MIOCENE TECTONIC AND STRATIGRAPHIC EVOLUTION OF THE CENTRAL BAJA CALIFORNIA PENINSULA AND RAPID MARINE FLOODING INTO THE GULF OF CALIFORNIA AT CA. 6.3 MA (Invited Presentation)

The central Baja California peninsula contains a record of crustal deformation, sedimentation, magmatism, and geomorphic changes driven by tectonic evolution of the Pacific-North America plate boundary over the past 30 Myr. Here we summarize results from two collaborative studies: the Baja Basins program that trained undergraduate students in field-based research, and the Baja GeoGenomics project that is testing hypotheses for N-S genomic divergence in modern plants and animals along the peninsula. This abstract focuses on Miocene evolution of the central Baja California region as determined from geologic mapping, stratigraphic analysis, Ar/Ar and U-Pb ages of volcanic rocks, and U-Pb ages of detrital zircon (DZ) from sandstones and tuffs. We divide the Comondú Group into: (1) ~33–23 Ma sandstone and conglomerate including the El Salto Formation at the base; (2) ~23–19(±2) Ma tuff breccias and lahar deposits; and (3) ~19(±2)–11 Ma intermediate lavas, breccias, and pyroclastic rocks. The Comondú Group is interpreted to record westward migration of arc magmatism in response to steepening of the Farallon slab. The change to a dextral-oblique plate boundary ca. 12.3 Ma led to the end of subduction and a change to slab-window magmatism. Upper Miocene (12–6 Ma) rocks include tholeiitic basalts, bajaites, thick tuff breccias and dacite dome complexes, rift-transition bimodal volcanics near Santa Rosalía, and >10-Ma marine deposits (San Ignacio Formation) on the western peninsula below 200 m elevation. Age data define a magmatic gap and regional unconformity from 8.8 to 6.4 Ma, when plate-boundary dextral strain was becoming localized into the present-day Gulf of California prior to marine incursion. The Boleo Formation in the Santa Rosalía basin contains marine-deltaic gravels and sands that pass laterally into distal gypsum facies. DZ ages from the basal Boleo limestone yield a maximum depositional age of 6.35 ± 0.21 Ma, nearly 1 Myr younger than a previous age estimate. This dates earliest marine flooding into the central Gulf of California that resulted from linking of strike-slip faults and rapid subsidence in pull-apart basins during onset of sea floor spreading and related hydrothermal activity. Marine incursion was geologically instantaneous at ca. 6.3 Ma along ~1,000 km of the plate boundary, with unknown but potentially large impacts on Earth-life evolution.