Kinematics and Timing of Tertiary Extension in the Western Lake Mead Region, Nevada

Home , Aztec Sandstone, Fortification Hill, Frenchman Mountain, Horse Spring Formation, Muddy Mountains, Thumb member

Kinematics and timing of Tertiary extension in the western Lake Mead region, Nevada

ERNEST M. DUEBENDORFER Department of Geology, Northern Arizona University, Flagstaff, Arizona 86011 DAVID A. SIMPSON* Department of Geoscience, University of Nevada, Las Vegas, Nevada 89154

ABSTRACT postdate the major phase of extension and in footwall geometry (McClay and Ellis, right-slip faulting in the western Lake Mead 1987; Ellis and McClay, 1988), regional con- Explanation of the origin of the complex ar- area. strictional strains (Fletcher and Bartley, ray of structures in some extensional terrenes Dynamic models that invoke either a single 1991; Anderson and Barnhardt, 1993), or far- (including folds and normal, strike-slip, and stress Held or rotating stress fields to explain field compressive stresses unrelated to the reverse faults) includes many models that im- development of structures in the western Lake extensional process (Cakir and Aydin, 1990). plicitly assume kinematic compatibility be- Mead area are inconsistent with the kinematic Models proposed to explain the origin of di- tween and contemporaneous operation of these and age data. Similarly, kinematic models that verse structural assemblages in structurally structures. We present new stratigraphic and view all structures in the context of a single complex extensional terranes such as the age data from the highly extended western strain field are precluded by systematic cross- Lake Mead region, southern Nevada, have Lake Mead region, Nevada, together with an cutting relationships that demonstrate at least assumed implicitly that all structures are analysis of fault kinematics (technique of Mar- partial diachroneity of deformational styles. kinematically compatible and developed and rett and Allmendinger, 1990) to test the as- Large-magnitude extension south of the Las operated contemporaneously. These assump- sumptions of kinematic compatibility and con- Vegas Valley and Lake Mead fault zones ap- tions have not been tested rigorously. temporaneity of structures in an area of pears to have been followed by north-south Our purpose in this paper is threefold. excellent exposure and superb stratigraphic contraction that was highly localized near the First, using the Marrett and Allmendinger control. Our analysis indicates an overlapping region of greatest extension. We suggest that (1990) technique of fault-slip data analysis, but clearly distinct chronology of deformation. lateral pressure gradients arising from differ- we evaluate the kinematic compatibility of Early regional extension (> 187-13.5 Ma) is ential crustal thinning at the northern end of structures in the western Lake Mead area marked by development of a basin into which the Colorado River extensional corridor may (Fig. 1), a region that contains both exten- the middle Miocene lower Horse Spring For- have provided the driving mechanism for lo- sional and contractional structures. We eval- mation was deposited. In the western Lake calized contractional deformation. uate results of this analysis in the context of Mead region, this basin was disrupted by more new geochronological and fielddata . Second, areally restricted, post-13 Ma normal and kin- INTRODUCTION we present an assessment of the Miocene ex- ematically coupled right-slip faulting along the tension direction in the western Lake Mead Las Vegas Valley shear zone. Kinematic anal- Considerable controversy exists regarding region. Previous studies variably report the ysis of faults indicates an average regional ex- the origin and kinematic significance of the extension direction as southwest (Anderson, tension direction of nearly due west for the wide array of structures other than normal 1973; Bohannon, 1979; Weber and Smith, middle and late Miocene. faults in extensional tectonic regimes. These 1987), approximately west (Wernicke and Extension and right-slip faulting was fol- structures include strike-slip faults, reverse others, 1988; Rowland and others, 1990; lowed by development of dominantly south- faults, and folds. For example, strike-slip Fryxell and Duebendorfer, 1990; Duebendor- vergent contractional structures including faults in extensional settings have been con- fer and others, 1990), west-northwest (Long- tight, east-plunging folds and east-striking re- sidered (1) transfer faults that link areas un- well, 1974), or invoke changing extension diverse faults. These structures deform the post- dergoing differential extension (Anderson, rections with time (Angelier and others, 1985; 8.5 Ma Muddy Creek Formation; the Muddy 1971,1973; Davis and Burchfiel, 1973; Weber Choukroune and Smith, 1985). Third, we Creek Formation is not cut by the Las Vegas and Smith, 1987; Burchfiel and others, 1989), evaluate existing models for Miocene exten- Valley shear zone. Older faults, including the (2) first-order, deep-seated crustal structures sional tectonism in the western Lake Mead eastern Las Vegas Valley shear zone, reacti- that operate independently of extension region. Existing models for Tertiary deforma- vated as south-vergent reverse faults. North- (Ekren and others, 1976; Ron and others, tion in this area focus on one or two struc- east-striking left-slip faults cut folds and re- 1986), or (3) pre-existing barrier faults that tures (Anderson, 1973; Bohannon, 1979) or verse faults. These observations show that may compartmentalize later deformation provide an incomplete kinematic explanation north-south shortening and left-slip faulting (Bartley and others, 1992). Contractional for the varied structural elements present structures such as folds and reverse faults (Ron and others, 1986; Cakir and Aydin, may form because of listric, normal-fault ge- 1990; Campagna and Aydin, 1991). These ometry (Hamblin, 1965; Dula, 1991; Xiao and studies also were hampered by lack of suffi- *Present address: Thiel, Winchell, and Associ- cient age data to resolve adequately the tim- ates, Inc., 34 Lakes Boulevard, Dayton, Nevada Suppe, 1992), localized hanging-wall contrac- 89403. tile strains, (Brumbaugh, 1984), irregularities ing of various structures. We conclude by

Geological Society of America Bulletin, v. 106, p. 1057-1073, 17 figs., August 1994.

