Sequence and geochronology of Miocene rocks adjacent to the main gulf escarpment: southern Valle Chico, Baja California Norte, Mexico

Geologic mapping and K/Ar dating of Tertiary rocks in NE Baja California, at latitude 30° 30' N, reveal a history of volcanism from 20 Ma to at least 6 Ma. From 20 to 15 Ma, basalt, andesite and pyroclastic flows locally covered the Cretaceous and older batholithic and prebatholithic rocks. These deposits were followed by up to 300 m of additional pyroclastic flows, reworked tuff, basalt, sandstone and conglomerate. Subsequent eruptions between 11 and 6 Ma produced andesite to rhyolite flows and breccias. Eruptions at about 6 Ma covered the region with up to an additional 300 m of pyroclastic flows, ash fall deposits and welded vitric tuff. These units cap the high plateaus of the northwestern corner of the Puertecitos volcanic province. Local undated volcanic units may be younger than 6 Ma. The tilting, faulting and topographic relief associated with the Main Gulf Escarpment began developing sometime between 11 and 6 Ma; variations in thickness of the pre- I I Ma units arc not generally fault-related, although variations in thickness of the 6 Ma deposits arc. The alignment of four prc-6 Ma volcanic vents along a NNW trend parallel to the present escarpment suggests that ascent of the magmas may have been structurally controlled by Late Miocene extensional faults