SYNCHRONOUS WANING OF LARAMIDE SHORTENING AND INITIATION OF CENOZOIC EXTENSION IN NORTHWESTERN MEXICO: A STILL UNSOLVED ISSUE

Laramide deformation recorded in northwestern Mexico shows striking differences in structural style and age across the orogen. The western domain developed thick-skinned deformation characterized by crustal block uplifting, reverse faulting and regional shearing. The younger eastern part in contrast developed intense thin-skinned shortening and late reactivation of basement faults. The boundary between the two deformation styles is likely a wide transitional zone; however, its possible location is mainly concealed beneath the large mid-Tertiary Sierra Madre Occidental volcanic province. Based on available isotopic ages and geological observations, the time when the Laramide shortening ended to give place to the extensional tectonics that affected most southwestern North America during the Cenozoic is still difficult to discern. Nevertheless, the process that triggered detachment of the distal cooler and denser part of the Farallon slab and sustained retreat of the remaining fragment of the subducted plate may have controlled this tectonic change. Although extension may occur during most of the lifetime of magmatic arcs, the beginning of large-scale extension in southwestern North America appears synchronous with waning and cessation of the front of the orogen, and was characterized by low-angle detachment faulting and exhumation of ductile deformed rocks of the middle crust. Isotopic cooling ages of plutons associated with formation of metamorphic core-complex systems in Sonora are as old as 40 Ma, which is considered the time when the eastern Laramide shortening ended. This appears to coincide in time with a large degree of extension allowing exhumation of the metamorphic core complexes. Moreover, the outstanding metal accumulation associated with emplacement of the porphyry copper systems of southwestern North America, largely coincides in space with the metamorphic core complex belt. It may imply that both events, first the porphyry copper formation and secondly the exhumation of the metamorphic core complex, were linked in time and space to the initial stages of the tectonic process that finally caused the detachment of the Farallon slab.