SCEC Award Number 13091 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Tectonic Evolution of Transpressional Fault Systems - California Continental Borderland and Adjacent Coastal Region
Investigator(s)
Name Organization
Mark Legg Legg Geophysical
Other Participants 1 student may be involved, SCEC intern is feasible
SCEC Priorities 1, 4, 2 SCEC Groups SDOT, USR, Geology
Report Due Date 03/15/2014 Date Report Submitted N/A
Project Abstract
 A model for restraining bend development along an evolving continental transform margin is evaluated to explain the geometry and deformation history of major transpressional fault systems offshore southern California. The focus for this study was the Santa Cruz-Catalina Ridge (SCCR) fault zone that comprises the northern section of the larger San Clemente fault system. Santa Catalina Island at the southern end of the SCCR is recognized as a major restraining bend that developed from a major NW-trending transform fault that grew within the Inner Borderland during the middle Miocene rifting of the Western Transverse Ranges block from the North America continental margin. The island and ridge represent transpressional pop-up structures that developed following the late Miocene to early Pliocene clockwise rotation of the Pacific-North America relative motion vector. The Palos Verdes Anticlinorium and Wilmington Anticline are considered to represent other restraining bend pop-ups that formed by this mechanism within this system of right-stepping en echelon transforms and linking pull-apart basins of the Miocene Inner Borderland Rift. Interpretation of seismic reflection profiles along the SCCR show that the transpressional uplift began in late Miocene time, but also may have accelerated in late Pleistocene time, similar to uplift of the PVA. Triple junctions are created when new faults form to bypass the restraining bends and represent a distinctive feature of this process that resembles the geometry of the southern San Andreas fault system. Similarity in structural geometry represents an important aspect of the transpressional process and continental transform plate boundary evolution.
Intellectual Merit The tectonic model being tested in this project provides an elegant approach to attack a challenging problem in plate tectonics for one of the longest continental transform fault margins on Earth. The California Continental Borderland has been described as an “organized train wreck” (Charlie Hollister, pers. commun., 1998) which has “enjoyed” countless and widely-varying attempts to model its tectonic evolution – and too often described as being too difficult to understand. Hopefully, this research will help to remove the stigma that studies of the geology of the California Continental Borderland are “Provinicial” – and only of local interest.
Broader Impacts The project is focused on the geometry and history of deformation along major transpressional fault zones located offshore southern California, which represents a significant threat to the heavily populated coast due to large earthquakes and potential local tsunamis. However, the overall process of plate boundary evolution being modeled and tested in offshore basins is directly relevant to those located onshore. In particular, the geometrical similarity of the faulting within the San Clemente fault system to that of the San Andreas fault zone shows the obvious relevance of the model to understanding the full plate boundary in southern California. Furthermore, due to more than 1,000 km of right shear along the Pacific-North America plate boundary since early Miocene time, this model may provide important tools to understand the transform fault evolution from the Gulf of California to Cape Mendocino. Other complex plate boundaries along continental margins, e.g. British Columbia, Caribbean, Indonesia and Sumatra, may also involve similar processes that may find this model appropriate to help provide better models for tectonic deformation and earthquake hazards in those areas. Interaction with colleagues in academia, government, and industry represent at crucial component of this research, which would be impossible without such collaboration. One student (Sergey Ishutov) completed his M.S. thesis based on aspects of this research at CSU Long Beach in 2013. Dr. Legg is continuing this collaboration with Dr. Robert ‘Dan’ Francis and students at CSULB. For the exploration industry, results of this research will provide better understanding of the structural geometry and evolution of inverted strike-slip basins that hold significant quantities of hydrocarbons in southern California and elsewhere. This brief SCEC project represents the culmination of Dr. Legg’s Borderland research, yet is only a beginning of the more detailed plate boundary reconstruction. Future research may discover major flaws in the model, but may also provide additional confirmation or refinement that result in significant advances in our understanding of this complex active plate boundary.
Exemplary Figure Figure 1. Map showing geometric similarity of two major transform fault systems – San Andreas and San Clemente – of the Pacific-North America plate boundary in southern California. Focal mechanisms of moderate (M>5) offshore earthquakes are shown: black for strike-slip, purple for reverse, and red for normal faulting (Corbett, 1980; Legg, 1980; 1986; Hauksson and Jones, 1988; Hauksson and Gross, 1991; Hauksson et al, 2013).
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