Cenozoic volcanism and tectonics of the continental margins of the Upper Delfín basin, northeastern Baja California and western Sonora

Abstract

Pre- and syn-rift stratigraphy of conjugate rifted margins of the Upper Delfín basin provides a rare opportunity to explore proximal relationships between the loci of volcanism and rifting during formation of new ocean basin. The Upper Delfín basin is one of a series of youthful, en-echelon ocean basins that accommodate spreading between the Pacific and North American plates in the Gulf of California. Four groups of volcaniclastic stratigraphy are described from the Baja California rifted margin from the Pucrtecitos Volcanic Province to the Sierra San Felipe and from the conjugate Sonora rifted margin from Isla Tiburón to the adjacent mainland coastal region. Excluding the uppermost post-6 Ma group, these strata predate opening of the Upper Delfín basin and thus similar facies relationships occur on both conjugate rift margins. Pre-rift, mostly arc-related volcanism from 21 to 12 Ma built isolated volcanic centers over a regional Eocene(?) erosion surface cut onto pre-Tertiary basement rocks. The Puertecitos Volcanic Province formed as a concentration of arcrelated volcanism with a peak of activity at 18-15 Ma. Rift-related faulting and basin formation initiated after 12.6 Ma to the north and cast of the Puertecitos Volcanic Province, but extension largely bypassed the center of the volcanic province. Rather, rifting stepped eastward along the Matomi Accommodation Zone, perhaps by taking advantage of crust weakened by prior arc-related volcanism in the Puertecitos Volcanic Province. Later rift-related volca nism localized along intersections of the Mato mi Accommodation Zone and north-striking extensional faults. The crustal break that opened the Upper Delfín basin ca. 6 Ma coincides with the most voluminous exposures of late Miocene syn-rift volcanism and the vent area for 6.7-6.1 Ma rhyolite ignimbrites that blanketed the region just prior to the onset of marine sedimentation. Overall, the pattern of arc- and rift-related volcanism, sedimentation, and faulting on the margins of the Upper Delfín basin indicates a close association between continental extension and volcanism. These relationships support that magmatism and crustal rift structure evolved as a coupled system to localize Pacific-North America plate motion into the Gulf of California.