Tertiary Extension North of the Las Vegas Valley Shear Zone, Sheep and Desert Ranges, Clark County, Nevada

Tertiary extension north of the Las Vegas Valley shear zone, Sheep and Desert Ranges, Clark County, Nevada

PETER L. GUTH* Department of Earth and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139

ABSTRACT Stewart (1971) calculated that the horst and tension (Davis, 1979; Stewart, 1980). graben model requires about 10% exten- Strike-slip faults may be transform bound- Detailed mapping reveals the presence of sion across the entire Great Basin, assuming aries between regions of differential exten- high-angle extensional faults and low-angle that 60° dips exist on range-front faults. sion, such as the Garlock fault of southern gravity slides on the west side of the Sheep The tilted-block model of Morton and California (Hamilton and Myers, 1966; Range. Three major high-angle faulting Black (1975) seems to require 20-30% ex- Davis and Burchfiel, 1973). Recent work events each account for 20° of eastward ro- tension across the entire Great Basin, but suggests this model for faulting in the Lake tation and accommodate extension between local extension might exceed 100% Mead area of southern Nevada (Bohannon, the Las Vegas Range and the Desert Range. (Stewart, 1980). Inferred listric-fault 1979a). Both strike-slip and extensional Low-angle faults represent surficial slides in geometry leads to local estimates of 30% to faulting appear to be dominantly late Ter- response to topography produced by ex- over 100% extension (Anderson, 1971; tiary events in the Great Basin. tension on the high-angle faults. Faulting Wright and Troxel, 1973; Proffett, 1977), Armstrong (1972) reviewed widespread took place during the Miocene, synchron- although the model might not apply to the low-angle denudational faulting in the east- ously with deposition of the Horse Spring entire Great Basin. ern Great Basin. The faults generally place Formation and with displacement on the Las Strike-slip faults are related to exten- younger rocks on older, and Armstrong ad- Vegas Valley shear zone. The extension in sional faulting as boundaries of domains vocated a Tertiary gravity mechanism dis- the Sheep Range took place without vol- with differences in style or magnitude of ex- tinct from Mesozoic compression. In con- canism, intrusion, or metamorphism of the Paleozoic sedimentary rocks. Offset thrust faults suggest that the area west of the Sheep Range extended almost 100% during the Miocene, while the corresponding area south of the Las Vegas Valley shear zone did not extend significantly. The shear zone bounded the extending terrane on the south, acting as a transform fault. This extension west of the Sheep Range may in part balance that mapped by Ander- son (1971) in the Eldorado Mountains. The Las Vegas Valley shear zone and the Lake Mead fault system may have acted together to compensate for areas of localized extension between the Colorado Plateau and the vicinity of the Specter Range.

INTRODUCTION

The importance of extensional faulting in the development of structure and physiog- raphy in the Great Basin has been evident since the pioneering observations of Gilbert (1874). Recently Stewart (1971, 1980) reviewed and summarized the horst and graben, tilted-block, and listric-fault models Figure 1. Locality map of southern Nevada. Mountain ranges indicated and referred to commonly applied to the Great Basin. in the text are: ACR, Arrow Canyon Range; DR, Desert Range; EM, Eldorado Mountains; FM, Frenchman Mountain; LVR, Las Vegas Range; MM, Muddy Mountains; MoM, Mormon Mountains; PR, Pintwater Range; SR, Sheep Range; SdR, Spotted Range; SpR, * Present address: B Company, 34th Engineer Battalion (Combat) (Heavy), Fort Riley, Kansas Specter Range; and VM, Virgin Mountains. Thrust faults indicated are the Wheeler Pass 66442. (WPT), Keystone (KT), Gass Peak (GPT), Muddy Mountain (MMT), and Glendale (GT).

Geological Society of America Bulletin, Part I, v. 92, p. 763-771, 5 figs., 1 table, October 1981.

