Miocene paleomagnetism of Baja California Sur; evidence concerning the structural development of western Mexico

Abstract

An apparent lower to middle Miocene paleomagnetic pole at 85.4°N, 71.2°E (95=6.2°) has been determined from measurements of the remanent magnetization in volcanic rocks· at 16 sites in Baja California Sur. This pole is not significantly different from the reference middle Tertiary paleomagnetic pole for the North American craton; hence, probably no more than the 2° latitudinal northward movement of Baja California relative to interior North America has occurred since the Miocene, as indicated by magnetic anomalies in the mouth of the Gulf of California. Previous paleomagnetic studies have shown that Miocene and older rocks from mainland Mexico display a significant counterclockwise rotation with respect to Miocene paleomagnetic directions from both Baja California and cratonic North America. It is likely that the mainland Mexican counterclockwise shift occurred after the 12 m.y. initiation of Gulf of California rifting and did not affect the Baja California peninsula. Right-lateral shear along the peninsula associated with Gulf development may have caused localized fliocene to Recent clockwise rotation in Baja California Sur with respect to mainland Mexico.