LATE JURASSIC MOJAVE-SONORA MEGASHEAR VERSUS LATE CRETACEOUS LARAMIDE OROGENY, NW SONORA, MÉXICO–SW ARIZONA, USA

The so-called Caborca terrane could not have been emplaced in the Late Jurassic because the Early Cretaceous Bisbee Group accumulated in a low-lying topography; terrigenous sediments were derived mainly from a volcanic arc. In the Caborca region no sediments derived from Proterozoic-Paleozoic rocks have been reported.

The Upper Cretaceous is widely distributed throughout northern Sonora, but was previously assigned a Jurassic age. This is one of the reasons why the latest Cretaceous orogeny was thought to be a minor event. This sequence was deposited in continental foreland basins and has received several names: El Chanate Group, El Tuli Formation, Cabullona Group, Tarahumara Formation. The sequence consists of conglomerates, sandstone and mudstone, mostly derived from volcanic sources. Detritus derived from Neoproterozoic-Paleozoic rocks has been reported in few localities. These clasts could help in understanding the timing of the thrust-front position in northern Sonora. One of the problems which did not allow the proper identification of Cretaceous sequences is the fact that they display different styles and intensity of deformation, from just folded to metamorphosed into green schist facies. In fact, one of the arguments considered in the megashear hypothesis is that strongly stretched conglomerates were Jurassic as locally the Lower Cretaceous does not display such deformation. However, U-Pb ages in zircons from the Altar Schist and other stretched units indicate that the protolith is Late Cretaceous. These new findings strongly suggest that the Papago Terrane is the sole fault of an orogenic system and in Sonora we have part of the upper plate which has been disrupted by Cenozoic metamorphic core-complex exhumation and Basin & Range extension.