Quick Search:    advanced search

GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help

This Article Abstract Figures Only Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Langenheim, V.E. Articles by Powell, R.E. Search for Related Content GeoRef GeoRef Citation

Basin geometry and cumulative offsets in the Eastern Transverse Ranges, southern California: Implications for transrotational deformation along the San Andreas fault system

¹ U.S. Geological Survey, 345 Middlefield Road, Mail Stop 989, Menlo Park, California 94025, USA
² U.S. Geological Survey, 520 N. Park Avenue, Tucson, Arizona 85719, USA

	ABSTRACT

TOP ABSTRACT INTRODUCTION GEOLOGIC SETTING GEOPHYSICAL DATA GEOPHYSICAL ANOMALIES LATE CENOZOIC STRUCTURE DISCUSSION CONCLUSIONS REFERENCES CITED

 
The Eastern Transverse Ranges, adjacent to and southeast of the big left bend of the San Andreas fault, southern California, form a crustal block that has rotated clockwise in response to dextral shear within the San Andreas system. Previous studies have indicated a discrepancy between the measured magnitudes of left slip on through-going east-striking fault zones of the Eastern Transverse Ranges and those predicted by simple geometric models using paleomagnetically determined clockwise rotations of basalts distributed along the faults. To assess the magnitude and source of this discrepancy, we apply new gravity and magnetic data in combination with geologic data to better constrain cumulative fault offsets and to define basin structure for the block between the Pinto Mountain and Chiriaco fault zones. Estimates of offset from using the length of pull-apart basins developed within left-stepping strands of the sinistral faults are consistent with those derived by matching offset magnetic anomalies and bedrock patterns, indicating a cumulative offset of at most ~40 km. The upper limit of displacements constrained by the geophysical and geologic data overlaps with the lower limit of those predicted at the 95% confidence level by models of conservative slip located on margins of rigid rotating blocks and the clockwise rotation of the paleomagnetic vectors. Any discrepancy is likely resolved by internal deformation within the blocks, such as intense deformation adjacent to the San Andreas fault (that can account for the absence of basins there as predicted by rigid-block models) and linkage via subsidiary faults between the main faults.

	INTRODUCTION

TOP ABSTRACT INTRODUCTION GEOLOGIC SETTING GEOPHYSICAL DATA GEOPHYSICAL ANOMALIES LATE CENOZOIC STRUCTURE DISCUSSION CONCLUSIONS REFERENCES CITED

 
Large-scale crustal rotations followed by convergence have been proposed as the mechanisms responsible for the anomalous east-west orientation of the Transverse Ranges along the San Andreas fault system in southern California. Such rotations, modeled as rigid blocks, have implications for basin formation along the margins of the rotating domain and the amount and style of fault offset within the domain. For the western Transverse Ranges, abundant paleomagnetic data (Kamerling and Luyendyk, 1979; Hornafius et al., 1986; Luyendyk, 1991) corroborate clockwise rotation of 70°–90° inferred from patterns of disrupted lithotectonic belts (Jones et al., 1976; Crouch, 1979) and sediments of the Eocene Poway Group (Yeats et al., 1974). For the Eastern Transverse Ranges (Fig. 1), a sinistral domain adjacent to the San Andreas fault southeast of its big compressional bend where paleomagnetic data indicate ~40° of clockwise rotation (Carter et al., 1987), direct geologic constraints for the magnitude of rotation within the crystalline basement rocks are few. Although Ron and Nur (1996) suggested dike orientations supported 40°–50° rotation in the Eastern Transverse Ranges, more recent analysis of trends of dikes attributed to the Independence dike swarm is not conclusive (Hopson et al., 2008).

View larger version (53K): [in this window] [in a new window]   Figure 1. Index map showing sinistral domains marked by clockwise rotation (gray) modified from Dickinson (1996). BCF—Blue Cut fault; CF—Chiriaco fault; CM—Chocolate Mountains; ECSZ—Eastern California shear zone; PMF—Pinto Mountain fault; SCF—Salton Creek fault; SBM—San Bernardino Mountains; SGP—San Gorgonio Pass.

View larger version (14K): [in this window] [in a new window]   Figure 2. Schematic model of rigid-block rotation of the Eastern Transverse Ranges (A) before and (B) after 40° of clockwise rotation, simplified from Carter et al. (1987). Striped line represents marker that is sinistrally offset by faults that traverse the domain. Shaded areas are gaps that result from rotation resulting from dextral shear (arrows in B).

	GEOLOGIC SETTING

TOP ABSTRACT INTRODUCTION GEOLOGIC SETTING GEOPHYSICAL DATA GEOPHYSICAL ANOMALIES LATE CENOZOIC STRUCTURE DISCUSSION CONCLUSIONS REFERENCES CITED

 
The Eastern Transverse Ranges block (Fig. 1) is characterized by sinistral and/or left-oblique, east-striking faults that extend east from the Little San Bernardino Mountains for more than 70 km. This sinistral domain interrupts the dominant north-northwest–trending physiography and structure of the Mojave Desert to the north and east and of the Salton Trough and Peninsular Ranges to the west, all characterized by dextral faults. The San Andreas and Pinto Mountain fault zones, both major strike-slip faults, bound the Eastern Transverse Ranges sinistral domain on the southwest and north, respectively. Domain boundaries to the east and south, while abrupt, do not occur along single major faults (Figs. 1 and 3). The eastern boundary, marked by an abrupt change in physiographic and structural grain along a topographically low corridor defined by the eastern ends of the Pinto basin and Chuckwalla Valley, in part follows the northwest-striking Sheep Hole fault (Fig. 3). The southern boundary of the sinistral domain in part follows the Salton Creek fault (SCF in Fig. 1) between the Orocopia and Chocolate Mountains. Overall offsets on the sinistral faults are constrained by paleogeologic patterns comprising Proterozoic–lower Miocene rocks, briefly summarized here.

View larger version (95K): [in this window] [in a new window]   Figure 3. Shaded-relief topographic map of study area. Red dots are earthquake epicenters from Shearer et al. (2005). Blue stars denote locations of the 1992 Landers and Joshua Tree earthquakes. Gray lines—main faults from Howard (2002), Powell (1981, 2003), and Jennings (1994); CC—Cottonwood Canyon; EDC—El Dorado Canyon.

Metamorphosed Mesozoic strata occur in two lithostratigraphic units, the Orocopia Schist in the western Orocopia Mountains and the McCoy Mountains Formation in the Coxcomb Mountains. Various studies have suggested that Orocopia-like rocks underlie much of southern California (Haxel and Dillon, 1978; Magistrale and Zhou, 1996), including the Eastern Transverse Ranges (Powell, 1981), but Haxel et al. (2002) suggested that the schists were restricted to an east-west–trending belt prior to Neogene displacement on the San Andreas fault system and therefore may not underlie the Eastern Transverse Ranges. The McCoy Mountains Formation in the Coxcomb Mountains consists of phyllitic rocks metamorphosed from non-marine, clastic rocks.

Mesozoic plutonic suites occur in three north-northwest–trending belts (Powell, 1993, appendix 1). The western belt encompasses Cretaceous tonalite and monzogranite that are intruded by granitic rocks. The central plutonic belt consists of scattered bodies of sparse Triassic monzodiorite and monzonite accompanied by abundant plutons of Triassic–Cretaceous biotite monzogranite (Powell, 1981; Barth et al., 1997) and Late Cretaceous (Wooden et al., 1994) granodiorite. The eastern plutonic belt spans the boundary between the Eastern Transverse Ranges and Mojave Desert. This belt comprises mafic hornblende-rich plutonic rocks intruded by biotite-hornblende quartz monzonite and monzodiorite, all of Jurassic age. In the Coxcomb Mountains, the Jurassic plutonic rocks and the McCoy Mountains Formation are intruded by Late Cretaceous granitic rocks that constitute part of the Cadiz Valley batholith (Calzia et al., 1986; Howard, 2002; Barth et al., 2004).

Marine Eocene strata (Crowell and Susuki, 1959) and nonmarine Oligocene–lower Miocene strata of the Diligencia Formation (Spittler and Arthur, 1982; Law et al., 2001) are exposed south of the Chiriaco fault in the central Orocopia Mountains (Powell, 1993; Fig. 4). Newly discovered bodies of Eocene strata also crop out at the foot of the Cottonwood Mountains north of the Chiriaco fault (8 in Fig. 4). These strongly deformed Tertiary deposits are now exposed in uplifted and exhumed inverted basins that crop out in highlands of those mountain ranges.

View larger version (97K): [in this window] [in a new window]   Figure 4. Simplified geologic map of the Eastern Transverse Ranges. Green arrows indicate inferred rotations (relative to north) from paleomagnetic declination data of Carter et al. (1987). Numbers show correlated geologic features discussed in text. Faults, shown in red, are compiled from Jennings (1994), Howard (2002), and Powell (1981, personal observ.); solid lines are well located, dashed lines are concealed, and dotted lines are inferred. Note that more detailed mapping of faults is east of long 116°W. Light gray line is outline of Joshua Tree National Park. Dark gray and blue dashed lines are approximate contact between western and central belt and central and eastern belt plutonic rocks, respectively. CLF—Cleghorn Lake fault; CPF—Cleghorn Pass fault; CWF—Clemens Well fault; CSWF—Corn Springs Wash fault; DWF—Dog Wash fault; EVMF—East Valley Mountain fault; WVMF—West Valley Mountain fault. DB—Diligencia Basin paleomagnetic site; PM—Palen Mountains paleomagnetic site; PW—Pinto Wells paleomagnetic site.

Faults of Late Miocene and Younger Age
Sinistral Faults
From north to south, spaced ~20 km apart, the main, through-going sinistral fault zones of the Eastern Transverse Ranges province are the Pinto Mountain, Blue Cut, Chiriaco, and Salton Creek fault zones (Fig. 1). Additional subordinate sinistral faults include the Ivanhoe fault, which splays off the Pinto Mountain fault zone to the southeast, and the Corn Springs Wash fault, which splays southeast from the Chiriaco fault zone (Fig. 3). Between the Chiriaco and Blue Cut fault zones are two pairs of subordinate faults that do not connect with the main, east-striking, through-going faults or with the bounding San Andreas or Sheep Hole faults. The Porcupine Wash and Substation faults and the Smoke Tree Wash and Victory Pass faults are aligned pairs of faults that are separated by gaps across which geologic units and contacts are not disrupted.

Most maps in the Eastern Transverse Ranges show the sinistral faults as largely concealed continuous features that transect the province (Hope, 1966; Bacheller, 1978; Powell, 1981, 1993). Ongoing geologic mapping has shown, however, that each of these through-going fault zones, despite their great lengths and large displacements, is expressed at the surface as a series of left-stepping fault strands rather than as a continuous fault (Figs. 3 and 4). The locations of these strands are well constrained where they are exposed in basement rocks, moderately well constrained where fault-line scarps occur in basin fill, and poorly constrained where they are completely concealed by basin fill. Although overall displacement is likely to carry through at depth, displacements measured on the left-stepping surface and near-surface strands appear to diminish toward their ends as displacement is transferred from one strand to the next.