1057 DUEBENDORFER AND SIMPSON proposing an alternative tectonic model that honors all available kinematic and timing data. Our results show that the Marrett and All- mendinger (1990) technique, combined with development of a field-based structural chronology, is a powerful tool to establish kinematic compatibility or incompatibility between faults sets in complex areas and may be more meaningful in characterizing the overall strain pattern in complexly deformed regions than the commonly applied paleostress inversion techniques (for example, An- gelier and others, 1985; see also Pollard and others, 1993).

TECTONIC SETTING

A marked change in the style of Basin and Range extension occurs in the Lake Mead region. South of the Lake Mead area, the "core complexes" of the Colorado River extensional corridor expose ductilely deformed rocks in the lower plates of regional detachment faults (Davis and others, 1980, 1982, Figure 1. Map of the Lake Mead area showing principal geographic features and geologic 1986; Howard and John, 1987; many others) structures. CM = Callville Mesa, LW = Lovell Wash, SIF = Saddle Island fault. Light stipple and major transverse structures appear to be represents Lake Mead. rare. For 250 km to the north, exposures of ductilely deformed lower-plate rocks are rare and transverse structures are common (Due- The Saddle Island fault is a low-angle fault strike-slip faults in the area. The Rainbow bendorfer and Black, 1992). The Lake Mead that contains the characteristic elements of Gardens Member consists of a basal con- area contains both core complex-type de- classic metamorphic core complexes (Smith, glomerate that fines upward into evaporite tachment systems as well as two of the larg- 1982; Choukroune and Smith, 1985; Sewall, and lacustrine limestone. The unit predates est strike-slip (transverse) faults in the Basin 1988; Duebendorfer and others, 1990). Re- or records the earliest phase of extension and Range province. These faults mark the construction of structurally disrupted mid- in the region (Anderson, 1973; Bohannon, northern terminus of the northern Colorado Miocene volcanic-plutonic complexes sug- 1979). The overlying Thumb Member con- River extensional corridor (Faulds and oth- gests 20 km of post-13.4 Ma westward tains terrigenous clastic deposits, evapor- ers, 1990, 1992). translation of upper-plate rocks along the de- ites, air-fall tuff, and distinctive megabrec- The three principal Tertiary structures in tachment (Weber and Smith, 1987). cia deposits. The upper two units of the the Lake Mead region are the Lake Mead Horse Spring Formation, the Bitter Ridge fault system, the Las Vegas Valley shear STRATIGRAPfflC FRAMEWORK Limestone and Lovell Wash Members zone, and the Saddle Island fault (Figs. 1 (13.5-12.0 Ma; Bohannon, 1984; Dueben- and 2). The Lake Mead fault system is a zone Tertiaiy Stratigraphy dorfer and others, 1991) consist of lacus- of northeast-striking, left-slip faults that col- trine limestone and interbedded tuffaceous lectively accounted for between 20 and 65 km The western Lake Mead region contains siltstone and sandstone. These units show of slip between 17 and 10 Ma (Anderson, Tertiary sedimentary and volcanic rocks that marked differences in lithology and thick- 1973; Bohannon, 1979,1984). The northwest- lie with minor angular discordance on Trias- ness across the Las Vegas Valley shear striking Las Vegas Valley shear zone has sic to Cretaceous rocks (Figs. 2 and 3). Mio- zone, which probably record the onset of well-documented right-slip displacement of cene rocks of the Lake Mead region have major extensional deformation in the west- 48 ± 7 km (Longwell, 1960, 1974; Burchfiel, been divided into three unconformity- ern Lake Mead area. 1965; Fleck, 1970; Wernicke and others, bounded sequences (Bohannon, 1984). These The informally named red sandstone unit 1988). The Las Vegas Valley shear zone ap- are the Horse Spring Formation, the infor- (11.9 ± 1.2 to 8.5 ± 0.2 Ma; Bohannon, 1984; pears to be younger than 15 Ma (Fleck, 1970), mally named red sandstone unit and associ- Feuerbach and others, 1991), is composed of and Deibert (1989) and Duebendorfer et al. ated volcanic rocks of Callville Mesa, and the conglomerate, sandstone, siltstone, air fall (1991) suggested that significant movement Muddy Creek Formation. tuff, and basaltic andesite flows and flow postdated 13 Ma. These interpretations are Bohannon (1984) divided the Horse Spring breccias of the Callville Mesa volcanic field supported by paleomagnetic data from sedi- Formation into four members (Fig. 3). The (Feuerbach and others, 1991, 1993). Bohan- mentary units as young as 13.5 Ma, which Rainbow Gardens and Thumb Members (24- non (1984) recognized the unit in White basin indicate clockwise rotations to 70° (Sonder 13.5 Ma; Bohannon, 1984; Beard and Ward, and east of Frenchman Mountain (Figs. 1 and and others, 1989; Jones and others, 1991). 1993) occur both north and south of the major 2); Duebendorfer and Wallin (1991) docu-

1058 Geological Society of America Bulletin, August 1994 TERTIARY EXTENSION, LAKE MEAD REGION, NEVADA Age (Ma)

Fortification Hill Basalt 5.88'