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trast, some recent workers (for example, Davis and Coney, 1979) studying the metamorphic core complexes of the eastern Great Basin have concluded that low-angle faulting is intimately connected with intrusion, metamorphism, and crustal doming. Tertiary low-angle faulting need not be as- sociated with any metamorphic or igneous events, and low-angle extensional faulting can involve unmetamorphosed Paleozoic rocks (see Longwell, 1945). Extension on listric or low-angle normal faults occurs in the Sheep and Desert Ranges of Clark County, southern Nevada (Figure 1). This extensional terrane occurs north of the Las Vegas Valley shear zone, a Miocene strike-slip fault (Longwell, 1960, Figure 2. Map of the Sheep Range and vicinity. Low-angle fault blocks are indicated by 1974; Fleck, 1970a). In this paper I describe stipple pattern. A detailed geologic map of this region is available elsewhere (Guth, 1980). the terrane in the Sheep Range and the eastern Desert Range, in which low-angle extensional faulting occurs. The low-angle large blocks that sit atop the crest of the ALLOCHTHONOUS UNITS faults there are related to high-angle, Sheep Range. A complex terrane involving tilted-block or listric normal faults. Nearly older-on-younger faults occurs on the west Long Valley Block 100% extension may have occurred since side of the Sheep Range and has been in- (Ddg to Mis) the Miocene in this area. The Las Vegas formally designated the Hoodoo Hills HOODOO HILLS HAVOC Valley shear zone forms the southern boun- Havoc. Hanging wall rocks involved in dary to the extended region in the Sheep both types of low-angle faults are brec- Unit VI and Desert Ranges. I will also suggest that ciated and disrupted by numerous small (Oa to Dn) the Sheep Range extensional terrane in- faults. Dolomite behaves in a more brittle teracts with other extensional areas in a fashion than limestone, and the dolomites Unit V coordinated response to provincewide ex- are much more intensely brecciated. (Oes to Ddg) tension. High-Angle Faults Unit IV STRUCTURAL GEOLOGY (Oes to Mis) Three large, north-trending, west-dipping Geologic Setting faults, the Mormon Pass, Wildhorse Pass, Unit III and Alamo Road faults, cause progressive (Ddg to Mis) Uppermost Precambrian to Upper Mis- eastward rotation of strata with a sig- sissippian sedimentary rocks in the Sheep nificant change in bedding attitude at each 44 Unit II Range are more than 4000 m thick. These fault (Figs. 2 and 4B). Rocks in the Las (Dn to Ddg) rocks were thrust eastward over Permian Vegas Range are essentially subhorizontal limestone along the Gass Peak thrust during away from the trace of the Gass Peak Unit I / Unit lb the Mesozoic Sevier orogeny. The Gass thrust. To the west of the Mormon Pass (Ddg to Mis) Peak thrust has stratigraphic displacement fault, rocks in the Sheep Range dip an aver- of 5900 m and probable horizontal dis- age of 20°E; west of the Wildhorse Pass AUTOCHTHONOUS UNITS placement exceeding 30 km (Guth, 1980). fault, rocks in the Black Hills and Hoodoo Strata in the region appear to have been lit- Hills Havoc dip 40°E; and west of the QQ I Quaternary Alluvium tle deformed upon the completion of Alamo Road fault, rocks in the Desert Mesozoic thrusting, except near thrust Range dip 60°E. All three high-angle faults ramps where they dip steeply (Armstrong, are poorly exposed and largely covered by Tc Tertiary Conglomerate 1968). Strata along the trace of the Gass alluvium. Other high-angle faults with less Peak thrust retain the ramp geometry; displacement account for additional com- upper plate rocks dip up to 70°W at the plexities, but most of the rotation appears Mississippian 8 Devonian MDI Limestone (Ddg-Mp-Mj-Mis) thrust but are subhorizontal 4 km to the to be restricted to the three major faults. west. Devonian 8 Silurian High-angle, north-trending, west-dipping Low-Angle Faults DSd Dolomite (Sl-Dn) normal faults cut the strata of the Sheep

Range and have caused eastward dips in One type of low-angle fault consists of Ordovician Units bedding. In addition, two types of low- younger-on-older faults that form detached Ou (Op-Oa-Oe-Oes) angle fault occur: those that place younger klippen such as the Hidden Forest and Long rocks on older, and those that place older Valley blocks exposed along the crest of the Sheep Range (Figs. 2 and 3). The Long Val- Cambrian Dolomite rocks on younger. Rocks in the hanging -Gd (•€ b k--€n) walls of the younger-on-older faults form ley block is about 1 km wide and extends at

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A" jt Low-Angle Fault PICTURE -J- 10,000 RYE CANYON SHEEP RANGE PATC H FAULT WILDHORSE PASS BLACK FAU LT^U FAULT HILLS Unit III 60 0 0'

20 00

Figure 3. Geologic sketch map, south- Camp and is drawn without vertical exag- Gate Limestone; Mp, Pilot Shale; Mj, Joana western Sheep Range and Black Hills, Clark geration. Formation symbols used are: Cbk, Limestone; Mis, Indian Springs Formation; County, Nevada. The map covers all of the Bonanza King Formation; Cn, Nopah For- and PPbs, Bird Spring Formation. The unit Black Hills 7.5' quadrangle and the south- mation; Op, Pogonip Group undifferen- CpCu in the subsurface corresponds to the western portion of the Hayford Peak 15' tiated; Oa, Antelope Valley Formation; Oe, Eocambrian clastic wedge which crops out quadrangle. Section A-A' goes through the Eureka Quartzite; Oes, Ely Springs Dolo- in the Desert and Las Vegas Ranges. (Ex- Hoodoo Hills Havoc in the vicinity of Cow mite; Dn, Nevada Formation; Ddg, Devil's planation on facing page.)

