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For permission to copy, contact [email protected] 2002 Geological Society of America1108

GSA Bulletin; September 2002; v. 114; no. 9; p. 1108–1130; 12 figures; 4 tables; Data Repository item 2002112.

Paleomagnetic and 40Ar/39Ar geochronologic data bearing on thestructural evolution of the Silver Peak extensional complex,

west-central Nevada

Michael S. Petronis*John W. GeissmanDepartment of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131-1116, USA

John S. OldowDepartment of Geological Sciences, University of Idaho, Moscow, Idaho 83844-3022, USA

William C. McIntoshNew Mexico Geochronological Research Laboratory, New Mexico Institute of Mining and Technology, Socorro, New Mexico87801, USA

ABSTRACT

The Silver Peak extensional complex, lo-cated in the Silver Peak Range of west-central Nevada, is a displacement-transfersystem linking the Furnace Creek–FishLake Valley fault system and transcurrentfaults of the central Walker Lane. LateNeogene, northwest-directed motion of anupper plate, composed of lower Paleozoicsedimentary rocks and late Tertiary volca-nic and volcaniclastic strata, exhumed alower-plate assemblage of metamorphictectonites with Proterozoic and Mesozoicprotoliths.

Paleomagnetic investigation of Miocene–Pliocene pyroclastic and sedimentary rocksof the upper plate and Miocene mafic dikesin the lower plate reveals modest horizontal-axis tilting (northwest-side-up) and vertical-axis rotation (clockwise) within the exten-sional complex. Eight to ten samples fromeach of 123 sites were demagnetized; 95sites yielded interpretable results. Dual-polarity results from one population of maficdikes in the lower-plate assemblage indicatemoderate, northwest-side-up tilting (decli-nation D 5 3298, inclination I 5 378, a95 54.38, number N 5 30 sites; in situ) (a95 5the confidence limit for the calculated meandirection expressed as an angular radiusfrom the calculated mean direction). Somedikes yield exclusively normal-polarity re-

*E-mail: [email protected].

sults that are interpreted to indicate modestclockwise vertical-axis rotation (D 5 0218,I 5 578, a95 5 4.38, N 5 19 sites; in situ)concurrent with uplift of the lower-platerocks, and nine sites yield magnetization di-rections that are north-directed with posi-tive inclinations of moderate steepness, sim-ilar to an expected Miocene field. LateMiocene pyroclastic rocks in the upperplate yield normal-polarity magnetizationssuggestive of moderate, clockwise, vertical-axis rotation (D 5 0328, I 5 538, a95 5 8.88,N 5 10 sites). The apparent clockwise ro-tation is unlikely to result from incompletesampling of the geomagnetic field, becausethe overall dispersion of the VGP (virtualgeomagnetic pole) positions is high for thelatitude of the site location. Middle Miocenesedimentary rocks probably were remag-netized shortly after deposition. Of eight40Ar/39Ar determinations from mafic dikesin the lower plate, five groundmass concen-trates yield saddle-shaped age spectra, andone separate provided a plateau date of lowconfidence. Isochron analysis reveals thatall six groundmass concentrates contain ex-cess Ar. If rapid cooling and Ar retentionbelow ;250 8C are assumed, the preferredage estimate for mafic intrusions is provid-ed by isochron dates and suggests emplace-ment between 12 and 10.5 Ma. The 40Ar/39Ar age-spectrum data are consistent withexisting fission-track cooling and K-Ar iso-topic age information from lower-plate gra-nitic rocks and indicate rapid cooling of the

lower-plate assemblage from well above 3008C to 100 8C between 13 and 5 Ma. Rapidcooling may explain the overall distributionof paleomagnetic results from lower-plateintrusions such that the earliest acquiredmagnetizations reflect both northwest-side-up tilt and clockwise rotation and the youn-ger magnetizations reflect northwest-side-up tilt. Overall, the paleomagnetic datafrom the Silver Peak extensional complexare interpreted to suggest that vertical-axisrotation of crustal-scale blocks, associatedwith displacement transfer in the centralWalker Lane, may play an integral part inaccommodating strain within a continentaldisplacement-transfer system.

Keywords: ash-flow tuffs, extensional tec-tonics, geochronology, metamorphic corecomplexes, paleomagnetism, Walker Lane.

INTRODUCTION

Middle to late Tertiary extension in the cen-tral Basin and Range province is manifestedin north-northeast–trending mountain rangesand intervening valleys bounded by high-angle faults (e.g., Wernicke, 1988, 1992).Along the western margin of the Great Basin,near its boundary with the Sierra Nevada, thephysiographic expression of the tectonic prov-ince changes and is characterized by arcuate-shaped ranges underlying the northwest-trendingWalker Lane (Locke et al., 1940). The WalkerLane region has long been recognized as a

Geological Society of America Bulletin, September 2002 1109

STRUCTURAL EVOLUTION OF THE SILVER PEAK EXTENSIONAL COMPLEX

zone of diffuse intracrustal shear (Turner,1902), and Gianella and Callaghan (1934) at-tributed deformation to displacement within acomplex fault zone, the character of which issimilar to that of the San Andreas fault sys-tem. The Walker Lane is seismically active(Wallace, 1984; Rogers et al., 1991) and ex-hibits a complex array of coeval extensionaland transcurrent faults (Stewart, 1988; Oldow,1992) that are kinematically linked to theDeath Valley and Eastern California shearzones to the south (Dokka and Travis, 1990)and to the central Nevada seismic belt (Wal-lace, 1984) to the northeast. Combined, theactive belts of deformation form part of a dif-fuse continental boundary that accommodatesup to 25% of the relative movement betweenthe Pacific and North American plates (Savageet al., 1990; Dixon et al., 1995; Dixon et al.,2000; Miller et al., 2001; Hearn and Hum-phries, 1998; Bennett et al., 1999; Thatcher etal., 1999; Oldow et al., 2001).

The Walker Lane forms a boundary zoneseparating the central Basin and Range fromthe Sierra Nevada, which is moving northwestat 12–14 mm/yr relative to stable North Amer-ica (Argus and Gordon, 1991; Dixon et al.,2000). East of the southern Sierra Nevada, theactive belt of deformation is narrow and trendsnorth-northwest. Displacement is accommo-dated by kinematically linked faults boundedon the east and west by the Furnace Creek andOwens Valley fault systems, respectively (Fig.1A). At the latitude of the central Sierra Ne-vada, the belt of active deformation broadensand bifurcates. From south to north, the Fur-nace Creek and Owens Valley fault systemstransfer much of their displacement to theeast, to northwest-trending transcurrent faultsof the central Walker Lane (Fig. 1A). Farthernorth, as much as 6 mm/yr of contemporarymotion is transferred from the Walker Lane tonorth-northeast–trending extensional faults ofthe central Nevada seismic belt (Oldow et al.,2001). Displacement transfer from the Fur-nace Creek and Owens Valley fault systems ismarked by seismically active east-northeast–trending transcurrent and extensional faults(Hardyman and Oldow, 1991; Oldow, 1992;Oldow et al., 2001) forming an east-northeast–trending belt .50 km wide and 80 km long(Fig. 1A). The east-northeast–trending faultsare part of a large-scale, right step in the dom-inantly northwest-trending dextral fault sys-tem (Oldow, 1992).

Prior to the 3 Ma initiation of the OwensValley fault system, displacement transfer wasrestricted to structures linking the FurnaceCreek fault and transcurrent structures of theWalker Lane. The Furnace Creek fault system

stretches 250 km from eastern California to aterminus in northern Fish Lake Valley east ofthe White Mountains (Fig. 1A); displacementalong this fault system, the site of 50 km ofright-lateral strike-slip, steps east to the centralWalker Lane by way of east-northeast–trend-ing faults that merge into a shallow northwest-dipping extensional detachment (Oldow,1992; Oldow et al., 1994). Motion on the de-tachment fragmented an upper plate of supra-crustal rocks and exhumed lower-plate rocksof the Silver Peak–Lone Mountain extensionalcomplex (Oldow, 1992). The upper plate iscomposed of Paleozoic carbonate and clasticrocks overlain by preextensional Tertiary vol-canic rocks and upper Miocene to Pliocene(?)synorogenic sedimentary and volcanic rocks(Stewart and Diamond, 1990; Prestia and Ol-dow, 2000). The upper-plate assemblage restson a shallow-dipping detachment-fault systemthat is folded in doubly plunging, northwest-tending folds with half wavelengths of 5–10km and amplitudes of 1–1.5 km. Beneath thedeformed detachment, the lower-plate assem-blage is composed of metamorphic tectonitesrecrystallized at amphibolite facies and ex-posed as turtleback structures originally rec-ognized by Kirsch (1968). Metasedimentaryrocks have Proterozoic protoliths and were in-truded by late Mesozoic syntectonic graniticrocks (Kohler et al., 1993). On the basis offission-track analysis of zircon and apatite, ina range of lower-plate rocks, the lower-plateassemblage cooled below 220 8C and 100 8Cat 11 and 5 Ma, respectively (Oldow et al.,1994).

Displacement-transfer systems are well-established features in extensional and strike-slip tectonic regimes (e.g., Duebendorfer andWallin, 1991), and many are associated withvertical-axis rotation of structural blocks (Syl-vester, 1988). Large-magnitude block rota-tions are increasingly recognized as an inte-gral part of extensional deformation in manytectonic settings (Hudson and Geissman,1987; Hagstrum and Gans, 1989; Hudson andGeissman, 1991; Snow, 1992; Holm et al.,1993; Snow and Prave, 1994; Janecke et al.,1991; Avigad et al., 1998). In displacement-transfer systems, subsidiary structures transferdisplacement between offset segments of astrike-slip fault system and can be accompa-nied by vertical-axis rotation of crustal blocksthat is often paleomagnetically discernible(e.g., Holm et al., 1993; Snow et al., 1993;Snow and Prave, 1994). Evidence for suchblock rotation has not been recognized at thesouth end of the Walker Lane belt until thepresent study.

Upper-plate rocks of the Silver Peak exten-

sional complex consist principally of upperMiocene volcanic (5.9 6 0.5 Ma, Robinson etal., 1968) and sedimentary rocks and minorpre-Cenozoic rocks (Fig. 1B). The lower-platemetamorphic rocks, which themselves are in-appropriate composition for paleomagneticstudies, are intruded by mafic dikes that havenot been affected by ductile deformation. Therocks of the Silver Peak Range provide an ex-cellent opportunity to use paleomagneticmethods to assess the magnitude of horizontal-axis tilting and vertical-axis rotation of thelower plate associated with displacementtransfer within and across a continental-scalestrike-slip fault system, from the FurnaceCreek–Fish Lake Valley fault system to theWalker Lane fault zone. To better define theage of remanence acquisition in mafic intru-sions of the lower-plate assemblage, we reportnew 40Ar/39Ar geochronologic data from eightmafic intrusions. Paleomagnetic data from 95accepted sites in middle Miocene mafic intru-sions, Miocene to Pliocene(?) pyroclasticrocks, and middle Miocene sedimentary rocksare compared to middle Miocene reference di-rections (e.g., Mankinen et al., 1987) to quan-tify the magnitude of vertical-axis rotation andhorizontal-axis tilting.

FIELD RELATIONSHIPS ANDSTRUCTURAL SETTING

The Silver Peak Range is situated betweennorthwest-trending faults of the Walker Laneand the northern terminus of the FurnaceCreek–Fish Lake Valley fault zone (Fig. 1A).The range consists of three different regionsof distinct character (Fig. 1B). Two subparal-lel belts of lower Paleozoic sedimentary rocksintruded by middle Mesozoic plutons trendnorthwest and form the northeast and south-west parts of the range. They are separated bya central region of pre-Tertiary rocks overlainand dominated by Tertiary sedimentary andvolcanic rocks (Albers and Stewart, 1972).

The eastern Silver Peak Range was the fo-cus of this study and is divided into north-western and southeast structural domains. Inthe northeast, the Silver Peak Range is dom-inated by the northwest-trending MineralRidge, formed by a northwest-trending doublyplunging antiform exposing lower-plate meta-morphic tectonites of the Silver Peak exten-sional complex (Fig. 2). Along the margins ofthe metamorphic core and in isolated klippen,upper-plate rocks are separated from underly-ing metamorphic tectonites by the MineralRidge detachment fault (Stewart and Dia-mond, 1990; Oldow et al., 1994). To thenorthwest, the Mineral Ridge detachment fault

1110 Geological Society of America Bulletin, September 2002

PETRONIS et al.

Figure 1. (A) Major strike-slip and normal-slip fault systems in western Nevada and eastern California. FCF—Furnace Creek fault,OVF—Owens Valley fault, CWL—central Walker Lane, CNSB—central Nevada seismic belt. (B) Simplified geology of the Silver PeakRange, Nevada. Paleomagnetic sampling localities, including several discrete sites, shown as stars. Cross-section lines A–A9 and B–B9are shown in Figure 3. (modified from Albers and Stewart, 1972).

dips beneath brittlely deformed rocks of theupper-plate assemblage that forms the north-west structural domain (Fig. 3).

Pre-Tertiary rocks in the central Silver PeakRange are deformed in four generations ofpenetrative structures (Kohler et al., 1993;Satterfield et al., 2000). In the lower-plate as-semblage, the first structures are synmeta-morphic tight to isoclinal folds with a pene-

trative axial-planar foliation. First structuresare deformed by upright, northwest–trending,tight to open folds. In upper-plate rocks, twogenerations of penetrative structures also arefound.

In the upper plate, second-generation foldsin pre-Tertiary rocks are similar in geometryand physical conditions of formation to thosein the lower plate. In contrast, first-generation

structures in the upper-plate rocks are not ac-companied by major metamorphism (Oldowet al., 1994; Satterfield et al., 2000), and thekinematic relationship, if any, between first-generation structures in the upper- and lower-plate assemblages is not well understood (Sat-terfield et al., 2000). In both upper- andlower-plate assemblages, the first two gener-ations of structures are older than upper Mio-



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PETRONIS et al.

Figure 3. Schematic geologic cross sections of the Silver Peak extensional complex. (A–A9) Cross section oriented N608W along the axisof Mineral Ridge. (B–B9) Cross section oriented N608E that crosses the high point of the metamorphic basement on Mineral Ridge. SeeFigure 1 for location of section lines. The detailed geology is based on Prestia and Oldow (2000), which is not depicted in Figure 1.

cene sedimentary and volcanic rocks (Prestiaand Oldow, 2000).