.»ÄW.».".»'«;*';! Muddy Crwk Furmatlon

Volcanic Rucks of Callville Mfsa

10.9-Ï.6-1 Red Sandstone Unit M.9-10.64

1Î.0-1 Lovell Wish Mfrntxr

13.0'1 Bitter Kid ge Limestone Member ! ] 3-2 Explanation '•'-*- I lot I Quaternary and Tertiary (Muddy Creek Fm.) deposits (<8.5 Ma) Thumb Member t/iCh mg mJB Red sandstone unit of Bohannon (1984)( 11,9-8,5 Ma) a> z:uvc\ Volcanic rocks of Callville Mesa (10.5-8.5 Ma) i S hHTu I Older Tertiary volcanic rocks (middle Miocene) I Oft] Horse Spring Formation (>24-12.0 Ma) 17. Teniary and Prceambriun crystalline rocks Ruini»» (jsrdens FrXfrMl Paleozoic and Meso/oic rocks Member COO 1 : a f s i c n l . ' 't t^-l 'r Figure 2. Highly generalized geologic map of the study area. LWSZ = Las Vegas Valley shear Pre-Teniary Rocks zone, LMFS = Lake Mead fault system, SIF = Saddle Island fault, WBF = West Bowl of Fire fault. Heavy rectangle shows location of map, Figure 14. Figure 3. Generalized stratigraphic column for the western Lake Mead area (modified af- mented widespread exposures of the unit in the onset of active extension in the western ter Bohannon, 1984). Dates in Ma. 1 = K/Ar Boulder basin. Lake Mead region. plagioclase (Damon and others, 1978); 2 = The late Miocene Muddy Creek Forma- Frenchman Mountain Block. The French- K/Ar plagioclase (Feuerbach and others, tion (8.5-5.8 Ma; Damon and others, 1978; man Mountain block is a structurally intact 1991); 3 = K/Ar plagioclase (Duebendorfer Feuerbach and others, 1991) lies uncon- homocline that dips 45°-55°E. The block con- and others, 1991); 4 = fission track on air fall formably upon the red sandstone unit and tains the most complete Tertiary section in tuffs (Bohannon, 1984); 5 = K/Ar whole rock consists of boulder-cobble conglomerate, the study area (Fig. 4); however, the Bitter (Anderson and others, 1972). sandstone, siltstone, and gypsum. The unit Ridge Limestone pinches out to the north and probably represents continued infilling of contains more clastic detritus than its strati- extensional basins formed in the middle graphic counterpart on the Muddy Mountain conglomerate interfingers with lacustrine de- Miocene. block. The northern end of the block, there- posits of the Bitter Ridge Limestone and fore, may have been topographically higher Lovell Wash Members directly north of the Regional Stratigraphie Variations than the southern end from 13 to 13.5 Ma. Las Vegas Valley shear zone (Bohannon, The Bitter Ridge Limestone may actually 1984). These relations suggest that the Las The study area may be divided into three represent two separate basins, one in the Vegas Valley shear zone was active during blocks, each of which exhibits a depositional present Muddy Mountains and one at the deposition of the younger members of the and structural history distinct from its neigh- south end of Frenchman Mountain. Major Horse Spring Formation and may have bors. These are (1) the Frenchman Mountain stratal tilting occurred between 11.9 and 8.5 marked the southern margin of the basin, at block, (2) the Muddy Mountains block, and Ma (Figs. 4 and 5). that locality, into which the units were (3) the Boulder Basin block (Fig. 4, inset). Muddy Mountains Block. The Muddy deposited. The pre-13.5 Ma stratigraphy is similar on all Mountains block contains a complete section Boulder Basin Block. Within the Boulder blocks; their stratigraphie records and stratal of the Horse Spring Formation, including basin block, the Lovell Wash Member (200- tilt relations diverge markedly at about 13.0 thick sections of Bitter Ridge Limestone (375 250 m) conformably overlies the Thumb Ma (Fig. 4). The appearance of these differ- m) and Lovell Wash Members (250 m), but Member and contains an 80- to 100-m-thick ences in stratigraphy and stratal tilting signals few exposures of younger rocks. A boulder basaltic-andesite sequence dated at 12.0 ±

Geological Society of America Bulletin, August 1994 1059 DUEBENDORFER AND SIMPSON

Boulder Basin Muddy Mts. Frenchman Mt. Block .cS, Muddy Mountains Frenchman ^ LW Mt. Block CI>1 HM/O- Muddy Creek Muddy Creek v Boulder Basin . <10° Fm. 15° Fm. Las Vegas ^Blockr-» \ Muddy Creek Fm. Red sandstone Red sandstone Lake Mead- 40° \ ) 20 km unit unit (includes -360 ,15° Lovell Wash CaMte Mesa 10-45° Member Lovell Wash " V volcanic Member 40° V Wa sÎV^ 35-45° Member Bitter Ridge Tc o Limestone Member _t c o Bitter Ridge ra Thumb Figure 4. Generalized columns showing rel- — 13 Ma E Limestone Membe Member ative age and stratigraphie relations between O) Tertiary sections in the Muddy Mountains, c Thumb Member O) Frenchman Mountain, and Boulder basin Q. c S Thumb Member 15-25° I/) Q. blocks. Columns are referred to a 13-Ma da- to tum; thicknesses on columns do not corre- 40° spond to true stratigraphie thicknesses. Num- o bers to the right of each column indicate x 45-50° Mesozoic average range of dips for corresponding strat- Rainbow Gardens and igraphie unit. Inset shows location of French- Member Paleozoic Rainbow Gardens man Mountain, Muddy Mountains, and Boul- Member rocks der Basin blocks. LWSZ = Las Vegas Valley Mesozoic and shear zone; LMFS = Lake Mead fault system, Paleozoic rocks 45-50° Mesozoic and LW = Lovell Wash, CM = Callville Mesa; Paleozoic rocks 20-25° HM = Hamblin Mountain.