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least 5 km along the crest of the southern numerous small faults cutting the Devil's much of the west side of the Sheep Range. Sheep Range. Erosional remnants to the Gate and Joana Limestones, but I now The fault-bounded wedge of Unit VI occurs north increase the original length to at least consider Unit III to be an upward continua- between Unit V and autochthonous rocks 9.6 km. The Devonian Devil's Gate Lime- tion of the units exposed in the Black Hills. along the northeast part of the Hoodoo stone (Guilmette equivalent) and the Mis- Unit I structurally overlies Cambrian and Hills Havoc (Fig. 3). Unit VI rests on Unit V sissippian Joana Limestone make up the Ordovician bedrock of the Black Hills on along a low-angle fault, dipping 40°E upper plate. The Silurian Laketown Dolo- an alluvium-covered, low-angle fault. Unit (three-point solution in canyon) near its mite and the Devonian Nevada Formation la is situated on the east side of the Black southern boundary. Northward the attitude (Simonson Dolomite equivalent) form the Hills, and consists of breccia containing a of the fault must be subvertical to account footwall. About 300-400 m of section be- jumbled mixture of clasts from the Devil's for its straight outcrop pattern. longing to the upper Nevada Formation Gate and Joana Limestones. Both forma- Relations among the different units be- and the lower Devil's Gate Limestone is cut tions occur in Unit 1, but such mixing is not come obscure at the southern end of the out along the fault. Dolomite in the foot- characteristic of Unit I or other units within Hoodoo Hills Havoc, where rotation and wall is undeformed except for brecciation the Hoodoo Hills Havoc. Unit la could be deformation along the Las Vegas Valley immediately adjacent to the fault. Bedding either the chaotic toe of Unit I or represent shear zone have complicated structural re- is obscure and complexly disrupted in the unrelated later landslides. North of Black lations. Relations are most clear in the limestone of the hanging wall. Rocks in the Hills Gap, Unit la is faulted over Tertiary vicinity of Cow Camp (section A-A', Fig. 3; hanging wall dip 50-80°E, while those in conglomerate in depositional contact with Fig. 4B), where higher relief creates better the footwall dip only 20-30°E. The Ordovician limestone (Fig. 3). The exposed exposures of the various units. North of the allochthonous Long Valley block contains fault plane dips N80°E, 26°S, and grooves area shown in Figure 3, lack of outcrop and high and low-angle faults that dip east, and on the plane plunge S20°E. The fault plane low relief mask relations in the havoc. the basal detachment also dips east very is nearly parallel to the attitude of bedding Tertiary conglomerate underlies Unit la gently (5-10°E by three-point solution). The in the conglomerate, N85°E, 28°S. on the east side of the Black Hills (Fig. 3). similar Hidden Forest block is about 1.5 by A low-angle fault, which dips very shal- Small east-west—trending, high-angle faults 2.5 km in size and rests on Ordovician lowly to the east, separates Units I and II. cut Unit la. Tertiary conglomerate that dips rocks. Poorly exposed dolomites in Unit II must be 26°E overlies Unit IV and is cut by the Pic- Reconnaissance mapping in the Desert thickened by concealed faults, because for- ture Canyon fault. Poorly exposed patches Range has revealed several low-angle, mations exceed their normal stratigraphic of conglomerate, which grade into Quater- younger-on-older, tectonically emplaced thicknesses. Unit II and Unit III are sepa- nary alluvium, overlie Units V and VI. units, which probably represent surficial rated by alluvium, but the contact is in- Conglomerate that dips 45°N and rests on gravity slides. Rocks in the upper plates are ferred to be a fault because normal strati- Unit V is cut by high-angle faults on its not rotated relative to the lower plates but graphic thicknesses preclude a continuous northern and southern sides (Fig. 3). probably moved westward down shallowly section through the covered interval. Unit The Tertiary conglomerate is tentatively dipping faults (13°W measured for one III could be autochthonous, as it appears to assigned to the Horse Spring Formation of block). They can be restored to local source be part of a continuous stratigraphic section Miocene age. Nondiagnostic ostracods areas just east of their present locations on beneath the low-angle faults underlying occur in Horse Spring marls in the Hoodoo ridge crests (Guth, 1980). Units I and II. Although disrupted by Hills Havoc, as do undated biotite-bearing The second type of low-angle faulting, numerous small faults, this unit remains volcanic tuffs. A similar tuff in rocks also older-on-younger, creates the Hoodoo Hills much more coherent than other units in the called Horse Spring Formation near Gass Havoc, which extends for at least 16 km Hoodoo Hills Havoc. Peak, 10 km to the SE, yielded K-Ar ages of along the west side of the Sheep Range, be- The low-angle, east-dipping Rye Patch 15.2 and 15.9 m.y. (J. F. Sutter, 1967, per- tween the Black Hills and the Wildhorse fault separates Units III and IV (Fig. 3). This sonal commun.). Thus lithology and possi- Pass fault (Fig. 2). The informal name fault crops out for 7.5 km along strike, gen- ble temporal equivalence suggest correla- comes from an unpublished 1927 observa- erally with the Mississippian Joana Lime- tion with the lower clastic unit of Bohannon tion of C.R. Longwell that dolomites in the stone as the footwall and the Devonian (1979a). The Horse Spring Formation in terrane were tectonites that eroded to form Nevada Formation as the hanging wall. the Sheep Range consists of locally derived hoodoos. Dolomite in this faulted terrane is Hanging wall dolomite is thoroughly shat- conglomerate having subordinate sand- thoroughly shattered and recemented; units tered and recemented. Although the: at- stone, tuff, and marl. Paleozoic carbonate retain only gross formational characteris- titude of the Rye Patch fault varies along clasts predominate, and there are subordi- tics. The terrane contains as least six major strike, three-point solutions indicate an av- nate quartzite clasts. Detritus from structural units numbered I through VI (Fig. erage attitude of N10°E, 19-24°E. Grooves Eocambrian clastic units, igneous rocks, 3). The youngest rocks in the Hoodoo Hills measured on the fault surface trend S80°E, and metamorphic rocks have not been ob- Havoc are found in the western units, and plunging 25°E. served. successively higher and more easterly units Units IV and V are separated by the expose older rocks. Three units (I, II, and high-angle Picture Canyon fault, which dips Fault Timing IV) appear to represent low-angle slide 78°W. Although not well exposed, Unit V is blocks. Units V and VI appear to have been apparently thickened by numerous con- Normal faults postdate the emplacement caught between faults and thoroughly cealed faults. Unit V is juxtaposed with of the Gas Peak thrust, which is directly brecciated. Unit III was initially mapped as autochthonous Cambrian dolomite by the dated as post-Permian (Leonardian) and belonging to the Havoc because of the west-dipping Wildhorse Pass fault along pre-Miocene, and indirectly dated as pre-

