VERY LONG-BASE-LINE 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 vertic al axes with respect to the Pacific and North American plates. Minimum apparent rotations, given by the ratios between components of velocit y perpendicular to baseline vectors and the lengths of the baselines, imply minimum current rotation rates of a few degrees per million year s. The relevant VLBI receivers lie on different crustal blocks that ar e separated by major active faults. Both geologic and other geodetic o bservations imply north-south convergence between such blocks at sever al millimeters per year. Corrections to perpendicular components of ve locity for such relative movements between blocks yield likely clockwi se rotation rates of 6-degrees/m.y. +/- 2-degrees m.y., which are indi stinguishable from the average rate inferred from paleomagnetic declin ations of rocks in the Western Transverse Ranges with ages less than 1 5 m.y. Thus, rotation seems to have occurred continuously and apparent ly with only small variations in rate during a period when the tectoni cs of southern California changed dramatically. This apparent independ ence of the rotation rate on the changing surface kinematics is consis tent (1) with such rotation being a manifestation of continuous deform ation at depth in the lower crust and upper mantle, (2) with weak faul ts separating upper-crustal blocks, and (3) with the important resista nce to continental deformation lying in the upper mantle and/or lower crust.