0.3 Ma (K/Ar plagioclase; Duebendorfer and erned by major strike-slip faults, suggesting the Las Vegas Valley shear zone records de- others, 1991). The block contains the most that the principal period of movement on velopment of the extensional Boulder basin widespread and complete exposures of post- these structures occurred after Thumb time produced by movement along the Saddle Is- Horse Spring rocks in the Lake Mead area (Bohannon, 1984). The appearance of mega- land low-angle fault (Duebendorfer and Wal- (Fig. 4), suggesting that Boulder basin has breccia deposits within the 17- to 13.5-Ma lin, 1991). The Las Vegas Valley shear zone been a structural and topographic low since Thumb Member affords the earliest record of forms the northern margin of the Boulder ba- deposition of the red sandstone unit (Dueben- topographic relief associated with the onset sin and may have functioned as an exten- dorfer and Wallin, 1991). In the western and of shallow-crustal extension in the eastern sional transfer fault. The red sandstone unit central parts of the Boulder basin block, the Lake Mead region (Longwell, 1974; Bohan- contains Bitter Ridge Limestone clasts de- steeply dipping red sandstone unit lies above non, 1983,1984; Parolini, 1986; Rowland and rived from the north, indicating that a struc- the Lovell Wash Member without angular others, 1990). tural and topographic inversion had occurred discordance. In the eastern part of the basin, Marked differences in the stratigraphie across the shear zone by —11.9 Ma. the stratigraphically high parts of the red record at 13.5-13.0 Ma signal the onset of ac- sandstone unit are only slightly tilted and rest tive extension in the western Lake Mead re- STRUCTURE with significant (40°) angular discordance on gion. Specifically, the abrupt southward fa- the Thumb Member. The red sandstone unit cies transition within the Bitter Ridge Kinematic Analysis of Fault-Slip Data is cut by many faults that do not cut the over- Limestone and Lovell Wash Members from lying Muddy Creek Formation. These obser- lacustrine deposits to coarse clastic detritus Introduction. We employ the graphical vations indicate that the Boulder basin was directly north of the Las Vegas Valley shear method for kinematic analysis of fault-slip characterized by basin development and zone indicates that the Muddy Mountains data developed by Marrett and Allmendinger disruption, volcanism, normal faulting, and block was topographically and structurally (1990) to analyze faults exposed in a 250-km2 stratal tilting between 12.0 and 8.5 Ma. low relative to a southern block between 13.5 area north of Lake Mead, east of Frenchman and 13.0 Ma. There is currently no obvious Mountain, and west of Hamblin Mountain Discussion source for the lower Paleozoic clasts within (Fig. 1). The goal of this analysis is to (1) de- the conglomerate facies of the Bitter Ridge termine the kinematics of individual fault sets The pre-Tertiary "basement" on all blocks Limestone; however, restoring 20 km of to infer principal shortening and extension was tilted < 10° at the time of deposition of the movement along the Saddle Island fault axes for each set, (2) assess the degree of kin- basal Tertiary units (Fig. 4). Facies distribu- places Frenchman Mountain in a position to ematic compatibility or incompatibility of dif- tions within the lower units of the Horse shed these clasts into the Bitter Ridge basin. ferent sets of faults to determine which sets Spring Formation do not appear to be gov- Presence of the red sandstone unit south of may have functioned in concert to accommo-

1060 Geological Society of America Bulletin, August 1994 TERTIARY EXTENSION, LAKE MEAD REGION, NEVADA

date regional strains, and (3) infer the collec- tive kinematics of faults both north and south of the Las Vegas Valley shear zone to determine whether this regional geologic boundary also separates kinematically distinct domains of deformation. Sense of slip on individual faults was determined by a combi- nation of fault striae and stratigraphic sepa- ration, brittle kinematic indicators (Petit, 1987) where available, and drag features. The Marrett and Allmendinger (1990) method utilizes a seismological approach to three-dimensional incremental strain analysis. For each fault datum (fault surface and N = 55 striae orientation combined with sense of Ijiwcr Horse Spring Formation slip), a principal incremental shortening and Pre-Tcrtiary rocks extension axis is determined graphically. One reading is taken from each fault. Shortening and extension axes are then contoured for a set of fault data to determine the regional kinematic axes represented by that fault set. We emphasize that this technique is funda- mentally different than widely used stress-inversion techniques (for example, Angelier and others, 1985) in that it does not seek to determine the orientation of principal paleostress directions, but rather reflects the kinematics of motion on a fault. This approach partially circumvents the problem of re- activation or reorientation of faults in that a pre-existing fault may be reactivated under a different stress regime even though its orientation is not optimal for development of frac- c -— ^ N = 42 tures predicted by the Coulomb fracture criterion. In the dynamic approach, such a fault Upper Horse Spring Formation Kwi sandstone unii could yield a spurious paleostress direction; whereas in the kinematic approach, the sense of slip would reflect a component of the overall kinematic framework independent of the regional stress field at the time of faulting. Assumptions. Kinematic analysis of faults involves several assumptions (Marrett and Allmendinger, 1990). The most important assumptions that must be evaluated are that (1) fault kinematics are scale-independent or in- variant, (2) early-formed faults are not reoriented during later deformation, and (3) reac- tivation of early faults has not occurred under a later stress regime. The validity of the first assumption is im- possible to evaluate in the Lake Mead region. N = 21 Absolute magnitude of displacement on most Muddy Creek Formation faults is an elusive quantity, and estimates of finite displacement based on fault width and Figure 5. Lower-hemisphere, equal-area projection of poles to bedding of rocks of the French- gouge thickness (for example, Cox and man Mountain block. (A) Mesozoic and Paleozoic rocks. (B) Lower Horse Spring Formation. (C) Scholz, 1988; Hull, 1988) or fault length (El- Upper Horse Spring Formation. (D) Red sandstone unit. (E) Muddy Creek Formation. Data liott, 1976; Walsh and Watterson, 1988) are from Longwell (unpublished mapping) and Duebendorfer (unpublished mapping). The maxi- problematic in the study area because of large mum pre-Tertiary tilt of the Paleozoic and Mesozoic section on Frenchman Mountain is 9°E. There differences in mechanical properties of units is no significant discordance between the red sandstone unit and the Horse Spring Formation. juxtaposed along faults (for example, Evans,

Geological Society of America Bulletin, August 1994 1061 DUEBENDORFER AND SIMPSON

