Tectonophysics, 191 (1991) 223-236

Elsevier Science Publishers B.V., Amsterdam

223

Relationships between kinematics of arc-continent collision and kinematics of thrust faults, folds, shear zones, and foliations

in the Nevadan orogen, northern Sierra Nevada, California

Steven H. Edelman

Department of Geological Sciences, University of Tennessee, Knoxville, TN 37996-1410, USA

(Received November 30, 1987; revised version accepted August 10, 1988)

ABSTRACT

Edelman, S.H., 1991. Relationships between kinematics of arc-continent collision and kinematics of thrust faults, folds, shear

zones, and foliations in the Nevadan orogen, northern Sierra Nevada, California. In: A. Perez-Estaun and M.P. Coward

(Editors), Deformation and Plate Tectonics. Tectonophysics, 191: 223-236.

The Mesozoic Nevadan orogeny in the northern Sierra Nevada metamorphic belt, California, may be attributed to

arc-continent collision. stratigraphic data and macroscopic cross cutting relations suggest successive accretion of two arcs

along an active continental margin. The younger accretion event involved the Early Jurassic Slate Creek terrane, which is a

3-5 km thick pseudostratigraphic arc fragment. The Slate Creek thrust, an isoclinally folded fault with a subhorizontal median

surface, carries the Slate Creek terrane at least 40 km eastward (continentward) over the pre-existing continental margin

terrane amalgam. No rock units can be correlated across the Slate Creek thrust which is thus interpreted as an arc-continent

suture. In addition to the Slate Creek thrust, the Nevadan orogen includes a major east-vergent imbricate thrust set east of and

beneath the Slate Creek thrust, and steep west-vergent reverse shear zones, macroscopic upright folds, and steep foliations that

overprint and cut the east-vergent structures.

These data suggest a mode1 for the relationships between the kinematics of arc collision and Nevadan erogenic structures.

The Slate Creek arc terrane accreted by westward partial subduction of the continental margin along the east-vergent Slate

Creek thrust. The continental margin was imbricated along an east-vergent thrust set. The structurally higher, inactive Slate

Creek thrust-suture was deformed by steep west-vergent shear zones, folds, and foliations which may have accommodated

shortening of the east-vergent thrust sheet. This deformation occurred within an active, continental margin arc that probably

initiated by subduction flip after collision of the Slate Creek arc. This kinematic model is consistent with the structural geometry and chronology of the Nevadan orogen while qualitatively

maintaining lithosphere-scale strain compatibility. This model has implications for problems related to emplacement of large

crystalline thrust sheets, displacements beneath and at the margins of shortened crustal segments, and interaction of

oppositely-verging structures. The Nevadan orogen is a slate belt, and the structural-plate tectonic mode1 presented for the

Nevadan orogeny may be testable in slate belts of other orogens.

Introduction

An understanding of the relationships between plate tectonic processes and the structure of con- tinental crust has remained obscure despite ad- vances in both fields independently. This paper proposes hypotheses for some of these relation- ships by developing a structural-plate tectonic model for the Mesozoic Nevadan orogeny in the northern Sierra Nevada, California (Fig. 1). The Nevadan orogen contains structurally complex, Paleozoic and Mesozoic continental margin and

oceanic rocks. A model for Nevadan arc accretion and deformation is presented in this paper as a case study of how simatic crust may accrete to a continental nucleus and deform to form crust of continental thickness and structure. The Nevadan orogeny is defined by thrust faults, steep shear zones, upright folds, and steep foliations.

Although every orogen is unique, certain re- gional structural associations are observed re- peatedly in many erogenic belts, for example fore- land fold-thrust belts. An understanding of the common structural associations is certainly a pre-

0040-1951/91/$03.50 0 1991 - Elsevier Science Publishers B.V.

S.H. EDELMAN

Fig. 1. Generalized geologic map of the northern part of the

Sierra Nevada metamorphic belt showing tectonic terranes and

major Nevadan faults. SCT= Slate Creek terrane (200 Ma);

SC = Smartville Complex (160 Ma volcanic and plutonic rocks

and 200 Ma basement); Ju = CalJovian and older Jurassic

volcanogenic rocks. Easr-oergent faulrs: F, = Ft thrust faults;

GM = Grizzly Mountain thrust; HCW = Higgins Comer

window; LOW= Lake Oroville window; SC? = Slate Creek

thrust; TT = Taylorsville thrust. Terrunes in suture zone: CT =

Calaveras terrane; FRT = Feather River terrane; TRT =

Tuolumne River terrane; Red Ant terrane of Fig. 2 is not

separable at this scale. Cross section A-B and COCORP line

X-Y are shown in Fig. 2. From Day et al. (1985), Pdelman

and Sharp (1986. 1989), Pdelman et al. (1989) Hietanen (1981);

Ricci et al. (1985), and other sources.

requisite for elucidating erogenic and continental

crust-forming processes. The Nevadan orogen

contains three types of structures that are reported

repeatedly from the internal parts of mountain

belts but which pose fundamental questions of

large-scale strain compatibility and tectonic set-

ting: (1) Large horizontal thrust sheets of crystal-

line rock, including ophiolites (Iverson and Smith-

son, 1982; Hatcher and Williams, 1986)-how are

they detached from their original crystalline sub-

strates and transported without opening spaces

behind them? (2) Extensive regions of steep slaty

cleavage and upright folds (e.g. Cambrian slate

belt of North Wales and Carolina slate belt of the

southeastern U.S.; Hobbs et al., 1976, pp. 403-

405)-how is the implied horizontal shortening in

these belts accommodated at depth and at the

ends of the shortened zones? (3) Thrust faults and

folds of opposite vergence (e.g. Alps, Milnes and

Pfiffner, 1980; Canadian Cordillera, Price, 1986)

-how do these operate without interfering with

one another, and what controls the dominant ver-

gence direction and the order of overprinting?

