Uplift of the Palos Verdes Tectonic Block from 600 ka, As Evidenced by Mapping of the Palos Verdes Fault, Pre- and Syn-Uplift Deposition, and Related Structures: Results of High-Resolution Offshore Seismic Imaging
Robert D. Francis1 and Mark R. Legg2
1Department of Geological Sciences, California State University, Long Beach, 1250 Bellflower Blvd., Long Beach, CA 90840
2Legg Geophysical, Huntington Beach, California, 16541 Gothard St., Suite 107, Huntington Beach, CA 92647
Mapping, using 1,400 km of high-resolution seismic reflection data and previously published USGS multibeam bathymetry, suggests that the uplift of the Palos Verdes Peninsula and nearby offshore San Pedro shelf was episodic, occurring largely in the last 600,000 years, rather than at a more or less constant rate over 2 or 3 million years. We consider that the Palos Verdes Peninsula and shelf west of the Palos Verdes fault (PVF) make up a more or less coherent tectonic unit, the Palos Verdes tectonic block. Prior to 600 ka this block underwent some uplift, with erosion mostly keeping up, producing a widespread 600 ka lowstand surface. After 600 ka, arching of the Palos Verdes tectonic block on the PVF caused the emergence of the Peninsula, erosion into Middle Miocene units on the shelf, and up warping of the 600 ka surface. Thus, an unconformity formed (our “unconformity A”) on the west side of the PVF, dipping into the fault and onlapped by Pleistocene Timms Point Silt and equivalent 600-220 ka sediments. West of unconformity A (which wraps around the peninsula and extends to a point 17 km offshore), Pliocene and older rocks are exposed in the Palos Verdes anticlinorium (PVA). The segment of the PVF bounding the Palos Verdes tectonic block (Palos Verdes peninsula to the shelf edge) is a transpressional segment 37-40 km long. On the slope to the south is an apparent transtensional, or nearly pure strike-slip zone, called by us the San Gabriel Transitional Zone (SGTZ); still farther south is another apparent transpressional segment involving Lasuen Knoll. Such segmentation along trend is typical of major strike-slip faults in the California Continental Borderland.
AAPG Search and Discovery Article #90076©2008 AAPG Pacific Section, Bakersfield, California