Offshore Geology Framework for the Community Rheology Model

Mark R. Legg, & Michael E. Oskin

Published August 15, 2018, SCEC Contribution #8742, 2018 SCEC Annual Meeting Poster #151

The California Continental Borderland comprises the 250-km wide offshore part of the Pacific-North America transform plate boundary in southern California. The Offshore Geologic Framework (GF) for the SCEC Community Rheology Model (CRM) is based on the large-scale crustal blocks represented by the tectono-stratigraphic terranes (Howell and Vedder, 1981). These terranes represent basement lithology and structure derived from the Farallon-North America subduction zone and dismembered and rearranged during the continental margin transform plate boundary evolution. Four blocks south of the Transverse Ranges are included in the simple model. The Inner Borderland is represented by the offshore Catalina oblique-rift terrane and the coastal Santa Ana (Peninsular Ranges) terrane. The Outer Borderland is represented by the Nicolas forearc and Patton accretionary wedge terranes. A fifth terrane consists of the Pacific oceanic lithosphere adjacent to the Patton Escarpment. Published geological and geophysical cross-sections of the California Continental Borderland provide data for creating the simple 1-D lithospheric columns. The block boundaries for the simple model were originally envisioned as high-angle strike-slip fault zones (Vedder, 1987). Borderland GF block boundaries are derived from major faults that exist in the SCEC Community Fault Model (CFM). Most faults are high-angle, strike-slip in character. However, known major low-angle faults (detachments) are important components of a 3-D block boundary definition. The low-angle faults imaged by reflection seismology and mapped throughout the Inner Borderland Rift include the Thirtymile Bank and Oceanside/Coronado Bank structures. In the Outer Borderland terranes, one major low-angle fault is represented by the former subduction megashear that dips eastward from the base of the Patton Escarpment. Low-angle faults exist in the CFM, and these will be updated as needed to define significant block boundaries in the GF. Outer Borderland faults missing from the CFM are being compiled and prepared for loading into the updated Community Models. We consider upgraded GF models to include important sub-terranes that describe more complex structure within each of the simple blocks. Four sub-terranes within the Inner Borderland Rift include: 1) Catalina core complex; 2) Santa Monica-San Diego Trough, adjacent rift basins; 3) coastal rifted continental crust (Coronado Bank); 4) San Clemente Basin, a highly extended rhombochasm.

Key Words
Offshore geology, crustal structure, rheology, Borderland

Citation
Legg, M. R., & Oskin, M. E. (2018, 08). Offshore Geology Framework for the Community Rheology Model. Poster Presentation at 2018 SCEC Annual Meeting.

Related Projects & Working Groups
SCEC Community Models (CXM)