y .


R E F E R E N C E S C I T E D

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Hose, R. K . , and Danes, Z. E . , 1973, Development of late Mesozoic to early Cenozoic structures ofthe eastern Great Basin; ;// DeJong, K. A . , and Scholten, R., eds., Gravity and Tectonics: New York, John Wiley and Sons, p. 429-441.

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Part I, Geology: Nevada Bureau of Mines and Geology Bulletin 85, 105 pp.

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Lee, D . E . , Marv in , R. F . , Stern, T. W. , Mays, R. E. , and Van Loenen, R. E . , 1968, Accessory zircon from granitoid rocks of the Mount Wheeler mine area, Nevada in Geological Survey Research, 1968: U.S. Geological Survey Professional Paper 600-D, p. D197-D203.

Lee, D . E . , Marv in , R. F . , Stern, T. W. , and Peterman, Z. E . , 1970, Modification of K - A r ages by Tertiary thrusting in the Snake Range, White Pine County, Nevada, in Geological Survey Research, 1970: U.S. Geological Survey Professional Paper 700-D, p. D93-D102.

Lee, D . E . , Marv in , R. F. , and Mehnert, H . LL, 1980, A radiometric age study of Mesozoic-Cenozoic metamorphism in eastern White Pine County, Nevada and nearby Utah, U.S . Geological Survey Professional Paper 1158C, p. C17-C28.

Lee. D. E . , Kistler, R. W „ and Robinson, A . C , (in press). The strontium Isotope Composition of Granitoid Rocks of the southern Snake Range, Nevada, Shorter Contributions to Isotope Research, U.S. Geological Survey, Professional Paper.

Lee, D . E . , Stacey, J. S., and Fischer, L . , (in press), Muscovite-phenocrystic two-mica granites of northeastern Nevada are late Cretaceous in age: Shorter Contributions to Isotope Research, U.S. Geological Survey Professional Paper.

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U G M S E D I T O R I A L A N D I L L U S T R A T I O N S S T A F F K l a u s D . Guts-el Ed i to r T rena L . Foster , Nancy A . C l o s e Ed i to r i a l Staff Brent R . J o n e s Sen io r I l lustrator D o n a l d Powers Il lustrator James W . Parker Car tographer

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Geologic Excursions in the Overthrust Belt and Metamorphic ... · Geologic Excursions in the Overthrust Belt and Metamorphic Cor Complexee s of the Intermountai Region n . GUIDEBOOK