The northern faults show evidence of younger movement than do the southern faults (Powell, 1993, p. 60). The Pinto Mountain, Blue Cut, Porcupine Wash, and Smoke Tree Wash faults are characterized by scarps in Pleistocene alluvium (Hope, 1966; Bacheller, 1978) and the latter three (including the aligned Substation and Victory Pass faults) are apparently seismically active (Powell, 1981; Shearer et al., 2005; Fig. 3). Last surface rupture on parts of the Chiriaco fault zone and the subsidiary Corn Springs Wash fault appears to be older because pavemented, varnished, and incised Pleistocene fan deposits are not disrupted by the fault (Powell, 1981). The inception of faulting is most likely late Miocene, because there is no evidence for faulting older than Miocene on these faults. For example, the Chiriaco and Salton Creek fault zones truncate Eocene strata and upper Oligocene and lower Miocene rocks (Powell, 1981). Miocene (6–7 Ma, R. Fleck, 2007, written commun.) basalt fields, generally located near the sinistral faults, are displaced by normal faults associated with the Blue Cut fault zone, and are offset sinistrally by the Smoke Tree Wash fault (Hope, 1966; Powell, 1993). These relations demonstrate that faulting postdated volcanism; thus, much, and likely most, movement along the sinistral faults was as young as or younger than 6 Ma (Powell, 1993).

Bounding Dextral Faults
We take the San Andreas fault as forming the western boundary of the Eastern Transverse Ranges block, although the transition between left- to right-slip domains is complex in detail (Powell, 1981). The east-striking sinistral faults that traverse the block, with the exception of the Pinto Mountain fault zone, cannot be mapped all the way to the San Andreas fault zone, but instead lose definition within a broad cataclastic zone in the Little San Bernardino Mountains that is highly fractured and brecciated. This diffuse boundary coincides with the 1992 Joshua Tree M_w6.1 earthquake and aftershocks (Fig. 3) and represents a zone where slip from the San Andreas fault zone may be transferred northward onto the faults of the Eastern California shear zone (Rymer, 1997). As mentioned earlier, the eastern boundary of the sinistral domain is considered to be the Sheep Hole fault between the Pinto Mountain and Chiriaco fault zones. The type locality of the Sheep Hole fault is along the western margin of the Sheep Hole Mountains, but this fault has been extended south along the western margin of the Coxcomb Mountains to Chuckwalla Valley, where there is a basement discontinuity between the Pinto and Coxcomb Mountains (Powell, 1981).

Paleomagnetic Studies
Paleomagnetic data on samples from late Miocene basalts from four fields in the Eastern Transverse Ranges and one in the Palen Mountains in the Mojave Desert indicate clockwise rotation of 41.4° ± 7.7° (Fig. 4; Carter et al., 1987). Data from the Pinto Wells basalt field (PW in Fig. 4), however, show no rotation for the sampled flows (Carter et al., 1987) and were not included in their calculation. Although all these sites have abnormally high values of the precision parameter (kappa), suggesting that secular variation is not averaged, the regional value calculated by Carter et al. (1987) should average secular variation. We, however, concur with the analysis of Dickinson (1996) that excludes the Palen Mountains site (PM in Fig. 4) because it is clearly outside the Eastern Transverse Ranges domain. The revised amount of rotation is 44° ± 7°.

South of the Chiriaco fault zone, paleomagnetic data from early Miocene flows in the Diligencia Formation indicate clockwise rotation of as much as 164° (Carter et al., 1987). These data from two sites in the Diligencia basin (DB in Fig. 4) sampled a long enough time period to average out secular variations in the Earth's magnetic field. The extreme apparent clockwise rotation of rocks from the Diligencia basin may reflect proximity to the Clemens Well fault rather than a regionally significant rotation (Carter et al., 1987). Paleomagnetic data from Cretaceous(?) microdiorite dikes in the Cottonwood and eastern Orocopia Mountains indicate 40° clockwise rotation (Terres, 1984; Carter et al., 1987).

	GEOPHYSICAL DATA

TOP ABSTRACT INTRODUCTION GEOLOGIC SETTING GEOPHYSICAL DATA GEOPHYSICAL ANOMALIES LATE CENOZOIC STRUCTURE DISCUSSION CONCLUSIONS REFERENCES CITED

 
For our study, we concentrate on that part of the Eastern Transverse Ranges north of the Chiriaco fault zone (Fig. 3) because (1) it is this region where paleomagnetic data (Carter et al., 1987; green arrows in Fig. 4) indicate a clockwise rotation of 44° (Dickinson, 1996), (2) our gravity data set does not extend with sufficient coverage south of the fault, and (3) heat flow and seismicity suggest a change in the geophysical setting across the Chiriaco fault zone. The Chiriaco fault zone coincides with the boundary between lower heat flow in our study area and higher heat flow to the south (Lachenbruch et al., 1985). The area north of the Chiriaco fault zone is more seismically active (Williams et al., 1990; Shearer et al., 2005) and generally higher in elevation (Fig. 3). These observations lead us to suggest that despite many geologic similarities, an important distinction in geophysical signature developed north and south of the Chiriaco fault zone during late Cenozoic crustal evolution of the province.

Nearly 900 new gravity stations along and north of the Blue Cut fault zone were added to earlier coverage (Biehler and Rotstein, 1979; Biehler et al., 2004; Chapman and Rietman, 1974; Roberts et al., 2002; Bracken and Simpson, 1982) for this study. In addition, data from various theses from the University of California, Riverside, were reprocessed to a common datum and formula. Together, these nearly 6800 gravity stations (Langenheim et al., 2007) were used to create an isostatic residual gravity map of the region (Fig. 5). The isostatic correction, using a sea-level thickness of 25 km, a crustal density of 2670 kg/m³, and a mantle-crust density contrast of 400 kg/m³, removes the long-wavelength effect of deep crustal and/or upper mantle masses that isostatically support regional topography, assuming an Airy-Heiskanen model of isostatic compensation (Jachens and Griscom, 1985). Station distribution is on average 1 station per 2 km², although the station density is as low as 1 station per 25 km² within parts of the wilderness areas of Joshua Tree National Park.

View larger version (124K): [in this window] [in a new window]   Figure 5. Isostatic gravity map shown with faults (red) and Joshua Tree National Park boundary (black). Contour interval is 3 mGal. Dark blue dots are density boundaries derived from the maximum horizontal gradient method. Gray shaded region in southeast corner denotes area affected by 15° clockwise rotation of Sheep Hole fault. A–A' is location of model profile (Fig. 9).

View larger version (104K): [in this window] [in a new window]   Figure 6. (A) Thickness of basin fill from inversion of gravity data. Black dashed lines are basin-bounding faults, locations of which are based on analysis of maximum horizontal gravity gradients. CLF—Cleghorn Lake fault; CPF—Cleghorn Pass fault; DWF—Dog Wash fault; EVMF—East Valley Mountain fault; WVMF—West Valley Mountain fault.

View larger version (110K): [in this window] [in a new window]   Figure 7. Aeromagnetic map. Dark blue dots are magnetization boundaries derived from the maximum horizontal gradient method; smaller dots reflect smaller-amplitude gradients. Black dashed lines are basin-bounding faults from gravity analysis (see Fig. 6A). 3MI shows areas outlined by white lines covered by poorer resolution data. A–A' is location of model profile (Fig. 9). ELB—Emerson Lake body; Jv—hydrothermally altered Jurassic volcanic and hypabyssal rocks. a–a' through i–i' denote offset anomaly pairs and are listed in Table 2. Anomalies marked 1 and 2 constrain apparent horizontal displacement on the Sheep Hole fault to be small.

View larger version (108K): [in this window] [in a new window]   Figure 8. Magnetic potential (also known as pseudogravity) map. J and K denote areas dominated by Jurassic and Cretaceous plutonic rocks, respectively; sch shows area of Orocopia Schist; ELB is Emerson Lake body. Arrows point to offset magnetic potential gradients by the Eagle Mountains strand of the Blue Cut fault zone.

	GEOPHYSICAL ANOMALIES

TOP ABSTRACT INTRODUCTION GEOLOGIC SETTING GEOPHYSICAL DATA GEOPHYSICAL ANOMALIES LATE CENOZOIC STRUCTURE DISCUSSION CONCLUSIONS REFERENCES CITED

Magnetic properties vary widely within the basement blocks, except for the Orocopia Schist. Magnetic susceptibility measurements indicate that the Orocopia Schist should not produce significant magnetic anomalies, and this is evident by the smooth magnetic field with generally low values and low amplitudes (sch in Fig. 7) observed over the Orocopia Mountains. This relatively featureless magnetic anomaly pattern contrasts with the ~50-km-wide band of magnetic anomalies northeast of the San Andreas fault zone in Joshua Tree National Park. Griscom and Jachens (1990) attributed the source of these anomalies to Precambrian igneous and metamorphic rocks and Mesozoic plutons that roughly coincide with the mapped distribution of the western and central plutonic belts (Powell, 1993). Jurassic intrusive rocks, predominant in the eastern plutonic belt, are generally more magnetic than the Cretaceous plutons, as indicated by high-amplitude magnetic anomalies in the Eagle and Pinto Mountains and the generally low magnetic values present over the Cadiz Valley batholith, east and north of the sinistral domain. The most prominent magnetic high in the study area is attributed to a mostly concealed mass of Jurassic hornblende diorite named the Emerson Lake body (ELB in Fig. 7; Langenheim and Jachens, 2002). An exception to the generally magnetic Jurassic rocks are Jurassic hydrothermally altered hypabyssal and volcanic rocks exposed in the eastern Pinto Mountains (Howard, 2002) that coincide with pronounced magnetic lows (Jv in Fig. 7), similar to those observed in the Cretaceous rocks of the Coxcomb Mountains.

Both the gravity and magnetic data indicate a major basement boundary between the Coxcomb Mountains on the east and the Pinto and Eagle mountains on the west. The basement gravity field shows a gradient of 25 mGal, which can be accounted for by a density contrast of 80 kg/m³ that extends to 7–8 km depth using an infinite slab approximation. This major boundary, which appears to be offset sinistrally by the Blue Cut fault zone, is even better expressed on the magnetic potential map (arrows in Fig. 8). A forward gravity and magnetic model (Fig. 9) across the fault shows that low magnetic values north of the fault are not caused by deepening of the basement surface below a basin. Instead, observed magnetic variations are a better fit if basement north of the fault is not magnetic, and thus is most likely composed of weakly magnetic, lower density basement similar to that exposed in the Coxcomb Mountains (Fig. 9) and in the hydrothermally altered eastern Pinto Mountains. The model also suggests that the magnetic source rocks for the belt of magnetic anomalies extend to depths of at least 10–15 km, consistent with the base of seismicity. Because the Orocopia Schist is weakly magnetic, the magnetic data suggest that if such rocks are present northeast of the Clemens Well fault zone, they are at or below the base of seismicity.

