Complex History of San Pedro Basin Fault and the Development of San Pedro Basin; Preliminary Results from High-Resolution Seismic Reflection Data

R. D. Francis¹ and M. R. Legg^2
1California State University, Long Beach, [email protected]
²Legg Geophysical, Huntington Beach, California, [email protected]

We mapped the San Pedro Basin fault (SPBF) across San Pedro Basin. The fault divides the basin into two parts with possibly different tectonic histories. The fault has deformed sediments younger than 70 ka, suggesting activity into latest Pleistocene or Holocene time.

The stress regime of the SPBF has varied over time and along strike. Dip discordance across the fault occurs as deep as 2.5-3 km below the seafloor. In the southern part of the Basin strata dip toward the fault on both sides forming a trough, suggesting a transtensional regime in this area. This condition persisted until at least late Pleistocene time (70 to 300 ka). Very deep sediments (more than 1.5 km below the seafloor; age unknown) are poorly imaged, but apparently the transtensional regime was weaker or non-existent when they were deposited.

In contrast, strata in the northern part of the Basin east of the SPBF dip away from the fault, forming a large depocenter about 4 km to the northeast. This argues against transtension along the fault in this area. Highly disturbed rocks associated with Redondo Knoll occur west of the fault. Only the youngest sediments (about 70 ka) continue over the fault and a subsurface projection of Knoll rocks southwest of the fault. This projection stops about 5 km southeast of the Knoll, southeast of which the trough previously described exists.

Sediment packages are found on some of the slopes surrounding San Pedro Basin, including Redondo Knoll, Catalina Ridge, and the slope south of San Pedro Sea Valley. These strata have divergent dips toward the Basin, and onlap up-slope, away from the Basin. This suggests uplift and tilting, particularly west of the SPBF. Ages of these sediments, and relationship of this uplift to the SPBF and formation of the Basin, are still to be determined.

AAPG Search and Discovery Article #90088©2009 Pacific Section Meeting, Ventura, California, May 3-5, 2009