Evolution of slip partitioning in a major continental margin strike-slip fault system during a transition to oblique plate-margin tectonics: Insight into the evolution of the Garlock fault zone, California (USA)

Many tectonic boundaries on Earth have oblique deformation (Woodcock, 1986; Philippon and Corti, 2016; Brune et al., 2018) and follow an earlier period of pure extension. The obliquity of these belts is at odds with the classical view of plate tectonics in which plate boundaries are discrete end-member contractional, extensional, and transcurrent faults. Oblique extension resolves as non-mantle convection plate tectonic forces, which should be minimal except where preexisting rheologic anisotropies of the lithosphere allow localization of deformation, thus leading to a decrease in the plastic yield strength (Brune et al., 2018).

This study examines the faultslip history of an area within a well-exposed and well-studied zone of continuing dextral deformation in eastern California, USA. We discuss the Garlock fault zone (Fig. 1), a large transversely oriented set of sinistral strike-slip faults. These faults separate the transtensional Walker Lane belt (Stewart, 1988) from the transpressional Eastern California shear zone (Bartley et al., 1990; Dokka and Travis, 1990). Our goal is to determine the deformation history of the Garlock fault zone in the context of regional deformation and how slip on this non-ideally oriented fault functions in the deformation zone in which it is embedded. This study interprets the slip magnitude and history using identified offsets of geologic markers for faults in the eastern segment of the Garlock fault zone to examine accommodation mechanisms of the NNW-directed regional shear crossing the continuous-trace, WSW-striking Garlock fault.

The Garlock fault has been envisioned as a transform fault between the extension of Basin and Range Province and the dextral deformation of the Mojave Desert (Davis and Burchfiel, 1973; Garfunkel, 1974). These ideas are valid, but the overall evolution is more complex, with the Basin and Range Province evolving from extension to transtension and the Mojave Desert under transpression. The Garlock fault has been interpreted more recently to be a passively acting (Dokka and Travis, 1990; Gan et al., 2003; Andrew et al., 2015), deep-rooted structure along a preexisting lithospheric anisotropy (Chapman et al., 2010; Monteiller and Chevrot, 2011) that is now transverse to regional plate-boundary deformation, thus leading to a reduction in plastic yield strength and promoting low-stress failure and an increase in deformation rates (Brune et al., 2018). This active dextral shear system of the Walker Lane belt, Eastern California shear zone, and Garlock fault zone forms an important example of the factors and processes involved in creating and evolving an oblique deformation system.

Assessing the seismic hazards of the eastern Garlock fault zone requires accurate knowledge of all the faults involved in this zone. Seismic hazards are informed by the slip history interpreted by neotectonic studies, but neotectonic studies treat the Garlock fault zone as a simple single-stranded system (e.g., McGill et al., 2009; Hatem and Dolan, 2018). With its kinks, bends, strands, and complex structural features, the Garlock fault zone needs to be better understood as these sites are rupture initiation points, rupture stall points, or areas of lateral transfers of rupture propagation.

A note on the terminology used in this paper: The Walker Lane belt and Eastern California shear zone labels are variably and inconsistently applied to this zone of dextral deformation. Their original definitions define two distinct areas and styles of deformation (Stewart, 1988; Dokka and Travis, 1990). To honor the original definitions and simplify the text, we refer to this combined dextral deformation zone as an abbreviated composite system—Walker Lane belt–Eastern California shear zone (WLEC)—focusing on the crosscutting Garlock fault zone.

The WLEC is a 40- to 100-km-wide zone of dispersed strike-slip, normal, and oblique faulting and associated deformation that is inboard of the plate-boundary, dextral strike-slip San Andreas fault. Geodetic data show that this zone accounts for a quarter of the active plate boundary relative movement (Miller et al., 2001). It overprints the distributed normal fault system of the middle Miocene Basin and Range orogeny (e.g., Stockli et al., 2002; Andrew and Walker, 2009) and cuts across the Mojave Desert, which only experienced early Miocene extension. The WLEC is characterized by a complex system of faults with many different geometries, scales, and kinematics (e.g., Stewart, 1988; Dokka and Travis, 1990; Oldow et al., 1994; Schermer et al., 1996; Stockli et al., 2002; Oskin and Iriondo, 2004; Wesnousky, 2005; Andrew and Walker, 2009, 2017; Lee et al., 2009; Sizemore et al., 2019).

This study focuses on the sinistral Garlock fault zone, where it cuts transversely across the faults of the WLEC (Fig. 1). The Garlock fault zone is a prominent structure, but it does not disrupt the overall dextral shear of the WLEC, as demonstrated by geodetic data (Peltzer et al., 2001). The Garlock fault connects southwestward into the San Andreas fault (Fig. 1) as a conjugate fault (Davis and Burchfiel, 1973) at the “Big Bend” of trace of the San Andreas fault. Our group has studied other sites along the central segment of the Garlock fault (Andrew et al., 2015) to evaluate the interactions of dextral strike-slip faults that intercept the fault; we have pursued further study and have accessed the closed-access military areas to the east.

This study explores these new areas to test our earlier hypotheses of how NNW-directed shear of the active WLEC is accommodated in the region of the Garlock fault zone. We seek to add to our understanding of the Garlock fault zone by examining the structural evolution of the eastern Garlock fault zone over its polyphase deformation lifetime to compare to the slip history of the central Garlock fault zone (Andrew et al., 2015).

Our work enables us to identify and separate the deformation of earlier events on the Garlock fault to interpret the kinematics of recently active structures in the complex, multi-stranded eastern Garlock fault zone. This information is crucial to seismic hazard assessment, which requires an understanding of how strain is distributed between faults in complex, multi-stranded segments and how fault slip varies spatially and temporally (e.g., Norris and Cooper, 2007; Bennett et al., 2004; Dair and Cooke, 2009; Onderdonk et al., 2015; Choi et al., 2018; Burgette et al., 2020; Yao et al., 2022).