Possible answers to these questions are offered for

the Nevadan orogeny and are integrated into a

plate tectonic model.

The Nevadan orogen in the northern Sierra Nevada

Stratigraphy, general structure, and tectonics

The Nevadan orogeny has long been recognized

as a Jurassic fold-, fault-, and cleavage-forming

event in the wall rocks of the Sierra Nevada

batholith in California (Blackwelder, 1914; Knopf,

1929; Bateman and Clark, 1974). Because many

structures considered to be Nevadan have been

shown to range in age from Middle Jurassic to

Cretaceous (Nokleberg and Kistler, 1980; Tobisch

and Fiske, 1982; Paterson et al., 1987; Edelman et

al., 1989), the term “Nevadan orogeny” will be

used loosely here to include all these structures

(compare Schweickert et al., 1984a).

The Nevadan orogeny is best represented in the

Paleozoic through Late Jurassic (early Rim-

meridgian) oceanic and continental margin

metasedimentary and metaigneous rocks of the

western Sierra Nevada metamorphic belt. A sim-

plified map of the northern part of the metamor-

phic belt is shown in Fig. 1. In the eastern part of

the metamorphic belt, continentally-derived quartz

sandstone and other rocks of the Shoo Fly Com-

plex are unconformably overlain by Upper De-

vonian through Middle Jurassic volcanogenic and

sedimentary strata in the Northern Sierra terrane

(schweickert et al., 1984b; Harwood, 1988) (Fig.

1). The Slate Creek terrane, in the central and

western parts of the belt, consists of an

LowerJurassic (200 Ma) pseudostratigraphic se-

quence interpreted as an oceanic arc (Edelman et

al., 1989; Edelman and Sharp, 1989; Saleeby et

al., in press; M.E. Bickford and H.W. Day, un-

published U-Pb zircon data).

A structurally complex suture zone which con-

sists of several oceanic terranes (FRT, CT, TRT in

KINEMATICS OF ARC COLLISION AND NEVADAN OROGENIC STRUCTURES 225

E3 5 k”

” I

SLATE CREEK TEARANE kl)

HORIZON1 AL SCALE=VtRTICAL SCALE _,.

(c) I

Fig. 2.(a) Cross section A-B (see Fig. 1 for location) across part of the northern Sierra Nevada metamorphic belt. BB = Big Bend

fault; D = Dowmeville fault; DP = Dogwood Peak fault; GC = Goodyears Creek fault; RAT = Red Ant terrane (not distinguished

in Fig. 1); other abbreviations as in Fig. 1. Querries in Smartville Complex (SC) reflect uncertainty of the extents of allochthonous

200 Ma Slate Creek rocks and autochthonous 160 Ma rocks. (b) Schematic structural succession before emplacement of Slate Creek

terrane. Symbols as in (a). (c) Simplified line drawing of COCORP seismic profile (X-Y in Fig. 1); from Nelson et al. (1986). Surface

locations of some faults are shown (K-GM are projected along strike).

Fig. 1) lies between the continental Northern Sierra

terrane and the oceanic Slate Creek terrane. The

Feather River terrane is a Paleozoic ophiolite, the

Red Ant terrane (not shown in Fig. 1; see RAT in

Fig. 2) is an early Mesozoic blueschist, the

Calaveras terrane is a late Paleozoic-Triassic

chert-argillite melange, and the Tuolumne River

terrane is a Paleozoic ophiolitic melange overlain

by early Mesozoic arc volcanic rocks, argillite, and

chert.

The well known Smartville Complex (Xeno-

phontos and Bond, 1978; Day et al., 1985; Beard

and Day, 1987) in the western part of the terrane

is a 160 Ma volcanic-plutonic complex built into

200 Ma Slate Creek terrane basement (Fig. 1).

Plutonic rocks with ages of about 160 Ma intrude

the other Sierran terranes (Snoke et al., 1982) and

coeval volcaniclastic rocks occur east of the

Taylorsville thrust (Ju in Fig. 1). These ca. 160

Ma intrusive rocks and coeval (Callovian-Kim-

meridgian) volcanogenic rocks postdate amalga-

mation of the terranes they intrude and overlie,

and thus represent the initial stages of a Middle

Jurassic-Cretaceous continental margin arc which

culminated in emplacement of the Sierra Nevada

batholith (Hamilton, 1969).

Plate models for the Nevadan orogeny pro-

posed collisions of oceanic arcs (Moores, 1970,

1972; Schweickert and Cowan, 1975; Edelman,

1985, 1987; Ingersoll and Schweickert, 1986) or

imbrication of arcs formed in situ (Burchfiel and

Davis, 1981; Saleeby, 1981, 1983; Sharp, 1985).

The collisional models proposed a Late Jurassic

collision between the 160 Ma Smartville Complex

(including its basement rocks) and the rock units

to the west. However, this model appears to be

disproved by new evidence that the 200 Ma Slate

Creek Complex accreted before 165 Ma, and that

the Smartville Complex is younger than this accre-

tion and is built into Slate Creek basement. Thus,

the Smartville Complex arc apparently formed in

situ.

The Slate Creek terrane may represent an oce-

anic arc that collided before 165 Ma. Edelman

(1987) proposed two successive arc collisions, with

the ensimatic Tuolumne River arc terrane (Fig. 1)

accreted during the earlier collision. This chronol-

ogy is indicated by the following:

(1) The Slate Creek Complex tectonically over

lies both the Tuolumne River and Calaveras ter-

ranes along the “Slate Creek thrust” (Figs. 1 and

2).

