ABSTRACT

The relative motion between the Pacific and North American plates in early Miocene time was not parallel to the overall NW strike of the transform, but was instead oblique and transtensional. Dokka and Ross (1995) have recently proposed that in response to this divergence, the western edge of the North American plate east of the transform gravitationally collapsed and moved 100-150 km to the southwest (S50°-60°W). The region of collapse covered an area of nearly 106 km2 and included what is now southern California, southwestern Arizona, and northwestern Mexico. A major structure facilitating collapse between 21 and 18 Ma was the Trans Mojave-Sierran shear zone (TMSSZ). This east-west shear zone linked the classic detachment fault terranes and metamorphic core complexes of the Mojave desert, southeastern California, southern Arizona, and Sonora, Mexico to the transtensional plate boundary.

To more fully understand the nature and kinematic significance of the TMSSZ and its role in facilitating early Miocene fragmentation of the North American plate, a palinspastic reconstruction of the Mojave desert was performed in order to remove the disruptive effects of the TMSSZ and younger tectonic events. Features formed just prior to movements along the TMSSZ were used as markers to assess the TMSSZ deformation. Our analysis indicates that TMSSZ deformation was distributed across an ~90 km wide band; restoration of markers to their original positions imply that >80 km of dextral shear occurred along the TMSSZ. First-order dextral shear deformation within the TMSSZ is expressed internally by clockwise vertical axis rotations of large areas that were facilitated by second order zones of sinistral shear that separated the blocks. These second order sinistral zones apparently exploited older transfer zones of the 24-21 Ma Mojave Extensional Belt.

INTRODUCTION

Plate tectonic principles have been widely and successfully applied to explain Neogene deformation of coastal regions of the southern Cordillera (e.g., Atwater, 1970; Ingersoll, 1982; Stock and Molnar, 1988; Severinghouse and Atwater, 1990; Nicholson et al., 1994; Bohannon and Parsons, 1995; Figure 1). Early attempts to apply these principles to explain the distribution and origin of the classic extensional terranes within the interior of the North American plate were thwarted because of unresolved uncertainties of older plate models and by the lack of clear-cut geometric and kinematic links to the global plate circuit. Recent plate tectonic reconstructions (Stock and Molnar, 1988; Atwater, 1989; Severinghouse and Atwater, 1990), have improved our comprehension of the uncertainties and behavior of the Pacific-North American portion of the global plate circuit, and have resulted in bringing us closer than ever to fulfilling the promise of fuller tectonic understanding set forth in Tanya Atwater's (1970) plate tectonic tour-de-force. The nagging mechanical question of direct, physical linkage of the global plate circuit with contemporaneous early Miocene tectonic systems of the southern Cordillera (e.g., extension in southern California-Arizona, the San Andreas fault system, the Sierran orocline) has been apparently solved by the recognition of the Trans Mojave-Sierran shear zone (TMSSZ; Figure 1), a broad (~90 km), ~E-W dextral shear zone that passes through the southern Sierra Nevada region and Mojave desert (Dokka and Ross, 1995; this paper). This link was detected and identified from regional analysis of field structural and paleomagnetism data. Dokka and Ross proposed that the TMSSZ formed along the northern edge of a large fragment of the North American plate that detached in early Miocene time in response to transtension developed along the Pacific-North American plate boundary. In addition to providing the kinematic linkage between the global plate circuit and contemporaneous early Miocene tectonic systems of the southern Cordillera (e.g., extension in southern California-Arizona, the San Andreas fault system, the Sierran orocline), the TMSSZ has been important in rearranging the position of older paleogeographic and tectonic elements.

This paper presents the results of a structural analysis of the Mojave desert that was performed to gain insights into the original geometry and kinematic history of the Trans Mojave-Sierran shear zone (TMSSZ) and its role in the early Miocene transtensional collapse of southwestern North America. Our study consisted of two steps. Before we could analyze the structure of the TMSSZ, we first needed to restore the Mojave desert back to its early Miocene (~18 Ma) configuration, just after the end of movement along the TMSSZ. This required that subsequent translations, rotations, and strains associated with the 0-13 Ma Eastern California shear zone be accounted for and restored (Figure 2). Upon reaching this point, we used well constrained 24-21 Ma markers to observe and measure the deformational effects of the TMSSZ.

