Mesozoic tectonics and metamorphism in the Pequop Mountains and Wood Hills region,northeast Nevada:Implications for the architecture and evolution of the Sevier orogen

Phyllis A. Camilleri* Department of Geology and Geophysics,University of Wyoming,Laramie,Wyoming 82071 Kevin R. Chamberlain }

ABSTRACT reached a maximum. During 84Ð75Ma miogeoclinal rocks in the Basin and Range prov- another minor pulse of shortening and thick- ince in eastern Nevada (Fig.1). In this region, The Pequop MountainsÐWood HillsÐEast ening along the Independence thrust was fol- Mesozoic contractional structures are over- Humboldt Range region,northeast Nevada, lowed by partial exhumation of the metamor- printed by Cenozoic extensional structures and exposes a nearly continuous cross section of phic rocks and as much as 10km of crustal fabrics thereby leaving a fragmental record of Precambrian to Mesozoic strata representing thinning along the Pequop fault. Thus the in- contractional tectonics. There is thus uncertainty middle to upper crustal levels of the Mesozoic terval from 84to 75Ma in northeast Nevada and debate about the large-scale Mesozoic struc- hinterland of the Sevier orogen. These rocks marks a fundamental,and apparently perma- tural geometry and magnitude of thrust faulting preserve the transition from unmetamor- nent,change from horizontal contraction to in the middle to upper crust (e.g.,Miller et al., phosed Mesozoic upper crust to partially extension in the upper to middle crust in the 1983; Bartley and Wernicke,1984). Isolated melted middle crust. Integration of new struc- hinterland. Final exhumation of the metamor- contractional structures in the hinterland have tural,metamorphic,and U-Pb thermochrono- phic rocks was accomplished by the Tertiary been documented to be Late Jurassic to Late logic data from the Wood Hills and Pequop Mary’s River fault system. Cretaceous in age,indicating that hinterland de- Mountains,coupled with a regional tectonic Our data indicate that much of the meta- formation is older than,but in part temporally reconstruction,reveals substantial Cretaceous morphism and some of the contraction in the overlaps,foreland deformation (Allmendinger et metamorphism,contraction,and extension in Sevier hinterland in northeast Nevada,which al.,1984; Glick,1987; Miller et al.,1987; Miller the Sevier hinterland in northeast Nevada. We was previously thought to be largely Late et al.,1988; Snoke and Miller,1988; Miller and report two phases of contraction not previ- Jurassic,is actually Cretaceous in age. Fur- Gans,1989; Hudec,1992; Miller and Hoisch, ously recognized that are accommodated by thermore,the data indicate that widespread 1992,1995). Contractional deformation in the top-to-the-southeast thrust faults,the Winder- metamorphism of the middle crust is a by- hinterland was accompanied by sparse regional- mere and Independence thrusts. Contraction product of tectonic burial,and that hinter- contact metamorphism adjacent to syntectonic was succeeded by two phases of extension land and foreland thrust faulting were coeval, plutons (Glick,1987; Miller et al.,1987; Miller along west-rooted normal faults,the Late Cre- suggesting that thrust faults in the Sevier oro- et al.,1988; Snoke and Miller,1988; Miller and taceous Pequop fault and Tertiary Mary’s gen do not form a simple foreland younging Gans,1989; Hudec,1992; Miller and Hoisch, River fault system. sequence. 1992,1995; Taylor et al.,1993). The earliest phase of thrust faulting resulted The Ruby MountainsÐEast Humboldt Range, in as much as 30km of crustal thickening and INTRODUCTION Wood Hills,and Pequop Mountains (Fig.1),in an estimated minimum of 69km of shortening the hinterland in northeast Nevada,expose a rel- along an inferred fault called the Windermere The Sevier orogenic belt (Armstrong,1968; atively continuous crustal cross section of rocks thrust. The timing of this thrusting event is Fig.1) forms part of the North American,Meso- from the Mesozoic metamorphosed middle crust bracketed between Late Jurassic (ca. 153Ma) zoic to Tertiary Cordilleran fold and thrust belt to unmetamorphosed upper crust,and afford a and Late Cretaceous (84Ma). Relaxation of that extends from northern Alaska to Mexico. rare opportunity to assess the large-scale struc- crustal isotherms following and perhaps dur- The Sevier foreland fold and thrust belt (Fig.1) ture,tectonics,and nature of metamorphism in ing thrusting resulted in Barrovian-style meta- spans the Early Cretaceous to early Tertiary and the Mesozoic crust. These ranges are in a part of morphism of footwall rocks,and partial melt- contains a foreland younging sequence of thin- the hinterland where Mesozoic contractional de- ing of metapelite at deep levels. Peak skinned thrust systems (Armstrong and Oriel, formation and metamorphism are perceived to metamorphism was attained ca. 84Ma,and by 1965; Armstrong,1968; Royse et al.,1975; be largely Late Jurassic in age,predating fore- this time hinterland crustal thickening had Dixon,1982; Wiltschko and Dorr,1983; All- land deformation (Allmendinger and Jordan, mendinger,1992; Heller et al.,1986; Royse, 1981; Glick,1987; Miller et al.,1987; Snoke and 1993). Perhaps one of the least structurally un- Miller,1988; Hudec,1990,1992; Miller and *Present address:Department of Geology and Ge- derstood parts of the Sevier orogen is its hinter- Hoisch,1992,1995). However,the precise age ography,Austin Peay State University,P.O. Box 4418, Clarksville,Tennessee 37044; E-mail:camillerip@ land that extends through a thick sequence of of much of the Mesozoic deformation and meta- apsu01.edu. Data Repository item 9720 complexly deformed,Mesozoic and Paleozoic morphism in the Ruby MountainsÐEast Hum-

GSA Bulletin; January 1997; v. 109; no. 1; p. 74Ð94; 16 figures; 3 tables.

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We document two phases of top-to-the-southeast thrust faulting. The early phase accommodated the bulk of crustal thickening and shortening and the latter only minor thickening and shortening. We also document a period of normal faulting that was coeval with the decompression event in the Ruby MountainsÐEast Humboldt Range. A surprising result of our thermochronologic and metamorphic data was the revelation that the first phase of thrusting resulted in widespread Late Cretaceous tectonic-burial metamorphism of middle crust and that the later phase of thrusting is Late Cretaceous in age.

GEOLOGIC SETTING

Rocks within the Pequop Mountains, Wood Hills, and Ruby MountainsÐEast Humboldt Range region are Proterozoic to Triassic miogeo- clinal strata and Tertiary volcanic and clastic rocks. The miogeoclinal sequence is composed of metamorphosed and unmetamorphosed carbon- ate and siliciclastic strata that was at least 12 km thick prior to Mesozoic contractional deformation (Figs. 2A and 3). In places, unmetamorphosed up- Figure 1. Tectonic map of Nevada, Utah, and western Wyoming showing the position of the per Paleozoic to Mesozoic strata are overlain by Sevier fold and thrust belt and its hinterland as referred to in this paper. Map is modified after Eocene volcanic rocks (39Ð41 Ma; Brooks et al., Royse (1993) and Speed et al. (1988). 1995), and these rocks are in places overlain dis- conformably by the Oligocene to Miocene Hum- boldt Formation, a sequence of synextensional boldt Range, Wood Hills, and Pequop Moun- Pequop Mountains, which are adjacent to the volcaniclastic and clastic strata (Figs. 2A and 3) tains is unknown. The purpose of this paper is to Ruby MountainsÐEast Humboldt Range, consti- (Mueller and Snoke, 1993a, 1993b). present a structural reconstruction of the Sevier tute a collage of complexly faulted metamor- The metamorphosed miogeoclinal rocks were hinterland in the Ruby MountainsÐEast Hum- phosed and unmetamorphosed miogeoclinal metamorphosed and deformed during the Meso- boldt Range, Wood Hills, and Pequop Moun- strata that lack extensive extensional overprint zoic and then exhumed by normal faults during tains region and to elucidate the timing, style, (Thorman, 1970; Camilleri, 1994) and collec- the Tertiary. Metamorphic rocks in the Wood and magnitude of contractional deformation and tively contain a continuum of rocks representing Hills and Ruby MountainsÐEast Humboldt metamorphism. middle to uppermost levels of the Mesozoic crust. Range were exhumed by the Tertiary, west- The Ruby MountainsÐEast Humboldt Range The remarkable preservation of Mesozoic fea- rooted, Mary’s River fault system (Mueller and (Fig. 1) expose the structurally deepest metamor- tures in the Wood Hills and Pequop Mountains Snoke, 1993a). The modern faults bounding the phic rocks in the Sevier hinterland (McGrew, makes these ranges critical to assessing the timing East Humboldt Range and Snake Mountains are 1992). Although this metamorphic complex and validity of the regional crustal thickening and the youngest segments of the Mary’s River fault mainly records late Tertiary ductile, extensional thinning event implied by barometric data, and to system, and low-angle normal faults in the East deformation (Snoke, 1980; Snoke and Lush, reconstructing the Mesozoic crust. We investi- Humboldt Range, Wood Hills, and Windermere 1984), it also contains evidence for a major crustal gated the Wood Hills and Pequop Mountains to Hills are older extinct, rotated segments of this thickening event of uncertain age and a subse- address the following fundamental questions: system (Mueller and Snoke, 1993a; Fig. 3). quent decompression event inferred to be Late (1) how many phases of thrust faulting are re- These low-angle normal faults contain a west- Cretaceous or early Tertiary in age (McGrew, sponsible for the large-magnitude crustal thicken- dipping sequence of unmetamorphosed Paleo- 1992; Hodges et al., 1992; Peters and McGrew, ing event, (2) what are the kinematics and ages of zoic strata, Eocene volcanic rocks, and the Hum- 1994; McGrew and Snee, 1994). The crustal thrust faulting, (3) what is the origin and age of boldt Formation in their hanging walls (Fig. 3). thickening and decompression events are based metamorphism in the Wood Hills and Pequop The Humboldt Formation represents part of the on barometric data from miogeoclinal rocks that Mountains, and (4) is there evidence for early Ter- synextensional basin that developed along the require burial to depths of ≈30 km and subsequent tiary or Cretaceous normal faulting? Mary’s River fault system, whereas the volcanic decompression to depths of ≈20 km (McGrew, In this paper we present new structural, meta- rocks predate extension and are inferred to have 1992; Hodges et al., 1992). The barometric data morphic, and U-Pb thermochronologic data from blanketed a topographic surface of low relief imply major crustal thickening by thrust faulting the Wood Hills and Pequop Mountains, which prior to normal faulting (Thorman et al., 1992; and subsequent extension by normal faulting, but we integrate with data from the Ruby Moun- Mueller and Snoke, 1993a, 1993b). Mueller and no large-magnitude thrusts or CretaceousÐEarly tainsÐEast Humboldt Range to produce the first Snoke (1993a) indicated that slip along the Tertiary normal faults have been documented in, detailed structural and metamorphic reconstruc- Mary’s River fault involved isostatically induced or adjacent to, the complex. The Wood Hills and tion of the Sevier hinterland in northeast Nevada. rebound of its footwall and consequent excision