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150 m.y. (Guth, 1980). Normal faulting INTERPRETATION horizon near the base of the Eocambrian appears to have occurred synchronously clastic wedge, the favored interpretation with deposition of the Horse Spring For- Development of the Extensional Terrane (Guth, 1980; compare with Fig. 6B of mation. Tertiary conglomerate underlies Burchfiel and others, 1974). This requires the fault below Unit la at one locality. The Figure 4 is an interpretative cross section that a slice of the next more easterly thrust high-angle Picture Canyon fault and other of the region at the close of Mesozoic (Dry Lake) exists beneath the Gass Peak smaller faults cut the Horse Spring at other thrusting and at the present time. The sec- allochthon. localities. Tertiary rocks in the Hoodoo tion contains no vertical exaggeration, and Figure 4B shows the present configura- Hills Havoc dip an average of 20°E, and goes approximately through the location of tion. The high-angle faults are drawn as lis- appear to have been rotated by faulting. section A—A', Figure 3. tric with westward dips of 60° near the sur- Longwell and others (1965) suggested that Figure 4A shows the presumed condi- face, but exact attitudes are uncertain. Fault the Horse Spring Formation along the west tions in the region at the close of Sevier displacement rotates rocks to the west in side of the Sheep Range was deposited thrusting. The trace of the Gass Peak thrust the hanging wall about 20°. Fault activity is against an active fault scarp. in the Las Vegas Range is shown essentially presumed to have migrated eastward; older Clockwise rotation of about 90° at the as it appears today, and the upper plate of fault blocks have experienced multiple ro- extreme southern end of the Sheep Range, the thrust is drawn only to include units as tations to account for the increasingly related to drag along the Las Vegas Valley high as the uppermost Mississippian. Any greater dips of the sedimentary rocks. shear zone, appears to be the latest struc- younger rocks that may once have been 1 interpret high-angle faulting to have oc- tural event. The Gass Peak thrust and the present (probably at least Permian and curred in three stages. In the first stage, the Wildhorse Pass fault both change trend perhaps basal Mesozoic) have been eroded Alamo Road fault created the Desert Range from north to southwest, and the strike of from the region without leaving any indica- structural block. Faulting on the Wildhorse sedimentary bedding in the Black Hills and tion of their former extent. The Gass Peak Pass fault next created the Black Hills struc- the Las Vegas Range is also bent. thrust has been shown with a décollement tural block, which includes the Hoodoo

