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

The westernmost part of the Mediterranean Alpine Belt is represented by the Betic-Rif orogenic belt, around the Gibraltar Arc, which in turn surrounds the Alboran Basin. In the Betic Chain, early and middle Miocene crustal thinning of the Alboran basement is well established, as extensional low-angle normal faults and detachment faults, developed in both ductile and brittle conditions, thinned a previously thickened crust. In the Alboran Domain of the central Betics, two main extensional episodes are evidenced: a Langhian one, with a north-northwestward transport direction, followed by a west-southwestward extension, Serravallian in age. Therefore all the units heretofore considered to be thrust nappes are, in reality, extensional units bounded by low-angle normal faults. The cortical segment studied formed the basement of the Miocene Alboran Basin, in which progressively deeper basement units were covered by younger marine sediments as a result of extensional denudation processes. The age of these sediments clearly dates the faulting. The extensional evolution during the Miocene is much more complex than the past models suggest. During the upper Miocene, these extensional systems were folded as the result of a compressive regime, which allowed them to be well exposed. Compression in the Gibraltar Are is nearly contemporaneous with extension, and the westward migration of the compression through its footwall is related with the extensional spreading.