Rapid localization of Pacific-North America plate motion in the Gulf of California

Correlation of late Miocene volcaniclastic strata across the northern Gulf of California shows that the Pacific-North America plate boundary localized east of the Baja California peninsula ca, 6 Ma. Dextral offset of the 12.6 Ma Tuff of San Felipe and a pair of overlying ca. 6.3 Ma pyroclastic Bows indicate at least 255 +/- 10 km of displacement along an azimuth of 310 deg rees. Isopach and facies trends of the Tuff of San Felipe support no more t han a few tens of kilometers of additional dextral displacement between 12. 6 and 6.3 Ma. These constraints indicate that nearly all of the dextral dis placement between the Pacific and North American plates prior to 6.3 Ma was accommodated outside of the gulf region, and by 4.7 Ma, the plate boundary motion was localized in the Gulf of California. Although continental exten sion has accounted for a component of plate boundary motion in northwestern Mexico since cessation of subduction offshore of southern Baja California at 12.5 Ma, transfer of Baja California to the Pacific plate was delayed by at least 6-7 m.y.