CRETACEOUS TECTONICS IN SONORA: CONSTRAINING TIME OF THE SHORTENING EVENTS

New ages on igneous rocks and cross-cutting relations indicate that at least two shortening events took place during Cretaceous time in northern Sonora. A middle Cretaceous phase recorded by NE-vergent folds and thrust faults that affected Lower Cretaceous strata of the Bisbee and Lampazos Groups has been widely recognized. Although this event has been assigned by many authors to the Laramide orogeny, Rangin (1986) first named it as the Oregonian deformation and interpreted it as Cenomanian-Turonian in eastern Sonora (Pubellier and Rangin, 1995). In north-central Sonora we refine the age of this older event by recognizing that strata of the Bisbee Group as young as late Albian are thrust over syntectonic alluvial conglomerate of the Cocóspera formation, which contains detritus derived from the Bisbee Group. A ⁴⁰Ar/³⁹Ar age of ~93 Ma from an andesite flow interbedded within this conglomerate indicates that shortening began by the end of Cenomanian time. However, the ~4 km thick Tarahumara Formation, which composes the record of the Laramide volcanic arc, unconformably overlies these deformed strata and does not record shortening. U-Pb ages from the lowermost part of the Tarahumara Formation are as old as ~80 Ma and thus indicate that deformation took place between ~93 and 80 Ma. This event is probably of a thin-skinned style that needs yet to be documented, but a thick-skinned Laramide deformation which formed the Cabullona basin is slightly younger on the basis of new U-Pb detrital zircon ages that date the oldest strata of the Cabullona Group at ~73 Ma, roughly coeval with onset of thrusting in northwesternmost Sonora between 75 and 60 Ma (Iriondo et al., 2005).