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Figure 2. (A) Generalized stratigraphic column of Protero- zoic to Tertiary strata in northeast Nevada. The column show- ing Proterozoic to Mesozoic stratigraphy depicts average strati- graphic thicknesses and depths prior to Mesozoic deformation and metamorphism, and also shows correlations of the map units PTr, OM, and ZCU shown in Figure 3. Data used in con- structing this column are from Camilleri (1994); McCollum and Miller (1991); Miller (1984); Glick (1987); Thorman (1970); Robinson (1961); and Fraser et al. (1986). (B) Schematic cross section depicting stratigraphic, structural, and metamor- phic relationships in the northern Pequop Mountains.

and rotation of fault slices of its hanging wall. are pervasively intruded by Late Jurassic, Late sure increase gradationally northward from The low-angle normal faults in the Ruby Moun- Cretaceous, and Tertiary granitic dikes, sills, and ≈4.5 kbar in the southern Ruby Mountains to tainsÐEast Humboldt Range and Wood Hills are plutons (Snoke and Lush, 1984; Wright and ≈8.6 kbar in the northern East Humboldt Range underlain by a gently dipping, top-to-the-west- Snoke, 1986, 1993; McGrew, 1992; Fig. 4). (Fig. 4; Hudec, 1990, 1992; McGrew, 1992; Pe- northwest mylonitic shear zone (Fig. 4; Snoke However, locally the metamorphic rocks pre- ters and McGrew, 1994). Metamorphosed strata and Lush, 1984; Lister and Snoke, 1984; Hudec, serve Mesozoic metamorphic and deformational in the Wood Hills and Pequop Mountains differ 1990; Snoke et al., 1990; McGrew, 1992; Camil- features. Late Jurassic and Late Cretaceous from those in the Ruby MountainsÐEast Hum- leri, 1994). The mylonite zone represents the metamorphism and deformation have been doc- boldt Range because they are of lower metamor- ductile, exhumed part of the Mary’s River fault umented in the southernmost Ruby Mountains phic grades (Fig. 4); they are not associated with system (Mueller and Snoke, 1993a). and northern East Humboldt Range, respectively plutons; they are sparsely intruded by granitic Most of the metamorphosed miogeoclinal (Hudec, 1992; McGrew, 1992; Snoke, 1994). dikes and sills; and they mainly record Meso- strata in the Ruby MountainsÐEast Humboldt Metamorphic and thermobarometric data indi- zoic, rather than Tertiary, metamorphism and de- Range are overprinted in the mylonite zone and cate that Mesozoic metamorphic grade and pres- formation (Thorman, 1970; Camilleri, 1994).

76 Geological Society of America Bulletin, January 1997

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METHODS AND CONVENTIONS

Reconstructing the Mesozoic tectonic history of the Pequop MountainsÐWood HillsÐEast Humboldt Range region involved tracing and correlating structures, fabrics, and stratigraphy across the region as well as determining style and the grades of metamorphism in the Wood Hills and Pequop Mountains. This was accomplished in part by detailed geologic and metamorphic mapping of the Wood Hills and northern Pequop Mountains at a scale of 1: 24 000 accompanied by reconnaissance observations in Spruce Moun- tain, southern East Humboldt Range, and Win- dermere Hills region (Fig. 4; Camilleri, 1994). Our data were integrated with published data from the Ruby MountainsÐEast Humboldt Range and the result of our integration is presented in simplified regional geologic and metamorphic maps in Figures 3 and 4. The extent and geom- etry of structures and metamorphic facies were determined by constructing a series of regional cross sections that depict structure as well as the distribution of metamorphic facies (Fig. 4). Met- amorphic facies in the Wood Hills, Windermere Hills, and Pequop Mountains were delineated by studying more than 200 oriented thin sections of metacarbonate and metapelite. Metamorphic mineral abbreviations used in the text are shown in Table 1. Pressure (kilobars) to depth (kilome- ters) conversions presented in this paper were calculated using a conversion of 3.8 km/kbar (Carter and Tsenn, 1987).

STRUCTURE AND METAMORPHISM IN THE WOOD HILLS AND PEQUOP MOUNTAINS REGION

The PequopÐSpruce MountainÐWood HillsÐ Figure 4. Metamorphic map and cross sections of the Pequop Mountains, Wood Hills, Spruce Windermere Hills region (Figs. 3 and 4) contains Mountain, and Ruby MountainsÐEast Humboldt Range. For each line of section two cross sec- a complex array of thrust and normal faults and tions are shown. The first cross section shows stratigraphy and structure and utilizes the same ductile fabrics. The most important structural stratigraphic units depicted in Figure 3. The second cross section depicts the distribution of features in this area are a prograde regional metamorphic facies with respect to stratigraphic and fault contacts. Data from the Wood metamorphic fabric, the Independence thrust, HillsÐWindermere Hills and northern Pequop Mountains are simplified from Camilleri (1994). the mylonite zone, and low-angle normal faults Data from the southern Pequop Mountains are integrated from Coats (1987); Spruce Mountain (Figs. 3 and 4). The regional metamorphic fabric data are from Hope (1972), Coats (1987), and our observations; the Ruby Mountains and East is the oldest structural feature and is exposed in Humboldt Range data are from Snoke and Lush (1984), Howard et al. (1979), Taylor (1981), the Wood HillsÐWindermere Hills, central Pe- Hudec, (1992), McGrew (1992); Snake Mountains from Coats (1987). The position of the silli- quop Mountains, and in the southern part of manite-in isograd is constrained by data from Howard (1966), Taylor (1981), Hurlow et al. Spruce Mountain. In the Pequop Mountains, the (1991), McGrew (1992), Snoke (1992), and Hodges et al. (1992). Barometric data shown for the metamorphic fabric is cut by the Independence Wood Hills are from Hodges et al. (1992); data for the East Humboldt Range are from McGrew thrust, and in the northern Wood Hills it is over- (1992) and Peters and McGrew (1994); and for the Ruby Mountains are from Hudec (1992). printed in the mylonite zone (Camilleri, 1994). The low-angle normal faults include a principle segment of the Mary’s River fault system in the northern Wood Hills and the Pequop fault in the faults that cut the low-angle normal faults as well discuss the low-angle normal faults first because Pequop Mountains (Camilleri, 1994; Fig. 3). as the modern range-bounding normal faults they establish the large-scale structural frame- These faults postdate metamorphism and the In- (Fig. 3). The major structural features are dis- work and provide important crosscutting rela- dependence thrust. The youngest structural fea- cussed below in relative age of formation with tionships relevant to the discussion of thrust tures in the region are minor high-angle normal the exception of the low-angle normal faults. We faults and regional metamorphism.

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Figure 4. (Continued.)

Low-angle Normal Faults and Evidence for ceded development of the low-angle normal faults (Fig. 3). The Mary’s River and Pequop a Major Thrusting Event faults. Collectively, the Mary’s River and Pequop low-angle normal faults therefore duplicate the faults separate and duplicate two different Paleo- miogeoclinal section, but everywhere place un- The Mary’s River and Pequop low-angle nor- zoic sections. A nearly continuous, regionally metamorphosed strata over metamorphosed mal faults and their crosscutting relationships metamorphosed to unmetamorphosed Precam- strata (Figs. 3 and 4), which indicates that normal provide important relative age constraints on brian to Triassic sequence lies beneath the low- faulting postdates metamorphism and suggests a contractional features. The Mary’s River fault angle normal faults (Figs. 3 and 4). The entire se- significant amount of throw across the faults. The system and the Pequop fault (Fig. 3) represent quence is completely metamorphosed in the repetition of section across the low-angle normal two distinct generations of low-angle normal Wood Hills, but in the Pequop Mountains and faults, however, requires that the miogeoclinal se- faults that are differentiated by whether they pre- Spruce Mountain only Ordovician and older quence was duplicated by thrust faulting prior to date or postdate deposition of the Eocene vol- strata are metamorphosed (cf. Figs. 3 and 4). Al- normal faulting. This is further supported by the canic rocks. The Mary’s River fault system post- though the Paleozoic to Mesozoic section be- observation that the metamorphosed section and dates, and the Pequop fault predates, the neath the low-angle normal faults contains some structurally overlying unmetamorphosed Paleo- volcanic rocks. unmetamorphosed strata, for simplicity, we here- zoic strata differ stratigraphically. The most Evidence for a Major Thrusting Event. The after refer to it as the metamorphosed section. In salient stratigraphic difference is the presence of most important aspect of both generations of contrast to the metamorphosed section, unmeta- different facies of the Silurian Roberts Mountain low-angle normal faults is that they provide evi- morphosed Ordovician to Triassic strata are pre- Formation. An eastern dolomitic facies of the for- dence for a major thrust faulting event that pre- sent in the hanging walls of the low-angle normal mation is present in the metamorphosed section and a western platy limestone facies occurs in the hanging wall of the Pequop fault, and regionally TABLE 1. MINERAL ABBREVIATIONS in the hanging walls of the low-angle normal AND andalusite EP epidote SILL sillimanite faults associated with the Mary’s River fault sys- ALL allanite GT garnet SP sphene ALS aluminosilicate HNB hornblende ST staurolite tem (Thorman, 1970; Berry and Boucot, 1970; BI biotite IL ilmenite TLC talc Sheehan, 1979; Camilleri, 1994; Fig. 2B). The CC calcite MU muscovite TOUR tourmaline CHL chlorite PL plagioclase TR tremolite implication here is that an extensionally dismem- DI diopside RT rutile KY kyanite bered western miogeoclinal section currently is DO dolomite SER sericite Q quartz structurally above a relatively continuous but

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Figure 5. Simplified geologic map and cross sections of the northern Pequop Moun- tains. Data are from Camilleri (1994). Rocks composing the hanging wall of the Pequop fault on the map are denoted by a diagonally ruled pattern superimposed on stratigraphic unit patterns.

metamorphosed eastern miogeoclinal section, and 5). Hanging-wall and footwall strata of the wall and footwall. The Pequop fault cuts, and is and that these two sections were duplicated by Pequop fault are depositionally overlapped by therefore younger than, an unnamed, east-strik- thrust faulting prior to normal faulting. the Eocene volcanic rocks (Fig. 5), indicating ing, north-dipping thrust in its hanging wall Pequop Fault and Crosscutting Relation- that the fault predates 41 Ma. (Fig. 5, sections A-A′ and E-E′). This thrust ships. The Pequop fault forms the base of a The Pequop fault provides relative age con- places unmetamorphosed Ordovician to Missis- klippe of Ordovician to Permian strata (Figs. 3 straints on contractional features in its hanging sippian strata on Permian strata (Fig. 5). The