EARLY MIOCENE PALINSPASTIC RECONSTRUCTION OF THE TRANS MOJAVE-SIERRAN SHEAR ZONE

The Need for Reconstruction

It is fundamental to the concept of structural analysis that the effects of younger events in an area be sequentially removed before one can accurately assess the nature of older events. In intricately deformed areas such as the Mojave desert, this requires that all translations, vertical axis rotations, and strains associated with each event be known and explained. As will be discussed below, several models for Mesozoic and Cenozoic tectonics of the Mojave desert region are incorrect because they fail to account for the disruptive effects of all Miocene and younger deformations.

During late Cenozoic time, the Mojave experienced three structurally different and temporally separated intervals of deformation: (1) ~N-S directed opening of the ~24-21 Ma Mojave Extensional Belt (Dokka, 1989; Ross, 1995); (2) ~E-W striking, dextral shearing (~21-18 Ma) along the Trans Mojave-Sierran shear zone (Dokka and Ross, 1995, 1996; this paper); and (3) the ~13-0 Ma Eastern California shear zone (Dokka and Travis, 1990ab; Dokka, 1993). Subsequent to its time of activity 21 to 18 Ma, the Trans Mojave-Sierran shear zone (TMSSZ) was truncated and disrupted by the 0-13 Ma Eastern California shear zone (ECSZ [Figure 2]; Dokka and Travis, 1990a; Dokka, 1993). Approximately 65 km of slip (resolved along an ~N40°W line) has occurred within this ~80 km wide belt of distributed right shear since its inception (Dokka and Travis, 1990a); Pezzopane and Weldon (1993) propose that the ECSZ continues through northern California and Nevada to eastern Oregon where it may connect with the Cascade Range. Faults of the southern portion of the ECSZ join with those of the San Andreas system near the Pinto Mountain fault and final merger is completed in western Sonora, Mexico. The physical connection of faults of the ECSZ with the San Andreas fault system demonstrates that the ECSZ is a key element of the Pacific-North American plate boundary; the ECSZ has and continues to accommodate between 9% and 23% of the total relative plate motion [Dokka and Travis, 1990b; Savage et al., 1990; Sauber et al., 1986, 1994]. Given these disruptive effects, palinspastic reconstruction of the region is thus essential before meaningful structural analysis of the TMSSZ can be done.
 
 

Methodology

Reconstruction of the Mojave desert region to remove the effects of the 0-13 Ma Eastern California shear zone (ECSZ) followed the protocol established by Dokka and Travis (1990a) and Dokka (1993). The improved reconstruction presented here, as well as the original Dokka and Travis (1990a) model, is constructed primarily to explain regional, two-dimensional, surface (x,y) relations; vertical strain implications of the ECSZ such as local crustal extension and contraction are discussed in Dokka (1993). The resolution of the reconstruction presented here is approximately ±1 km.

Analysis of the ECSZ has shown that strain in the Mojave desert region is inhomogeneous and is partitioned into several domains of similar deformation that are separated by strike-slip faults and extensional zones (Garfunkel, 1974; Dokka and Travis, 1990a; Dokka, 1993). Strike-slip translation along dominantly NW striking faults, along with tectonic rotations of blocks bounded by ~E-W, left-slip faults are reckoned to have occurred within this broad, regional zone of right shear.

The earlier models, as well as this reconstruction, are founded on several explicit and implicit assumptions that need to be discussed. First, we assume that all deformation is brittle and involves translations and rotations of rigid, fault-bounded bodies. Internal strain within individual fault blocks is assumed to be negligible. In order to simplify the reconstruction, fault systems composed of multiple strands are considered as single faults with a composite net slip. For example, the Camp Rock fault system which includes several, locally named strands (Camp Rock, Emerson, Old Woman Springs, etc.), is treated as one continuous fault. The model considers that the amount of lateral translation of one fault block relative to an adjacent block is equal to its strike-slip. In addition, the amount of vertical axis rotation of a fault block is reckoned equivalent to the declination anomaly implied from paleomagnetic studies collected from within that block or from a kindred block within the same structural domain; for example, in a group of tilted dominos deformed in simple shear, the rotation of all can be inferred from the measurement of any one domino. Finally, the recognition of the style of deformation in the region (overall inhomogeneous deformation composed of domains of homogeneous simple shear) permits us to use simple geometry and trigonometry to facilitate and describe the restoration of translations and rigid body rotations.

