Abstract Introduction Faults Formed by Shearing Along Preexisting

Home , Aztec Sandstone, Horse Spring Formation

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

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 setting (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). The King Formation over the AZS (Bohannon, 1983b). In the earlier Summit-Willow Tank thrust places stratigraphically Buffington Window of the Muddy Mountains, Brock and lower red AZS over upper orange AZS, as well as the over- Engelder (1979) estimated the thickness of the Muddy Mountain thrust sheet to be 2-5 km. At this time in the Val- ley-of-Fire, the AZS was buried by at least 1.2 km of over- cover Jar lying Cretaceous sediments (Carpenter and Carpenter, Jar Cretaceous units, undifferentiated 1994). Jurrassic Aztec sandstone Jao - upper orange unit Jab - middle buff unit Cenozoic Extensional and Strike-slip Deformation Jar - lower red unit Cenozoic Basin and Range extensional and strike-slip de- Triassic units, undifferentiated formation followed the Sevier orogeny. The Valley-of-Fire lithologic contact region was apparently unaffected by deformation associ- thrust fault (teeth on upper ated with the late Cretaceous - early Tertiary Laramide orog- hanging wall of fault block) eny (Bohannon, 1983a). The first recorded Cenozoic event strike-slip/normal fault (arrows to occur in the vicinity of the Valley-of-Fire was the un- indicate lateral slip sense; conformable deposition of non-marine Tertiary sediments ticks on downthrown block) atop the Cretaceous Baseline sandstone (Bohannon, 1983a). Jar road 1 kilometer At this time, much of the southern Nevada region was struc- Jao turally characterized by a broad, gently north plunging arch, of which the Valley-of-Fire was on the gently north-north- Jao easterly dipping side (Bohannon, Plate 1, 1984). Strati- graphic dips between the Cretaceous units and the overlying basal Tertiary units differ by only 5-10º. Jao Figure 5 The Tertiary sediments in the Valley-of-Fire region in- Jab clude the Rainbow Gardens and Thumb Members of the Horse Spring Formation [26 Ma - 13.5 Ma (Bohannon, Jar Jab Jab 1984; Beard, 1996)], and the Muddy Creek Formation [10 Jao Ma - 4 Ma (Bohannon et al., 1993)] (Bohannon, 1983a). Intervening upper members of the Horse Spring Formation and the red sandstone unit of Bohannon (1984) are notably

Bas missing in the Valley-of-Fire. Tilting of the Valley-of-Fire Jar el

Jab ine Mesa f of up to 25º in a northeasterly direction occurred prior to deposition of the Muddy Creek Formation. Present day bedding dips in the Horse Spring Formation are approxi- ault mately 30º to the northwest, while in the overlying Muddy

Water Pocket fault Pocket Water Jar Creek Formation they are approximately 5º to the northeast (Carpenter, Plate 1.3, 1989). Jar Three major structural features related to Miocene N Basin and Range deformation define the landscape in the vicinity of the Valley-of-Fire (Figure 1). These are: (1) The left-lateral Lake Mead Fault System (LMFS) (Anderson, Figure 2. Geologic and structure map of the Valley of Fire State Park. 1973; Bohannon, 1979), which consists of several strands Adapted from Carpenter and Carpenter (1994), Myers (1999), and that show cumulative left-lateral offset of approximately Taylor (1999). 65 km (Bohannon, 1984). Activity along the LMFS oc-

Stanford Rock Fracture Project Vol. 13, 2002 D-3 Geologic Time Scale Tr Jurassic Cretaceous Tertiary Q

20 Ma 15 Ma 10 Ma 5 Ma

deposition, burial, Sevier Orogeny5 Basin and Range Extension8 diagenesis of VoF1,2

major left-lateral slip along LMFS10,11,12 regional(?) uplift local(?) burial of regional tilting and local VoF by foreland exposure of VoF(?)6,7 and exposure major right-lateral slip along LVVSZ12,13 of VoF3,4 basin deposits6,7 VoF Reigion VoF burial of VoF local burial of VoF by Geologic Events, by Sevier Tertiary sediments(?)6,9 thrust sheets4,6,7

gentle folding3 gentle folding7

pervasive grain-scale pressure solution14,15

thick deformation bands (compactive)14,16,17 faulting by shearing along joint zones18,19 thin deformation bands (shearing)14,16 Aztec sandstone

