Miocene shearing along the northern boundary of the Jalisco Block and the opening of the southern Gulf of California

New geologic and structural mapping in western Mexico shows that the tectonic boundary between the Eocene-early Miocene Sierra Madre Occidental volcanic province and the Cretaceous-Paleocene Jalisco block was reactivated as a transfer zone accommodating a limited amount of lateral motion in middle and late Miocene time. Transpressional folding, left-lateral strike-slip faulting, and minor thrusting occurred between 14.5 and 11.5 Ma during initial rifting at the mouth of the Gulf of California. Inferred right-lateral transtension between approximately 12 and 9 Ma took place concurrently with widespread alkaline volcanism and east-west extension at the gulf mouth and east of the Jalisco block. These events can be explained in light of the long extensional history that led to the separation of Baja California from mainland Mexico since the birth of the Magdalena microplate, about 14.5 Ma. Left-lateral shearing at the Sierra Madre Occidental-Jalisco block boundary indicates that initial (14-12 Ma) rifting at the mouth of the gulf was accomplished mainly by east-southeast motion of the block. This was caused by differential stresses applied to the North America plate at the two sides of the subducting Magdalena-Cocos plate boundary as a consequence of the waning of subduction of the Magdalena plate. Subsequent right-lateral transtension at the Sierra Madre Occidental-Jalisco block boundary and extension to the east imply a limited west-northwest motion of the block, which possibly accounts for the discrepancy between slow rifting at the mouth of the gulf and fast northwest motion of Baja California.