The Garlock fault, a large continental strike-slip fault, exhibits the typical long straight fault segments separated by structurally complex steps (Sylvester, 1988; Norris and Cooper, 2007; Cunningham and Mann, 2007; Klinger, 2010). The junction of the straight sections along large strike-slip faults with complex zones is an observed initiation point of fault rupture and can also act as a boundary to rupture propagation (King and Nabelek, 1985; Wesnousky, 1988, 2006; Harris et al., 1991; Shaw, 2006; Cunningham and Mann, 2007). The neotectonic activity along the Garlock fault has been examined at many sites along the length of the fault (Clark and Lajoie, 1974; McGill, 1992; McGill and Sieh, 1993; Carter, 1994; McGill and Rockwell, 1998; McGill et al., 2009; Ganev et al., 2012; Crane, 2014; Rittase et al., 2014), leading to interpretations that the slip rate decreases eastward (McGill et al., 2009; Ganev et al., 2012; Hatem and Dolan, 2018) in contrast to the relatively consistent total slip of Pliocene and younger deformation found by Andrew et al. (2015) from examining geologic slip. These neotectonic studies along the Garlock fault have focused on the slip history of single fault strands, whereas the central and eastern segments of the Garlock fault zone are noted for their complex multi-stranded aspect (Miller et al., 2014; Andrew et al., 2015; Rittase et al., 2020). A slip budget for seismic hazard assessment requires that the fault slip of each fault strand in a complex fault system is known (Weldon and Sieh, 1985; Dair and Cooke, 2009; Daout et al., 2016; Matrau et al., 2019; Burgette et al., 2020). Slip on the multiple strands of the central and eastern Garlock fault zone would add to the total slip of these segments, potentially enough to negate the hypothesized significant eastward decrease in slip. Thus, understanding the complexity and kinematics of the multi-stranded segments of the fault system is needed to evaluate the overall deformational model of the Garlock fault zone and to inform the seismic hazard assessment.

Although the areas adjacent to the eastern Garlock fault are unpopulated, significant infrastructure exists along and astride the eastern parts of the Garlock fault zone, with the U.S. Naval Air Weapons Station China Lake and the Fort Irwin U.S. Army base. In the nearby town of Ridgecrest (Fig. 1), the M6.4 and M7.1 paired strike-slip earthquakes of 2019 (Barnhart et al., 2019) caused an estimated $4 billion of damage to the facilities at China Lake (Klein, 2022), and earthquakes on the Garlock fault itself could yield larger magnitude earthquakes.

The Garlock fault zone has westward, stepwise increasing amounts of total sinistral slip from 58 to 67 km, with slip initiating at or after 10.7 Ma and continuing today (Monastero et al., 1997; Andrew et al., 2015). We use the term Garlock fault zone when referring to the entire ensemble of sinistral faults and related structures and use Garlock fault for describing the specific fault in the sense of Hess (1910). The Garlock fault zone contains active segments marked by a single through-going fault (e.g., eastern Summit Range to Christmas Canyon segment of the Garlock fault zone of Andrew et al. [2015]) with many other areas along the strike that are more complicated and contain several active fault strands (e.g., the segment of the Garlock fault zone along the El Paso Mountains; Fig. 1). This complexity is characteristic of the Garlock fault zone during Pliocene and Quaternary times, with slip partitioned on different faults at various times (Andrew et al., 2015). New work and re-examination of published mapping show that the Garlock fault zone from the southern Slate Range to the eastern Quail Mountains is composed of three prominent strike-slip faults: the Marine Gate fault in the southern Slate Range and western Quail Mountains, the Breccia Canyon fault (informally named from Muehlberger’s [1954] geologic map) in the Quail Mountains, and the Garlock fault.

The Marine Gate fault was first identified in the western Quail Mountains, California (Fig. 2; Muehlberger, 1954; Andrew, 2002, 2007) and was shown to occur farther west along the southern Slate Range by Andrew et al. (2014). Andrew (2002) identified low-rake slickenlines along the fault in the Quail Mountains. He interpreted the Marine Gate fault to have a Late Cenozoic left-lateral offset of 15–20 km so that it may account for a third of the ~60 km total sinistral offset of this part of the Garlock fault zone. Furthermore, Rittase (2012), Andrew et al. (2014), and Rittase et al. (2020) showed that the Marine Gate fault in the Slate Range is now active as a dip-slip, high-angle fault with a relative north-side-up offset of 1 km since middle Pliocene time. Finally, the Marine Gate fault interacts with the Searles Valley fault zone to the west to create a kinematic inconsistency of fault blocks in the southwestern Slate Range.

We use the methods and approach of Andrew et al. (2015) to establish the timing and amounts of lateral offset on the Marine Gate, Breccia Canyon, and associated Garlock fault strands. The total offset on each fault is determined using a now-dismembered Late Jurassic thrust fault intersecting these faults at a high angle. Younger slip is bracketed by examining the depositional source relationships of the Pliocene to Pleistocene rocks of the Pilot Knob Valley formation (Rittase et al., 2020).

The Marine Gate fault in the western Quail Mountains separates distinctive assemblages of Jurassic rocks and cuts Miocene volcanic rocks and Pliocene and younger sedimentary rocks and deposits (Fig. 2A; Muehlberger, 1954; Andrew, 2007). The north side of the Marine Gate fault has a suite of Jurassic metasedimentary, metavolcanic, and hypabyssal rocks overlain by ca. 14 Ma volcanic rocks (Smith et al., 1968; Andrew, 2007). The southern side of the fault is a Late Jurassic thrust complex that places a Jurassic mafic to felsic mixed plutonic suite over Late Jurassic leucocratic augen orthogneiss (Andrew, 2007). The Marine Gate fault in the western Quail Mountains is a strike-slip fault with low-rake fault striae and left-lateral slip indicators. Rocks along the fault are extensively cataclastically deformed (Fig. 3A). The fault cuts Mesozoic to middle Miocene volcanic rocks, late Miocene–Pliocene quartzite boulder conglomerate, and Quaternary alluvial deposits (Fig. 2A).

The Marine Gate fault in the southern Slate Range (Fig. 1) separates Jurassic rocks and a thrust complex on the north from Pliocene–Pleistocene rocks of the Pilot Knob Valley formation to the south (Fig. 4; Andrew et al., 2014; Rittase et al., 2020). Unlike the Quail Mountains, no exposures juxtapose different packages of Mesozoic rocks. The Slate Range north of the Marine Gate fault contains the Late Jurassic Layton Well thrust complex (Dunne and Walker, 2004) with similar lithologies, textures, kinematics, and crystallization ages to those in the western Quail Mountains on the south side of the Marine Gate fault. The Marine Gate fault in the Slate Range is a poorly exposed contact between Jurassic igneous and metamorphic rocks and weakly consolidated alluvial and lacustrine deposits. A steeply dipping, synthetic splay fault along the southwestern face of the Slate Range shows a left-lateral separation of 700 m of Jurassic gneisses and plutonic rocks (Andrew et al., 2014). The 3.3 Ma deposits in the Pilot Knob Valley formation are insignificantly offset laterally by the Marine Gate fault. Instead, they are offset north-side-up by dip-slip along the Marine Gate fault along the southern Slate Range (Rittase et al., 2020).

The westward continuation of the Marine Gate fault is difficult to interpret due to young alluvial cover, but it may be the steeply dipping fault imaged in a seismic line 13 km west of the Slate Range (Monastero et al., 2002). Monastero et al. (2002) named this structure the Spangler Hills thrust, but Rittase et al. (2020) correlated this imaged structure to the Marine Gate fault.

