Structural Geology of the Upper Plate of the Bullfrog Hills Detachment Fault System, Southern Nevada

Structural geology of the upper plate of the Bullfrog Hills detachment fault system, southern Nevada

FLORIAN MALDONADO U.S. Geological Survey, M.S. 913, Box 25046, Denver Federal Center, Denver, Colorado 80225

ABSTRACT strata; and an upper plate composed of Miocene volcanic, volcaniclastic, and sedimentary rocks. Rocks of the upper plate are widespread, but An extremely distended terrane containing two detachment faults exposures of lower- and middle-plate rocks are limited to poor scattered and an overlying complex of normal faults is exposed in the Bullfrog outcrops. Exposures of the detachment faults are also poor; however, the Hills, southern Nevada. Shallow crustal rocks have been extended geometry of the detachment faults is inferred to be low angle from several along the detachment faults by listric and planar-rotational normal measured exposures and from fault trace patterns (Fig. 2). The depths to faults. The detachment faults define three structurally discordant the detachment faults are projected and inferred from surface exposures plates. The lower detachment fault separates a lower plate of meta- and are queried in the geologic sections (Fig. 3). Although exposures of the morphosed Late Proterozoic rocks from an overlying middle plate, upper-plate faults merging with or truncated by the detachment faults are composed of slivers of lower and middle Paleozoic clastic and carbon- poor, exposed geologic and geometric relationships strongly indicate merg- ate rocks. The middle-plate rocks are brecciated and essentially ing or truncation of upper-plate faults. unmetamorphosed, and the stratigraphic succession is incomplete and The presence of a low-angle fault in the Bullfrog Hills has been highly attenuated. The upper detachment fault separates the middle- recognized and mapped by several geologists. The first to recognize the plate rocks from an upper-plate succession of block-faulted Miocene low-angle fault was Ransome and others (1907, 1910). In their study volcanic, volcaniclastic, and sedimentary rocks. (1910) of the eastern half of the Bullfrog Hills area, they mapped what Miocene rocks of the upper plate dip at moderate to steep angles they termed the "Original Bullfrog fault," a low-angle fault that separates into the upper detachment fault, or, where the middle plate has been underlying Paleozoic rocks from overlying Tertiary rocks. This fault corre- tectonically removed, into the lower detachment fault. The rocks are lates with the upper of the two detachment faults shown in Figure 2. broken, tilted, and repeated in blocks bounded by normal faults that Cornwall and Kleinhampl (1961b, 1964) also mapped this upper low- terminate against, or flatten and merge into, the upper detachment angle fault in the Bullfrog Hills area. On the basis of his detailed mapping fault or, where the middle plate has been removed, the lower detach- in and around the Grapevine Mountains, Reynolds (1969) recognized the ment fault. The normal faults in the upper plate are (1) planar- upper detachment fault in the Bullfrog Hills. Monsen (1983) also recog- rotational faults that form imbricate map patterns and (2) listric faults nized a low-angle normal fault in Fluorspar Canyon, at the north end of that are characterized by oval and horseshoe map patterns. These fault Bare Mountain, east of the Bullfrog Hills (Fig. 1). The Bullfrog Hills map patterns may be due to (1) curvilinear intersection of a listric or detachment fault system has been correlated (Carr and Monsen, 1988) planar fault with an antithetic fault, (2) complex intersection of two or with the Boundary Canyon (Reynolds, 1986) and Fluorspar Canyon fault more faults of different ages, (3) rotated listric faults, (4) extremely systems (Fig. 1). rotated planar normal faults that resemble listric faults, (5) nearly Ransome and others (1907,1910) interpreted the normal faults to be flat-lying normal faults, or (6) topographic and erosional effects. tectonic in origin and presented a classic discussion on the genesis of Attenuation of the Late Proterozoic and Paleozoic strata indi- normal faults. The normal faults have also been interpreted to be related to cates large movement on the detachment faults; the upper plate has a late Cenozoic caldera in the Bullfrog Hills (Cornwall, 1962; Cornwall been extended more than 100% and possibly more than 275% locally. and Kleinhampl, 1961b, 1964). Reynolds (1969) reinterpreted the upper The geometry of the normal faults and the repetition and dip direction fault in terms of low-angle faulting and doming. He stated (1969, p. 146), of the Miocene rocks indicate that major extension, at least of the "Pre-Tertiary rocks exposed in the center of the Bullfrog Hills structure are upper plate, was west-northwest-east-southeast and occurred mostly faulted against the welded tuffs along low-angle normal faults. These between about 10 and 8 Ma. pre-Tertiary rocks are the exposed center of a late structural dome, unre- lated to local volcanic activity. Weak Tertiary rocks probably slipped from INTRODUCTION the crest of the dome along low-angle faults against the older rocks." My detailed comparison of the Bullfrog Hills volcanic rocks with those of the The Bullfrog Hills, west of Beatty, Nevada (Fig. 1), comprise a com- Timber Mountain-Oasis Valley caldera complex (Byers and others, 1976) plex, structurally distended terrane that contains two detachment faults indicates that the Timber Mountain-Oasis Valley caldera complex is the (following usage of Reynolds and Spencer, 1985) that are herein referred source for most of the ash-flow tuffs in the Bullfrog Hills. Detailed map- to as the "Bullfrog Hills detachment fault system." This detachment fault ping (scale 1:24,000) (Maldonado, 1990; Maldonado and Hausback, system is overlain by a complex of listric and planar-rotational normal 1990) supports the interpretation that structural features in the Bullfrog faults (Maldonado, 1985, 1988). The detachment faults separate three Hills area resulted from regional extensional faulting and not from caul- structural plates: a lower plate composed of Late Proterozoic metamor- dron collapse or resurgent doming. phic rocks; a thin, discontinuous middle plate of Paleozoic miogeoclinal The Bullfrog Hills area is within the Walker Lane fault zone, which

Geological Society of America Bulletin, v. 102, p. 992-1006, 11 figs., 2 tables, July 1990.

992

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Figure 1. Map showing location of the Bull- frog Hills study area (modified from Carr and Monsen, 1988).