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Pequop fault also truncates gentle folds in the eral assemblages indicative of classic Barrovian neath the transition zone, the progressive appear- metamorphosed section in its footwall (Fig. 5, style metamorphism, and hence allow delin- ance of biotite and garnet in metapelite indicates cross sections A-A′ and E-E′). These crosscutting eation of greenschist facies and lower and upper that metamorphic grade increases with strati- relationships require that the Pequop fault post- amphibolite facies metamorphic zones (Fig. 4). graphic depth. The biotite-in isograd for metapel- dates contractional deformation. The Pequop fault Our metamorphic data indicate that the metamor- ite lies between the Kanosh Shale and upper is interpreted as a normal fault because it omits phic grade of the metamorphosed section in- Cambrian Dunderberg Shale, and the garnet-in section and metamorphic grade. For example, the creases to the northwest (Fig. 4; additional data isograd is between the Dunderberg Shale and the unmetamorphosed Permian section in the hang- are presented in the data repository). Lower Cambrian Prospect Mountain Quartzite ing wall of the Pequop fault lies structurally above The Pequop Mountains and Spruce Mountain (Fig. 6). The first appearance of tremolite in sili- Silurian, Devonian, and metamorphosed Ordovi- form the lowest grade part of the metamorphosed ceous carbonate coincides approximately with the cian rocks (Figs. 4 and 5). We infer the sense of section, and in this region the transition zone garnet-in isograd (Fig. 6). slip on the Pequop fault to be top-to-the-west to from unmetamorphosed to metamorphosed strata No thermobarometric data exist for garnet and northwest because it cuts down-structure and is exposed (Fig. 4, cross sections A-A′, B-B′, and biotite zone rocks in the Pequop Mountains, but down-section in this direction. C-C′). The boundary between metamorphosed an estimate of pressures and temperatures can be and unmetamorphosed strata is based solely on inferred from structural and metamorphic rela- Regional Metamorphism field relationships and is placed between the last tionships. The metamorphosed section lies struc- appearance of shale and the first appearance of turally beneath the Pequop fault, which dupli- The metamorphosed section in the Wood phyllite. cates much of the Paleozoic section and indicates Hills, Pequop Mountains, and Spruce Mountain The nearly complete Paleozoic section in the that the metamorphosed section was structurally (Fig. 4) records a single prograde, regional meta- footwall of the Independence thrust in the Pequop buried (Figs. 2B and 3). The hanging wall of the morphic event. The metamorphic fabric com- Mountains provides a continuous cross section Pequop fault exposes strata to a stratigraphic prises S and S-L tectonites wherein foliation is that best illustrates metamorphic changes charac- depth of ≈7 km (base of the Ordovician Pogonip parallel or at a low angle to bedding, and lin- teristic of this region (cross section B-B′ in Figs. 3 Group; Camilleri, 1994; Fig. 2, A and B), sug- eation is defined by grain shape or porphyroblast and 6). In this and other sections, the transition be- gesting a minimum of ≈7 km of structural over- elongation (Camilleri, 1994). Tectonite develop- tween unfoliated, unmetamorphosed rocks and burden. Adding ≈7 km to a stratigraphic depth of ment during metamorphism accommodated vari- foliated, metamorphosed rocks lies between ≈8 km for the Dunderberg Shale (Fig. 2A), which able attenuation of stratigraphic units (Camilleri, shales of the Mississippian Chainman ShaleÐ is in proximity of the biotite-in isograd (Fig. 6), 1994; see thicknesses of units in cross sections in Diamond Peak Formation and chlorite zone phyl- suggests a burial depth of at least 15 km or an ap- Fig. 4). Metapelites within this region yield min- lites of the Ordovician Kanosh Shale (Fig. 6). Be- proximate pressure of ≈4 kbar for the biotite-in

Figure 6. Detailed cross section through the footwall of the Independence thrust in the Pequop Mountains illus- trating metamorphic mineral assemblages (see Table 1) in various units. Location of section is shown in Figure 3. ZCpm = Prospect Mountain Quartzite; Cks = Killian Springs Formation; Ct = Toano Limestone; Ccl = Clifside Limestone; Cu = Upper Cambrian limestone, undivided; Cd = Dunderberg Shale; Cnp = Notch Peak Formation; Op = Pogonip Group; Oe = Eureka Quartzite; OMu = Or- dovician, Silurian, Devonian, and Lower Mississippian strata, undivided; Mdpc = Diamond Peak Formation and Chainman Shale, undivided; Pe = Ely Limestone.

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that exceeds its approximate original pressure at stratigraphic depths by ≈3 to 4 kbar (see Fig. 7). Because andalusite does not occur in the Dunder- berg Shale, we infer a clockwise P-T path for the Dunderberg Shale (Fig. 7). Assemblages in metamorphosed siliceous dolomite within the Wood Hills show a distinct zonation and are divided into talc, tremolite, and diopside zones separated by northeast-trending tremolite-in and diopside-in isograds (Fig. 8). The position of isograds indicates a west-north- west increase in metamorphic grade. Some of the calc-silicate assemblages in the Wood Hills can be used to infer spatial thermal variations across isograds. Figure 9 is a plot of reaction relation-

ships in the system CaO-MgO-SiO2-CO2-H2O in T-XCO2 space. The plot assumes end-member mineral compositions and a pressure of 6 kbar, which is derived from Hodges et al.’s (1992) pressure estimate of 5.5 to 6.4 kbar. Samples numbered 5, 6, and 7 in the talc zone (Fig. 8) con- tain the assemblage CC-DO-Q-TLC, which is Figure 7. Diagram illustrating inferred pressure-temperature (P-T) paths for metapelites in stable along reaction 1 and requires temperatures the Pequop Mountains, Wood Hills, East Humboldt Range, and Ruby Mountains. Zm = Prot- <≈470 ¡C (Fig. 9). The tremolite zone encom- erozoic schist from the McCoy Creek Group (P-T paths are denoted by a bold dashed line); passes exposures of the kyanite-bearing schists ZCpm = Prospect Mountain Quartzite (P-T path is denoted by a gray line); and Cd = Dunder- that record a metamorphic temperature of berg Shale (P-T paths are denoted by a light dashed line). Shaded circles or polygons represent ≈540Ð590 ¡C (zone a in Fig. 9). Tremolite bear- thermobarometric data and solid circles represent the approximate pressure (at stratigraphic ing assemblages within the tremolite zone are depth; see Fig. 2A) and temperature (assuming an initial thermal gradient of ≈25 ¡C/km) of stable within a region in T-XCO2 space delimited rocks prior to thrust faulting. Thermobarometric data from Proterozoic schist in the north- by reactions 2, 4, and 7 (zone b in Fig. 9). In ad- western East Humboldt Range (NEHR [B]) are from McGrew (1992) and Peters and McGrew dition, in proximity of the kyanite-bearing schist, (1994); for northeastern East Humboldt Range (NEHR [A]) are from Hodges et al. (1992); and sample 10B (Fig. 8) has an assemblage of for the southern Ruby Mountains (SRM) are from Hudec (1992). Thermobarometric data from TR-CC-Q, which has a more restricted zone of the Dunderberg Shale in the southern Wood Hills (SWH) are from Hodges et al. (1992). NPM = stability bounded by reactions 2, 4, and 5, and re- inferred P-T paths for the Prospect Mountain Quartzite (NPMA) and the Dunderberg Shale quires a temperature of <≈620 ¡C, compatible (NPMB) in the northern Pequop Mountains. Aluminosilicate curves and curve 1b are from with temperature estimates derived from the Spear and Cheney (1989); curve 1a is from Carmichael (1978); and biotite- and garnet-in iso- kyanite schists. Diopside zone assemblages grads are from Yardley (1989). See Table 1 for mineral abbreviations. (Fig. 8), exclusive of those overprinted in the my- lonite zone, lie on or above curves representing reactions 5 and 6 (Fig. 9). In proximity of the isograd. We utilize Yardley’s (1989) inferred po- assemblage in the Dunderberg Shale is BI-ST- diopside-in isograd, sample 18 (Fig. 8) in the sitions of the biotite- and garnet-in isograds in KY-GT-MU-Q-PL-ALL-RT-IL (see Table 1), in- southern Wood Hills has the assemblage DI-TR- pressure-temperature (P-T) space to infer meta- dicating that these rocks are in the kyanite zone or CC-Q, which lies along reaction 5 and requires a morphic temperatures. According to Yardley upper amphibolite facies. Textural relationships temperature <≈620 ¡C. Away from the tremolite- (1989; Fig. 7), the temperature of the biotite-in indicate that kyanite in this assemblage is a by- in isograd, in the northwestern Wood Hills, sam- isograd at ≈4 kbar is ≈400Ð430 ¡C. Because product of the breakdown of staurolite by a reac- ples 19 and 21 have the assemblage DI-DO-Q metapelite in the Pequop Mountains lack an- tion such as Q + MU + ST = BI + GT + KY. The- (Fig. 8), which is stable on curve 6 and suggests a dalusite, we infer the P-T path for rocks at the bi- oretical data of Spear and Cheney (1989) suggest temperature between ≈620 and 640 ¡C. This otite-in isograd to define a clockwise trajectory that this reaction requires a pressure >8 kbar and comparison of equilibrium calc-silicate assem- (curve NPMB in Fig. 7). Figure 7 (curve NPMA) temperature of ≈700 ¡C (Fig. 7, curve 1b), but em- blages in the Wood Hills with the isobaric

also shows the inferred P-T path for rocks at the pirical data of Carmichael (1978) require a pres- T-XCO2 plot (Fig. 9) suggests a west-northwest garnet-in isograd. sure > ≈4.8 kbar and temperature of ≈540 ¡C increase in temperature. In contrast to the Pequop Mountains, the Wood (Fig. 7, curve 1a). Hodges et al. (1992) presented In summary, the metamorphic data from the Hills expose a higher grade and structurally thermobarometric analyses of kyanite-bearing Pequop MountainsÐWood Hills region indicate deeper level of metamorphosed section. Prograde Dunderberg Shale in the easternmost kyanite lo- that metamorphic grade increases to the north- metamorphic chlorite is absent from all metapel- cality (Figs. 4 and 8). Their analyses yielded a west and, on the basis of structural and baromet- ite in the Wood Hills. The only diagnostic meta- pressure range of 5.5Ð6.4 kbar and temperatures ric data, we infer that metamorphism records pelite in the Wood Hills is the Upper Cambrian of ≈540Ð590 ¡C, which are more consistent with clockwise P-T paths. Despite the lack of contin- Dunderberg Shale, which crops out in two locali- Carmichael’s (1978) observations (Fig. 7). The uous exposure of the metamorphosed section be- ties in the central Wood Hills (Fig. 8). A typical Dunderberg Shale therefore records a pressure tween the Pequop Mountains and Wood Hills,

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Figure 8. Metamorphic map of the Wood Hills showing lo- cations of kyanite-bearing Dunderberg Shale and the ap- proximate locations of the tremolite-in and diopside-in iso- grads. Numbers refer to calc-silicate samples used to delineate positions of isograds; a list of assemblages in these samples is in the data repository. Sample location 22 is the location of U-Pb sample 97C.

there is a relatively smooth gradation from un- metamorphosed to greenschist to lower and up- per amphibolite facies from east to west (Fig. 4). Moreover, our plotting of the metamorphic fa- cies on regional cross sections (Fig. 4) reveals a systematic pattern to the distribution of the fa- cies with respect to stratigraphic contacts. Meta- morphic grade increases vertically with strati- graphic depth, and within a given stratigraphic unit, grade increases laterally to the northwest (Fig. 4, cross sections A-A′,B-B′, and C-C′).