Mafic dikes of diverse orientations cut pre-Tertiary crystalline and metasedimentaryrocks of the lower plate. The dikes are typi-cally ,1 m thick and display chilled marginswith their host rocks. No consistent crosscut-ting relationships between dikes have beenobserved.

Upper Miocene to Pliocene(?) rocks of theupper plate and pre-Tertiary rocks, both of theupper and lower plates, are deformed in twoyounger generations of folds. The earlier of thetwo fold sets is northeast trending and exhibitsa spaced axial-planar cleavage. Superposed onnortheast-trending structures are open to gentle,northwest-trending folds also characterized bya spaced axial-planar cleavage.

The Mineral Ridge detachment is deformedin broad north-northeast– and northwest–trending superposed folds to form the turtle-back structure that controls exposure of thelower-plate rocks and the distribution of Ce-nozoic and pre-Tertiary rocks in the centralSilver Peak Range. The northwest-trendingturtleback formed during distributed simpleshear associated with displacement transferfrom the Furnace Creek fault system to thenorthwest-trending transcurrent faults of thecentral Walker Lane (Oldow et al., 1994). Thepresent-day long axis of the turtleback,N608W, is consistent with modest simple shear(g 5 1.7) (g 5 tan C, where C is the angular

shear strain) and 258 clockwise rotation withrespect to an initially more east-west orienta-tion (see Discussion) and the N408W-trendingFurnace Creek fault, which served to accom-modate simple shear. The structure of the upper-plate assemblage is complicated and records ahistory of late Miocene to Pliocene(?) py-roclastic and volcaniclastic deposition dur-ing active extension (Prestia and Oldow,2000). The upper Miocene to Pliocene(?)sections are characterized by abrupt changesin distribution and thickness owing to ac-cumulation in fault-bounded basins activeduring northwest-directed displacement of theupper-plate assemblage.

Neogene volcanic and sedimentary rockslying between the two belts of pre-Cenozoicstrata form the topographically highest part ofthe mountain range and involve two deposi-tional sequences: one predating upper-plate ex-tension and the other deposited during exten-sion. Preextensional Tertiary rocks are locallypreserved in the central Silver Peak Range andrest depositionally on lower Paleozoic carbon-ate and siliciclastic rocks. The Tertiary succes-sion consists of an upper Oligocene to lowerMiocene basal fluvial conglomerate overlainby rhyolitic tuff and andesitic volcanic rocks(Diamond, 1989; Robinson et al., 1968; Bai-ley, 1951) varying in thickness from severalhundred meters to zero over distances of a fewkilometers. These strata are unconformablyoverlain by middle to upper Miocene coarse-

grained detrital rocks, poorly welded rhyoliteand latite pyroclastic flows, and basalt to dac-ite lavas. The thickness of the overlying suc-cession varies dramatically but reaches a max-imum of .1.0 km. The younger Tertiarysection accumulated in half-graben basins ac-tive during displacement of the upper-plate as-semblage of the Silver Peak extensional com-plex (Prestia et al., 1998; Prestia and Oldow,2000). These strata contain numerous internalangular unconformities with differential tiltsof 108 to as much as 358. The Tertiary sectionand internal unconformities dip southeast,consistent with tilt about a northeast-trendingaxis. The pattern of tilt, spatial distribution,and differential thickness of the upper Tertiarysuccession is clearly syndepositional and wascontrolled by northeast-trending half-grabensformed during upper-plate extension (Prestiaand Oldow, 2000) (Fig. 3).

Both older and younger systems of north-to northeast-trending normal faults dissect thecentral Silver Peak Range and have a consis-tent northwest-side-down sense of throw (Fig.3, A–A9). The younger system of faults cut allpre-Tertiary and Tertiary rocks and are locallyactive. The younger faults preserve scarps inalluvium up to 10 m high along the westernrange front (Reheis and Sawyer, 1996). In thecentral Silver Peak Range, the range-frontfault system of the southern part of the rangebifurcates and forms a series of subparallel,north-trending structures that divide the cen-



Figure 4. Lower-hemisphere equal-area projections of in situ site-mean directions frommafic dikes. (A) All interpretable site-mean directions, based on demagnetization data. (B)Group-mean directions (GPM) calculated on the basis of 58 accepted out of 76 sites (seetext for explanation). (C) In situ site-mean directions from sites with a northwest-directedmagnetization. Gray star—the group-mean direction of all interpretable northwest-directed site-mean directions (D 5 329.28, I 5 37.78, a95 5 4.38, k 5 37.5, n/N 5 30 sites).(D) In situ site-mean directions from sites with a north-northeast–directed magnetization.The group-mean direction of all interpretable north-northeast–directed site-mean direc-tions (D 5 021.48, I 5 57.28, a95 5 4.38, k 5 51.1, n/N 5 19 sites). Solid square—normal-polarity site mean, open square with X—reversed-polarity site mean inverted through theorigin, gray diamond—remagnetization circle site mean (not included in group-mean cal-culations), gray sun—Miocene expected direction.

Figure 5. Lower-hemisphere equal-area projections of (A) in situ and (B and C) correctedsite-mean directions from all interpretable sites in pyroclastic rocks. Three sites yielduninterpretable results and are not plotted. (A) In situ site-mean directions from all in-terpretable sites. Gray star—the in situ group mean of sites with a northeast-directedmagnetization (D 5 032.98, I 5 52.58, a95 5 9.58, k 5 26.8, n/N 5 10/14 sites). Gray X—the in situ group mean of sites with a northwest-directed magnetization (D 5 328.28, I 550.08, a95 5 9.68, k 5 64.7, n/N 5 5/7 sites). (B) Corrected site-mean directions from siteswith northwest-directed magnetizations. Gray star—the group-mean direction of all ac-ceptable sites (D 5 327.88, I 5 49.08, a95 5 13.98, k 5 31.3, n/N 5 5/7 sites). (C) Correctedsite-mean directions from sites with northeast-directed magnetizations. Gray star—thegroup-mean direction of northeast-directed site-mean directions (D 5 030.38, I 5 53.58,a95 5 6.58, k 5 52.3, n/N 5 10/14 sites). See Figure 4 for description of symbols. Siteswith labeled site numbers lie more than two standard deviations from the mean and havebeen excluded from group-mean calculations.

tral Silver Peak Range into north-trendingstructural panels. Traced to the north, the arrayof faults progressively swings northeast acrossthe mountain range.

RESULTS

Paleomagnetism

Of the 123 sites sampled, 95 sites provideinterpretable NRM (natural remanent magne-tization) data (Figs. 4–6). Paleomagnetic sitesare divided into four groups, principally on thebasis of rock type but also by geographic lo-cation. Results are summarized by rocktype—mafic dikes, pyroclastic rocks, and la-custrine rocks and basalt flows—in Tables 1and 2, and GSA Data Repository Tables DR1and DR21, respectively. Representative de-magnetization diagrams (Fig. 7) reveal thegenerally high quality of the progressive-demagnetization data for these rocks. Inter-pretable demagnetization data have been ob-tained for 58 of 76 sites in mafic dikes, 21 of24 sites in pyroclastic rocks, 13 of 18 sites inlacustrine sedimentary rocks, and 2 of 3 sitesin Holocene basalt flows, and the 1 site in Ho-locene sedimentary rocks. One site (SP87) indeformed marble had very high between-sample dispersion and did not yield an inter-pretable demagnetization result.

Mafic Dikes and SillsOf the 76 sampling sites established in maf-

ic dikes (Fig. 2), 58 sites yielded interpretabledemagnetization data that are well grouped(a95 confident estimate values , 10.08) at thesite level (Fig. 4). Demagnetization data from30 of these 58 sites yield northwest declina-tion and moderate and positive inclinations forsite-mean directions or antipodes (Fig. 4C).Demagnetization data from 19 of these 58sites yield north-northeast declination andmoderate and positive inclination (Fig. 4D).Nine sites yield magnetizations with direc-tions that are north directed with moderate,positive inclinations, similar to expected Neo-gene field directions (Fig. 4A). For the re-maining sites, 11 are associated with unac-ceptably high dispersion (Table 1), and 7 donot yield interpretable demagnetization re-sponse. Representative demagnetization dia-grams of typical behavior of mafic dikes showthe overall high quality of the demagnetizationbehavior (Fig. 7, M–X).

1GSA Data Repository item 2002112, summaryof 40Ar/39Ar age determinations from mafic intru-sions, Mineral Ridge, is available on the Web athttp://www.geosociety.org/pubs/ft2002.htm. Re-quests may also be sent to [email protected].

Site-mean directions from mafic dikes arecharacterized by two statistically distinct group-mean populations (Fig. 4, C and D). The firstincludes 30 sites with northwest-directed mag-

netizations (group-mean population A, Fig.4B). The other includes 19 sites with north-northeast–directed magnetizations (group-meanpopulation B, Fig. 4B). Five sites used to define


PETRONIS et al.

Figure 6. Lower-hemisphere equal-area projections of (A) in situ and (B) corrected site-mean directions from all interpretable sites in Neogene lacustrine sedimentary rocks. (A)In situ site-mean directions. Solid symbols are from sites 34, 35, 36, 37, 46, 47, and 48collected on both limbs of an open, upright, northeast-trending anticline on the west sideof the Silver Peak Range (Figs. 1 and 2). Open symbols with X are sites 39, 40, and 41collected in a uniformly southeast-dipping sequence, and sites 102, 103, and 105 collectedin a uniformly northwest-dipping sequence of lacustrine sedimentary rocks along the eastside of Red Mountain (all site means have been inverted to lower-hemisphere projections).In situ group-mean directions: gray star—D 5 001.08, I 5 63.08, a95 5 10.78, k 5 32.9,n/N 5 7 sites; gray X—D 5 150.08, I 5241.08, a95 5 14.18, k 5 23.5, n/N 5 6 sites. (B)Corrected site-mean directions: gray star—D 5 336.08, I 5 62.08, a95 5 25.98, k 5 6.4, n/N 5 7 sites; gray X—D 5 162.08, I 52488, a95 5 31.88, k 5 5.4, n/N 5 6 sites. Afterapplying outcrop-scale structural corrections, which varied between sites, to restore bed-ding to horizontal, the significant increase in dispersion between in situ and corrected site-mean directions, at both localities, is interpreted to suggest that lacustrine sedimentaryrocks were remagnetized during or after deformation of the Silver Peak Range. The rockshave a slight greenish color, suggesting the presence of iron-bearing clays around detritaloxides, and abundant hematite was observed surrounding grains in thin sections. Overall,our working hypothesis for the Miocene sedimentary rocks in the Silver Peak Range isthat they have been differentially remagnetized over a time spanning at least one polarityreversal. That the upper-plate sedimentary rocks are remagnetized is curious, as upper-plate pyroclastic rocks do not show evidence of remagnetization. It is plausible that thehigh porosity and permeability of the sedimentary deposits and their shallow depth facil-itated migration of meteoric fluids through the sedimentary rocks.

the group-mean population A direction yieldmagnetizations of southeast declination andmoderately shallow, negative inclination; thesedata have been inverted through the origin forthe purpose of comparison (open squares, Fig.4C). A paleomagnetic reversal test on sitesfrom group-mean population A (McFadden andMcElhinny, 1990) yields a critical angle for thetwo populations of 12.18, and the observed an-gle between the mean of the normal-polarity setand the mean of the reversed-polarity set is17.48, indicating a negative reversal test. Allsites used in calculating group-mean populationB have normal-polarity magnetizations.

Field sampling of mafic dikes focused onthe best possible exposures. At several sites,comprehensive sampling for paleomagneticcontact tests was carried out with the goal ofassessing paleomagnetic stability and magne-tization antiquity. The host rock at all contactsites was either deformed granitic rocks or fel-sic gneiss. The results of all contact tests areinconclusive. Demagnetization behavior ofhost rock samples adjacent to contacts is typ-ically difficult to interpret and magnetizations

are poorly defined (Fig. 7, K–L). We interpretthe results to indicate poor paleomagneticcharacter of the host rocks, which are penetra-tively deformed and weakly magnetized. Al-though results of field tests do not negate thepossibility of pervasive remagnetization of thelower-plate rocks after emplacement of maficintrusions, the mafic dikes show only modestchloritization and clearly were intruded afterpenetrative deformation of the host rocks.Also, if pervasive remagnetization of mafic in-trusions occurred, it is likely that the distri-bution of the site-mean data, interpreted to inpart reflect paleo–secular variation, wouldshow less scatter than actually observed. Rep-resentative samples of the mafic dikes exam-ined petrographically and in electron-microprobebackscattered-electron images show onlyslight to moderate chloritic alteration as haloesaround amphiboles, and low-Ti magnetitegrains are fresh to weakly oxidized. Ten rep-resentative samples from the mafic dikes wereanalyzed by using X-ray fluorescence to de-termine the major oxides and calculated nor-mative mineral abundances (Table 3). Five

samples are classified (Streckeisen and Le-Maitre, 1979) as monzodiorite, one as quartzmonzodiorite, one as gabbro, one as monzo-gabbro, one as nepheline gabbro, and one asnepheline monzogabbro. Representative pol-ished thin sections of specimens were exam-ined in transmitted and reflected light in low-to medium-power magnification. Small,generally euhedral to subhedral, equant grainsof magnetite in a matrix of hornblende 1quartz 1 plagioclase feldspar 6 potassiumfeldspar 6 calcite are typical of most speci-mens. Textural relationships between magne-tite grains and silicate phases—such as the oc-currence of magnetite as inclusions infeldspars and amphiboles and as an interstitialgroundmass phase in the more porphyriticrocks—are interpreted to indicate that mag-netite is of primary, magmatic origin.

Rock-magnetic experiments indicate thatthe principal magnetic phase is cubic and ofmoderately high coercivity. The typically highlaboratory unblocking temperatures (unblock-ing between ;5458 and 580 8C) are consistentwith a relatively fine grained, low-impuritymagnetite phase, although we have not carriedout the appropriate thermomagnetic tests toconfirm this interpretation. These observationsand the quality of the demagnetization datafrom accepted sites (Fig. 7, M–X) suggest thatthe sampled intrusions possess characteristicremanent magnetizations (ChRMs) that aregeologically stable and of primary thermorem-anent origin.