jointing(?) jointing14,18,19 Local Deformation Features,

Figure 3. Summary of geologic and tectonics events for the Aztec sandstone in the vicinity of the Valley of Fire, southern Nevada. References are as follows: 1Poole (1964); 2Marzolf (1983); 3Fleck (1970); 4Brock and Engelder (1977); 5Armstrong (1968); 6Bohannon (1983a); 7Carpenter and Carpenter (1994); 8Zoback et al. (1981); 9Bohannon (1984); 10Beard (1996); 11Campagna and Aydin (1994); 12references in Duebendorfer et al. (1998); 13Langenheim et al. (2001) and referenced therein; 14Taylor (1999); 15Flodin et al. (in press); 16Hill (1989); 17Sternlof (this volume); 18Myers (1999); and, 19this study. curred between 16 and 5 Ma (references in Duebendorfer, Two phases of deformation bands are recognized: early 1998). (2) The right-lateral Las Vegas Valley Shear Zone deformation bands that show little to no shear offset (Set (LVVSZ) (Longwell, 1960), which consists of several right I), and late deformation bands that show shear offsets on stepping segments that form the Las Vegas Valley pull-apart the order centimeters to decimeters (Set II) (Hill, 1989; basin (Campagna and Aydin, 1994; Langenheim et al., 2001). Taylor, 1999). Set I deformation bands are at high angle to Slip along the LVVSZ is estimated to be from 40 to 65 km bedding, group within three general orientations (north- (Bohannon, 1984), and occurred between 14 and 7.5 Ma northeast, north-northwest, and northwest), are continuous (references in Duebendorfer, 1998). Both the LMFS and for lengths greater than 100 m, and appear thick (centime- the LVVSZ were active contemporaneously based on mu- ters) in outcrop (Hill, 1989). Set II deformation bands tual crosscutting relationships at their intersection in the crosscut and offset the earlier formed set. The Set II de- Gale Hills (Çakir et al., 1998). However, as mapped by formation bands are found in both low and high-angle ori- Anderson et al. (1994), the LVVSZ abuts against the more entations with respect to bedding. The low-angle Set II de- extensive LMFS. (3) The Virgin River depression, an anoma- formation bands are short and discontinuous, show offsets lously deep and complex extensional basin atypical of other of 1-3 cm, and are generally found localized along strati- basins in the Basin and Range province (Bohannon et al., graphic bedding planes (Hill, 1989). In contrast, high-angle 1993). Subsidence of the Virgin River depression began Set II deformation bands are continuous for hundreds of slowly around 24 Ma, with rapid subsidence occurring be- meters, and are found as zones of deformation bands with tween 13-10 Ma (Bohannon et al., 1993). Subsidence is cumulative offsets on the order of decimeters. In outcrop, ongoing as evidenced by offset Quaternary deposits in the Set II deformation bands are readily recognized as they form eastern reaches of the Virgin River depression (e.g., resistive ridges up to one meter wide and as tall as a few Billingsley and Bohannon, 1995). Basin depths in parts of meters. the Virgin River depression are estimated to be as great as Hill (1989) speculated the Set I deformation bands 10 km (Langenheim et al., 2000). were the result of horizontal compression associated with the earliest stages of Sevier shortening. Hill also attrib- Local Deformation, Aztec Sandstone uted the formation of the Set II deformation bands to Sevier Deformation features in the AZS record two phases of de- related thrusting and showed that the slip direction on the formation, early compression and later extension. The ear- low-angle deformation bands share the same east-directed liest formed localized deformation features in the AZS were vergence with the Sevier thrusts. The high-angle Set II de- deformation bands. Possibly coeval with the formation of formation bands, which show right-lateral offset, are only the deformation bands was pervasive micro-scale pressure found in the vicinity of the right-lateral strike-slip tear fault solution (Taylor, 1999) and gentle folding (Carpenter and at the southern boundary of the Sevier Summit-Willow Tank Carpenter, 1994). thrust.

