Active faulting and block rotations in the Western Transverse Ranges, California

Abstract

The geometry of faulting, slip vectors of major earthquakes, and results of very long baseline interferometry (VBLI) are consistent with active clockwise rotation of crustal blocks in the western Transverse Ranges about vertical axes. Oblique left‐lateral thrust faults trending roughly east‐west across the western Transverse Ranges apparently separate long east‐west blocks of upper crust, and the large components of lea‐lateral shear on east striking planes suggest that the blocks rotate with respect to the San Andreas fault in the Mojave region (between the western Transverse Ranges and the Mojave Desert). Paleomagnetic declinations indicate some 35° (and possibly 60°) of clockwise rotation since about 10 Ma, and more importantly, the relative velocity between the only two, frequently measured, VBLI sites in the western Transverse Ranges is consistent with a rotation rate of 6°/Ma (±3°/Ma), during a 5‐year period in the 1980s. Azimuths of slip vectors of major earthquakes are essentially orthogonal to the segment of the San Andreas fault across the western Mojave Desert, indicating that crustal shortening across the region is absorbed by crustal thickening and not by lateral translation (extension) of material parallel to that segment of the fault. Plate motions, Late Quaternary offsets on the San Andreas and San Jacinto faults, and geodetic measurements in the Mojave region yield a calculated direction of relative motion between the San Gabriel Mountains and the Pacific plate (across the western Transverse Ranges) that also is perpendicular to the San Andreas fault in the Mojave region. Rotation rates associated with simple mechanisms for applying essentially normal tractions to the lateral margins of blocks are not consistent with the observed geometry of faulting and with average rotation rates. The rotation rate of 2.8–5.3°/Ma calculated assuming that the long, east‐west blocks rotate in response to a uniform compression of a viscous substratum to the western Transverse Ranges, however, overlaps the range of observed average rotation rates. Thus, the hypothesis that blocks of continental crust move as if carried passively by a substratum undergoing continuous deformation passes one test that can be made of it.