EVOLUTION OF A STRIKE-SLIP FAULT NETWORK IN SANDSTONE Eric Flodin Stanford University, Stanford, CA 94305 email: [email protected] Abstract Faults formed by shearing along preexisting This paper is a progress report on research concerning discontinuities the evolution of strike-slip networks in sandstone. In the A solid basis exists in the literature for understanding the Valley-of-Fire State Park of southern Nevada, the Juras- initiation and evolution of faults formed along preexisting sic Aztec sandstone is deformed by two strike-slip fault discontinuities. Much of the early work concerning fault sets with different orientations and opposite slip sense. formation and evolution were focused in granites (Segall One fault set is oriented north-northeasterly and shows and Pollard, 1983; Granier, 1985; Martel et al., 1988; apparent left-lateral displacements up to 2.4 km. The other Martel 1990). Other workers have extended these concepts fault set is oriented northwesterly and shows apparent to other lithologies including carbonates (e.g., Willemse right-lateral offsets up to 50 m. At a regional scale, most et al., 1997; Mollema and Antonellini, 1999; Graham et of the right-lateral faults terminate against the larger- al., in review), shales (e.g., Engelder et al., 2001), and sand- offset left-lateral faults and are found localized both be- stones (e.g., Cruikshank et al., 1991; Zhao and Johnson, tween step regions along individual left-lateral faults, 1992; Myers, 1999; Davatzes and Aydin, in review). as well as at the ends of the larger left-lateral faults. At a Joint-based faulting as a prominent deformation smaller scale, right- and left-lateral faults show mutu- mechanism in sandstone was first described by Myers ally abutting relationships. Also, mode I splay fractures (1999). Earlier studies focused on faulted joints with cen- related to fault slip are observed sharing the same ori- timeter-scale slip magnitudes (e.g. Cruikshank et al., 1991), entation and abutting relationships as both fault sets. A while Myers (1999) was able to show that a similar pro- working model for the evolution of the strike-slip fault cess of faulting along joint zones operated over a wide range network in the Valley-of-Fire is presented whereby the of slip magnitudes, from centimeters to hundreds of fault network forms via progressive splay fracturing be- meters. Using a series of detailed maps of faults with dif- tween fault segments and later shearing of the splay frac- ferent joint configurations and slip magnitudes, Myers tures. At the scale of observation made in this study, at developed a conceptual model that describes a hierarchi- least five generations of structures are identified. cal process of fault evolution beginning with shearing along preexisting joint zones. Shearing of joints in turn creates Introduction fragmentation zones at their intersections, tiplines, and This study focuses on the evolution of a strike-slip fault stepovers. This process is repeated as localized shear strain network that developed within the otherwise extensional continues to accumulate. Fragmentation zones are further Cenozoic Basin and Range orogen of the western United crushed to form isolated pockets of fault rock along small States. A field-based approach is used to study an ancient faults. Eventually, a through going slip surface develops fault network developed in Aztec sandstone now exposed at and the once discontinuous fault rock pockets coalesce to the surface in the Valley of Fire State Park of southern Ne- form a continuous fault rock zone. Further evolution re- vada (Figure 1). quires new coalescence and results in greater fault dimen- In this paper, I first introduce concepts relevant to fault sions. nucleation and growth. Particular focus is made to models Myers (1999) further recognized that these faults developed by Myers (1999) for faults that form by shear- evolved in different ways depending on the original preex- ing along preexisting joint zones in sandstone. Because of isting joint configuration. He developed a classification the relative importance of previously formed structural fea- scheme to illustrate the idealized evolution of three end- tures to the latest stage of strike-slip faults, an effort is member joint configurations: en echelon joint zones that made to detail the geologic history of the Aztec sandstone have step-sense opposite to shear-sense (e.g., right-step- in the vicinity of the Valley of Fire, Nevada. As research for ping and left-lateral shearing), en echelon joint zones that this work is still at an early stage, a concentration is put on have step-sense similar to shear-sense (e.g., right-stepping presenting observations as they relate a working concep- and right-lateral shearing), and subparallel joint zones char- tual model. A discussion of the field data in terms of rel- acterized by a large joint-length to joint-spacing ratio. For evant concepts concerning fault formation and growth is en echelon joint