LATE OLIGOCENE TO MIDDLE MIOCENE RIFTING AND SYN-EXTENSIONAL MAGMATISM IN THE SOUTHWESTERN SIERRA MADRE OCCIDENTAL, MEXICO: THE BEGINNING OF THE GULF OF CALIFORNIA RIFT

Although Basin and Range-style extension affected several areas of western Mexico since the Late Eocene, extension in the Gulf of California region (the Gulf Extensional Province ­ GEP) is thought to have started as subduction waned and ended at ~14­-12.5 Ma. A general consensus also exists in considering the mid-Miocene Comondú group as a supra-subduction volcanic arc. Our new integration of the geology of the south­east Gulf region, backed by 43 new Ar-­Ar and U-­Pb mineral ages and geochemical studies, document a widespread phase of extension in the southern GEP between latest Oligocene and Early Miocene that subsequently focused in the region of the future Gulf in the Middle Miocene. Upper Oligocene to Lower Miocene rocks across the southern Sierra Madre Occidental (SMO) (northern Nayarit and southern Sinaloa) were affected by major ~N­-S to NNW striking normal faults prior to ~21 Ma. Then, between ~21 and 11 Ma, a system of NNW-­SSE high­ angle extensional faults continued extending the southwestern side of the SMO. Rhyolitic domes, shallow intrusive bodies, and lesser basalts were emplaced along this extensional belt at 20-­17 Ma. In northern Sinaloa, large grabens were floored by huge dome complexes at ~21-­17 Ma and filled by continental sediments with interlayered basalts dated at 15-­14 Ma, a setting and timing very similar to Sonora. Early to Middle Miocene volcanism, including the largely volcaniclastic Comondú strata in Baja California Sur, was thus emplaced in rift basins and was likely associated to decompression melting of upper mantle (inducing crustal partial melting) rather than to fluxing by fluids from the young subducting plate. Along the Nayarit and Sinaloa coast, flat-­lying basaltic lava flows dated at 11­-10 Ma are exposed just above the present sea level. Here, crustal thickness is almost half that in the unextended core of the SMO, implying significant lithosphere stretching before ~11 Ma. Our study shows that rifting began much earlier than Late Miocene and provided a fundamental control on the style and composition of volcanism from at least 30 Ma. We envision a sustained period of lithospheric stretching and magmatism during which the pace and breadth of extension changed at ~20-18 Ma to be narrower and likely more rapid, and again at ~12.5 Ma, when the kinematics of rifting became more oblique.