STRATIGRAPHY OF MID-MESOZOIC ARC-RELATED BASINS, NORTH-CENTRAL SONORA, MEXICO: NEW TEMPORAL AND GEOGRAPHIC CONSTRAINTS ON MAGMATIC EVOLUTION AND BASIN DEVELOPMENT IN THE SOUTHWESTERN CORDILLERA

Geologic mapping and new U-Pb geochronology on tuffs and detrital zircons in sandstones reveal an extensive Lower Jurassic through Lower Cretaceous stratigraphic section in north-central Sonora. These strata accumulated in and near volcanic arcs, but largely escaped repeated intrusive overprinting characteristic of southwestern Cordilleran arc systems; therefore, they provide important insight into the distribution and evolution of arc magmatism on North America's SW margin. The strata, exposed in stratigraphic continuity on the east flank of the Sierra de Cucurpe, form 4 separate genetic successions: (1) Lower Jurassic (189-186 Ma) volcanogenic strata of deep-water(?) arc-proximal affinity (Basomari Formation) intruded by 181 Ma quartz-feldspar porphyry; (2) a Middle Jurassic (170-166 Ma) succession of volcaniclastic conglomerate and litharenite interbedded with intermediate volcanic rocks, rhyodacitic ash-flow tuff, cross-bedded quartz arenite, and subordinate lacustrine limestone (Rancho San Martin Formation); (3) Upper Jurassic (159-150 Ma) deep-marine shale and turbiditic volcanic intrarenite dominated by syneruptive tuffaceous grains (Cucurpe Formation); (4) widespread Lower Cretaceous (Aptian-Albian) shallow marine and continental strata (Bisbee Group). Succession 1, although dominated by detritus from contemporaneous volcanic sediment sources, also contains boulders of Proterozoic granite and gneiss (1746 Ma and 1736 Ma) derived from nearby North American basement and Devonian and Late Permian grains of likely Gondwanan origin. Succession 2 is part of a widespread system of intra-arc basins that received sediment primarily from the Jurassic erg of the Colorado Plateau and active arc sources nearby, whereas succession 3 was deposited in a narrow basin northeast of the Caborca block that was fed primarily from active magmatism to the west and northwest. Succession 4, which onlaps Caborca, received sediment from North America, an arc source to the southwest, and Caborca itself. Successions 1-3 record westward migration of the Cordilleran continental-margin arc during the Jurassic; basal strata of the Lower Cretaceous succession were deposited following uplift of an outboard arc terrane (Guerrero arc) that collided with Caborca near the beginning of the Cretaceous.