30 KILOMETERS

consists of northwest-striking, right-lateral strike-slip faults. The zone is not EXPLANATION recognizable in the upper plate in this area; however, the upper plate has been intensely fragmented by extensional normal faults. Strike-slip faults Quaternary and Tertiary alluvial deposits may be present deeper in the crust. A possible area for strike-slip faults is Cenozoic volcanic and sedimentary rocks beneath a wide alluvium-filled valley between the Bullfrog Hills and the Grapevine Mountains (Fig. 1). Paleozoic and Late Proterozoic sedimentary and metamorphic rocks STRUCTURAL PLATES Detachment fault—Dotted where inferred, queried where uncertain Rocks of the lower plate are exposed in two areas: in a structural culmination south of Bullfrog Mountain where they have been interpreted blages that indicate metamorphic amphibolite facies (Monsen, 1983). as a metamorphic core complex (McKee, 1983) and in the southeast These rocks have been penetratively foliated and lineated; principal folia- corner of the study area, south of Beatty (Figs. 2, 3, and 4). The lower- tions dip 25° to 60° eastward, and mineral lineations plunge east (M. D. plate rocks south of Bullfrog Mountain consist of mylonitic quartzofeld- Carr and S. A. Monsen, 1985, written commun.). The rocks have been spathic gneiss, biotite schist, marble, and amphibolite dikes that are tentatively correlated with the Johnnie(?) Formation of Late Proterozoic intruded by many granitic pegmatite dikes (M. D. Carr and S. A. Monsen, age (B. W. Troxel, 1986, oral commun.). Mineral separates of these rocks 1985, written commun.). The rocks contain staurolite and kyanite assem- have been dated and are as follows.

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Figure 2. Generalized geologic map, Bullfrog Hills, Nye County, Nevada. Geologic section lines along A-A', B-B', and C-C' shown in Figure 3; D-D' shown in Figure 10. Geology mapped by Florian Maldonado (1984,1985). Mapping of Paleozoic rocks from M. W. Reynolds (1984, written commun.). Mapping of Late Proterozoic rocks from Monsen (1983).

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FAULT QTac Alluvium and colluvium QUATER- S NARY PALE- Unconiormity FVs Sedimentary rocks, undivided AND TER- OZOIC FAULT Tss Spearhead Member of Stonewall TIARY Tuff and basalt lava flow Zsm Sedimentary and metamorphic Unconformity rocks—Stirling and lower TI Latite lava flows member of Wood Canyon LATE Formation as mapped by Y PROTER- Unconformity Monsen (1983) OZOIC Trr Rhyoliie lava flows and tuffs of Zm Metamorphic rocks—Correlative Rainbow Mountain with the Johnnie (?) Formation^ Tld Latite, dacite, and rhyodacite lava flows Contact Tie Tuff of Leadfield Road Normal fault—Dotted where Tma Ammonia Tanks Member of concealed, bar and ball on Timber Mountain Tuff downthrown side, queried Tmr Rainier Mesa Member of Timber where uncertain Mountain Tuff Upper detachment fault- Tpc Tiva Canyon Member of Paint- I TER- Dashed where approximately brush Tuff TIARY located or concealed, queried Tcb Bullfrog Member of Crater Flat where uncertain, barbs on Tuff upper plate Tlr Lithic Ridge Tuff and andesite • Lower detachment fault— Dashed where approximately lava flow, undivided Tql located or concealed, queried Quartz latite lava flow where uncertain, barbs on Ts upper plate Sedimentary rocks DF Trp • Detachment (?) fault—Inferred -X X- Rhyolite porphyry dikes beneath Late Proterozoic rocks Tst Tuff of Sawtooth Mountain and sedimentary rocks, undivided -4- Thrust fault Tbs Tuff of Buck Spring, sedimentary rocks, and bedded and ash- Structural culmination flow (?) tuffs, undivided 8BH-3, Paleomagnetic sample location Tas Ash-fall tuff and tuffaceous sedimentary rocks

116°45'

Figure 2. (Continued).

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METERS

1800 - Tma

1500 -

1200 - ^ Upper detachment fault 900 7 zm Lower detachment fault "" Zm

B B' O METERS METERS I- Bullfrog Mountain 1500 -, o I- 1500 LU QTac VL Tma QTac ^rir\Tcb\Tpc\ Tmr 1200 - - 1200 QTac Upper detachment fault " f \ Tql Ts 900 - -900 Lower detachment fault \ , , , Lower detachment fault Zm Upper detachment fault Zm 600 600

Montgomery-Shoshone Fault

METERS

1200

Upper detachment fault O 300 600 900 1200 1500 METERS

Figure 3. Geologic sections for lines A-A', B-B', and C-C'. Section lines shown in Figure 2.

