SEDIMENTARY BASINS OF THE BACKARC REGION IN NORTH-CENTRAL SONORA, MEXICO: A RECORD OF LATE JURASSIC EXTENSION AND EARLIEST CRETACEOUS ARC ACCRETION

Upper Jurassic and Lower Cretaceous rocks of the Altar-Cucurpe basin in north-central Sonora reveal a tectonic history of Oxfordian to Tithonian intra-arc or back-arc extension represented by the Cucurpe Formation followed by deformation, uplift and erosion resulting from arc accretion prior to deposition of the Aptian La Colgada and Morita formations. New U/Pb zircon data collected at the Stanford/USGS SHRIMP lab and detrital zircon data from the University of Arizona LA-ICPMS lab form the basis for new understanding of the age range of this critical stratigraphic interval. The Cucurpe Formation unconformably overlies Middle Jurassic volcanic arc rocks and contains voluminous pumiceous and tuffaceous turbidites that record syndepositional siliceous volcanism. Pumiceous debris flows, tuffs, and volcanic lithic sandstones of the Cucurpe Formation were deposited between 161.2+/-3.2 and 150.6+/-1.2 Ma. The age of the Cucurpe Formation coincides with postulated rollback-induced extension during westward and southwestward retreat of the Jurassic arc.

The Lower Cretaceous La Colgada and Morita formations unconformably overlie the more intensely folded Cucurpe Formation. Differential deformation, uplift and erosion indicate that a shortening event occurred after Late Jurassic deposition and prior to deposition of the shallow marine La Colgada Formation at about ~120 Ma. Detrital zircon U/Pb ages indicate that sediment influx during the Aptian was from nearby Jurassic arc rocks as well as from Precambrian to Triassic basement and miogeoclinal rocks of the Caborca block to the south. Graphic granitic grains in the lower Morita Formation may be derived from the Aibo granite of the Caborcan basement. Andesite clasts with U/Pb ages averaging 142.2+/-1.8 Ma in Morita conglomerates were likely derived from accreted arc rocks to the south and west of the Caborca block. A tuff bed in the uppermost Morita formation has an age of 115.5+/-0.7 Ma. These data are consistent with accretion of the oceanic Guerrero arc terrane and attendant uplift between 140 and ~120 Ma. Late Aptian tuffs then record early stages of continental-margin arc volcanism along the west coast of Mexico. Aptian and Albian strata in Sonora probably record post-accretion thermotectonic subsidence similar to that of the Bisbee basin in the southwestern US.