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

Abstract

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.

Geology of the Northern Simbruini Mts. (Abruzzo – Italy)

“…In the late Miocene the study area became involved in the Apennine chain orogenic phase, which caused the definitive halt of shallow-water carbonate sedimentation and the development of a thick syn-orogenic (foredeep) terrigenous succession, essentially Tortonian-early Messinian in age (Bigi et al, 2003;Carminati, Fabbi, & Santantonio, 2014;Compagnoni et al, 2005 and references therein). Due to the orogenic uplift, the majority of the area emerged during the Pliocene, and the outcropping continental deposits are essentially Quaternary in age.…”

“…(Devoto, 1967) is a lateral equivalent of the terrigenous units (Fabbi & Rossi, 2014 and references therein). This unit is markedly clastic, lithologies ranging from pure breccias to rudite-arenite-pelite associations, and reflects the existence of a prominent structural high in the area (Figure 6), whose margins underwent dismantling (Carminati et al, 2014;Critelli et al, 2007;Fabbi, 2013;Fabbi & Rossi, 2014). These margins were submarine escarpments which could be sites of mineralization (i.e.…”

“…For a description and analysis of pre-orogenic faults and paleogeographic setting of the area see Carminati et al (2014) and Fabbi and Rossi (2014).…”

“…can be described as a wide monocline, with beds roughly dipping toward the NE; bed attitude abrubtly steepens, up to vertical, close to the thrust front of the structure. The monocline is cut to the E and NE by the Simbruini thrust front and dismembered to the west by a large system of major (regional) SW-dipping extensional faults (Figure 1), whose plains crop out outside the study area (Carminati et al, 2014 and references therein).…”

“…Remarkably, in the westernmost portion of the map the succession is dissected by several small faults, making it difficult to determine the kinematics and the deformation history of this sector. The intense tectonization and fragmentation is possibly due to the superimposition of subsequent tectonic phases (Carminati et al, 2014;Fabbi, 2013).…”