Very long baseline interferometry and active rotations of crustal blocks in the Western Transverse Ranges, California

Changes in baseline vectors between very long baseline interferometry (VLBI) receiving stations in the Western Transverse Ranges imply that east-west blocks of crust in this region rotate clockwise about vertical axes with respect to the Pacific and North American plates. Minimum apparent rotations, given by the ratios between components of velocity perpendicular to baseline vectors and the lengths of the baselines, imply minimum current rotation rates of a few degrees per million years. The relevant VLBI receivers lie on different crustal blocks that are separated by major active faults. Both geologic and other geodetic observations imply north-south convergence between such blocks at several millimeters per year. Corrections to perpendicular components of velocity for such relative movements between blocks yield likely clockwise rotation rates of 6°/m.y. ± 2°/m.y., which are indistinguishable from the average rate inferred from paleomagnetic declinations of rocks in the Western Transverse Ranges with ages less than 15 m.y. Thus, rotation seems to have occurred continuously and apparently with only small variations in rate during a period when the tectonics of southern California changed dramatically. This apparent independence of the rotation rate on the changing surface kinematics is consistent (1) with such rotation being a manifestation of continuous deformation at depth in the lower crust and upper mantle, (2) with weak faults separating upper-crustal blocks, and (3) with the important resistance to continental deformation lying in the upper mantle and/or lower crust.