Kinematic evolution of a large-offset continental normal fault system, South Virgin Mountains, Nevada Robert Brady* Brian Wernicke Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA Joan Fryxell Department of Geological Sciences, California State University, San Bernardino, California 92407, USA ABSTRACT examination of other areas suggests that the John and Foster, 1993); others may have ini- evolutionary sequence seen in the South tiated with steep dips and remained active as The South Virgin Mountains and Grand Virgin Mountains may, in fact, be widely they tilted to shallow dips. Tilting of these Wash trough comprise a mid-Miocene nor- applicable. faults is probably due to combined domino- mal fault system that de®nes the boundary style tilting and isostatically driven footwall between the unextended Colorado Plateau Keywords: Egan Range, kinematics, Lemi- ¯exure (Fig. 1C). Many gently dipping normal to the east and highly extended crust of the tar Mountains, normal faults, South Virgin faults might be best interpreted this way, rath- central Basin and Range province to the Mountains, Yerington. er than invoking rotation by younger struc- west. In the upper 3 km of the crust, the tures. system developed in subhorizontal cratonic INTRODUCTION Detailed geologic studies were conducted in strata in the foreland of the Cordilleran the South Virgin Mountains of southeastern fold and thrust system. The rugged topog- Geologic mapping in the Basin and Range Nevada and northwestern Arizona (Fig. 2), a raphy and lack of vegetation of the area af- province has shown that gently dipping nor- region well suited for testing existing kine- ford exceptional three-dimensional expo- mal faults are common (e.g., Longwell, 1945; matic models of normal faulting. Extensional sures. Compact stratigraphy and well-de- Anderson, 1971; Proffett, 1977; Miller et al., faults within the Grand Wash trough and ad- ®ned prefaulting con®guration of the rocks 1983; Wernicke et al., 1985; John and Foster, jacent South Virgin Mountains de®ne a rela- permitted a detailed reconstruction of the 1993), but theoretical fault mechanics sug- tively sharp transition from the unextended system. Reconstruction of cross sections gests that such faults should not slip (cf. An- Colorado Plateau to the strongly extended Ba- 2 based on more than 300 km of detailed derson, 1942; Sibson, 1985; Nur et al., 1986; sin and Range (stretching factor b ø 3.5; Wer- mapping at a scale of 1:12 000 shows that Agnon and Reches, 1995). These faults have nicke et al., 1988). The plateau consists of the fault system accommodated more than been explained in a manner consistent with high-grade Proterozoic basement nonconform- 15 km of roughly east-west±directed Mio- mechanical theory by invoking tilting to lower ably overlain by a thin (;2 km) Paleozoic cene extension. Extension was initially ac- dips after cessation of slip (e.g., Proffett, cover, and forms a headwall from which steep- commodated on moderately to steeply dip- 1977; Miller et al., 1983; Buck, 1988). For ly east tilted normal fault blocks, bounded by ping listric normal faults. As the early example, in the Snake Range, the Lemitar both steep and shallow normal faults, includ- faults and fault blocks tilted, steeply to Mountains, and the Yerington district, gently ing both basement and cover, were derived. moderately dipping faults initiated within dipping faults have been interpreted as the re- The mapped area comprises a series of eight the fault blocks, soling into the early faults. sult of multiple generations of crosscutting, tilted fault blocks deformed by multiple gen- Some of the early faults were active at dips steeply dipping normal faults, with older erations of normal faults in middle Miocene of ,208. Isostatically driven tilting is su- faults being tilted to shallow dips above youn- time (Fig. 3). These fault blocks are bounded perimposed on tilting due to active slip and ger faults (Fig. 1A; Proffett, 1977; Chamber- by normal faults with 1 km or more of throw, domino-style rotation of the fault blocks. lin, 1982, 1983; Miller et al., 1983). Gently and are internally disrupted by numerous Collectively these processes rotated origi- dipping normal faults have also been attribut- smaller offset faults. Four of these blocks, in- nally steeply dipping faults to horizontal ed to the rotation of originally steeply dipping cluding (from east to west) the Wheeler orientations. The kinematics are inconsis- faults after abandonment above an up¯exing, Ridge, Iceberg Ridge, Indian Hills, and Con- tent with the widely accepted view that tectonically denuded, footwall (Fig. 1B; Buck, noly Wash blocks, straddle the eastern end of many near-horizontal normal faults were 1988). Lake Mead, Nevada (Fig. 3). The next block rotated to their present orientations by lat- However, many gently dipping normal to the west, interpreted to be structurally con- er, crosscutting normal faults. However, re- faults can not be explained by either of these tinuous with the Gold Butte crystalline block, mechanisms. Some of them probably initiated is the Azure Ridge block. The next three *E-mail: [email protected]. with shallow dips (e.g., Wernicke et al., 1985; blocks to the west, including Tramp Ridge, GSA Bulletin; September 2000; v. 112; no. 9; p. 1375±1397; 16 ®gures; 1 table. 