Stanford Rock Fracture Project Vol. 13, 2002 D-4 Based on crosscutting relationships, jointing of the AZS these two fault sets. However, I first detail the major faults is generally thought to have followed deformation banding that bound the exposures of AZS in the Valley-of-Fire in an and to have preceded the latest sheared-joint fault stage attempt to define the boundary conditions that led to the (Myers, 1999; Taylor, 1999). In outcrop, many joints are devolvement of the two fault sets internal to these bound- found both localized along and abutting against deforma- ing faults. tion bands, indicating that joint propagation was influenced by the presence of the earlier formed deformation bands. Large-offset Bounding Faults Taylor identified a north-south oriented joint set not asso- Two major faults bound exposures of AZS in the Valley-of- ciated with faults. He also identified an en echelon joint Fire, the Water Pocket fault to the west and the Baseline set that trends 30º both east and west of north, noting that Mesa Fault to the east (Figure 2). Previous workers inter- the northeast orientation was more common. Both Myers preted these structures as normal faults (Longwell, 1949; and Taylor speculate that the joints formed during the ear- Bohannon, 1983b; Carpenter and Carpenter, 1994). How- liest stages of Miocene extension. It is also possible that ever, new evidence implies a major strike component of an earlier jointing event occurred during unroofing and slip on both faults. In addition to the evidence presented possible gentle folding (Fleck, 1970) of the AZS in late below, inferences about slip motion may be drawn from Jurassic or early Cretaceous time prior to burial by analogous faults to the north. Other workers have mapped synorogenic Cretaceous sediments and the Sevier thrust strike-slip faults sharing the same orientation and slip sense sheets. as these two bounding faults to the north of the Valley-of- Faults formed along preexisting joints and joint zones, Fire in the southern and eastern Mormon Mountains and sometimes deformation bands, are the latest recorded (Anderson and Barnhard, 1993a) and the Tule Spring Hills deformation feature in the AZS (Myers, 1999). At the ini- (Axen, 1993) (Figure 1). tiation of faulting, the AZS was buried by at least 1.6 km of The Water Pocket fault on the eastern boundary off- Cretaceous and Tertiary sediments (Bohannon, 1983a). The sets the tectonic contact between the upper plate of the youngest unit that these faults deform is the Thumb Mem- Summit-Willow Tank thrust and the underlying AZS in an ber of the Miocene Horse Spring Formation (Carpenter, apparent left-lateral sense by 2 km. In contrast, consider- Plate 1.3, 1989). The structural characteristics of these ing only a normal component of slip, the offset along the faults are detailed below. Water Pocket fault would be only 30-50 m (Longwell, 1949). The main fault plane strikes generally north and dips Mapping Methods 50-60º to the west. Damage related to slip along this fault Mapping for this research was carried out at scales ranging is in some places severe; fault gouge zone exposures wider from 1:5 to 1:30,000. Outcrop scale maps were made us- than 15 m were found. ing base photographs taken from a pole-mounted camera at Stratigraphic and tectonic relations on either side of heights ranging from 2-3 m. Meso-scale maps (1:200 and the Water Pocket fault indicate a strike-slip component of 1:825) were made using basemap enlargements of custom, motion. The sub-horizontal Summit-Willow Tank thrust low-altitude aerial photographs taken at scales of 1:1600 (Longwell, 1949) intersects the underlying AZS in same and 1:6600. The low-altitude aerial photo-basemaps were stratigraphic position in the upper orange unit of the AZS georeferenced in the field by locating map control points on both sides of the Water Pockets fault. Present day el- using a differentially corrected GPS. The smallest-scale evations of the thrust contact are at approximately 600 m mapping (1:30,000) was carried out using digitally above sea level on both sides. If the Water Pocket fault orthorectified aerial-photo quads (DOQ). Using GIS soft- were primarily a normal fault, the Summit-Willow Tank ware, the georeferenced basemaps made it possible to thrust trace would be buried in the subsurface and not ex- merge data collected on different basemaps and at differ- posed, as it is presently, in the supposed down-thrown block. ent scales into a single project. The western boundary of the AZS in the Valley-of-Fire is demarcated by the Baseline Mesa fault. This fault off- Strike-slip Fault Network: Descriptive Analysis sets the stratigraphic contact between the AZS and the over- Two strike-slip faults sets are found in the AZS of the Val- lying Cretaceous Willow Tank Formation by 2.4 km in an ley-of-Fire (Figure 2). A north-northeast trending fault set apparent left-lateral sense. The main trace of the Baseline showing left-lateral slip with a normal component, and a Mesa fault trends approximately 20º east of north in the northwest trending fault set showing right-lateral slip with northern portion of the fault, and approximately north-south a normal component. Longwell (1949) first recognized in the southern portion of the fault. Main fault plane dips these faults, making note of offsets along the contact be- were found to be approximately 60-70º to the west. In the tween AZS and the underlying Triassic units. He did not northern part of the fault, a younger generation of left-lat- state what mode of slip these faults were (i.e., normal, re- eral faults oriented north-northwest cut the main trace of verse, or lateral), but did interpret them to be related to the Baseline fault, although offsets along these younger crestal collapse along the Valley-of-Fire anticline. faults are minor. Damage associated with the Baseline Mesa In this section, field data is presented that documents fault is even more severe than the damage associated with the geometric, kinematic, and timing relationships between the Water Pocket fault. In places, fault gouge zone widths

Stanford Rock Fracture Project Vol. 13, 2002 D-5 are greater than 25 m meters. Large-offset Left-Lateral Faults Field evidence indicates a strike-slip component of slip Three principle north-northeast oriented faults are identi- for the Baseline Mesa fault. Minor to major drag folds im- fied in the map shown in Figure 5. The central, throughgoing plying left-lateral slip are common on both sides of the fault is named the Lonewolf fault (Myers, personal com- fault. Figure 4 shows a major fault-related fold of a con- munication, 1998), while the faults to the west and east are glomeratic layer in the Cretaceous Willow Tank Forma- named the Wall fault and the Classic fault, respectively tion (also noted by Bohannon, 1983b). On either side of (Figure 5b). In the northern half of the area shown in Fig- the fold crest shown in Figure 4, bedding in the conglom- ure 5, the next major structure to the west is about 700 m erate layer changes strike by 123º in a counterclockwise distant, while the next major structure to the east is at least manner over a distance of about 20 m. Figure 4 also shows 1 km distant (cf. Figure 2). This is contrasted by the south- a tectonically thickened section of a siltstone layer in the ern portion of Figure 5, where the next major fault west of Willow Tank Formation. Measured parallel to the strike of the Wall fault is 150 m away, and the next major fault east the Baseline Mesa fault, the apparent map thickness of the of the LWF is just outside of the mapped area (cf. Figure Willow Tank siltstone changes from 90 m at a distance away 2). from the fault (left side of Figure 4), to more than 300 m The Lonewolf fault (LWF) has an overall exposed near the fault. length of roughly 2 km and consists of eight sub-parallel, linked segments that have lengths ranging from 200 to 550 Strike-slip Fault Network m. Mean strikes for the segments range from 193º to 201º, Fault maps at scales of 1:825 and 1:200 are used as a basis while mean dips ranged from 59º to 70º (Figure 5c). The for interpretation of fault network evolution. The smaller- location of the southern end of the LWF as shown in Fig- scale map (Figure 5) is located primarily within the middle ure 5b is based on the recognition of a slip magnitude mini- and upper member of the AZS (Figure 2), while the larger- mum. Given that much of the fault in this area is covered, scale map focuses on a region within the smaller-scale map and that another fault roughly in-line with the LWF contin- (cf. Figure 5b). Note that all structural orientation data in ues to the south (cf. Figure 2), the location of actual end of the ensuing sections are presented using the right-hand- the LWF is not clear. rule convention. Most of the segments that comprise the LWF are rela- tively in-line with each other and are characterized by seg-