These previous studies show that (1) the Marine Gate fault can be traced for 45 km from the western Quail Mountains to the west of the Slate Range; and (2) Jurassic rocks and structures south of the Marine Gate fault in the western Quail Mountains may match similar rocks and structures in the southern Slate Range on the north side of the fault. These Jurassic rocks and structures appear to be repeated in the eastern Quail Mountains and south of the Garlock fault farther eastward in the Granite Mountains (Fig. 1). The restoration of these Jurassic features from the Granite Mountains to the Slate Range forms one of the principal offset markers for total offset on the Garlock fault zone (Davis and Burchfiel [1973], building on work of Smith [1962] and Smith and Ketner [1970]).

The amount of published detailed geologic data is now sufficient to understand when and how geologic features are offset across the Marine Gate fault and to interpret the slip history of these faults. Correlations for offset features are based on the recently published geologic mapping, structural, sedimentologic, and geochronologic studies of Dunne and Walker (2004), Andrew (2007), Andrew et al. (2014), Walker et al. (2014), and Rittase et al. (2020).

Jurassic faults provide key markers for the total offset along the Garlock fault zone (Smith, 1962). The Jurassic Layton Well thrust is well documented in the southern Slate Range (Smith et al., 1968; Dunne and Walker, 2004) and is correlated with the Drinkwater Lake thrust fault in the Granite Mountains (Fig. 1) for an estimate of ~64 km total offset across the Garlock fault zone (Davis and Burchfiel, 1973; Walker and Glazner, 1999).

The Layton Well thrust is exposed for 16 km and has >500 m of exposed structural relief along the south face of the Slate Range (Fig. 4A). It has a 50°–70° southwest-dipping segment that abruptly changes eastward to a consistently gently dipping segment (for geologic relationships and subarea stereonet plots of orientations of fabrics, see Figs. 4A–4J). This overall structure is similar to that of a ramp-flat as seen in thrust fault systems in well-bedded sedimentary rocks, even though the Layton Well thrust has a hanging wall that carries plutonic and metamorphic rocks over a footwall of layered metavolcanic rocks intruded by leucogranite (Fig. 4B). The steeply southwest-dipping “ramp” zone of the Layton Well thrust is the more typical expression of Late Jurassic thrust faults in the region (see summary in Dunne et al., 1998 and Dunne and Walker, 2004). The “flat’ section of the Layton Well thrust is a unique exposure of the Late Jurassic thrusts in the region and has an average dip of 10°–20° to the east (Dunne and Walker, 2004; Andrew et al., 2014; Walker et al., 2014). The original structure was a steeper “ramp” and roughly horizontal “flat” after accounting for the ~15°–20°eastside downtilting of the range that occurred in the Pliocene (Andrew and Walker, 2009). The footwall varies from voluminous alaskite below the ramp and ramp-flat transition, changing to meta-hypabyssal and metavolcanic rocks farther eastward. We consider this ramp-flat structure to be the original geometry and not a later folding event based on the spatial abruptness, the footwall lithologic variations, and the lack of kinematically coordinated folding in the footwall rocks (Fig. 4F). Still, a complete discussion of this is beyond the scope and goal of this paper. We refer to this structure in the rest of the paper as the ramp-flat structure with the caveats stated.

The hanging-wall rocks of the ramp zone define a great circle of poles to foliations with a β-axis gently plunging to the SSE with dominantly SW-dipping foliations (Fig. 4B) that parallel the ramp portion of the thrust fault. Associated stretching lineations plunge to the SW (Fig. 4C), which fits with the top- to- the- northeast transport direction of the hanging wall (Dunne and Walker, 2004). A similar set of foliations with slightly more clockwise orientations occurs in the central Slate Range (Fig. 4E), 10 km to the north. The hanging wall in the thrust flat zone has gently dipping foliations without strong folding of the foliations (Fig. 4D). The footwall metavolcanic rocks in this area show only moderate degrees of folding about an east-plunging β-axis (Fig. 4F). Footwall coarse-grained alaskite has gneissic and mylonitic fabrics along the thrust with steep dips at the ramp and low dips along the flat (Fig. 4J). A thrust duplex of metavolcanic rocks occurs near the ramp-flat transition with gently dipping foliations with a significant amount of azimuth dispersion (Fig. 4I). The southwestern-most part of the Slate Range has a different pattern of foliations with two main populations of NE and SW dips (Fig. 4G) and distinctly different stretching lineations with gentle plunges to the SSE and NNW (Fig. 4H). These NNW-SSE gently plunging, stretching lineations are similar to those found in the nearby Panamint Range that are interpreted as being due to Late Cretaceous contractional deformation (Andrew, 2002, 2019).

Rocks in the hanging wall consist of a mixed, maficfelsic plutonic complex with minor metavolcanic and metasedimentary pendants and screens. U-Pb zircon geochronology (Dunne and Walker, 2004) yielded crystallization ages of 151 ± 1 Ma for a biotite monzogranite with weak deformation near the Layton Well thrust, 177 ± 1 Ma for an undeformed monzogranite, and 184 ± 1 Ma for a mylonitic granodiorite adjacent to the Layton Well thrust. The footwall rocks consist of 152 ± 1 Ma, coarse-grained alkali feldspar porphyritic alaskite and 152 ± 1 Ma, metavolcanic rocks, including a hypabyssal rhyolitic intrusion. These rocks grade into mylonite toward the Layton Well thrust with SW-plunging, stretching lineations (Dunne and Walker, 2004). The footwall coarse-grained alaskite has steeply dipping gneissic and mylonitic fabrics along the thrust ramp with low dips to the east along the thrust flat (Fig. 4J).

The footwall rocks in the thrust-flat section have ultramylonitic, mylonitic, and cataclastic fabrics that overprint earlier quartz-ductile shear fabric. S-C fabrics and asymmetric folds show a top- to- the- NE sense of shear. Emplacement of the mixed plutonic complex over the alaskite and metavolcanic rocks by the Layton Well thrust occurred at ca. 151 Ma based on the similar ages of metavolcanic rocks and deformed alaskite in the footwall and plutonic rocks in the hanging wall. A pendant of the hanging-wall mylonite also is present in the footwall alaskite, indicating synthrusting intrusion of the Late Jurassic alaskite (Dunne and Walker, 2004).