• 16.3 ± 0.4 Ma, K-Ar, muscovite from gneiss (W. J. Carr, 1980, divided Hidden Valley Dolomite and Lost Burro Formation (Silurian and written commun.). Devonian), and Eleana Formation (Devonian and Mississippian). These • 11.2 + 1.1 Ma, K-Ar, muscovite from gneiss (McKee, 1983). rocks represent an extremely incomplete stratigraphic succession in the • 10.5 ± 0.3 Ma, K-Ar, biotite from schist (M. D. Carr and S. A. Bullfrog Hills as compared to the nearly normal succession approximately Monsen, 1985, written commun.). 7,400 m thick at Bare Mountain (Cornwall and Kleinhampl, 1961a; These ages represent K-Ar closure temperatures of these minerals and Monsen, 1983). therefore may suggest that the metamorphic complex cooled owing to The slivers of Paleozoic rocks have been interpreted (Carr and uplift during extension in late Miocene time. Monsen, 1988) as slices of the Grapevine Mountain block left behind as The lower-plate rocks south of Beatty consist of metasedimentary the block moved west-northwest from Bare Mountain to its present posi- rocks that have been identified by Monsen (1983) as the Late Proterozoic tion west of the Bullfrog Hills (Fig. 1). The actual amount of tectonic Stirling Quartzite and the lower member of Wood Canyon Formation. thinning that occurred during the movement of the Grapevine Mountain These rocks are penetratively deformed and metamorphosed to amphibo- block is indeterminable because a Mesozoic thrust fault in the Bullfrog lite facies as indicated by staurolite-biotite-garnet assemblages (Monsen, Hills (M. W. Reynolds, 1984, written commun.) has placed Devonian and 1983) and are considered to be part of the metamorphic complex exposed Silurian or Ordovician rocks on Mississippian rocks. The thrust fault has south of Bullfrog Mountain (Fig. 2). These two units are approximately been correlated with the Last Chance thrust fault (Reynolds, 1971), a fault 1,100 m thick at Bare Mountain (Monsen, 1983) but have been tectoni- exposed in the Last Chance Range approximately 75 km northwest of the cally eliminated south of Bullfrog Mountain. map area. The middle plate is wedge shaped and highly attenuated; it comprises The upper plate is composed of Miocene rocks that dip moderately to fragmented slivers of Paleozoic rocks bounded above and below by de- steeply (Fig. 4) into the upper (Fig. 5) or the lower detachment fault. A tachment faults (Fig. 2). Where exposed, the structural wedge is at least complete stratigraphic succession is approximately 5,600 m thick; it com- 250 m thick but may be thicker in the subsurface. The rocks are extremely prises silicic ash-flow tuff sheets interlayered with ash-fall tuff and inter- brecciated and sheared, essentially unmetamorphosed, and tilted (25°- bedded volcaniclastic rocks, conglomerate, shale, and sandstone. Lava 65°). The rocks have been mapped and described by Cornwall and Klein- flows of rhyolitic, intermediate, and basaltic compositions are also in the hampl (1961b, 1964) and by M. W. Reynolds of the U.S. Geological succession (Table 1). The oldest unit (Tas) is correlative with part of a Survey (1984, written commun.) and consist, in ascending order, of the similar unit ("unnamed tuff sequence" of Reynolds, 1969) in the Death Bonanza King Formation (Cambrian), Pognonip Group (Ordovician), Valley-Grapevine Mountains area. The youngest dated rock unit in the Eureka Quartzite (Ordovician), Ely Springs Dolomite (Ordovician), un- succession is the Spearhead Member of the Stonewall Flat Tuff (previously

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assigned as the Spearhead Member of Thirsty Canyon Tuff, but reassigned described by workers in other terranes in the Basin and Range province by Noble and others, 1984), dated at 7.6 Ma (Deino and others, 1989). (Reynolds, 1969, 1974; Anderson, 1971; Proffett, 1977; Wernicke, 1981; This unit overlies the upper plate of tilted Tertiary rocks unconformably Gross and Hillemeyer, 1982; Wernicke and Burchfiel, 1982; Bohannon, and is not involved in the major deformation. The youngest tuff (undated) 1983; Chamberlin, 1983; Gans and others, 1983). in the volcanic succession is the tuff of Sober-up Gulch. The unit crops out The planar-rotational normal faults in the Bullfrog Hills form imbri- approximately 6 km north of the map area, below essentially flat-lying cate blocks (Ladd Mountain-Velvet Peak area, Fig. 2) or what has been gravel deposits informally designated as the gravel of Sober-up Gulch referred to as "dominoes" (diagram C of Fig. 6). These faults may have (Maldonado and Hausback, 1990). Throughout the succession, the units developed in an early stage of detachment faulting (pre-rotation) as mod- are conformable except for major unconformities below a sequence of erate (60°-70°) breaks in the upper plate, propagating from the underlying latitic lava flow (T1 in Table 1), the Spearhead Member, and the tuff of detachment fault in response to early movement (Fig. 7A). Continued Sober-up Gulch. movement along the underlying detachment fault further extended the The Miocene rocks have been hydrothermally altered and may have upper plate, tilting the normal faults and the strata between them and been subjected to several hydrothermal episodes. Intensity of alteration progressively flattening the faults and steepening the stratal dips (Fig. 7B). decreases up section. Rocks older than 10 m.y. old (rhyolite lava flows and As movement progressed, displacements were taken up along the normal tuffs of Rainbow Mountain) have been affected, but rocks younger than 10 faults with shearing and brecciation at the detachment fault level, truncat- m.y. old have not, indicating that age of last alteration was about 10 Ma. ing both the lower part of the strata and the basal tips (toes) of the faults. In Studies by Jackson and others (1988) gave an age between 13 and 10 Ma some areas, at least 1 km of strata (Tertiary volcanic rocks) has been for the alteration. The source of the hydrothermal fluids may be related to tectonically removed at these levels. Proffett (1977) described a sequence detachment faulting as suggested for other areas by Bartley and Glazner of faulting when early-formed faults were rotated to a shallower dip by (1985), possibly to intrusive bodies (Maldonado and Hausback, 1990) that subsequent faults. This situation may also hold for the Bullfrog Hills area. are part of the rhyolite lava flows and tuffs of Rainbow Mountain, al- Listric faults in the Basin and Range province have been commonly though no isotopic data are available to support this hypothesis, or to a described as spoon or shovel shaped. In the Bullfrog Hills area, there are larger unknown intrusive body in the subsurface. faults that resemble listric faults that commonly have oval, or horseshoe, Gold is present in the upper-plate rocks. On the south flank of Bull- map patterns (Fig. 4). The oval patterns reflect intersection of the curvilin- frog Mountain (Fig. 5), the volcanic sequence (quartz latite lava flows and ear segment of the fault with the land surface. The pattern of the fault may the Lithic Ridge Tuff) hosts a quartz vein complex referred to as the be due to (1) topographic and erosional effects, (2) curvilinear intersection "Original Bullfrog Vein" by Ransome and others (1910). This complex of a listric or rotated planar normal fault with an antithetic fault (Fig. 6A), contains disseminated gold and fragments of the Eleana Formation, quartz (3) intersection of two generations of faults as described by Proffett (1977) latite lava flows(?), and Lithic Ridge(?) Tuff. The complex is intensely and Miller and others (1983) (in this interpretation, the antithetic fault brecciated and is truncated by the upper detachment fault. Gold has also shown in diagram Fig. 6A would now be interpreted as an earlier-formed been found along faults at other localities within the upper plate. Localiza- fault rotated through the horizontal to an apparent reverse fault position), tion of gold in the upper plate may be related to normal faults in the upper (4) a rotated original listric fault, (5) extremely rotated planar normal plate acting as conduits for mineralizing fluids. Both the occurrence of gold faults that resemble listric faults (Fig. 6C), or (6) nearly flat-lying faults along faults in the upper plate and gold occurrence truncated by the (toe level of a listric fault or extremely rotated planar normal fault). detachment fault indicate that the gold is probably synchronous with Similar patterns have been described as crestal collapse grabens by extension. McClay and Ellis (1987) in laboratory experiments using sand as model- ing material. Other experimental analogues of extensional deformation STRUCTURES IN THE UPPER PLATE (Cloos, 1955, 1968; Kautz and Sclater, 1988) have also shown similar types of features. The oval patterns in the Bullfrog Hills may represent the Upper-plate rocks form tilted blocks above the detachment faults. natural occurrence of the lower structural levels of grabens formed in the The tilted blocks are separated by gently to moderately dipping planar laboratory experiments. The horseshoe patterns resemble the oval patterns normal faults that have been rotated and terminate against the upper or the but are much smaller and in most cases dip in the same direction as the lower detachment fault. The tilted blocks may be separated by listric faults strata. These patterns overlie individual rotated fault blocks and may be that flatten and merge with the upper detachment or lower detachment gravity-slide blocks that formed as a consequence of tilting of the fault fault, where the middle plate has been removed. Wernicke and Burchfiel blocks (Fig. 6B) and are interpreted as late-stage features. In areas where (1982) have proposed that listric faults can be distinguished from planar the horseshoe patterns do not dip in the same direction as the strata, they faults by differential tilt between imbricate fault blocks (uniformly tilted may be listric faults or early-formed faults that have been extremely ro- blocks are in most cases bounded by planar faults). This criterion holds for tated to their present position. The listric faults also form imbricate pat- listric faults locally (along section D-D', Fig. 2), but not for most areas terns (area along section D-D', Fig. 2) similar to those of the imbricated (along section C-C', Fig. 2). In the Bullfrog Hills area, it is difficult to planar-rotational normal faults (area along section C-C', Fig. 2). differentiate listric faults from rotated planar normal faults, owing to poor Faults in the Bullfrog Hills are poorly exposed or are covered by exposures. This situation is further complicated by the extreme rotation of alluvium (Fig. 2). For those unexposed faults, the dips were determined by the planar normal faults to low angles resembling the geometry of listric assuming original nearly horizontal strata and original fault-to-bedding faults. Listric faults, if present, may be truncated by the upper detachment angle of 60° to 70°. This angle is kept constant through rotation of fault fault, leaving only their steep, planar upper segments, and thus resemble and strata and is constrained by amount of dip of exposed strata that make planar-rotational faults. up tilted blocks (Fig. 4). Dips on the normal faults in the Bullfrog Hills The planar-rotational normal faults were first described in a classic range from 10° to about 80°. A wide range of attitudes is present, but study by Emmons (1907) and Emmons and Garrey (in Ransome and north-northeast strikes are predominant; the faults dip west-northwest, others, 1910) for faults in the eastern part of the Bullfrog Hills area. Since counter to the stratal dip (Fig. 8). East of Bullfrog Mountain (Figs. 2 and then, both planar-rotational normal faults and listric faults have been 4), the structure is systematic; strata dip eastward, and the faults, poorly