View larger version (41K): [in this window] [in a new window]   Figure 9. Model across the Eagle Mountain strand of the Blue Cut fault. D—density in kg/m3; S—magnetic susceptibility in SI units. Profile location is shown in Figures 5 and 7.

	LATE CENOZOIC STRUCTURE

TOP ABSTRACT INTRODUCTION GEOLOGIC SETTING GEOPHYSICAL DATA GEOPHYSICAL ANOMALIES LATE CENOZOIC STRUCTURE DISCUSSION CONCLUSIONS REFERENCES CITED

Cumulative Fault Displacements
In order to better constrain offsets on the sinistral faults and the northwest-striking Sheep Hole fault, we compare disruptions in the regional pattern of gravity and magnetic anomalies to mapped disruptions in basement paleogeologic patterns at the Earth's surface. Geologic studies include several that used geologic mapping to document magnitude and timing of fault movements (Hope, 1966, 1969; Dibblee, 1975, 1992; Bacheller, 1978; Powell, 1981, 1982, 1993; Howard, 2002). Offset paleogeologic features in the pre-Cenozoic rocks provide measures of total cumulative offset along the sinistral fault zones.

Basement gravity and magnetic anomalies also constrain the amount of cumulative strike-slip offset (horizontal separation) on the left-lateral strike-slip faults because these anomalies are sourced by physical property variations within the crystalline basement rocks. Basement gravity variations typically give only a rough idea of cumulative offset because these variations are poorly constrained across the basins and in some of the wilderness areas. Estimates of offset based on magnetic data are more reliable because these data are more evenly spaced throughout the study area and yield more sharply defined anomalies, except for the surveys with a flightline-spacing of 5 km (areas marked 3MI, i.e., 3 mi, in Fig. 7). This approach offers a means of evaluating and refining estimates based on geologic data and of identifying additional offset features where magnetically transparent Cenozoic sedimentary deposits conceal bedrock patterns that otherwise would provide geologic evidence for offset. Magnetic and gravity anomalies used to infer offset carry ambiguity where the anomalies cannot be tied directly to observed geologic features. In some cases, anomalies sourced at depth may be inaccurately correlated. By using multiple sets of displaced anomalies, we posit that we can identify false correlations.

The specific technique we use to identify offset magnetic anomalies is analogous to that used to identify offsets of exposed geologic features. We search along the trace of a candidate fault for elongate magnetic anomalies that are subperpendicular to the fault and whose magnetic sources appear to be truncated at the fault trace. We then search the opposite fault block for similar anomalies that are truncated against the opposite side of the fault and estimate the magnitude of offset by measuring the along-fault separation between corresponding distinctive features of the anomaly. This technique has been used to measure offsets along faults in the Mojave Desert north of the Pinto Mountain fault zone (Jachens et al., 2002), yielding reasonable results for right-lateral offset along faults north of the Pinto Mountain fault zone (Jachens et al., 2002), when compared to geologic data, with an estimated uncertainty of ~0.6 km. Given the resolution of aeromagnetic data in our study area, 0.6 km is a reasonable estimate of uncertainty except for areas covered by lower-resolution data (3MI in Fig. 7), where the uncertainty could be as great as several kilometers. In the Eastern Transverse Ranges, strike-slip offsets defined on the basis of magnetic and basement gravity data are in good agreement with those from geologic data for the major through-going sinistral faults (Table 2).

Based on offsets of distinctive basement types, the Pinto Mountain fault zone has 16–20 km of left-lateral displacement (1–1', 2–2', 3–3' in Fig. 4; Dibblee, 1975; Bacheller, 1978). A best-fit reconstruction of all basement units across the central reach of the Pinto Mountain fault zone yielded a slip measurement of 16 ± 2 km (Powell, 1993). The range of offsets indicated by geologic features is generally consistent with estimates from matching magnetic anomalies of 13–18 km (Fig. 7; Table 2). The offset estimate from b-b' (Fig. 7) is less than the geologic estimate 3–3' (Fig. 4) based on the contact of gneiss with quartz monzonite; the northern contact is not well exposed and could be 2 km farther east, making the two offset estimates more consistent. It is important to note, though, that whereas the offset of anomaly a-a' is along the same reach as the offset geologic features, anomalies b-b' and c-c' are offset along more eastern segments of the fault zone, and suggest eastward-diminishing overall displacement from 18 ± 1 km to ~14 ± 2 km.

Gravity and magnetic gradients indicate that the Pinto Mountain fault zone extends east of its intersection with the northwest-striking Mesquite Lake fault, contrary to Dibblee (1992), who argued against such an eastward continuation because of the apparent southward continuity of the Mesquite Lake fault. Although most recent rupture along the Pinto Mountain fault zone is clearly older than Holocene east of the Mesquite Lake fault (Bacheller, 1978), the continued linear escarpment along the northern front of the Pinto Mountains is suggestive of a major fault. Reconstruction of matching offset magnetic anomalies along this eastern reach (c-c', Fig. 7) also indicates offset along this proposed eastern extension of the Pinto Mountain fault zone that is comparable to offset along the previously mapped part of the fault zone west of the Mesquite Lake fault. This eastern extension also is consistent with mapping by Howard (2002) that shows the fault zone extending nearly 15 km east of its intersection with the Mesquite Lake fault.

Blue Cut Fault Zone
The Blue Cut fault zone is composed of four left-stepping strands, named here, from west to east, the Blue Cut, Hexie Mountains, west Pinto basin, and Eagle Mountains strands (Fig. 6A). Strands of the Blue Cut fault zone are exposed only in the vicinity of the Blue Cut in the Little San Bernardino Mountains and near El Dorado Canyon in the Hexie Mountains (Fig. 3), and the Eagle Mountains strand is tightly constrained by mapped units along the north front of the Eagle Mountains. Where well located by geologic mapping, there is excellent correspondence with the locations indicated by the gravity and magnetic data. Where there is little or no surface expression of fault strands, the geophysical data provide much more accurate fault locations, such as in the western Pinto basin. At the east end of the Blue Cut fault zone, gravity data clearly show that the Eagle Mountains strand extends east of Pinto Wells to 1 km east of the inferred extension of the Sheep Hole fault (Fig. 5).

The first studies that measured offsets along the Blue Cut fault zone matched contacts between Mesozoic granitic rocks and Proterozoic gneiss and within the gneiss to show offsets of 3 km across the western reach of the Blue Cut fault strand (4-4' in Fig. 4) and 5–6 km across what we call the Hexie Mountains fault strand (5-5' and 6-6' in Fig. 4; Hope, 1966, 1969). Across the west Pinto basin–Eagle Mountains fault strands, Hope measured 7–9 km by matching a displaced section of quartzite and pelitic rocks intruded by Jurassic quartz monzonite and monzodiorite. Subsequent geologic mapping identified a more detailed array of geologic units and contacts, including not only the metamorphosed sedimentary section and Jurassic pluton, but also the Proterozoic granite gneiss of Joshua Tree, layered paragneiss and orthogneiss, and Jurassic hornblende gabbro and diorite (Powell, 1981, 2002a, 2002b). Reconstructing the paleogeologic pattern of this whole array appears to indicate an offset of 5–6 km across that reach of the fault (7-7', Fig. 4; Powell, 1981, 1982, 1993), somewhat less than the offset Hope measured by matching just the metasedimentary part of the array. These findings led to a best-fit reconstruction of all basement units across the Pinto basin reach of the Blue Cut fault zone, assuming no variation of offset, by restoring a displacement of 5 km (Powell, 1993), which is a minimum with a reasonable uncertainty of as much as 2 km.

The geophysical data show an increasing magnitude of displacement in basement anomalies from west to east along several strands of the Blue Cut fault zone. In our study, measurements obtained from matching magnetic anomalies in the Eagle Mountains with anomalies revealed beneath the Pinto basin indicate offsets of 8 ± 1 km and 9 ± 1 km (f-f', g-g', Fig. 7) across the west Pinto basin and Eagle Mountains fault strands, respectively. These findings are closer to Hope's measurement than to mine (Powell's). Given the greater robustness of the paleogeologic pattern including both geologic features 7-7' and 8-8 (Fig. 3), one of us (Powell) argues that the uncertainties in reconstructing the magnetic anomaly patterns are >1 km and that the geophysical and geologic data are all compatible with a slip measurement of ~7 km. Displaced magnetic anomalies in the Pleasant Valley area indicate an offset of 6 km on the Blue Cut fault strand, consistent both with Hope's and my (Powell's) earlier findings. An offset anomaly (d-d', Fig. 7) along the western reach of the Blue Cut fault strand indicates a fault displacement of 3 km, consistent with Hope's measured offset there.

The kinematics of fault displacement along the Blue Cut fault zone is not completely understood. Hope (1966, p. 88) attributed the smaller amount of offset at the west end to contraction on splay faults that branch northward from the Blue Cut fault, whereas Richard (1993) suggested that the increase in offset eastward may be accommodated by extension south of the fault zone. Based on physiographic expression, geologic and structural mapping, or geophysical data, we find no evidence that supports eastward-increasing extension south of the Blue Cut fault zone, although it may be difficult to identify such a small amount of extension (3–5 km) over a 60-km-long region. Kinematic considerations require, however, that eastward from the array of geologic and geophysical offset markers in the northeastern Eagle Mountains and south-central Pinto Mountains, displacement on the Blue Cut fault zone diminishes to zero, presumably at its junction with the Sheep Hole fault. Gravity data indicate that the Blue Cut fault zone extends ~1 km east of the inferred location of the Sheep Hole fault, whereas geologic mapping does not show the Blue Cut fault zone extending east into the Coxcomb Mountains (Hope, 1966). It is also clear that at its west end displacement along the Blue Cut fault zone either joins the San Andreas fault zone or is accommodated by deformation within the structurally complex basement terrane of the Little San Bernardino Mountains, or a combination of the two.

Chiriaco Fault Zone
The Chiriaco fault zone consists of three left-stepping strands, from west to east, the Shavers Valley, Chiriaco, and Hayfield faults. The west end of the Chiriaco strand is exposed where it cuts through the crystalline basement in the Cottonwood Mountains, and buried fault-line scarps occur along the Shavers Valley fault strand in piedmont of the north-central Orocopia Mountains. Where surface expression of a fault trace is present, geologically and geophysically mapped fault locations are coincident, but where there is no surface evidence for the fault location the mapped faults tend not to be coincident.