The following steps were conducted during the reconstruction. Fault blocks were first defined on the basis of mapped, late Cenozoic faults (Figure 3); where such data were lacking, the position of the boundaries were inferred on the basis of seismicity or topography. Fault blocks were then restored to prefaulting and prerotation positions using fault slip (Table 1) and paleomagnetic declination vector constraints described in Dokka and Travis (1990a); these constraints, as well as more recent contributions, include data presented in Golombek and Brown (1988), Ross (1988), Ross et al. (1989), Wells and Hillhouse (1989), MacFadden (1990ab), Valentine et al. (1993), MacConnell et al. (1994), Ross (1995), and Lu and Dokka (1995). The geometry of and motion along unconstrained boundaries were then adjusted to eliminate the few geologically unexplained overlaps or gaps. The model was then run forward to test how well it could predict topography, present-day deformation patterns, and geologic relationships. Successive iterations of this procedure were performed to obtain a geometric best fit to observed relationships.

Figure 3. Fault map of the Mojave desert highlighting the location of late Cenozoic faults and associated features mentioned in text. DKSFZ, Desert King Spring fault zone; MLF, McLean Lake fault; FIFZ, Fort Irwin fault zone; GLFZ, Goldstone Lake fault zone; PF, Paradise fault; CCFZ, Coyote Lake fault zone; TMF, Tiefort Mountain fault; BLF, Bicycle Lake fault; GSF, Garlic Spring fault; AM, Alvord Mountain; AW, Avawatz Mountains; BM, Bristol Mountains; CM, Calico Mountains; CdM, Cady Mountains; CP, Cajon Pass; GM, Granite Mountains (south); MH, Mud Hills; MM, Marble Mountains; NM, Newberry Mountains; OM, Ord Mountain; PR, Paradise Range; RM, Rodman Mountains; SBM, San Bernardino Mountains.
 
 
 
 

In order to assess the effects of the Trans Mojave-Sierran shear zone, we assembled a group of older markers deformed by the TMSSZ and restored them to their pre-ECSZ configuration according to the reconstruction. An index map showing the locations of these markers is presented in Figure 4. The most important of these markers include structures and kinematic indicators of the 24-21 Ma Mojave Extensional Belt (Dokka, 1989) as well as paleomagnetic vectors recorded in lower Miocene strata (see Ross [1995] and Geological Society of America Data Repository item 9530 for more complete discussion of paleomagnetics data for the Mojave desert).

Results

The results of our reconstruction is presented in Figure 5. Figure 5a shows the present-day configuration of the early Miocene markers that we used to detect the Trans Mojave-Sierran shear zone (TMSSZ) in the Mojave desert. Figure 5b is a reconstructed view that removes the effects of the Eastern California shear zone (ECSZ) at ~1Ma, whereas Figure 5c displays the configuration of the TMSSZ just after its time of activity (~18 Ma). Figure 5d depicts the Mojave desert Block at ~21 Ma showing the position of older features prior to deformation by the TMSSZ.

The reconstruction produces several interesting effects. First, the restoration of 65 km of distributed right shear along the ECSZ returns the now fragmented Mojave Extensional Belt (MEB) into a single coherent unit (Figure 5c). Structural domains and intervening transfer zones of the MEB such as the Kane Springs, Baxter Wash, and Lane Mountain faults become whole and continuous. All areas that are inferred to be young extensional basins created by the ECSZ (in black, Figure 5) during its history become closed in the restoration. Second, although intact, the MEB remained severely distorted in the reconstruction, as indicated by the warped traces of transfer faults of the Mojave Extensional Belt and associated 24-21 Ma kinematic indicators (Figure 5c). Paleomagnetic declination vectors in 24-21 Ma rocks as well as kinematic indicators from the Mojave Extensional Belt show similar disorientation (Figure 5c). All of these distorted linear elements occur within an ~E-W, ~90 km wide zone that defines the region of effect of the Trans Mojave-Sierran shear zone (Figure 5c). We conclude that this 18 Ma reconstruction reflects the configuration of the TMSSZ just following its time of activity.
 
