Late Miocene extension in coastal Sonora, México: Implications for the evolution of dextral shear in the proto-Gulf of California oblique rift

These results support an emerging model in which dextral strain was not ubiquitous across Sonora and did not initiate immediately following the ~ 12.5 Ma transition from subduction to oblique rifting. Instead, strain east of the Baja California microplate at this latitude evolved from extension-dominated transtension prior to ~ 8 Ma to dextral shear-dominated transtension by ~ 7-6 Ma. The onset of dextral shear in coastal Sonora likely resulted from an increase in rift obliquity due to a change in relative plate motion direction at ~ 8 Ma. The increase in rift obliquity and resultant onset of significant strike-slip faulting played a crucial role in facilitating subsequent plate boundary localization and marine incursion in the northern Gulf of California by ~ 6 Ma and continental rupture at ~ 2-1 Ma.