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Figure 4. Generalized tectonic map, Bullfrog Hills, Nevada. Unlabeled areas are underlain by Tertiary and Quaternary deposits.

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exposed, dip mainly to the west. West of Bullfrog Mountain, the structure strike-slip faults. In general, most faults in the Bullfrog Hills area strike is less coherent; the assumed strata-fault couplets face in various directions north-northeast and presumably dip west-northwest (Fig. 8). This orienta- (Fig. 2). These variable attitudes west of Bullfrog Mountain may be due to tion suggests that extension is predominantly west-northwestward, down- (1) ramps resulting in undulation of the detachment-fault surface, dropping, and repeating strata generally in that direction. (2) rotation of stranded blocks about a vertical axis overlying an Displacements on the normal faults are generally dip-slip, but some unexposed detachment-fault surface, (3) landslide blocks from the uplifted appear to be oblique-slip. The Montgomery-Shoshone fault (Fig. 2), for Grapevine Mountain block (Fig, 1), or (4) movement along unexposed example, has been interpreted as a regional left-lateral strike-slip fault (Carr, 1984) that traverses the area. I suggest that the fault, which may be of local extent rather than regional, may be a listric or rotated planar fault EXPLANATION that apparently truncates imbricate planar-rotational normal faults (Fig. 4). The fault is thought to merge downward into the underlying Trp -K *- Tertiary rhyolite prophyry dikes detachment fault or to be truncated by it. This apparent lateral offset is common along curving or shallow-dipping normal faults throughout the Rzs Paleozoic sedimentary rocks Bullfrog Hills area. Offset of beds in the Bullfrog Hills area is variable and Zsm Late Proterozoic sedimentary and metamorphic may indicate locally as much as 3,000 m of apparent stratigraphic rocks, undivided separation. Extension of the upper-plate rocks in the Bullfrog Hills is variable, Zm Late Proterozoic metamorphic rocks but generally the extension is more than 100% and perhaps more than 275%. Figures 9 and 10 are palinspastic restorations of geologic sections Normal fault—Dotted where concealed, bar and ball that assume original nearly horizontal attitudes for upper-plate strata, orig- on downthrown side, queried where uncertain inal 60° to 70° dips for planar faults, and original low to moderate atti- Upper detachment fault—Dashed where approxi- tudes for listric faults. Figure 9 shows restoration according to a model of a mately located or concealed, queried where rotated planar fault (as used by Ransome and others, 1910), where both uncertain, barbs on upper plate, arrow indicates fault and strata are rotated. Figure 10 shows a listric fault model, where the direction and amount of dip I strata are rotated along a fault without rotation of that fault. ?— Lower detachment fault—Dashed where approximately located or concealed, queried where Restoration of geologic sections in estimating amount of extension uncertain, barbs on upper plate, arrow indicates results in apparent voids, between and at the base of the fault blocks, that direction and amount of dip are difficult to explain. The voids may represent areas of brecciation and DF Detachment fault—Inferred beneath Late Proterozoic extreme internal deformation (interbed shear) formed during rotation of rocks the blocks. Some of the voids may be partly reduced if the following factors are known and incorporated into the models: (1) attitude of the —»— Thrust fault detachment fault surfaces (undulating fault surfaces form ramps that result -J Inferred antiform in higher angles on the detachment fault surfaces), (2) depth to the detachment fault surfaces (thickness of upper plate would be altered if Structural culmination detachment faults are deeper than indicated in geologic section), (3) atti-

