INTERPLAY BETWEEN VOLCANISM AND STRUCTURE OF THE NORTHERN BASINS WITHIN THE GULF OF CALIFORNIA RIFT SYSTEM

The northern Gulf of California (Upper and Lower Delfin basins) reveals a complex history of relationships between Miocene and Pliocene volcanic centers and Pacific-North America plate boundary fault zones, superposed on a zone of structural weakness due to earlier subduction-related volcanism. A zone of early Miocene core complexes does not appear to have acted as a zone of weakness during rift localication. The rift roughly coincides with subduction-related andesitic vents (21-14 Ma), although in the region of the Upper and Lower Delfin basins, these vents present an irregular pattern in space and time. However, the younger pre-rift vent of the ca. 12 Ma Tuff of San Felipe appears to have localized the zone of highest strain in the early development of these plate boundary basins. This tuff is a large volume high-Si rhyolitic ignimbrite whose proximal facies are preserved on both sides of the Pacific-North America plate boundary, in Punta Chueca (Sonora), Tiburon Island, Delicias (Baja California) and Sierra Libre (Sonora) suggesting that the original vent region is now dismembered within a highly extended and faulted part of coastal Sonora. Major rotation of this unit is present in regions near the modern coastline, although outcrops are buried or nonexistent beneath younger units of the Sonoran coastal plain and the Puertecitos Volcanic Province (PVP) of Baja California. In the PVP, rhyolite eruptions occurred farther west at 6.4-6.1 Ma along a stepover in the original fault systems of the rift; this in turn may have caused plate boundary motion to localize in the Upper Delfin Basin. Eruptions extending along strike from the Upper Delfin Basin then occurred at 3.3-2.7 Ma in the PVP. These may have further weakened the region, with three related consequences: localization of extension in the lower Delfin basin, relocation of the plate boundary into a previously unrifted region, and transfer of the Isla Angel de la Guarda block to the North America plate. We conclude that although the rift initiated after 12 Ma within a preexisting weak zone (the early Miocene volcanic arc), it did not stay confined to this zone. Transfer of large crustal blocks extended beyond the original weak zone into adjacent regions, outside the boundaries of the original rift, driven by interplay between volcanism and faulting on a local scale.