A similar fault to the Layton Well thrust occurs in Jurassic rocks between the Marine Gate and Breccia Canyon and Garlock faults in the western Quail Mountains (Fig. 2A; Andrew, 2002, 2007). This fault, referred to as the Old Jackass fault for a nearby historic Cu-Au mine, juxtaposes two Jurassic plutonic packages. The hanging wall consists of a mixed mafic and felsic plutonic rock complex (Muehlberger, 1954; Andrew, 2007). The mixed complex consists of abundant diorite intruded by numerous light-pink–colored, porphyritic quartz monzonite bodies in an intricate pattern that is too complex to map separately. This quartz monzonite yielded a U-Pb zircon age of 174.3 ± 2.3 Ma, and a 3-m-wide mylonitic shear zone within the mixed complex is cut by a 162.7 ± 1 Ma mafic dike (Andrew, 2002). The footwall consists of 151 ± 3 Ma coarse-grained alkali feldspar porphyritic alaskite with textures that range from igneous to augen gneiss to mylonite and ultramylonite. Fabric orientations are shown in Figures 2B to 2I. These mylonites are LS- and L-tectonites with stretching lineations that plunge gently in a range of trends from NE-SW to SE-NW (Fig. 2H). Foliations dip moderately to the SSW (Fig. 2E), although some have been locally folded about a shallowly west-plunging fold axis to steep NNE dips. A few small, sheared inclusions of metavolcanic rocks occur within the eastern exposures of the augen gneiss alaskite (Fig. 2A).

The Old Jackass fault varies in orientation (Fig. 2I) across the western Quail Mountains: the southwestern exposures dip steeply to the SW, and the eastern exposures dip gently to the SW with 100–500 m wavelength broad undulations of dips (Fig. 2A). The fault and the footwall augen gneiss and mylonite show a multiphase deformation history (Fig. 3B): D1 is top- to- the- northeast transport on the main gneissic and mylonitic foliation, and D2 is a reactivation of the mylonitic foliation with top- to- the- NW asymmetric folds only on the west side of the undulating mylonitic foliation. Although there is low relief on outcrops of this fault, the fault is well exposed (Figs. 3C and 3D). Jurassic and Miocene rocks within 100 m of the Marine Gate fault are cataclastically crushed and folded about shallowly plunging WNW-trending fold axes (Fig. 3A).

The Late Jurassic thrust faults in the Slate Range (Layton Well thrust) and Quail Mountains (Old Jackass thrust) have similar hanging-wall and footwall lithologies, crystallization ages, deformation styles, metamorphic grades, subsidiary structures, and geometric relationships with a few notable differences. (1) The Layton Well thrust comprises only local mylonitic fabrics of the coarse-grained alaskite proximal to the thrust fault, as opposed to the abundant gneissic to mylonitic fabrics exposed in the footwall of the Old Jackass fault. This could be due to the limited amount of exposed structural relief in the western Quail Mountains. The structurally lowest (southwestern-most) exposures of the Layton Well thrust are composed of a body of foliated coarse-grained alaskite in a fault duplex. (2) The Layton Well thrust includes a voluminous section of metavolcanic rocks in the footwall, whereas the Old Jackass fault does not. This discrepancy may be due to the limited lateral exposure in the western Quail Mountains along the thrust transport direction. The structurally lowest and southwestern-most exposures of the Layton Well thrust are similar to the western Quail Mountains in juxtaposing mixed complex plutonic rocks directly over sheared alaskite. (3) The Layton Well thrust has top- to- the- ENE displacement (Dunne and Walker, 2004, and Figs. 4B and 4E), and the Old Jackass fault has top- to- the- NE displacement (Fig. 2E). The western Quail Mountains have pervasive brittle deformation textures and folding that we interpret to be caused by the adjacent Marine Gate, Breccia Canyon, and Garlock faults. (4) The Old Jackass fault was reactivated with top-westward extensional slip, but extensional reactivation of the Layton Well thrust was not noted by Dunne and Walker (2004). Dunne and Walker (2004) did note a mylonitic reactivation of older ductile fabrics locally in the Slate Range, and the Layton Well fault duplex (Fig. 4A) contains the intrusive contact of alaskite with metavolcanic rocks that appears to be offset westward relative to the footwall, opposite to the top-ENE thrust transport direction. These differences are minor compared to the overall similarity of hanging-wall and footwall rocks, deformation fabrics, and the northeastward-directed shear and thrusting. These aspects lead us to correlate the Old Jackass fault with the Layton Well thrust. Additionally, no other similar thrust faults are exposed in the nearby region.

To restore slip on the Marine Gate fault, we use the thrust ramp zone as the offset marker feature to align the ramp geometry and footwall foliated alaskite of the Layton Well thrust to the steeper, western-most exposures of the Old Jackass fault in the western Quail Mountains. This restoration gives left-lateral offset of 19.4 ± 0.3 km. This offset and others discussed below are shown in Figure 5 and Table 1 (A0 to A1 in Fig. 5 shows offset for the Layton Well and Old Jackass faults). The assigned error was geometrically constructed and measured using the scale of the ramp-flat transition, the geometry of augen alaskite in the Slate Range exposures, and the horizontal projection distances to the Marine Gate fault.

The Pilot Knob Valley formation is a Pliocene–Pleistocene (>3.6–0.64 Ma) sequence of clastic and evaporitic rocks flanking the southern Slate Range between the Marine Gate and Garlock faults (Fig. 4; Andrew et al., 2014; Rittase et al., 2020). This unit contains distinctive clastic rock types tied to sources now displaced laterally by the Garlock fault zone (Rittase et al., 2020). The clasts in member 2 of this formation show a west- to- east lateral change going upsection that is interpreted to be a change in the source area of clasts—from a southern source of the Eagle Crags volcanic field to northern sources of bed-rock from the Slate Range. A rock-avalanche deposit in the upper part of the lacustrine facies of member 2 consists of angular clasts of coarse-grained alaskite. This rock-avalanche deposit matches the coarse-grained alaskite in the Layton Well thrust footwall ramp and ramp transition. The transport direction of the rock-avalanche deposit is unknown except as an inference using the steep south-facing slope of the range today; so we have applied a larger uncertainty to account for potential deviations and deflections of the path of the rock-avalanche deposit. We interpret the rock-avalanche deposit to indicate small (<1 km) to no lateral slip on the Marine Gate fault since ca. 3.3 Ma—the age of this member of the Pilot Knob Valley formation) (B0 to B1 in Fig. 5 and Table 1).

This rock-avalanche deposit indicates that the lateral offset of member 2 of the Pilot Knob Valley formation relative to the Slate Range is much less than the total slip of the Marine Gate fault. Because of the conformable contact between members 1 and 2 (Rittase et al., 2020), we consider most of the lateral offset on the Marine Gate fault to have occurred before ca. 4 Ma (interpreted age of member 1; see next section). Any lateral offset between 4 and 3.3 Ma would have disrupted the basin for these sedimentary rocks, given that the Marine Gate fault is <3 km north of the avalanche deposit and the basal member 2 contact. In addition, Rittase et al. (2020) interpreted the lacustrine deposits of member 2 to result from basin isolation by vertical slip on the Marine Gate fault. Exhumation of the south face of the Slate Range thus would have occurred along the dipslip reactivated Marine Gate fault that would eventually expose the Slate Range bedrock source areas and create steep slopes for the rock-avalanche deposit. This exhumation occurred synchronously with normal slip on the Searles Valley fault along the west side of the Slate Range and associated east-side-down tilting of the Slate Range (Didericksen, 2005; Walker et al., 2014).