Well-constrained measurements of offset on the Chiriaco fault zone are found along the Hayfield fault strand, which displaces a distinctive vertical propylitized dacite dike (11-11', Fig. 4), the steeply dipping western contact between a pluton of Mesozoic biotite monzogranite and the Proterozoic granite gneiss of Joshua Tree (10-10', Fig. 4), and a steeply dipping folded fault between that granite gneiss and the Proterozoic augen gneiss of Monument Mountain (9-9', Fig. 4), all offset ~11 km (Powell, 1981). Newly discovered exposures of Eocene strata along the base of the Cottonwood Mountains (Fig. 4) indicate 2–3 km offset across the west end of the Chiriaco fault strand, where offset is likely to decrease as it steps left onto the Shavers Valley fault strand. The mapped distribution of Eocene strata on either side of the Chiriaco fault zone is consistent with an offset of ~11 km across the Shavers Valley fault strand (8-8', Fig. 4). Measurements made by matching magnetic anomalies are also ~11 km (h-h', i-i', Fig. 7; Table 2).

Other Sinistral Fault Zones
For the lesser sinistral faults with displacements of 3 km or less, the resolution of the aeromagnetic data along most of the mapped extent of the Porcupine Wash and Smoke Tree Wash faults precludes offset estimates, except west of long 116°W, where a linear north-trending gradient shows no left-lateral offset, consistent with geologic mapping that shows that these faults do not extend that far west. Aeromagnetic data suggest ~1 km of sinistral offset on the Victory Pass fault, 3 km on the Substation fault, 2 km on the Ivanhoe fault, and 2.5 km on the Corn Springs Wash fault, all consistent with estimates based on realignment of basement contacts (Hope, 1966; Powell, 1981, 1982, 1993; Howard, 2002; Table 2). We do not find evidence for much sinistral offset (>1 km) on the inferred fault in the central part of the eastern Pinto basin or the fault splay along the northern margin of the west Pinto basin. These faults are mapped in part by their topographic expression and therefore may be normal or normal-oblique faults.

Dextral Faults
We also estimate cumulative offsets along dextral faults that bound the Eastern Transverse Ranges. On the western margin of the sinistral domain in the Little San Bernardino Mountains, mapping by Rymer (2000) indicates that the basement rocks are cut by several geologically young, north-northwest–striking faults with small cumulative (<1 km), dextral offsets. Some of these faults showed triggered slip during the 1992 Joshua Tree and Landers earthquakes (Rymer, 2000). The continuity of the prominent magnetic anomaly bounded on the south by the Mission Creek strand of the San Andreas fault zone suggests that these faults, which may form a link to the Eastern California shear zone to the north, have little cumulative offset. Lack of pronounced offset of the magnetic anomaly is consistent with mapped evidence for youthfulness and distributed deformation along these faults. It is significant that the continuity of the anomaly severely limits offset on any other fault sets observed or inferred in the Little San Bernardino Mountains (Hope, 1966; Powell and Matti, 2006) and might indicate frequent fault reorganization between these faults and the Mission Creek fault.

On the eastern margin of the sinistral domain, clasts in older alluvial deposits offset from their sources north of the Pinto Mountains indicate 1.2–2.5 km of displacement on the Sheep Hole fault (Howard and Miller, 1992; Jagiello et al., 1992). Jagiello et al. (1992) argued for a maximum dextral offset of 4.5 km along the Sheep Hole fault based on matching outcrops of quartz monzonite in the Pinto and Bullion Mountains. Offset apparently decreases southward because the presence of a relatively rare metamorphosed quartz monzodiorite to quartz diorite on both sides of the fault in the Clarks Pass area shows little separation across the fault (Howard and Miller, 1992) and because the fault apparently does not offset an aeromagnetic high along the southwestern front of the Sheep Hole Mountains (1 in Fig. 7; Simpson et al., 1984). Similarly, to the south along the inferred extension of the Sheep Hole fault, the continuity of magnetic anomalies in northern Chuckwalla Valley (2 in Fig. 7) suggests little (<1 km) dextral offset along this fault. Clast provenance of Miocene deposits in the Pinto Wells area is also consistent with little or no right offset (<4–5 km) along the Sheep Hole fault between the Pinto Wells area and the south end of the Coxcomb Mountains.

Basin and Fault Geometry
Inversion of gravity data using the modified method of Jachens and Moring (1990) reveals several sediment-filled basins along the major through-going sinistral faults (Fig. 6A). Our modeling suggests that many of these basins have a rhombic morphology that is suggestive of having formed at releasing steps in a strike-slip fault zone (Crowell, 1974). Below we discuss these basins, define the strands of the sinistral fault zones that bound them, and then describe basins that are along the margins of the sinistral domain.

Basins in Left Steps
The gravity inversion shows two basins along the Pinto Mountain fault zone west of the Mesquite Lake fault. Both basins are narrow (no more than 2 km wide), elongate, and filled with low-density sediment <1 km thick. Both occur between left-stepping strands of the Pinto Mountain fault zone (Fig. 6A). The western basin extends ~17 km west of the town of Yucca Valley and is bounded by the Yucca Valley strand to the south and the Joshua Tree strand to the north. The eastern basin, centered on the town of Twentynine Palms, is shallower (<500 m deep) and is only 13–15 km long, possibly because of (1) the interaction between the Pinto Mountain and Mesquite Lake faults that results in distributive strain and uplift (Bacheller, 1978), or because (2) displacement is diminishing eastward as a result of kinematic interaction between left slip on the Pinto Mountain fault zone and right slip on the north-northwest–striking faults of the Mojave Desert. The latter is supported by 13–15 km offset of magnetic anomalies (b-b' and c-c' in Fig. 7) along this reach of the fault zone.

Along the Blue Cut fault zone, two mapped, left-stepping strands of the Blue Cut fault, the Blue Cut and Hexie Mountains strands, bound a basin beneath Pleasant Valley (Fig. 6A). Gravity, aeromagnetic, and electrical data show these faults to be steeply dipping and basin sediments to be as much as 1–1.5 km thick (McPhee et al., 2008). The length of the basin (6 km) equals the amount of left-lateral offset from matching bedrock patterns and magnetic anomalies. Thus, we argue that this basin is an extensional strike-slip basin.

Gravity data show that two other strike-slip basins along the Blue Cut fault zone underlie the Pinto basin on either end of the gravity-defined west Pinto strand. The basin-bounding faults mapped by geophysical data, however, do not coincide everywhere with mapped faults and scarps. For example, the west Pinto strand is well expressed by magnetic and gravity gradients, but is as much as 2–3 km south of the fault mapped by its topographic expression at the southern margin of the Pinto Mountains. However, the geophysical gradients coincide with a belt of tilted Pliocene and/or Pleistocene sedimentary rocks that poke up through the younger alluvial deposits (Powell, 2002a). The southern basin-bounding strand locally coincides with mapped scarps (Powell, 2002a). The gravity-defined fault strands represent most of the sinistral offset, whereas mapped traces may be related secondary fault zones or splays.

The central basin within the Pinto topographic depression has similar dimensions to the west Pinto basin, but this basin is nearly bisected by subtle scarps (Powell, 2002b). The basin-bounding strands as defined by gravity gradients do not coincide with these mapped scarps. The Eagle Mountains strand on the north and the west Pinto strand on the south again form a left-stepping geometry. Sandbox models of strike-slip basins show that faulting in extensional stepovers evolve such that younger fault strands commonly bisect the strike-slip basin (Dooley and McClay, 1997).

Along the Chiriaco fault zone are two basins, one beneath Shaver Valley and the other beneath Hayfield playa. Our results for the geometry of the basin beneath Hayfield playa are consistent with those of Blackman (1988), but our results are an improvement in that we use a three-dimensional inversion of gravity data that removes a regional field that explicitly accounts for density variations in the basement. Thus, our basin depth is almost 1 km greater than that of Blackman (1988). We concur with Blackman (1988) that the Hayfield basin is bounded by left-stepping strands of the Chiriaco fault and thus is a strike-slip basin. We also suggest that the Shaver Valley basin is a strike-slip basin exhibiting similar geometry. For both basins, the basin-bounding faults also coincide with linear aeromagnetic gradients. Faults based on topographic breaks in slope do not coincide in some places with the buried basin-forming fault strands identified by geophysical analysis.

Basins at the Eastern Ends of the Sinistral Faults
Areally larger and deeper basins from our analysis are located at the east ends of the three through-going sinistral faults (Fig. 6A). Along the Pinto Mountain fault zone, east of its intersection with the Mesquite Lake fault, the gravity inversion reveals a complex basin configuration centered on Dale Lake Valley. The southern margin of the basin coincides with the Old Dale fault, but then diverges southward to the west to connect to the Pinto Mountain strand where the basin narrows and is only ~1 km deep. The northern basin margin is irregular, presumably reflecting interaction with northwest-striking dextral faults, such as the Cleghorn Pass and Cleghorn Lake faults (CPF and CLF in Fig. 4). The eastern margin roughly coincides with the Sheep Hole fault; where gravity data cross the eastern basin margin, the steepest gradient is located ~1 km west of the mapped Sheep Hole fault. The western basin margin does not coincide with any mapped or inferred faults, but gradually shallows to a basement ridge on the upthrown side of the East Valley Mountain fault (EVMF in Fig. 4). Diffuse seismicity in this area (Fig. 3) suggests that the basin margins may still be tectonically active. A small depression in the basement surface is evident on the downthrown (west) side of the East Valley Mountain fault and north of the Dog Wash fault, considered by Bacheller (1978) to be the more recently active part of the Pinto Mountain fault zone east of the Mesquite Lake fault. We suggest, on the basis of gravity and magnetic gradients that the Dog Wash fault steps to the right eastward onto the Pinto Mountain fault and form the East Hills.

At the east end of the Blue Cut fault zone is a roughly triangular basin that is as much as 2.5 km deep. The southern margin of the basin corresponds to the steeply dipping Eagle Mountain strand of the Blue Cut fault zone (Figs. 6A and 9). The northeastern basin edge is concave to the southwest, indicating that the southeast extension of the Sheep Hole fault may not be as linear as inferred.

Another deep (>2 km) triangular basin is at the east end of the Chiriaco fault zone. The eastern margin is roughly aligned with a linear Sheep Hole fault as inferred from gravity data, and we propose that the Chiriaco fault forms the southern basin margin, which is ~8–9 km long.

At the east ends of the Porcupine Wash–Substation and the Smoke Tree–Victory Pass faults is an asymmetric basin more than 750 m deep in northern Chuckwalla Valley. The eastern margin of the basin coincides with the inferred extension of the Sheep Hole fault (Rotstein et al., 1976) and an east-striking buried basement ridge that extends 4 km beyond basement outcrops forms the southern margin of the basin, which is aligned with the Substation fault. Based on this correlation, Rotstein et al. (1976) suggested that the Substation fault extends, buried, most of the way across the valley. Rotstein et al. (1976), based on sparser station coverage, could not justify extending the Victory Pass fault beyond basement outcrops; based on the denser station array of this study, the Victory Pass fault coincides with the edge of a shallow east-west–elongated basin. We thus argue that the Victory Pass fault also extends another 4 km east beneath the alluvial floor of Chuckwalla Valley where it intersects northward-striking faults.

