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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

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).

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.

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.

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 (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.

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.

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.

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.

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).