NSF Award Abstract - #0230439

Neogene Evolution of the Sonoran Margin: The Transition from Backarc Extension to Transtensional Rifting

Abstract

The fundamental processes that govern continental rifting and ultimately lead to rupturing of continental lithosphere and opening of an ocean are still poorly understood. The transtensional Gulf of California represents a superb natural laboratory to investigate these processes because it provides both along-strike and across-strike views of the rifting process, from distributed continental extension to the final rupturing of the North American lithosphere and the capture of Baja California by the Pacific Plate. However, models for the kinematics and dynamics of this rift depend critically on understanding exactly how extensional and transcurrent strains are distributed in space and time on the adjacent continental margins - particularly the Sonoran Margin. This project, supported by the Tectonics Program and the International Program, is quantifying the distribution, magnitude, timing, and style of Neogene deformation and magmatism across the Sonoran rifted margin in order to evaluate the kinematics of the transition from earlier (pre-12 million years ago) distributed backarc extension to post-12 million year old transtensional deformation associated with the cessation of subduction and transfer of Baja California to the Pacific Plate. This multi-disciplinary study has brings together scientists and students from the U.S. and Mexico to unravel the history of extension and strike-slip faulting across the widest and most completely exposed, but least well understood rifted continental margin adjacent to the Gulf of California. New geologic mapping is being carried out to identify the important fault systems and to work out the details of local structural, volcanic, and sedimentary histories. Investigators are deciphering in detail the structural evolution of a 60 by 100 km area in southwestern Sonora, from the coast near Guaymas to the Sierra Mazatan metamorphic core east of Hermosillo. In addition, structural and stratigraphic studies are being carried out on two representative Neogene basins in eastern Sonora to assess whether significant late Miocene deformation affected areas further inboard. Approximately 60 new high-precision Ar-Ar age determinations are being obtained on key pre-, syn-, and post-tectonic volcanic units to document local volcanic and sedimentation histories and to bracket the timing of structural events. K-feldspar Ar-Ar multi-domain diffusion, apatite (U-Th)/He, and fission track analyses are being employed to determine low temperature cooling histories in the footwalls of large normal fault systems to assess the timing, magnitude, and rates of extensional slip.