Other Basins
The gravity inversion highlights other basins that are not associated with sinistral faulting, such as a major basin north of the sinistral domain north of Twentynine Palms. The basin is elongated in a northwest direction and is ~1.5 km deep. Although Bacheller (1978) suggested that the basin is bounded on the east by the Mesquite Lake fault, the Mesquite Lake fault may not be the main basin-bounding fault unless it is a reverse fault, because its trace is located in the bottom of the gravity low. Jachens et al. (2002) suggested local, recent changes in stress regime from extensional to compressional ~20 km northwest of our study area along the projection of the Mesquite Lake fault, because of evidence of steeply dipping basin deposits above the deepest part of a gravity-defined strike-slip basin. The Mesquite Lake fault could therefore be a reverse fault; however, evidence for contraction in our study area awaits more detailed study. Based on the maximum horizontal gravity gradient, the main basin-bounding fault is concealed and located ~2 km to the northeast of the mapped trace of the Mesquite Lake fault (Fig. 6A).

	DISCUSSION

TOP ABSTRACT INTRODUCTION GEOLOGIC SETTING GEOPHYSICAL DATA GEOPHYSICAL ANOMALIES LATE CENOZOIC STRUCTURE DISCUSSION CONCLUSIONS REFERENCES CITED

 
Basin Structure and Fault Kinematics
The basins along the major sinistral faults likely formed in a transtensional faulting regime in the Eastern Transverse Ranges, given the present-day stress field. Focal mechanisms in this area are predominantly strike-slip, normal, or oblique-normal faulting (Williams et al., 1990), with nearly horizontal T axes (minimum stress direction) trending west-northwest and P axes (maximum stress direction) ranging from near-horizontal to near-vertical and trending north-northeast to northeast. The focal mechanisms thus indicate a transtensional faulting regime in the Eastern Transverse Ranges. Williams et al. (1990) suggested that the main east-striking faults may have been rotated into positions no longer favorable for further slip, although focal mechanism orientations include N80°E strikes.

Basins at the east ends of these faults do not require a significant component of dextral slip on the Sheep Hole fault, and can result from the simple termination of sinistral offset at the east boundary of the rotated domain (Fig. 10). The combination of gravity and magnetic data suggests that the inferred Sheep Hole fault is characterized more by vertical (2 km) than by horizontal (<1 km) offset. Thus, the inferred Sheep Hole fault in the Pinto basin is a normal or normal-oblique fault that accommodates space created as the Eastern Transverse Ranges rotate and as the block north of the Eagle Mountain strand moves west with respect to the Eagle Mountains to the south (see also Powell, 1993), an expected outcome of rigid-block rotation (Luyendyk, 1991).

View larger version (77K): [in this window] [in a new window]   Figure 10. Vertical displacement predicted from sinistral displacement resulting from an earthquake rupturing through five strands of the Blue Cut fault zone (white rectangles) using Coulomb 3.0 (Toda et al., 2005; Lin and Stein, 2004). At the far eastern end of the fault zone, note subsidence on the northern side of the fault, consistent with the location of the East Pinto basin, and uplift on the southern side of the fault, coincident with the Eagle Mountains. At the western end of the fault zone, the opposite pattern is predicted, but no deep basin is present at the southwest end of the fault.

View larger version (40K): [in this window] [in a new window]   Figure 11. Comparison of the perspective view of the topography of the Eastern Transverse Ranges (A) with the perspective view of the pre-Cenozoic bedrock surface (B), illustrating how narrow the subsurface basins are relative to the physiographic depressions. Vertical exaggeration is 4 x. View is to the northwest.

The estimates of cumulative sinistral offset based on geophysical data are consistent with matching bedrock patterns across the main through-going sinistral faults in the Eastern Transverse Ranges. The magnetic anomaly patterns do not indicate any significant (>3 km) offsets on the subsidiary sinistral faults. Another constraint on offset of the major through-going faults is the length of strike-slip basins. Thermo-mechanical modeling suggests that the length of strike-slip basins equals the cumulative horizontal displacement if the crust is weak (Petrunin and Sobolev, 2006). The basin lengths for the Pinto Mountain, Blue Cut, and Chiriaco fault zones are also similar to estimates of cumulative offsets from matching magnetic anomaly and bedrock patterns (Table 2).

Apparent increases in displacement eastward on the Blue Cut fault zone and westward on the Pinto Mountain fault zone suggest a possible link through the Pinto Mountains, perhaps along the Ivanhoe fault or along northwest-striking faults projecting southeastward from the Mesquite Lake fault (Fig. 3). Such a link would be contractional in nature and reverse faults mapped along the eastern margin of Twenty-nine Palms Mountain (Howard, 2002) may reflect such a link. More speculative is the link southward into the Pinto basin, but geomorphic analysis indicating increased uplift and incision of the Pinto Peak block (Figs. 3 and 5A; Young, 1968) may reflect how offset is transferred into the Pinto basin. Similarly, displacement may be bled off of the Chiriaco fault zone onto the Corn Springs Wash fault (CSWF in Fig. 4), as suggested by a decrease in basin length along the Chiriaco fault zone east of its intersection with the Corn Springs Wash fault from 11 km to 8–9 km. Alternatively, slip gradients along these faults may result from rotation, depending on which end of the fault is pinned (Luyendyk et al., 1980); given the rotation and initial angle between sinistral and dextral domains for the Eastern Transverse Ranges, the calculated slip gradient is <1 km.

Seismicity in the area unbroken by the Porcupine Wash–Substation and Smoke Tree Wash–Victory Pass faults (Fig. 3), coupled with geophysical evidence to extend the Victory Pass and Substation faults farther east (Fig. 6A), suggests that these faults, although not entirely through-going at the surface, probably are through-going at depth and thus contribute to transrotation. These data, together with evidence for links between the Pinto Mountain and Blue Cut fault zones, indicate that the tectonic blocks cannot be modeled solely as rigid, and require more complex models than those applied by Carter et al. (1987) and subsequent workers. Our analysis of the geophysical and geologic data indicates an optimal average value of ~34–35 km of sinistral offset on the main through-going sinistral faults in our study area and as much as 4.5 km on the aligned subsidiary faults. The geophysical data do not indicate any additional faults with more than 1 km of sinistral offset.

Tectonic Rotation
Studies that model the sinistral fault domain in the Eastern Transverse Ranges must address not only geologic constraints on fault displacements, but also paleomagnetic declination data that indicate a clockwise rotation of 41° ± 8° since the late Miocene (Carter et al., 1987), recalculated herein as 44° ± 7° exclusive of the Mojave Desert Palen Mountains site. Four such studies illustrate a range of approaches in dealing with the implications of those data. Carter et al. (1987), in presenting their paleomagnetic findings, noted that known geologic displacements indicated a rotation of ~35° and concluded that the geologic and paleomagnetic data were in rough agreement at the 95% confidence level. One of us (Powell, 1993) examined the sinistral faulting domain in the context of a balanced reconstruction in which displacements on all late Cenozoic strike-slip faults in southern California were restored, avoiding overlaps and minimizing gaps between blocks of crystalline basement. Powell found that rotation of the fault blocks in the Eastern Transverse Ranges is required to achieve a balanced reconstruction. The indicated rotation for the Pinto Mountains block between the Pinto Mountain and Blue Cut fault zones is 15°. That rotation is increased to 20° for the Eagle Mountains and Orocopia Mountains blocks, allowing differential block rotation, and in effect removing extension by partially closing the Pinto basin in the reconstruction (Powell, 1993). Compared with rotation determined by palinspastic reconstruction, the paleomagnetically determined rotation is a factor of two larger. The Powell (1993) geometric rigid-block model to test the self-consistency of the reconstruction yielded rotations of 15°–20°: for the sinistral faults, Powell restored minimal displacements from range of estimates for each, and did not include slip from subsidiary faults.

Richard (1993) developed a tectonic model for southeasternmost California that also showed that simple rigid block models using a rotation of 40° overestimate the sinistral displacements across the entire Eastern Transverse Ranges block. Richard (1993), in order to honor the paleomagnetic data, modified the boundary conditions to allow the Sheep Hole fault to rotate 15° clockwise south of its intersection with the Blue Cut fault zone. He assumed that the Sheep Hole fault originally was straight with an azimuth of N46°W (measured north of the Pinto Mountain fault zone). Geophysical data do not readily support the rotation of the eastern boundary. First, gravity data indicate that the overall fault zone does not change strike and that, at least in the Pinto basin, the Sheep Hole fault is not linear, but curved and suggestive of normal faulting. Second, rotation of the eastern boundary should result in extension in the domain east of the Sheep Hole fault. The amount of extension should increase southward from the pivot point in the southeastern corner of the Pinto basin, with as much as 6 km at lat 33°45' (see shaded region in Fig. 5). Instead, a basement ridge is located between the Palen Valley and Chuckwalla Valley basins in the area that should be extended, according to Richard (1993, his Fig. 12). The basin beneath Palen Valley could be a potential location for this extension except that the basin should become wider to the south. Instead the basin is arguably wider at its north end. Perhaps the repercussions of rotating the Sheep Hole fault are distributed over a large area and thus are difficult to recognize in either the geology or in the existing geophysical data. We prefer the interpretation that the Sheep Hole fault is a loci of fixed points along which the Eastern Transverse Ranges fault panels swung without changing the orientation of the San Andreas edge of the domain (i.e., Dickinson, 1996).

View larger version (35K): [in this window] [in a new window]   Figure 12. Comparison of predicted offsets from equations in Dickinson (1996) and Powell (1993), assuming the width of the rotation domain is 50 km (our study area, the block between Pinto Mountain and Chiriaco faults). The red arrow points to the predicted angle of rotation given the amount of displacement discussed in Powell (1993). The black arrow and arrows in various shades of blue are the predicted angles of rotation for the various models given by Powell (1993) and Dickinson (1996) case II, case I, and case III models, respectively, for the amount of optimal cumulative offset from this paper for the region between the Pinto Mountain and Chiriaco faults. The vertical gray band represents the range of rotation indicated by the paleomagnetic data of Carter et al. (1987) at the 95% confidence level, but recalculated to exclude the Mojave Desert Palen Mountains site (Dickinson, 1996).

The rotation models all have shortcomings, which we attribute to components of non-rigid behavior of the blocks between the through-going faults. None of these models accurately predicts the amount of offset for all of the individual faults because all these models assume rigid-block behavior and that the amount of offset on the fault is proportional to the fault spacing (and amount of rotation). Spacing between the main through-going faults is roughly the same, ~20–25 km, but offset on the Pinto Mountain fault is considerably more than that on the other through-going faults (Table 2). The greater displacement on the Pinto Mountain fault may be related to deformation in the Mojave block or to the deflection of the San Andreas fault in San Gorgonio Pass.

