Evolution of part of the Nevadan orogen in northwestern Mexico
The geology of the polyorogenic El Fuerte region is described here, with particular reference to the effects of the late Mesozoic Nevadan orogeny. On a gneissose basement of probable Pre-cambrian age, possible upper Paleozoic sediments and silicic volcanic materials were deposited, isoclinally folded (D s 1) and eroded prior to the deposition of a thick sequence of Cretaceous mafic volcanic rocks and subordinate limestones. The West Mexican batholith is represented by prekinematic mafic plutons and late-kinematic to postkinematic granitoids. Upright isoclines (D N 1) dominate the structure, but two later fold phases are recognized. The youngest phase may be partly related to strike-slip displacement along the Torreon-Monterrey lineament. Granite emplacement contemporaneous with or postdating D N 3 folding has produced widespread contact metamorphism overprinting the lower greenschist facies assemblages that were stable during the earlier Nevadan folding. By Paleocene time, erosion had exhumed the granite-metamorphic complex, which was then covered by volcanic equivalents of still later granites, gently folded, eroded, and recovered by Miocene ignimbrite sheets. Quaternary basalts and fluviatile deposits complete the succession.