,45 Dips and direction of strata

116°45'

Ì ^^yw//V\ 1 \ / \ '-v _ ! w/ V r' .ÀA ) ftoRSËéHÔÊs^ATTERN/ Beatty t30 Ì

Rhyolite V7" / 49 \

If 32y / ,'-\\58W 4 / C(~A ) Is? ^ i&J _v Ylf Tit ! " O.. (1 $ TCriUts MONTGOMERY-SHOSHONE FAULT /

j /

¿L L 36° 52' 30"

Figure 4. (Continued).

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/102/7/992/3381094/i0016-7606-102-7-992.pdf by guest on 25 September 2021 Figure 5. Photograph of Bullfrog Mountain, showing block of Miocene volcanic rocks (upper plate) dipping into the upper detachment fault (hachured dashed line). QTac, alluvium and colluvium (Quaternary and Tertiary); Tql, quartz latite lava flows (Tertiary); Tlr, Lithic Ridge Tuff (Tertiary); Tcb, Bullfrog Member of Crater Flat Tuff (Tertiary); Tpc, Tiva Canyon Member of Paintbrush Tuff (Tertiary); Pzs, slivers of Paleozoic sedimentary rocks; and qtz, quartz vein complex (Original Bullfrog Vein). View is looking northwest.

Oval Horseshoe

Upper detachment Upper fault detachment Lower fault detachment Lower fault detachment fault

Lìstric C Planar Imbricate

Figure 6. Schematic block diagrams showing oval, horseshoe, and imbricate patterns. Unit 1 is youngest and unit 8 is oldest. Pzs, Paleozoic sedimentary rocks; Zm, Late Proterozoic metamorphic rocks.

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Map-unit General description Approximate K-Ar age* symbol thickness (meters)

Tcb Bullfrog Member of Crater Flat Tuff- 250 13.9 Ma TABLE 1. STRATIGRAPHIC TABULATION OF THE MAJOR CENOZOIC ROCK. UNITS OF THE Ash flow, grayish-pink, very light (sanidine, BULLFROG HILLS ARF.A, SOUTHWESTERN NEVADA gray to medium-light gray, and pale Marvin and red, nonwelded to densely welded, others, 1970; crystal rich, with sanidine, Carr and Map-unit General description Approximate K-Ar age* plagioclase, quartz, biotite, and others, 1986) symbol thickness hornblende. Underlain by bedded (meters) ash-fall tuff

QTac Alluvium and colluvium- intercalated Variable Tlr Lilhic Ridge TufT and andesite lava 435 gravel, sand, and silt flow, undivided - Ash flow, grayish- UNCONFORMITY orange-pink, very light gray, to light bluish-gray, base is mottled Tss Spearhead Member of Stonewall Flat 0-30 7.6 Ma moderate red, crystal rich, with Tuffand basalt flow, undivided— («Ar/39Ar sanidine, quartz, biotite, and Ash (low. grayish-pink, nonwelded sanidine, sphene, rich in lithic fragments to partly welded, crystal poor wilh Deino and of lava and tuff. Underlain by sanidirie. plagioclase, quartz, others. 1989) bedded ash-fall tuff. The andesite ctirvopyroxene. olivine, sodic lava flow overlie the Lilhic amphibole, and Fe-Ti oxides. The Ridge Tuff in some areas basalt lava flow, dated at 8.1 Ma (whole rock, R. F. Marvin, 1986, Tql Quartz latite lava flows—Medium- 0 245 wriiten commun.), underlies the light gray and light brownish- Spearhead Member gray, flow-banded, porphyritic, UNCONFORMITY crystal rich, with plagioclase, biotite, alkali feldspar, n Latitic lava flows—Medium-light gray 0-245 10.0 0.4 Ma hornblende, pyroxene, and sphene and grayish-brown, porphyritic, (biotite, R. F. crystal rich, wilh plagioclase, Marvin. 1985, written biotite, and pyroxene commun.) Ts Sedimentary rocks—Very light gray to 150-300 light gray, light olive-gray, and UNCONFORMITY pale olive, sandstone, shale, and conglomerates. Locally includes a Trr Rhyolite lava flows and tuffs of 0-600 10,0 Ma pale greenish-yellow ash-fall tuff Rainbow Mountain—Multicolored (Carr. 1984, complex of interbcdded lava flows, average of K-Ar ash-fall tuff, breccias, and debris age of 11.1 Ma Tst Tuff of Sawtooth Mountain and 850 flows and sedimentary rock. Includes and zircon sedimentary rocks undivided local intrusive rocks and ash-flow fission-lrack Composed of upper and lower tuff tuffs age of 9.0 Ma) of Sawtooth Mountain separated by sedimentary rocks; upper tuff is yellowish-gray and very light gray, Tld Lame, dacite, and rhyodacite lava 0 215 — partly to moderately welded, moderate flows— Light gray to very light crystal content, with sanidine, gray, and grayish-red. porphyritic. plagioclase, quartz, and crystal rich hornblende(?); lower tuff is pinkish-gray, very light gray, and pale red, partly to moderately Tie Tuff of Leadfield Road -Ash flow, 0-275 welded, crystal poor, with sanidine, grayish-orange-pink, nonwelded to plagioclase, and quartz. Sedimentary partly welded, crystal poor, with rocks are approximately plagioclase, sanidine, quartz, 30 m thick, medium-light gray, biotite, hornblende, and pyroxene, light olive-gray, grayish-pink, fragments or rhyolite lava, tuff, and pale greenish-yellow sandstone siltslone, and mudstone common. and conglomerate Southwest of area, unit overlies gravity-slide blocks of Paleozoic carbonate rocks Tbs Tuff of Buck Spring, sedimentary 800 rocks, and bedded and ash-flow(?) Tina Ammonia Tanks Member of Timber Mountain 395 11.4 Ma tuff, undivided Composed of upper Tuff Ash flow, light red, grayish-red, (sanidine. and lower tuff of Buck Spring light brownish-gray to very light gray, Kistler. separated by ash-fall tuffand tuffaceous sandstone approximately nonwelded to densely welded, crystal 1968) 6 m thick- The upper tuff is pale rich, with sanidine, quart/, yellowish-green, nonwelded to plagioclase. biotite, and sphene. partly welded, contains plagioclase, Basalt fragments common, overlain by sanidine, quartz, biotite, and hornblende. basalt lava flows dated at 10.3 The lower tuff is pale yellow-green, Ma (whole rock, R. F. Marvin, 1985. pale yellowish-green, light gray, written commun.). Underlain by and very pale orange, nonwelded bedded ash-fall tuff to densely welded, crystal rich, with plagioclase, sanidine, quartz, biotite, and hornblende. Tmr Rainier Mesa Member of Timber 435 11.6 Ma The lower unit is correlative Mountain Tuff—Ash flow, moderate (sanidine, with the tuff of Yucca Rat brown, pale red, and light gray, Marvin and (Carr and others, 1986). The nonwelded to densely welded, others, 1970) crystal rich, with quartz, sanidine, upper tuff of Buck Spring is overlain plagioclase. and biotite. Overlain by by a sequence of sedimentary rocks, basalt lava flows. Underlain by approximately 180 m thick, composed bedded ash-fall tuff of conglomerate, shale, and sandstone, and graviiy-slide blocks of Paleozoic carbonate rocks. The lower tuff of Tpc Tiva Canyon Member of Paintbrush 275 12.9 Ma Buck Spring is underlain by a sequence Tuff—Ash flow, grayish-pink, light (sanidine, of greenish-gray to light greenish-gray gray, light brown-gray, moderate Marvin and bedded tuff, tuffaceous sandstone, and brown, and grayish-red, nonwelded to others, 1970) ash-flow(?) tuff, approximately 300 m thick densely welded, crystal poor, with plagioclase, sanidine, biotite, clinopyroxene, and trace of sphene. Tas Ash-fall tuff and tuffaceous 300 Underlain by bedded ash-fall tuff sedimentary rock—Dark gray to medium-light gray, crystal-rich ash fall intercalated with yellowish- gray, tuffaceous sandstone, with minor dark gray limy shale. Unit correlative with the "unnamed tuff sequence" of Reynolds (1969)