1990) and because many faults are localized with the general eastward tilt of strata in the Figure 6. Lower-hemisphere, equal-area within highly incompetent evaporite units. area. A notable example of this fault set is the projections showing orientation and kinematic We justify this assumption by noting the West Bowl of Fire fault (Fig. 2), a 45° west- data for faults north of the Las Vegas Valley many studies that demonstrate the scale-in- dipping normal fault that places Bitter Ridge shear zone. Fault data (column 1) and kine- variant kinematics of fault systems (for ex- Limestone over Jurassic Aztec sandstone, matic analyses (columns 2 and 3) are presented ample, King, 1983; Turcotte, 1986; Barton resulting in deletion of at least 1200 m of in terms of 45°-orientation domains. Col- and others, 1988). The second assumption is section. umn 1: plots of faults (great circles), striae clearly invalid for part of the study area. Ad- Both the left-slip and normal faults yield a (dots), and slip direction (arrow shows move- jacent to the Las Vegas Valley shear zone, statistical extension axis (Fig. 6) of 265°, thus ment of hanging wall block). N = number of structures are rotated conspicuously clock- raising the possibility of kinematic compati- measurements. Column 2: contoured diagram wise. The paleomagnetic results of Jones and bility of the two fault subsets. Kinematic of extension axes. Column 3: contoured dia- others (1991) show, however, that post-14- compatibility of faults with different orienta- gram of shortening axes. Contour interval = Ma vertical axis rotations decrease sharply tions requires that faults either slip parallel to 2

1062 Geological Society of America Bulletin, August 1994 Faults north of the Las Vegas Valley shear zone

Domain Faults and Striae Extension Axes Shortening Axes

North-northeast- striking faults

N = 44

East-northeast- striking faults

N = 44

North-northwest- striking faults

N = 23

West-northwest- striking faults

N s 26

Geological Society of America Bulletin, August 1994 1063 DUEBENDORFER AND SIMPSON

data from these fault sets to evaluate the pos- Faults north of the Las Vegas Valley shear zone sibility that north-northeast- and north-northwest-striking faults could represent a single set of kinematically coordinated, conjugate faults (Fig. 7). The extension direction de- fined by the combined data set is horizontal Domain Extension Axes Shortening Axes and east-west. Two distinct shortening-axes maxima, one north-south and subhorizontal and one nearly vertical, are present. The two maxima are separated by 15°-45° "low-den- sity" zones. This observation argues against true constrictional strain in which shortening Combined NNE and should be expressed equally in all directions NNW striking faults normal to the finite extension direction. Therefore, although the right- and left-slip faults could represent a kinematically linked conjugate fault set (but see discussion of tim- N =67 N = 67 ing), the normal faults may reflect a separate phase of deformation with kinematics distinct from one or both sets of strike-slip faults. This interpretation is most consistent with timing relations between fault sets as discussed below. Combined East-Northeast- and West- Northwest-Striking Faults. We combined data from these fault sets to evaluate the pos- Combined ENE and sibility that east-northeast- and west-north- WNW striking faults west-striking faults could be kinematically related (Fig. 7). A strong maximum of subhorizontal extension axes is oriented at 293° (Fig. 7). Minor submaxima at vertical and N = 70 N = 70 north-south horizontal, however, suggest a non-uniform kinematic picture that is incon- Figure 7. Lower-hemisphere, equal-area projections showing orientation and kinematic data sistent with a single strain field. This inter- for combined north-northeast- and north-northwest-striking faults (row 1) and for combined pretation is supported by the plot of shorten- east-northeast- and west-northwest-striking faults (row 2) north of the Las Vegas Valley shear ing axes (Fig. 7) in which the primary and zone. Column 1: contoured diagram of extension axes. Column 2: contoured diagram of short- secondary shortening directions, north-south ening axes. Contour interval = 2

1064 Geological Society of America Bulletin, August 1994 Faults south of the Las Vegas Valley shear zone

Domain Faults and Striae Extension Axes Shortening Axes

North-northeast- striking faults

WsltllB1'"'

N = 12 N = 12

All northwest- striking faults

N = 18 —i——" N = 18 N = 18 Figure 8. Lower-hemisphere, equal-area projections showing orientation and kinematic data for faults south of the Las Vegas Valley shear zone. Fault data (column 1) and kinematic analyses (columns 2 and 3) are presented in terms of 45°-orientation domains. Column 1: plots of faults (great circles), striae (dots), and slip direction (arrow shows movement of hanging wall block). N = number of measurements. Column 2: contoured diagram of extension axes. Column 3: contoured diagram of shortening axes. Contour interval = 2o- (after Kamb, 1959).

Geological Society of America Bulletin, August 1994 1065 DUEBENDORFER AND SIMPSON

directions suggest a constrictional strain pattern during extension, a change in finite shortening direction during extension, or compo- nents of both.

Domain Extension Axes Shortening Axes Folds

North- to Northeast-Trending Folds. North- to northeast-trending folds deform rocks as young as the Bitter Ridge Limestone north of the Las Vegas Valley shear zone (Fig. 2) (Bo- Combined NNE and hannon, 1983). These folds are open, upright, NNW striking faults generally symmetrical, and variably spaced. Most of the folds are parallel to and lie in the hanging walls of north- to northeast- striking normal faults. The traces of fold axial surfaces are rotated clockwise near N = 15 N = 15 the Las Vegas Valley shear zone. The folds differ markedly in distribution, orientation, and geometry from the younger, east-trending folds described below. The origin of the north- to northeast-trending folds is enigmatic. Based on their geometry, spatial distribution, and similarity in orientation and timing with north-northeast- Combined ENE and striking normal faults, we interpret these WNW striking faults folds as either rollover structures or extensional fault-bend folds formed in the hanging walls of normal faults (McClay and Ellis, 1987; Ellis and McClay, 1988; Xiao and Suppe, 1992). We reject explanations that N =29 these folds are related to either a shortening Figure 9. Lower-hemisphere, equal-area projections showing orientation and kinematic data event or strike-slip faulting. In the first case, for combined north-northeast- and north-northwest-striking faults (row 1) and for combined the fold orientation requires regional short- east-northeast- and west-northwest-striking faults (row 2) south of the Las Vegas Valley shear ening that is approximately east-west. This zone. Column 1: contoured diagram of extension axes. Column 2: contoured diagram of short- kinematic situation is difficult to reconcile ening axes. Contour interval = 2

1066 Geological Society of America Bulletin, August 1994 TERTIARY EXTENSION, LAKE MEAD REGION, NEVADA

Shortening iS Extension Axes nent of constrictional strain during extension, or (3) were formed as a result of far-field stresses at the apex of the wedge bounded by the Las Vegas Valley shear zone and Lake Mead fault system (for example, Cakir and Aydin, 1990). We evaluate these models below.

