NEOGENE VOLCANO-TECTONIC EVOLUTION OF THE CENTRAL BAJA CALIFORNIA PENINSULA, MÉXICO: TRANSITION FROM SUBDUCTION TO RIFTING AND MARINE INCURSION

Ongoing study of the central Baja California peninsula by the Baja GeoGenomics Consortium yields insights into the Neogene volcano-tectonic evolution of northwest Mexico. Recent geologic mapping of a ~4,000 km² region combined with ~150 new geochronologic dates and ~350 new geochemical analyses provide a vivid record of the transition from Farallon-North America subduction to the Gulf of California transform rift. Subduction is recorded by calc-alkaline volcanic rocks of the Comondú Group (CG). NW of Santa Rosalía, volcaniclastic strata of the Lower CG (~24–23 Ma) pass conformably upsection into massive andesitic tuff breccias of the Middle CG (~23–22 Ma), and in turn andesitic lavas and pyroclastic rocks representing proximal facies of composite volcanoes of the Upper CG (~22–14 Ma). This lithostratigraphy matches CG sequences up to 400 km along strike to the SE and supports SW migration of the Miocene Comondú arc. Younger mafic lavas of the Sierra San Jorge (12.5–11 Ma) and mafic-intermediate domes and tuff breccias of the Sierra San Francisco (11.5–9 Ma) are enriched in Mg and more mafic (basaltic andesite and basalt) than the CG, signaling an increasing flux of more primitive melts after subduction ceased ~12 Ma. Incipient extension and regional tilting are recorded by a minor angular unconformity (5–10°) that formed ~12–10 Ma, synchronous with this mafic, Mg-enriched volcanism but prior to emplacement of the extensive 9.9-Ma tholeiitic Esperanza basalt. Extensional monoclines began to form ~7–6 Ma above blind, NE-dipping normal faults along the western margin of the Gulf of California transform rift. NE monoclinal block tilting and hanging-wall breakup resulted in subsidence of the >60 km-long Santa Rosalía basin and initial marine incursion at 6.3 Ma. West of the peninsular divide, NNW-striking normal faults align with lineaments and feeder dikes of Late Miocene-Pliocene volcanic centers, including dozens of 5–4 Ma high-Mg monogenetic volcanoes. The NNW-striking dextral Bonfil fault offsets a 11.6 Ma volcano >500 m. Most faulting on the peninsula at this latitude was complete by ~4 Ma. Paleogeographic reconstruction of volcanism, faulting, and marine environments constrains the Pacific and Gulf of California shorelines and illustrates the 6-7 Myr transition from subduction to rifting and marine incursion.