•Age determinations corrected for new K-Ar constants (Dalrymple, 1979).

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u o Figure 7. Schematic section illustrating pro- u. a

Upper detachment fault

tude of faults in the subsurface (faults may be shallower dipping than nation, with oldest Miocene rocks near the culmination changing from indicated in geologic section), (4) fault geometry in the subsurface (fault predominantly east-dipping strata east of the culmination to northwest- surfaces may change shape or form splays in the subsurface to accommo- and southwest-dipping strata west of the culmination. An area southwest date internal deformation), and (5) amount and type of internal deforma- of the culmination deviates from this westerly dip and is characterized by tion within the individual fault blocks. Unfortunately, the determination of south- and southeast-dipping strata that may represent the relict pre- these factors in the Bullfrog Hills area may not be resolvable, owing to doming, easterly dipping attitude. Doming of the lower plate may be due poor exposures; thus, present data are insufficient to explain the voids to tectonic denudation or to the intrusion of a Tertiary magma at depth. adequately. Available evidence does not permit a confident choice. Doming of the lower-plate rocks has apparently deformed the upper- plate and detachment faults, forming a culmination. The doming is indi- AGE OF EXTENSION cated by (1) antiformal configuration of detachment faults over the metamorphic rocks and (2) Miocene rocks dipping away from the culmi- Major extension of the upper plate in the Bullfrog Hills area occurred between 10 and 8 Ma. The Spearhead Member of the Stonewall Flat Tuff, dated at 7.6 Ma by Deino and others (1989), is exposed in four scattered outcrops in the map area (Fig. 2). Foliation measurements of the tuff indicate shallow to steep dips that suggest possible deformation; however, recent paleomagnetic data (Table 2 and Fig. 11; M. R. Hudson, 1988, written commun.) indicate that the Spearhead Member was not involved in the major deformation as previously thought (Maldonado, 1985,1988). Four outcrops were sampled for paleomagnetic measurements and indicate the following. In geographic coordinates, the mean directions of paleomagnetic data of three sample sites with shallow- to moderate-dipping foliation (8BH-1, 8BH-3, and 9BH-6; see Fig. 2 for locations) have north- erly declinations. These are similar to declinations reported elsewhere (Noble and others, 1984) and indicate no tectonic tilting. In contrast, the mean direction from one site (8BH-2) with steep east-dipping foliation has a northwest declination (Fig. 11). The northwest declination of site 8BH-2 is best explained by an eastward tectonic tilt of about 25° (M. R. Hudson, 1988, written commun.). The relationship of this tectonic tilting of this particular outcrop to the major episode of extension is unclear but may be related to faulting associated with younger deformation to the west of the Bullfrog Hills, where Hamilton (1988) indicated deformation around 4 Ma for the Funeral Range area. This younger period of deformation may also be related to local faulting, as suggested by the occurrence of a faulted thin lens of ash-fall tuff located approximately 600 m north of site 8BH-2 and 300 m south of Currie Well (Fig. 2). The ash-fall tuff is intercalated with alluvium of Quaternary-Tertiary age, is approximately 0.5 m thick, and contains approximately 13% crystals of plagioclase, sanidine, quartz, (210 faults) and biotite and approximately 1% lithic fragments. The lens is slightly Figure 8. Dip directions (taken from map) of selected normal offset approximately 100 cm and may simply indicate minor faulting of faults, Bullfrog Hills area. local extent.

