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

Abstract

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 eaststriking 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 defi ne basin structure for the block between the Pinto Mountain and Chiriaco fault zones. Estimates of offset from using the length of pullapart 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% confi dence 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.

CVM-S4.26

“…In the western Transverse Ranges, paleomagnetic data suggest cumulative clockwise rotation of about 70 o -90 o (e.g., Kamerling and Luyendyk 1985; Terres and Luyendyk 1985;Hornafius et al 1986;Luyendyk 1991), which is in agreement with earlier geological studies of disrupted lithotectonic belts (e.g., Jones et al 1976;Crouch 1979) and sediments of the Eocene Poway Group (Yeats et al 1974). For the eastern Transverse Ranges, paleomagnetic data record about 40 o of clockwise rotation between 10 and 4.5 Ma (Carter et al 1987;Powell 1993;Richard 1993;Dickinson 1996) which is in agreement with geophysical constraints in (Langenheim and Powell 2009). In Pliocene to Pleistocene, the southern segment of the Pacific-North America plate boundary shifted eastward into the Gulf of California and the plate boundary had to break a new path across southern California (e.g., Atwater and Stock 1998).…”

“…3b in Lee et al 2014b), which also show the east-west trend of the geomorphic features. In the eastern Transverse Ranges, surface rock types include Pliocene and Quaternary sediments, Proterozoic and Mesozoic plutonic and metamorphic rocks (Powell 1981(Powell , 1982Langenheim and Powell 2009). A series of large and mediumsized earthquakes occurred in the eastern Transverse Ranges during the 1980's and early 1990's ( Fig.…”

Subsurface Geometry of the San Andreas Fault in Southern California: Results from the Salton Seismic Imaging Project (SSIP) and Strong Ground Motion Expectations

“…This contrast is consistent with the average density contrast predicted by the velocity structure within the basin using the relationship of Gardner et al (1974) to convert velocity to density. Densities and magnetic susceptibilities assigned to basement rocks are guided by measurements of hand samples (Anderson et al, 2004;Langenheim and Powell, 2009). The Orocopia Schist (lower plate of the Chocolate Mountains fault, interpreted to be overturned in Figure 6.…”

“…10d and 11). Sedimentary rocks were chosen to have a density contrast with basement of between 300 and 400 kg=m 3 , and densities and magnetic susceptibilities for basement rocks were guided by measurements of hand samples (Anderson et al, 2004;Langenheim and Powell, 2009). Modeling of Figure 9.…”

A New Perspective on the Geometry of the San Andreas Fault in Southern California and Its Relationship to Lithospheric Structure

“…In modeling the magnetic data, we assume a planar contact between rocks having differing susceptibilities that intersects the land surface at the Holocene trace of the SAF. Physicalproperty measurements of basement rocks in these areas (Anderson et al, 2004;Langenheim and Powell, 2009) are used in our analysis.…”