Baseline Mesa fault Jar

folded conglomerate layer

thickened siltstone

Kwt

Figure 4. View north along the Baseline Mesa fault showing evidence for strike-slip motion. Note the fold developed in the conglomerate layer in the midground (long-dashed black line), as well as the tectonically thickened section of siltstone in the foreground (outlined by dotted black lines). The white dashed line is the approximate trace of the Baseline Mesa fault. Black arrow points to a two-lane dirt road for scale. Jar = lower member of the Jurassic Aztec sandstone; Kwt = Cretaceous

Stanford Rock Fracture Project Vol. 13, 2002 D-6 Kwt 11cm (A) 87 17m

N 89 100 meters Kwt 90 Map Legend for part A 80 left lateral fault (>1m slip) 88 left lateral fault (<1m slip) 10cm Kwt 143m 2m 89 right lateral fault (>1m slip) 1m 89/ 5 right lateral fault (<1m slip) 88 20cm joint or joint zone 23m lithologic contact 83 84/ dip/rake (where indicated) 80cm 83 3 57/ 6 2m slip magnitude 2cm 15cm cover 62/ 160 road 75 87 40cm

3m 50cm 1m 20cm 1.8m 89 22m* 85/ 2m 25 30cm 1m 2.2m 80 4m

5m 75 1m 85 85 1.7m 4cm 84/ 1m 80 15 2m 1.3m 80 80 78/ 20 40cm 1m 3m 16m 19m 80 7cm 1m *3-plane solution slip vector = 14o 174m 70

80 65 80 4m 75 90cm 75 75

1.5m Two RL fault 68 (B) orientations, 165 and CF 180. Faults in the 180 orientation share direction with older DBs. 75 69m LWF-north 2m region 1

region 2 4cm

60 3cm

62/ 15 region 3 region 4 LWF-central 58 Figure 5. (A) Structure map of a portion 75/ of the Valley-of-Fire State Park 165 63/ region 5 163 Figure 8 60 originally mapped at a scale of 1:825. 4cm 76 Given the scale of mapping, not all joints 84 are explicitly shown on this map. See ~80m Figure 2 to locate this map within the 57/ 10 LWF-south 74 58/ 20 context of the Valley-of-Fire. (B) Index

65/ map for part A. Highlighted regions are 171 1m referenced to in the text. Also, note the 72 WF location of the detail map shown in

egion 6 Figure 8. Fault names are as follows: 70 CF = Classic fault, LWF = Lonewolf fault, WF = Wall fault. (C) Equal-area

70 70 stereonet plot of structural data shown 38m region 7 in part A. The green great circles are 65 lot #1 N mean orientations of the segments that 65/ (C) 1m 20 comprise the left-lateral LWF (individual 4cm data shown as green plus symbols), 65 85 while the red great circle is the mean

65 orientation of secondary right-lateral 3m faults associated with the major left- 4m

70 lateral faults. 65/ 21 4m

3m Stereoplot Legend left lateral faults (LWF; mean planes) 86 right lateral faults (secondary; mean plane) left lateral faults (LWF; poles) 88 left lateral faults (secondary; poles) 1m right lateral faults (secondary; poles) n=113 84