Thrust Faults

Two main thrust faults are exposed in the Wood HillsÐPequop Mountains region. One of these is the aforementioned unnamed thrust con- tained in the hanging wall of the Pequop fault, and the other is the Independence thrust in the metamorphosed section in the footwall of the Pequop fault (Fig. 5). Independence Thrust. The Independence thrust transects nearly the entire Paleozoic sec-

tion and cuts S1 and the metamorphic gradient such that it emplaces higher grade over lower grade rocks (Fig. 4). The Independence thrust cuts down section toward the west and therefore roots beneath the Wood Hills metamorphic se- quence (Fig. 4, cross sections C-C′ and B-B′). The formation of minor hanging wall contrac- tional structures was associated with the devel- Figure 9. T-XCO2 plot for the system CaO, MgO, SiO, H20, CO2 at 6 kbar. Grid generated opment of the Independence thrust, and these by GEO-CALC (Perkins et al., 1986) using the data base of Berman (1988). Plot assumes end- structures are useful for inferring shortening di- member compositions. See text for explanation, Table 1 for abbreviations. rections. Bedding and S1 in the hanging wall of the Independence thrust in the Wood Hills and

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inference postdates development of the Indepen- dence thrust (Fig. 5, sections A-A′ and E-E′). Unnamed thrust. The unnamed thrust cuts strata in its hanging wall and footwall at a mod- erate angle, juxtaposing Ordovician to Missis- sippian strata on Permian strata (Fig. 5). Hang- ing-wall and footwall cut-offs trend northeast in the thrust plane, suggesting a top-to-the-south- east sense of slip. The unnamed and Indepen- dence thrusts are similar because they cut upper Paleozoic strata at a moderate angle and both ap- pear to have a top-to-the-southeast sense of slip.

Structural Summary

Our structural data suggest that the Wood Hills and Pequop Mountains region underwent at least two episodes of thrust faulting prior to normal faulting. The repetition of section across the low-angle normal faults requires that the miogeocline was duplicated by thrust faulting prior to extension. Furthermore, stratigraphic data require that this thrusting event involved emplacement of a western miogeoclinal se- Figure 10. Equal-area, lower hemisphere projections of lineations and poles to foliation and quence over an eastern miogeoclinal section. We bedding in the hanging wall and footwall of the Independence thrust. Stippled regions are do- believe that this event represents the first major mains from which orientation data are derived. A is from the footwall and B, C, D, and E are episode of contraction, and that it is recorded in from folds in the hanging wall. the barometric data from the Wood Hills, which require that the metamorphosed section was buried ≈10Ð16 km in excess of premetamorphic Pequop Mountains are deformed by a series of Wood Hills contains two anticline-syncline stratigraphic depths (Fig. 7). In this interpreta- minor north- to northeast-trending back thrusts pairs; the anticlines are cored by small-displace- tion, metamorphism was coeval with or followed and northwest-vergent back folds with axes that ment thrust faults (Figs. 3 and 11). The fold pair this thrusting event. The second episode of thrust plunge shallowly to the northeast (Figs. 3 in the southern Wood Hills is asymmetric and faulting involved development of the Indepen- and 10, cross sections B-B′ and C-C′). The style overturned, and farther north the fold pair is re- dence thrust and contractional structures in its of folding varies with metamorphic grade and cumbent. Locally, an axial-planar grain-shape hanging wall. The Independence thrust must

reflects lower temperature conditions in the Pe- foliation overprints S1 in the hinge regions of the postdate the earlier episode of thrust faulting be- quop Mountains to higher temperature condi- folds in the Wood Hills. We infer the sense of cause it deforms the metamorphic fabric. It is tions in the Wood Hills. Outcrop- to map-scale slip on the Independence thrust to be top-to-the- unknown whether the unnamed thrust in the folds in the lower grade Pequop Mountains are southeast, perpendicular to the regional trends of hanging wall of the Pequop fault formed during predominantly kink folds that lack an axial pla- fold axes and back thrusts in its hanging wall. the first or second episode of thrust faulting. nar cleavage (Fig. 5). One of the folds in the Pe- We infer that the Independence thrust predates quop Mountains appears to be a fault-bend fold the Pequop fault on the basis of crosscutting re- U-Pb THERMOCHRONOLOGY above the Independence thrust where it cuts lationships. The Pequop fault truncates flexures footwall upper Ordovician to Devonian strata at in the hanging wall of the Independence thrust, We utilized U-Pb methods to better constrain a moderate angle (Fig. 5). The higher grade indicating that it postdates back folding and by the timing of peak metamorphism and the cool-

Figure 11. Simplified northwest-southeast cross section of the Wood Hills. See Figure 3 for location of section.

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ing history of metamorphic rocks in the Wood possible inheritance in one fraction that was se- blende with equilibrium contacts, and thus is Hills and Pequop Mountains. We combine new lected with visible cores (4, Table 2). The nonlin- probably a product of the prograde reaction that U-Pb sphene dates with published 40Ar/39Ar earity of the data from this sample (Fig. 12A) that produced hornblende. Sphene grains from this dates from metamorphic micas to produce tem- occurs even among the four abraded fractions re- sample are pale yellow and range in size from perature-time paths that help constrain the tim- flects either multiple sources of inheritance or 0.40 to 0.05 mm. This sample is between the ing of metamorphism and faulting. We also pre- varying amounts of recent Pb loss, or combina- biotite-in and garnet-in isograds for pelitic rocks sent a U-Pb zircon date from a granitic dike that tions of both. We estimate the age of the dike as (Fig. 6). The temperature interval for the first ap- places an upper limit on the timing of metamor- 154 ± 5 Ma based on the lower intercept of a pearance of biotite and garnet is ≈400Ð475 ¡C phism. U-Pb sample locations and analytical chord between fractions 4 and 5 (Fig. 12A), the (Fig. 7). Thus, during the sphene-producing re- procedures are given in the appendix. Sample lo- fractions with the least evidence for recent Pb action the temperature of this sample probably cations are also shown in Figures 5 and 8. loss and the most and least evidence of inheri- did not exceed 475 ¡C. tance, respectively. The uncertainty is assigned From five sphene analyses we have obtained a U-Pb Zircon Upper Age Limit on rather than calculated, and is based on our esti- spheneÐwhole-rock 238U/206Pb isochron for this Metamorphism (Sample 151P) mation of the maximum range that the lower in- sample (Fig. 12B). The low concentration of U tercept could vary due to any remaining Pb-loss in sphene in this sample (15 to 74 ppm, Table 3), Sample 151P is from a foliated granitic dike domains within the analyzed abraded grains. The which is typical of low-grade metamorphic that intrudes Cambrian strata in the hanging wall inherited Pb component is at least 1753 Ma sphene, resulted in low radiogenic to initial Pb of the Independence thrust in the Pequop Moun- (207Pb/206Pb age of 4) and is possibly as old as ratios and difficulty in extracting useful age esti- tains (Fig. 5). The dike and enveloping Cam- Early Archean. The crystallization age of the dike mates from the 235U/207Pb data, so neither con- brian strata are deformed into a prograde meta- places an upper limit of 154 ± 5 Ma on the incep- cordia diagrams nor 207Pb/206Pb isochron plots

morphic biotite zone S-tectonite (S1). Low-grade tion of metamorphism. are presented. There is a general trend of in- 238 204 (biotite zone) conditions for development of S1 creasing U/ Pb with decreasing grain size in in the dike are reflected in the observations that Temperature-Time Path for the Pequop this sample. This trend is observed even in the much of the feldspar in the dike has reacted to Mountains two fractions that were air abraded (4 and 6, white mica and that igneous biotite has reacted Table 3) and suggests to us that the U concentra- to chlorite. There is no evidence for synmag- The temperature-time path for the Pequop tion of the sphene-producing fluids evolved dur- matic deformation; so the age of the dike places Mountains (Fig. 13) is derived from U-Pb and ing metamorphism. A linear regression of the an upper limit on the timing of metamorphism. 40Ar/39Ar data from garnet and biotite zone sphene data alone, which includes a range of an- Pb and U data from five analyzed zircon frac- Cambrian units in the footwall of the Indepen- alyzed grain sizes, agrees with the spheneÐ tions (Table 2) plot in a nonlinear array that dence thrust. 40Ar/39Ar data from muscovite in whole-rock data within error (Fig. 12B), and is trends from ca. 154 Ma to ca. 2400 Ma metapelite indicate that these rocks cooled interpreted as evidence that the system has re- (Fig. 12A). Zircon grains in this sample are gen- through ≈300 ¡C at 75 Ma (Thorman and Snee, mained closed since the time of sphene growth. erally colorless, euhedral, elongate, small 1988). We dated metamorphic sphene (sample Thus, the slope of the spheneÐwhole-rock mix- (120 µm × 30 µm) and clear, although some of 127AP) from a metacarbonate to assess the ing line (Fig. 12B) is interpreted to reflect the the grains contain visible cloudy white cores. The higher temperature history of these rocks. age at which the system closed to Pb diffusion. data are interpreted to reflect mixtures of zircon Sample 127AP is derived from the Cambrian On the basis of experimental estimates of dif- that grew at ca. 154 Ma mixed with varying Toano Limestone (Figs. 6 and 5) and contains fusion parameters for Pb in sphene (Cherniak, amounts of inherited Pb. Grains for four of the the assemblage BI-CC-HNB-Q-DO-CHL-SP. 1993), the lowest closure temperatures for

fractions were air abraded (Krogh, 1982) to The rock contains a foliation (S1) and a lineation sphene in this sample range from 526 ¡C to smooth ellipsoids to minimize the effects of re- (L1) defined by hornblende and biotite. Sphene 577 ¡C for geologically reasonable cooling rates cent Pb loss, and to accentuate the presence of is typically included within or adjacent to horn- between 2 ¡C/m.y. and 40 ¡C/m.y., respectively

