Late Miocene Faults and Basins in Coastal Sonora, Mexico

Participants in this NSF-funded study include Tom Peryam (Ph.D. student here at the UO), Molly Keogh (UO undergraduate student), my colleague Mike Oskin at U.C. Davis, Mike's student Scott Bennett, and our Mexican colleagues Arturo Martín, John Fletcher, and Mario Gonzalez at CICESE in Ensenada, and Alex Iriondo at UNAM in Juriquilla. In this project we are studying a belt of well exposed sedimentary and volcanic rocks in coastal Sonora (including Isla Tiburon, see map below), to understand the timing, style, and kinematics of Late Miocene faulting that led to continental rupture and opening of the Gulf of California. This portion of the Gulf of California rift margin offers a unique opportunity to correlate the onland geologic record to offshore faults and basins imaged with existing seismic reflection data. We have started our investigation of the onland record through analysis of syntectonic faults and basins, mapping and measuring of sedimentary and volcanic basin-fill, dating of pre-, syn- and post-basinal volcanic rocks, and paleomagnetism. These data will be integrated with a concurrent study of offshore faults and basins in the adjacent Gulf of California and on the conjugate rifted margin in NE Baja California, in collaboration with our Mexican colleagues at CICESE. The results of our multi-disciplinary analysis will be compared to predictions of hypotheses for the kinematic development of the Pacific-North American plate boundary in this region, and will allow us to assess the role of transtensional strain in localizing lithospheric rupture that created the modern Gulf of California.

The map above shows the topography, bathymetry, and active faults and basins of the northern Gulf of California. Previous work by Mike Oskin documented that the Tuff of San Felipe (12.6 Ma) and Tuffs of Mesa Quadrada (6.3 Ma) are exposed in both coastal Sonora (including Isla Tiburon) and the Puertecitos area of northern Baja California, and were separated by oblique rifting across the northern Gulf after 6.3 Ma. In addition, however, studies by Mike Oskin and Phil Gans have yielded preliminary evidence for Late Miocene (6-12 Ma) slip on NW striking strike slip faults and associated CW rotation and crustal tilting in coastal Sonora, which suggests that some dextral shear occurred in this region prior to 6 Ma. But the details of that faulting episode are poorly understood. This sets the stage for our present study, which seeks to document the timing, style and kinematics of Late Miocene faulting and related basin evolution in coastal Sonora.

The seismic reflection line above, from Aragon and Martin (2007), provides an example of the high quality seismic data that are available - but have not yet been processed - in the northern Gulf of California. Our collaboration with colleagues Arturo Martín, John Fletcher, Mario Gonzalez and their students at CICESE will allow us to carry out a fully integrative, multidisciplinary study of the major Late Miocene structures and basins that formed in response to oblique trantension and strike-slip faulting, and culminated in opening of the Gulf along the modern Pacific - North America plate boundary. The photos below provide a few glimpses of the rocks and scenery of coastal Sonora.

The above material is based upon work supported by the National Science Foundation (NSF, Grant No. EAR-0738723). Any opinions, findings or conclusions expressed herein are those of the authors and do not necessarily reflect the views of the NSF.

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This page was last updated May 23, 2008, by Becky Dorsey.