Structural Chronology

I All structures described above deform rocks as young as the 12.0-Ma Lovell Wash Member. Because we cannot determine N = 137 when these structures began forming, some of these structural sets may have been active contemporaneously; however, unambiguous crosscutting relationships exist that permit development of a structural chronology based on the most recent phase of movement of each set (Figs. 14,15, and 16). We discuss below the following sequence of deforma- South of Las Vegas tion, from oldest to youngest: (1) North- Valley shear zone northeast-striking normal faults and northeast-trending folds, (2) right-slip faults associated with and including the Las Vegas Valley shear zone, (3) east-northeast- to west-northwest-striking reverse faults and associated folds, and (4) northeast-east- Figure 11. Lower-hemisphere, equal-area projections showing shortening (column 1) and northeast-striking left-slip faults. extension (column 2) axes for all faults north (row 1) and south (row 2) of the Las Vegas Valley Evidence for early normal faulting and as- shear zone. Contour interval = 2cr (after Kamb, 1959). sociated folding is indirect, but compelling, based on the following observations: (1) Nor- mal faults and folds are rotated clockwise associated with reverse faults, and crosscut- shear zone strongly suggests a kinematic con- near the Las Vegas Valley shear zone, sug- ting relations suggest that these two struc- nection. Three possibilities are that the folds gesting that these structures partly predate tures are contemporaneous. and associated reverse faults (1) formed at a displacement along the shear zone. (2) Young Spatial coincidence of folds with the east- restraining bend in the right-slip shear zone, east-trending folds plunge consistently east striking segment of the Las Vegas Valley (2) reflect localized shortening or a compo- (Fig. 13). The simplest way to accomplish this

A B C

Figure 12. Relation of north- to northeast-trending folds to hypothetical wrench-fault systems. (A) A N45°-60°E-striking dextral wrench system could produce N15°E-trending folds. Dextral faults of this orientation are not present in the region. (B) A N15°-30°W-striking sinistral wrench system could produce N15°E-trending folds. Sinistral faults of this orientation are not present in Figure 13. Lower-hemisphere, equal-area the region. (C) North-striking sinistral wrench system could produce N15°E-trending folds, pro- projection of fold axes directly north of the Las vided folds were rotated strongly into near-parallelism with the shear couple. This would produce Vegas Valley shear zone. Contour interval = strongly appressed folds, which are not observed. 2tr (after Kamb, 1959).

Geological Society of America Bulletin, August 1994 1067 Kilometer Figure 14. Geologic map at the east end of the Las Vegas Valley shear zone (LWSZ). Map shows critical crosscutting relations between right-slip LVVSZ, east-trending folds and reverse faults, left-slip faults, and stratigraphie units that bracket the timing of these structures. Field relations of these structures relative to stratigraphie units of known age provide the basis for the structural chronology presented in Figure 15. Area of Figure 14 includes eastern part of Government Wash and western part of Callville Bay 7.5' quadrangles. The mapped stratigraphie units include, from youngest to oldest, Qal, alluvial deposits; Qoa, older alluvial deposits; Tmc, Muddy Creek Formation (8.5-5.8 Ma); angular unconformity; Trs, Red sandstone unit (11.9-8.5 Ma); local unconformity; Thb, Bitter Ridge Limestone Member (13.5-13.0 Ma) (Horse Spring Formation); Tht, Thumb Member (17.2-13.5 Ma) (Horse Spring Formation); Thtc, Thumb Member, conglomerate facies; Thr, Rainbow Gardens Member (24-18 Ma); angular unconformity; Kbs, Baseline Sandstone (sandstone facies); Kbc, Baseline Sandstone (conglomerate facies); unconformity; Ja, Aztec Sandstone; TRm, Triassic rocks (Moenkopi and Chinle Formations); Pkt, Kaibab and Toroweap Formations; and Prb, Permian red beds. (Note duplication where cut.)

1068 Geological Society of America Bulletin, August 1994 1068 Strike and dip of bedding Faults (arrows show fault dip; line shows trend and plunge of © fault striae)

inclined horizontal vertical overturned well located approximately located presence Depositlonal contacts uncertain

well located approximately located concealed reverse or thrust strike slip fault other faults Folds

Tie bar (connects areas underlain by same unit) anticline syncline mesoscopic fold profile (with plunge) (with plunge) with downplunge profile

Geological Society of America Bulletin, August 1994 1069 DUEBENDORFER AND SIMPSON

Event >13 Ma 13 12 11 10 9 8 <8 Ma youngest rocks with the total clockwise ro- I I I 1 1 1 tation (Thumb Member) and control over fa- Lower Horse Spring Fm ^ des distributions in rocks of the upper Horse LVVSZ Spring Formation. East-striking reverse faults and associated Basin N of LVVSZ* tight folds clearly deform rocks of the red Basin S of LVVSZ** sandstone unit (Fig. 14). This relationship re- Oroclinal flexure quires that north-south shortening postdated N-S shortening Left-slip faulting ? — — — extension-related basin development. The Muddy Creek Formation • Las Vegas Valley shear zone may have been reoriented during shortening to a dip of 40°- Figure 15. Chronology of m^jor structures in the western Lake Mead area based on field 60° north, an unusually low angle for a major observations of crosscutting relations between units or structures of known age. Dashed lines strike-slip fault. South of the Las Vegas Val- indicate uncertainty in timing. LWSZ = Las Vegas Valley shear zone. *Refers to post-lower ley shear zone, several small reverse faults Horse Spring Formation basin north of the LWSZ. **Refers to post-lower Horse Spring For- place Triassic strata or the red sandstone unit mation basin south of the LWSZ. over the Muddy Creek Formation (Fig. 16). Slip dies out up-section within the Muddy systematic plunge is to fold strata with a pre- The Las Vegas Valley shear zone and as- Creek Formation, suggesting that reverse existing, regional eastward tilt developed by sociated right-slip faults were clearly active at faulting was waning during Muddy Creek slip on west-dipping normal faults. (3) Devel- the time of deposition of the youngest units of time. Northeast- to east-northeast-striking opment of White and Boulder basins (Bohan- the Horse Spring Formation and the red sand- left-slip faults consistently cut all folds and non, 1984; Duebendorfer and Wallin, 1991) at stone unit. The shear zone cuts the red sand- fault sets in the area, including the Las Vegas 11.9-10 Ma suggests active normal faulting stone unit but is lapped over by the Muddy Valley shear zone. These faults also cut rocks during that time. We suggest that northeast- Creek Formation, indicating activity between of the Muddy Creek and, therefore, were ac- striking normal faults partly preceded but 11.9 and 8.5 Ma (Fig. 14). The inception of tive in part after 8.5 Ma. Timing of inception largely coincided with major displacement movement is poorly and only indirectly of left-slip faulting in the western Lake Mead along the Las Vegas Valley shear zone. bracketed between 13.5 and 12.0 Ma by the area is unknown.

