ALTERNATING CONTRACTIONAL AND EXTENSIONAL EVENTS IN THE ALPUJARRIDE NAPPES OF THE ALBORAN DOMAIN (BETICS, GIBRALTAR ARC)

In the western Alpine system, Neogene extensional tectonics triggered the development of marine basins on the concave side of tight orogenic arcs, as happened within the Alboran Crustal Domain, the hinterland o f the Gibraltar Arc. A detailed analysis of the structural and metamor phic records of one of the main Alboran Domain complexes, however, pla inly reveals a complex tectonic evolution prior to the development of the Miocene arc/back are system, which includes a major intraorogenic extensional event. This large-scale subvertical shortening, that can b e assessed from the PT paths of representative tectonic units, was sub sequent to the continental crust subduction inferred from high pressur e-low temperature mineral asssemblages. The crustal section was thinne d in nearly isothermal conditions, its thickness being reducted to at least 1/3 of the initial value. Yet still before the Miocene, a second contractional event led to the overthrusting of high-grade metamorphi c rocks over other low-grade rocks, accompanied by subordinate overtur ning of the metamorphic zones. Since migration of the Gibraltar Are is roughly balanced by crustal spreading in the back are, available data concerning Miocene extension suggest that the Alboran Domain can be r estored to its appropriate position several hundred kilometers to the east. Thus a collision belt that underwent significant intraorogenic e xtension must have existed in what is now the western South-Balearic b asin.