S.H. EDELMAN 226

(2) The Slate Creek thrust is cut by a 165 Ma pluton (SP in Fig. 3).

The Nevadan orogeny is defined by two oppo- sitely-verging sets of structures (Fig. 2). The younger and more conspicuous set consists of steep west-vergent (east-dipping) reverse faults and related folds and cleavages. The other set consists of shallow east-vergent (west-dipping) thrust faults, including the Slate Creek thrust, and rare overturned folds. The east-vergent structures are systematically overprinted by the west-vergent structures (Speed and Moores, 1980; Moores and Day, 1984; Day et al., 1985; Edelman et al., 1989). The summary of Nevadan structure presented be- low is chiefly from Day et al. (1985) and Edelman et al. (1989) unless otherwise cited.

Steep west-vergent structures

Faults

The throughgoing steep faults of the Foothills fault system (Clark, 1960) are the most obvious of the Nevadan structures (“later steep faults” in Fig. 1). In cross section A-B (Fig. 2a) these faults are represented by the Downieville, Goodyears Creek, Dogwood Peak, and Big Bend faults. The steep faults are defined by curvilinear zones of intense foliation within which contrasting rock units are juxtaposed (Fig. 3). Consistently steep stretching and streaking lineations in the foliations indicate that they are dip-slip shear zones (stereo- grams in Fig. 3). Separations of stratigraphic and structural planar features indicate dip-slip dis- placements in the range of l-10 km with east sides up. The faults are thus west-vergent reverse faults. Their curvilinear traces indicate that they are folded, with map-scale bends of up to 80 o and more in the northwestern Sierra Nevada (Fig. 1).

The steep faults are superposed on an earlier west-vergent thrust system which is largely cryptic and predates the east-vergent structures. This earlier thrust system stacks, in descending struct- ural order, the Northern Sierra terrane, Feather River terrane, Red Ant terrane, Calaveras terrane, and Tuolumne River terrane (Fig. 2b). The open barbs on the later steep faults in Fig. 2a mark segments of these later faults that occupy the structural positions of earlier faults (compare Fig.

Fig. 3. Foliation trend map and cross section for area outlined

in Fig. 1. Stereograms show stretching and mineral streaking

hneations; contoured diagrams show number of measurements,

with contours at 0 (dashed), 5, 10, 15, 20 (black) times a

random distribution. Diagrams are located in, or have arrows

that point to, the general areas represented. Note that strongest

foliations occur along mapped later steep faults, indicating that

these faults are at least in part shear zones. Steep lineations

suggest shear zones are dip-slip. SC? = Slate Creek thrust

(barbed on map); SP = 165 Ma Scales pluton. From Fxlelman

et al. (1989).

2a and b). The locations and orientations steep faults are strongly controlled by the faults which were apparently reactivated.

Foldr

of the earlier

Macroscopic folds are defined mainly by fold- ing of the Slate Creek thrust (Fig. 2a and see below). These folds are tight to isoclinal with steeply east-dipping hinge surfaces and shallow, doubly plunging hinge lines (Fig. 3). Folding out-

KINEMATICS OF ARC COLLISION AND NEVADAN OROGENIC STRUCTURES 227

lasted faulting as indicated by the warped traces of the later faults (Fig. 1). In addition, the folding probably steepened the later faults by an un- known amount.

Mesoscopic folds are uncommon and difficult to relate to a specific larger-scale fold set. The hinge lines of some mesoscopic folds are subverti- cal and parallel to stretching lineations in steep shear zones.

Foliations

Foliations are most strongly developed along the later steep faults (Fig. 3). Between the faults, the foliation is weaker and/or occurs in narrow zones separated by zones hundreds of meters wide with weak foliation. The foliation is generally steeply east-dipping. The strong foliations along the faults are interpreted as shear zone foliations; steeply plunging stretching and streaking linea- tions indicate they are dip-slip shear zones. Throughout the Sierra Nevada metamorphic belt, steep lineations (Nokleberg and Kistler, 1980, their Fig. 5) and steep strain X-axes (Tobisch et al., 1977; Paterson et al., 1987) suggest regional verti- cal extension and dip-slip shear. In the Northern Sierra terrane (Fig. l), kinematic analysis of folds suggests steep Nevadan extension axes (Varga, 1985). Foliations between faults mark minor shear zones and/or axial plane foliations associated with folds.

Macroscopic strain accommodated by steep west-

vergent structures

The steep Nevadan structures taken together define a steep west-vergent fold-thrust system. Be- cause the faults are folded in map view, they probably initiated at shallower dips and rotated toward steeper dips during folding. The orienta- tions and locations of the faults were controlled in large part by the inherited, west-vergent, terrane- bounding faults (Fig. 2b).

The macroscopic strain accommodated by the steep Nevadan structures is east-west horizontal shortening (perpendicular to foliations and fold hinge surfaces) and vertical extension (crustal thickening). The amount of macroscopic strain is unknown, but judging from the cross section (Fig. 2) the across strike stretch is probably between

0.5 and 0.1. The intermediate principal strain (Y) is unknown but is probably insignificant. The large-scale problems to be addressed below are how this deformation is accommodated at depth and how the implied displacements at the margins of the shortened zone are accommodated without creating space problems.