The Pilot Knob Valley formation also has implications for interpreting offset on the Garlock fault. Member 1 of the Pilot Knob Valley formation has distinctive cobble to boulder clasts sourced from the central Eagle Crags volcanic field, located south of the Garlock fault (Fig. 1; Rittase et al., 2020). Although member 1 is not directly dated, it is overlain by member 2, which contains 3.6–3.3 Ma ash beds. Following the arguments of Rittase et al. (2020), we assign an age of ca. 4 Ma to member 1. Restoration of left-lateral offset on the Garlock fault for member 1 to a location adjacent to the clastic transportation valley systems from the Eagle Crags volcanic rocks has a maximum offset of 29.5 km (C0 to C2 on Fig. 5 and Table 1). Member 1 does not contain granitoid clasts. Thus, the offset depositional source area has an eastern limit before the mostly granitoid clasts-bearing alluvial fans from the western Granite Mountains. A minimum Garlock fault offset of 24 km (C0 to C1 in Fig. 5) of member 1 is needed to not align with a Paleozoic metasedimentary detrital source since these rock units are not present as clasts in member 1. This minimum offset also accounts for the offset amount preserved in a narrow fault sliver of member 1 exposed southeast of the Slate Range (Figs. 2A and 4A). We interpret these sources and deposits in member 1 to record the Garlock fault offset from 4 to 0 Ma of 26.9 ± 2.8 km (C0 to C3 in Fig, 5 and Table 1).

The Old Jackass fault in the western Quail Mountains is cut at its southern end by the Breccia Canyon fault (Fig. 2A; A2 on Fig. 5). The Breccia Canyon fault is noted for Holocene slip (Miller et al., 2014). It sinistrally displaces Jurassic and Pliocene features (Fig. 2A). We could not directly investigate the eastern Quail Mountains, because this area has been inaccessible as an active military area. To examine the slip in the eastern Quail Mountains, we used the geologic mapping and descriptions of Muehlberger (1954), who mapped both the western and eastern Quail Mountains. We compared the data of Muehlberger (1954) to that of Andrew (2002, 2007) for the western Quail Mountains and examined aerial and satellite imagery data for both eastern and western exposures.

We are confident in matching the Jurassic geologic units and steeply dipping thrust fault ramp from the western to the eastern Quail Mountains. Lateral offset from the western to eastern Quail Mountains occurs on the combined Breccia Canyon, Southern Panamint Valley, and Owl Lake sinistral faults (A2 to A3 in Fig. 5). Alignment of the apparent thrust ramp from the western to the eastern Quail Mountains yields a value of 13.5 km of sinistral offset as the eastern Quail Mountains do not clearly expose the ramp-flat transition, and the dip-slip component of the faults in this area cannot be thoroughly evaluated except as relatively steeply southwest-dipping from map relationships of Muehlberger (1954). The matching fault in the eastern Quail Mountains correlated with the Old Jackass fault is cut southward by the Garlock fault (A4 on Fig. 5).

The offset continuation of the Layton Well and Old Jackass faults on the south side of the Garlock fault is the Drinkwater Lake fault in the Granite Mountains (A5 on Fig. 5), a correlation made by Davis and Burchfiel (1973). The Drinkwater Lake fault has two strands and places Jurassic mixed plutonic complex rocks over a thick section of Jurassic metavolcanic rocks with an intervening fault duplex of granitic rocks. The Drinkwater Lake fault has moderate to steep dips. It is interpreted to be reactivated by top-westward extension (Brady, 1986), similar to the Old Jackass fault (Andrew, 2002) and potentially the Layton Well thrust (this study). The offset of the Old Jackass fault from the eastern Quail Mountains to the Granite Mountains is 24.3 ± 0.9 km on the Leach Lake segment of the Garlock fault (A4 to A5 in Fig. 5 and Table 1) with the caveat that this is a correlation of 50°–70° west-dipping thrust faults without a control on the dip-slip component. We consider the dip-slip components of these offsets to be much smaller than the strike-slip components. There is less than 500 m of topographic relief in this section of the Garlock fault zone, and similar rock packages are exposed in the different ranges.

These fault correlations combined with detailed mapping of the Garlock fault trace (McGill and Sieh, 1991) yield a total left-lateral offset of 57.2 ± 1.2 km (A0 to A5 in Fig. 5 and Table 1; errors added in quadrature following Andrew and Walker, 2017). This offset restores the Layton Well thrust ramp in the Slate Range to the structurally lower Drinkwater Lake fault in the Granite Mountains, partitioned onto the Marine Gate, Breccia Canyon, and Garlock faults.

The total offset of the Garlock fault zone east of the intersection with the Southern Panamint Valley fault is partitioned on the Southern Panamint Valley, Owl Lake, Denning Spring, and Garlock faults (Fig. 5). The Leach Lake segment of the Garlock fault has 24 km of offset (this study); the Owl Lake sinistral fault has 4 km (Andrew and Walker, 2022); and the dextral Southern Panamint Valley fault contributes its 10.5 km (Andrew and Walker, 2009) of dextral slip into the Garlock fault zone via an interpreted zipper-like process that effectively stretches the north side of the Garlock fault zone east of its intersection, reducing the total offset along the Owlshead Mountains by its slip amount of 10.5 km. Offset on the Denning Spring fault is unknown, but exposures of rocks to the north and south do not match (Brady, 1986); therefore, the fault experienced significant slip. Integrating the known total offset amounts on faults other than the Denning Spring along the eastern Garlock fault zone leaves 18.4 km of unaccounted-for offset to apply to the Denning Spring fault. The similar offset amount, relative timing (i.e., the Denning Spring fault has no young lateral offset [Miller et al., 2014]), and similar azimuth lead us to tentatively correlate all or part of the Denning Spring fault to be an offset segment of the Marine Gate fault. Also similar to the Marine Gate fault, the Denning Spring fault is now active as a dip-slip fault associated with Pliocene and younger north-south contraction (Miller et al., 2014). Although these offset amounts are internally consistent, understanding the evolution and partitioning of slip on these eastern Garlock region faults is incomplete and requires additional study of the Denning Spring fault to fully unravel an explanation.