DISCUSSION

Implications for Tectonic Models

Any tectonic model for middle- to late-Ter- tiary deformation in the northwestern Lake Mead area must honor the following kinematic and timing data: (1) Rocks as young as the 12.0- to 13.0-Ma Lovell Wash Member show evidence for all episodes of deformation described above. (2) Between —12 and perhaps 10 Ma, extension, probably along a combined the Saddle Island-Las Vegas Val- ley shear zone system, produced Boulder basin (Duebendorfer and Wallin, 1991). (3) The narrow, 300-m corridor of east-plunging folds and east-striking reverse faults adjacent to the Las Vegas Valley shear zone suggests that contraction was a local phenomenon. Contractional structures postdate all red sandstone unit deposition and most displacement along the Las Vegas Valley shear zone and accommodate < 1 km of shortening. (4) Figure 16. Photograph looking east, nearly parallel to strike of the Las Vegas Valley shear Left-slip faulting and possible associated nor- zone, showing contractional structures. Note folds in Bitter Ridge Limestone on peak near center mal or oblique-slip faulting outlasted all other of the photograph. Heavy lines indicate faults; arrows indicate relative sense of movement. Light deformational events. line shows overturned unconformity between Triassic Moenkopi Formation (left) and Miocene red sandstone unit (right). Bedding attitudes dip steeply north and are overturned. Moenkopi is Existing Models thrust over Muddy Creek Formation at extreme right side of photo; just out of view, younger Muddy Creek sediments lap over reverse fault. Pkt = Permian Kaibab-Toroweap Formations, We summarize and evaluate three pub- TRm = Triassic Moenkopi Formation, Trs = Miocene red sandstone unit, Tmc = Miocene lished models for origin and kinematics of Muddy Creek Formation. View is —200 m across in foreground. major fault systems in the western Lake