East-oergent low-angle structures

Northeastern Sierra Nevada

East-vergent structures in the northeasternmost Sierra Nevada have been known for some time (McMath, 1966; D’Allura et al., 1977; Speed and Moores, 1980). They include the Taylorsville and Grizzly Mountain thrusts (Figs. 1 and 2a) as well as east-vergent folds (Speed and Moores, 1980). These structures are deformed by west-vergent folds (Speed and Moores, 1980; Day et al., 1985). The Taylorsville and Grizzly Mountain thrusts cut and imbricate the Northern Sierra terrane strati- graphic succession, and the Taylorsville thrust cuts rocks as young as Callovian.

Slate Creek thrust

The Slate Creek thrust is defined in the area of section A-B (Fig. 2a) by the contact between the Slate Creek terrane and the earlier thrust pile of Fig. 2b. For the purposes of this paper, the term “Slate Creek thrust” is extended to include all such contacts in the northern Sierra Nevada (F,

LOW, HCW, SCt in Fig. 1). The Slate Creek thrust is overprinted by the later steep structures discussed above; the thrust is tightly folded and is cut by steep west-vergent faults (Fig. 2a). The Slate Creek thrust is an east-vergent thrust fault with a displacement of more than 40 km. The thrust is cut by the 165 Ma Scales pluton (SP in Fig. 3; U-Pb zircon ages, Saleeby et al., 1988; M.E. Bickford and H.W. Day, pers. commun., 1988) and is of course older than the steep west- vergent folds and faults that deform and cut it.

The Slate Creek thrust carries the Slate Creek terrane pseudostratigraphic sequence of ultra- mafic-intermediate igneous rocks in its hanging- wall. The preserved thickness of this sequence is 3-5 km. Volcanic, volcaniclastic, plutonic, and cumulate ultramafic rocks comprise the sequence,

228 S.H. EDELMAN

and ultramafic rocks of mantle origin (harsburgite

or lherzolite) are rare. In broad terms, the Slate

Creek terrane is an areally extensive, thin thrust

sheet of oceanic arc rocks that is detached from its

original mantle substrate and thrust eastward over

supracrustal rocks (TRT, CT, RAT in Fig. 2). The

presence of serpentinized cumulate ultramafic

rocks along the Slate Creek thrust suggests that

the Slate Creek terrane was detached along the

ancient petrologic Moho. Because the Slate Creek

thrust juxtaposes mutually exclusive rock assem-

blages (Slate Creek terrane is juxtaposed against

the other Sierran terranes), the Slate Creek thrust

can be interpreted as a suture (Edelman et al.,

1983; Edelman, 1985).

Kinematics of east-vergent Nevadan structures

The east-vergent structures constitute a set of

Nevadan structures distinctly different in style,

orientation, and relative age from the later steep

west-vergent structures. The east-vergent struc-

tures are consistently cut and folded by the steep

faults and folds.

The east-vergent thrust system constitutes a

structural succession of, from bottom to top: (1)

autochthonous Northern Sierra rocks below (east

of) the Taylorsville thrust; (2) allochthonous rocks

between the Taylorsville and Slate Creek thrusts;

(3) the allochthonous Slate Creek terrane above

the Slate Creek thrust. The Slate Creek thrust is

pre-165 Ma and the Taylorsville thrust cuts rocks

as young as Callovian (Imlay, 1961; 1699163 Ma,

time scale of Palmer, 1983), so the Slate Creek

thrust is probably the oldest of the east-vergent

faults.

Relationship between the east- and west-vergent

structures

Because the east-vergent structures are cut and

folded by the west-vergent structures, one could

propose two sequential deformational events of

opposite vergences. However, recent work in thrust

belts has shown that much of the deformation in

thrust sheets occurs as they override lower thrusts

(Suppe, 1983; Hatcher, this volume), and Coward

(1983) suggested that earlier faults may steepen

due to folding above lower decoupling zones. In

the Nevadan orogen in the northern Sierra Nevada,

the Slate Creek thrust may have been deformed by

west-vergent faults and folds that accommodated

shortening of the Grizzly Mountain-Taylorsville

thrust sheet, which in turn may have been de-

formed by west-vergent folds that accommodated

shortening of a still lower thrust sheet that is not

exposed in this region.

This proposed diachronous development of the

two systems of Nevadan structures is supported

by the fact that the Slate Creek thrust is more

tightly folded than the Grizzly Mountain and

Taylorsville thrusts (Fig. 2b); deformation of

thrust hangingwalls predicts that structurally

higher thrust sheets would be deformed more than

lower ones because higher sheets must accumulate

all the deformation of the lower thrusts. In ad-

dition, the age constraints on the steep faults and

the Taylorsville and Grizzly Mountain thrusts per-

mit them to be coeval. Thus, the steep west-ver-

gent faults, folds, and foliations in the Nevadan

orogen may reflect shortening of the hangingwall

of the Taylorsville, Grizzly Mountain, and possi-

bly lower east-vergent thrusts.

This interpretation cannot be proven with exist-

ing data. The critical test of this idea, that is,

whether the steep structures that deform the Slate

Creek thrust truncate against the Grizzly Moun-

tain and Taylorsville thrusts, lies deep in the sub-

surface. Deep seismic reflection by COCORP

shows oppositely dipping reflectors at depth that

disappear just above their projected intersections

(Nelson et al., 1986; see Fig. 1 for location of

seismic line X-Y and Fig. 2c for simplified

migrated section). Further, the reflectors that could

correspond to Nevadan faults have “true” dips of

32-54” (Nelson et al., 1986) much shallower than

the 80-90” dips of these faults observed at the

surface. The surface dips are based on dips of

shear zone foliations and the straight map traces

of the faults across deep canyons. Therefore, it is

possible that at least some of the COCORP reflec-

tions are not Nevadan faults, as appreciated by

Nelson et al. (1986). The seismic data do not

address the problem at hand.