Pyroclastic RocksOf the 24 paleomagnetic sampling sites in

pyroclastic rocks (Figs. 1 and 2), 21 yieldedinterpretable demagnetization data (Fig. 5).Magnetizations are all well grouped at the sitelevel; their k values range from 10 to 188 (Ta-ble 2, Fig. 5A). Demagnetization behavior ofpyroclastic rocks (Fig. 7, A–D) is generally ofhigh quality. In situ site means for 13 out of24 sites in pyroclastic rocks, all from RhyoliteRidge, are of northeast declination and mod-erate, positive inclination. No reverse-polaritymagnetizations were observed in samplesfrom these sites. The 13 northeast-directedsites from Rhyolite Ridge were used to cal-culate an in situ group mean for the pyroclas-tic rocks (Fig. 5A). After moderate structuralcorrections (Table 2), these sites yielded a cor-rected group-mean direction having declina-tion D 5 0328 and inclination I 5 538 (Fig.5C). Of the remaining eight sites in pyroclas-tic rocks elsewhere, three sites yielded in situsite means with northwest declination andmoderate to moderately steep, positive incli-nation, two sites yielded in situ southeast dec-



TABLE 1. PALEOMAGNETIC DATA FROM MAFIC DIKES AND HOST ROCKS

Site Rocktype

N/N0 a95 k D(8, insitu)

I(8, insitu)

S/D(8/8)

North(Y)

East(X)

Interpretation

Coyote Canyon13 Td 9/11 11.8 20.1 25.9 43.6 N.A. 4 182 389.1 435 028.0 Rotated14 Td 8/8 11.8 17.6 30.6 52.2 N.A. 4 182 452.5 434 955.2 Rotated15 Td 10/10 14.7 11.8 50.8 27.1 N.A. 4 182 034.1 435 013.5 Rotated16 Td 9/9 9.1 29.0 150.9 218.9 075/90 4 182 332.2 435 209.6 Tilted17 Td 12/12 8.1 27.3 330.5 29.8 050/08 4 181 904.7 435 479.2 Tilted18 Td 11/11 6.6 44.5 329.5 25.1 065/05 4 181 793.8 435 472.5 Tilted19 Td 10/10 4.3 112.0 332.5 34.5 065/05 4 181 719.7 435 493.9 Tilted20 Td 8/10 13.7 17.4 332.4 21.5 N.A. 4 181 719.7 435 491.0 Tilted21* Td 7/13 21.4 8.8 279.3 210.4 N.A. 4 180 177.3 435 746.1 Tilted55 Td 4/7 8.5 116.3 334.5 28.7 300/60 4 180 442.7 435 381.1 Tilted59 Td 8/9 5.2 116.7 16.0 62.3 105/22 4 180 610.5 436 393.9 Rotated61 Td 6/8 7.3 85.3 339.3 39.8 157/44 4 180 511.4 436 058.4 Tilted62 Td 8/8 8.0 49.4 341.4 45.6 139/48 4 180 511.4 436 058.4 Tilted64 Td 8/9 9.4 35.4 341.9 48.3 180/30 4 180 511.4 436 058.4 Tilted65 Td 9/8 7.0 54.4 336.3 45.1 130/64 4 180 511.4 436 058.4 Tilted66 Td 7/7 6.5 87.4 329.8 36.0 N.A. 4 180 267.7 436 486.7 Tilted67 Td 10/10 3.7 169.7 1.2 54.5 205/53 4 180 267.7 436 486.7 TiltedTarantula Canyon22* Td 4/7 20.6 20.9 137.0 15.6 315/90 4 182 055.0 437 091.9 Tilted23* Td 4/7 29.1 10.9 146.2 226.6 180/80 4 181 871.6 437 137.5 Tilted24 Td 6/9 11.1 37.6 23.9 62.6 315/75 4 181 871.3 437 168.3 Rotated25 Td 8/9 5.0 123.0 3.1 58.2 035/90 4 181 814.5 437 106.2 No T/R26 Td 8/9 8.9 39.3 137.2 217.0 080/90 4 181 796.2 437 153.1 Tilted27 Td 11/11 6.4 51.7 143.3 216.0 230/35 4 181 682.8 437 152.2 Tilted28 Td 9/12 9.3 31.7 332.1 41.7 170/05 4 181 529.5 437 133.4 Tilted107 Td 7/8 4.6 170.3 360.0 56.7 170/34 4 181 569.9 437 171.9 No T/R108 Td 7/8 9.0 45.5 11.0 53.8 065/60 4 181 539.7 437 250.9 Rotated109 Td 8/9 9.8 32.9 355.7 54.5 215/30 4 181 539.7 437 250.9 No T/R110 Td 5/10 9.2 70.2 13.9 58.5 208/20 4 181 339.5 437 315.4 Rotated112 Td 9/8 3.9 178.0 10.5 59.6 145/35 4 181 414.5 437 172.2 Rotated113 Td 9/8 7.3 50.3 351.9 50.4 160/30 4 181 141.1 437 382.9 No T/R114 Td 7/8 12.9 22.9 0.2 47.6 155/20 4 181 157.5 437 405.1 No T/R115 Td 8/9 9.0 39.0 124.6 229.7 180/58 4 181 157.5 437 405.1 TiltedVanderbilt Peak70 Td 7/11 10.4 34.5 20.0 60.7 053/45 4 178 594.2 440 684.8 Rotated72 Td 9/9 6.3 59.9 329.8 46.9 165/55 4 178 594.2 440 684.8 Tilted73 Td 10/10 9.1 28.8 320.0 40.2 165/55 4 178 594.2 440 684.8 Tilted74 Td 10/10 6.5 50.5 330.9 50.7 165/55 4 178 594.2 440 684.8 Tilted75 Td 7/8 8.6 50.3 320.9 41.1 165/55 4 178 594.2 440 684.8 Tilted76 Td 8/8 7.2 59.6 320.3 45.1 165/55 4 178 594.2 440 684.8 Tilted77 Td 9/9 9.7 29.1 314.4 31.5 165/55 4 178 594.2 440 684.8 Tilted78 Td 7/8 5.8 108.7 325.7 51.7 165/55 4 178 594.2 440 684.8 Tilted79 Td 10/10 6.7 53.1 334.7 52.5 165/55 4 178 594.2 440 684.8 Tilted80 Td 7/7 8.3 53.6 319.0 37.7 165/55 4 178 594.2 440 684.8 TiltedNorth Star mine82 Td 6/10 10.0 45.7 329.5 47.8 080/90 4 183 851.5 435 315.4 TiltedMary mine84 Td 11/10 6.3 54.1 354.9 56.4 125/15 37.79 117.71 No T/R86 Td 5/7 9.7 63.8 355.2 63.4 240/65 37.79 117.71 No T/R87* PcWF 065/40 37.79 117.71 Rejected88 Td 9/9 3.9 174.8 142.4 239.9 055/30 37.79 117.71 Tilted90 Td 9/10 4.2 155.2 8.8 58.5 240/45 37.80 117.70 Rotated91 Td 8/8 5.6 99.0 357.4 54.3 240/45 37.80 117.70 No T/R92 Td 8/8 6.3 77.8 355.9 55.0 319/50 37.80 117.70 No T/R93 Td 6/7 12.0 32.2 327.1 42.5 317/35 37.80 117.70 Tilted94 Td 7/9 16.0 15.2 58.4 71.4 110/90 37.80 117.70 Rotated96 Td 8/10 8.2 46.9 20.8 58.4 170/45 37.80 117.70 Rotated97 Td 8/9 5.4 105.0 25.0 59.4 315/00 37.80 117.70 Rotated98 Td 9/9 9.2 32.3 8.5 51.5 175/45 37.80 117.70 Rotated99 Td 10/10 8.6 32.5 23.8 52.0 155/50 37.80 117.70 Rotated100 Td 7/10 5.9 106.2 7.8 57.6 155/50 37.80 117.70 Rotated101 Td 7/12 9.5 41.7 10.1 55.1 320/00 37.80 117.70 RotatedNorth Springs117 Td 8/8 4.4 160.6 340.1 29.3 330/90 37.82 117.78 Tilted118 Td 9/8 3.8 188.0 46.8 11.9 020/90 37.82 117.78119 Td 9/8 2.0 639.4 354.0 46.0 350/90 37.82 117.78 No T/R

Note: Td—Tertiary dikes; PcWF—Precambrian Wyman Formation; N/No—ratio of samples used (N) to samplescollected (No); a95—95% confidence interval about the estimated mean direction, assuming a circular Fisherdistribution; k—best estimate of (Fisher) precision parameter; D and I—in situ declination and inclination; S/D—strike and dip, dip direction is clockwise from strike; N.A.—not available (strike and dip indeterminable); North(Y) and East (X), site location in Universal Transverse Mercator (UTM) coordinate system, except for Mary mineand North Springs, for which the first number is degrees north latitude, and the second number is degrees westlongitude; tilted—tilted group mean; rotated—rotated group mean; no T/R—no net tilt or rotation.

*Dispersion at site level unacceptably high and therefore rejected from further analysis.

lination and negative inclination, one siteyielded a north declination and moderate, pos-itive inclination, and two sites yielded an un-usual magnetization with an east declinationand negative inclination (Fig. 5B).

Sedimentary RocksA total of 18 sites was established in fine-

grained, Miocene detrital sedimentary rocks.The sedimentary rocks are chiefly light grayto grayish-green, thin- to medium-bedded,moderately sorted, well-indurated, calcite-cemented, fine-grained lithic sandstones. Sitesin these strata were established at two locali-ties. Ten sites were collected along the westside, and eight sites were collected along thesoutheast side, of the central Silver PeakRange (Figs. 1 and 2). Thirteen sites yieldedinterpretable paleomagnetic results with bothnormal and reverse polarity (Fig. 6A). Fivesites, at the sample level, did not yield inter-pretable demagnetization data (Table DR1).Representative demagnetization diagrams forthe fine-grained sedimentary rocks (Fig. 7, E–H) show that AF demagnetization responsevaried significantly between sites and betweensamples from the same sites. After a simplestructural correction, the dispersion of site-mean directions for both the normal-polarity(NP) and reversed-polarity (RP) data increasessignificantly (NP: Kin situ 5 32.8, RP: Kin situ 523.5; NP: Kcorrected 5 6.4, RP: Kcorrected 5 5.3)(Fig. 6B).

Rock Magnetism

Several rock-magnetic experiments wereconducted to assess the mineral constituentsand domain state of the principal magnetiza-tion carriers in rock types sampled in the Sil-ver Peak Range. The isothermal remanentmagnetization (IRM) acquisition and backfielddemagnetization results (Fig. 8) and modifiedLowrie-Fuller tests (Johnson et al., 1975) aresummarized by rock type in this section.

Mafic Dikes and Host RocksSpecimens from mafic dikes reach at least

95% saturation in fields between 90 and 200mT (Fig. 8A, specimen SP18b). Only onespecimen (site 16) required a slightly higherfield (300 mT) to reach saturation. These re-sults indicate that magnetite is the main mag-netic phase carrying the remanence in the maf-ic dike samples. One specimen from graniticcountry rock (site SP23) reached 60% satu-ration by 180 mT, but continued to acquire40% of its IRM up to 600 mT, indicating con-siderable hematite as a magnetic phase (Fig.8A, specimen SP23h).


PETRONIS et al.

TABLE 2. SUMMARY OF PALEOMAGNETIC DATA FROM PYROCLASTIC ROCKSIN THE SILVER PEAK RANGE

Site Rocktype

N/No a95 k D(8, insitu)

I(8, insitu)

S/D(8/8)

D(8, cor)

I(8, cor)

North(Y)

East(X)

Comments

Rhyolite Ridge1 Tv 9/14 7.1 53.0 331.5 62.4 020/07 336.6 69.0 4 186 466.9 428 126.0 Titled2 Tv 13/13 10.9 15.4 324.4 43.8 020/07 325.7 50.7 4 187 153.3 428 946.1 Rotated3 Tv 13/13 4.6 75.6 13.9 16.7 020/07 15.9 20.2 4 186 116.4 429 519.6 Rotated4 Tv 6/11 6.3 113.5 22.2 66.8 020/07 38.8 68.6 4 186 180.5 429 370.5 Rotated5 Tv 16/16 3.6 107.6 15.9 45.2 020/07 22.6 48.2 4 186 318.4 429 044.5 Rotated6 Tv 8/10 7.1 61.2 13.4 45.5 020/07 20.1 48.8 4 186 390.2 429 081.8 Rotated7 Tv 10/10 3.3 188.2 11.5 47.5 020/07 18.6 51.0 4 186 423.0 429 130.5 Rotated8 Tv 13/13 3.4 152.5 10.3 56.1 020/07 20.0 59.6 4 186 297.0 428 951.9 Rotated9 Tv 5/13 13.0 35.5 43.7 40.8 020/07 48.7 40.6 4 186 286.7 428 860.9 Rotated10 Tv 6/10 14.8 21.5 30.9 61.0 020/07 43.7 61.9 4 186 362.5 428 861.5 Rotated11 Tv 8/9 7.9 49.7 65.9 53.4 020/07 73.1 50.5 4 186 269.6 428 912.1 Rotated

12 Td 9/9 2.3 457.9 15.9 63.8 in situ 4 186 077.3 429 331.5 Rotated

29 Tv 12/13 7.9 31.3 343.8 49.0 220/20 335.3 31.5 4 185 725.5 427 048.6 Tilted30 Tv 7/14 13.8 19.9 16.3 44.9 220/20 1.9 34.5 4 185 658.0 426 946.8 Rotated31 Tv 12/13 7.1 38.5 68.1 45.5 220/20 46.1 51.5 4 185 220.7 427 256.6 Rotated32 Tv 15/17 5.1 112.5 56.1 52.6 220/20 29.0 53.5 4 185 725.5 427 048.6 Rotated33 Tv 15/15 11.6 10.4 50.0 45.9 220/20 19.1 45.7 4 185 658.0 426 946.8 Rotated

42 Tv 7/10 9.4 41.9 81.6 53.2 263/26 51.7 45.5 4 188 055.1 422 859.9 Rotated43* Tv 5/6 20.7 14.5 105.3 215.6 195/27 105.3 11.4 4 187 650.4 426 669.6 RejectedIce House Canyon49* Tv 5/12 67.5 2.2 86.7 235.7 in situ 4 187 091.8 423 229.3 Rejected50* Tv in situ 4 181 206.9 414 971.5 Rejected51 Tv 7/8 7.8 60.5 98.9 244.4 020/25 88.0 268.5 4 181 206.9 414 971.552 Tv 5/8 10.4 54.9 147.9 248.7 in situ 4 181 206.9 414 971.5 Tilted53 Tv 9/11 4.1 155.6 136.1 244.3 in situ 4 181 206.9 414 971.5 Tilted54 Tv 10/10 5.9 54.7 56.1 287.9 in situ 4 181 206.9 414 971.5

Note: Tv—Tertiary pyroclastic rocks; N/No—ratio of samples used (N) to samples collected (No); a95—95%confidence interval about the estimated mean direction, assuming a circular Fisher distribution; k—best estimateof (Fisher) precision parameter; D/I (in situ)—in situ declination and inclination; S/D—strike and dip, dip clockwisefrom strike; D/I (cor)—corrected declination and inclination, in situ—here in situ refers to sites, in pyroclastic rocksonly, in which a bedding attitude could not be determined; North (Y) and East (X), site location in UniversalTransverse Mercator (UTM) coordinate system; tilted—tilted group mean; rotated—rotated group mean; no T/P—no net tilt or rotation.

