OLIGOCENE TO PLIOCENE MAGMATIC AND STRUCTURAL EVOLUTION OF THE GULF OF CALIFORNIA MARGIN IN CENTRAL BAJA CALIFORNIA (MEXICO)

The volcanic stratigraphy of the central Gulf of California margin of the Baja California Peninsula preserves a valuable record of the transition from subduction of the Farallon plate (24-12 Ma) to oblique rifting (<12 Ma). We use geologic mapping, petrography, geochemistry and ⁴⁰Ar/³⁹Ar geochronology to reconstruct the tectonic evolution of this region in a series of time slices.

Time slice 1 –Distal forearc deposition (Lower Comondú Group, 28-22 Ma): distal ignimbrites erupted in the Sierra Madre Occidental, interstratified with fluvial and eolian sandstones. Time slice 2 - Proximal forearc extension (Middle Comondú Group, 22-15 Ma): andesitic volcanic debris flow deposits in sections 100’s of meters thick, with a lateral extent of 500 km. These were shed from large andesitic arc stratovolcanoes that lay on the footwall of a normal fault system that lay to the west of the Baja California Peninsula. Localized, small-volume magmatism occurred within the proximal forearc basin, preserved as lava domes, block-and-ash-flow tuffs, plugs, and dike swarms, with a geochemistry that is distinct from younger arc axis magmas. Time slice 3 - Westward sweep of the arc axis and normal faults into the Baja California Peninsula (Upper Comondú Group, 14-11 Ma). Calcalkaline lavas thicken dramatically eastward toward the Bahía Concepción normal fault zone, which became active at 14 Ma (and remains active today, cutting alluvial fans). Time slice 4 – Arc volcanism ceases, and a dextral-oblique strike slip fault becomes active, the newly-recognized Potrero fault (post-11 Ma). The Potrero fault is a subvertical dextral strike-slip fault with a >800 m down-to-the east component of slip. It is the first onshore strike-slip fault mapped in central Gulf of California margin of the Baja California Peninsula, although they are common on the sea floor. The Bahía Concepción normal fault zone to the east remains active, perhaps recording strain partitioning. Time slice 5 - Emplacement of a widespread sheet of high Sr/Y trachyandesite lava (4 Ma), a previously unidentified part of the central Baja California post-subduction suite of “bajaites”. We tentatively infer that parent magmas of the high-Sr/Y trachyandesite lava were generated at higher pressures, from a more garnet-rich source, than the low Sr/Y calcalkaline volcanic rocks.