1070 Geological Society of America Bulletin, August 1994 TERTIARY EXTENSION, LAKE MEAD REGION, NEVADA

Mead area. The first two are dynamic and Model 2 is based on strict Coulomb failure tion largely as transfer faults between differ- involve either north-south-directed com- theory and addresses only the strike-slip entially extending terranes, and contractional pression (Model 1) or a reorientation of prin- faults in the area. We reject this model for the structures reflect the south-directed "flow" cipal stress axes during the middle to late Mio- following reasons: (1) There is no evidence of rocks into an area of high-magnitude ex- cene (Model 2). The third model is kinematic that major strike-slip faults must form in re- tension. Our model views north-south con- and proposes a constrictional strain fielddur - sponse to regional stresses in accord with the traction as a response to large-magnitude ex- ing Tertiary extension (Model 3). Mohr-Coulomb theory. Strike-slip faults in tension in the northern Colorado River Model 1 postulates a remote north-south both compressional and tensional regimes extensional corridor. maximum principal stress direction during can form subparallel to the tectonic transport We follow the early suggestion of Ander- Miocene (Cakir, 1990; Cakir and Aydin, direction, where they function as tear or son (1973) that Miocene deformation in the 1990) and interprets observed structures in transfer faults (for example, Rich, 1934; Lake Mead area was dominated by large- the context of the Coulomb fracture criterion. Davis and Burchfiel, 1973; Wernicke and oth- scale westward translation of blocks away The model explains development of north- to ers, 1982). (2) The paleostress determinations from the Colorado Plateau. We suggest that northeast-striking normal faults as the result themselves are suspect, both in terms of ori- extension south of the Las Vegas Valley of east-west tension, the east-west-trending entation of principal stress axes and timing of shear zone was associated largely with the folds and reverse faults as the result of north- rotation. Angelier and others (1985) deter- Saddle Island fault (Weber and Smith, 1987; south compression, and the strike-slip faults mined paleostress orientations in an area < 1 Duebendorfer and others, 1990), occurred as conjugate shears. By viewing all structures km2; hence, the regional applicability of their between 13 and —10.0 Ma, and is recorded by as dynamically related, the model requires determinations is questionable. In addition, sedimentary rocks of the red sandstone unit. that all structures formed and were active these authors do not address the timing of the Coeval extension occurred north of the Las contemporaneously. inferred change of paleostress direction. (3) Vegas Valley shear zone but was of lesser We consider this interpretation as un- Finally, by failing to incorporate the devel- magnitude as shown by modest stratal tilts likely for the following reasons: (1) Our opment of east-west-trending folds and re- and the more limited extent of red sandstone- data suggest significant diachronous move- verse faults, the model incompletely de- age deposits. North-south shortening clearly ment on all faults present in the area. (2) scribes the geometry and kinematics of the postdated deposition of the red sandstone The model does not and cannot explain the region; i.e., it is not comprehensive. unit. The constrictional model, which re- formation of north-northeast-trending Anderson and Barnhardt (1993) present requires contemporaneous shortening and ex- folds. (3) The two major strike-slip fault gional evidence for significant north-south tension, would require intraformational un- zones intersect at an angle of —130°. This shortening during Tertiary extension. They conformities within the red sandstone unit, is far larger than the angle predicted by argue that coeval north-south shortening, which are not seen. The spatial coincidence simple Coulomb fracture theory and that vertical structural attenuation, and east-west between the highly extended western Lake observed for conjugate shears in natural extension are manifestations of a regional Mead region and locus of greatest docu- settings. (4) It is not clear why far-field constrictional strain field and that the vertical mented shortening strains suggests a kine- stresses would produce localized deforma- and north-south horizontal contractions com- matic relation between the extension and tion at the apex of the wedge bound by the bine to form constrictional strains whose contraction. The difference in timing, how- Las Vegas Valley and Lake Mead fault magnitudes approximately balance east-west ever, between the two deformational events systems rather than more pervasive short- extension (Model 3). Our detailed, but areally argues that north-south contractional strain ening structures within the wedge. (5) The more limited, data set suggests that north- may be a consequence of extension rather model approximates uniaxial shortening south shortening clearly post-dated, rather than a cause (Cakir and Aydin, 1990) or and does not account for the large-magni- than coincided with, large-magnitude east- "equal partner" (Anderson and Barnhardt, tude lateral translations that are docu- west extension. Furthermore, the docu- 1993) in the regional strain picture. mented in the Lake Mead area (for exam- mented east-west extension in the northwest Our observations, data, and interpreta- ple, Anderson, 1973; Bohannon, 1979). Lake Mead area is nearly an order of magni- tions are consistent with the suggestion of Model 2 invokes reorientation of principal tude greater than the maximum possible Wernicke et al. (1988) that substantial differ- stress directions (for example, Zoback and north-south shortening recorded in the rocks ential extension between two regions can in- others, 1981; Angelier and others, 1985) to of the area. While we acknowledge the im- duce gravitational flow of crustal material explain observed differences in timing be- portance of north-south shortening, our data from the thick, less-extended terrane toward tween the two major strike-slip fault zones show that these strains are neither contem- the thinner and more highly extended ter- (Rowland, 1989). In this model, the Las Ve- poraneous with nor of sufficient magnitude to rane. As emphasized by Block and Royden gas Valley shear zone was active when the represent true constrictional strain coincident (1990) and modeled by Kruse and others maximum compressive stress was oriented with east-west extension. (1991), the driving mechanism for crustal N20°W, and the Lake Mead fault system op- flow is simply the lateral pressure gradient erated when the maximum compressive Our Model created by differential near-surface loads be- stress was oriented N30°E. Because Model 2 tween any two crustal columns. The viscous invokes a rotation of the maximum com- The models discussed above fail to address flow model developed by Wdowinski and pressive stress axis around a due-north ori- adequately either the timing of development Axen (1992) to calculate isostatic rebound as- entation, it is broadly consistent with the of principal sets of structures or the kinematic sociated with tectonic denudation clearly development of structures associated with role of documented detachment faults in the predicts a horizontal flow component of ma- both north-south shortening and east-west western Lake Mead region. We propose a terial from undenuded crust toward tectoni- extension. model whereby major strike-slip faults func- cally denuded crust (Fig. 17). Although they

Geological Society of America Bulletin, August 1994 1071 DUEBENDORFER AND SIMPSON

d = 5 km SUMMARY AND CONCLUSIONS

fïïTT Vue m 1.00 The earliest post-Horse Spring Formation structures in the region are north-northeast- Vic =1.00 striking normal faults, associated extensional fault-bend folds, and right-slip faults associ- »7ml =1.00 ated with the Las Vegas Valley shear zone. These structures operated in a kinematically coordinated fashion to accommodate approximately due east-west extensional strain Vma = 1 00 between 13.0 and 10.0 Ma. North-south shortening clearly postdated the main phase of extension in the western Lake Mead area. North-south shortening may reflect localized (a) 1.0 50 km southward flow of crustal material into the region of extreme extension at the northern end of the Colorado River extensional corri-

d = 5 km dor. Left-slip faults associated with the Lake Mead fault system represent the youngest structures in the region and may be related to Vue =1.00 extension to the northeast in the Mesquite Vic =0.01 Basin-Virgin River depression region (Wer- nicke and others, 1988; Bohannon and others, 1993). The Las Vegas Valley shear zone Vml =2.00 and Lake Mead fault system neither initiated nor functioned as conjugate shears related to a single regional paleostress direction.

Vma = 0.01 ACKNOWLEDGMENTS

This work was supported in part by Na- tional Science Foundation Grant EAR- 9017629 and a University Research Council Figure 17. Flow field within the crust and mantle in response to tectonic denudation (Fig. 4 in grant from the University of Nevada, Las Wdowinski and Axen, 1992; see that article for discussion). Top = velocity field for constant Vegas. We thank E. I. Smith, R. E. Ander- viscosity. Bottom = velocity field for more realistic viscosity structure (Wdowinski and Axen, son, J. E. Faulds, W. J. Taylor, and R. J. 1992). Note flow of material from undenuded toward denuded terrane in upper and lower crust. Dorsey for numerous discussions on exten- Sections are constructed with tectonic transport from left to right. We suggest that similar flowage sional tectonism over the past several years; must occur across transverse structures that separate highly extended from weakly extended however, these individuals may not agree terranes. In this case, the tectonic transport direction would be away from the observer (into the with the conclusions of this study. Michael page). Flowage of material across transverse structure toward highly extended block could pro- Wells introduced us to the technique of R. duce localized contractional deformation near the transverse structure—in this case, the Las Marrett and R. Allmendinger, and we appre- Vegas Valley shear zone. ciate their generosity in providing the soft- ware free of charge. Constructive reviews by A. Glazner, J. Steven, and B. Bohannon im- proved the manuscript significantly. modeled extension-parallel, two-dimensional extensional corridor, was active as a

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