70 Stanford Rock Fracture Project Vol. 13, 2002 D-7 ment-steps that are short in both width and overlap. How- with a trend for progressively steeper rakes from north to ever, two large right steps are recognized that effectively south along the LWF. An example of shallow plunging slip divide the LWF into three principle segments: north, cen- indicators on a primary plane of the LWF is shown in Fig- tral, and south (Figure 5b). Region 1 (Figure 5b) highlights ure 6. Meter wavelength fault groove marks are shown with the location of the right-step that separates the northern rakes plunging 2º to 4º to the south. Smaller, centimeter and central segments of the LWF. These two segments are wavelength groove marks with rakes plunging from 4º to not joined by a throughgoing left-lateral fault. Rather, they 14º overprint the larger grooves. By analogy to the LWF, are linked by a dense network of secondary right-lateral and in addition to the recognition of some offset markers faults (discussed below). In contrast, the right-step that sepa- and kinematic indicators, left-lateral slip is prescribed to rates the central and southern segments of the LWF is linked the two faults described below. by a throughgoing left-lateral fault (region 5, Figure 5b). The Classic fault (CF) is to the east of the LWF (Fig- Two independent lines of evidence suggest a major left- ure 5). In the mapped area, the CF consists of essentially lateral component, with a minor normal component, of slip one major throughgoing segment. However, many short, along the LWF. In the northern part of the fault, a three- subparallel segments that merge with the CF are also iden- plane solution between the fault plane, the shallow dipping tified in the northern reaches of the fault. Region 3 (Fig- contact between the middle and upper members of the AZS, ure 5b) highlights a particularly large left-lateral segment and a steeply dipping Set II deformation band zone indicate that merges with the CF. South of where these two seg- a slip rake of 14º, as projected onto the west-dipping fault ments merge, cover conceals much of the CF. To the south plane. Where identified, slickenlines and groove marks on of this area, the next major exposure is the tip region of primary fault surfaces yielded rakes between 2º and 21º, the CF. Slip near the fault tip is accommodated along at least two subparallel segments. To the north, the last evidence of the CF is the offset of the overlying basal conglomerate member of the Cretaceous Willow Tank For- mation. The Wall fault (WF) is to the west of the LWF (figure 5). Much of this fault disappears beneath cover to the north and must end prior to reaching the overlying Cretaceous sediments, as the basal conglomerate member is not offset where the WF projects (cf. Figure 2). However, the WF does continue for more than a kilometer to the south. Where the WF was mapped, it consists of many sub-parallel strands (also noted by Myers, 1999). Slip profiles for all three faults are presented in Fig- ure 7. Offset markers in the northern part figure 5a are abundant and consist of nearly vertical Set II deformation bands, as well as unit and lithologic boundaries. However, offset markers in the central portion of the mapped area were sparse, making it difficult to divide slip between the southern and central segments of the LWF (vicinity of re-

220 horizontal scale cumulative slip profile 200 200 meters 180

160

140 CF ? 120 Figure 6. Meter wavelength fault grooves overprinted by 100 slip magnitude (m) smaller discontinuous centimeter wavelength grooves on a 80 LWF-S/C primary plane of the Lonewolf fault. Rake of the large grooves 60 is 2-4º, while the smaller grooves rakes ranged from 4-14º, both 40 indicating left-lateral, oblique-normal slip. The black arrow LWF-N 20 ? points to an intact body of fault rock that is dislodged from the WF ? 0 main fault surface. The curvature of the large wavelength fault South North surface is also seen in this fault rock body (black dotted line). The average dip of the fault plane is 88º to the west. This Figure 7. Fault slip profiles for the three large left-lateral faults photograph was taken near the north end of the LWF. The shown in Figure 5. LWF-N = Lonewolf fault – north branch; white arrow points to a horizontally oriented Brunton compass LWF-S/C = Lonewolf fault – south and central branches; CF = for scale. View is to the south. Classic fault; WF = Wall fault.

Stanford Rock Fracture Project Vol. 13, 2002 D-8 722000

(A) N N (B) rl mean: 10 meters 161/74 ll mean: 69 197/67

72 4038600

left-lateral fault right lateral fault splays from left-lateral faults splays from right-lateral faults n=71 6cm

10cm

60/ 2 171 SF 3cm

7cm RL2

SF1 11cm LL1

60/ 10cm 171 SF3

5cm LL3 63/ 60 163 65/ 167

75/ 165

15cm Map Lgend 67/ left lateral fault 163 4038500 right lateral fault 5cm joint or joint zone 3m deformation bands 5cm zone of intense damage LL5 81/ 55/ 158 RL4 170 dip/rake (where indicated) 14cm 2m fault offset magnitude SF4 cover

UTM Zone 11N 62/ 155

722050 Figure 8. (A) Detail structure map of a portion of the Lonewolf fault originally mapped at a scale of 1:200 (see Figure 5b for map location). Note the shared orientation between splay fractures and right-lateral faults originating from the LWF, as well as the shared orientation between splay fractures and left-lateral faults originating from the secondary right-lateral faults. At least five generations of splay fracturing and shearing of splay fractures are identified on this map. Examples of each generation are as indicated: SC = splay fracture; LL = left-lateral fault; RL = right-lateral fault; numbers indicate generation (e.g., SC1 = first generation splay fracture). Given the scale of mapping, not all joints are explicitly shown on this map. (B) Equal-area stereonet plot of structural data shown in part A. Green and red great circles are the mean orientations of left- and right-lateral faults mapped in part A. The black-dashed line separates like-oriented left-lateral faults and splays from right-lateral faults from right-lateral faults

