ABSTRACT: Westward Tilting of the Tejon Embayment, Southernmost San Joaquin Valley, CA

Robert B. West

The Tejon embayment lies within the fault-bounded Tehachapi block, which lies between the Sierra Nevada and Transverse ranges. Previous studies of subsurface imaging have characterized the embayment as part of the northern fold and thrust belt of the Transverse Ranges. Geomorphic indicators support a Sierra Nevada-style block tilting character for the Tehachapi block.

The geomorphic history of alluvial fans of the northern Tehachapi Mountains, recorded under the influence of climatic and tectonic forces, may reflect the style of tectonic activity characteristic of the Tehachapi block as a whole. Fan-head entrenchment of alluvial fans east of Pastoria Creek resulted in stabilization of upper fan segments and subsequent pedogenesis of alluvial deposits. Soils on the upper segments of these fans show similar strength of hue, structure, clay films, and depth, indicating simultaneous stabilization of these surfaces.

Incision of each of these fans occurred on their west flanks and present drainage patterns on lower fan segments are to the west, indicating a common tectonic influence. Westward tilting can be inferred from these geomorphic features and the study area's proximity to the westward tilting Sierra Nevada. Elevations on a fan in the Tejon Embayment were compared to an untitled theoretical fan model. The results of this comparison are a measure of Late Pleistocene tilt of the Tehachapi block encompassing the Tejon embayment and average to 5.84m/Km with a maximum expected range of between 2.7 m/km and 7.0 m/km.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990