Pacific-north America plate motion and opening of the Upper Delfin Basin, northern Gulf of California, Mexico

Correlation of conjugate rifted margins of the Upper Delfin basin constrains the timing and amount of transtensional opening along the Pacific-North America plate boundary in the northern Gulf of California. Lithologic, geochemical, paleomagnetic, and geochronologic data from a set of four ignimbrites, consisting of eight distinctive cooling units, are shown to correlate from northeastern Baja California to Isla Tiburon and adjacent areas of western Sonora. These matching ignimbrites are the ca. 12.6 Ma tuff of San Felipe, the 6.3 +/- 0.2 Ma tuffs of Mesa Cuadrada (Tmr3 and Tmr4), the tuffs of Dead Battery Canyon (Tmr5), and the 6.1 +/- 0.5 Ma tuffs of Arroyo El Canelo. Offset distributions and facies patterns of these ignimbrites support 255 +/- 10 km of opening between conjugate rifted margins of the Upper Delfin basin. Addition of deformation from the continental margins of this basin indicates at least 276 +/- 13 km of Pacific-North America plate motion between coastal Sonora and the main gulf escarpment in Baja California since ca. 6 Ma; a further 20 +/- 10 km of northwestward displacement of Isla Tiburon relative to coastal Sonora occurred sometime after 12.6 Ma. These reconstructions agree with earlier estimates of slip across the Gulf of California and on the San Andreas fault system of southern California, but require that the Pacific-North America plate boundary became localized in the gulf at ca. 6 Ma. The restored continental margins of the Upper Delfin basin show that only a 20-25 km width of upper continental crust has foundered beneath this part of the northern Gulf of California. This result suggests that most of the crustal area formed by opening of the Upper Delfin basin was either exhumed from lower-crustal levels or is new transitional oceanic crust.