*Dispersion at site level unacceptably high and therefore rejected from further analysis.

Results from modified Lowrie-Fuller tests(Johnson et al., 1975) reveal that the normal-ized stability of saturation isothermal rema-nent magnetization (SIRM) relative to NRMand anhysteretic remanent magnetization(ARM) varies among different rock types andamong sites in the same rock type. Briefly,results can be described in terms of three dif-ferent categories based on the shape of the de-magnetization curves (Fig. 8B): (1) the mag-netization is carried by single-domain topseudo-single-domain magnetite (Fig. 8B,specimen SP27b), (2) it is dominated by mul-tidomain magnetite (Fig. 8B, specimenSP91h), and (3) the tests are inconclusive withthree distinct behaviors (Fig. 8B, specimensSP103c, SP113d, and SP5i). Inconclusive re-sults likely indicate that an assemblage ofmultidomain to single-domain magnetitegrains are present in the samples and that abroad range of grain sizes and shapes contrib-ute to the magnetization.

Pyroclastic RocksSpecimens from pyroclastic rocks reach at

least 95% of SIRM in fields between 160 and

200 mT (Fig. 8A, specimen SP5i), indicatingthat magnetite is the principal remanence car-rier for pyroclastic rocks. Specimens fromsites 1 and 33, however, do not reach satura-tion by 1.25 T, indicating that a part of theremanence is likely carried by a hematite.

Sedimentary RocksSpecimens from lacustrine sedimentary

rock required fields between 160 and 400 mTbefore 95% saturation was reached (Fig. 8A,specimen SP39g), indicating that magnetite isa principal magnetic phase in the sedimentaryrocks, but that a higher-coercivity phase isalso present. Specimens from two sites, how-ever, never reached saturation in fields up to1.25 T, suggesting hematite as an importantmagnetic phase.

40Ar/39Ar Results

Resistance-furnace incremental-heating40Ar/39Ar analyses of six groundmass-concentrate samples yielded somewhat dis-turbed age spectra, but form relatively well-

defined isochrons with moderately elevated40Ar/36Ar intercepts (Fig. 9, A–F). Age spec-tra for five of the six samples (Fig. 9) aresaddled shaped, with central apparent-ageminima generally ranging from 14 Ma to 10Ma. The apparent ages of high- and low-temperature steps in these spectra are olderthan the central age minima, ranging fromas old as 50 Ma to 15 Ma. The central part ofthe age spectrum of the sixth groundmass con-centrate (Fig. 9E, SP-96) is relatively flat, but,in contrast to the saddle-shaped spectra, theapparent ages of the high- and low-temperaturesteps are slightly younger. For all six ground-mass samples, weighted-mean apparent ageswere calculated for the nearly concordantsteps comprising the relatively flat central partof the age spectrum. In most cases, these stepsdo not satisfy typical ‘‘age plateau’’ criteria,such as those defined by Fleck et al. (1977).

The 40Ar/39Ar results from these six ground-mass samples (Fig. 9, A–F) appear more con-cordant when plotted on isotope-correlationplots (36Ar/40Ar vs. 39Ar/40Ar), which do notdepend on the assumption that all argontrapped at the time of emplacement is of at-mospheric composition (40Ar/36Ar 5 295.5).With the exception of anomalous high- andlow-temperature steps, the isotopic ratios de-fine moderately well correlated arrays, and theisochron intercept ages range from 12.4 6 1.9Ma to 10.5 6 0.8 Ma, and 40Ar/36Ar interceptsrange from 300 6 1 to 399 6 6 (Fig. 9, A–F; Table 4). The isochrons of two specimens(SP-55 and SP-75, Fig. 9, A and C) have rel-atively low MSWD (mean square of weighteddeviates) values (2.1 and 2.5, respectively) andyield relatively precise isochron intercept ages(11.6 6 0.4 Ma and 11.3 6 0.2 Ma, respective-ly). Results from three groundmass-concentratespecimens (SP-69, SP-92, SP-112, Fig. 9, B, D,and F) are somewhat less well correlated, withMSWD values ranging from 3.8 to 9.6. Errorestimates for these samples were increased bymultiplying by the square root of the MSWD,yielding less precise isochron intercept agesranging from 11.4 6 0.6 Ma to 10.5 6 0.8Ma. Data from the sixth groundmass concen-trate (SP-96, Fig. 9E) are relatively poorlycorrelated, yielding an MSWD value of 36and an imprecise isochron intercept age of12.4 6 1.9 Ma.

The results from the six groundmass-concentrate isochron analyses are consistentwith simple, closed-system isotopic behaviorfollowing trapping of small amounts of excess40Ar at the time of cooling, yielding moder-ately elevated and relatively homogeneous40Ar/36Ar intercept values. By this interpreta-tion, the intercept ages of these isochrons are



considered accurate estimates of the coolingages of the mafic intrusions. For most of thesamples, the isochron intercept age agreeswithin error with the weighted-mean age cal-culated for the flat midsections of the agespectra (Table 4). Similar saddle-shaped agespectra and well-defined isochrons with mod-estly elevated 40Ar/36Ar values are not uncom-mon in mafic sills, dikes, and lavas that havebeen intruded into or erupted through thickcontinental crust elsewhere (e.g., Lanphereand Dalrymple, 1976; Harrison and McDou-gall, 1981; Sugden et al., 1999).

The incremental heating data obtainedfrom groundmass concentrates in this studychiefly represent laboratory degassing ofgroundmass plagioclase, which thin-sectionand electron-microprobe observations showto be the predominant K-bearing phase inthese samples. Although the closure tem-perature of plagioclase has not been system-atically determined experimentally (Mc-Dougall and Harrison, 1999), studies ofslowly cooled plutons containing multipledatable phases typically suggest closuretemperatures of ;250 8C for some plagio-clase (e.g., Harrison et al., 1979). It is likelythat the 40Ar/39Ar isochron ages determinedfor Silver Peak mafic intrusions representthe time that they cooled below ;250 8C.The isochron intercept ages of the sixgroundmass concentrates all overlap at the2s level and are consistent with emplace-ment and cooling of the sampled intrusionsbetween 12.0 and 10.5 Ma (Fig. 10).

In addition to the groundmass-concentratesamples, hornblende separates from two maficintrusion samples were also analyzed by 40Ar/39Ar incremental heating. Age spectra fromthese two hornblendes (SP-64 and SP-108,Fig. 9G–H) are highly disturbed and fail toform well-defined linear arrays on isotope-correlation diagrams (Fig. 9, G–H; MSWDvalues of 190 and 20 000, respectively). Thesediscordant hornblende results are probablydue to combined factors including excess 40Ar,39Ar recoil, and loss of K and Ar during chlo-rite alteration (Lo and Onstott, 1989). Datafrom these two hornblende analyses did notyield reliable emplacement-age estimates andare not discussed further.

DISCUSSION

Paleomagnetic Data

Mafic intrusive and pyroclastic rocks in theSilver Peak Range provide interpretable paleo-magnetic data that bear on both the formationof the Silver Peak extensional complex and,

possibly, syn- to postextensional vertical-axisrotation of the range, or parts thereof. On thebasis of a general tilt test (Fig. 6), lacustrine-facies sedimentary rocks appear to contain lit-tle, if any, primary remanent magnetizationand are not discussed further. The interpret-able paleomagnetic results are discussed in de-tail in the following sections by rock type andassociated magnetizations.

Mafic DikesA key concern addressed when sampling

the mafic intrusions of the lower plate was toassess whether the structural attitude of eachdike could be correlated with the site-meanremanence direction or the character of theremanence (i.e., demagnetization behavior). Acorrelation between site-mean direction andstructural attitude may imply that the dikeswere emplaced episodically into an evolvingstrain field in the footwall rocks during theirpartial exhumation. Overall, we see no con-sistency between structural attitude and site-mean magnetization direction or character ofthe remanence (Fig. 11A). Sites in dikes inclose proximity (,2 m) to one another withdifferent orientations yield high-quality de-magnetization data with statistically indistin-guishable site-mean directions (e.g., compareSP55 with SP59, Table 1). Conversely, intru-sions with similar orientations commonlyyield different site-mean directions (e.g., com-pare SP92 with SP93, Table 1). A plausibleexplanation for the differences in site-meandirections is that the dikes were emplaced overan extended period of time. The lack of con-sistent crosscutting relationships among dikesof different orientations suggests that theywere emplaced along preexisting fractures orprimary anisotropy of the lower-plate rocks.The paleomagnetic data from 30 sites in maficintrusions (25 normal polarity, 5 reverse po-larity) yielded magnetizations with north-northwest declinations and moderately steepto shallow, positive inclinations (group-meanpopulation A, Fig. 4B; D 5 329.28, I 5 37.78,a95 5 4.38, (5 37.5). This group mean is dis-cordant in both declination and inclinationfrom Miocene expected directions and exhib-its an inferred rotation (R) of 224.88 (65.38)and a minimum inclination flattening (F) of16.38 (6 4.28) (Beck, 1980; Demarest, 1983)compared to the expected direction of Man-kinen et al. (1987) (D 5 3588, I 5 1558, a95

5 6.08). Because 5 of 30 sites in mafic intru-sive rocks used to calculate group-mean pop-ulation A have reversed-polarity, mafic mag-matism must have spanned at least one polarityreversal between ca. 12 and 10.5 Ma. Four ofthe five mean directions for the negative-polarity

sites have shallower inclinations than normal-polarity sites. The remanence in reverse-polarity sites may be contaminated by a su-perimposed normal-polarity remanence thatwas incompletely removed during demagne-tization (Fig. 4C). In addition, it is possiblethat the northwest-directed and moderate tomoderately steep, positive inclination (nor-mal-polarity) remanence is also contaminatedby an unremoved secondary magnetization orhigh-coercivity viscous overprint.

Given the fact that most mafic dikes sampledin the lower plate are thin (0.5–2 m), they prob-ably cooled quickly and acquired a magneti-zation rapidly relative to paleo–secular varia-tion (PSV) of the geomagnetic field. By usingPSV models (e.g., Merrill and McElhinny,1983) for the average latitude of the study area(37.58), the VGP (virtual geomagnetic pole) an-gular standard deviation is predicted to be ;138to 158. The VGP angular standard deviation ofour data set is 10.9 8; thus, we cannot refutethe possibility that the group-mean directionmay be in part biased by incomplete samplingof the field. The relatively low angular disper-sion of the ensemble of data from group-meanpopulation A may be related, at least in part,to using AF demagnetization to isolate theremanence directions from most samples, asthermal demagnetization generally yieldsshallower remanence directions at somesites. This behavior suggests considerableoverlap in unblocking temperature spectra ofthe early-acquired TRM (thermal remanentmagnetization) and an overprint of steeper, pos-itive inclination. In some samples at some siteswe note a progressive shallowing of the incli-nation isolated at higher peak fields or labora-tory unblocking temperatures (e.g., above 75mT or 400–450 8C). Our estimate of the meaninclination for this population may be biasedtoward a slightly steeper inclination and thustoward less of an inferred discordance (andhigher dispersion) than actually exists.

Paleomagnetic data from 19 mafic intru-sions (normal polarity) yield magnetizationshaving north-northeast declinations and mod-erate, positive inclinations (group-mean pop-ulation B, Fig. 4B; D 5 021.48, I 5 57.28, a95

5 4.38, k 5 51.1). This mean is discordant,in declination only, to the middle Miocene ex-pected direction (R 5 127.48, 67.08). TheVGP angular standard deviation for these re-sults is 16.28, larger than the predicted 138 to158, suggesting that the group-mean directionis a realistic average of PSV and that we haveobtained a representative sample of a middleMiocene geomagnetic field. We note, howev-er, that these data are exclusively of normalpolarity. Nine sites (all normal polarity) yield


PETRONIS et al.

Figure 7. Representative modified, in situ, orthogonal demagnetization diagrams (Zijderveld, 1967; Roy and Park, 1974) of results forrocks collected in the Silver Peak Range. The natural remanent magnetization (NRM) is shown as a star in all plots. Solid (open) symbolsrepresent the projection onto the horizontal (true vertical) plane. AF (alternating field) demagnetization steps are given in milliteslas,and thermal demagnetization steps are shown in degrees Celsius. Typically, AF and thermal demagnetization results from two specimensof the same sample are shown for comparison. Diagrams are designated with a site location (SP number), sample letter (-i), method oftreatment (AF or Th), and rock type. Intensity in A/m is given for each sample. (A–D) Representative AF and thermal demagnetizationdiagrams of pyroclastic rocks. (E–H) Representative AF and thermal demagnetization diagrams of lacustrine sedimentary rocks. (I–J)Representative diagrams from basalt and lacustrine sedimentary rocks, respectively, from the monocline area east of Mineral Ridge.(K–L) Results from a marble layer (site SP87) within the Wyman Formation. No interpretable results could be obtained from this site.See text for discussion, Figures 1 and 2 for site locations, and Tables 1, 2, DR1, and DR2 for rock descriptions. (Caption continued onp. 1119.)