TABLE 2. U-Pb DATA No. Sample Weight U Pb comPb Corrected atomic ratios 206Pb/238U 207Pb/235U 207Pb/206Pb Rho (mg) (ppm) (ppm) (ppm) 206Pb/204Pb 206Pb/238U 207Pb/235U 207Pb/206Pb Age Age Age (corr.) (rad.) %err (rad.) %err (rad.) %err (Ma) (Ma) (Ma) 151P Granitic dike—Pequop Mountains 154 ± 5 Ma 1 Z d-1 0.750 245 6.8 0.54 752 0.0256 0.25 0.2143 0.14 0.0608 0.13 163 197 631.7 ± 28 0.54 2 Z d-1 aa I 0.140 617 15.4 0.17 5709 0.0249 0.23 0.1833 0.63 0.0534 0.55 158 171 347.3 ± 13 0.50 3 Z d-1 aa II 0.060 381 10.2 0.02 37974 0.0266 0.23 0.2227 0.56 0.0608 0.48 169 204 631.9 ± 10 0.51 4 Z d-1 aa 0.023 527 28.7 0.25 6847 0.0515 0.25 0.7618 0.42 0.1073 0.33 324 575 1753.5 ± 6 0.63 w/cores 5 Z d-1 aa IV 0.140 267 6.7 0.17 2459 0.0245 0.23 0.1722 0.73 0.0509 0.67 156 161 236.0 ± 15 0.43

97c Metacarbonate, Roberts Mountain Formation—Wood Hills 75 ± 1 Ma 6 S nm5 II 0.303 642 10.6 0.30 177 0.0116 0.16 0.0768 0.58 0.0478 0.53 74.6 75.1 91.7 ± 11 0.43 0.15 mm 7 S nm5 III 0.900 1082 17.4 0.45 198 0.0118 0.24 0.0767 0.57 0.0473 0.49 75.4 75.0 64.5 ± 10 0.50 0.15 mm 8 S nm5 IV 0.160 929 15.1 0.42 185 0.0117 0.26 0.0777 0.18 0.0481 0.17 75.1 76.0 102.5 ± 41 0.52 0.10 mm Notes: Sample: Z (zircon), S (sphene), d and nm (angles of diamagnetic and paramagnetic susceptibility on a barrier style Franz separator), aa (air-abraded fraction), and mm (diameter in millimeters). ComPb: common Pb concentration corrected for blank. 206Pb/204Pb (corr.): 206Pb/204Pb corrected for blank and mass discrimination. %err: 2 sigma in percent.

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A B calc-silicate equilibria, and because data from the Pequop Mountains yield a metamorphic peak ca. 84 Ma, we infer a similar timing of peak metamorphism for the Wood Hills (Fig. 13).

Synthesis of Thermochronologic Data

The thermochronologic data are interpreted to indicate that Barrovian metamorphism began af- ter 154 ± 5 Ma and peaked at ca. 84 Ma (Late Cretaceous). The data also indicate that the Inde- pendence thrust and Pequop fault must postdate C 84 Ma because both cut the metamorphic fabric. Following peak metamorphism, rocks at all Figure 12. (A) U-Pb concordia dia- structural levels underwent cooling at moderate gram of zircon analyses from sample rates from the Late Cretaceous to Tertiary 151P. (B) U-Pb spheneÐwhole-rock iso- (Fig. 13). Apparent cooling rates derived from chron is from sample 127AP. (C) U-Pb the thermochronologic data range from 14Ð19 concordia diagram of sphene analyses and 13Ð15 ¡C/m.y. for shallow and intermediate from sample 97C. structural levels, respectively.

REGIONAL STRUCTURAL AND METAMORPHIC SYNTHESIS

(calculated with an effective radius of 0.025 mm, orless one. Much of the color in the light amber We incorporate our new observations and data the smallest observed). Because the metamor- grains is heterogeneous and may be staining, so from the Wood Hills and Pequop Mountains re- phic temperature in sample 127AP probably did these grains may be a variant of the colorless gion with data from the Ruby MountainsÐEast not exceed ≈475 ¡C, the sphene grew below its ones. However, the reddish-brown population is Humboldt Range to establish a regional struc- closure temperature, and we interpret the distinct; the grains exhibit an unusual dispersion tural and metamorphic framework. This frame- spheneÐwhole-rock isochron age of 84.1 ± of plane-polarized light and contain a few weight work provides the basis for reconstructing the 0.2 Ma (Fig. 12B) to be the timing of metamor- percent aluminum. The reddish-brown color, metamorphic and structural history of the Meso- phism rather than a younger cooling age. high aluminum content, and distorted crystal zoic middle to upper crust in the next section. In The combined U-Pb and 40Ar/39Ar data from structure are all consistent with growth at high this section, we first correlate metamorphic the Pequop Mountains yield a temperature-time pressure during metamorphism. The reddish- events in the metamorphosed section and then curve with a metamorphic peak of ≈425-475 ¡C brown population makes up approximately one- summarize the evidence for thrust faulting. We (inferred temperature of the garnet-in isograd) at half of all sphene grains. There are no core-over- then correlate and discuss extensional events. 84.1 ± 0.2 Ma followed by cooling through growth relationships between any of these ≈300 ¡C at 75 Ma (Fig. 13). populations. Fractions were picked separately to Mesozoic Metamorphism test for any age differences; however, only the Temperature-Time Path for the Wood Hills reddish-brown population had sufficient ura- The metamorphic section extends from the nium to produce age information. Three frac- Wood Hills into the Ruby MountainsÐEast Hum- The temperature-time path for the Wood Hills tions of reddish-brown sphene were analyzed. boldt Range beneath klippen of the Mary’s River (Fig. 13) is derived from U-Pb and 40Ar/39Ar The sphene data plot close to concordia and in- fault system (Figs. 3 and 4). The Cretaceous Bar- data from tremolite and diopside zone rocks in dicate a date of 75 ± 1 Ma (Fig. 12C and rovian metamorphic pattern developed in the the Wood Hills. 40Ar/39Ar data from biotite and Table 2). Sphene grains are ~0.10 to 0.15 mm in Wood Hills and Pequop Mountains region ex- muscovite in metapelite indicate that these rocks size; thus the closure temperature for Pb diffu- tends into the northern Ruby Mountains and East cooled through ≈300 ¡C between 56 and 47 Ma sion is in the range of 550Ð600 ¡C for reasonable Humboldt Range, but is obscured in the south- (Thorman and Snee, 1988). In order to assess the cooling rates of 2 and 40 ¡C/m.y., respectively. ernmost Ruby Mountains. We have extended the cooling history at higher temperatures we dated Because this sample is from the northwestern tremolite-in and diopside-in isograds from the metamorphic sphene (sample 97C) from a part of the diopside zone (Fig. 8), where equilib- Wood Hills into the southern part of the East metacarbonate. rium assemblages suggest a metamorphic tem- Humboldt Range. The extensions of these iso- Sample 97C is from the Silurian Roberts perature of ≈620Ð640 ¡C, we interpret this date grads are based on Taylor’s (1981) descriptions Mountains Formation and contains an equilib- as a cooling age that records cooling through of rocks and metamorphism in the southern East rium assemblage of CC-DO-DI. This sample ≈580 ¡C. Humboldt Range, which are similar to those 40 39 contains S1 and L1, which is defined by diopside The combined U-Pb and Ar/ Ar data for described herein for the Wood Hills. Because porphyroblasts. Sphene occurs in trace amounts the Wood Hills yield a temperature-time curve Mesozoic sillimanite is reported in the northern and is typically included within or adjacent to that documents cooling through ≈580 ¡C at Ruby Mountains and northernmost East Hum- diopside porphyroblasts. There are three distinct 75 Ma followed by cooling through ≈300 ¡C at boldt Range, we have placed a sillimanite-in iso- populations of sphene in this sample, a reddish- 56Ð47 Ma (Fig. 13). We assumed a peak meta- grad between these areas and the southern East brown population, a light amber one, and a col- morphic temperature of ≈620 ¡C on the basis of Humboldt Range and the Wood Hills (Fig. 4).

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graphic depth of ≈11 km (Fig. 2A), or a pressure of ≈3 kbar (Fig. 7) indicating that metamor- phism records a substantial pressure increase of ≈5 kbar (see curve NEHR in Fig. 7). McGrew (1992) reported that anatectic granite in this re- gion is constrained to be 70Ð90 Ma (J. E. Wright, unpublished data cited by McGrew [1992] and McGrew and Snee [1994]) indicating a Late Cretaceous peak metamorphism (Fig. 13). We correlate the metamorphic event in the East Humboldt Range with that in the Wood HillsÐ Pequop Mountains region because both are Bar- rovian style and are of similar age. Moreover, both involved pressure increases. Regional metamorphism in the southernmost Ruby Mountains differs from the classic Barro- vian sequence in the Pequop Mountains, Wood Hills, and East Humboldt Range in that Meso- zoic prograde metamorphism overlapped in time with emplacement of a Late Jurassic (ca. 153 Ma) pluton (Hudec, 1992; Fig. 4). Hudec (1992) recognized two progressive regional- contact metamorphic events that define a coun- terclockwise P-T path (curve SRM in Fig. 7). The earlier event is characterized by andalusite in metapelite and occurred at a pressure <3.8 kbar. This event was followed by an increase in pres- sure to 4Ð4.7 kbar and temperature of 560Ð630 ¡C and growth of sillimanite in meta- Figure 13. Temperature-time paths for various structural levels in the footwall of the Win- pelite (Fig. 7). Hudec (1992) estimated a thermal dermere thrust. The deep curve represents rocks in the northern East Humboldt Range and is gradient of 35Ð60 ¡C/km for the final phase of based on data from McGrew (1992) and McGrew and Snee (1994). The intermediate curve rep- metamorphism, and his interpretation of contact resents the Wood Hills, and the shallow curve, the Pequop Mountains. The intermediate and metamorphism is reinforced by comparison with shallow curves combine our U-Pb sphene data with Thorman and Snee’s (1988) 40Ar/39Ar data the Pequop Mountains terrain, which was proba- from micas. Note that the “deep” curve in this figure corresponds to the NEHR (B) curve on the bly at similar pressure, but much lower tempera- P-T plot in Figure 7, and the “shallow” curve in this figure to the NPM curves on the pressure- ture and metamorphic grade. Because there is a temperature (P-T) plot. The “intermediate” curve in this figure represents a structurally deeper continuity of metamorphic section from the high level than the SWH curve on the P-T plot in Figure 7. P-T Cretaceous Barrovian metamorphism in the East Humboldt Range to the low-PÐhigh-T Juras- sic contact metamorphism in the southern Ruby We correlate the Cretaceous Barrovian meta- growth of sillimanite (McGrew, 1992; Snoke, Mountains (Hudec, 1992), we envision that dur- morphic event in the Wood Hills and Pequop 1992; Hodges et al., 1992). Thermobarometric ing the Cretaceous the southern Ruby Mountains Mountains with the earliest phase of metamor- data from Proterozoic schist indicate that the were at a pressure and possibly temperature sim- phism recorded in the northern East Humboldt peak pressure and temperature recorded by silli- ilar to those of the Pequop Mountains. The pres- Range. The earliest metamorphic event in the manite-bearing assemblages are ≈8.6 kbar and ence of Late Jurassic contact metamorphism in northern East Humboldt Range is represented by 780 ¡C (McGrew, 1992; Peters and McGrew, the southern Ruby Mountains locally obscures growth of kyanite and staurolite in metapelite, 1994). We estimate that the Proterozoic schist, the regional Cretaceous Barrovian metamorphic which was then followed by partial melting and prior to metamorphism, was probably at a strati- pattern.