 

STRUCTURE OF THE TRANS MOJAVE-SIERRAN SHEAR ZONE

Geometry

The 18 Ma reconstruction (Figure 5c) indicates that the Trans Mojave-Sierran shear zone (TMSSZ) was oriented ~E-W (N82°W) at its final stage of movement and that it spanned the Mojave desert and the southern Sierra Nevada region. Comparison of the 18 Ma and 21 Ma restorations suggests that the TMSSZ may have rotated 30-40° clockwise during deformation from an original trend that was WSW-ENE (Figure 5d). Figure 5d also suggests that the original trend and position of the TMSSZ was inherited from the slightly older Mojave Extensional Belt. Early Miocene regional extension had weakened the lithosphere through tectonic thinning (10-17 km vertical) and magmatism (Dokka, 1989; Henry and Dokka, 1992). Additional evidence for reactivation (usage from Holdsworth et al., 1997) is presented below.

Figure 5c shows that early Miocene markers such as paleomagnetic declination vectors and the Mojave Extensional Belt kinematic indicators and transfer faults are consistently and regularly deflected across the central Mojave desert. The sense of this deflection implies dextral shear and is thus consistent with the conclusion of Dokka and Ross (1995, 1996) regarding the nature of first-order shear along the TMSSZ. Although the now ~E-W belt was dominantly right-slip in character, dextral shears parallel to the TMSSZ are few. This, of course, is not a requirement of a right slip zone (cf. Glazner et al, 1996). Closer examination of the TMSSZ shows that internal strain is complex and facilitated subregionally by several mechanisms (Dokka and Ross, 1995, 1996).

Figure 6 illustrates our concept of how deformation occurred within the TMSSZ. Throughout much of the Mojave, dextral shear deformation is expressed by clockwise rotations of large areas about vertical axes (Golombek and Brown, 1988; Ross et al., 1989; Ross, 1995). Across the central Mojave, deformation is generally continuous and the distribution of shear strains across the TMSSZ inferred from vertical axis rotations is asymmetrical with respect to the center of the shear zone; the maximum apparent shear strain occurs south of the center of the TMSSZ (Figure 7). Because of the lack of E-W faults and the observation that shear strains (rotations) gradually and continuously decrease towards the outer limits of the shear zone, we conclude that much of the dextral shear in the TMSSZ was accomplished by continuous oroclinal folding about vertical axes. This, however, was not the entire story.

A growing body of paleomagnetic and field structural data indicate that sinistral shear zones played a critical role in the development of the TMSSZ. We propose that these cryptic left shear zones facilitated the clockwise rotation of large intact crustal blocks within the TMSSZ (Figure 6). Motions along these now NE trending zones also produced local effects such as counterclockwise rotations about vertical axes, megascopic folding, and faulting (Valentine et al., 1993; Lu and Dokka, 1995; Dokka and Ross, 1995, 1996; Temple, 1997). These shear zones were first detected by Valentine et al (1993) who showed that rocks along the now NE striking Lane Mountain fault of the west-central Mojave were rotated 23°-57° counterclockwise between 22 and 18 Ma (Figure 8). Reevaluation of the Kramer Hills, a well exposed area that lies adjacent to the Lane Mountain fault and a key area cited by Valentine et al. (1993) and Golombek and Brown (1988), reveals important details on the sequence of 21-18 Ma rotational events (Figure 8; Lu and Dokka, 1995; this paper). Our studies show that rocks of the Kramer Hills were rotated ~80° counterclockwise about a vertical axis just prior to 21 Ma (R2, Figure 8). This occurred during the same time interval that the area to the east (central Mojave) was rotating clockwise (Ross, 1995). At ~21 Ma and continuing until ~18 Ma, the Kramer Hills area rotated ~45° clockwise; again, this occurred synchronously with clockwise vertical axis rotations in the central Mojave desert (Ross, 1995). These relations support our view that the early counterclockwise rotation was due to local strain during initial sinistral shearing. As soon as a through going fault was formed, local strain ceased and the Kramer Hills rotated clockwise along with the rest of the region.
 