SHEEP RANGE DE SERT BLACK LAS VEGAS RANGE Wildhorse RANGE HILLS Picture Pass Mormon A la m o Canyon Fault Pass Gass Road Fau 11 / Fault Peak Faul » I / Thrust /III —T-. , / -///// /-r

B. PRESENT Figure 4. Schematic model for development of the extensional terrane in the Sheep and Desert Ranges. The section is a generalized extension of section A-A' of Figure 3; that section details relations for the surficial low-angle slide blocks. Faults are shown flattening into the Dry Lake thrust fault, but could instead flatten into the shallower Gass Peak thrust or at deeper levels. Conservation of volume pres- ents a problem at the detachment horizon; if that level resembles the chaos of the Death Valley region (Wright and Troxel, 1973), complexly anastamosing listric faults reflect adjustment within the sedimentary sequence. Faulting is inferred to occur in three stages: (1) the Alamo Road and related faults in the Desert Range; (2) the Wildhorse Pass fault and then low-angle surficial gravity slides; and (3) the Mormon Pass, Picture Canyon, and related faults in the Sheep Range.

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Hills Havoc terrane, and caused further ro- for volume conservation along listric faults. reasons: 1) the conglomerates appear to tation in the Desert Range. The final stage, Other faults within the Sheep Range suggest have been derived from the east, and might faulting along the Mormon Pass and related merely a shuffling of blocks as in the have been deposited with an initial west- faults, rotated the Sheep Range 20° and in- tilted-block model. ward dip, which would increase the amount creased the rotation of the older structural The high-angle faults might flatten into a of rotation; and 2) the conglomerates may blocks. master detachment fault, but the possible not predate the rotation but rather may be The low-angle, younger-on-older faults level for this décollement remains unclear. contemporaneous with it, and hence may (Unit 1, and the Long Valley and Hidden The Gass Peak thrust surface, probably not fully reflect the amount of rotation. Forest Blocks) have no firm age constraints. present about 1500 m below sea level, By using the estimates based on rotation The present shallow eastward dips on the could have been reactivated to form a Ter- of Paleozoic sedimentary rocks, a 9.2-km faults suggest that they predate at least the tiary detachment. The Dry Lake thrust fault extension from the crest of the Las Vegas latest phase of high-angle faulting. The should be about 2500 m deeper and offers a Range to the west side of the Desert Range low-angle faults initially dipped to the west second possible décollement horizon for can be calculated. This corresponds to and have been rotated to their present posi- Tertiary listric faults. Tertiary reactivation over-all average extension of 44% for an tion. Eastward rotation is implied by the of Mesozoic thrusts has been proposed to area now 30 km wide. Excluding the Las eastward dips of the Horse Spring Forma- explain other areas with much better sub- Vegas Range, the area west of the Mormon tion, and by anticlines indicating westward surface control (Bally and others, 1966; Pass fault that experienced the extension ac- transport in Units 1 and II. The older-on- Royse and others, 1975). Reactivation of a tually extended by 58%, and extended pro- younger Rye Patch fault beneath Unit IV is décollement horizon inherited from a gressively more toward the west. inferred to postdate the Wildhorse Pass Mesozoic compressional regime has been fault and is cut by the Picture Canyon fault. suggested to explain Tertiary listric faulting REGIONAL SIGNIFICANCE With its steep 78°W attitude, the Picture in the Great Basin (Eaton, 1980). If thrust Canyon fault belongs to the most recent surfaces were reactivated for Tertiary fault- Nellis Bombing and Gunnery Range generation of faults. If the Rye Patch fault ing, the phenomenon requires a thin-skin (4 postdates the Wildhorse Pass fault, Unit IV to 8 km or so) tectonic style. The Desert and Pintwater Ranges west of could have had a source in the footwall Thrust reactivation might not be the only the Sheep Range have been mapped only in block of the Wildhorse Pass fault. West- possible mechanism, however. In the same reconnaissance. Mapping of similar detail ward motion could have carried Unit IV paper in which he described the Desert failed to reveal the low-angle faulting in the across the fault plane of the Wildhorse Pass Range faults, Longwell (1945) described Sheep Range. Existing regional maps show fault, to come to rest on hanging wall rocks the region around Iceberg Canyon on the a large (1x5 km) block of Cambrian with the older-on-younger relation. Unit II, Colorado River. East of the recognized limit rocks, probably derived from the Desert older-on-younger above Unit I, appears to of Mesozoic thrusting, that area appears to Range, which overrides Tertiary rocks in postdate Unit 1 and to have come from a have multiply rotated fault blocks anala- the Pintwater Range (Longwell and others, similar, perhaps lower source area. gous to those in the Sheep and Desert 1965; Tschanz and Pampeyan, 1970). This The greatest westward displacement oc- Ranges. In the Mormon Mountains, listric suggests the toe of a west-moving, low- curs at the western end of the section, where faults appear to follow a décollement hori- angle slide. Both the slide and the underly- fault blocks in the Desert Range suffered zon in the Precambrian basement below the ing Tertiary strata dip eastward toward the multiple displacements. Two distinct types Mesozoic thrusts (Wernicke, 1981). These presumed source of the slide, suggesting of faulting occur in the Sheep Range exten- southern Nevada examples show that hori- later deformation. sional terrane: high-angle faults upon zons other than Mesozoic thrust surfaces The Prospector fault in the Desert Range which extension occurs, and low-angle are possible for Tertiary listric faulting. separates Ordovician rocks in the hanging faults that originated during surficial grav- wall from Eocambrian rocks in the footwall ity gliding. The low-angle faults formed as a Amount of Extension (Longwell, 1945). Longwell suggested that response to topography produced by ex- this fault was a low-angle (17°), west- tension along the high-angle faults. The tilted-block model allows the dipping fault; the hanging wall had been amount of extension to be calculated by eroded north of the fault trace to expose the Possible Geometry of High-Angle measuring the rotation of bedding. If footwall with the Eocambrian section. Extensional Faults sedimentary bedding in the Paleozoic rocks Longwell's map (1945, Fig. 8) shows about is assumed to have been horizontal at the 15° to 20° rotation of the hanging wall rel- Faults in the Sheep Range could exhibit close of Mesozoic thrusting (Armstrong, ative to the footwall. 1 would interpret this either listric or tilted-block geometries. 1968), the relation of Morton and Black fault as being similar to the extensional Erosional levels do not reveal the flattening (1975, p. 62) gives the following estimates faults in the Sheep Range; the Prospector of any normal faults in the Sheep Range, of extension: Sheep Range, about 33%; fault could in fact be the northward exten- although Longwell (1945) reported seeing Black Hills, about 70%; and Desert Range, sion of the Alamo Road fault. upwardly concave fault surfaces in the Des- about 150%. The rotation of Tertiary sed- The Pintwater Range appears to mark the ert Range, with radii of curvature between iments would provide a more reliable indi- western boundary of the terrane of east- 4500 and 6000 m. The observed stratal ro- cation (Stewart, 1980), but suitable Ter- dipping sedimentary rocks exposed in the tation along the Mormon Pass, Wildhorse tiary sediments occur only in the Black Sheep and Desert Ranges. Longwell (1945) Pass, and Alamo Road faults suggests a lis- Hills-Hoodoo Havoc structural block and mapped a large anticline whose axis ran tric nature. Observed intraformational suggest roughly 33% extension. This figure through the Pintwater Range. In the ter- faulting could reflect adjustments necessary should be regarded as a minimum, for two minology of Stewart (1980), that anticline

