A reconstruction of late Miocene to Recent transtension in the southwestern United States and northwesternmost Mexico
John A. Dembosky, Jr., Thomas H. Anderson, 2005. "A reconstruction of late Miocene to Recent transtension in the southwestern United States and northwesternmost Mexico", The Mojave-Sonora Megashear Hypothesis: Development, Assessment, and Alternatives, Thomas H. Anderson, Jonathan A. Nourse, James W. McKee, Maureen B. Steiner
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A retrodeformable reconstruction of late Miocene and younger deformation based on a transtensional model, which combines extension and strike-slip motion, is proposed herein. The reconstruction incorporates and expands upon previous restorations of late Miocene (Basin and Range) strain east of the San Andreas transform and is supported by Pacific–North America plate kinematics since 15.1 Ma. The study region extends south from the Garlock fault across the Mojave and Sonoran Deserts into southwestern Arizona and Sonora, Mexico. In the reconstruction, proposed late Miocene north-trending physiographic basins are treated as pull-apart structures at releasing steps within a system of linked northwest-striking, right-lateral strike-slip faults. Pull-apart structures are closed by restoring strike-slip fault displacements obtained from published fault offsets where available. In areas where published data are unavailable, restoration is based upon consideration of geologic markers, seismic interpretation, depth to basement studies, and studies of basin geometry. Rotations of crustal blocks in California are modeled after Powell (1993) and have not been recognized in southwestern Arizona and northwestern Sonora, Mexico. The late Miocene reconstruction shows total east-west elongation strain to be 9% (50 km). About 43 km of right-lateral strike-slip displacement is accommodated by northwest-trending faults east of the San Andreas fault between the Gila River and the Garlock fault. Total right-lateral separation of 77 km results from the incorporation of clockwise block rotations in the model.
The reconstruction of late Miocene to Recent transtension: (1) reveals the partial effect of extensional disruption of the surficial trace of the Mojave-Sonora megashear, and (2) partially resolves some of the late Miocene “slip discrepancy” of dextral offsets between the Pacific and North American plates (∼635 km), and that recorded in the Gulf of California (∼300 km). Furthermore, the reconstruction assesses the relationship between transtensional strain in the desert region of the southwest and faults comprising the San Andreas transform plate boundary.