 

Similar relations have recently been observed along the Baxter Wash transfer zone in the northern Cady Mountains (Temple, 1997). Here, lower Miocene rocks yield paleomagnetic declination vectors suggesting >80° of local counterclockwise rotation that occurred in association with development of large folds along the now NE striking Baxter Wash fault (Temple, 1997). Higher in this section, upper lower Miocene rocks (>18 Ma) yield evidence of a large (20°-67°) regional clockwise rotation (MacFadden et al., 1990a; Ross, 1995; Temple, 1997). Finally, reevaluation of folds along the Kane Springs fault in the Newberry Mountains suggests that yet another Mojave Extensional Belt transfer zone was reactivated during 21-18 Ma and facilitated TMSSZ deformation. These megascopic folds were originally explained as reverse drag folds that formed along the Kane Springs fault during extension (Dokka, 1980, 1986). The problem with this earlier interpretation, however, is that the limbs of some folds are locally overturned, a situation unlikely to have occurred due exclusively to reverse drag. Even though the argument for initial formation due to reverse drag remains compelling, a more likely scenario is one where these earlier formed folds were modified and tightened by sinistral shear during TMSSZ time.

Comparison of these sinistral shear zones reveals several common characteristics worth noting. First, each of these sinistral shear zones coincides spatially with major transfer faults of the slightly older Mojave Extensional Belt; these older faults originated as right slip faults (Dokka, 1986, 1989). We propose that these older discontinuities may have influenced the deformation as weak zones reactivated by the younger deformation. Previously, Valentine et al. (1993), as well as Bartley and Glazner (1991), speculated that early Miocene clockwise and counterclockwise rotations were coincident and that both occurred concurrently with regional extension. This interpretation was shown to be incorrect by Ross (1995) who found that early Miocene extension and regional vertical axis rotations were not coincident. This was accomplished through examination of paleomagnetic declination anomalies in a well exposed stratigraphic section containing extended and post-extensional rocks of the Cady Mountains. Second, the slip on each fault similar (sinistral) and some (all?) have substantial displacement. For example, the width (>16 km) of the shear zone and degree of shearing inferred from paleomagnetism and structural studies along of the Lane Mountain fault in the Kramer Hills (~80° counterclockwise) suggest that >22 km of distributed left slip occurred. This is based on the simple relationship, NS>wfp/180, where NS is the net slip, w is the width of the shear zone, and f is the rotation produced by shear. Preliminary studies in the northern Cady Mountains suggests that ~13-25 km of left slip occurred along the Baxter Wash fault (Temple, 1997). Finally, these sinistral shear zones die out to the north and south at the inferred outer limits of the TMSSZ.
 
 

Amount of Displacement

Determination of the amount of dextral displacement along the Trans Mojave-Sierran shear zone (TMSSZ) between 21 and 18 Ma is not easy or straightforward because of the lack of appropriate piercement lines with which to gauge the integrated net slip. Previously, Dokka and Ross (1995) proposed that 41-66 km of right slip occurred along the TMMSZ based on the assumption of regional simple shear; they calculated this value using an average vertical axis rotation as a measure of the shear strain. This method likely underestimates the total net slip by a factor of perhaps two because it neglects the contribution made by sinistral faults to the overall deformation. To at least partially address this shortcoming, we present an improved method to estimate net slip that relies on the palinspastic reconstruction presented above (Figure 9). This method compares the positions of selected elements of the Mojave Extensional Belt (MEB) at 18 Ma and 21 Ma. The net slip is taken as the average displacement of selected points in the shear zone.