TABLE 3. U-Pb DATA FROM 127AP No. Sample Weight U Pb Corrected atomic ratios Rho Rho Rho Rho (mg) (ppm) (ppm) 206Pb/204Pb 207Pb/204Pb 208Pb/204Pb 238U/204Pb 235U/204Pb 6/4-7/4 6/4-8/4 αеβÐν %err %err %err %err 127AP Metacarbonate, Toano Limestone—Peqoup Mountains 84.1 ± 0.23 Ma (MSWD = 1.39) 1 whole rock 0.81 0.8 0.5 0.19 0.12 0.15 0.18 0.40 0.24 0.975 0.50 0.0707 0.998 0.997 0.245 0.247 2 S nm10 0.40 mm 0.52 0.1 0.1 0.29 0.14 0.16 0.10 0.39 0.10 0.72 0.29 0.5222 0.804 0.786 0.444 0.887 3 S nm.5a 0.35 mm 0.23 0.4 0.3 0.31 0.43 0.16 0.37 0.40 0.41 0.94 0.76 0.6831 0.835 0.800 0.506 0.859 4 S nm10 aa 0.25 mm 0.61 0.3 0.2 0.35 0.10 0.16 0.32 0.40 0.35 0.11 0.19 0.8691 0.624 0.480 0.526 0.986 5 S nm.5a 0.20 mm 0.51 0.3 0.2 0.33 0.27 0.16 0.41 0.39 0.40 0.10 0.63 0.7402 0.763 0.686 0.724 0.998 6 S nm10 aa 0.05 mm 0.43 0.7 0.4 0.39 0.63 0.16 0.25 0.40 0.29 0.14 0.12 0.1081 0.573 0.461 0.436 0.968 Notes: Sample: S (sphene), nm (angles or amperage [a] of paramagnetic susceptibility on a barrier style Franz separator), aa (air-abraded fraction), mm (diameter in millimeters). Corrected ratios: measured values corrected for blank and mass discrimination. %err: 2 sigma in percent.

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Several observations about the Cretaceous Thrust Faulting the hanging wall of the Windermere thrust were metamorphism suggest that this metamorphic originally thrust to the east or southeast and then event was largely due to structural burial and sub- Structural and metamorphic data indicate that normal faulted back to the west or northwest. A sequent thermal relaxation (e.g., Thompson and at least two episodes of thrust faulting occurred top-to-the-southeast sense of slip for the Win- England, 1984; England and Thompson, 1984), in this region: an earlier episode that duplicated dermere thrust is consistent with Mesozoic and only a small component of heat was due to the miogeocline and resulted in burial and meta- shortening directions elsewhere in northeast magmatism. First, the variations in metapelitic morphism, and a younger episode that involved Nevada (e.g., Hudec, 1992; Miller and Hoisch, and calc-silicate assemblages indicate that there is development of the Independence thrust. We 1992, 1995). a consistent northwest increase in both metamor- emphasize that the Independence thrust must phic grade and pressure for a given stratigraphic postdate the earlier episode of thrust faulting, be- Extensional Features unit (Fig. 4). Pressure increases from ≈5Ð6 kbar in cause it deforms the prograde metamorphic fab- the Wood Hills to ≈8.6 kbar in the northern East ric. Moreover, the earlier episode of thrust fault- Two generations of top-to-west to northwest Humboldt Range, indicating a depth dependence ing is not documented by the presence of a low-angle normal faults are responsible for ex- of the distribution of metamorphic facies and iso- mapped thrust fault, but rather is inferred from humation of the footwall of the Windermere grads. Second, at all structural levels metapelites structural and barometric data. In this section we thrust: the Tertiary Mary’s River fault system and record apparent clockwise P-T paths (Fig. 7), and summarize the evidence for the early episode of its associated mylonite zone, and the pre-41 Ma the apparent thermal gradient during metamor- thrust faulting and infer a thrust fault called the Pequop fault. Our age bracket of 84 to 41 Ma for phism does not appear to be excessive, consistent Windermere thrust. the Pequop fault is similar to the timing of a de- with thermal relaxation. Using the P-T estimates The main evidence for the first episode of compression event in the northern East Hum- of 8.6 kbar and 780 ¡C for the northern East Hum- thrust faulting is the repetition of the miogeocli- boldt Range. Barometric and thermochronologic boldt Range and 5.5Ð6.4 kbar and 540Ð590 ¡C for nal section across all of the low-angle normal data from the structurally deepest part of the the Wood Hills results in relatively low estimated faults, which requires that the miogeocline was northern East Humboldt Range indicate that, fol- gradients of ≈24 ¡C/km and ≈23Ð29 ¡C/km, re- duplicated by thrust faulting prior to normal lowing peak metamorphism at 70Ð90 Ma, rocks spectively. Third, the duplication of the miogeo- faulting. Furthermore, barometric data from the underwent decompression and cooling along a clinal section across the low-angle normal faults metamorphosed section require that strata were relatively straight P-T path from 8.6 kbar and requires that the metamorphosed section was buried to depths that greatly exceed their pre- 780 ¡C to 5.5 kbar and ≈689 ¡C and then cooled structurally buried. All of these observations are metamorphic stratigraphic depths (Fig. 7). The through 480Ð580 ¡C at 63Ð49 Ma (McGrew, consistent with metamorphism due to tectonic pressure or burial depth of the metamorphosed 1992; Peters and McGrew, 1994; McGrew and burial and thermal relaxation, but they do not pre- section increases to the northwest, and hence this Snee, 1994; McGrew, 1996, personal commun.; clude a contribution of heat due to magmatism at pressure gradient requires that the metamor- see P-T path NEHR-B in Fig. 7 and T-time path depth. On the basis of McGrew’s (1992) recogni- phosed section once lay beneath a southeast- for deep structural level in Fig. 13). The timing of tion of Cretaceous partial melting at deep struc- tapering wedge of thrusted material that in- decompression is therefore bracketed between tural levels, some component of heat from mi- cluded miogeoclinal strata. The thrust fault(s?) 70Ð90 Ma and 63Ð49 Ma (McGrew, 1992; Mc- grating middle to lower crustal partial melts responsible for duplication of the miogeocline Grew and Snee, 1994). Hodges et al. (1992) also probably contributed to metamorphism. Nonethe- and burial metamorphism is not exposed and postulated a decompression event on the basis of less, we envision that metamorphism is largely a must have been excised during extension along barometric data from a structurally shallower byproduct of tectonic burial and that burial proba- the Pequop and Mary’s River fault systems. level of the East Humboldt Range, and their data bly induced partial melting in the deep crust re- Windermere Thrust. Because structural and suggest final equilibration at 5.0Ð6.4 kbar and sulting in rising melts that transferred and con- barometric data require that the metamorphosed 550Ð630 ¡C (curve NEHR-A in Fig. 7), similar to tributed some heat necessary for mid-crustal section must have composed the footwall of a that derived by McGrew (1992). Hodges et al. metamorphism. thrust fault, we name this thrust the Windermere (1992) and McGrew and Snee (1994) both attrib- On the basis of our U-Pb data, we infer that thrust (the name “Windermere” was chosen out uted the decompression event to extension, but Barrovian burial metamorphism in northeast of necessity; it is one of the few geographic Hodges et al. (1992) preferred a Cretaceous age, Nevada began sometime after 154 ± 5 Ma and names in the region that have not been appended and McGrew and Snee (1994) preferred an early peaked ca. 84 Ma (Fig. 13). Emplacement of the to something geologic). Unmetamorphosed Pa- Tertiary age. Nonetheless, we interpret the Pe- tectonic load responsible for metamorphism leozoic and Mesozoic strata currently in the quop fault and the decompression event to be must have preceded peak metamorphism at hanging walls of the low-angle normal faults products of the same extensional event because 84 Ma, and data from the southern Ruby Moun- represent remnants of the former hanging wall of they share similar age constraints. tains indicate that tectonic burial began by the Windermere thrust. 153 Ma (Hudec, 1992). Thus we bracket the tim- A southeast taper to the hanging wall of the MESOZOIC TO TERTIARY TECTONIC- ing of tectonic loading and subsequent meta- Windermere thrust suggests that transport of the METAMORPHIC RECONSTRUCTION morphism as 154Ð84 Ma. Exactly when the bulk hanging wall was to the southeast, and we infer of the tectonic load was generated is unknown, a top-to-the-southeast sense of slip for the Win- Using the metamorphic and structural data, but because relaxation of isotherms and conse- dermere thrust. This inference is further sup- we present a tectonic reconstruction to illustrate quent metamorphism following loading may ported by stratigraphic constraints. Unmetamor- the approximate original geometries of the Win- take place over several millions to tens of mil- phosed remnants of the hanging wall of the dermere and Independence thrusts, and their lions of years (e.g., England and Thompson, Windermere thrust compose a western miogeo- subsequent exhumation by the low-angle normal 1984), it is likely that much of the loading sig- clinal section, whereas the footwall of the Win- faults. A northwest-southeast line of section nificantly preceded peak metamorphism at dermere thrust composes a more eastern mio- (A-A′; Fig. 3) was chosen for reconstruction be- 84 Ma. geoclinal sequence. This requires that rocks in cause it transects maximum exposure of the