Another shortcoming shared by the rigid block rotation models is that gaps predicted to open along the margins of the rotating domain (Fig. 2) are not always observed. Some of these predicted gaps coincide with deep basins, such as those at the east ends of the sinistral faults. Gaps predicted at the western ends of the sinistral faults do not, however, coincide with deep basins. If deep basins were formed during transrotation along the western margin of the domain, they may have been subsequently highly modified by the San Andreas fault, as suggested by highly folded and deformed Quaternary deposits in the Mecca and Indio Hills. Alternatively, intense distributed deformation along the western boundary of the rotating panel, perhaps reflecting the width of dextral shear associated with the San Andreas fault, did not allow for the creation of the deep basins there. Numerical two-dimensional modeling of crustal rotations in the Eastern Transverse Ranges shows that gaps or misfits at the margins of the rotating block can be reduced or even eliminated assuming elastic or elastoplastic deformation (e.g., non-rigid behavior; Schelle and Grünthal, 1996).

According to the rigid block rotation models, triangular gaps should also be located along the northern margin of the sinistral domain (Fig. 2), where northwest-striking right-lateral faults impinge on the Pinto Mountain fault zone. The only gaps that coincide with deep (>1 km) basins are the Dale Lake basin and the Mesquite Lake basin (Fig. 6A). Both basins are elongated and not obviously triangular. Furthermore, the Dale Lake basin is wider at its western end, contrary to the eastward-increasing width of the basins predicted by the rigid-block models (Fig. 2). The geometry of the Mesquite Lake basin suggests that the original basin geometry may have been modified by younger deformation. The eastern margin of the basin does not coincide with the Quaternary Mesquite Lake fault. The southern margin of the Mesquite Lake basin should coincide with the Pinto Mountain fault zone, but instead is located 3 km north of the fault and appears to dip gently toward the basin, as if the original basin margin had been arched by subsequent deformation. Such deformation, however, has yet to be documented.

Whereas rigid-block models assume that deformation is limited to through-going faults and cannot extend into the intervening blocks, our findings indicate that this assumption is not valid for fault blocks between sinistral faults in the Eastern Transverse Ranges. Our combined geophysical and geologic data offer insight into mechanisms and magnitude of non-rigid behavior. The distribution and extent of the sinistral faults, and displacements on them, are not uniform throughout the province. The Pinto Mountain and the Chiriaco faults each show diminishing displacement eastward along the fault, whereas the Blue Cut shows increasing displacement eastward, suggesting possible linkage between the faults. Thus, we sum displacements on all sinistral faults on the west side of the province and all those on the east side (Table 3), and show that displacements on the subordinate faults roughly balance the diminishing eastward displacements on both the major faults in the study area. The data in Table 3 suggest that the balanced overall displacement between east and west is 34–40 km for the study area, translating to rotation ranges of ~27°–34° for the Powell (1993) model to ~35°–39° for Dickinson's (1996) case III model (Fig. 12). Geologic data to the south of our study area show that displacement on the through-going Salton Creek fault diminishes eastward and is likely taken up by subsidiary sinistral faults, such as the Ship Creek and Corn Springs Wash faults, in the eastern part of the domain (see Powell, 1993).

Other data also indicate that the blocks between the faults are not rigid. Available detailed mapping (Powell, 2001a, 2001b, 2002a, 2002b) shows a web of subsidiary northwest-striking, west-striking, and north-striking faults between the main through-going faults. Although the offsets on these faults are demonstrably small, their cumulative effect may contribute to resolving the discrepancy between total displacement and amount of rotation indicated by the paleomagnetic data.

Given our constraints on sinistral offsets for the block between the Pinto Mountain and Chiriaco fault zones (34–40 km), the rigid-block rotation models predict a rotation in the range of 27°–39° (Table 3; Fig. 12). Depending on the rigid-block model, the upper end of the rotational range either shows a slight gap from, or overlaps, the lower end of the rotational range at the 95% confidence level indicated by the paleomagnetic data for this region (Fig. 12). Our analysis shows that the geophysical, geologic, and paleomagnetic data are in agreement only within the 37°–39° rotational range, corresponding to an overall displacement of ~40 km. This overlap, however, does not preclude that less rotation has occurred, at least locally, especially given paleomagnetic data for the Pinto Wells site (PW in Fig. 4). This site, arguably the best paleomagnetic site in the Eastern Transverse Ranges, despite its high-precision parameter, because it has both normal and reversed directions, is not rotated. Carter et al. (1987) attributed the lack of rotation at Pinto Wells to three factors: (1) it is outside the rotated domain, (2) its proximity to both sinistral and dextral faults may have resulted in no net rotation, or (3) the Pinto Wells basalt may be younger than the other basalts, thus postdating the rotation. Gravity data confirm that the Blue Cut fault zone extends east of the Pinto Wells site, so the site should be in the sinistral domain and thus be rotated. Both geologic and geophysical data do not support an equal and opposing amount of dextral offset along the Sheep Hole fault that results in no net rotation at Pinto Wells. The remaining alternative, that the Pinto Wells basalt is younger than the rotation, suffers from ambiguity in that ages for the Pinto Wells center range from 4.5 to 7.8 Ma and that preliminary analysis of GPS data indicates ongoing sinistral displacement on the Blue Cut fault zone (Bennett et al., 2006). Clearly this site would benefit from detailed geologic mapping and additional geochronologic and paleomagnetic studies.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION GEOLOGIC SETTING GEOPHYSICAL DATA GEOPHYSICAL ANOMALIES LATE CENOZOIC STRUCTURE DISCUSSION CONCLUSIONS REFERENCES CITED

 
Geophysical data provide markers of displacement on the sinistral faults in the Eastern Transverse Ranges that document variable displacement along the faults. These data are consistent with and augment displacements measured using offset geological features. Sinistral offsets derived from integrating best fits for the geophysical and geologic data indicate 16–18 km along the west-central Pinto Mountain fault, diminishing eastward to 13–16 km; 7–9 km on the eastern Blue Cut fault, diminishing westward to 3 km; and 11 km along most of the length of the Chiriaco fault to 8 km near the east end of the fault. Offsets on subsidiary faults sum to ~7 km in the study area. Of the subsidiary faults, the Smoke Tree Wash–Victory Pass and Porcupine Wash–Substation faults discontinuously transect the Eastern Transverse Ranges sinistral domain, having a combined displacement of 4–4.5 km. We group these fault pairs kinematically with the above through-going major faults as boundaries between rotational fault blocks. The remaining subsidiary faults are located in the eastern half of the sinistral domain and represent a principal mechanism of non-rigid fault-block behavior compensating for variations in magnitude of displacement along the through-going faults.

Patterns of alternating westward increasing and decreasing slip along the main through-going faults and the location and slip on the minor subsidiary faults are consistent with our interpretation of the minor faults as one mechanism of non-rigid fault-block behavior in the Eastern Transverse Ranges. Any discrepancy between our data and the 95% confidence window of the paleomagnetic data can be eliminated if blocks are allowed to further deform, a condition that is consistent with faults mapped within the blocks and by the absence of basins at the west ends of the sinistral faults, where rocks are intensely deformed in the Little San Bernardino Mountains.

Geophysical data indicate that sinistral faults in the Eastern Transverse Ranges form zones characterized by left-stepping segments and pull-apart basins in the stepovers. The basin geometry is consistent with a transtensional stress regime since inception of faulting, most likely associated with opening of the Salton Trough and the birth of the modern San Andreas system in this region ca. 5–6 Ma. The lengths of the pull-apart basins match offset estimates on the faults based on matching bedrock patterns and magnetic anomalies, with a cumulative sinistral offset of 34–40 km across the fault blocks between the Pinto Mountains and Chiriaco fault zones, calculated by adding displacements on the minor faults to compensate for slip deficiencies along the major faults. Regardless of the rigid-block rotation model used, this amount of cumulative offset is consistent with a clockwise rotation of 27°–39°, generally less than indicated by paleomagnetic measurements on late Miocene basalts, but sharing a small overlap with the 95% confidence range of 37°–51° for the paleomagnetic declination data.

The deepest basins along the sinistral fault zones are located at the eastern ends of the faults, near their intersections with the northwest-striking Sheep Hole fault, as predicted by rigid-block models. Gravity and magnetic data suggest that the Sheep Hole fault is primarily characterized by vertical, rather than horizontal, displacement. The basins at the eastern ends of the sinistral faults do not necessarily reflect interaction between sinistral and dextral faults, but rather accommodate space created at the ends of the sinistral faults due to block rotations.

View larger version (107K): [in this window] [in a new window]   Figure 6. (continued). (B) Basement gravity map. J and K denote areas dominated by Jurassic and Cretaceous plutonic rocks, respectively; sch shows area of Orocopia Schist. ELB is Emerson Lake body. Shaded gray areas are poorly constrained.

	ACKNOWLEDGMENTS

 
We thank Dave Miller, Bob Jachens, Jack Hill-house, Jon Matti, and Laura Serpa for their helpful comments on the manuscript. Bruce Luyendyk provided not only useful comments on the manuscript, but also data, unpublished reports, and advice. We gratefully acknowledge Luke Sabala of the Joshua Tree National Park for his unflagging logistical support and interest in this work. We recognize Shawn Biehler and his legion of students for measuring gravity not only in this area, but throughout southern California, and we also thank Darcy McPhee, Meg Anderson, Janet Tilden Watt, Bruce Chuchel, Craig McCabe, and Phil Stoffer, all of or formerly with the U.S. Geological Survey (USGS), for their field efforts in the park. Last, but not least, we thank the National Cooperative Geologic Mapping program of the USGS and the National Park Service for financial support.

MANUSCRIPT RECEIVED BY THE SOCIETY March 29, 2008

REVISED MANUSCRIPT RECEIVED September 17, 2008

MANUSCRIPT ACCEPTED October 2, 2008

	REFERENCES CITED

TOP ABSTRACT INTRODUCTION GEOLOGIC SETTING GEOPHYSICAL DATA GEOPHYSICAL ANOMALIES LATE CENOZOIC STRUCTURE DISCUSSION CONCLUSIONS REFERENCES CITED

 

Bacheller, J. 1978, Quaternary geology of the Mojave Desert–eastern Transverse Ranges boundary in the vicinity of Twentynine Palms, California [M.S. thesis]: Los Angeles University of California 160 p.