For this approach to be valid, the restoration must deal effectively with several geological constraints. First, variably oriented kinematic indicators of the MEB should become parallel and oriented north. Because detailed paleomagnetism studies have demonstrated that original N-S extension of the MEB was not accompanied by rotations about vertical axes (Ross, 1995; Dokka, 1989), we conclude that the observed disorientation of these features at 18 Ma is an artifact of TMSSZ deformation. Second, transfer zones of the MEB should also become aligned and parallel to the extension direction implied by MEB kinematic indicators. Third, paleomagnetic declination vectors recorded in 24-21 Ma rocks of the region should become reoriented to north. Fourth, the severely warped traces of Mesozoic thrust systems (e.g., Snow, 1992; Figure 9) and older Precambrian-Paleozoic paleogeographic trends that cross the region (e.g., Burchfiel and Davis, 1981) should straighten; although the degree of straightening needed is uncertain because we do not know the original configuration of the Mesozoic and older features, the complex form of the features suggests that some smoothing is required. Fifth, restoration should involve and undo sinistral motions along the "reactivated" transfer zones of the MEB. As shown in Figure 6a, these sinistral shear zones can significantly contribute to the overall dextral shear along the TMSSZ; because each left slip zone adds its own contribution to the total right slip, we expect the net slip along the TMSSZ to increase to the west. At present we lack net slip data on all of these faults to make a complete restoration. Our reconstruction at 21 Ma (Figure 5d and 9) uses the values calculated for the Lane Mountain and Baxter Wash faults and suggest that the TMSSZ has a minimum dextral net slip of 80 km; because we assume that no slip occurred on the Rodgers Lake and Kane Springs faults, the resultant total net slip value for the TMSSZ is understated.
 
 

DISCUSSION

Recognition of the Trans Mojave-Sierran shear zone (TMSSZ) is significant for two main reasons. First, the TMSSZ is a major, crustal-scale break whose net slip is among the largest in the southwestern Cordillera. Surprisingly, this major structure lay undetected until recently in one of the world's most intensively studied areas. This was likely the result of the masking by younger deformations and due to the shear zone's broad and subtle effects. It is clear that discovery would not have likely occurred were it not for the application of paleomagnetic declination analysis pioneered in the region by Luyendyk et al. (1980), Golombek and Brown (1988), Ross et al. (1989), Wells and Hillhouse (1990), MacFadden et al. (1990ab), Valentine et al (1993), and Ross (1995).

Perhaps the true significance of the TMSSZ is more philosophical in nature than structural. As discussed below, modern plate tectonic reconstructions tell us that the Pacific-North American plate boundary was transtensional in early Miocene time. These models specify the total amount integrated displacement that should be apparent in the rocks across the boundary. Such models, however, offer no insights on how, where, and when the continent responded.

In our view, the TMSSZ represents the last remaining major piece of a long standing tectonic puzzle that includes the metamorphic core complexes and detachment faults of the California-Arizona-and Sonora region, the San Andreas fault system, and the Sierra Nevada orocline. When viewed collectively, the geometry and kinematics of this coordinated system of moving parts is consistent with motions along the transtensional plate boundary. We expand on this theme below.
 
 

Plate Tectonic Controls on Continental Deformations

The early Miocene plate tectonic history of the northeastern Pacific region offers several important insights into the origin of concomitant deformation of the North American plate (e.g., Atwater, 1970; Severinghaus and Atwater, 1990; Stock and Molnar, 1988; Figure 10). It is now clear that the relative motion between the Pacific and North American plates was not purely strike-slip, but was instead oblique to the plate boundary (e.g., Atwater, 1970, 1989; Severinghaus and Atwater, 1990; Stock and Molnar, 1988). These movements resulted in transtension across the transform and the WNW migration of the Pacific plate away from the NW striking transform (Dokka and Ross, 1995; Bohannon and Parsons, 1995). As illustrated in Figure 10, 340±200 km of transform-normal migration should have occurred since ~30 Ma (Atwater, 1989), with as much as ~175 km developing between 20 and 16 Ma. Evidence for such divergence would be expected to be observed within the Pacific plate, along the boundary between the two plates, or within the North American plate (Atwater, 1989). Magnetic anomaly mapping of the Pacific plate offshore of central California shows no indication of extension, i.e., seafloor spreading, within the Pacific plate or between the two plates (Lonsdale, 1991), whereas the North American plate is highly extended (e.g., Davis and Coney, 1979; Davis et al., 1980; Davis and Lister, 1988; Dokka, 1989; Wernicke, 1981). On this basis, Dokka and Ross (1995) concluded that as the Pacific plate migrated WNW away from the transform during the Miocene (24-5 Ma), the North American plate extended laterally to maintain contact. Dokka and Ross viewed this divergence as the triggering mechanism that set up gravity-driven movement of a large fragment of the North American plate to the southwest toward the transform. Through what they termed, the "Collapse", they sought to integrate all early Miocene deformational events in the continent and relate them to the divergence occurring along the transform. As will be discussed below, the TMSSZ was an integral structure of this system of coordinated early Miocene deformations.

