EXTENSION VERSUS COMPRESSION DURING THE MIOCENE TECTONIC EVOLUTION OFTHE BETIC CHAIN - LATE FOLDING OF NORMAL-FAULT SYSTEMS

The westernmost part of the Mediterranean Alpine Belt is represented b y the Betic-Rif orogenic belt, around the Gibraltar Arc, which in turn surrounds the Alboran Basin. In the Betic Chain, early and middle Mio cene crustal thinning of the Alboran basement is well established, as extensional low-angle normal faults and detachment faults, developed i n both ductile and brittle conditions, thinned a previously thickened crust. In the Alboran Domain of the central Betics, two main extension al episodes are evidenced: a Langhian one, with a north-northwestward transport direction, followed by a west-southwestward extension, Serra vallian in age. Therefore all the units heretofore considered to be th rust nappes are, in reality, extensional units bounded by low-angle no rmal faults. The cortical segment studied formed the basement of the M iocene Alboran Basin, in which progressively deeper basement units wer e covered by younger marine sediments as a result of extensional denud ation processes. The age of these sediments clearly dates the faulting . The extensional evolution during the Miocene is much more complex th an the past models suggest. During the upper Miocene, these extensiona l systems were folded as the result of a compressive regime, which all owed them to be well exposed. Compression in the Gibraltar Are is near ly contemporaneous with extension, and the westward migration of the c ompression through its footwall is related with the extensional spread ing.