Abstract

The geological-structural study carried out along the Gran Sasso-M. Cappucciata salient thrust system allowed us to define: - the structural setting; - the role played by the Mesozoic palaeomargin architecture during the Neogene thrust system development (foreland deformation, foredeep physiography and geometry of the thrusts and related folds). The Gran Sasso-M. Cappucciata thrust system forms a spectacular salient structure in the Central Apennines, overthrusting the Triassic-Miocene carbonate unit belonging to the Lazio-Abruzzi carbonate platform and related slope domain onto the siliciclastic Miocene deposits belonging to the Laga Unit and deposited over the Jurassic-Miocene carbonate sequence of the Marche-Abruzzi pelagic realm. In the study area, the E-W trending thrust front is represented by two main thrust planes, trending about E-W, that isolate an intermediate tectonic body, involved by N-verging recumbent folds tightening to the east. To the west of the Corno Grande, the above mentioned thrusts progressively loose displacement, display the character of blind-thrusts and terminate at the periclinal tip of the M. Corvo anticline. In the western sector, an en-echelon relationship is present between the Mt. Corno anticline and the Mt. Jenca anticline which continues westward, together with the associated thrust, until Mt. S. Franco. To the east, the thrust front assume a N-S trend, parallel to the hanging wall anticline axial direction and to the Mesozoic paleomargin distribution of carbonate platform-pelagic basin domains. Along this N-S thrust front minor tectonic lenses are present, represented by Miocene foreland deposits and the pre-orogenic carbonate sub-stratum (Colle Madonna). NW-SE trending normal faults affected the Gran Sasso-M. Cappucciata hanging wall unit. Eastward, the N-S trending Teramo thrust shows the Messinian post-gypsum Laga Formation on its hanging wall and Lower Pliocene on the footwall. In the N-S segment of the Gran Sasso-M. Cappucciata salient, Middle-Upper Miocenic calcarenite and calcirudite unconformably overlie the Jurassic-Cretaceous carbonate sequence. These relationships are interpreted as connected with the development of a structural high in the foreland, coinciding with carbonate platform area. On the basis of normal faults trending WSW-ESE, that control thickness and facies variation of Miocene carbonate deposits, and of palaeomagnetic data, that underline counterclockwise rotations about vertical axes in the study area, it is supposed a structural association characterized by right and left strike-slip faults, trending respectively E-W and N-S, that reactivate Mesozoic palaeomargin faults, interconnected with normal faults. The integrated analysis of the geological, stratigraphic and structural data allowed us to point out: - an Oligocene-Miocene p.p. transcurrent/transpressive deformation involving the foreland domain which reactivated the Mesozoic palaeomargin architecture; - an physiography of the syn-orogenic Messinian basin, characterized by a depocenter localised in correspondence with the Marche-Abruzzi pelagic palaeodomain (M. della Laga area); - the correspondence of the study area with the syn-sedimentary structural high, localised on the Lazio-Abruzzi carbonate platform palaeodomain, that bordered to the south the foredeep depocenter, inherited from foreland tectonics; - the control of foredeep physiography on the geometry and location of the Gran Sasso thrust ramps and related folds; - a progressive deformation during the syn-orogenic history defined by the superposition of three main incremental strain steps, represented by transcurrent/transpressive foreland tectonics, crustal/lithospheric buckling vs bending, during syn-orogenic basin development, thrusting and related folding process, and Quaternary normal faults development. Our study illustrates that Messinian syn-orogenic basin in Marche-Abruzzi area is characterized by the presence of two crustal/lithospheric flexures with an amplitude exceeding 50 km, with antiform geometry for Lazio-Abruzzi carbonate platform and syn-form geometry for Marche-Abruzzi pelagic basin. A similar anti-form, located onto the Mesozoic pelagic high (present Umbria-Marche ridge), bordered the depocenter of Laga basin westward. We believe that this flexural geometry may result from combined action of flexural bending model and crustal/lithospheric buckling connected with horizontal stress and controlled by palaeomargin architecture. In this context, the Mesozoic palaeomargin architecture (carbonate platform distribution) controlled the foreland deformation, Messinian syn-orogenic basin physiography and geometric structure of chain, with development of characteristic salients, including the thrust system of Gran Sasso d'Italia and M. Sibillini, with strong implication for the relationship between structural elevation and amount of orogenic contraction.

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