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A basalt lava flow, dated at 8.1 Ma (R. F. Marvin, 1986, written to emplacement of the Spearhead Member and underlying basalt flow. commun.), underlies the Spearhead Member. Paleomagnetic data are not This is suggested by their distribution along a north-trending basin where available for this basalt flow, but the flow does not appear to be deformed the detachment faults are exposed (Fig. 2). The units overlie tilted upper- except for sample site 8BH-2, where the Spearhead Member is tectonically plate Tertiary rocks and are inferred to overlie middle-plate rocks (Fig. 2) tilted. unconformably, and this constrains the age of major deformation. The topography of the Bullfrog Hills was essentially developed prior The approximately 5.5-km-thick section of Tertiary volcanic and / / / / ' / ' //<$>/ @ / <2> /© t i1 , / ' Extended width / I J' // T-n, I ' /TTa/ V/r™ .'V -250 percent extension) Original width

Original width Figure 9. Palinspastic restoration of geologic section line C'-C" (shown in Fig. 2), showing idealized extension according to rotated planar fault model. (A) Reconstructed section, (B) reconstructed section rotated to the horizontal. Dotted line indicates erosional surface. See Figure 2 for map-unit symbols used in this figure.

Montgomery-Shoshone Fault c Ladd Mountain Velvet METERS I QTac Peak 1200 -i/^ Ph, ®

Tpc

Upper detachment fault 600 1200 1500 METERS _J

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) Di D Extended width Original (~275 percent extension) width

Figure 10. Palinspastic restoration of geologic section line D-D' (shown in Fig. 2), showing idealized extension according to a listric fault model. Dotted line indicates erosional surface. (A) Schematic section showing reconstruction of original width, (B) reconstructed section. See Figure 2 for map-unit symbols used in this figure.

1500

1200

0 300 600 900 1200 1500 METERS

sedimentary rocks indicates a structural basin for accumulation of these Several interpretations are possible in correlating lower and upper rocks. The inception of basin formation and early extension predates the detachment fault surfaces with timing of deformation. One possible inter- Tertiary succession. This early extension is further supported by the pres- pretation is that the two detachment faults represent two episodes of ence of a sedimentary unit, in the lower part of the Tertiary succession extension. The lower detachment fault is a mid-crustal fault that represents (included in unit Tbs, Table 1), that contains conglomerate and gravity- an older episode where lower- and middle-plate rocks were extended, slide blocks of Paleozoic carbonate rocks. The blocks suggest that Paleo- forming a basin in which the upper-plate rocks were deposited. The upper zoic rocks were exposed as topographic highs in fault-block ranges, caused detachment fault is an upper-crustal fault that represents a younger episode by extension, adjacent to the structural basin. (between 10 and 8 Ma), during which the upper-plate rocks were ex-

N in the Bullfrog Hills area may be as follows: (1) formation of detachment faults, (2) nucleation of imbricate planar normal faults, propagating from the detachment faults, (3) rotation of these imbricate planar normal faults (Ladd Mountain-Velvet Peak area, Fig. 2), with continued movement along the detachment faults, (4) continuation of rotation with propagation of later planar normal faults and possible listric faults, resulting in truncation of the earlier imbricate rotated planar normal faults (Montgomery- Shoshone fault-Ladd Mountain area, Fig. 2) (at this stage, the earlier-rotated planar faults are considered inactive), and (5) formation of antithetic faults that may cut the earlier-formed faults or propagate from them. Most of the normal faults in the upper plate in the Bullfrog Hills may be rotated planar normal faults that resemble listric faults in geometry. Restorations of geologic sections of the upper plate in the Bullfrog Hills area almost always result in voids in the sections. In most cases, this may be attributed to internal deformation not incorporated into the restorations; however, there are local situations where this may not be the case entirely. Some of the faults may have formed after intense brecciation close to the detachment faults, and subsequent sliding of blocks created voids in the geologic section. The voids reappear when sections are restored. Com- plete restoration may be impossible in extremely extended terrane owing to complexities coupled with poor exposures, and attempts will be only approximate. Evidence presented indicates that mineralization and hydrothermal Figure 11. Equal-area projection of paleomagnetic data from the alteration may be related to extensional deformation in the Bullfrog Hills Spearhead Member of the Stonewall Flat Tuff (M. R. Hudson, 1988, area. written commun.). All symbols are projections on the lower hemi- Formation of the upper-plate structures that form the oval, horseshoe, sphere. Solid circles are means of sites from the Bullfrog Hills. Squares and imbricate map patterns suggests extreme extension on shallow near- are means from Noble and others (1984). Cones about means are 95% surface detachment faults. In other terranes in the Basin and Range prov- statistical confidence limits. The northwest declination of the site mean ince where detachment faults are not exposed, these types of upper-plate from 8BH-2 suggests that this site has been tilted eastward about 25° fault configurations may be used to interpret detachment faults in the since emplacement of the Spearhead. subsurface.

ACKNOWLEDGMENTS TABLE 2. PALEOMAGNETIC DATA FROM THE SPEARHEAD MEMBER OF THE STONEWALL FLAT TUFF The author would like to acknowledge M. W. Reynolds and M. D.

Site latitude N Longitude D 1 a95 k Carr and to thank them for lengthy discussions in the field and for their no. comments and suggestions on the manuscript. Thanks are also extended to

8BH-I 36.9678°N 116.9204°W 8 357° 53° 1.3° 1835 K. F. Fox, Jr., and D. L. Schmidt for numerous discussions. The manu- 8BH-2 36.9603°N 116.9208°W 10 326° 55° 3.5° 189 script was considerably improved by the critical reviews of W. B. Myers, 8BH-3 36.8966"N II6.9I53°W 8 3° 60° 1.5° 1300 9BH-6 36.9177°N 116.9153°W 8 0° 61° 2.1° 696 D. L. Schleicher, and R. G. Bohannon and the Geological Society of America reviewers J. H. Diles and J. D. Walker. The research Note-. M. R. Hudson, 1988, written commun.; all samples subjected to alternating-field demagnetization. Sample directions determined by principal-component analysis (Kirschvink, 1980). N, number of samples; D, I, was sponsored by the U.S. Geological Survey's Yucca Mountain Project,

declination and inclination, respectively, of site mean in geographic coordinates; ag5, cone of 95% confidence; k, precision parameter (Fisher, 1953). supported by the U.S. Department of Energy (Interagency Agreement DE-AI08-78ET44802).