Figure 7. (Continued.) (Caption continued from p. 1118.) In general, most samples contain a single characteristic remanent magnetization(ChRM) that is well grouped at the site level, but some samples also contain additional magnetizations that are readily removed (Q–R).AF demagnetization effectively randomizes any remanence overprinted on the ChRM by 20 mT. The ChRM, above 20 mT, decays alonga roughly univectorial path to the origin with ,10% of the NRM intensity remaining by 120 mT. About 20% of the samples exhibitdemagnetization behavior characteristic of two components of magnetization, consistent with rock-magnetic results. Typically, a viscousremanent magnetization (VRM) or secondary magnetization of varying relative intensity is superimposed on the ChRM. (R9) Occasion-ally, the ChRM could not be isolated by AF demagnetization, as the specimen did not decay fully to the origin and up to 20% of theNRM remained after 120 mT fields. Thermal demagnetization was then used to more fully isolate the ChRM. Typically, thermaldemagnetization removes low-coercivity VRMs or secondary magnetizations by 300 8C, depending on the nature of the carrier of theVRM or secondary magnetization (cf. E–F with C–D). Maximum VRM unblocking temperatures ranged from 200 8C up to 400 8C. (M–X) Representative AF and thermal demagnetization diagrams of mafic intrusions (see Fig. 2 for site locations). (M–N) Sites located inthe Vanderbilt Peak area of Mineral Ridge. (O–P) Site in a dike intruding pyroclastic rocks on Rhyolite Ridge. (Q–R, R9) Site locatedin the vicinity of the Mary mine. (S–V) Sites located in Coyote Canyon area of Mineral Ridge. (W–X) Site in the Tarantula Canyonarea of Mineral Ridge. See text for discussion and Tables 1, 2, and DR1 for rock descriptions.

magnetization directions that are north direct-ed and have moderate, positive inclinations(Fig. 4B; D 5 3578, I 5 568). These resultsare statistically indistinguishable from thetime-averaged Quaternary axial geocentric di-pole field and the Miocene expected direction.We interpret these data to indicate that eitherthe sampled intrusions are (1) relativelyyoung, postdating deformation of the SilverPeak Range, (2) were recently remagnetized,or (3) record no net tilting or rotation.

Overall, paleomagnetic results from the maf-ic dikes cutting the lower-plate rocks are ofhigh quality. The data suggest that magmatismspanned at least one field reversal and are in-terpreted to record at least part of the defor-mational history of the lower plate. The un-certainty in the cooling history provided by40Ar/39Ar data from these intrusions, which arethe only geochronologic data available forthese rocks, makes interpretation of the paleo-magnetic data somewhat ambiguous. Interpre-

tation of paleomagnetic data always dependson the age of the rocks sampled and, in turn,the age of magnetization acquisition. When allaccepted paleomagnetic results are consideredas a single population acquired during a pe-riod spanning ;1.5 m.y. during the Miocene,the spatial distribution of the entire ensembleof site-mean directions is less straightforwardto explain. On the basis of 40Ar/39Ar data, as-suming that the Ar budget is controlled by thedegassing of plagioclase, we suggest that maf-


PETRONIS et al.

TABLE 3. CHEMICAL ANALYSES AND MINERAL NORMS, MAFIC DIKES

Sample: SP55 SP67 SP69 SP75 SP83 SP85 SP88 SP99 SP110 SP111

Bulk chemical analysis (by % oxides) of representative mafic intrusive rocksSiO2 42.02 48.80 47.06 46.44 44.49 47.78 42.81 58.71 48.81 44.44Al2O3 12.62 14.63 14.58 14.96 14.88 14.11 11.63 16.25 14.36 13.77P2O5 0.23 0.46 0.28 0.41 0.46 0.39 0.28 0.17 0.46 0.36CaO 10.38 7.84 7.55 8.32 9.47 7.79 8.78 4.15 7.01 9.23TiO2 1.14 1.14 1.03 1.50 1.43 1.04 0.86 0.59 1.09 0.96Fe2O3 10.78 8.75 8.70 10.19 10.57 8.36 10.18 4.84 8.54 9.25Na2O 2.32 3.02 3.40 3.35 2.86 2.70 1.87 4.17 3.10 1.96K2O 0.89 1.91 2.02 1.83 2.08 2.42 1.13 3.28 2.18 2.14MgO 11.22 7.16 8.18 7.95 8.80 6.69 14.70 2.06 7.78 9.78MnO 0.18 0.16 0.14 0.17 0.12 0.16 0.15 0.11 0.14 0.15% oxide 91.76 93.87 92.96 95.11 95.16 91.43 92.39 94.34 93.48 92.05LOI H2O (1) 7.68 5.13 6.36 2.86 2.79 6.37 5.77 3.88 4.94 7.15H2O (2) 0.30 0.11 0.38 0.08 20.04 0.18 0.25 0.08 0.22 0.35Total 99.44 99.00 99.32 97.97 97.95 97.80 98.16 98.22 98.42 99.20% unknown 0.56 1.00 0.68 2.03 2.05 2.20 1.84 1.78 1.58 0.80Normative mineral abundance (%) of representative mafic intrusive rocks%Anorthite 54.14 44.78 39.21 41.82 51.57 45.71 55.83 31.11 41.79 57.53Quartz 0 1.09 0 0 0 0.97 0 11.85 0 0Orthoclase 5.73 12.02 12.84 11.37 12.92 15.64 7.23 20.55 13.78 13.73Albite 19.75 27.22 30.94 29.8 21.34 24.99 17.13 37.4 28.06 18.02Anorthite 23.31 22.08 19.97 21.43 22.72 21.04 21.65 16.89 20.14 24.4Nephelite 0.89 0 0 0 2.22 0 0 0 0 0Diopside 21.49 9.81 11.75 11.18 14.6 11.81 16.28 1.58 8.07 15.41Hypersthene 0 14.45 0.86 0 0 12.75 4.47 4.7 16.67 4.950livine 14.35 0 10.94 10.95 11.4 0 19.35 0 0.22 10.06Ilmenite 0.42 0.36 0.32 0.38 0.27 0.37 0.35 0.25 0.32 0.35Hematite 11.75 9.32 9.36 10.71 11.11 9.14 11.02 4.75 9.14 10.05Titanite 0 2.51 2.3 2.73 0 2.31 1.84 1.21 2.45 2.11Apatite 0.58 1.14 0.7 1 1.12 0.99 0.7 0.42 1.14 0.91Perovskite 1.74 0 0 0.45 2.32 0 0 0 0 0Total 100.01 100 99.98 100 100.02 100.01 100.02 99.6 99.99 99.99

Note: Major oxide and calculated normative mineral abundances analyzed by using X-ray fluorescence toassess possible variation in bulk chemical composition of mafic dikes. No correlation is apparent in whole-rockchemistry and dike orientation, geographic location, or demagnetization behavior between the analyzedspecimens. LOI—loss on ignition.

Figure 8. Rock-magnetic tests. (A) Acquisi-tion of isothermal remanent magnetization(IRM) and back-field demagnetization of thesaturation IRM. (B) Modified Lowrie-Fullertest (Johnson et al., 1975) that comparesAF demagnetization response of naturalremanent magnetization (NRM), anhyster-etic remanent magnetization (ARM), andsaturation isothermal remanent magneti-zation (SIRM). AFT—ash flow tuff, Ts3—Tertiary sedimentary unit 3 of Stewartand Diamond (1989). There are no unitswith normalized magnetization. The inten-sity of the magnetizations at each progres-sive demagnetization step has been nor-malized to the initial NRM intensity. Thenormalized magnetization usually has val-ues from one to zero.

M

ic magmatism and remanence acquisition tookplace from ca. 12 to ca. 10.5 Ma. This agerange is consistent with early stages of upper-plate extension and partial exhumation of lower-plate rocks (Stewart and Diamond, 1990;Kohler et al., 1993; Oldow et al., 1994). The40Ar/39Ar data—when combined with field re-lationships on the basis of mapping in the up-per plate (Prestia and Oldow, 2000) and zirconand apatite fission-track dates (11.0 6 0.7 Maand 5.8 6 0.5 Ma, respectively) from lower-plate metamorphic rocks (Kohler et al.,1993)—suggest that the lower-plate rockscooled rapidly during active extension anddike emplacement. By using geothermal gra-dients of 20 8C/km to 30 8C/km (Lachenbruchand Sass, 1978), cooling rates suggest partialexhumation of the lower plate, at rates rangingbetween 1.0 and 0.6 km/m.y., so that the lower-plate rocks were at depths of between 10 and6.5 km by 10.5 Ma.

The distribution of paleomagnetic data indikes of the lower plate is also best explainedby remanence acquisition spanning a period ofrapid intrusion and partial exhumation. In ac-cordance with dike emplacement spanning atleast 1.5 m.y., we hypothesize that intrusionsunderwent differential tilt and rotation during

progressive deformation associated with par-tial exhumation of the shallowly northwest-dipping lower plate and formation of thenorthwest-plunging turtleback structures (e.g.,Mineral Ridge). Although we cannot conclu-sively demonstrate a correlation between agedistribution and paleomagnetic declination,we note that mafic intrusions having northeastdeclination yield isotopic ages between 12.5Ma and 11 Ma, whereas intrusions havingnorthwest declination are tightly grouped be-tween 11.5 Ma and 10.5 Ma (Fig. 10). Wehypothesize that older (ca. 12.5 Ma) intrusionsacquired a remanence and were tilted and ro-tated clockwise by a greater magnitude thanyounger (,12.5–10.5 Ma) intrusions. Alter-natively, if we discount the imprecise resultsfrom SP-96, the groundmass age data all over-lap at 2s. Excluding the imprecise age wouldlead to rapid emplacement of all the intrusionsbetween 11.5 Ma and 11 Ma, and, therefore,the data do not support the hypothesis that theobserved paleomagnetic declinations are afunction of age. Nonetheless, the data are fullyconsistent with the following interpretivestructural history (Fig. 12). Initially, the re-gional detachment surface dipped to the north-west (Oldow et al., 1994) (Fig. 12A). The

shallowly dipping detachment was first sub-jected to modest west-side up tilting about anorth-trending axis as the lower-plate rockswere deformed into an early generation ofnorth-trending folds, on the basis of penetra-tive structures in the lower plate (Fig. 12B).Additional reorientation of structures withinthe lower-plate rocks during progressive de-velopment of the northwest-trending turtle-back involved shortening in a simple-shearcouple aligned at about N408W (Oldow et al.,1994). Although folding about a N858W axisand clockwise vertical-axis rotation through258 took place simultaneously, we separate thephenomena to illustrate them as discrete, su-perposed deformations (Fig. 12, C and D).The predicted redistribution of late Miocenemagnetizations in the dikes by this superposeddeformation (Fig. 12E) compares favorablywith observed distributions (Fig. 12F). Theprogression that results in a simple-shear ro-tation (g 5 ;1.7), as outlined in Figure 12D(lower diagram), shows one of many possibleways to generate the observed paleomagneticdata and deformation of the lower- and upper-plate rocks in the Silver Peak extensionalcomplex. We note that applying a componentof pure shear reduces the magnitude of simpleshear required to generate a similar predicteddistribution of late Miocene magnetization di-rections in dikes exposed on Mineral Ridge(Fig. 12).

An alternative interpretation of the ob-served distribution of the paleomagnetic datafrom the mafic dikes in the lower plate in-volves variable magnitude of tilt, in differentdirections, about horizontal to subhorizontalfold axes, without any component of vertical-axis rotation. We find this hypothesis less con-




PETRONIS et al.

Figure 9. (A–H) 40Ar/39Ar age spectra and isochron results from mafic intrusions in the Mineral Ridge area of the Silver Peak Range.**—MSWD (mean squared weighted deviates) values above 95% confidence level.



Figure 9. (Continued.)


PETRONIS et al.

Figure 9. (Continued.)

sistent with the observed data. Dikes withinseveral specific parts of Mineral Ridge yielda range of magnetization directions (Fig. 10).We would not expect this observation if allthe dikes within a particular part of MineralRidge were emplaced over a relatively shortperiod of time and then uniformly tilted.

Pyroclastic RocksPaleomagnetic data from 13 sites in late

Miocene pyroclastic rocks (5.9 6 0.5 Ma,Robinson et al., 1968) from the RhyoliteRidge area of the Silver Peak Range (all nor-mal polarity) yield in situ north-northeast dec-linations and moderate positive inclinations.After application of a modest structural cor-rection (N238E strike, 78SE dip) (Table 2), thegroup mean is D 5 032.98, I 5 52.58, a95 59.58, k 5 26.8 (Fig. 5C). This corrected groupmean is discordant to the Miocene expected

direction (R 5 138.38 6 12.18 and F 5 0.986 7.48). The data are consistent with moderatevertical-axis rotation of the upper-plate rocks.Nevertheless, we cannot discount the possi-bility that the data set, of uniform polarity,does not adequately sample the geomagneticfield. On the basis of low-precision K-Ar agedeterminations (Robinson et al., 1968) andfield relationships (Prestia and Oldow, 2000),these rocks were likely emplaced over lessthan ;1 m.y. Several factors support modesttectonic rotation of the upper-plate rocks. Oursampling sites were established in several dis-tinct pyroclastic deposits constituting .350 mof section at Rhyolite Ridge. The pyroclasticdeposits contain internal erosional surfacesand change character up section; therefore,these deposits are clearly not the product of asingle eruption or a series of quickly spacederuptions. The VGP angular standard devia-

tion value of 21.28 for the Rhyolite Ridge datais greater than predicted and suggests that wehave sampled considerable field variation inproviding a group-mean direction. A single(;1.5 m wide) mafic dike (SP 12) intrudespyroclastic rocks exposed at Rhyolite Ridgeand also yields a (normal polarity) site meanthat is consistent with a modest clockwise ro-tation (Table 2). Alternatively, the high VGPangular standard deviation may reflect a dif-ficulty in distinguishing initial, depositionalorientations from those produced by defor-mation. This contribution, however, is consid-ered minimal as each flow unit has a uniformdip and dip direction of ;78W, on the basisof field measurements.

