Abstract

In the Southern Apennines, Plio-Quaternary post-orogenic extensional tectonic phases locally permitted normal tectono-gravitational reactivation of thrust ramps, accommodated by normal antithetic neo-formational structures. Reactivated ramps and neo-formational structures delimit wide rock masses, affected by pervasive fracture cleavage, known in the literature as "Gravity-Accommodated Structural Wedges" or "GASW" ("SCATG" in Italian; Iovine et alii, 1996; 1997; Iovine & Tansi, 1998). GASW are characterised by typical morphological elements of deep-seated gravitional slope deformation (Dramis, 1984; Dramis et alii, 1985; Sorriso-Valvo, 1995), and show a lozenge shape in plan and a triangular shape in section (fig. 3). The Acri tectono-gravitational phenomenon, described in this paper, is located in Northern Calabria (Italy) in a very complex structural domain (figs. 1 and 2), characterised by the transition between two distinct regional faults systems-trending N-S and NW-SE (Lanzafame & Tortorici, 1981; van Dijk et alii, 2000; Iovine & Tansi, 2002). In particular, the phenomenon developed on the eastern border of a crystalline-metamorphic horst (the Sila Massif), along thrust ramps arranged into a transpressive push-up, connected to a regional left-lateral strike-slip fault (the Petilia-Luzzi Fault) belonging to the NW-SE System. The Acri structure is typologically composite, being constituted by three distinct deformational elements ("a", "b", and "c" in figs. 2 and 4): it represents the widest example of tectono-gravitational deformation in the Apennine chain. In particular, the element "a" is a GASW; the remaining elements ("b" and "c") are ascribed to lateral spreading. The widespread presence of deep-seated and tectono-gravitational phenomena observed in the Apennine chain underline their fundamental role in the morpho-dynamic evolution of chain systems, during late phases of uplift and dissection. The proper recognition and interpretation of such phenomena is therefore essential for correct strategies of mitigation and reduction of slope-movement related risk.

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