1375 BRADY et al. Figure 2. Location map, showing the South Virgin Mountains (stippled) and surrounding area, as well as major structural features. Light gray indicates the location of mountainous terrane. BMÐBlack Mountains, CPÐColorado Plateau, FMÐFrenchman Mountain, GPTÐGass Peak thrust, KTÐKeystone thrust, VDÐVirgin detachment, LMFSÐLake Mead fault system, LVRÐLas Vegas Range, LVVSZÐLas Vegas Valley shear zone, MMÐMuddy Mountains, MMTÐMuddy Mountains thrust, NVMÐNorth Virgin Moun- tains, SRÐSheep Range, SVMÐSouth Virgin Mountains. Lime Ridge, and the Maynard Spring block, to the thin-skinned Sevier thrust belt (Burch- are north of and structurally above the crys- ®el et al., 1974), which developed within the talline block, which is a 15-km-wide basement sedimentary rocks of the Paleozoic miogeo- dome, unroofed by a west-dipping normal cline. The easternmost thrusts seem to have fault (Fryxell et al., 1992). been localized along the hinge zone of the Most of the faults within the South Virgin miogeocline, which trends roughly northeast- Figure 1. Schematic cross sections of the up- Mountains cut through Paleozoic to Tertiary southwest, and runs just to the west of Lake per crust showing three models of normal strata. The stratigraphic section includes more Mead. Accordingly, the easternmost thrust fault evolution, all of which result in both than 30 mappable units in about 3 km of sec- sheet at the latitude of Lake Mead is exposed shallowly and more steeply dipping normal tion. The South Virgin Mountains were little nearby, to the west, in the Muddy Mountains faults. Incipient faults are shown as dashed deformed by Mesozoic compression, as evi- (Fig. 2; Longwell, 1949; Longwell et al., lines, active faults are shown as heavy black denced by the lack of contractional structures 1965; Bohannon, 1983). South of Lake Mead, lines, and inactive faults are shown as thin- within the region, and the very gentle sub- the thrust belt changes to a northwest-south- ner black lines. (A) Multiple generations of Tertiary unconformity across the South Virgin east trend, and thrusting involves Precambrian domino faults; only moderately to steeply and Beaver Dam Mountains (Bohannon, 1984; crystalline basement rocks (Burch®el and Da- dipping faults are active; shallowly dipping Wernicke and Axen, 1988). The faulted sec- vis, 1975). faults have been rotated above younger tions of the South Virgin Mountains must re- Following Cretaceous thrusting, the Lake crosscutting faults (cf. Proffett, 1977; Miller store so as to be continuous with the ¯at-lying Mead region seems to have been tectonically et al., 1983). (B) Rolling hinge model; ini- strata of the Colorado Plateau. Because the stable until Neogene time. However, early tially steeply dipping faults are ¯exurally ro- faults within the South Virgin Mountains are Miocene to Eocene deposits ®lling northeast- tated above an uplifting footwall, becoming well exposed, have accommodated large-mag- ward-draining paleocanyons that cut into the inactive at low dips. New, steep faults break through the intact hanging wall as old faults nitude extension, and must restore to a well- southwestern part of the Colorado Plateau become inactive and are abandoned on the de®ned structural datum, the area provides an show that a basement high had formed to the uplifted footwall (Buck, 1988). (C) Synchro- exceptional opportunity to study the kinematic south of Lake Mead by Eocene time (Young, nous slip model; after some slip and rotation evolution of a continental normal fault system. 1966, 1979). Paleozoic and Mesozoic strata of initially steep faults, younger steep faults were erosionally removed from this basement break and sole into the rotated older faults. GEOLOGIC SETTING high. The exact age and extent of this high All faults remain active and rotate domino- remain unclear, due to incomplete understand- style, as well as rotating as a result of iso- During Late Jurassic and Cretaceous time, ing of the age and preextensional con®gura- static uplift of the thinning region. the Lake Mead area was part of the foreland tion of the sub-Tertiary unconformity. 1376 Geological Society of America Bulletin, September 2000 KINEMATIC EVOLUTION OF A LARGE-OFFSET CONTINENTAL NORMAL FAULT SYSTEM The present physiographic features of the area result primarily from Miocene extension. During Miocene time, extension initiated at the edge of what is now the Colorado Plateau, accommodated on a set of west-dipping nor- mal faults. These faults strongly control the topography of the South Virgin Mountains, as discussed above and illustrated in Figure 3. The timing of extension in the South Virgin Mountains is constrained by ®ssion-track ages from across the Gold Butte crystalline block.