Tectonic evolution of transpressional fault systems: California Continental Borderland and adjacent coastal areas

Mark R. Legg

Published September 2013, SCEC Contribution #2050

Major transpressional fault systems in the southern California coastal and offshore region pose severe earthquake hazards to the region with the potential for strong ground shaking and local tsunami inundation. Geological structure inherited from Neogene oblique rifting of the Inner Borderland was reactivated by post-Miocene oblique convergence when the locus of plate boundary deformation jumped eastward to the southern San Andreas and Gulf of California transform fault systems. Crustal thinning during oblique rifting followed by basin inversion and uplift raised lower crustal structure to shallow depths or the seafloor where imaging with high-resolution geophysics enable detailed mapping of complex fault systems. Linked networks of middle Miocene transtensional structures including low-angle normal faults (detachments), high-angle strike-slip faults (transforms) and oblique-normal faults may result in large complex earthquake ruptures. Three major transpressional fault systems are investigated to map the complex faulting and related deformation: 1) San Clemente fault system (Santa Cruz-Catalina Ridge); 2) Palos Verdes fault system (PV, San Pedro Bay); 3) Newport-Inglewood-Rose Canyon fault system (NIRC, South Coast). Uplifted basin sediments record the evolution and character of the deformation.
A model for transpression and restraining bend development is based on changing relative plate motions and boundary geometry. In Phase 1, right-stepping en echelon transform faults form to link extensional “pull-apart” basins during oblique-rifting. In Phase 2, northward rotation of the relative plate motion vector and the eastward jump in the plate boundary create transpression on NW-trending transform faults and inversion of Miocene basins. The THUMS-Huntington Beach fault may represent the last stage where a former transform fault was abandoned as new faults by-pass the restraining bend. The PV and NIRC fault zones provide straighter paths for right-slip. The same process occurred at the Santa Catalina restraining bend and is active now at the San Bernardino Mountains segment of the San Andreas fault. Repeated structural style reveals the active process of transpressional fault system evolution within the PAC-NAM plate boundary in southern California. Process details, including strain partitioning between strike-slip and dip-slip, provide important criteria to estimate the character of large complex earthquakes that may strike this heavily populated region.

Legg, M. R. (2013, 9). Tectonic evolution of transpressional fault systems: California Continental Borderland and adjacent coastal areas. Poster Presentation at SCEC Annual Meeting 2013.