Shallow Structure and Geomorphology of the Hosgri Strike-Slip Fault, Offshore Central California

Johnson, Samuel Y.; Watt, Janet T.; and Hartwell, Stephen R.
[email protected]

We mapped a ~94-km-long portion of the right-lateral Hosgri Fault from Point Sal to Piedras Blancas in offshore central California using high-resolution seismic reflection profiles, marine magnetic data, and multibeam bathymetry. The database includes 121 seismic profiles across the fault zone and is perhaps the most comprehensive reported survey of the shallow structure of an active strike-slip fault. These data document the location, length, and near-surface continuity of multiple fault strands, highlight fault-zone heterogeneity, and demonstrate the importance of fault trend, fault bends, and fault convergences in the development of shallow structure and tectonic geomorphology.

The Hosgri Fault is continuous through the study area passing through a broad arc in which fault trend changes from ~ 338° to 328°. The southern ~40 km of the fault zone in this area is more extensional, resulting in accommodation space that is filled by deltaic sediments of the Santa Maria River. The central ~24 km of the fault zone is characterized by oblique convergence of the Hosgri Fault with the more northwest-trending Los Osos and Shoreline faults. Convergence between these faults has resulted in the formation of local restraining and releasing fault bends, transpressive uplifts, and transtensional basins of varying size and morphology. We present a hypothesis that links development of a paired fault bend to indenting of the Hosgri Fault by a strong crustal block translated to the northwest along the Shoreline Fault.

Two diverging Hosgri Fault strands bounding a central uplifted block characterize the northern ~30 km of the Hosgri Fault in this area. The eastern Hosgri strand passes through releasing and restraining bends; the releasing bend is the primary control on development of an elongate, asymmetric, "Lazy Z" sedimentary basin. The western strand of the Hosgri Fault passes through a significant restraining bend and dies out northward where we propose that its slip transfers to active structures in the Piedras Blancas fold belt.

AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013