Figure 4 shows a model for the kinematic inter-

action of the oppositely verging structures. The

first order boundary condition is the initiation of

KINEMATICS OF ARC COLLISION AND NEVADAN 0R0i35NtC STRUCTURfiS 229

Map of tmbricate

______Z&&_f______- lmbricate thrust

(d)

1 - I .-

4

Fig. 4. Conceptual diagram illustrating accusation of dif-

ferential displacement along an imbricate thrust by thrust sheet

shortening. and how the thrust sheet deformation produces

structures. (a) Initiation of imbricate thrust, perhaps during a

collision in which subduction dicp% left. Black triangles mark

adjacent material points that will be displaced along the im-

b&ate thrust. The suture or other higher thrust at this stage is

an inactive structure residing in the imbricate thrust sheet. (b)

After an increment of differential displacement, structures that

accommodate thrust sheet shortening initiate, including back-

thrusts (steep shear zones) that terminate at the imbricate

thrust, foliations with down dip stretching and streaking linea-

tions formed in noncoaxial deformation in backthrust shear

zones, and backfolds and associated foliations formed in coaxial

spinning deformation between the backthrusts. Schematic finite

strain ellipses (XZ sections, Y = 1) are shown. Small rotation

arrow near strain ellipsoid indicates rotation of principal strain

axes relative to material (noncoaxiahty or “internal rotation”);

larger arrows outside circles indicate rotation with respect to

imbricate thrust (“spin” or “external rotation”). (c) Differen-

tial displacement and thrust sheet shortening continue to accu-

mulate resulting in increased finite strains and steepening of

structures. (d) Backthmsts steepen to the point that they can

no longer efficiently accommodate horizontal shortening. Fur-

ther shortening is macroscopically coaxial and produces fold

tightening, steepening and folding of faults, and crenulation of

favorably oriented inactive fabrics. Along-strike strain gradi-

ents during this phase of deformation produce map-view warps

in faults, fold hinges, and foliations. Compare this model

structural style to Fig. 3. Foliation traces omitted from cross

sections for clarity.

an imbricate thrust (Taylorsville-Grizzly Moun-

tain) beneath a collisional suture or other structur-

ally higher thrust (Slate Creek thrust), with a

negative displacement gradient along the im-

bricate thrust in its transport direction (Fig. 4a).

Material below the thrust sheet is considered to be

rigid during its deformation; if it were not rigid,

the thrust would probably deform and lock.

An increment of differential displacement along

the imbricate thrust imposes an increment of strain

upon the thrust sheet (Fig. 4b). The macroscopic

strain field is characterized by shortening parallel

to the transport direction of the imbricate thrust

and vertical extension. This thrust sheet deforma-

tion is accommodated internally by two deforma-

tion mechanisms: (1) backthrusts that are shear

zones with noncoaxial internal deformation, and

(2) macroscopically coaxial deformation of

material between the shear zones accommodated

by folding and cleavage formation. The two defor-

mation mechanisms are envisioned as ideal “end

members,” with progressive, superimposed coaxial

and noncoaxial deformations, or with composite

deformations in which some combination of pure

shear (coaxial plane strain) and simple shear are

imposed during each increment of deformation.

These two deformation mechanisms are similar to

the “backthrusting” and “coupling” modes of

thrust sheet shortening defined by Dunne and

Ferrill (1988). The shortening of the thrust sheet

requires steepening of all planes, which is mani-

fested by a “spin” or “external rigid rotation”

(Fig. 4b) of all structures with respect to the

imbricate thrust. Thrust sheet deformation by these

two mechanisms is continued for another defor-

mation increment in Fig. 4c.

The backthrusts eventually steepen such that

they can no longer efficiently accommodate shor-

tening of the synthetic thrust sheet (Fig. 4d). Ex-

ternal rotation of inter-shear zone material de-

creases commensurate with decreasing internal ro-

tation (noncoaxiality) of intra-shear zone material,

and the macroscopic deformation approaches

coaxial irrotational strain. All material planes and

lines steepen as they rotate toward the XY-plane

and the X-axis, respectively, of the macroscopic

strain ellipsoid. In particular, folds tighten, faults

steepen, and foliations and lineations acquire

230

steeper dips and plunges. Along-strike strain

gradients may produce map view warps in faults,

fold hinge surfaces, and foliations, and variable

plunges in fold hinge lines (Fig. 4d) (Ramsay,

1967, fig. 7-105; Wood, 1974, pp. 383). This model

produces the warped shear zones, folds, and folia-

tions that characterize the steep west-vergent set

of Nevadan structures as described above. Com-

pare the map view structural style in Fig. 4d to

Fig. 3. The model also explains the steep dips of

the later faults which would accommodate only

local vertical uplifts, without crustal thickening or

thinning, in their present orientations.

Structural and plate tectonic evolution

nevadan orogen

of the

In Fig. 5 are shown paleotectonic cross sections

of the Jurassic arc-continent collisions inferred

W

H=V

0 a.-..,- _.

(b)

SCT set

d Active fault

Inactive fault

’ + active arc magnatism

Pab-topogaphic OT 170-150 Ma-----

-bathymetric surface A B

0 --1 Petrdogic Mdla (e)

Base of ktbsphere -100 I

150~(?)120 MS“------.__ + active arc magnetism

S.H. EDELMAN

for the northern Sierra Nevada. The sections are

drawn to scale, using average oceanic crustal

thicknesses, lithospheric thicknesses, and esti-

mated unstrained dimensions of northern Sierra

Nevada rock units. These sections are not tightly

constrained or quantitatively balanced, but they

yield more insight into structural processes than

do “plate tectonic cartoons.” Possible strike-slip

displacements are omitted from this simple two-

dimensional model although they may have been

important in the post-collisional structural history

(Edelman and Moores, 1984; Harper et al., 1985).