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might form the antiformal boundary for the Vegas Range, the present erosional level faults are spaced approximately equal dis- tilt domain including the Sheep and Desert exposes basal Cambrian quartzite on tances on each side of the shear zone, Ranges. The synformal boundary would lie Leonardian limestone. Any unique stratig- suggesting that most extension occurs be- between the Sheep and Las Vegas Ranges. raphic juxtaposition should occur only once tween the Specter and Las Vegas Ranges. Stewart (1980) suggested that faulting on the thrust ramp surface, and it forms a (Some extension may be present between progressed from the antiformal toward the unique linear feature suitable for use in the Las Vegas and Arrow Canyon Ranges.) synformal boundary, the sequence pre- palinspastic reconstructions. Topographic differences across the shear ferred for the Sheep Range. This distance from the anticline to the zone could reflect the difference in exten- Gass Peak thrust north of the shear zone is sion. The Spring Mountains to the south Las Vegas Valley Shear Zone and 42 km, while south of the shear zone, the consists of a single mountain range that has Regional Extension Wheeler Pass thrust is only 25 km from the not undergone Tertiary extension (Burchfiel anticline. Reconstruction of the two thrusts and others, 1974). North of the shear zone, Longwell (1960) described the Las Vegas as a single plate requires displacement of 17 valleys filled with alluvium separate several Valley shear zone as a major northwest- km north of the shear zone, corresponding mountain ranges. trending, right-slip fault buried in Las to extension of 80%. I suggest that motion The Las Vegas Valley shear zone is trun- Vegas Valley. Estimates of displacement are on the shear zone accommodated this dif- cated eastward by the Lake Mead fault sys- up to 64 km (Stewart and others, 1968; ferential extension north of the shear zone. tem, a major northeast-trending, left-slip Longwell, 1974). Displacement by surface This 80% extension is required between system that was active in late Tertiary time fracture and oroflexural bending took place the Gass Peak thrust and the Pintwater (Bohannon, 1979a, 1979b). The Lake 17 to 11 m.y. B.P. (Ekren and others, 1968; Range. The Morton and Black model Mead system and the Las Vegas Valley Fleck, 1970a; Longwell, 1974). Based on suggests extension of 44% between the shear zone have similar estimates of dis- analogies elsewhere in the Great Basin, An- Gass Peak thrust and the Desert Range, placement and time of active strike-slip derson (1973) and Davis and Burchfiel which produces 9.2 km of the 17 km re- motion. The Lake Mead system forms the (1973) suggested that the shear zone might quired by the offset of the thrust trace. Be- northern boundary of the Eldorado represent a strike-slip boundary between cause extension increases toward the west, Mountains extensional terrane (Anderson regions of differential extension. Mapping extreme extension between the Desert and 1971, 1973). The Las Vegas Valley shear in the Specter Range (Burchfiel, 1965) es- Pintwater Ranges would dramatically in- zone forms the southern boundary of an ex- tablished that the shear zone did not extend crease the total for the region. Thus two ar- tensional terrane with perhaps 17 km of ex- beyond that point as a surface fracture, guments are in substantial agreement that tension in the Sheep and Desert Ranges, and although large-scale oroflexural bending is significant extension occurs in the Sheep with up to a total of 40 km of extension present in the Specter Range. Range extensional terrane. possible between the Las Vegas and Specter Table 1 lists comparative distances be- The northern Spring Mountains-Pint- Ranges. The two strike-slip systems may tween correlative Mesozoic structural fea- water anticline also appears to have been have acted together to control extension be- tures across the Las Vegas Valley shear dragged by movement along the shear zone, tween the Colorado Plateau and the vicinity zone. Two correlations are of particular in- and extension west of the Pintwater Range of the Specter Range (Fig. 5). The over-all terest: that of the northern Spring Moun- appears likely. This extension should occur region might have extended at least 25%, tains anticline with the Pintwater anticline east of the Specter Range where no surface but most of the extension appears to be (B.C. Burchfiel, 1981, personal commun.), displacement occurs on the Las Vegas Val- concentrated in a few areas of extreme ex- and that of the Wheeler Pass and Gass Peak ley shear zone (Burchfiel, 1965). East of the tension. Additional extensional terranes thrusts. Both thrusts are exposed as ramps Gass Peak thrust, the correlations do not along the strike-slip faults — the Virgin cutting up section in the lower plate, and in support large differential extension across Mountains and the area to the south (Sea- both the Spring Mountains and the Las the shear zone (Table 1). Correlated thrust ger, 1970; Longwell, 1945), the Mormon Mountains (Wernicke, 1981), and French- man Mountain (Longwell and others, 1965 TABLE 1. DISTANCES TO CORRELATED MESOZOIC STRUCTURAL FEATURES — could greatly increase the amount of ACROSS THE LAS VEGAS VALLEY SHEAR ZONE over-all regional extension. South side of shear zone North side of shear zone