zones that have step-sense opposite to reserved for a future effort and is not included in the present slip-sense, the overlapping region between stepping joints contribution. is subject to a localized contractional strain which results Stanford Rock Fracture Project Vol. 13, 2002 D-1 Western USA BDM TSH 4100000 NV MRM Basin study area er Virgin Riv VM SR STUDY AREA LVVSZ MM LMFS Las GB Vegas SM FM 4000000 Mead Lake 20 kilometers UTM Zone 11N 650000 784700 Figure 1. Generalized map of Cenozoic faults in the Lake Mead region of southern Nevada. Note the north to north-east trending left- lateral strike-slip faults and the northwest trending right-lateral strike-slip faults. Heavy lines are faults. Arrows indicate lateral slip sense. Ball and tick symbols indicate normal fault hanging walls. Inset: Map of the western United States. LMFS = Lake Mead Fault System, LVVSZ = Las Vegas Valley Shear Zone, BDM = Beaver Dam Mountains, FM = Frenchman Mountain, GB = Gold Butte, MM = Muddy Mountains, MRM = Mormon Mountains, SM = Spring Mountains, SR = Sheep Range, TSH = Tule Spring Hills, VM = Virgin Mountains. Base image is a mosaic of 1:250,000 USGS DEMs. Fault geometries adapted from Stewart and Carlson (1978), Anderson and Barnhard (1993a and b), Axen (1993), Campagna and Aydin (1994), Beard (1996), and this study. in the formation of small discontinuous deformation bands al., in press). Within the Valley-of-Fire, the AZS has a strati- and promotes the frictional breakdown of host rock mate- graphic thickness of approximately 800 m (Longwell, rial. In contrast, en echelon joint zones that have the same 1949) and is divided into three sub-units based on rock step- and slip-sense are subject to a localized dilational color (Figure 2) (Taylor, 1999). From the stratigraphically strain that results in the fragmentation of rock that spans lowest position, the sub-units are lower red unit, middle the overlapping en echelon joints (Myers, 1999). buff unit, and upper orange unit. The lower red unit is well cemented and has a low average porosity; the middle buff Geologic Setting unit is poorly cemented and has a high average porosity; This study focuses on strike-slip faults found within the the upper orange unit is moderately cemented and has a aeolian Jurassic Aztec sandstone (AZS) exposed in the Val- high average porosity (Flodin et al., in press). This study ley of Fire State Park (Valley-of-Fire) in the Northern focuses on faults found in the middle and upper units of the Muddy Mountains of southern Nevada (Figure 1). The AZS AZS (cf. boxed area in Figure 2). was deposited in early Jurassic time in a back-arc basin set- ting (Marzolf, 1983) and was part of a more continuous Post-Depositional History aeolian erg system that included the Navajo sandstone of Mesozoic Contractional Deformation the Colorado Plateau (Poole, 1964; Blakey, 1989). Since deposition, the AZS has experienced a long and var- The AZS in the Valley-of-Fire is a fine to medium ied deformation history (Figure 3). The earliest stage of grained sub-arkose characterized by large-scale tabular-pla- deformation is attributable to regionally extensive, thin- nar and wedge-planar cross-strata (Marzolf, 1983; Marzolf, skinned east-directed thrusting associated with the Creta- 1990). Host rock sandstone porosities range from 15-25%, ceous Sevier orogeny (Armstrong, 1968). Following depo- while permeabilities range from 100-5900 mD (Flodin et sition and likely burial, the AZS was once again exhumed Stanford Rock Fracture Project Vol. 13, 2002 D-2 to the surface in late Jurassic to early Cretaceous time. lying synorogenic Cretaceous Willow Tank Formation and Fleck (1970) attributes the local exposure of the AZS to the white member of the Baseline sandstone (Figure 2) regional uplift, possibly accompanied by gentle folding, (Bohannon, 1983a). Longwell (1949) estimated slip along associated with the earliest stages of the Sevier Orogeny. a portion of this fault to be on the order of a few kilome- In the Valley-of-Fire, the upper stratigraphic contact of the ters. The Summit-Willow Tank thrust was in turn over-rid- AZS is a slight angular unconformity with the overlying den by the regionally extensive and far traveled Muddy synorogenic Cretaceous Willow Tank Formation and Mountain thrust (Longwell, 1949; Bohannon, 1983a). Baseline Sandstone (Bohannon, 1983a; Carpenter and Car- In the Valley-of-Fire, erosion has either removed the penter, 1994). These units were deposited in a foreland Muddy Mountain thrust sheet or the thrust sheet was never basin associated with the advancing Sevier thrust front emplaced over the Valley-of-Fire, as speculated by (Armstrong, 1968). Bohannon (Plate 1, 1984) and Taylor (1999). However, a At least two large thrust faults were emplaced over the few kilometers south of the study area in the Muddy Moun- AZS in the Valley-of-Fire in middle Cretaceous time tains, the Muddy Mountain thrust places Cambrian Bonanza (Bohannon, 1983a; Carpenter and Carpenter, 1994).