Terrane Boundary Definition for the Borderland Geologic Framework SCEC5 Community Rheology Model

Mark R. Legg, & Marc J. Kamerling

Submitted August 15, 2019, SCEC Contribution #9862, 2019 SCEC Annual Meeting Poster #318

A Geologic Framework (GF) for the offshore southern California region (Borderland) is developed for the Community Rheology Model. The Borderland region of fault-bounded basins and ridges formed during the Miocene evolution of the PAC-NAM transform plate boundary when the subduction forearc region rifted obliquely away from the continental margin. Four major crustal blocks comprise the GF based on a tectonostratigraphic terrane model of the subduction zone architecture. The Patton accretionary wedge, Nicolas forearc basin, and Santa Ana volcanic arc represent the former subduction margin. The Catalina terrane of exhumed Catalina Schist subduction complex is wedged between the forearc and arc terranes along the coast. Initial terrane boundaries are modeled as simple high-angle fault zones. Some low-angle faults are included to model structure active during oblique rifting of the Catalina terrane. Borderland deformation accommodates >100 km of oblique-slip between major crustal blocks. Refinement of the Borderland GF defines segmentation of the boundary faults and distinct sub-terranes of the initial model. Segmentation of the Inner Borderland rift breakaway was produced by uneven lateral growth of the transform plate boundary due to the zigzag geometry of the East Pacific Rise as the Rivero Triple Junction migrated southward. Microplate capture of remnant Farallon plate slabs produced major jumps in the transform fault system growth. The resulting right-stepping echelon transform fault and pull-apart basin geometry resembles the modern Gulf of California transform fault system. Major lateral slip within the Gulf of California and the Inner Borderland occurs along right-slip faults within the rift, which must be included in the Borderland GF, whereas the boundary faults appear to preserve the initial rift boundary configuration. Volcanism along the rift boundaries produced varying subsurface rheology that may control subsequent fault activity. Reactivation of breakaway terrane boundaries may accommodate subsequent deformation within the region as the broader plate boundary configuration changes. Such diffuse fault systems may sustain large complex earthquake fault ruptures. Major low-angle faults associated with subduction and oblique rifting provide direct linkages between multiple high-angle strike-slip (oblique) faults to accommodate significant displacements during large earthquakes along complex plate boundary fault systems.

Key Words
Borderland, rheology, fault, tectonic evolution

Citation
Legg, M. R., & Kamerling, M. J. (2019, 08). Terrane Boundary Definition for the Borderland Geologic Framework SCEC5 Community Rheology Model. Poster Presentation at 2019 SCEC Annual Meeting.


Related Projects & Working Groups
SCEC Community Models (CXM)