Stanford Rock Fracture Project Vol. 13, 2002 D-9 gion 5, Figure 5b). Because of this, these two segments are lateral faults, both emanating from right-lateral faults. lumped together in the slip distribution (Figure 7). Offset Some field relations do deviate from these generalizations, markers in the southern part of the mapped area consist which are discussed below. solely of primary and secondary dune bounding surfaces. The two structural sets are not randomly distributed. Of the three faults, the CF has the largest maximum offset Rather, they are preferentially located around the end re- of 173 m, while the LWF shows a maximum offset of about gions of both the larger left-lateral faults and the segments 80 m. The largest offset measured along the WF is 7 m. that comprise the large faults. At the scale of mapping However, we project slip along the WF to increase towards shown in Figure 5a, the northwest set consists primarily the south based on the trend for increasing slip in this di- of right-lateral faults that are bound by segments of the rection. Similarly, we project the decreasing slip trends for larger left-lateral faults, while the north-northwest set con- the buried portions of the LWF and CF to the north. sists of both fault bound left-lateral faults and splay fractures. In regions 6 and 7 (Figure 5b), the north-northeast Small-offset Discontinuous Structures set is only slightly less abundant than the northwest trend- In contrast to the few large-offset, throughgoing faults dis- ing set. Structures in region 6 are localized between the cussed in the previous section, many more structures exist end of the LWF and the next large left-lateral fault east of that are discontinuous, have offsets of no more than 50 m, the mapped area, while the structures in region 7 are local- and are usually bound in extent by the larger structures. In ized between the Wall fault and the start of the fault that is this class of structures, two general sets with different ori- in-line and south of the LWF. Another region similar in entations are recognized: (1) a more abundant set with north- style, but smaller in scale is located between the end of west oriented splay fractures and right-lateral faults, both the CF and the juncture of the south and central segments emanating from left-lateral faults; and, (2) a less abundant of the LWF (area covered by Figure 8). set with north-northeast oriented splay fractures and left- Three other regions are identified as having a more

1 2

legend preexisting joint splay fracture 1st generation left-lateral fault 2nd generation right-lateral fault 3rd generation left-lateral fault 4th generation right-lateral fault

100 m

3 4 5

Figure 9. Conceptual model for the evolution of the strike-slip fault network in the Valley-of-Fire. The first scene shows the preexisting joints stage prior to faulting. Scenes 2-5 show the progressive stages of splay fracturing and shearing of splay fractures that evolve into set of left- and right-lateral faults.