A System of Coordinated Continental Deformations

Dokka and Ross (1995) considered that all major early Miocene tectonic systems active in the southwestern part of the North American plate were physically interconnected and kinematically linked to the global plate circuit (Figure 11 and 12). Collapse began near 24 Ma and continued until ~18 Ma resulting in southwesterly translation of the lithospheric fragment. The region of collapse covered an area of nearly 106 km2 and included what is now southern California, southwestern Arizona, and northwestern Mexico. Strain, both laterally and vertically, was not uniform. Strain at the surface was concentrated in belts around the perimeter of the large collapsed fragment (Arizona metamorphic core complexes; Colorado River Extensional Corridor, Mojave Extensional Belt, Trans Mojave-Sierran shear zone), whereas areas within the fragment away from its highly extended margins remained largely intact (i.e., no major lateral lengthening). The collapsed fragment decoupled in the middle and lower crust and moved southwest along subhorizontal detachment faults and ductile shear zones; recognition of this mid-crustal zone of shearing is based on extensive industry and university seismic reflection profiles of southeastern California and southern Arizona that show that the middle crust and deeper is characterized by low angle reflectors (e.g., Hamilton, 1987; Frost et al., 1987; Cheadle et al., 1986; Dokka, 1989; McCarthy et al., 1991; Morris, 1993). In the northern Mojave desert, middle crustal reflectors correlated with collapse-related detachment faults do not continue at depth past the region of surface extension (Serpa and Dokka, 1992). Major brittle-ductile detachment faults that breach the surface in the highly extended Colorado River Extensional Corridor and southern Arizona can be traced downward to the northeast where they merge with the zone of decoupling (e.g., Hamilton, 1987; Frost et al., 1987).

Figure 11 shows the sequential tectonic evolution of the southwestern USA and Mexico during early Miocene time and the role played by the TMSSZ. Both northern and eastern edges of the collapsed region underwent extension between ~24 and ~21 Ma. Although extension in southeastern California and Arizona was coaxial with the overall southwest collapse direction, local kinematics along the northern boundary (MEB) were complicated by the combined effects of the SW collapse and the independent ~NW translation of the region to the north. These two motions resulted in ~N-S extension across the MEB. Collapse continued during the interval 21-16 Ma, with major extension of southeastern California and Arizona occurring between 20 and 18 Ma. Cessation of NW motion of the region to the north at ~21 Ma caused the kinematics of the northern boundary to change to dextral shear and minor extension (Fig. 11b). This aspect of the model is confirmed by the results of this study that demonstrate that motions along the TMSSZ were dextral and substantial (>80 km). Distributed right shear along the TMSSZ resulted in oroclinal folding of rocks about vertical axes along an ~90 km wide, ~E-W belt that included the southern Sierra Nevada and the central Mojave desert.
 
 

Amounts of Early Miocene Deformation

Evidence presented in this paper for >80 km of 21-18 Ma dextral slip along the Trans Mojave-Sierran shear zone (TMSSZ) provides an important test of the notion that events in the continent were kinematically linked to the divergence along the transform. The Dokka and Ross (1995) theory requires that the magnitudes and rates of displacement in each of the interconnected tectonic elements should be equivalent. These elements include: 1) SW directed extension in southeastern California and southern Arizona; and 2) the amount of the North American plate truncated and transferred to the Pacific plate (e.g., Salinian Block, etc.). The >80 km of slip on the TMSSZ reported here falls within the limits of the amount of divergence predicted between Pacific and North American plates (175 ±100 km; Stock and Molnar, 1988); our comparison does not include the amount contributed by the 24-21 Ma Mojave Extensional Belt. The magnitude and timing of TMSSZ slip is also similar to those associated with extension occurring along the eastern edge of the collapsed region. The amount of extensional lengthening recorded in rocks of southern Arizona-southeastern California from the edge of the stable Colorado Plateau in early Miocene time is 86±13 km (Spencer and Reynolds, 1991). Similar structures lying to the southwest along the Salton trough that are linked in the middle and lower crust by detachment faults imply that the total lengthening is even greater.