Barth, A.P., Tosdal, R.M., Wooden, J.L., and Howard, K.A. 1997, Triassic plutonism in southern California: Southward younging of arc initiation along a truncated continental margin: Tectonics, v. 16, p. 290– 304, doi: 10.1029/96TC03596.[CrossRef][Web of Science][GeoRef]

Barth, A.P., Wooden, J.L., Jacobson, C.E., and Probst, K. 2004, U-Pb geochronology and geochemistry of the McCoy Mountains Formation, southeastern California: A Cretaceous retroarc foreland basin: Geological Society of America Bulletin, v. 116, p. 142– 153, doi: 10.1130/B25288.1.[Abstract/Free Full Text]

Bennett, R.A., Anderson, M.L., Hreinsdottir, S., Buble, G., Spinler, J., and Thompson, S. 2006, GPS constraints on crustal deformation in the eastern Transverse Ranges, southern California: Eos (Transactions, American Geophysical Union), Fall meeting supplement, abs. G43B–0994.

Biehler, S., and Rotstein, Y. 1979, Bouguer gravity map of California, Salton Sea sheet: California Division of Mines and Geology, scale 1:250,000.

Biehler, S., Kovach, R.L., and Allen, C.R. 1964, Geophysical framework of northern end of Gulf of California structural province, in van Andel T., Shor G.G. Jr. eds., Marine geology of the Gulf of California: American Association of Petroleum Geologists Memoir 3, p. 126– 143.

Biehler, S., Langenheim, V.E., Sikora, R.F., Ponce, D.A., Chapman, R.H., Beyer, L.A., and Oliver, H.W. 2004, Bouguer gravity map of California, Santa Ana sheet: California Geological Survey Map Series, scale 1:250,000.

Blackman, T.D. 1988, Geophysical investigations of the Hayfield Dry Lake area in the western Chuckwalla valley, southern California [M.S. thesis]: Riverside University of California 101 p.

Blakely, R.J., and Simpson, R.W. 1986, Approximating edges of source bodies from magnetic or gravity anomalies: Geophysics, v. 51, p. 1494– 1498, doi: 10.1190/1.1442197.[CrossRef][Web of Science][GeoRef]

Bracken, R.E., and Simpson, R.W. 1982, Principal facts for gravity stations in the Sheep Hole/Cadiz Wilderness study area (CDCA-305), California: U.S. Geological Survey Open-File Report 82–882, 30 p.

Calzia, J.P., DeWitt, E., and Nakata, J.K. 1986, U-Th-Pb age and initial strontium isotopic ratios of the Coxcomb Granodiorite and a K-Ar date of olivine basalt from the Coxcomb Mountains, southern California: Isochron West, v. 47, p. 3– 8.

Carter, J.N., Luyendyk, B.P., and Terres, R.R. 1987, Neogene clockwise rotation of the eastern Transverse Ranges, California, suggested by paleomagnetic vectors: Geological Society of America Bulletin, v. 98, p. 199– 206, doi: 10.1130/0016-7606(1987)98<199:NCTROT>2.0.CO;2.[Abstract/Free Full Text]

Chapman, R.H., and Rietman, J.D. 1974, Bouguer gravity map of California, Needles sheet: California Division of Mines and Geology, scale 1:250,000.

Cordell, L., and Grauch, V.J.S. 1985, Mapping basement magnetization zones from aeromagnetic data in the San Juan Basin, New Mexico, in Hinze W.J. ed., The utility of regional gravity and magnetic anomaly maps: Tulsa, Oklahoma Society of Exploration Geophysicists p. 181– 192.

Crouch, J.K. 1979, Noegene tectonic evolution of the California Continental Borderland and western Transverse Ranges: Geological Society of America Bulletin, v. 90, part I, p. 338– 345, doi: 10.1130/0016-7606(1979)90<338:NTEOTC>2.0.CO;2.[Abstract/Free Full Text]

Crowell, J.C. 1974, Origin of late Cenozoic basins in southern California, in Dott R.H., Shaver R.J. eds., Modern and ancient geosynclinal sedimentation: Society of Economic Paleontologists and Mineralogists Special Publication 19, p. 292– 303.

Crowell, J.C., and Susuki, T. 1959, Eocene stratigraphy and paleontology, Orocopia Mountains, southeastern California: Geological Society of America Bulletin, v. 70, p. 581– 592, doi: 10.1130/0016-7606(1959)70[581:ESAPOM]2.0.CO;2.[Abstract/Free Full Text]

Dibblee, T.W.. Jr. 1975, Late Quaternary uplift of the San Bernardino Mountains on the San Andreas fault in southern California, in Crowell J.C. ed., San Andreas fault in southern California: A guide to San Andreas fault from Mexico to Carrizo Plain: California Division of Mines and Geology Special Report 118, p. 127– 135.

Dibblee, T.W.. Jr. 1992, Geology and inferred tectonics of the Pinto Mountain fault, eastern Transverse Ranges, California, in Richard S.M. ed., Deformation associated with the Neogene Eastern California shear zone, southwestern Arizona and southeastern California: San Bernardino County Museum Special Publication 92–1, p. 28– 31.

Dickinson, W.R. 1996, Kinematics of transrotational tectonism in the California Transverse Ranges and its contribution to cumulative slip along the San Andreas transform fault system: Geological Society of America Special Paper 305, 46 p.

Dooley, T., and McClay, K. 1997, Analog modeling of pull-apart basins: American Association of Petroleum Geologists Bulletin, v. 81, p. 1804– 1826.[Abstract]

Grauch, V.J.S. 1984, Aeromagnetic map of the Eagle Mountains area, Riverside County, California: U.S. Geological Survey Open-File Report 84–502, scale 1:62,500.

Griscom, A., and Jachens, R.C. 1990, Crustal and lithospheric structure from gravity and magnetic studies in Wallace R.E. ed., The San Andreas fault system, California: U.S. Geological Survey Professional Paper. 1515, p. 239– 259.

Harris, R.A., and Day, S.M. 1993, Dynamics of fault interaction: Parallel strike-slip faults: Journal of Geophysical Research, v. 98, p. 4461– 4472, doi: 10.1029/92JB02272.[GeoRef]

Haxel, G.B., and Dillon, J.T. 1978, The Pelona-Orocopia schist and Vincent-Chocolate Mountain thrust system, southern California, in Howell D.G., McDougall K.A. eds., Mesozoic paleogeography of the western United States: Pacific Coast Paleogeography Symposium 2: Pacific Section, Society of Economic Paleontologists and Mineralogists, p. 453– 469.

Haxel, G.B., Jacobson, C.E., Richard, S.M., Tosdal, R.M., and Grubensky, M.J. 2002, Orocopia schist in southwest Arizona: Early Tertiary oceanic rocks trapped or transported far inland, in Barth A. ed., Contributions to crustal evolution of the southwestern United States: Geological Society of America Special Paper 365, p. 99– 128.

Hope, R.A. 1966, Geology and structural setting of the eastern Transverse Ranges, southern California [Ph.D. thesis]: Los Angeles University of California 201 p.

Hope, R.A. 1969, The Blue Cut fault, southeastern California: U.S. Geological Survey Professional Paper 650-D, p. D116– D121.

Hopson, R.F. 1999, A mechanical model for the Pinto Mountain fault, southern California: Preliminary results: Geological Society of America Abstracts with Programs, v. 31, no. 6, p. A63.

Hopson, R.F., Hillhouse, J.W., and Howard, K.A. 2008, Dike orientations in the Late Jurassic Independence dike swarm and implications for vertical axis tectonic rotations in eastern California, in Wright J.E., Shervais J.W. eds., Ophiolities, arcs, and batholiths: Geological Society of America Special Paper 438, p. 481– 498, doi:10.1130/2008.2438(17).

Hornafius, J.S., Luyendyk, B.P., Terres, R.R., and Kamerling, M.J. 1986, Timing and extent of Neogene tectonic rotation in the western Transverse Ranges, California: Geological Society of America Bulletin, v. 97, p. 1476– 1487, doi: 10.1130/0016-7606(1986)97<1476:TAEONT>2.0.CO;2.[Abstract/Free Full Text]

Howard, K.A. 2002, Geologic map of the Sheep Hole Mountains 30 by 60 quadrangle, San Bernardino and Riverside counties, California: U.S. Geological Survey Miscellaneous Field Studies Map MF-2344, http://pubs.usgs.gov/mf/2344.

Howard, K.A., and Miller, D.M. 1992, Late Cenozoic faulting at the boundary between the Mojave and Sonoran blocks: Bristol Lake Area, California, in Richard S.M. ed., Deformation associated with the Neogene Eastern California shear zone, southwestern Arizona and southeastern California: San Bernardino County Museum Special Publication 92–1, p. 37– 47.

Jachens, R.C., and Griscom, A. 1985, An isostatic residual gravity map of California—A residual map for interpretation of anomalies from intracrustal sources, in Hinze W.J. ed., The utility of regional gravity and magnetic maps: Tulsa, Oklahoma Society of Exploration Geophysicists p. 347– 360.

Jachens, R.C., and Moring, B.C. 1990, Maps of the thickness of Cenozoic deposits and the isostatic gravity over basement for Nevada: U.S. Geological Survey Open-File Report 90–404, 15 p.

Jachens, R.C., Langenheim, V.E., and Matti, J.C. 2002, Relationship of the Hector Mine and Landers ruptures to offsets on Quaternary faults and distribution of late Cenozoic basins in the Eastern California Shear Zone: Seismological Society of America Bulletin, v. 92, p. 1592– 1605, doi: 10.1785/0120000915.[Abstract/Free Full Text]

Jagiello, K., Christie, J.M., and Blom, R.G. 1992, Horizontal separation of major late Cenozoic strike-slip faults in the Twentynine Palms region, Mojave Desert, California, in Richard S.M. ed., Deformation associated with the Neogene Eastern California shear zone, southwestern Arizona and southeastern California: San Bernardino County Museum Special Publication 92–1, p. 48– 53.

Jennings, C.W. 1994, Fault activity map of California and adjacent areas: California Division of Mines and Geology Geologic Data Map 6, scale 1:750,000.

Jones, D.L., Blake, M.C.. Jr., and Rangin, C. 1976, The four Jurassic belts of northern California and their significance to the geology of the southern California borderland, in Howell D.G. ed., Aspects of the geologic history of the California Continental borderland: American Association of Petroleum Geologists Miscellaneous Publication 24, p. 343– 362.

Kamerling, M.J., and Luyendyk, B.P. 1979, Paleomagnetism and Neogene tectonics of the northern Channel Islands, California: Journal of Geophysical Research, v. 90, p. 12,485– 12,502, doi: 10.1029/JB090iB14p12485.

Lachenbruch, A.H., Sass, J.H., and Galanis, S.P.. Jr. 1985, Heat flow in southernmost California and the origin of the Salton Trough: Journal of Geophysical Research, v. 90, no. B7 p. 6709– 6736, doi: 10.1029/JB090iB08p06709.[GeoRef]

Langenheim, V.E., and Hauksson, E. 2001, Comparison between crustal density and velocity variations in southern California: Geophysical Research Letters, v. 28, p. 3087– 3090, doi: 10.1029/2001GL013392.[CrossRef][Web of Science][GeoRef]

Langenheim, V.E., and Jachens, R.C. 2002, The Emerson Lake body: A link between the Landers and Hector Mine earthquakes, southern California, inferred from gravity and magnetic anomalies: Seismological Society of America Bulletin, v. 92, p. 1606– 1620, doi: 10.1785/0120000906.[Abstract/Free Full Text]

Langenheim, V.E., Biehler, Shawn, McPhee, D.K., McCabe, C.M., Watt, J.T., Anderson, M., Chuchel, B.A., and Stoffer, P. 2007, Preliminary isostatic gravity map of Joshua Tree National Park and vicinity, southern California: U.S. Geological Survey Open-File Report 2007–1218, scale 1:100,000.