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footwall of the Windermere thrust and the low- angle normal faults, and it is roughly parallel to shortening and extension directions. The recon- struction is divided into five stages, A through E (Fig. 14), which represent the present (A) to the Mesozoic (E). The following data and assump- tions were used in the reconstruction. 1. Barometric data from the Wood Hills and East Humboldt Range were used to constrain Mesozoic burial depths of the footwall of the Windermere thrust. The barometric data were projected approximately parallel to metamor- phic isograds into section A-A′ (projections are represented by solid circles along section A-A′ in Fig. 3). Based on the barometric data, burial depths of ≈33 km and 23 km (Fig. 7) were used; these data are shown in the restored cross section E in Figure 14. Although no barometric data ex- ist for the metamorphic section in the Pequop Mountains, this section was probably buried at least 7 km in excess of stratigraphic depths. This is based on the observation that Paleozoic rocks once composing the hanging wall of the Winder- mere thrust that are currently in the hanging wall of the Pequop fault, are exposed to a strati- graphic depth of ≈7 km. We assumed a depth of 10 km to the Windermere thrust in the Pequop Mountains (Fig. 14E), which is 3 km in excess of the minimum. This assumption appears to be reasonable because it is less than the burial depths required by barometric data in the higher grade Wood Hills (Fig. 7). 2. We correlate the unnamed thrust in the hanging wall of the Pequop fault with the Inde- pendence thrust in the footwall of the Pequop fault. The intersection of the unnamed thrust with the Pequop fault (Fig. 5, cross sections A-A′ and E-E′) provides a hanging-wall cut-off for the Pequop fault. Although the footwall cut- off is not exposed, the Independence thrust is the only visible candidate for correlation with the unnamed thrust; in fact, both of the thrusts have a similar amount of separation (Fig. 5). There- fore, we assume that the unnamed thrust is a dis- placed segment of the Independence thrust (cf. Fig. 14, B, C, and D). 3. Eocene volcanic rocks formed a regionally distributed, relatively flat lying sheet (Thorman et al., 1992) prior to dismemberment by the Mary’s River fault system (Fig. 14, cross sec- tion C). 4. The location of the initial Mary’s River fault breakaway is assumed to be approximately Figure 14. Tectonic reconstruction along line A-A′ (Fig. 3) from the present (A) to the Meso- that shown in Figures 4 and 14B. The breakaway zoic (E). Units are the same as in Figure 3 with the exception of the unmetamorphosed mio- must lie east of exposures of the Humboldt For- geoclinal section above the low-angle normal faults. In Figure 3 unmetamorphosed miogeocli- mation in the hanging wall of the Mary’s River nal strata above the low-angle normal faults are depicted with a northeast-southwest fault in the northern Wood Hills, but west of in diagonally ruled pattern, whereas in this figure they are unpatterned. Note that the cross sec- situ exposures of the Eocene volcanic rocks in tion in A does not show unit TQu between the Mary’s River Valley and the Wood Hills because the northern Pequop Mountains (Fig. 3). Our it is too thin to show at the scale of the cross section.

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placement of the breakaway above the Pequop Mountains (Figs. 4 and 14B) reflects the obser- vation that the presence of ductile fault rock (mylonite) along the Mary’s River fault in the Wood Hills suggests that the initial breakaway was well to the east of the Wood Hills. Reconstruction of the Windermere and Inde- pendence thrusts involved four steps. First, the minor high-angle normal faults in cross-section 14A were retrodeformed, and eroded units and structures were projected above the topographic surface, resulting in cross section 14B. The sec- ond step was to retrodeform the Mary’s River fault system. Retrodeformation was accom- plished by restoring the Eocene volcanic rocks to a horizontal, continuous sheet, and also involved bringing units and structures that are currently to Figure 15. Cross section of the footwall of the Windermere thrust showing distribution of the west of cross-section 14B into the retrode- metamorphic facies prior to formation of the Independence thrust. Note that the Paleozoic sec- formed cross-section in 14C. The third step in tion is thinner in the footwall of the Windermere thrust. This is because units were ductilely at- restoration involved removal of the volcanic tenuated during prograde metamorphism (Camilleri, 1994). rocks and restoring units and structures eroded prior to the deposition of the volcanic rocks (Fig. 14D). The fourth step, retrodeformation of the Pequop fault, involved matching the offset segments of the Independence thrust (Fig. 14, cross sections D and E). Minimum estimates of Mesozoic shortening morphic isograds extended into the hanging wall gional relations permit speculation. The surface can be derived from cross-section 14E, which of the Windermere thrust, consistent with relax- trace of the thrust must have been situated to the shows the Windermere and Independence thrusts ation of isotherms following burial. east of the Pequop Mountains, but probably west just prior to extension. The Independence thrust of the Toano and Pilot ranges (Fig. 1). The Toano accommodated ~4 to 5 km of shortening (heave) Windermere Thrust Geometry and and Pilot ranges expose a miogeoclinal sequence along its up-dip portions. This estimate does not Regional Extent that is stratigraphically similar to the sequence include the amount of shortening accommodated composing the footwall of the Windermere by back folds and thrusts. An estimate of the Although the structural and metamorphic data thrust in the Wood Hills and Pequop Mountains; amount of shortening accommodated by the require that the hanging wall of the Windermere however, the sequence in the Toano and Pilot Windermere thrust can be made by measuring thrust was wedge shaped and tapered to the ranges was not tectonically buried during the the line length along the Windermere thrust be- southeast, its internal structural geometry is un- Mesozoic (Miller and Hoisch, 1992, 1995). Thus tween points Z′ and Z″. Point Z′ is the vertical known (Fig. 14E). The structural data indicate we envision the Mesozoic trace of the thrust to projection of point Z, which represents the east- that at least one miogeoclinal section was dupli- have extended to the north between the Pequop ernmost known occurrence of rocks composing cated across the Windermere thrust. However, Mountains and Pilot and Toano ranges (Fig. 1). the hanging wall of the Windermere thrust. Point more structural overburden than can be derived To the southwest of the Pequop Mountains, the Z″ is the projection of the footwall of the Win- by just one duplication of the miogeoclinal sec- surface trace of the Windermere thrust probably dermere thrust from the northern East Humboldt tion is required to achieve the burial depths in the turned westerly to a southwest orientation, as Range into the line of section. The line length be- structurally deeper part of the footwall of the suggested by a decrease in pressure recorded by tween Z′ and Z″ is ≈69 km. Some of this length Windermere thrust. It is possible that burial was footwall rocks in the southern Ruby Mountains. may be due to ductile attenuation of the footwall accomplished by additional, structurally higher The extent of the Windermere thrust to the south during metamorphism, which would lead to an thrust faults that imbricated more than one Pale- of the Ruby Mountains is unknown. overestimation of the amount of shortening by a ozoic section. Alternatively, the Windermere few kilometres; however, we have not factored thrust may be solely responsible for burial. If this Thermochronologic Constraints on the this in because the lengths that the Windermere is the case, then the hanging wall of the Winder- Timing of Faulting thrust extends beyond points Z′ and Z″ are un- mere thrust would have consisted of one Paleo- known. Thus, 69 km is the best estimate of the zoic section and a significant amount of base- The thermochronologic data provide con- amount of shortening accommodated by the ment above the structurally deeper part of the straints on the timing of thrust and normal fault- Windermere thrust. footwall. A detailed structural study of regional ing. Our age constraint of 154Ð84 Ma (Fig. 13) Figure 15 shows a cross section of the foot- exposures of rocks once composing the hanging on tectonic burial (loading) of the footwall of the wall of the Windermere thrust and the distribu- wall of the Windermere thrust is required to de- Windermere thrust brackets the age of the Win- tion of metamorphic facies just prior to forma- termine which scenario is valid. dermere thrust. Our data indicate that the Inde- tion of the Independence thrust. As shown in the The areal extent of the Windermere thrust in pendence thrust and younger Pequop fault both cross section, we envision that at depth the meta- the Sevier hinterland is unknown, however, re- formed between 84 and 41 Ma. This age bracket

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elucidates some regional differences. The 84 Ma peak of metamorphism in northeastern Nevada overlaps temporally with 70Ð90 Ma (Miller and Gans, 1989) peak metamorphism to the south in east-central Nevada, but the two styles of meta- morphism are different. In east-central Nevada, regional metamorphism is attributed to proxim- ity of Late Cretaceous plutons (Miller and Gans, 1989), whereas in northeast Nevada it can be as- cribed to tectonic burial. Our data suggest that crustal thinning, accom- modated by the Pequop fault, in northeast Nevada began at some time between ca. 84 and 75 Ma. Early or Late Cretaceous extension has been documented elsewhere in the Sevier hinter- land, and is attributed to overthickening of the crust resulting in gravity-driven extension (Wells et al., 1990; Wells, 1992; Hodges et al., 1992; Hodges and Walker, 1992). The age constraint of 84Ð75 Ma on extension in northeast Nevada overlaps temporally with intrusion of Late Cre- taceous (ca. 83Ð75 Ma) granites throughout the hinterland, including northeast Nevada, that are regarded as products of melting of the crust due to crustal thickening (Miller and Gans, 1989; Figure 16. Schematic reconstruction of the Sevier orogen in northern Nevada and Utah. Note Patino Douce et al., 1990; Wright and Wooden, that emplacement of late PaleozoicÐearly Mesozoic terranes (dark gray) predates, and is unre- 1991). However, our age constraints on exten- lated to, development of the Sevier orogen. sion bring into question whether some of the Late Cretaceous partial melting of crust could be in part due to decompression, as well as crustal thickening.

Evolution of the Sevier Orogen in Northeast can be further narrowed to 84 to 63-49 Ma by DISCUSSION Nevada and Northeast Utah considering that the decompression event in the East Humboldt Range, which we attribute to the Our structural and chronologic data yield im- We integrate our results with data from other Pequop fault, occurred at or before 63Ð49 Ma portant new constraints on the Mesozoic evolu- parts of the hinterland and foreland to present a (McGrew and Snee, 1994). Partial exhumation tion of the Sevier hinterland in northeast Nevada. series of schematic cross sections (Fig. 16) de- of the metamorphic rocks by the Pequop fault Previously, contractional deformation and meta- picting the evolution of the Sevier orogenic belt should ultimately result in cooling, and we inter- morphism in this region were perceived to be in northern Nevada and northeastern Utah from pret the Late Cretaceous to early Tertiary cooling largely Late Jurassic, with metamorphism being the Late Jurassic to Late Cretaceous (Fig. 16). paths at all structural levels (Fig. 13) to be a re- ascribed to regional contact metamorphism adja- For simplicity, in our cross sections we have as- sult of extension along the Pequop fault. The ear- cent to syntectonic plutons. Because no middle sumed that the hanging wall of the Windermere liest record of cooling is at 75 Ma; therefore, we to upper crustal Cretaceous normal or thrust thrust is not internally deformed, and we have interpret the formation of the Pequop fault to faults were known to exist in this region, the Cre- adapted and simplified Sevier thrust belt recon- have occurred ≥75 Ma but after peak metamor- taceous appeared to be relatively quiescent with structions from DeCelles (1994) and Yonkee et phism ca. 84 Ma (Fig. 13). Slip along the Pequop respect to faulting, although extension during al. (1992). fault could be entirely Late Cretaceous in age, in this time was postulated on the basis of baromet- By ca. 153 Ma (Late Jurassic) substantial which case early Tertiary cooling would be pri- ric data (Hodges et al., 1992). Our data reveal shortening had occurred adjacent to the mag- marily due to reequilibration of thermal gradi- Late Cretaceous metamorphism related to tec- matic arc in western Nevada (Oldow, 1984; ents following extension. Alternatively, slip tonic burial, thrust, and normal faulting in this Speed et al., 1988; Fig. 16A). Eastern Nevada along the Pequop fault and cooling of its foot- region. underwent regional contact metamorphism adja- wall could be protracted, extending from the cent to sparse 165Ð153 Ma syntectonic plutons Late Cretaceous to the early Tertiary, but prior to Regional Implications for Hinterland and minor crustal shortening and thickening dur- ca. 49 Ma. We emphasize that although the initi- Tectonics, Metamorphism, and Plutonism ing the Late Jurassic (Miller et al., 1987; Glick, ation of slip on the Pequop fault is bracketed be- 1987; Snoke and Miller, 1988; Hudec, 1992; tween ca. 84 Ma and ca. 75 Ma, the Indepen- Our recognition of substantial Cretaceous Miller and Hoisch, 1992, 1995; Fig. 16A). Our dence thrust also shares this same age constraint metamorphic and tectonic events in northeast chronologic data do not establish the precise tim- because it cuts the metamorphic fabric, but pre- Nevada indicates contemporaneity of similar ing of the development of the Windermere thrust dates the Pequop fault. events throughout the Sevier hinterland, but also between 154 and 84 Ma, but the data indicate that