We follow the regional tectonic model of Andrew et al. (2015) in using three intervals of slip history for the Garlock fault zone to interpret the Marine Gate fault: (1) the early history from 11 to 7 Ma, (2) middle history from 7 to 4 Ma, and (3) the late development from 4 to 0 Ma. The initiation of sinistral slip on the Marine Gate fault started after ca. 14 Ma because volcanic rocks of that age (Andrew and Walker, 2009) are cut by the fault (Fig. 2). While we can match the Jurassic markers with the full 19.4 km of sinistral offset, no middle Miocene volcanic rocks are exposed on the fault’s south side to evaluate their offset. The Pliocene rocks (member 2 of the Pilot Knob Valley formation) south and along the southern Slate Range indicate little or no lateral offset (B to B1 in Fig. 5). For this reason, lateral slip on the Marine Gate fault is bracketed between 14 and 4 Ma. This age estimate overlaps with the regionally interpreted initiation age of the Garlock fault zone at 11 Ma (Andrew et al., 2015); therefore, we interpret the Marine Gate fault as starting at or after 11 Ma.

The most recent time interval (4–0 Ma) on the Marine Gate fault included no lateral offset with a south-side-down, dip-slip reactivation of the fault. The interpreted western continuation of the Marine Gate fault in the seismic section west of the Slate Range shows a similar south-side-down throw on a steeply dipping fault (Monastero et al., 2002) with an unknown component of lateral offset. Along the west side of the Slate Range, the low-angle normal Searles Valley fault was active during 4–0 Ma with 4 km of relative westward slip of the hanging wall (Walker et al., 2014). The geologic mapping of Andrew et al. (2014) and this study shows several other young faults that cut the Pliocene and Quaternary members of the Pilot Knob Valley formation and younger deposits and surfaces; the significance of these faults is explored in a subsequent section on the kinematics of this complex system.

No lateral offset of the Marine Gate fault during 4–0 Ma (Rittase et al., 2020; this study) implies that the 19.4 km of sinistral offset on the Marine Gate fault must have occurred during the earlier periods of activity on the Garlock fault zone. Andrew et al. (2015) interpreted that the central segment of the Garlock fault zone experienced 10.5 km of sinistral offset during 11–7 Ma along the central segment, west of the Slate Range, The transform fault interpretation of the Garlock fault zone during 11–7 Ma (Andrew et al., 2015) has undergone extension on the Slate Range detachment along the west side of the Slate Range (Walker et al., 2014). This extension implies that the sinistral offset on the Garlock fault zone is reduced east of the Slate Range detachment by the offset of the westward hanging-wall transport detachment fault. The Slate Range detachment has 4 km of top- to- the- west displacement (Walker et al., 2014); therefore, the Marine Gate fault could have 0–6.7 km (subtract 4 km from 10.7 km) of sinistral offset during 11–7 Ma. The remainder of the sinistral slip on the Marine Gate fault must have occurred during 7–4 Ma (12.7–19.4 km of offset).

Using these observations, the sinistral slip of the Garlock fault zone is inferred to be 19.1 km during 7–4 Ma. This interpretation is based on the similarity in the values of the Marine Gate fault lateral offset with the slip on the Garlock fault zone attributed to this time interval by Andrew et al. (2015). Earlier lateral slip cannot be ruled out for the Marine Gate fault, but it can only account for 35% of the sinistral slip of this fault. The time interval 7–4 Ma represents a transitional stage for the slip history of the Garlock fault zone between the transform fault with Basin and Range westward extension during 11–7 Ma and the passive accommodation of regional NNW-directed dextral shear in the WLEC during 4–0 Ma.

The fault system in the southwestern Slate Range during 4–0 Ma is complex with low-angle normal faulting of the Searles Valley fault, dip-slip on the Marine Gate fault, and sinistral slip on the Garlock fault. The lacustrine member 2 of the Pilot Knob Valley formation was deposited during the early part of 4–0 Ma as the uplift of the Slate Range occurred on the Searles Valley fault and dip-slip movement on the Marine Gate fault (Figs. 6A and 6B). The Searles Valley fault intersection with the Marine Gate fault creates an unstable triple junction analogous to a ridge-ridge-transform or ridge-trench-transform fault, with the differences being due to the normal versus reverse fault interpretations of the 4–0 Ma Marine Gate fault.

We have re-examined the geologic mapping of the western Marine Gate fault by Andrew et al. (2014) using remote sensing imagery and light detection and ranging (lidar) data (available at https://www.google.com/maps/, https://www.mapbox.com/, and https://opentopography.org/). Andrew et al. (2014) show the continuation of the Marine Gate fault west of the Slate Range as a questionable-inferred and questionable-approximate quality structure with the Pliocene lacustrine facies member 2 of the Pilot Knob Valley formation to the north and south. These authors considered the fault a through-going feature continuing west of the known fault exposures along the Slate Range bedrock face. Our new analysis using better resolution imagery does not show this westward continuation of the Marine Gate fault through Pliocene rocks. A NW-striking dextral fault along the range front, to the northwest of the last westward definitive exposures of the Marine Gate fault, was mapped by Andrew et al. (2014) in Pleistocene alluvial fans, linking the Marine Gate fault to the south end of the SSW-striking Searles Valley fault (Figs. 4A and 5). This dextral fault zone is 3.5 km long, cutting Pliocene to Quaternary rocks, with Holocene alluvium covering the northern exposures. Faulting to the west, within Searles Valley, can only be inferred from sparse outcrops. We locally observe a WSW-striking fault that strikes toward the intersection of the dextral fault with the south end of the Searles Valley fault (Fig. 5). The relative movement sense is unknown, although this fault is parallel to the topographic break from Searles Lake to the uplifted Pliocene sedimentary rocks to the south.

The complex kinematics of time interval 4–0 Ma can be viewed using these new interpretations in the context of an evolving unstable triple junction (Fig. 6). This unstable triple junction would break down into two diverging triple junctions, separated by a progressively lengthening NW-striking dextral fault (Figs. 6B and 6C). The northern triple junction has the Searles Valley fault intersecting with the new dextral fault and the western continuation of the Marine Gate fault. The western portion of the Marine Gate fault would have sinistral slip or normal oblique sinistral slip as it accommodates the differential slip of the hanging wall of the Searles Valley fault relative to the rocks to the south of the Marine Gate fault. The southern triple junction has the new, lengthening, NNW-striking dextral fault intersecting with the dip-slip reactivated Marine Gate fault to the east and a new dextral or dextral-oblique fault to the south that connects to the Garlock fault.

We can use a simple geometric model to determine how far the western segment of the Marine Gate fault could be offset northward by the dextral fault between the diverging triple junctions. Walker et al. (2005) and Numelin et al. (2007) interpret a westward extension direction for the Searles Valley fault from data farther north. Still, sparse data from the southern exposures of the Searles Valley fault show WNW-directed extension (Andrew et al., 2014). A triple-junction model with west-directed extension on the Searles Valley fault yields a dextral fault offset on the SW corner of the Slate Range of ~2 km, whereas WNW-directed extension yields ~3 km.

