Slab bending, syn-subduction normal faulting, and out-of-sequence thrusting in the Central Apennines

摘要

In the Central Apennines, a Tertiary-Present eastward-vergent fold and thrust belt, thrusting mainly occurred in-sequence, cutting across a Mesozoic to Early Neogene succession, deposited in rifted (Early Jurassic) to passive continental margin, to foredeep (late Miocene) conditions. However, the Simbruini Mts were characterized by the occurrence of regional out-of-sequence thrusts (the Vallepietra-Filettino-Monte Ortara thrust fault) and ofTertiary prethrusting normal faults, associated with the deposition of thick breccia deposits (Brecce della Renga, Tortonian-early Messinian). A direct link between these two features is here proposed. Evidence for a prethrusting normal faulting origin for the Brecce della Renga is amply discussed. Although evidence for subduction-related normal faulting is growing in the Apennines literature, nowhere in the region are known extensional systems so vast and their syn-tectonic deposits so thick as in the Simbruini area. We propose that the Simbruini prethrusting normal faults were subduction related, i.e., they developed in the foreland due to anomalous bending of the lithosphere subducting in Late Tertiary time under the Apennines in the Simbruini Mts area. The associated breccia deposits were subsequently deformed during the development of the Simbruini Mts thrust front. We speculate that the prethrusting normal faults allowed the penetration of fluids within the subducting crust and possibly also in the mantle, weakening this part of the subducting plate. As a consequence, the weaker plate was anomalously down-bent, leading to a subcritical state of the Apennines wedge. The regional scale out-of-sequence thrust faults developed in order to bring the Apennines wedge back to the critical state.