Stanford Rock Fracture Project Vol. 13, 2002 D-10 dominant presence of the northwest trending right-lateral ally sheared to form a third generation of left-lateral faults set. A dense set of right-lateral faults is localized between and related splay fractures. Higher order generations con- closely spaced left-lateral faults segments in regions 1 and tinue to form as shear strain accumulates across the sys- 3 (Figure 5b). In both of these areas, right-lateral faults tem and the rock is broken into smaller and smaller frag- range in orientation from 125º to 170º. In region 4, many ments. right-lateral faults span the step between the end of a large left-lateral fault and the LWF. Like regions 1 and 3, mul- Acknowledgements tiple orientations of right-lateral faults are identified trend- I was ably assisted in the field by Fabrizio Agosta, Billy ing from 135º to 180º. Most of the faults in the 180º ori- Belt, Stephan Bergbauer, and Frank Schneider. The staff of entation are localized along earlier formed Set I deforma- the Valley of Fire State Park, Nevada Parks Division is tion bands that share the same orientation. Discontinuous thanked for their friendly assistance. Support for this re- left-lateral faults with trends from 185º to 190º are also search was provided by the Stanford Rock Fracture Project found in region 4. In contrast to regions 1 and 3 where the and the U.S. Department of Energy, Office of Basic En- right-lateral faults are structurally bound, right-lateral faults ergy Sciences (DE-FG03-94ER14462 to Atilla Aydin and on the west side of region 4 extend outside of the larger David D. Pollard). left-lateral faults. A detailed map illustrating relations between the right- References and left-lateral fault sets is shown in Figure 8. The largest Anderson, R.E., 1973, Large-magnitude late Tertiary strike-slip structure in this area is LWF (heavy north-northeast ori- faulting north of Lake Mead, Nevada: U.S. Geological ented green line, Figure 8a). Emanating from the LWF are Survey Professional Paper 794, 18 p. similarly oriented splay fractures and right lateral faults. Anderson, R.E., and Barnhard, T.P., 1993a, Heterogeneous Two of the right-lateral faults extend though the map area Neogene strain and its bearing on horizontal extension and horizontal and vertical contraction at the margin of the to the southwest where they join the end of the CF outside extensional orogen, Mormon Mountains area, Nevada and of the mapped area. Emanating from these two right-lateral Utah: U.S. Geological Survey Bulletin, B2011, 113 p. faults, are both splay fractures and left-lateral faults. Fur- Anderson, R.E., and Barnhard, T.P., 1993b, Aspects of three- ther branching occurs from these left-lateral faults in the dimensional strain at the margin of the extension orogen, form of splay fractures and second generation of right-lat- Virgin River depression area, Nevada, Utah, and Arizona: Geological Society of America Bulletin, 105, 1019-1052. eral faults. At least five generations of splay fractures and Anderson, R.E., Barnhard, T.P., and Snee, L.W., 1994, Roles of sheared joint faults are identified in the mapped area (cf. plutonism, midcrustal flow, tectonic rafting, and horizontal Figure 8). collapse in shaping the Miocene strain field of the Lake Mead Structural orientation data for the map shown in Fig- region area, Nevada and Arizona, Tectonics, 13, 1381-1410. ure 8a are presented in Figure 8b. Mean orientations of the Armstrong, R.L., 1968, Sevier orogenic belt in Nevada and Utah: Geological Society of America Bulletin, 79, 429-458. left-and right-lateral faults differ by 36º. Right-lateral faults Axen, G.J., 1993, Ramp-flat detachment faulting and low-angle and splays fractures formed along left-lateral faults share normal reactivation of the Tule Springs thrust, southern the same general orientation. The same observation is made Nevada: Geological Society of America Bulletin, 105, for left-lateral faults and splays fractures formed along 1076-1090. right-lateral faults. The two groupings of orientations dif- Beard, L.S., 1996, Paleogeography of the Horse Spring Formation in relation to the Lake Mead fault system, Virgin Mountains, fer by 35º. Nevada and Arizona: Geological Society of America Special Paper, 303, 27-60. Conceptual Model Billingsley, G. H., and Bohannon, R. C., 1995, Geologic map of the A model is presented for the development of the strike- Elbow Canyon quadrangle, northern Mohave County, Arizona: U.S. Geological Survey Open-File Report 95-560, slip fault network in the Valley-of-Fire based in part on fault scale 1:24,000, 16 p. evolution models developed by Martel (1990) and Myers Bohannon, R.G., 1979, Strike-slip faults of the Lake Mead region (1999) (Figure 9). The fault network evolution model be- of southern Nevada: SEPM Pacific Coast Paleogeography gins with preexisting joint zones that are sheared to form Symposium, 3, 129-139. first generation throughgoing left-lateral faults and asso- Bohannon, R.G., 1983a, Mesozoic and Cenozoic tectonic development of the Muddy, North Muddy, and northern Black ciated mode I splay fractures that form along the periphery Mountains, Clark County, Nevada: Geological Society of of the left-lateral faults, across shorter spans of overlap- America Memoir, 157, 125-148. ping fault segments, and across the longer spans between Bohannon, R.G. 1983b, Geologic map, tectonic map and structure fault ends and a neighboring faults. Second generation right- sections of the Muddy and Northern Muddy Mountains, Clark lateral shear is eventually imposed across the short and long County, Nevada: U.S. Geological Survey Miscellaneous Investigations Series, Map I-1406, 2 sheets. splay fracture zones as shear strain continues to accumu- Bohannon, R.G., 1984, Non-marine sedimentary rocks of Tertiary late along the larger left-lateral faults. Slip along these right- age in the Lake Mead region, southeastern Nevada and lateral faults forms second generation splay fractures along northwestern Arizona: U.S. Geological Survey Profes- their periphery, as well as across spans between neighbor- sional Paper, 122, 72 p. ing right-lateral faults. The new splay fractures are eventu-