In Icehouse Canyon of the western SilverPeak Range, four of six sites in pyroclasticrocks yield well-defined results at the site lev-el but are difficult to interpret (Table 2). Sites



TABLE 4. SUMMARY OF 40Ar/39Ar AGE DETERMINATIONS FROM MAFIC INTRUSIONS, MINERAL RIDGE

Sample Location Material Strike/dip(8/8)

Site meandec/inc

nt np ni Spectrum age(Ma 6 2s)

40Ar/36Ari

(62s)MSWD* Isochron age

(Ma 6 2s)

Groundmass analysesSP-55 Coyote Canyon groundmass 300/60 334/28 9 4 6 14.4 6 0.8 399 6 6 2.1 11.6 6 0.4SP-69 Vanderbilt Peak groundmass 053/45 124/5 9 7 9 11.8 6 0.8 300 6 1 6.7 11.4 6 0.6SP-75 Vanderbilt Peak groundmass 165/55 320/45 9 4 4 12.1 6 2.2 321 6 2 2.5 11.3 6 0.2SP-92 Mary mine groundmass 319/50 355/55 11 6 9 10.8 6 0.4 300 6 1 3.8 10.5 6 0.8SP-96 Drinkwater mine groundmass 170/45 021/58 9 6 8 13.6 6 1.3 329 6 6 36.0 12.4 6 1.9SP-112 Tarantula Canyon groundmass 145/35 010/59 9 2 7 10.9 6 0.1 314 6 1 9.6 10.9 6 0.3Hornblende analyses (rejected)SP-64 Coyote Canyon hornblende 180/30 341/48 11 11 11 26.4 6 2.2 309 6 2 190 23.6 6 2.4SP-108 Tarantula Canyon hornblende 065/60 011/54 11 11 11 50.0 6 1.0 376 6 2 2000 47.5 6 9.8

Note: Site mean is paleomagnetic remanence; nt is number of incremental heating steps; np is number of steps used in spectrum age; ni is number of steps used inisochron age. Sample preparation and irradiation: Hornblende was separated with standard heavy liquid, Franz magnetic, and handpicking techniques. Groundmassconcentrates were treated with dilute HCl, washed and handpicked. Groundmass concentrates and hornblende separates were loaded into a machined Al disc and irradiatedfor 7 or 3 h in D-3 position, Nuclear Science Center, College Station, Texas. Neutron-flux monitor was Fish Canyon Tuff sanidine (FC-1), with an assigned age of 27.84Ma (first used by Deino and Potts (1990), equivalent to Mmhb-1 at 520.4 Ma (Samson and Alexander, 1987). We note that the 40Ar/39Ar dating community has yet to reacha consensus on monitor ages. Renne et al. (1998) make a strong case for a calibration scheme with a Fish Canyon age of 28.02 Ma, although some other workers (e.g.,Lanphere and Baadsgaard, 2002) have argued for calibrations yielding a Fish Canyon age as young as 27.51 Ma. For consistency with previous results, the New MexicoGeochronology Laboratory intends to continue using a Fish Canyon Tuff monitor age of 27.84 Ma until a consensus is reached in the 40Ar/39Ar community. The effect ofthe choice of monitor age on calculated ages for Silver Peak samples is on the order of 61%, considerably less than the precision of weighted-mean ages obtained inthis study. Instrumentation: Mass Analyzer Products 215–50 mass spectrometer on line with automated all-metal extraction system. Samples step-heated in Mo double-vacuum resistance furnace. Heating duration 7 min. Reactive gases removed by reaction with three SAES GP-50 getters, two operated at ;450 8C and one at 20 8C.Gas also exposed to a W filament operated at ;2000 8C. Analytical parameters: Electron-multiplier sensitivity averaged 1 3 10–16 mol/pA. Total system blank andbackground for the furnace averaged 386, 5.3, 0.5, 2.6, 1.6 3 10–18 mol at masses 40, 39, 38, 37, and 36, respectively, for temperatures of ,1250 8C. J-factors determinedto a precision of 60.1% by CO2 laser-fusion of four single crystals from each of six radial positions around the irradiation tray. Correction factors for interfering nuclearreactions were determined by using K-glass and CaF2 and are as follows: (40Ar/39Ar)K 5 0.00020 6 0.0003; (36Ar/37Ar)Ca 5 0.00026 6 0.00002; and (39Ar/37Ar)Ca 5 0.000706 0.00005. Age calculations: Spectrum age and errors for groundmass concentrates were calculated for selected steps representing relatively flat parts of age spectra,weighting each step by the inverse of the variance using the method of Taylor (1982), and multiplying the error by the square root of the MSWD where MSWD valuesexceeded the 95% confidence levels. Spectrum ages and errors for hornblendes are integrated ages of all steps calculated by weighting individual steps by the fractionof 39Ar released. Isochron ages, 40Ar/36Ari and MSWD values calculated by using regression methods of York (1969). For spectrum and isochron ages where MSWD valuesexceeded the 95% confidence levels (Mahon, 1996), errors were increased by multiplying by the square root of the MSWD. Decay constants and isotopic abundancesafter Steiger and Jager (1977). All errors reported at 62s, unless otherwise noted.

*MSWD (mean squared weighted deviates) isochron values above 95% confidence level, where errors were increased accordingly as described below. Ages shown inbold are interpreted as the most accurate determinations of mafic-intrusion cooling ages.

Figure 10. 40Ar/39Ar age distribution of Silver Peak mafic dikes based on paleomagneticsite-mean directions. We note that the correlation between site-mean direction and ap-parent age is highly speculative. Additional, high-precision, age determinations from themafic dikes could improve the apparent correlation. The age data from five of the sixsamples overlap at 2s, and if the imprecise result from SP-96 is discounted, the ground-mass data suggest rapid emplacement of all intrusions between 11.5 and 11 Ma.

51 and 54 are from separate andesite lavaflows in the preextensional section. Both sitesyield reversed-polarity, east-directed magne-tizations. Sites 52 and 53 are from rhyolite

ash-flow tuffs that are lithologically similar tothe pyroclastic rocks exposed on RhyoliteRidge. These two sites yield well-grouped,southeast-directed, moderate negative inclina-

tion magnetizations. Because of their reversedpolarity, they are not lateral equivalents of thenormal-polarity deposits sampled on RhyoliteRidge and likely represent a different eruptivesequence with a similar composition.

Tectonic Evolution

The Neogene history of the Silver PeakRange is characterized by progressive defor-mation associated with northwest-directed ex-tension related to displacement transfer fromthe Furnace Creek fault system to the trans-current faults of the central Walker Lane (Re-heis and Nolar, 1989; Oldow, 1992; Reheisand Sawyer, 1996; Petronis, 1998; Petronis etal., 1997; Prestia et al., 1998). The deforma-tion involved middle Miocene to Pliocene for-mation of the Silver Peak extensional complexand was intimately linked to the evolution ofnorthwest-trending turtleback structures. Pa-leomagnetic data contribute to our understand-ing of deformational processes by resolvingcomponents of horizontal-axis tilting and ver-tical-axis rotation. They may provide an ex-ternal reference frame to assess the orientationof the detachment during acquisition of theremanent magnetization. The observed rema-nence recorded by the dikes was acquired


PETRONIS et al.

Figure 11. Lower-hemisphere projections of paleomagnetic site-mean directions from allintrusions sampled in the Mineral Ridge area and mafic dike orientation (p [right] and(b [center] diagrams). (A) Left-hand projection shows site-mean directions, center projec-tion shows orientation of planar mafic intrusions represented as great circles (b diagrams),and right-hand projection shows orientation of planar mafic intrusions represented asnormals to planes (p diagrams). (B) Upper diagrams show paleomagnetic results frommafic dikes and sills defined by geographic location; lower diagrams show correspondingorientations of intrusions (b diagrams). Ellipses around paleomagnetic site means denotegroups of data shown in Figure 4. See Figures 1 and 2 for sampling localities and Tables1, 2, and DR1 for individual orientation of specific dikes sampled.

while the lower plate was at a depth of ;10km and is inconsistent with a steeply dippingextensional decollement. Rather, inclinationdata imply a modest northwest-up tilting ofthe lower plate and support earlier models of ashallowly northwest-dipping decollement onthe basis of fault-displacement rates and ther-

mochronology of the lower-plate assemblage(Oldow et al., 1994). Furthermore, the paleo-magnetic data are consistent with emplacementof mafic dikes while the lower-plate underwentprogressive deformation and attendant partialexhumation.

The Silver Peak Range forms part of a

broad displacement stepover in a northwest-trending transcurrent fault system. Displace-ment transfer is accomplished by northwest-directed motion of brittlely deformingupper-plate rocks separated from the lower-plate rocks by a shallow, northwest-dippingdetachment. Zircon and apatite fission-trackcooling dates indicate that lower-plate rockscooled through ;220 8C and 100 8C, respec-tively, during this time period (Kohler et al.,1993). The 40Ar/39Ar geochronologic data re-ported here indicate that cooling of maficdikes through ;300 to 250 8C (Harrison et al.,1979) occurred between 12 and 10.5 Ma.These results are consistent with the initiationof extension recorded by synorogenic depo-sition of fluvial and lacustrine sedimentary de-posits (Stewart and Diamond, 1990) that con-tinued into the latest Miocene or earlyPliocene (Prestia and Oldow, 2000). Paleo-magnetic data from some mafic dikes (group-mean population A, Fig. 4C; D 5 3298, I 5378) are consistent with northwest-side-up tilt-ing of the footwall about a north-northeast–trending tilt axis, concurrent with sedimenta-tion and detachment faulting. Movement alongthe detachment and associated horizontal-axistilting ceased by the late Miocene to early Pli-ocene, as pyroclastic and fluvial rocks of lateMiocene and younger and overlying the foot-wall metamorphic rocks are broadly de-formed, with the underlying detachment fault,into regional-scale, northwest-trending foldswith half wavelengths of ;10 km.

During and possibly shortly after the initia-tion of broad-scale folding, at least parts of therange may have rotated in a clockwise senseabout a vertical axis in response to displace-ment transfer across the region. Some paleo-magnetic results from mafic intrusions andmost of the pyroclastic deposits exposed onRhyolite Ridge are consistent with a modestdegree of clockwise vertical-axis rotation ofthis part of the Silver Peak Range. The Rhyo-lite Ridge pyroclastic data are discordant indeclination, but not in inclination, with theMiocene expected direction (R 5 132.78 67.78); (Fig. 4C). Clockwise rotation of the Sil-ver Peak Range could have been facilitated byleft-lateral movement along antithetic splayfaults of the Furnace Creek fault zone thatbound the range to the northwest (EmigrantPeak fault zone) and southeast (Clayton Valleyfault zone), respectively (Fig. 1). At present, theEmigrant Peak fault zone and Clayton Valleyfault zone serve as oblique west-side-down ex-tensional structures normal to the present-daymaximum extension direction (Reheis andSawyer, 1996). The Clayton Valley fault zone,however, continues to have left-lateral offset, as



Figure 12. Interpretive structural evolution of the Silver Peak extensional complex, based on penetrative fabrics in the lower plate onMineral Ridge, paleomagnetic data, and stratigraphic relationships in the upper-plate assemblage. Equal-area projections (upper four plots)correspond to specific geometric models (lower four structure diagrams). Dotted oval represents schematic location of Mineral Ridge thatformed during the structural evolution of the extensional complex; WLFZ—Walker Lane fault zone, FV-DVFZ—Furnace Creek–DeathValley fault zone. (A) Great-circle lower-hemisphere projection shows inferred original northwest dip of the regional detachment as shownin associated structure diagram; black triangle is the normal to the detachment-fault plane; black diamond is an ‘‘average’’ normal-polaritymagnetization of mafic intrusions in the lower plate, acquired between ca. 13 and 10 Ma. (B) Initial east-side-down tilt through (308, usinga tilt axis (black circle) based on orientation of penetrative structures, with 258 of clockwise rotation removed. (C) Second north-side-uptilt related to unrotated position of turtleback axis structures using a N858W axis. (D) Clockwise vertical-axis rotation of all elementsrelated to simple shear strain (258 of clockwise rotation corresponds to g 5 1.7). We note, however, that a component of pure shear willreduce the simple shear needed to rotate the fold-axis ;258. If 10% pure shear is added, the simple-shear strain is reduced to g 5 1.5. If20% pure shear is added, then g 5 1.2. (E) Predicted distribution of late Miocene normal-polarity magnetization directions by this sequenceof superimposed deformation. (F) Observed paleomagnetic data from mafic intrusions on Mineral Ridge. Shaded region is the outline ofthe predicted distribution of the data from (e) that would result from the sequence of superimposed deformations. Black diamond—normalto the detachment plane, black triangle—average normal polarity magnetization, black circle—initial tilt axis with 258 of clockwise rotationremoved.

indicated by historic earthquakes and offsetQuaternary deposits (Reheis and Nolar, 1989).The available data are consistent with the SilverPeak Range and the surrounding area acting asa zone of diffuse intracrustal shear associatedwith displacement transfer from the FurnaceCreek–Fish Lake Valley fault zone to the faultsof the Walker Lane belt during the middle Mio-cene. Field mapping (Reheis and Sawyer, 1996;Prestia et al., 1998) and contemporary seismic-ity attest to continuing displacement transferaffecting the region.

CONCLUSIONS

On the basis of the new paleomagnetic dataand 40Ar/39Ar dates reported, we reach the fol-

lowing conclusions bearing on the structuraland magmatic activity affecting the SilverPeak Range. Miocene mafic dikes were em-placed into the actively deforming footwallrocks of the Mineral Ridge metamorphic corecomplex during northwest-directed detach-ment faulting along the Mineral Ridge detach-ment system. 40Ar/39Ar isochron dates and zir-con fission-track age estimates suggest that themafic intrusions and footwall rocks cooledthrough ;250 8C at 10.5 Ma. Paleomagneticdata from some of the mafic dikes preserve arecord of the northwest-side-up tilting of thefootwall associated with extension, whereasother dikes seem to preserve a record of anearlier phase of modest clockwise vertical-axisrotation associated with displacement transfer

in the region, which may have been manifest-ed as crustal-scale block rotation of the SilverPeak Range. Preliminary paleomagnetic datafrom elsewhere in the range and adjacentranges also support this event (Petronis et al.,2001). Miocene pyroclastic rocks at RhyoliteRidge contain a single polarity magnetizationthat is likely a thermoremanent magnetization.The group-mean direction from these rocks isconsistent with a modest degree of clockwisevertical-axis rotation, although these data maybe biased owing to the limited number of dis-tinct eruptive units available for sampling.Middle Miocene lacustrine sedimentary rocksare interpreted to have been remagnetized andto thus preserve little information on the tec-tonic history of the region.


PETRONIS et al.