This omission is not critical because there is no

evidence for major strike-slip displacements in

Nevadan structures.

The Nevadan orogeny is interpreted as a funda-

mental consequence of Jurassic collisional accre-

tion of a fringing arc system (Fig. 5a). Two dis-

tinct arc segments are represented by the

Tuolumne River and Slate Creek terranes. The

Tuolumne River arc collided first along a west-

vergent suture (Fig. 5b); the earlier west-vergent

Fig. 5. Paleotectonic cross sections showing the interpreted

plate tectonic and structural evolution of the northern Sierra

Nevada in the Jurassic and Cretaceous. Note that the sections

are drawn to scale. (a) Early Jurassic convergent continental

margin composed of active arc and its basement in the North-

em Sierra terrane (NST), Feather River terrane ophiolite

(MT), blueschist of Red Ant terrane (RAT). and melange of

Calaveras terrane (CT). Active oceanic arcs represented by the

Tuolumne River terrane (TRT) and the pseudostratigraphic

Slate Creek terrane (SCT). (b) Arc-continent collision of the

Tuolumne River terrane along an oceanward-directed thrust.

(c) Arc-continent collision of the Slate Creek terrane along the

continentward-directed Slate Creek thrust (SCr). Age con-

straints are the same as for the collision in (b), but cross-cut-

ting relations (Fig. 2) indicate that the Slate Creek arc accre-

tion is the younger collision within those constraints. (d) Imbri-

cation of the continental margin along the Taylorsville-Grizzly

Mountain thrust set (TT-GM) beneath and in front of the

Slate Creek thrust-suture. Shortening of the Taylorsville-

Grizzly Mountain thrust sheet is accommodated by west-ver-

gent faults and folds. Active arc magmatism above an east-di-

pping subduction zone west of and below the cross section is

omitted for clarity. (e) Continued crustal shortening, reflected

by steepening of previous structures and folding of the

Taylorsville and Grizzly Mountain thrusts, may have occurred

above a deeper, hypothetical detachment (querried fault). Ac-

tive arc magmatism above an east dipping subduction zone is

omitted for clarity. A-B corresponds to section in Fig. 2a.

KINEMATICS OF AKC COLLISION AND NEVADAN OROGENIC STRUCTURES 231

thrust set (Fig. 2b) formed at this time or earlier

(Fig. 5b). This collision was followed by the Slate

Creek arc collision (Fig. 5~). The Slate Creek arc

was detached from its mantle substrate along the

Moho and thrust eastward as a tectonic flake over

the other Sierran terranes. This process might be

favored by the buoyancy of the continental margin

and the mechanical weakness of the Moho (White

and Bretan, 1985). In any case, the position of the

Slate Creek thrust in the Slate Creek pseudo-

stratigraphy (Fig. 2a) requires the geometry de-

picted in Fig. 5c. Cumulate ultramafic rocks, pre-

sumably formed near the subarc Moho, are the

deepest level of the arc preserved in the Slate

Creek thrust sheet. The collisional subduction

probably dipped west, synthetic to the Slate Creek

thrust-suture, but because there is no independent

evidence for subduction dip direction it is omitted

from the present model.

In Fig. 5d is shown imbrication of the con-

tinental margin beneath and in front of the Slate

Creek thrust along the Taylorsville-Grizzly

Mountain thrust set. The later steep structures

that deform the Slate Creek thrust are depicted as

initiating at this time as a west-vergent set of

backthrusts and backfolds. Note that the faults

above TT-GM in Fig. 5d are depicted as re-

activating many of the earlier west-vergent thrusts

from Fig. 5b. Arc magmatism represented by the

Smartville Complex, Callovian volcaniclastic rocks

in the northeasternmost Sierra Nevada (JV in

Fig. l), and widespread coeval intrusive rocks,

occurred during this deformation. This magma-

tism probably reflects eastward, subcontinental

subduction initiated after collision of the Slate

Creek arc. Continued shortening reflected by fold-

ing and steepening of the west-vergent faults and

folds, and by folding of the Taylorsville and

Grizzly Mountain thrusts, is depicted as a kine-

matic continuation of the same deformation above

a deeper east-vergent thrust shown hypothetically

in Fig. 5e (querried fault). Arc magmatism con-

tinued through the Cretaceous to form the Sierra

Nevada batholith (Hamilton, 1969).

Discussion

The structural-plate tectonic model outlined

above, though speculative, is consistent with a

large body of existing structural and stratigraphic

data and is testable. The Nevadan orogen in the

northern Sierra Nevada is fundamentally an east-

vergent thrust system (Day et al., 1985). The Slate

Creek thrust-suture and Taylorsville-Grizzly

Mountain thrust set are “synthetic” to the colli-

sional subduction (Roeder, 1973) or at least to the

partial subduction of the continental margin (Fig.

SC), in that they have the same dip directions and

senses of shear. The later steep structures have the

opposite dips and senses of shear and are thus

“antithetic.” Reactivation of earlier structures

contributed to the westward vergence of the steep

backthrusts. The observation that the east-vergent

thrusts do not reactivate earlier structures sup-

ports the proposition that their locations and

orientations are controlled by an externally pre-

scribed kinematic field, presumably the Slate Creek

arc collision depicted in Fig. 5c.