Stanford Rock Fracture Project Vol. 13, 2002 D-11 Bohannon, R.G., Grow, J.A., Miller, J.J., and Blank, R.H. Jr., 1993, Langenheim, V.E., Grow, J.A., Jachens, R.C., Dixon, G.L., and Seismic Stratigraphy and tectonic development of Virgin Miller J.J., 2001, Geophysical constraints on the location and River depression and associated basins, southeastern Nevada geometry of the Las Vegas Valley shear zone, Nevada: and northwestern Arizona: Geological Society of America Tectonics, 20, 189-209. Bulletin, 105, 501-520. Longwell, C.R., 1949, Structure of the northern Muddy Mountain Brock, W.G., and Engelder, T., 1979, Deformation associated with area, Nevada: Geological Society of America Bulletin, 60, the movement of the Muddy Mountain overthrust in the 923-967. Buffington window, southeastern Nevada: Geological Longwell, C.R., 1960, Possible explanation of diverse structural Society of America Bulletin, 88, 1667-1677. patterns in southern Nevada: American Journal of Science, Çakir, M., Aydin, A., and Campagna, D.J., 1998, Deformation 258-A, 192-203. pattern around the conjoining strike-slip fault systems in the Martel, S.J., 1990. Formation of compound strike-slip fault zones, Basin and Range, southeast Nevada: the role of strike-slip Mount Abbot quadrangle, California. Journal of Structural faulting in basin formation and inversion: Tectonics, 17, 344- Geology, 12, 869-882. 359. Martel, S.J, Pollard, D.D., and Segall, P., 1988, Development of Campagna, D.J. and Aydin, A, 1994, Basin genesis associated with simple strike-slip fault zones in granitic rock, Mount Abbot strike-slip faulting in the Basin and Range, southeastern quadrangle, Sierra Nevada, California: Geological Society Nevada: Tectonics, 13, 327-341. of America Bulletin, 100, 1451-1465. Carpenter, D.G., 1989, Geology of the North Muddy Mountains, Marzolf, J.E., 1983, Changing wind and hydraulic regimes during Clark County Nevada, and regional structural synthesis: deposition of the Navajo and Aztec sandstones, Jurassic (?) fold-thrust and Basin and Range structure in southern southwestern United States, In M.E. Brookfield and T.S. Nevada, southwest Utah, and northwest Arizona: [M.S. Ahlbrandt, (eds.) Eolian Sediments and Processes: Elsevier, Thesis] Oregon State University, 145 p. Amsterdam, Netherlands, 635-660. Carpenter, D.G., and Carpenter, J.A., 1994. Fold-thrust structure, Marzolf, J.E., 1990, Reconstruction of extensionally dismembered synorogenic rocks, and structural analysis of the North early Mesozoic sedimentary basins; Southwestern Colorado Muddy and Muddy Mountains, Clark County, Nevada. In: Plateau to the eastern Mojave Desert, In Wernicke, B.P. Dobbs, S.W., and Taylor, W.J. (eds.). Structural and (ed.) Basin and range extensional tectonics near the stratigraphic investigations and petroleum potential of latitude of Las Vegas, Nevada: Geological Society of Nevada, with special emphasis south of the Railroad America Memoir, 176, 477-500. Valley producing trend: Nevada Petroleum Society Mollema, P.M., and Antonellini, M., 1999. Development of strike- Conference II, 65-94. slip faults in the dolomites of the Sella Group, northern Italy: Cruikshank, K.M., Zhao, G., and Johnson, A.M., 1991. Analysis of Journal of Structural Geology, 21, 273-292. minor fractures associated with joints and faulted joints: Myers, R., 1999, Structure and hydraulics of brittle faults in Journal of Structural Geology, 13, 865-886. sandstone: [Ph.D. Thesis] Stanford University, 176 p. Davatzes, N.C., and Aydin, A., in review, Overprinting faulting Poole, F.G., 1964, Paleowinds in the western United States, In mechanisms in sandstone: Journal of Structural Geology. Nairn, A.E.M., Problems in Paleoclimatology: London, John Duebendorfer, E.M., Beard, L.S., and Smith, E.I., 1998, Restora- Wiley, 394-406. tion of Tertiary deformation in the Lake Mead region, Segall, P. and Pollard, D.D., 1983, Nucleation and growth of southern Nevada: the role of strike-slip transfer faults, In strike-slip faults in granite: Journal of Geophysical Accommodation zones and transfer zones: regional segmen- Research, 88, 555-568. tation of the Basin and Range province, Faults, J.E. and Stewart, J.H., 1980, Geology of Nevada: Nevada Bureau of Stewart, J.H. (eds.): Geological Society of America Mines and Geology Special Publication, 4, 136 p. Special Paper, 323, 127-148. Stewart, J.H., and Carlson, J.E., 1978, Geologic map of Nevada, Engelder, T., Haith, B.F., and Younes, A., Horizontal slip along 1:500,000 scale: U.S. Geological Survey Map MF-930. Alleghanian joints of the Appalachian plateau: evidence Taylor, W.L., 1999, Fluid flow and chemical alteration in showing that mild penetrative strain does little to change the fractured sandstones: [Ph.D. Thesis] Stanford University. pristine appearance of early joints: Tectonophysics, 336, 31- Willemse, E.M.J., Peacock, D.C.P., and Aydin, A., 1997, Nucle- 41. ation and growth of strike-slip faults in limestones from Fleck, R.J., 1970, Tectonic style, magnitude, and age of deforma- Somerset, U.K.: Journal of Structural Geology, 19, 1461- tion in the Sevier orogenic belt in southern Nevada and 1477. eastern California: Geological Society of America Bulletin, Zhao, G. and A. M. Johnson, 1992, Sequence of deformations 81, 1705-1720. recorded in joints and faults, Arches National Park, Utah, Flodin, E.A., Prasad, M., and Aydin, A., in press, Petrophysical Journal of Structural Geology, 14, 225-236. constraints on deformation styles in Aztec Sandstone: Pure Zoback, M.L., Anderson, R.E., and Thompson, G.H., 1981, and Applied Geophysics. Cainozoic evolution of the state of stress and style of Graham, B., Antonellini, M., and Aydin, A., in review, Formation tectonism of the Basin and Range province of the western and growth of normal faults in carbonates within a compres- united States: Philosophical Transactions of the Royal sive environment: Geology. Society of London, A-300, 407-434. Granier, T., 1985, Origin, damping, and pattern of development of faults in granite: Tectonics 4, 721-737. Hill, R., 1989, Analysis of deformation bands in the Aztec sandstone, Valley of Fire State Park, Nevada: M.S. Thesis, Geosciences Department, University of Nevada, Las Vegas, Nevada. Langenheim, V.E., Glen, J.M., Jachens, R.C., Dixon, G.L., Katzer, T.C., and Morin, R.L., 2000, Geophysical constraints on the Virgin River depression, Nevada, Utah, and Arizona: U.S. Geological Survey Open File Report 00-407, 26 p.

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