Deformation of the Silver Peak Range, atleast in part documented by the paleomagneticresults presented here, is consistent with a tec-tonic model in which the Fish Lake ValleyFault system, the northern extent of the Fur-nace Creek-Death Valley fault system, is cou-pled with faults of the Walker Lane belt andEastern California shear zone, which accom-modate part of the Pacific–North Americanrelative plate motion (Dokka and Travis, 1990;Reheis and Sawyer, 1996). The northwesternend of the Furnace Creek fault terminates atthe north end of Fish Lake Valley (Fig. 1).Upper Pliocene to Quaternary sedimentaryrocks in the northern part of the valley are cutby numerous north-striking structures andshort splay faults off the main fault zone.These faults appear to transfer displacement inan extensional right step to normal faults ofthe Emigrant Peak fault zone (Reheis andMcKee, 1991; Reheis and Sawyer, 1996; Saw-yer, 1991). The Emigrant Peak fault zone inturn transfers slip from the Furnace Creekfault zone to the right-lateral Bettles Well andSoda Spring faults of the Walker Lane belt tothe northeast (Stewart, 1988). Displacementtransfer in the Silver Peak Range is interpretedto have been active since the middle Mioceneand appears to have been accommodatedalong large-scale detachment faults and large-wavelength folds and, as documented here,has involved modest clockwise vertical-axisrotation of crustal-scale blocks that was con-current with or slightly earlier than modest,northwest-side-up crustal tilting.

The paleomagnetic data have important im-plications for the evolution of the westernmargin of the Basin and Range province. Pa-leomagnetic results from several areas northof the Silver Peak Range show little to no ev-idence for appreciable middle to late Cenozoicvertical-axis rotation (Geissman et al., 1984;Bell et al., 1987; Callian et al., 1988). If theproposed rotation of at least parts of the SilverPeak Range proves to be a range-scale phe-nomenon, as to a limited degree substantiatedhere, this outcome will validate the hypothesisof vertical-axis rotation associated with dis-placement transfer in the Walker Lane belt.Additional data from Miocene and youngerpyroclastic rocks in the Silver Peak Range aswell as in nearby areas of the south-centralWalker Lane belt may provide clues to theoverall nature of deformation in the region.Further work may reveal that vertical-axis ro-tation as well as crustal tilting play an integralpart in accommodating displacements inboardof a continental transform margin.

APPENDIX. PALEOMAGNETIC SAMPLINGAND ANALYTICAL METHODS, ROCKMAGNETISM METHODS, AND 40Ar/39ArMETHODS

Paleomagnetism

SamplingA total of 123 sites in late Tertiary volcanic, vol-

caniclastic, and intrusive rocks was sampled in theSilver Peak Range. Oriented samples were collectedby using a portable gas-powered drill with a non-magnetic, diamond tipped drill bit. Each sample wasindividually oriented by using both solar and mag-netic compasses. Middle Miocene mafic dikes weresampled at 76 sites at Mineral Ridge (Fig. 2) in thelower-plate assemblage. From east to west, these in-clude 22 sites in Coyote Canyon (SP13–SP21,SP55–SP67), 13 sites near the Vanderbilt mine(SP68–SP80), four sites near North Spring at thebase of Rhyolite Ridge (SP116–SP119), one sitefrom a north-trending intrusion located on the eastside of Rhyolite Ridge (SP12), three sites in thevicinity of the North Star mine (SP81–SP83), 10sites from the Drinkwater mine (SP84–SP86, SP95–SP101), 7 sites from the Mary mine (SP88–SP94),and 16 sites in Tarantula Canyon (SP22–SP28,SP107–SP115). We were not able to sample dikesin the easternmost part of Mineral Ridge, as nodikes are exposed. A single marble from lower-platerocks was sampled at one site in the vicinity of theMary mine (SP87). Late Miocene volcanic rocks inthe upper-plate assemblage were sampled at 24 sites(Fig. 1). These included 18 sites in pyroclastic rockson or near Rhyolite Ridge (SP1–SP11, SP42, SP43)and 6 sites from Icehouse Canyon on the westernside of the range (SP49–SP54). Two sites (SP51 andSP54) in Icehouse Canyon are from andesite lavaflows, and the remaining four sites are from pyro-clastic rocks. A total of 18 sites was collected inupper Miocene to Pliocene(?) sedimentary strata(Figs. 1 and 2). These include 10 sites on the westside of Rhyolite Ridge (SP34–SP38, SP44–SP48)and 8 sites east of Rhyolite Ridge near the base ofRed Mountain (SP39–SP41, SP102–SP106). A fur-ther 3 sites (MC1, MC2, MC4) were obtained frombasalt flows and 1 site was located in lacustrine de-posits (MC5) in Clayton Valley just east of MineralRidge (Fig. 2).

Analytical MethodsEach core sample was cut into 2.5 by 2.5 cm

right-cylinder specimens with a diamond-tipped,nonmagnetic saw blade. Up to three specimens persample were obtained. Remanent magnetizations ofall samples were measured by using a three-axis 2-G Enterprises magnetometer with an integrated al-ternating field (AF) demagnetizing unit. Specimenswere progressively AF demagnetized, typically in15–20 steps, to a maximum field of 120 mT. Sam-ples with high coercivity were treated with thermaldemagnetization up to 615 to 680 8C, depending onrock type and behavior. Thermal demagnetizationson replicate specimens, to compare with AF behav-ior, were conducted in a Schonstedt TSD-1 thermaldemagnetizer. Principal component analysis (PCA;Kirschvink, 1980) was used to determine the best-fit line or plane through selected demagnetizationdata points for each sample. For most samples, asingle best-fit line could be fit to the demagnetiza-tion data points. Best-fit magnetization vectors, inmost cases, involved 10–15 data points, but as fewas 3, and as many as 25, were used. Magnetization

vectors with maximum angular deviation (MAD)values of .158 were not included in site-mean cal-culations. Sample MAD values for linear data typ-ically were ,48, but ranged from as low as 18 to ashigh as 138. For ,10% of the demagnetization re-sults, it was necessary to anchor the magnetizationvector to the origin. Individual sample directionswere considered outliers and rejected from the site-mean calculation if the angular distance between thesample direction and the estimated site-mean direc-tion was .158. Remagnetization circle analysis wasattempted on demagnetization data from 11 sites inmafic dikes where most samples from each site didnot yield stable endpoint behavior. The resultant di-rectional data, however, usually did not yield a con-sistent solution, and these results were excluded inany group-mean calculation. The remagnetizationcircle data from these sites, summarized in TableDR2, will not be discussed further.

Rock-Magnetic Experiments

Rock-magnetic experiments include AF demag-netization of anhysteretic remanent magnetization(ARM), acquisition of isothermal remanent mag-netization (IRM), direct current backfield demag-netization of saturation isothermal remanent mag-netization (SIRM), and AF demagnetization ofSIRM. ARMs were induced at 95 mT in a 0.1 mTdirect field. If the dominant carrier of the magneti-zation is magnetite, the shape and position of theSIRM AF demagnetization relative to the ARM AFdemagnetization curve and the shape of the back-field demagnetization of the SIRM curve indicatewhether the sample is dominated by single-domain(SD), pseudosingle domain (PSD), or multidomain(MD) grain sizes (Dunlop, 1972, 1973, 1981; Lew-chuk and Symons, 1990; Symons and Sangster,1991).

40Ar/39Ar Geochronology

Six groundmass concentrates and two hornblendeseparates from eight samples of mafic dikes and sillswere prepared and irradiated in the D-3 position atthe Nuclear Science Center reactor, College Station,Texas; Fish Canyon Tuff sanidine was used as aneutron-flux monitor (FC-1, assigned age 5 27.84Ma, Deino and Potts, 1990). Incremental-heatinganalyses for 40Ar/39Ar age spectra were performedat the New Mexico Geochronology Research Lab-oratory. Sample preparation, irradiation, and ana-lytical methods are summarized in Table 4 foot-notes. Results are presented as age spectra andisotope-correlation plots in Figure 9 and summa-rized in Table 4. 40Ar/39Ar analytical data are in theGSA Data Repository. All errors are reported at 2s,unless otherwise noted. Decay constants and isoto-pic abundances are those reported by Steiger andJager (1977).

ACKNOWLEDGMENTS

We greatly appreciate the help of K. Helper, L.Ferranti, J. Ashby, and D. Henderson for assistancewith paleomagnetic sample collecting in sometimesintolerable conditions, Mineral Ridge Inc. for per-mission to sample on mine property, Dave Hembree(senior mine geologist) for assistance in locatingsampling areas and fruitful discussions on the ge-ology of Mineral Ridge, and the owner (Kenny) ofThe Alternative in Silver Peak, Nevada, for alwaysgraciously providing drill and driller fluid. An ear-



lier version of the manuscript was greatly improvedby comments from Paul Renne and two anonymousreviewers. Funded in part by grants from Sigma XIand the Geological Society of America, by a Uni-versity of New Mexico Research, Project, and Trav-el Grant (Petronis), and by support from the NewMexico Technology Geochronological Laboratoryand the University of New Mexico PaleomagnetismLaboratory.

REFERENCES CITED

Albers, J.P., and Stewart, J.H., 1972, Geology and mineraldeposits of Esmeralda County, Nevada: Nevada Bu-reau of Mines and Geology Bulletin, v.78, 80 p.

Argus, D.F., and Gordon, R.G., 1991, Test of the rigid-platehypothesis and bounds on intraplate deformation usinggeodetic data from very long baseline interferometry:Journal of Geophysical Research, v. 101, no. 6,p. 13555–13572.

Avigad, D., Baer, G., Heimann, A., 1998, Block rotationsand continental extension in the central Aegean Sea;palaeomagnetic and structural evidence from Tinosand Mykonos (Cyclades, Greece): Earth and PlanetaryScience Letters, v. 157, no. 1–2, p. 23–40.

Bailey, P.A., 1951, Geology of the southern part of MineralRidge, Esmeralda County, Nevada (Ph.D. thesis): PaloAlto, California, Stanford University, 186 p.

Beck, M.E., 1980, Paleomagnetic record of plate-margintectonic processes along the western edge of NorthAmerica: Journal of Geophysical Research, v. 85,p. 7115–7131.

Bell, M.J., Geissman, J.W., Callian, J.T., Harlan, S.S., andOldow, J.S., 1987, Tectonic implications of paleomag-netic data from Mid-Cretaceous plutons, ExcelsiorMountains, Nevada, Eos, Transactions: AmericanGeophysical Union 67, no. 44, p. 922.

Bennett R.A., Davis J.L., Wernicke B.P., 1999, Present-daypattern of Cordilleran deformation in the westernUnited States: Geology, v. 27, p. 371–374.

Callian, J.T., Geissman, J.W., and Keller, R., 1988, Paleo-magnetism and deformational response to regional ex-tension; Walker Lane region, western Great Basin,Eos, Transactions: American Geophysical Union 69,no. 44, p. 1164.

Deino, A., and Potts, R., 1990, Single-crystal 40Ar/39Ar dat-ing of the Olorgesailie Formation, Southern KenyaRift: Journal of Geophysical Research, v. 95,p. 8453–8470.

Demarest, H.H., 1983, Error analysis for the determinationof tectonic rotations from paleomagnetic data: Journalof Geophysical Research, v. 88, p. 4321–4328.

Diamond, D.S., 1989, Detachment faulting and basin for-mation in the northern Silver Peak Range, EsmeraldaCounty, Nevada: Geological Society of America ab-stracts with programs, v. 19, p. 372.

Dixon, T.H., Robaudo, S., Lee, J., and Reheis, M.C., 1995,Constraints on present-day Basin and Range defor-mation from space geodesy: Tectonics, v. 14, no. 4,p. 755–772.

Dixon, T.H., Miller, M., Frederic, F., Wang, H., and John-son, D., 2000, Present-day motion of the Sierra Ne-vada block and some tectonic implications for the Ba-sin and Range Province, North American Cordillera:Tectonics, v. 19, no. 1, p. 1–24.

Dokka, R.K., and Travis, C.J., 1990, Late Cenozoic strike-slip faulting in the Mojave Desert, California: Tecton-ics, v. 9, p. 311–340.

Duebendorfer, E.M., and Wallin, E.T., 1991, Basin devel-opment and syntectonic sedimentation associated withkinematically coupled strike-slip and detachmentfaulting, southern, Nevada: Geology, v. 19, p. 87–90.

Dunlop, D.J., 1972, Magnetite: Behavior near the singledomain threshold: Science, v. 27, p. 41–43.

Dunlop, D.J., 1973, Thermoremanent magnetization in sub-microscopic magnetite: Journal of Geophysical Re-search, v. 78, p. 7602–7613.

Dunlop, D.J., 1981, The rock magnetism of fine particles:Physics of the Earth and Planetary Interiors, v. 26,p. 1–26.

Fleck, R.J., Sutter, J.F., and Elliot, D.H., 1977, Interpreta-tion of discordant 40Ar/39Ar age-spectra of Mesozoictholeiites from Antarctica: Geochimica et Cosmo-chimica Acta, v. 41, p. 15–32.

Geissman, J.W., Callian, J.T., Oldow, J.S., and Humphries,S.E., 1984, Paleomagnetic assessment of oroflexuraldeformation in west-central Nevada and significancefor emplacement of allochthonous assemblages: Tec-tonics, v. 3, p. 179–200.

Gianella, V.P., and Callaghan, E., 1934, The earthquake ofDecember 20, 1932, at the Cedar Mountain, Nevada,and its bearing on the genesis of the Basin and Rangestructure: Journal of Geology, v. 42, no. 1, p. 1–22.

Hagstrum, J.T., and Gans, P.B., 1989, Paleomagnetism ofthe Oligocene Kalamazoo tuff: Implications for mid-dle Tertiary extension in east central Nevada: Journalof Geophysical Research, v. 94, p. 1827–1847.

Hardyman, R.F., and Oldow, J.S., 1991, Tertiary tectonicframework and Cenozoic history of the central WalkerLane, Nevada, in Raines, G.L., Lisle, R.E., Schafer,R.W., and Wilkinson, W.H., eds., Geology and ore de-posits of the Great Basin, Reno Nevada: Geological So-ciety of Nevada, Symposium Proceedings, p. 279–301.

Harrison, T.M., and McDougall, I., 1981, Excess (40)Ar inmetamorphic rocks from Broken Hill, New SouthWales; implications for (40)Ar/(39)Ar age spectra and thethermal history of the region: Earth and Planetary Sci-ence Letters, v. 55, p. 123–149.

