Shear Zone Faulting, Basin Evolution, and Volcanism along a Continental Margin Transform System: San Pedro Basin, Offshore Southern California
Authors: Francis Robert D. , Legg Mark R. , Castillo Chris M. 
Published: May 2020

Study of shear zones and associated basins within an oblique rift can shed light on the development of a young transform continental margin. San Pedro Basin lies within the Inner Borderland Rift offshore southern California, where it is bisected by the San Pedro Basin Fault (SPBF). Based on seismic reflection and multibeam bathymetry data, we show that the SPBF attained continuity with the San Diego Trough Fault (SDTF) between 1 Ma and 800 ka, to form a 350-km-long shear zone. Prior to that time, the SDTF was linked to the Catalina Fault, forming a restraining bend that contributed to the uplift of Catalina Ridge.

Seismically defined depositional sequences in San Pedro Basin record a multistage history of uplift and subsidence for the basin. Young, flat-lying sequences filling a sigmoidal depocenter indicate that subsidence has been occurring since about 1 Ma. This date is corroborated by a series of submarine lowstand depositional terraces surrounding Santa Catalina Island. A 5 Ma to 1 Ma progressively tilted sequence, onlapped by the flat-lying strata, is confined to the present basin. Folded sequences older than ca. 5 Ma extend beyond the present basin onto Catalina Ridge and are correlated to Mohnian and Luisian strata on Santa Catalina Island and Palos Verdes Peninsula.

From these data, we interpret the growth history of San Pedro Basin to involve at least three successive, nested basins. The first, which we call the “San Pedro (SP) protobasin,” formed before 5 Ma and was of indeterminate size, including within its boundaries areas flanking the current basin that were subsequently uplifted, Catalina Ridge and Palos Verdes Anticlinorium. Between 5 Ma and 1 Ma, approximately, a second basin, nested within the first, formed as the two flanking structural highs initiated. Finally, a third basin, nested within the first two, began to form when the SPBF–SDTF link was established and rapid local subsidence began; this is the depocenter of the current San Pedro Basin, and its southwestern boundary is occupied by the trace of the SPBF.

Our model of basin formation begins with the initial oblique Inner Borderland (IB) Rift, which formed during rotation and translation of crustal blocks away from the continental margin (about 20 Ma). The IB Rift was segmented due to preexisting structural configurations. Published reconstructions show that the SP protobasin was originally in a narrow zone flanked by active volcanoes. Continued extension widened and deepened the rift, while volcanism continued along the flanks of the rift until about 10 Ma. As the rift widened, nested basins formed within the original protobasins along the axis of the rift. These basins were later fragmented (after 5 Ma for the SP protobasin) by transpressive processes associated with the shift of the transform plate boundary to the southern San Andreas Fault. New nested basins also formed during this time as shear zones reorganized to shortcut restraining geometries.