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crustal thickening in the hinterland had reached a wall, and peak metamorphism occurred ca. spiked with a mixed 208Pb/235U tracer. Pb and U from maximum by 84 Ma, culminating in Barrovian- 84 Ma. Between 84 and 75 Ma another minor zircon samples were purified by HCl chemistry anion ≥ column separations modified from Krogh (1973). Pb style burial metamorphism of the middle crust pulse of shortening ( 5 km) along the Indepen- from sphene samples was purified by HBr-HCl anion (Fig. 16B). By this time Sevier foreland thrusts dence thrust was followed by as much as 10 km column chemistry after Tilton (1973), and U was puri- were well developed (Fig. 16B). Because the old- of crustal thinning along the Pequop normal fied from the HBr washes by using the same HCl est Sevier foreland thrust system (Parris-Willard fault. Regional erosion, and possibly residual ex- chemistry as for zircon samples. Laboratory Pb blanks improved during the period of this research from system; Fig. 1) is constrained to be Early Creta- tension along the Pequop fault, occurred from 75 100 pg to less than 10 pg for zircon and from 150 pg to ceous (ca. 119Ð113 Ma [Aptian]; Heller et al., to 41 Ma. Between 41 Ma and recent, extension less than 30 pg for sphene procedures. U blanks re- 1986) and therefore formed within the age brack- along the Mary’s River fault system resulted in mained constant for both at less than 5 pg. Pb was ets for the Windermere thrust, it is unknown exhumation of structurally shallow to deep lev- loaded onto rhenium filaments using silica gelÐphos- whether the Windermere thrust formed prior to, els of the Mesozoic crust. phoric acid technique (Cameron et al., 1969); U was loaded onto Re with graphite and run as a metal. Iso- after, or contemporaneously with, the inception The results of this study provide important topic measurements were performed on a VG Sector of foreland thrusting. new constraints on the evolution of the Sevier mass spectrometer in multiple collector mode. Mass- The interval from 84 to 75 Ma in the hinterland hinterland. The data indicate that in the Sevier discrimination factors for Pb and U were determined marks a fundamental, and apparently permanent, hinterland in northeast Nevada (1) much of the by multiple analyses of NBS SRM 981 and U-500, re- spectively, and were 0.048% ± 0.06%/amu for Pb and change from horizontal contraction to extension metamorphism and at least some of the thrust 0% ± 0.06%/amu for U. PBDAT by Ludwig (1988a) in the upper to middle crust. The 84Ð75 Ma Inde- faulting is Cretaceous in age, rather than Late was used to reduce the raw mass spectrometer data, pendence thrust represents the last, albeit minor, Jurassic; (2) widespread Cretaceous metamor- correct for blanks, and calculate uncertainties. Initial vestige of contraction (Fig. 16C), and is the only phism is largely a byproduct of tectonic burial Pb corrections for zircon (sample 151P) and sphene documented Late Cretaceous thrust in the north- rather than contact metamorphism; and (3) thrust (sample 97C) data were made by using Stacey and Kramers (1975) model values for 150 Ma and mea- eastern part of the hinterland. Following defor- faulting was at least partially coeval with fore- sured ore values from Oligocene deposits at Gold Hill mation associated with the Independence thrust, land thrusting, indicating that thrust faults in the (Stacey and Zartman, 1978), respectively. Two sigma but prior to 75 Ma, thinning of hinterland crust Sevier orogen do not completely form a fore- uncertainties for Pb/U and Pb/Pb values are given in began along the Pequop fault (Fig. 16D). The last land-younging sequence. Tables 2 and 3. Concordia intercepts and isochron ages were calculated by ISOPLOT (Ludwig, 1988b). pulse of shortening and the switch to extension in the hinterland was synchronous with a major epi- ACKNOWLEDGMENTS REFERENCES CITED sode of foreland shortening (84Ð75 Ma; De- Celles, 1994). Shortening continued episodically P. A. Camilleri was supported by the Geolog- Allmendinger, R. W., 1992, Fold and thrust tectonics of the western United States exclusive of accreted terranes, in Burchfiel, B. C., in the foreland from 69 to 50 Ma (DeCelles, ical Society of America, American Association Lipman, P. W., and Zoback, M. L., eds., The Cordilleran orogen: 1994). In the hinterland, the interval of time fol- of Petroleum Geologists, Sigma Xi, Wyoming Conterminous U.S.: Boulder, Colorado, Geological Society of America, The geology of North America, v. G-3, p. 583Ð607. lowing cessation of slip along the Pequop fault Geological Association, Shell Oil Company, Allmendinger, R. W., and Jordan, T. E., 1981, Mesozoic evolution, hin- ca. 75(?) Ma and deposition of the Eocene vol- U.S. Geological Survey, Chevron USA, and Na- terland of the Sevier orogenic belt: Geology, v. 9, p. 308Ð313. Allmendinger, R. W., Miller, D. M., and Jordan, T. E., 1984, Known and canic rocks beginning ca. 41 Ma must have tional Science Foundation grant EAR 87-07435 inferred Mesozoic deformation in the hinterland of the Sevier belt, involved regional erosion. The unconformity be- (to A. W. Snoke). Our project has benefited from northwest Utah, in Kerns, J. G., and Kerns, R. L., Jr., eds., Geol- ogy of northwest Utah, southern Idaho, and northeast Nevada: neath the volcanic rocks in the Pequop Moun- discussions with: B. Ron Frost, Allen McGrew, Utah Geological Association Publication 13, p. 21Ð34. Armstrong, R. L., 1968, Sevier orogenic belt in Nevada and Utah: Geo- tains transects footwall and hanging-wall strata David M. Miller, Karl Mueller, Larry Snee, logical Society of America Bulletin, v. 79, p. 429Ð458. of the Pequop fault with little relief (Fig. 5), sug- Arthur W. Snoke, and Chuck Thorman. Reviews Armstrong, F. C., and Oriel, S. S., 1965, Tectonic development of the Idaho-Wyoming thrust belt: American Association of Petroleum gesting that any footwall topographic high cre- and suggestions by Peter Crowley, Peter Vrolijk, Geologists Bulletin, v. 49, p. 1847Ð1866. ated as a consequence of slip along the Pequop and Doug Walker greatly improved this manu- Bartley, J. M., and Wernicke, B. P., 1984, The Snake Range decollement interpreted as a major extensional shear zone: Tectonics, v. 3, fault was eroded. Moreover, no synextensional script and are much appreciated. p. 647Ð657. basin fill is preserved in the hanging wall of the Berman, R. G., 1988, Internally consistent thermodynamic data for min- erals in the system Na O-K O-CaO-MgO-FeO-Fe O -Al O - APPENDIX 2 2 2 3 2 3 Pequop fault beneath the unconformity. SiO2-TiO2-H2O-CO2: Journal of Petrology, v. 29, p. 445Ð522. Berry, W. B. N., and Boucot, A. J., 1970, Correlation of the North Amer- U-Pb Sample Locations ican Silurian rocks: Geological Society of America Special CONCLUSIONS Paper 102, 289 p. Brooks, W. E., Thorman, C. H., and Snee, L.W, 1995, The 40Ar/39 Ar ages Sample 127AP (sphene, Pequop Mountains): and tectonic setting of the middle Eocene northeast Nevada volcanic The Sevier hinterland in northeast Nevada lat 40¡56′24″N, long 114¡38′10″W. field: Journal of Geophysical Research, v. 100, p. 10 403Ð10 416. Sample 151P (zircon, Pequop Mountains): Camilleri, P. A., 1994, Mesozoic and Cenozoic tectonic and metamorphic underwent at least two main phases of Meso- evolution of the Wood Hills and Pequop Mountains, Elko County, lat 40¡59′39″N, long 114¡36′45″W. zoic thrust faulting followed by two periods of Nevada [Ph.D. dissert.]: Laramie, University of Wyoming, 196 p. Sample 97C (sphene, Wood Hills): Cameron, A. E., Smith, D. H., and Walker, R. L., 1969, Mass spectrome- extension from the Late Cretaceous to Tertiary. lat 41¡01′44″N, long 114¡52′49″W. try of nonogram-size samples of lead: Analytical Chemistry, v. 41, p. 525Ð526. Mesozoic shortening was accomplished by top- Carmichael, D. M., 1978, Metamorphic bathozones and bathograds: A to-the-southeast thrust faults and extension by U-Pb Analytical Procedures measure of the depth of post-metamorphic uplift and erosion on a top-to-the-west to northwest normal faults. regional scale: American Journal of Science, v. 278, p. 769Ð797. Heavy minerals were separated from crushed sam- Carter, N. L., and Tsenn, M. C., 1987, Flow properties of the continental The first phase of thrust faulting resulted in lithosphere: Tectonophysics, v. 136, p. 27Ð63. ples for U-Pb dating by standard density and magnetic Cherniak, D. J., 1993, Lead diffusion in titanite and preliminary results ≈30 km of crustal thickening and an estimated separation techniques at the University of Wyoming. on the effects of radiation damage on Pb transport: Chemical Geol- minimum of 69 km of shortening and is brack- Grains for U-Pb analysis were free of cracks and in- ogy, v. 110, p. 177Ð194. Coats, R. R., 1987, Geology of Elko County, Nevada: Nevada Bureau of eted between 154 and 84 Ma. Crustal thickening clusions. Zircon fractions were dissolved in HF and Mines and Geology Bulletin 101, 112 p. HNO in Teflon microbombs after Parrish (1987). was accomplished either wholly or in part by the 3 DeCelles, P. G., 1994, Late CretaceousÐPaleocene synorogenic sedi- Sphene fractions were dissolved in HF and HCl in mentation and kinematic history of the Sevier thrust belt, northeast Windermere thrust. Emplacement of the hanging open beakers on a hot plate (100 ¡C). Dissolved sam- Utah and southwest Wyoming: Geological Society of America Bulletin, v. 106, p. 32Ð56. wall of the Windermere thrust resulted in Bar- ples of zircon and sphene were converted to chloride Dixon, J. S., 1982, Regional structural synthesis, Wyoming salient of the rovian-style metamorphism of rocks in its foot- solutions and aliquots of the sample solutions were western overthrust belt: American Association of Petroleum Geol-

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