SCEC Award Number 19205 View PDF
Proposal Category Individual Proposal (Data Gathering and Products)
Proposal Title Develop Geological Model of Offshore Southern California (California Continental Borderland) for the Community Rheology Model - Continuation
Investigator(s)
Name Organization
Mark Legg Legg Geophysical
Other Participants
SCEC Priorities 3a, 1b, 5b SCEC Groups CXM, Geology, SDOT
Report Due Date 03/15/2020 Date Report Submitted 04/03/2020
Project Abstract
 The California Continental Borderland Geologic Framework is based upon the tectonostratigraphic terranes that define the major crustal blocks related to the former subduction zone. The initial model includes the Patton accretionary wedge, Nicolas forearc, Catalina subduction complex, and Pacific oceanic crust. Refined definitions of the complex boundaries are recommended to describe the fault character more accurately. Major terrane boundary fault zones may be described as fault wedges formed by transpressive inversion of Miocene transtensional basins after oblique rifting ceased within the northern Borderland. These complex fault systems may produce large complex earthquakes involving multiple faults and fault segments that are difficult to model with the simple preliminary fault model. Subdivision of the original four crustal blocks is recommended to improve the description of complex offshore crustal character. Terrane subdivisions include splitting the Inner Borderland Rift Catalina terrane into 1) western Core Complex, 2) eastern tilted crustal blocks with Detachment Faults, and 3) southern San Clemente Basin rhombochasm terranes. Additional subdivisions for Outer Borderland terranes may be useful, such as a northern, central and southern Nicolas terrane. Tectonic regionalization of the southern California crust based on the 3D SCEC Community Velocity Model (CVM), shows anomalous features offshore where tomographic data resolution is good. Further refinement of the Borderland Geologic Framework, based on comprehensive knowledge of offshore structure and tectonic evolution, is necessary to capture important mid-crustal and deeper features missing from the CVM where sparse seismograph coverage limits tomographic resolution.
Intellectual Merit The tectonic model being developed in this project will be used to evaluate earthquake potential offshore southern California and related hazards including strong shaking, induced ground failures, and tsunami. The complex deformation recognized in the California Continental Borderland has been described as an “organized train wreck” (Charlie Hollister, 1998 pers. commun.) and as a tectonic “log jam” (Legg et al. 2015). Accurate geologic framework is needed to produce a realistic Community Rheology Model that may be used for dynamic fault rupture and crustal deformation models to evaluate earthquake hazards. Predictions from deformation models may be tested against the observations used to develop the Geologic Framework for the CRM. Errors or weaknesses in the model may be identified and experiments designed to obtain new data for updating and improving the model. Ultimately, a more accurate and higher-resolution description of the tectonic evolution of the complex Pacific-North America plate boundary will result. More detailed understanding of subduction, oblique-rifting, transpression, and strike-slip faulting may be developed.
Broader Impacts The project is focused on the geology of the major crustal blocks and intervening fault boundaries located offshore southern California, which represents a significant threat to the heavily populated coast due to large earthquakes and potential local tsunamis. The offshore region (Borderland) represents a significant active part of the southern California tectonic plate boundary which must be considered to understand the tectonic evolution of southern California and the regional seismic hazard. Interaction with colleagues in academia, government, and industry represent at crucial component of this research, which would be impossible without such collaboration. Dr. Legg is collaborating with Dr. Mike Oskin and student at UC Davis to prepare the Geologic Framework in a consistent manner for development of the Community Rheology Model. In addition, Dr. Legg collaborates with Dr. Chris Sorlien, Dr. Craig Nicholson, Dr. Marie-Helene Cormier and Dr. Marc Kamerling to share knowledge and improve understanding of the complex tectonic evolution of the Borderland. Interaction with scientists and others involved with the Ocean Exploration Trust in continuing deep sea exploration of the Borderland provides unique opportunities to obtain additional data (seafloor samples, video, etc.) and to engage the broader community (public) with the direct interaction possible through distance learning during the live webcasts of these exploration activities. Presentation of results in scientific meetings as well as in local meetings of geological societies and through interactions at local universities informs the larger audience of both technical and non-technical people interested in the regional geology and natural hazards. Presentations of data related to this project were also made to the Catalina Island Conservancy and to help NASA scientists train astronauts for geological sampling of the Moon and Mars.
Exemplary Figure Figure 1. Map showing the major fault terrane boundaries (red lines) and original tectonotratigraphic terranes (after Howell & Vedder, 1981) with magnetic anomalies (EMAG2) and waveform-relocated seismicity (Hauksson et al. 2012, 2019 update). Dashed white lines show possible region boundaries. PST = Punta Santo Tomás, SNI = San Nicolas Island. SDT = San Diego Trough, SMB = Santa Monica Basin, 60MB = Sixtymile Bank, SD = San Diego, SCL = San Clemente Island, SCZ = Santa Cruz Island, DP = Dana Point, NEB = Northeast Bank, CAT = Santa Catalina Island. Bathymetric contours on base map from Legg (1985).