Specter Range Specter Range Volcanism and Extension 22 km 48.5 km NW Spring Mountains anticline Pintwater anticline Previous workers (Fleck, 1970a; Liggett 25 km 42.5 km and Ehrenspeck, 1974; Liggett and Childs, Wheeler Pass thrust Gass Peak thrust 1977) have suggested that the Las Vegas 12.5 km 17.5 km Valley shear zone served as a transform Lee Canyon thrust Arrow Canyon syncline fault separating two offset segments of a 5 km 5 km ridge system or axis of crustal extension. Deer Creek thrust Dry Lake thrust They viewed extension as occurring in two 17.5 km 15 km volcanic provinces centered in the Nevada Keystone thrust Glendale thrust Test Site and the Lake Mead area. Correlations after Fleck, 1970b; Burchfiel and others, 1974; Axen, 1980; and B. C. Burchfiel, 1981, Evidence presented here for significant personal commun. extension of the sedimentary terrane west Distances measured perpendicular to structural strike. of the Sheep Range shows these models to

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Figure 5. Model for regional extension along the right- lateral NW-trending Las Vegas Valley shear zone and the left- lateral NE-trending Lake Mead fault system. Areas of known extension between the Specter Range and the Colorado Plateau are shown by arrows in the Sheep and Desert Range extensional terrane north of the Las Vegas Valley shear zone and the Eldorado Mountains south of the Lake Mead system. Additional extension: is likely at Frenchman Mountain (FM), in the Virgin Mountains (VM) and vicinity, and in the Mormon Mountains (MoM). Northward the extensional terranes are probably bounded by the Pahranagat shear system, while southward there must be a transition to the extensional terrane in the Whipple Mountains region. Thrust faults shown are the Wheeler Pass (WPT), Gass Peak (GPT), Keystone (KT), and the Glendale (GT). Relations between the two strike-slip systems after Bohannon (1979a, and 1981, personal commun.).