Harrison, T.M., Armstrong, R.L., Naeser, C.W., and Har-akal, J.E., 1979, Geochronology and thermal historyof the Coast Plutonic Complex, near Prince Rupert,British Columbia: Canadian Journal of Earth Sciences,v. 16, p. 400–410.

Hearn, E.H., and Humphries, E.D., 1998, Kinematics of thesouthern Walker Lane Belt and motion of the SierraNevada block, California: Journal of Geophysical Re-search, v. 103, no. 11, p. 27033–27049.

Holm, D.K., Geissman, J.W., and Wernicke, B.P., 1993, Tiltand rotation of the footwall of a major normal faultsystem: Paleomagnetism of the Black Mountains,Death Valley extended terrane, California: GeologicalSociety of America Bulletin, v. 105, p. 1373–1387.

Hudson, M.R., and Geissman, J.W., 1987, Paleomagneticand structural evidence for middle Tertiary counter-clockwise rotation in the Dixie Valley region, west-central Nevada: Geology, v. 15, p. 638–642.

Hudson, M.R., and Geissman, J.W., 1991, Paleomagneticevidence for the age and extent of middle Tertiarycounterclockwise rotation, Dixie Valley region, westcentral Nevada: Journal of Geophysical Research,v. 96, no. 3, p. 3979–4006.

Janecke, S.U., Geissman, J.W., and Bruhn, R.L., 1991, Lo-calized rotation during Paleogene extension in eastcentral Idaho: Paleomagnetic and geologic evidence:Tectonics, v. 10, p. 403–432.

Johnson, H.P., Lowrie, W., and Kent, D.V., 1975, Stabilityof anhysteretic remanent magnetization in fine andcourse magnetite and maghemite particles: Geophys-ical Journal of the Royal Astronomical Society, v. 41,p. 1–10.

Kirsch, S.A., 1968, Structure of the metamorphic and sed-imentary rocks of Mineral Ridge, Esmeralda County,Nevada: University of California, Berkeley, Ph.D.Dissertation.

Kirschvink, J.L., 1980, The least-squares line and plane andthe analysis of paleomagnetic data: Geophysical Jour-nal of the Royal Astronomical Society, v. 62,p. 699–718.

Kohler, G., Oldow, J.S., Sisson, V.B., and Donelick, R.A.,1993, Displacement transfer linking the FurnaceCreek and Walker Lane fault systems, west-centralNevada: Geological Society of America AbstractsWith Programs, v. 25, p. 63.

Lachenbruch, A.H., and Sass, J.H., 1978, Models of an ex-tending lithosphere and heat flow in the Basin andRange province, in Smith, R.B., and Eaton, G., eds.,Cenozoic tectonics and regional geophysics of thewestern Cordillera: Geological Society of AmericaMemoir 152, p. 209–250.

Lanphere, M.A., and Baadsgaard, H., 2001, Precise K-Ar,40Ar/39Ar, Rb-Sr and U/Pb mineral ages from the 27.5

Ma Fish Canyon Tuff reference standard: ChemicalGeology 175, no. 3–4, p. 653–671.

Lanphere, M.A., and Dalrymple, G.B., 1976, Identificationof excess 40Ar by the 40Ar-39Ar age spectrum tech-nique: Earth and Planetary Science Letters, v. 32 (2),p. 1451–148.

Lewchuk, M.T., and Symons, D.T.A., 1990, Paleomagne-tism of the late Precambrian Coldwell Complex, On-tario, Canada: Tectonophysics, v. 184, p. 73–86.

Lo, C.H., and Onstott, T.C., 1989, 39Ar recoil artifacts inchloritized biotite: Geochimica et CosmochimicaActa, v. 53, p. 2967–2711.

Locke, A., Billingsly, P.R., and Mayo, E.B., 1940, SierraNevada tectonic patterns: Geological Society ofAmerica Bulletin, v. 51, p. 513–540.

McDougall, I, and Harrison, TM, 1999, Geochronology andThermochronology by the 40Ar/39Ar Method: OxfordUniversity Press, New York, 269 p.

Mahon, K.I., 1996, The New ‘‘York’’ regression: Appli-cation of an improved statistical method to geochem-istry: International Geology Review, v. 38,p. 293–303.

Mankinen E.A., Larson, E.E., Gromme, C.S., Prevot, M.,and Coe, R.S., 1987, The Steens Mountain (Oregon)geomagnetic polarity transition, 3, Its regional signif-icance: Journal of Geophysical Research, v. 92,p. 8057–8076.

McFadden, P.L., and McElhinny, M.W., 1990, Classifica-tion of the reversal test in paleomagnetism: Geophys-ical Journal International, v. 103, p. 725–729.

Merrill, R.T., and McElhinny, M.W., 1983, The Earth’smagnetic field: Its history, origin, and planetary per-spective: International Geophysical Series, AcademicPress, v. 32, 401 p.

Miller, M.M., Johnson, D., Dixon, T.H. et al., 2001, Refinedkinematics of the Eastern California shear zone fromGPS observations, 1993–1998: Journal of Geophysi-cal Research, v. 106, no. 2, p. 2245–2263.

Oldow, J.S., 1992, Late Cenozoic displacement partition-ing in the northwestern Great Basin, in Graig, S.D.,ed., Structure, tectonics, and mineralization of theWalker Lane: Reno, Nevada: Geological Society ofNevada, Walker Lane Symposium Proceedings Vol-ume, p. 17–52.

Oldow, J.S., Kohler, G., and Donelick, R.A., 1994, Low-angle displacement transfer system linking the Fur-nace Creek and Walker Lane fault zones, west-centralNevada: Geology, v. 22, p. 637–640.

Oldow, J.S., Aiken, C.L.V., Hare, J.L., Ferguson, J.F., andHardyman, R.F., 2001, Active displacement transferand differential block motion within the central Walk-er Lane, western Great Basin: Geology, v. 29, no. 1,p. 19–22.

Petronis, M.S., 1998, A reconnaissance paleomagnetic in-vestigation bearing on vertical-axis rotation associatedwith a simple-shear transfer system in the Silver PeakRange, west-central Nevada: University of New Mex-ico, [M.S. thesis], p. 273.

Petronis, M.S., Geissman, J.W., and Oldow, J.S., 1997, Pa-leomagnetism and structural history of the Silver PeakRange, west-central Nevada: Geological Society ofAmerica abstracts with programs, v. 29, no. 6, p. 380.

Petronis, M.S., Geissman, J.W., and Oldow, J.S., 2001, Pa-leomagnetic data from upper Tertiary volcanic rocks inthe central Walker Lane: Crustal-scale block rotationand transitional field directions: EOS, v. 82, no. 47,p. F313.

Prestia and Oldow, 2000, Upper-plate fragmentation in theSilver Peak extensional complex during transcurrentdisplacement transfer, central Walker Lane, westernGreat Basin: Geological Society of America Abstractswith Programs, v. 32, no. 7, p. A166.

Prestia et al., 1998, Synorogenic deposition of volcanic andvolcaniclastic rocks during upper-plate disarticulationof the Silver Peak-Lone Mountain extensional com-plex, east-central, Nevada: v. 19, no. 1, p. 80.

Reheis, M.C., and McKee, E.H., 1991, Late Cenozoic his-tory of slip on the Fish Lake Valley fault zone andsurrounding areas of Nevada and California, in Re-heis, M.C. et al., eds., Late Cenozoic stratigraphy andtectonics of the Fish Lake Valley, Nevada and Cali-


PETRONIS et al.

fornia: U.S., Geological Survey Open-File Report 91–290, p. 26–45.

Reheis, M.C., and Nolar, J.S., 1989, New perspectives onQuaternary faulting in the southern Walker Lane, Ne-vada and California, in Ellis, M.A., ed., Late Cenozoicevolution of the southern Great Basin: Selected papersfrom the workshop: Reno, Nevada Bureau of Minesand Geology, Open File 89-1, p. 57–61.

Reheis, M.C., and Sawyer, T.L., 1996, Late Cenozoic his-tory and slip rates of the Fish Lake Valley, EmigrantPeak, and Deep Springs fault zones, Nevada and Cal-ifornia: Geological Society of America Bulletin,v. 109, p. 280–299.

Renne, P.R., Swisher, C.C. et al., 1998, Intercalibration ofstandards, absolute ages and uncertainties in 40Ar/39Ardating: Chemical Geology 145(1–2), p. 117–152.

Robinson, P.T., McKee, E.H., and Moiola, R.J., 1968, Ce-nozoic volcanism and sedimentation, Silver Peak re-gion, western Nevada and adjacent California: Geo-logical Society of America Memoir 116, p. 577–611.

Rogers, A.M., Harmsen, S.C., Corbett, E.J., Priestly, K.,and dePolo, D., 1991, The seismicity of Nevada andsome adjacent parts of the Great Basin, in Slemmons,D.B., Engdahl, E.R., Zoback, M.D., and Blackwell,D.D., eds., Neotectonics of North America: Boulder,Colorado, Geological Society of America, DecadeMap Volume 1, p. 153–184.

Roy, J.L., and Park, J.K., 1974, The magnetization processof certain red beds: Vector analysis of chemical andthermal results: Canadian Journal of Earth Sciences11, p. 437–471.

Samson, S.D., and Alexander, E.C. Jr., 1987, Calibration ofthe interlaboratory 40Ar-39Ar dating standard, MMhb-1, New developments and applications in isotope geo-science Chemical Geology: Isotope Geoscience Sec-tion 66, no. 1–2, p. 27–34.

Satterfield, J.I., Oldow, J.S., and McClelland, W.C., 2000,Exhumation of midcrustal rocks of the Silver Peakextensional complex during transcurrent displacementtransfer, central Walker Lane, western Great Basin:Geological Society of America Abstracts with Pro-grams, v. 32, no. 7.

Savage, J.C., Lisowski, M., and Prescott, W.H., 1990, Anapparent shear zone trending north-northwest across

the Mojave Desert into Owens Valley: GeophysicalResearch Letters, v. 17, p. 2113–2116.

Sawyer, T.L., 1991, Late Pleistocene and Holocene paleo-seismicity and slip rates of the northern Fish LakeValley fault zone, Nevada and California, in Reheis,M.C., ed., Guidebook for field trip to Fish Lake Val-ley, California-Nevada: Golden, Colorado, PacificCell, Friends of the Pleistocene, p. 114–138.

Snow, J.K., 1992, Large-magnitude Permian shortening andcontinental margin tectonics in the southern Cordil-lera: Geological Society of America Bulletin, v. 104,p. 80–105.

Snow, J.K., and Prave, A.R., 1994, Covariance of structuraland stratigraphic trends: Evidence for anticlockwiserotation within the Walker Lane Belt Death Valley re-gion, California and Nevada: Tectonics, v. 13,p. 712–724.

Snow, J.K., Geissman, J.W., and Wernicke, B.P., 1993, Pa-leomagnetic data from Paleozoic sedimentary rocksand Permo-Triassic intrusions, Death Valley area,southeast California: Implications for Mesozoic andCenozoic Deformation [abs.]: Eos (Transactions,American Geophysical Union), v. 74, p. 206.

Steiger, R.H., and Jager, E., 1977, Subcommission on geo-chronology: Convention on the use of decay constantsin geo- and cosmochronology: Earth and PlanetaryScience Letters, v. 36, p. 359–362.

Stewart, J.H., 1988, Tectonics of the Walker Lane belt,western Great Basin: Mesozoic and Cenozoic defor-mation in a shear zone, in Ernest, W.G., ed., Meta-morphism and crustal evolution of the western UnitedStates: Englewood Cliffs, New Jersey, Prentice-Hall,p. 681–713.

Stewart, J.H., and Diamond, D.S., 1990, Changing patternsof extensional tectonics: Overprinting the basins of themiddle and upper Miocene Esmeralda Formation inwestern Nevada by younger structural basins, in Wer-nicke, B.P., ed., Basin and Range extensional tectonicsnear the latitude of Las Vegas, Nevada: GeologicalSociety of America Memoir 176, p. 447–476.

Streckeisen, A.L., and LeMaitre, R.W., 1979, A chemicalapproximation to the modal QAPF classification of ig-neous rocks: Neus Jahrbuch fur Mineralogie Abhan-dlungen, v. 136, p. 169–206.

Sugden, D.E., Summerfield, M.A., Denton, G.H., Wilch,T.I., McIntosh, W.C., Marchant, D.R., and Rutford,R.H., 1999, Landscape development in the Royal So-ciety Range ; southern Victoria Land: Antarctica: Sta-bility since the mid-Miocene: Geomorphology, v. 28,p. 181–200.

Sylvester, A.G., 1988, Strike-slip faults: Geological Societyof America Bulletin, v. 100, no. 11, p. 1666–1703.

Symons, D.T.A., and Sangster, D.F., 1991, Paleomagneticage of the central Missouri barite deposits and its ge-netic implications: Economic Geology, v. 86, p. 1–12.

Taylor, J.R., 1982, An introduction to error analysis: Thestudy of uncertainties in physical measurements: MillValley, California, University Science Books, p. 270.

Thatcher, W., Foulger, G.R., Julian, B.R., Svarc, J., Quilty,E., and Bawden, G.W., 1999, Present-day deformationacross the Basin and Range Province, Western UnitedStates: Science, v. 283, no. 5408, p. 1714–1718.

Turner, H.W., 1902, A sketch of the historical geology ofEsmeralda County, Nevada, American Geologist,v. 29, p. 261–272.

Wallace, R.E., 1984, Variations in slip rates on faults in theGreat Basin Province, Earthquake Notes, v. 55, no. 1.

Wernicke, B.P., 1992, Cenozoic extensional tectonics of theU.S. Cordillera, in Burchfiel, B.C., Lipman, P.W., andZoback, M.L., eds., The Cordilleran Orogen: Conter-minous U.S.: Boulder, Colorado, Geological Societyof America, Geology of North America, v. G-3,p. 553–583.

Wernicke, B.P., Axen, G.J., and Snow, J.K., 1988, Basinand Range extensional tectonics at the latitude of LasVegas, Nevada: Geological Society of America Bul-letin, v. 100, p. 1738–1757.

York, D., 1969, Least squares fitting of a straight line withcorrelated errors: Earth and Planetary Science Letters,v. 5, p. 320–324.

Zijderveld, J.D.A., 1967, A.C., Demagnetization of rocks:Analysis of results, in Collinson, D.W., Creer, K.M.,and Runcorn, S.K., eds., Methods in palaeomagne-tism: Amsterdam, Elsevier, p. 254, 286.

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