The crustal thickening depicted in Fig. 5e is

supported by the present 50 km depth to the

Moho in the northern Sierra Nevada (Speed and

Moores, 1980). Considering that at least 10 km of

overburden has been eroded during and after

Nevadan deformation, as indicated by exposure of

plutons and metamorphic rocks and by huge

thicknesses of Late Jurassic-Recent sediment de-

rived from the Sierra Nevada in the Great Valley,

the crust was probably at least 60 km thick in

Nevadan time. Collision is the best-documented

mechanism for producing such crustal thicknesses.

Three general problems of orogenesis were

mentioned in the Introduction: (1) large crystal-

line thrust sheets, (2) steep foliations and folds,

and (3) interaction of oppositely-verging struc-

tures. The implications of the model (Figs. 4 and

5) for these problems are pointed out here.

(1) The Slate Creek thrust displays a minimum

displacement of 40 km. The thrust sheet, i.e. the

Slate Creek terrane, is about 3-5 km thick. Be-

cause the igneous arc rocks of the Slate Creek

terrane rest directly on the thrust, the thrust can-

not be a subduction zone fault. The original sub-

arc mantle is missing along the Slate Creek thrust.

The present model explains this omission by hav-

ing the suture cut horizontally westward along the

subarc Moho and partially subduct the subarc

mantle (Fig. 5~). The problem of removing the

232

original substrate of the crystalline thrust sheet is

solved in this way without creating space prob-

lems. This solution is similar to the “vanishing

crust” model of Iverson and Smithson (1982) and

explains the features attributed to “flake tectonics”

(Oxburgh, 1972) without splitting the lithosphere.

The suggested geometry is similar to Butler’s (1986,

fig. lib) model for Moho detachment of continen-

tal crust.

(2) The problem of how large horizontal shor-

tening strains may be accommodated at depth is

solved in the model by allowing the shortening to

occur above a subhorizontal fault (imbricate thrust

in Fig. 4; TT-GM in Fig. 5d). Shortening of

thrust sheets during thrust propagation and mo-

tion may be a common phenomenon (Williams

and Chapman, 1983). The displacements at the

margins of the shortened zone are accommodated

by differential slip along the fault.

(3) The problem of the interaction between

oppositely verging structures is addressed in the

model by restricting the steep west-vergent struc-

tures, at any given instant during deformation, to

the material above the currently active east-ver-

gent thrust. In the Nevadan orogen, the mecha-

nisms by which hangingwall deformation is

accommodated is strongly influenced by pre-exist-

ing structure in the deforming rocks, whereas the

east-vergent structures cut across the earlier struc-

ture and by inference are fundamentally con-

trolled by the vergence of collision. Shortening of

a thrust sheet by faults with vergence opposite to

that of the main thrust fault is a form of “tectonic

wedging” (Price, 1986). The Taylorsville and

Grizzly Mountain thrusts are folded, though not

as strongly as the Slate Creek thrust Folding of

the Taylorsville and Grizzly Mountain thrusts and

the later steep faults, and the last increments of

folding of the Slate Creek thrust, may have oc-

curred during eastward transport of the entire

east-vergent thrust stack along a younger, more

easterly, and structurally lower thrust east of the

Sierra Nevada (Fig. 5e).

The significance of the model presented in Figs.

4 and 5 lies in the fact that it relates the kine-

matics and orientations of structures observed on

scale orders from plate tectonic sutures to clea-

vages and lineations. The model accounts for the

S.H. EDELMAN

major structural and stratigraphic features of the

Nevadan orogen while qualitatively maintaining

lithosphere-scale strain compatibility. The model

contains components that are not adequately

documented such as the displacement gradient

along the Taylorsville-Grizzly Mountain thrust

set, the amount of regional strain compared to

that predicted by the displacement gradient, and

the down-dip termination of the west-vergent

faults against east-vergent thrusts. These compo-

nents are predictions of the hypothesis to be tested

by future mapping, strain measurement, radiomet-

tic dating, seismic reflection and other geophysical

profiling, and scientific drilling.

The second and higher order structures in the

classical foreland thrust systems are mainly syn-

thetic imbricate fans and duplexes (Boyer and

Elliott, 1982) whereas the second and higher order

Nevadan structures are antithetic shear zones and

associated structures. The classical single-vergence

thrust systems are restricted to well-stratified fore-

land rocks. ‘The internal parts of mountain belts

commonly display more complex shear zones and

fold nappes. In particular, well known mountain

belts display late antithetic faults and folds

(“ backthrusts” and “backfolds”) (e.g. the Alps;

Milnes and Pfiffner, 1980). The geometric and

chronologic analogy of the Nevadan steep struc-

tures to backthrust-backfold structures in colli-

sional orogens was pointed out by Moores and

Day (1984). The present paper offers a mechanism

for producing these structures. In addition, recent

studies have pointed out the importance of second

order antithetic faults even in foreland settings

(Coward and Butler, 1985, their fig. 2; Price, 1986).

Coward (1983, p. 121) made the general observa-

tion that “sometimes the early thrust may be

steepened and folded by thrusts with a movement

direction opposite to that of the main thrust

movement.” In the northern Sierra Nevada, the

locations and orientations of at least some of the

backthrusts are controlled by pre-existing terrane-

bounding faults.