The Dokka and Ross (1995) model predicts that the amount of TMSSZ net slip and the amount of the North American plate truncated and transferred to the Pacific plate should be equivalent; the rationale for this is illustrated in Figures 11 and 12. According to Dokka and Ross, the transtensional nature of the Pacific-North American plate boundary led to the profound fragmentation of the west coast of California and Mexico by strike-slip faults of the San Andreas fault system. They reasoned that as the margin of North America collapsed to the west, the continental edge moved west past the locus of transform shear between the Mendocino and Rivera triple junctions. Thus, the portion of the plate lying west of the locus of shear became increasingly subject to the motion of the Pacific plate. Eventually, a new fault was propagated between the triple junctions. Over time, slivers of the North American continent such as the Salinian block (Page and Engebretson, 1984) were progressively truncated in meat slicer-like fashion and transferred to the Pacific plate (Figure 11 and 12). If the Dokka and Ross (1995) model is correct, then the TMSSZ net slip should be equivalent to the total map width of the fault blocks that were cut off and transferred to the Pacific plate by the San Andreas system. In central California, the combined width of these fault blocks lying between the edge of the continent and the San Andreas fault is ~150 km. We believe this discrepancy reflects the incomplete status of our analysis of the TMSSZ (i.e., >80 km) and suggest that the true value of the net slip along the TMSSZ may be closer to 150 km.
 
 

CONCLUSIONS

The following conclusions were reached in this analysis of the Trans Mojave-Sierran shear zone (TMSSZ):

1. The overall motion along the 21-18 Ma TMSSZ was dextral with at least 80 km of net slip. Although the first order character of the TMSSZ was right slip, internal deformation occurred by large clockwise vertical rotations of regions that were facilitated by movements along syntectonic sinistral shear zones (second order); motions along individual left slip zones may be large (>20 km) and are thus important contributors to the total deformation. The spatial coincidence of these sinistral shear zones with transfer zones of the 24-21 Ma Mojave Extensional Belt (MEB) suggests that the older structures may have controlled and guided TMSSZ deformation. It is also likely that the original trend and position of the entire TMSSZ may have been inherited from the MEB.

2. The original trend of the TMSSZ was likely ENE but rotated 30°-40° clockwise during its evolution.

3. The geometry and kinematics of the TMSSZ determined here are consistent with the unified theory of Dokka and Ross (1995) who proposed that the TMSSZ is a regional dextral shear zone that served as the kinematic link between inland extensional belts (Arizona metamorphic core complexes, Colorado River Extensional Corridor, Mojave Extensional Belt) and the transtensional Pacific-North American plate boundary (Atwater, 1970, 1989; Ingersoll, 1982; Stock and Molnar, 1988; Severinghaus and Atwater, 1990).
 
 

ACKNOWLEDGMENTS

Careful and constructive reviews were provided by T. Pavlis, B. Tikoff, and M. Woodburne. Insightful discussions were provided by Z. Garfunkel, C. Travis, D. Henry, R. Holdsworth, D. MacConnell, B. Temple, G. Sella, and C. Christensen. Special thanks are extended to the R. Holdsworth, J. Dewey, and R. Strachan, for their hard work and good cheer in the creation of this volume. Supported by grants from the National Science Foundation (EAR-9004339, EAR-9219191, and EAR-9418834).
 
 

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a, All values indicate the amount of lateral slip in kilometers; faults with left-slip shown in italics.

b, Estimate of right-lateral displacement by Dibblee [1968].

c, R. Dokka [unpublished mapping, 1988-1992].

d, Miller and Morton [1980].

e, Dokka [1983].

f, Includes 1.4 km of right shear expressed as strain.

g, Miller et al. [1982] documented >6 km of right separation.

h, Dokka [1989] documents 8-9 km of right separation in the Calico Mountains-Mud Hills.

i, R. Dokka, D. MacConnell, J. Ford [1992, unpublished map and report].

j, Left separation of distinctive marble unit.

k, Wright and Troxel [1967, 1970].

l, Brady [1990].

m, Use of higher model value for Southern Death Valley fault zone requires that the cumulative net slip for faults of Domain V be equal to 49.5 km. This also suggests that the total net slip for the entire ECSZ is 77 km.

n, Dibblee [1975].

p, Powell [1981].

q, Yount et al. [1994]

r, net slip included with other named left faults of the domain

.