Oblique rifting ruptures continents: example from the Gulf of California shear zone

Scott E. Bennett, & Gregory C. Beroza

Accepted 2013, SCEC Contribution #1823

We show that a belt of clockwise vertical-axis block rotation associated with dextral-oblique rifting in the Mexican Basin and Range province hosted the localization of plate-boundary strain that led to formation of the Gulf of California ocean basin. Paleomagnetism of Miocene ignimbrites distributed widely across the rift reveals the magnitude, distribution, and timing of rotation. Using new high-precision paleomagnetic vectors (alpha-95 ≤ 1°) from tectonically stable exposures of these ignimbrites in Baja California, we determine clockwise rotations up to 76° for intra-rift sites. Low reference site error permits isolation of intra-rift block rotation during proto-Gulf time, prior to rift localization ca. 6 Ma. We estimate 48% (locally 0% to 75%) of the net rotation occurred between 12.5 Ma and 6.4 Ma. Sites of large (>20°) block rotation define a ~100 km-wide belt, associated with strike-slip faulting, herein named the Gulf of California shear zone, which was embedded within the wide-rift Mexican Basin and Range province and kinematically linked to the San Andreas fault. After a protracted history of diffuse extension and transtension, rift localization was accomplished by focusing of Pacific-North America dextral shear into the Gulf of California, which increased strain rates and connected nascent pull-apart basins along the western margin of the province. Oblique rifting thus helped to localize and increase the rate of continental break up and strongly controlled the three-dimensional architecture of the resultant passive margins.

Citation
Bennett, S. E., & Beroza, G. C. (2013). Oblique rifting ruptures continents: example from the Gulf of California shear zone. Geology, (accepted).