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Abstract: Kinematics and Dynamics of the Western Transverse Ranges

Eugene Humphreys

The western Transverse Ranges consist primarily of clockwise-rotating elongate blocks separated by left-lateral and thrust faults (Luyendyk et al., 1985; Jackson and Molnar, 1990). However, the San Gabriel block, and probably the adjoining crust that lies north of the Ventura Basin, rotate slowly counterclockwise. This results in a narrow zone of convergence in the Ventura Basin region. West of the Ventura Basin region convergence is broadly distributed as the clockwise-rotating blocks shorten and thicken to become the crust of the Coast Ranges. This model has the crust of the Coast Ranges being shortened most rapidly at the time they pass by the vicinity of Big Bend in the San Andreas fault.

Tomographic images of the upper mantle suggest that the dense mantle lithosphere descends beneath the Transverse Ranges as far west as the Ventura Basin. In addition to the stresses guided through the crustal blocks, the sinking lithosphere contributes forces that drive Transverse Ranges convergence. The upper mantle structure is inconsistent with the hypothesis that a broad simple-shear flow field in the upper mantle (created by Pacific-North America plate interaction) drives crustal block rotation and convergence (Jackson and Molnar, 1990), at least for the Ventura Basin and regions to the east. The crust west of the Ventura Basin has been stripped of its mantle lithosphere and is thus weak and easily deformed into the Coast Ranges.

AAPG Search and Discovery Article #90981©1994 AAPG Pacific Section Meeting, Ventura, California, April 27-29, 1994