This evolving triple-junction model (Fig. 7) has implications for the narrow fault block between the Garlock and Marine Gate faults. N-S–directed contractional deformation affects the Pliocene and younger rocks in the exposures in this area (Figs. 7B to 7G). Members 2 and 3 of the Pilot Knob Valley formation show folding about gently E- or W-plunging β-axes (Figs. 7D and 7E). Members 5 and 4 (1.1 Ma and younger; Rittase et al. [2020]) do not have well-defined β-axes from folding, but there is a spread in the poles to bedding and obvious tilting on many beds (Figs. 7F and 7G). The difference in the amount of folding may be time-dependent with ongoing protracted deformation. It may, in part, be better expressed in the finely bedded lacustrine facies of members 2 and 3 (Figs. 7D and 7E).

The folding of members 1 and 2 is interpreted to be related to the accommodation of the 1–2 km of dextral slip of the SSE-striking Airport Lake fault zone (Walker and Andrew, 2019) at the west end of the exposures of these members (Fig. 7A). This fault intersects and ends at the straight trace of the Garlock fault. Local contraction east of this intersection could accommodate the relatively small amounts of dextral slip without deflecting or cutting the Garlock fault; thus, the Garlock fault acts as a backstop blocking through-going dextral faulting (Garfunkel, 1974; Gan et al., 2003).

The interpreted slip history of the Marine Gate and other faults in the Slate Range area, combined with the previously published slip histories from farther west along the central segment of the Garlock fault zone, shows a distinctive pattern of sidestepping for the main locus of Garlock fault slip. Our results, in this context, imply that the main slip surface transferred southward from the Marine Gate fault onto the Slate Range segment of the Garlock fault by ca. 4 Ma. Farther west, the locus of the Garlock fault zone’s sinistral slip transferred northward from the Little Bird fault (Fig. 5) of Andrew et al. (2015) to the Christmas Canyon segment of the Garlock fault at ca. 4 Ma. Even farther west, sinistral slip transferred northward from the Savoy to the Summit Range segment of the Garlock fault at ca. 7 Ma.

This sidestepping of the locus of the main slip of the Garlock fault zone results in a kink or deflection in the presumably once-straight trace of the Garlock fault zone (Gan et al., 2003). The location of this kink may correspond to where the western boundary of the region of distributed dextral shear of the northeastern Mojave Desert (Fig. 1; Garfunkel, 1974; Schermer et al., 1996) intercepts the Garlock fault zone from the south (East Goldstone Lake fault on Fig. 5). The northeastern Mojave Desert (Fig. 1) has deformed via block rotation and complex faulting to result in 30–35 km of Late Cenozoic NNW-trending dextral shear (Schermer et al., 1996), a portion of which could be accommodated northward at the Garlock fault zone by this sidestepping of the main sinistral slip surface by localization of a new slip surface on P-shear structures (Fig. 8). The lateral step between the Little Bird fault of Andrew et al. (2015) and the Marine Gate fault could accommodate ~9 km of NNW-directed dextral shear, based on the known map patterns and distances between the current Garlock trace and traces of the other two faults (see Fig. 8 for schematic evolution of these faults). The other ~21 km of NNW-directed dextral shear from the northeastern Mojave Desert (Schermer et al., 1996) must be transferred or accommodated across the Garlock fault via other mechanisms.

The temporal evolution of sidestepping of the main locus of sinistral slip in the Garlock fault zone needs additional data for better spatial and temporal resolution. Still, there appears to be an earlier sidestepping event at ca. 7 Ma—with the abandonment of the Savoy fault (Andrew et al., 2015)—and a younger event at 4–5 Ma (the interpreted once-contiguous Marine Gate and Little Bird faults). The Denning Spring fault is less studied, but it could have a similar slip and tectonic history as the Marine Gate fault and thus should be included in the sidestepping interpretation of the Garlock fault zone.

The initiation of WLEC deformation for the areas north and south of the Garlock fault zone has been interpreted as westward propagating systems (Andrew and Walker, 2009, 2017; and references therein). The locus of deformation shifted westward from Death Valley to Panamint Valley at ca. 4 Ma. The initiation and slip of the Southern Panamint Valley fault at ca. 4 Ma could lead to changes in the Garlock fault zone that resulted in the sidestepping event that abandoned the Little Bird–Marine Gate–Denning Spring fault. Still, testing and refining this idea requires more data from the Denning Spring and Owlshead Mountains. The detailed slip history of the Garlock fault zone is only well known from a few localities along the central segment of the Garlock fault zone; so it is difficult to examine the precise timing of events and correlation to regional events timings. The sidestepping of the main locus of slip of the Garlock fault zone may be currently active at Koehn Lake, west of the Summit Range, where the locus of sinistral slip may be transitioning between the Garlock and reactivated Savoy faults (Fig. 1).

The NNW-directed dextral strain of the WLEC crosses the active Garlock fault (Peltzer et al., 2001), and the individual offset of each of the dextral faults ends at the Garlock fault without cutting it. Offset of these dextral faults must be accommodated by mechanisms within the Garlock fault zone. The accommodation mechanism of sidestepping the locus of slip could result in the transfer of upper crustal blocks from the north side to the south side of the main Garlock slip structure; along the eastern segment of the Garlock fault, this could account for a third of the net 30 km NNW-directed dextral shear of the adjacent northeastern Mojave Desert region (Schermer et al., 1996).

Other mechanisms are needed to accommodate the net ~50 km of dextral shear of the Mojave Desert across the Garlock fault zone (Schermer et al., 1996; Andrew and Walker, 2017); these include oroclinal bending (Garfunkel, 1974; Gan et al., 2003), block rotation (Guest et al., 2003), local contractional and extension paired zones of structures at the intersection of dextral faults with Garlock fault (Andrew et al., 2015), and the lateral tectonic escape of fault slivers along the Garlock fault (Serpa and Pavlis, 1996; Andrew et al., 2015). The most apparent accommodation mechanism is oroclinal bending via vertical axis rotation of the trace of the Garlock fault, as the azimuth of the trace of the fault shows a difference of 49° between the 056° trending trace of the western segment and the 105° of the eastern segment (Gan et al., 2003). This bending occurs at two main kink points to give the Garlock fault three straight fault traces.

