Collaborative Research: Late Cenozoic Vertical Crustal Motions and Erosional Mass Transfer in the Southern San Andreas Fault Zone

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Vertical crustal motions are widely recognized in continental strike-slip fault zones, yet the underlying controls and surficial response to 3-dimensional strain in these settings are poorly understood. Observed patterns of uplift and subsidence often do no match the predictions of numerical models for oblique strain, suggesting that existing models for strike-slip faults are incomplete. Structural controls on development of sedimentary basins in strike-slip fault zones are similarly complex and incompletely understood. This project is addressing these problems with a multi-disciplinary, multi-investigator study of 3-dimensional strain and related surface processes in the San Andreas fault zone of southern California. The research team will use a diverse suite of methods to document rates and geometries of vertical crustal motions through time, and test two hypotheses for the evolution of the San Andreas fault: (1) that plate-motion obliquity exerts the primary control on the 3-dimensional and temporal evolution of the fault zone; and (2) that the fault zone experienced a major change at approximately 1.1 to 1.4 million years ago in response to tectonic reorganization of the plate boundary. Each hypothesis makes unique predictions about space-time patterns of uplift, erosion, subsidence, and sediment dispersal within the fault zone, that will allow the team to test the hypotheses with a systematic program of fieldwork, data analysis, and modeling. This project integrates diverse research methods including geologic mapping, stratigraphic and structural analysis, paleomagnetic studies of sediment age and block rotations, provenance analysis, detrital zircon dating, low-temperature (U-Th)/He dating of bedrock sources, geomorphic analysis, study of seismic and gravity data, and numerical modeling.This study seeks to fill large gaps in the understanding of the geologic evolution of the southern San Andreas fault system, a complex network of seismically active faults that define the Pacific-North America plate boundary in California. The history of deformation over geologic timescales (millions of years) is relatively poorly known, despite its critical role in shaping the crustal architecture and fault geometries that control earthquakes in this setting. This project's approach benefits from a unique collaboration of academic researchers and students from four universities with earth scientists at the U.S. Geological Survey. The team is also collaborating with geophysicists investigating processes of continental rupture beneath the Salton Sea, and scientists studying paleoseismology and fault slip rates on the San Andreas fault over shorter timescales. These collaborations provide an important avenue for engaging with and contributing new knowledge to the vibrant geoscience community in southern California. Lessons learned in this research will be used to develop new lab and teaching exercises that will reach thousands of students over the course of the project. Ultimately, the results of this study will shed new insights into dynamic linkages between crustal deformation, fault-zone complexity, growth of topography, erosion, and sediment dispersal within continental strike-slip fault zones at active plate boundaries.