The Nevadan orogen is a slate belt similar to

slate belts in other mountian chains (Edelman,

1985). Hobbs et al. (1976) gave a detailed account

of slate belt structures worldwide and identified

the following common attributes of these belts: (1)

KINEMATICS OF ARC COLLISION AND NEVADAN OROGENlt STRUCTURES 233

The most obvious fabric element is a steeply di-

pping slaty cleavage. (2) The slaty cleavage con-

tains a down-dip stretching lineation. (3) Macro-

scopic folds are tight to isoclinal with shallow

doubly-plunging hinge lines and steep hinge

surfaces. (4) Faults are parallel to cleavage. (5)

The cleavage is parallel to the flattening (XY)

plane, and the stretching lineation is parallel to

the long (X) axis, of the finite strain ellipsoid. The

steep Nevadan structures correspond closely to

those in other slate belts. and many slate belts

may reflect similar underlying tectonic processes.

The dominance of low grade volcanic, plutonic,

and ultramafic rocks in many slate belts (e.g.

Carolina slate belt in the U.S. Appalachians) sug-

gests that arc collisions may be an important

process for producing slate belts.

The net effect of the model for Nevadan defor-

mation is to repartition crustal displacements, in

the up-dip direction of the east-vergent imbricate

thrusts, from the east-vergent structures to the

west-vergent structures. It is thus possible that the

main east-vergent imbricate thrust (Taylorsville.-

Grizzly Mountain thrusts set) was “blind”, that is,

its displacement decreased to zero before intersect-

ing the Jurassic topographic surface (Dunne and

Fe&l, 1988). The total east-vergent displacement

would then be equal to the amount of west-ver-

gent shortening of the hangingwall (Williams and

Chapman, 1983).

The model presented here suggests that defor-

mation related to arc-continent collision con-

tinued after subduction reversal. Arc magmatism

related to the reversed, sub~ontinental subduction

occurred within the actively defor~ng collisional

orogen (Fig. 5d,e). The deformation, which thus

occurred in a subduction hangingwall as a “cordil-

leran-type” orogen (Dewey and Bird, 1970), is

related to an earlier collision rather than to the

active plate-kinematic regime.

The analysis presented in this paper is not a

report of strain measurement in the Nevadan

orogen7 a~thou~ constraints on some compo-

nents of the macroscopic strain field were pre-

sented. Rather, it is a model for part of the

displacement field history (Sanderson, 1982). It

represents a practical method of understanding

structural evolution at the scale attempted here.

The model is constrained by rock fabrics, large

scale structure, paleogeographic interpretations,

and plate kinematic theory. The model is offered

as a working hypothesis, subject to modification

or rejection as further data are collected, for the

lithosphere-scale kinematic processes by which

simatic material has accreted and deformed to

form crust of continental structure and thickness

in the Nevadan orogen.

Conclusions

The Mesozoic Nevadan orogeny in the north-

ern Sierra Nevada may be fundamentally linked to

two arc-continent collisions. The younger colli-

sion involved partial westward subduction of the

previously assembled terrane amalgam which con-

stituted the continental margin. This collision pro-

duced an east-vergent suture (Slate Creek thrust)

and east-vergent imbricate faults (Taylorsville-

Grizzly Mountain thrust set) within the continen-

tal margin beneath the suture. The material in the

Taylorsville-Grizzly Mountain thrust sheet shor-

tened by motion on west-vergent shear zones and

by tightening of folds. Continued shortening, in-

cluding folding of the Taylorsville and Grizzly

mountain thrusts, may have occurred above a

deeper east-vergent imbricate thrust. Displace-

ments along the east-vergent imbricate faults are

predicted to decrease in their transport directions.

The model addresses several general problems

of orogenesis.

(1) The problem of emplacing a large thrust

sheet of crystalline rock, in this case the Slate

Creek arc” terrane, is solved by underthrusting, or

subducting, the subarc mantle. The suture (Slate

Creek thrust) cut the subarc Moho and rooted

somewhere behind the arc, thus subducting the

subarc mantle wedge to make room for the collid-

ing and underthrusting continental margin which

presently underlies the Slate Creek arc flake.

(2) The problem of accommodating at depth

Nevadan horizontal shortening1 as reflected by

the steep fold hinge planes, steep faults, and steep

foliations, is solved by restricting the shortening to

the hangingwalls of thrusts beneath the Slate Creek

thrust. The displacements at the margins of the

shortened zone are accommodated by differential

displacement along the thrust faults.

234 S.H. EDELMAN

(3) The related problem of interaction of op-

positely-verging structures is solved in the same

way, that is, the steep west-vergent structures

truncate against or merge with the active east-ver-

gent thrust to form tectonic wedges.

Nevadan deformation occurred within an active

continental margin arc and resulted from an earlier

arc-continent collision which was followed by

subduction reversal to form the marginal arc. The

large-scale structural evolution of the northern

Sierra Nevada proposed here is a permissible

structural solution in that lithosphere-scale strain

compatibility is qualitatively maintained. The

Nevadan orogen is a slate belt, and the structural-

plate tectonic model presented here may be testa-

ble in slate belts in other orogens.

Helpful discussions were had with J.S. Beard,

H.W. Day. R.D. Hatcher, Jr., F. Koenemann,

E.M. Moores, J.B. Saleeby, R.A. Schweickert, and

W.D. Sharp. Field work was partially supported

by National Science Foundation Grant EAR 80-

19697 to H.W. Day and E.M. Moores, the Univer-

sity of California, Davis, Sigma Xi, and the Geo-

logical Society of America. Manuscript prepara-

tion was supported by the University of Kansas,

Lawrence, University of South Carolina, Colum-

bia, University of Tennessee, Knoxville, and Na-

tional Science Foundation grant EAR 84-17894

to Robert D. Hatcher, Jr. Reviews by H.W. Day,

EM. Moores, R.J. Twiss, and two anonymous

referees were very helpful.

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