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Low Rate of Quaternary Uplift Across the Hollywood Fault, Southern California: New Observations Based on a Mid-Pleistocene Marine Terrace

Thomas K. Rockwell, Scott C. Lindvall, & Gerry Kasman

In Preparation 2002, SCEC Contribution #656

Using nearly 4500 m (15,000 feet) of new hollow-stem auger core that we collected in the City of West Hollywood, California, we identify one and possibly two marine abrasion platforms in the subsurface beneath and south of Sunset Boulevard. The abrasion platforms cut a prominent sea cliff escarpment along the base of the Hollywood Hills, which has been heretofore interpreted as an escarpment marking the location of the main Hollywood fault. We have traced the abrasion platform southward from Sunset Boulevard and show that the main Hollywood fault must be at least 250 m south of Sunset Boulevard at La Cienega Boulevard. Similarly, we have traced the paleo-shoreline of the lower platform over a lateral distance of several hundred meters at about an elevation of 100 m, with a slightly higher elevation in an area of structural complexity. Overlying the lower marine abrasion platform is a sequence of alluvial deposits that are separated by well-developed soils. Comparison of the soils with dated soils in the southern California region suggests that the underlying abrasion platform is on the order of 400-900 ka in age. This age compares favorably with the age of the marine sediments identified in the La Brea Plain, south of our study area. Using this age range,
we calculate a long-term middle to late Quaternary uplift rate for the Hollywood Hills to be 0.11-0.25 mm/yr. Using the elevations and ages of the marine sediments (San Pedro Fm) in the La Brea Plain determined by Quinn et al. (2000), we calculate an uplift rate of the footwall of the Hollywood fault to be about 0.14 mm/yr. Thus, the differential uplift rate across the Hollywood fault on the order of 0-0.11 mm/yr. These results indicate that the component of shortening that can be attributed to the Hollywood fault is small and that other structures in the northern Los Angeles basin region must account for the 5-8 mm/yr of contraction between the Palos Verdes peninsula and Pasadena (JPL) observed in GPS measurements.

Rockwell, T. K., Lindvall, S. C., & Kasman, G. (2002). Low Rate of Quaternary Uplift Across the Hollywood Fault, Southern California: New Observations Based on a Mid-Pleistocene Marine Terrace. Geology, (in preparation).