Abstract
The geology and landscape of the central Baja California (CBC) peninsula reveals evidence of significant changes in Neogene to Quaternary paleotopography. Newly mapped marine tidal deposits E of San Ignacio give preliminary evidence for a nearly equal 3-4 Ma marine embayment or strait extending across most or all of the CBC, occupying a NE-SW oriented topographic saddle, the San Ignacio trough (SIT). Volcanic deposits and related structural features shed light on the origin, evolution and geometry of this transient marine embayment. Eruption of the Comondu Group (CG) volcanic arc dominated Baja California landscape during Farallon subduction approximately 24-12 Ma. In CBC major stratovolcanic centers constructed the topographic spine of the peninsula and are spaced 60-70 km apart. The SIT is a topographic low between two of the eruptive centers. At approximately 12 Ma subduction waned but volcanism continued as the Gulf Extensional Province (GEP) developed and the subducted slab foundered and broke up. Post-subduction volcanism in CBC is divided into three episodes that each formed distinctive topographic features. At approximately 10 Ma the basalt of Esperanza erupted in the GEP producing tholeiites which flowed 165 km across the peninsula down the SIT to the Pacific. approximately 11-3 Ma Mg-basaltic andesite lavas (MGA) erupted both N and S of the SIT and unconformably overlie CG. Within the SIT 3 Ma MGA overlie the Pliocene tidal deposits. MGA magmatism coincided with subsidence of the SIT, allowing tidal or marine submergence. Pliocene cross-peninsular marine connection to the GEP is uncertain but may have occurred where the SIT meets the modern peninsular divide. Just NE of the divide, the fault pattern suggests a NNE-oriented graben formed in a right step-over between NW-striking dextral faults. Quaternary Reforma and Aguajito calderas and the Las Tres Virgenes volcanic complex (LTV) erupted in the GEP and probably filled the proposed graben. Geothermal well logs record fossiliferous marine deposits and NE-oriented faulting beneath LTV volcanoes, supporting the existence of the graben and marine connection. LTV volcanoes are NNE-aligned within the graben. Quaternary magmatism resulted in crustal heating, inflation and uplift of the GEP NW of Santa Rosalia. Diverse volcanism and tectonism drove transient but significant changes to topography and paleogeography of CBC.