ABSTRACT

Development of strain partitioning in transtensional rifts may play a significant role in localizing continental rupture. Partitioning of extension and dextral shear in the proto-Gulf of California during Late Miocene time (12 to 6 million years ago) remains a fundamental unresolved problem that hinders understanding of the initial conditions for ocean basin formation. Preliminary evidence from coastal Sonora is supportive of a regional model that promotes dextral transtension in a zone of uncertain width prior to localization of plate-boundary motion in the Gulf of California approximately 6 million years ago. It is not known whether Late Miocene dextral shear was sufficiently fast or focused to alter rheological conditions and change the mode of rifting from distributed Basin-and-Range-style extension to localized lithospheric necking, subsidence, and continental rupture. In this project U.S. and Mexican scientists aim to constrain the structural distribution of dextral shear over time during localization of the plate boundary. Specifically, they will determine whether late Miocene transtension in the proto-Gulf of California was accommodated by diffuse 3-dimensional strain in a wide zone of coeval strike-slip and normal faults or occurred in a narrow zone of discrete dextral faults and/or rotating blocks embedded within a broader zone of East-West extension. To discriminate between these models, the team is investigating the geologic structure and basin development of the Sonoran continental margin, including adjacent Isla Tiburon and the offshore Sonora shelf by: (1) documenting the distribution of dextral shear through detailed structural mapping and paleomagnetic analysis; (2) quantifying transtensional strain rates using structural reconstructions, 40Ar/39Ar dating of intercalated volcanic rocks, and subsidence analysis of syntectonic sedimentary and volcanic basins; and (3) integrating the onshore geologic record with offshore structures and basins by interpretation of existing multi-channel seismic data from the offshore Tiburon basin.

The Gulf of California provides an excellent natural laboratory for studying processes of continental break-up. Previous studies have shown that warm continental crust tends to resist localized deformation, which is required for rupture of continental lithosphere. Data from this study will improve understanding of how and why the warm crust of western North America ruptured to form the Gulf of California, which has been opening since about 6 million years ago. This project is supported by the EAR Tectonics Program, the NSF Office of international Science and Engineering, and the NSF/OCE MARGINS program. The research involves significant participation of graduate and undergraduate students, collaboration with Native Americans, and strong international partnerships with research institutions in Mexico.

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