The clockwise, vertical-axis rotations of the central and eastern segments of the Garlock fault zone, which forms the boundary between the Walker Lane belt and Eastern California shear zone, must then be accommodated by deformation in a zone along both sides of the Garlock fault. This deformation creates the 10–30-km-wide Garlock fault zone of Andrew et al. (2015). The current study interprets some of these accommodation processes on the north side of the Garlock fault as locally tight contractional folding in Pliocene rocks, dip-slip faults (reactivated Marine Gate fault), and new fault systems, evolving triple-junction fault systems, and sidestepping. Serpa and Pavlis (1996) interpreted that a component of the Garlock slip is accommodated on the Wingate Wash fault zone, which appears likely since it is the boundary fault to a broad zone of Garlock-related accommodation zone of complex block rotation and faulting of the Owlshead Mountains (Guest et al., 2003; Luckow et al., 2005). We consider the Owl Lake fault as a tectonic escape feature expelling the southern Owlshead Mountains eastward between it and the Garlock fault. Further study is needed for other areas adjacent to the Garlock fault along the trace of the central and eastern segments of the Garlock fault zone to get a complete inventory of processes, timing, and accommodation amounts.

An eastward-decreasing neotectonic slip rate on the Garlock fault has been interpreted by McGill et al. (2009) and Hatem and Dolan (2018) to occur in two main steps—one at the western boundary of the WLEC and another east of the Slate Range (Hatem and Dolan, 2018). These active fault studies treat the Garlock fault zone as a single-stranded system, whereas our observations imply a wider deformation zone with potentially multiple active faults. Slip on the multiple strands of the central and eastern Garlock fault and associated accommodation-related faults could add to the total slip of these segments, possibly enough to negate part of the hypothesized large eastward decrease in slip. This interpretation complicates seismic hazard interpretations because it introduces more potential slip on the eastern Garlock fault zone and more kink points that could initiate, limit, or laterally shunt fault rupture propagation. It thus could imply an increase of the currently interpreted seismic hazards on the eastern portions of the Garlock fault zone. Our total slip and differentiated slip history interpret a stepwise decrease in total Garlock slip where the Miocene Basin and Range normal faults intersect the Garlock in the Miocene and another decrease where the NNW-striking, Pliocene dextral Southern Panamint Valley fault bends and zippers into the sinistral Garlock fault zone during Pliocene and younger deformation. Additional work is needed to understand the holistic complexity and kinematics of the fault system’s multi-stranded segments to evaluate the Garlock’s overall deformational model and to inform the seismic hazard assessment.

Seismic studies (e.g., Monteiller and Chevrot, 2011; Wang et al., 2020) have found that the Garlock fault zone is coincident with the boundary between lithospheres of different velocity and seismic anisotropy properties and that this contrast is “likely an inherited feature” (Monteiller and Chevrot, 2011). The Mojave Desert area is interpreted to have undergone an episode of subduction erosion of the lithosphere, emplacement of the Rand-Orocopia-Pelon schist in the Late Cretaceous–Early Paleogene, and subsequent extensional unroofing (Luffi et al., 2009; Chapman et al., 2010). Therefore, the Garlock fault is located along a zone of strong lithospheric contrast activated during Basin and Range extension and then reactivated during NNW-directed oblique shear of the WLEC. The complicated history of the strong contrast, sub-Garlock lithosphere and the subsequent middle Miocene creation and slip of the Garlock fault results in an evolving upper-crustal structure where the fault is at a non-ideal orientation in the stress regime. This non-ideal reactivation thus leads to a broad zone of accommodation, because the WLEC structures to the north and south of the Garlock fault cannot cut through the strong rheologic contrast of the deep parts of the Garlock fault zone.

This Garlock fault zone slip history has implications for regional tectonic models of the WLEC. The Garlock shows an overall increase in deformation rate with time (i.e., small amounts of slip during 11–7 Ma with larger amounts during 7–4 Ma and 4–0 Ma), and the obliquity of regional deformation also increases to form the dextral transtensional Walker Lane belt (northern part of the WLEC) and the dextral transpressional Eastern California shear zone (southern part of the WLEC). The evolution of the middle Miocene Basin and Range extension to modern oblique deformation in the Walker Lane belt is a scenario seen in many rifting systems, which Brune et al. (2018) attribute to a decrease of lithosphere strength with increasing obliquity as lithospheric contrasts localize deformation leading to decreasing plastic yield strength. We interpret a similar scenario but with the added importance of the Garlock zone being the lithospheric contrast that localizes deformation and then affects the extended and weakened crust of the western Basin and Range. In this scenario, the Garlock fault may drive the entire plate-inboard regional deformation system, forming the transtensional Walker Lane belt. The dextral transpression of the Eastern California shear zone (Mojave Desert) is thus envisioned as a compensating reaction to the oblique extension of the Walker Lane belt to the north of the Garlock. Therefore, the regional dextral shear may be due to the reactivation of lithospheric anisotropies, as previously suggested by other studies (e.g., Manatschal et al., 2015; Morley, 2016; Hodge et al., 2018; Phillips et al., 2018). We hypothesize that the sub-Garlock lithosphere anisotropy localizes the system. More work is needed on the Garlock and the structures in the WLEC to better define and constrain the complex slip history to explore the geodynamic evolution.

The set of offsets we identify for the eastern segment of the Garlock fault zone shows the slip history evolved from an earlier Miocene period of small displacement and a simple fault to a more complex zone with greater slip in Pliocene and younger time. Our reevaluation shows that strands other than the main Garlock fault can account for 30%–40% of the sinistral offset of the Garlock fault zone, and we interpret a process of how and why the locus of sinistral faulting shifts from one fault to another in the Garlock fault zone.

Our findings show that the eastern Garlock fault zone is wide and complex and accommodates the offsets of active transversely oriented dextral faults that intersect with but do not cut the Garlock fault. This complexity of multiple strands of the Garlock fault zone and the associated accommodation faulting is a much more complicated scenario than the current seismic hazards assessment model of a single-stranded Garlock fault and requires more investigation.

We postulate that the Garlock fault zone might be the pivotal structure that has controlled and created the active dextral Walker Lane belt and Eastern California shear zone because deformation reactivates along a strong lithospheric anisotropy, which the Garlock initially localized on when it initiated slip in the middle Miocene. The current reactivation along the sub-Garlock lithospheric contrast connects the dextral shear of the plate boundary San Andreas fault to the Miocene extension-weakened Mojave Desert and Basin and Range province to create the active oblique extension. The Eastern California shear zone in this model might play a passive role in balancing the deformation of the Garlock and Walker Lane and Eastern California shear zone.

This work was supported by the National Science Foundation EarthScope program (EAR 0643249) and by the Geothermal Program Office at the China Lake Naval Weapons Station. The Geothermal Program Office facilitated access. Frank Monastero, Andy Sabin, and Dan McClung provided help, hospitality, and guidance. We also acknowledge the significant improvements we have made to the paper using the suggestions of associate editor Terry Pavlis and reviewers Jeff Knott and Chris Menges.