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Chamberlin, R. M., 1983, Cenozoic domino-style crusta! extension in the Lemitar Mountains, New Mexico—A summary: Maldonado, F., and Hausback, B. P., 1990, Geologic map of the northern quarter of the Bullfrog 15-minute quadrangle, New Mexico Geological Society Guidebook, Field Conference, 34th, Socorro Region II, p. 111-118. Nye County, Nevada: U.S. Geological Survey Miscellaneous Investigations Series Map 1-2049, scale 1:24,000 Cloos, E., 1955, Experimental analysis of fracture patterns: Geological Society of America Bulletin, v. 66, p. 241-256. (in press). 1968, Experimental analysis of Gulf Coast fracture patterns: American Association of Petroleum Geologists Marvin, R. F., Byers, F. M., Jr., Mehnert, H. H., Orkild, P. P., and Stern, T. W„ 1970, Radiometric ages and stratigraphic Bulletin, v. 52, p. 420-444. sequence of volcanic and plutonic rocks, southern Nye and western Lincoln Counties, Nevada: Geological Society Cornwall, H. R , 1962, Calderas and associated volcanic rocks near Beatty, Nye County, Nevada, in Petrologic studies: of America Bulletin, v. 81, p. 2657-2676. Boulder, Colorado, Geological Society of America, Buddington volume, p. 357-371. McClay, K. R., and Ellis, P. G., 1987, Geometries of extensional fault systems developed in model experiments: Geology, Cornwall, H. R., and Kleinhampl, F. J., 1961a, Geology of the Bare Mountain quadrangle, Nevada: U.S. Geological v. 15, p. 341-344. Survey Geologic Quadrangle Map GQ-157, scale 1:62,500. McKee, E. H., 1983, Reset K-Ar ages—Evidence for three metamorphic core complexes, western Nevada: Isochron/ 1961b, Preliminary geologic map and sections of the Bullfrog quadrangle, Nevada-California: U.S. Geological West, no. 38, p. 17-20. Survey Mineral Investigations Field Studies Map MF-177, scale 1:48,000. Miller, L. E., Gans, P. 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W„ 1984, Stratigraphic relations Cat Members of the Stonewall Tuff, Nye County, Nevada: Eos (American Geophysical Union Transactions), and source areas of ash-flow sheets of the Black Mountain and Stonewall Mountains volcanic centers, Nevada: v. 70, no. 43, p. 1409. Journal of Geophysical Research, v. 89, no. B10, p. 8593-8602. Emmons, W. H„ 1907, Normal faulting in the Bullfrog district: Science, v. 26, p. 221. Proffett, J. M., Jr., 1977, Cenozoic geology of the Yerington district, Nevada, and implications for the nature and origin of Fisher, R. A., 1953, Dispersion on a sphere: Royal Society of London Proceedings, v. A217, p. 295-305. Basin and Range faulting: Geological Society of America Bulletin, v. 88, p. 247-266. Gans, P. B., Miller, E. L., and Garing, J., 1983, Styles of mid-Tertiary extension in east-central Nevada, in Guidebook Ransome, F. L., Garrey, G. H., and Emmons, W. 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Publishers, p. 257-265. 1971, The Grapevine thrust and its significance to right-lateral displacement in the northern Death Valley area, Hamilton, W. B., 1988, Death Valley tectonics—Hingeline between active and inactivated parts of a rising and flattening California: Geological Society of America Abstracts with Programs, v. 3, no. 2, p. 182-183. master normal fault, in Gregory, J. L., and Baldwin, E. J„ eds., Geology of the Death Valley region: South Coast 1974, Geology of the Grapevine Mountains, Death Valley, California—A summary: California Division of Mines Geological Society Annual Field Trip Guidebook 16, p. 179-205. and Geology Special Paper 106, p. 91-97. Jackson, Mac Roy, Noble, D. C., Weiss, S. I., and Larson, K. T., 1988, Timber Mountain magmaio-thermal event—An 1986, Geometry and chronology of late Cenozoic detachment faulting, Funeral and Grapevine Mountains, Death intense widespread culmination of magmatic and hydrothermal activity at the SW Nevada volcanic field: Geologi- Valley, California: Geological Society of America Abstracts with Programs, v. 18, no. 2, p. 175. cal Society of America Abstracts with Programs, v. 20, no. 3, p. 171. Reynolds, S. J., and Spencer, J. E., 1985, Evidence for large-scale transport on the Bullard detachment fault, west-central Kautz, S. A„ and Sclater, J. G„ 1988, Internal deformation in clay models of extension by block faulting: Tectonics, v. 7, Arizona: Geology, v. 13, p. 353-356. no. 4, p. 823-832. Wernicke, B., 1981, Low-angle faults in the Basin and Range province—Nappe tectonics in an extending orogen: Nature, Kirschvink, J. L„ 1980, The least-square line and plane and the analysis of paleomagnetic data: Royal Astronomical v. 291, p. 645-648. Society Geophysical Journal, v. 62, p. 699-718. Wernicke, B., and Burchfiel, B. C., 1982, Modes of extensional tectonics: Journal of Structural Geology, v. 4, p. 105-115. Kistler, R. W., 1968, Potassium-argon ages of volcanic rocks in Nye and Esmeralda Counties, Nevada, in Eckel, E. B., ed., Nevada Test Site: Geological Society of America Memoir 110, p. 251-263. Maldonado, F., 1985, Late Tertiary detachment faults in the Bullfrog Hills, southwestern Nevada: Geological Society of America Abstracts with Programs, v. 17, no. 7, p. 651. 1988, Geometry of normal faults in the upper plate of a detachment fault zone. Bullfrog Hills, southern Nevada: Geological Society of America Abstracts with Programs, v. 20, no. 3, p. 178. MANUSCRIPT RECEIVED BY THE SOCIETY FEBRUARY 21, 1989 1990, Geologic map of the northwest quarter of the Bullfrog 15-minute quadrangle, Nye County, Nevada: U.S. REVISED MANUSCRIPT RECEIVED NOVEMBER 9,1989 Geological Survey Miscellaneous Investigations Series Map 1-1985, scale 1:24,000 (in press). MANUSCRIPT ACCEPTED NOVEMBER 21,1989

Printed in U.S.A.

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