STRUCTURAL AND MAGMATIC EVOLUTION OF THE SONORAN RIFTED MARGIN: A PRELIMINARY REPORT

New geologic mapping and 40Ar/39Ar data from a transect across southern Sonora (Guaymas/San Carlos to the Sierra Madre Occidental - see also Blair et al, Wong et al, and MacMillan et al, this volume) provide constraints on the character and timing of Neogene deformation, magmatism, and sedimentation on this rifted margin. Neogene volcanic activity youngs westward toward the coast, but is episodic in detail. In eastern Sonora (e.g., Santa Rosa, Sahuaripa), voluminous 33-28 Ma rhyolitic volcanism of the SMO was followed by scattered eruptions of basalt and basaltic andesite at 29-25 Ma, ~ 20 Ma and 17-15 Ma. In central Sonora (Suaqui Grand to Rio Yaqui), voluminous 27-25.5 Ma mainly andesitic lavas and breccias represent the Late Oligocene arc, and were followed by basalt eruptions at 20 Ma and a distal 12.5 Ma rhyolite tuff. Coastal Sonoran volcanics (e.g., S. Libre, S. Santa Ursula, S. El Aguaje) include remnants of 22-12 Ma basalt to andesite lavas and breccias overlain by voluminous 12.5 to 10.7 Ma silicic lavas and tuffs, unconformably overlain by 11 to 8.5 Ma basalt lavas. Most of southern Sonora is highly extended and this deformation also youngs toward the coast. In the east, extension is mainly 25 to 20 Ma, in central areas (including S. Mazatan core complex) it is mainly 20 to 13 Ma, and in the coastal belt it is mainly 12 to 9 Ma. Gently tilted Neogene coarse basin fill deposits (loosely termed Baucarit Fm) post-date most local extension and are controlled by younger widely-spaced high-angle faults. These late-tectonic basins young westward, from 20-15 Ma in the east to post 12 Ma in the west. NW-trending high-angle faults throughout the transect commonly display evidence for right-slip, though the magnitude and timing of this transcurrent deformation is not yet clear. These preliminary observations suggest a model for the Gulf of California region, where most of extension on the Sonoran margin occurred prior to cessation of subduction at ~12 Ma, and was controlled in space and time by the westward retreat of the arc. Baja was captured by the Pacific Plate as soon as subduction ceased and subsequent transtensional deformation was distributed across the previously extended Basin and Range but mainly concentrated within and adjacent to the Gulf.