MIDDLE MIOCENE EXTENSION IN THE GULF EXTENSIONAL PROVINCE, BAJA-CALIFORNIA - EVIDENCE FROM THE SOUTHERN SIERRA-JUAREZ

New geologic mapping, structural studies, and geochronology of Miocene volcanic and sedimentary rocks in the southern Sierra Juarez, Baja Ca lifornia, shed light on the extensional history of the Gulf Extensiona l Province prior to sea-floor spreading in the Gulf of California, The southern Sierra Juarez is underlain by lower-middle Miocene rocks inc luding fluvial strata, intermediate composition volcanic deposits, bas alt lava flows and cinder cones, and dacite pyroclastic deposits and l avas that non-conformably overlie the Cretaceous Peninsular Ranges bat holith, The Ar-40/Ar-39 geochronology indicates that basaltic rocks ar e 16.90 +/- 0.05 Ma and dacite pyroclastic deposits are between 16.69 +/- 0.11 Ma and 15.98 +/- 0.13 Ma. These strata were subsequently cut by two generations of faults, First generation faults comprise a domin ant set of north-south-striking, west-dipping normal faults, a seconda ry set of north-south-striking, east-dipping normal faults, and a less er set of variably oriented strike-slip faults, All three fault sets a re temporally and spatially related and were produced by east-west ext ension, The dominant west-dipping faults, which are antithetic to and oblique to the east-dipping Main Gulf Escarpment, may have been a prec ursor or an early phase accommodation zone along the escarpment, West- dipping normal faults are cut by a 10.96 +/- 0.05 Ma dacite hypabyssal intrusion, thus bracketing the age of east-west extension between 15. 98 +/- 0.13 Ma and 10.96 +/- 0.05 Ma, Hence, this faulting event clear ly indicates a period of extension that predates the onset of oceanic rifting and even predates other dated Miocene extension within Baja Ca lifornia. Second generation faults, which are comprised of east-west-s triking strike-slip faults that cut first generation faults and associ ated northwest-striking, northeast-dipping normal faults, may be relat ed to early development of the Transpeninsular Strike-slip Province. G lobal plate reconstructions suggest that transtensional motion between the North American and Pacific plates along the western margin of Baj a California began during middle Miocene time, coeval with east-west e xtension in the southern Sierra Juarez. This observation supports a hy pothesis that middle Miocene transtensional plate motion was partition ed into two components: a strike-slip component parallel to active fau lts along the western offshore margin of Baja California, and an exten sional component normal to the margin, but located in what is now the Gulf Extensional Province, Hence, the onset of extension within the ci rcum-gulf region was in response to plate boundary processes.