Law, R.D., Eriksson, K., and Davisson, C. 2001, Formation, evolution, and inversion of the middle Tertiary Diligencia basin, Orocopia Mountains, southern California: Geological Society of America Bulletin, v. 113, p. 196– 221, doi: 10.1130/0016-7606(2001)113<0196:FEAIOT>2.0.CO;2.[Abstract/Free Full Text]

Lin, J., and Stein, R.S. 2004, Stress triggering in thrust and subduction earthquakes, and stress interaction between the southern San Andreas and nearby thrust and strike-slip faults: Journal of Geophysical Research, v. 109, B02303, doi: 10.1029/2003JB002607.[CrossRef]

Luyendyk, B.P. 1991, A model for Neogene crustal rotations, transtension, and transpression in southern California: Geological Society of America Bulletin, v. 103, p. 1528– 1536, doi: 10.1130/0016-7606(1991)103<1528:AMFNCR>2.3.CO;2.[Abstract/Free Full Text]

Luyendyk, B.P., Kamerling, M.J., and Terres, R. 1980, Geometric model for Neogene crustal rotations in southern California: Geological Society of America Bulletin, v. 91, p. 211– 217, doi: 10.1130/0016-7606(1980)91<211:GMFNCR>2.0.CO;2.[Abstract/Free Full Text]

Magistrale, H., and Zhou, H.-W. 1996, Lithologic control of the depth of earthquakes in southern California: Science, v. 273, p. 639– 643, doi: 10.1126/science.273.5275.639.[Abstract]

McPhee, D.K., Langenheim, V.E., Chuchel, B.A., and Pellerin, L. 2008, An integrated geophysical approach for groundwater and seismic hazard management in Joshua Tree National Park, southern California, inApplication of Geophysics to Engineering and Environmental Problems, Symposium, 21st, 2008, SAGEEP 2008: Atlanta, Georgia, Curran Associates, Inc., p. 510– 518.

Petrunin, A., and Sobolev, S.V. 2006, What controls thickness of sediments and lithospheric deformation at a pull-apart basin?: Geology, v. 34, p. 389– 392, doi: 10.1130/G22158.1.[Abstract/Free Full Text]

Powell, R.E. 1981, Geology of the crystalline basement complex, eastern Transverse Ranges, California: Constraints on regional tectonic interpretations [Ph.D. thesis]: Pasadena California Institute of Technology 441 p.

Powell, R.E. 1982, Crystalline basement terranes in the southern eastern Transverse Ranges, California, in Cooper J.D. complier, Geologic Excursions in the Transverse Ranges: Geological Society of America Cordilleran Section Guidebook, p. 109– 136.

Powell, R.E. 1993, San Andreas fault, in Powell R.E.., et al. eds., The San Andreas fault system: Displacement, palinspastic reconstruction, and geologic evolution: Geological Society of America Memoir 178, p. 1– 106.

Powell, R.E. 2001a, Geologic map and digital database of the Porcupine Wash Wells 7.5 minute quadrangle, Riverside county, California: U.S. Geological Survey Open-File Report 01–030, http://geopubs.wr.usgs.gov/open-file/of01-030/.

Powell, R.E. 2001b, Geologic map and digital database of the Conejo Wells 7.5 minute quadrangle, Riverside county, California: U.S. Geological Survey Open-File Report 01–031, http://pubs.usgs.gov/of/2001/of01-031/.

Powell, R.E. 2002a, Geologic map and digital database of the Pinto Mountain 7.5 minute quadrangle, Riverside county, California: U.S. Geological Survey Open-File Report 02–491, http://geopubs.wr.usgs.gov/open-file/of02-491.

Powell, R.E. 2002b, Geologic map and digital database of the San Bernardino Wash 7.5 minute quadrangle, Riverside County, California: U.S. Geological Survey Open-File Report 02–498, http://geopubs.wr.usgs.gov/open-file/of02-498/.

Powell, R.E. 2003, Digital geologic maps and databases in the Eagle Mountains 30 x 60 minute quadrangle, Riverside county, California: Geological Society of America Abstracts with Programs, v. 35, no. 6, p. 73.

Powell, R.E., and Matti, J.C. 2006, A geologic and geomorphic mapping approach to understanding the kinematic role of faulting in the Little San Bernardino Mountains in the evolution of the San Andreas fault system in southern California: Eos (Transactions, American Geophysical Union), v. 87, Fall meeting supplement, abs. T21B–0404.

Richard, S.M. 1993, Palinspastic reconstruction of southeastern California and southwestern Arizona: Tectonics, v. 12, p. 830– 854, doi: 10.1029/92TC02951.[Web of Science][GeoRef]

Roberts, C., Jachens, R., Katzenstein, A., Smith, G., and Johnson, R. 2002, Gravity map and data of the eastern half of the Big Bear Lake, 100,000 scale quadrangle, California and analysis of the depths of several basins: U.S. Geological Survey Open-File Report 02–353; http://geopubs.wr.usgs.gov/open-file/of02-353/.

Ron, H., and Nur, A. 1996, Vertical axis rotations in the Mojave: Evidence from the Independence dike swarm: Geology, v. 24, p. 973– 976, doi: 10.1130/0091-7613(1996)024<0973:VARITM>2.3.CO;2.[Abstract/Free Full Text]

Rotstein, Y., Combs, J., and Biehler, S. 1976, Gravity investigations in the southeastern Mojave Desert, California: Geological Society of America Bulletin, v. 87, p. 981– 993, doi: 10.1130/0016-7606(1976)87<981:GIITSM>2.0.CO;2.[Abstract/Free Full Text]

Rymer, M.J. 1997, Structural link between the San Andreas fault and the eastern California shear zone, Coachella Valley and Little San Bernardino Mountains, southern California: Geological Society of America Abstracts with Programs, v. 29, no. 5 p. 61– 62.

Rymer, M.J. 2000, Surface slips in the Coachella Valley associated with the 1992 Joshua Tree and Landers, California, earthquakes: Seismological Society of America Bulletin, v. 90, p. 832– 848, doi: 10.1785/0119980130.[Abstract/Free Full Text]

Schelle, H., and Grünthal, G. 1996, Modeling of Neogene crustal block rotation: Case study of southeastern California: Tectonics, v. 15, p. 700– 710, doi: 10.1029/95TC03687.[CrossRef][Web of Science][GeoRef]

Shearer, P., Hauksson, E., and Lin, G. 2005, Southern California hypocenter relocation with waveform cross-correlation, part 2: Results using source-specific station terms and cluster analysis: Seismological Society of America Bulletin, v. 95, p. 904– 915, doi: 10.1785/0120040168.[Abstract/Free Full Text]

Simpson, R.W., Bracken, R.E., and Stierman, D.J. 1984, Aeromagnetic, Bouguer gravity, and interpretation maps of the Sheep Hole-Cadiz wilderness study area (CDCA-305), San Bernardino County, California: U.S. Geological Survey Miscellaneous Field Studies Map MF 1618-B, 4 sheets, scale 1:62,500.

Simpson, R.W., Jachens, R.C., Blakely, R.J., and Saltus, R.W. 1986, A new isostatic residual gravity map of the conterminous United States with a discussion on the significance of isostatic residual anomalies: Journal of Geophysical Research, v. 91, p. 8348– 8372, doi: 10.1029/JB091iB08p08348.[GeoRef]

Spittler, T.E., and Arthur, M.A. 1982, The lower Miocene Diligencia Formation of the Orocopia Mountains, southern California: Stratigraphy, petrology, sedimentology, and structure, in Ingersoll R.V., Woodburne M.O. eds., Cenozoic nonmarine deposits of California and Arizona: Pacific Section, Society of Economic Paleontologists and Mineralogists, p. 83– 89.

Sweeney, R.E. 2002, Six aeromagnetic surveys in California, Nevada, and Arizona: A web site for distribution of data: U.S. Geological Survey Open-File Report 02–486 (http://pubs.usgs.gov/of/2002/ofr-02-486).

Terres, R.R. 1984, Paleomagnetism and tectonics of the central and eastern Transverse Ranges, southern California [Ph.D. thesis]: Santa Barbara University of California 323 p.

Toda, S., Stein, R.S., Richards-Dinger, K., and Bozkurt, S. 2005, Forecasting the evolution of seismicity in southern California: Animations built on earthquake stress transfer: Journal of Geophysical Research, v. 110, B05S16, doi: 10.1029/2004JB003415.[CrossRef]

U.S. Geological Survey 1981, Aeromagnetic map of the Needles 1° by 2° quadrangle, California and Arizona: U.S. Geological Survey Open-File Report 81–085, scale 1:250,000.

U.S. Geological Survey 1983, Aeromagnetic map of the Salton Sea area, California: U.S. Geological Survey Open-File Report 83–664, scale 1:250,000.

U.S. Geological Survey 1990, Aeromagnetic map of parts of the San Diego, Santa Ana, and adjacent 1° by 2° quadrangles, California: U.S. Geological Survey Open-File Report 90–206, scale 1:250,000.

Williams, P.L., Sykes, L.R., Nicholson, C., and Seeber, L. 1990, Seismotectonics of the easternmost Transverse Ranges, California: Relevance and seismic potential of the southern San Andreas fault: Tectonics, v. 9, p. 185– 204, doi: 10.1029/TC009i001p00185.[Web of Science][GeoRef]

Wooden, J.L., Tosdal, R.M., Howard, K.A., Powell, R.E., Matti, J.C., and Barth, A.P. 1994, Mesozoic intrusive history of parts of the eastern Transverse Ranges, California: Preliminary U-Pb zircon results: Geological Society of America Abstracts with Programs, v. 26, no. 2 p. 104– 105.

Yeats, R.S., Cole, M.R., Mershat, W.R., and Parsley, R.M. 1974, Poway fan and submarine cone and rifting of the inner southern California borderland: Geological Society of America Bulletin, v. 85, p. 293– 302, doi: 10.1130/0016-7606(1974)85<293:PFASCA>2.0.CO;2.[Abstract/Free Full Text]

Young, B.C. 1968, Geomorphology of the Pinto Basin, southern California [Ph.D. thesis]: Los Angeles University of California 294 p.

This Article Abstract Figures Only Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Langenheim, V.E. Articles by Powell, R.E. Search for Related Content GeoRef GeoRef Citation