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 field data. 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 di- verse 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 chro- nology, is a powerful tool to establish kine- matic compatibility or incompatibility be- tween faults sets in complex areas and may be more meaningful in characterizing the overall strain pattern in complexly deformed regions than the commonly applied paleo- stress 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 ex- tensional corridor expose ductilely deformed rocks in the lower plates of regional detach- ment 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-