be overly simplistic. Volcanism, intrusion, The offset of the Gass Peak and Wheeler tion Grant EAR 77-13637 awarded to B. C. and metamorphism do not appear to be di- Pass thrusts across the Las Vegas Valley Burchfiel, and the U.S. Army. Access and rectly required for extension of the upper shear zone suggests that the region between encouragement were provided by the Desert crust. The normal faulting in the Sheep the Sheep and Specter Ranges extended National Wildlife Range, U.S. Fish and Range region suggests almost 100% exten- almost 100% during the Miocene. The ex- Wildlife Service. Finally I thank my field sion of the upper crust without compensa- tensional faulting west of the Sheep Flange assistants and the Clones, who shared the tion by intrusion into the lower crust be- occurred in an area that experienced no joys and drudgery of the desert and the neath that region. The extensive volcanic Mesozoic or Tertiary plutonism, volcanism, drafting room. Without their support I activity in the Nevada Test Site simultane- or metamorphism. The Las Vegas Valley would never have completed the work. ous with extension along the Las Vegas Val- shear zone separated the extending terrane B. C. Burchfiel, J. B. Bartley, C. S. Came- ley shear zone may be evidence of the lower from the Spring Mountains to the south. ron, M. G. Haselton, K. V. Hodges, J. E. crustal compensation for the upper crustal The interaction of the shear zone with the Spencer, and B. Wernicke read various extension near the Sheep Range. This sep- Lake Mead fault system may link the Sheep drafts of the manuscript and helped nurse it aration of lower- and upper-plate crustal Range extensional terrane with a compen- along to completion. R. G. Bohannon, extension along a low-angle or horizontal sating region of extension in the Eldorado M.D. Crittenden, Jr., and W. S. Snyder detachment fault has been proposed by Mountains. The strike-slip faults of the constructively reviewed the manuscript. I Wernicke (1981 and unpub. data). Future Great Basin might join compensating re- have incorporated many of their sugges- models will have to account for the syn- gions of extreme extension separated by tions but remain responsible for the specu- chroneity of volcanism and extension in the large blocks that have not undergone major lations that remain. Great Basin, as well as for the fact that ex- extension. tension can occur in areas unaffected by Igneous and metamorphic events may not REFERENCES CITED thermal events. necessarily be directly required for extensional faults to develop. I am also suggest- Anderson, R. E., 1971, Thin-skin distension in Tertiary rocks of southeastern Nevada: CONCLUSIONS ing that regions with compensating exten- Geological Society of American Bulletin, sion in the upper crust may be separated v. 82, p. 43-58. The Sheep Range and the area to the west many kilometres by transform strike-slip 1973, Large-scale magnitude late Tertiary experienced two types of Tertiary faulting. faults. strike-slip faulting north of Lake Mead, High-angle normal faults caused rotation of Nevada: U.S. Geological Survey Profes- sional Paper 794, 18 p. sedimentary bedding to produce significant ACKNOWLEDGMENTS Armstrong, R. L., 1968, Sevier orogenic belt in extension. Three major faults each account Nevada and Utah: Geological Society of for 20° of eastward rotation, and the This paper results from research con- America Bulletin, v. 79, p. 429-458. tilted-block model of Morton and Black ducted between 1977 and 1979 for my 1972, Low-angle (denudation) faults, hinter- land of the Sevier orogenic belt, eastern (1975) estimates extension ranging from Ph.D. at the Massachusetts Institute of Nevada and western Utah: Geological So- 33% for the Sheep Range to 150% for the Technology, and would not have been pos- ciety of America Bulletin, v. 83, p. 1729- Desert Range. A listric-fault geometry is sible without the advice and encouragement 1754. also possible but does not provide quan- of my advisor, B. C. Burchfiel. I gratefully Axen, G. J., 1980, Geology of the La Madre titative estimates of extension. Large, low- acknowledge financial support from two Mountain area, Spring Mountains, Nevada: Unpub. M.S. thesis, Massachusetts Institute angle fault blocks represent surficial re- Geological Society of America Penrose of Technology, 170 p. sponses to topography produced by the ex- grants, a Fannie and John Hertz Founda- Bally, A. W., Gordy, P. L., and Stewart, G. A., tension. tion fellowship, National Science Founda- 1966, Structure, seismic data, and orogenic

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International symposium on the Afar region MANUSCRIPT RECEIVED BY THE SOCIETY DE- Fleck, R. J., 1970a, Age and possible origin of the and related rift problems, E. Schweizer- CEMBER 18, 1980 Las Vegas Valley shear zone, Clark and Nye bart'sche Verlagsbuchhandlung, Stuttgart, REVISED MANUSCRIPT RECEIVED APRIL 13,1981 Counties, Nevada: Geological Society of Germany, Proceedings, Scientific Report MANUSCRIPT ACCEPTED APRIL 20, 1981

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