Abstract

Structural analysis carried out in the outer zones of the Central Apennines of Italy, integrating surface and subsurface data with seismic profiles, has made it possible to define the main modes of interaction between the architecture of the Mesozoic palaeomargin of the Adria plate and the central-Apennines orogenic system (namely the thrust-foredeep-foreland system) during the Neogene-Quaternary contractional events. A complex architecture of the Mesozoic palaeomargin is well defined in the central Apennines by the occurrence of alternating Triassic-Miocene platforms and basins. These palaeogeographic domains were differentiated following the onset of extensional deformation during Upper Triassic and Middle Liassic times, and the consequent drowning of distinct parts of a formerly wider platform. Within the Apennine fold-and-thrust system, the main features recognised are: a) the distribution of the Triassic-Miocene platform and basin domains, which are transverse and longitudinal with respect to the trends of younger contractional structures; b) the correspondance of the Neogene-Quaternary syn-orogenic depocentres with older pelagic basins; c) the occurrence, in the thrust belt, of salient geometries, as seen in map view, corresponding to the palaleomargin architecture and to the physiography of the syn-orogenic basins. Balanced and restored cross-section construction across the Adriatic offshore area, i.e. the present foreland of the Apennine belt, and the axial zone of the thrust system through the orogenic front, has made it possible to outline the role of the architecture of the Mesozoic palaeomargin in the evolution of the Messinian-Quaternary syn-orogenic basins. These depressions are characterised by wide antiformal and synformal flexures with wavelength of 50-80 km, located respectively on carbonate platform and pelagic basin domains. The reconstructed geometry suggests that crustal-and/or lithospheric-scale buckling was more important than foreland bending as a driving deformation process. The development of reverse/transpressive and normal faults further accentuated the prominent physiographic differences within syn-orogenic basins. Some of these normal faults accommodated outer-arc extension and reactivated pre-orogenic extensional structures. During development of the thrust system, the palaeomargin architecture and pre/syn-orogenic normal faults controlled the location of thrust ramps, producing the salient geometries characteristic of the outer zones of the Apennine belt. In particular, pre-existing normal faults were truncated by thrusts that propagated upwards following "short-cut" trajectories, but also were rotated in steep fold limbs and/or partly reactivated. Therefore, the tectonic setting of the belt results from interaction of the orogenic system and the architecture of the palaeomargin. The syn-orogenic basins were characterised by structural highs and depocentres that appear controlled by crustal-lithospheric anisotropies related to buckling and foreland flexure induced by orogenic load. This geometry, in turn, controlled the final geometry of the thrust belt, where it is possible to document that thrust sheet culminations and depressions reflect syn-orogenic structural highs and basins, respectively. The modes of palaeomargin-foredeep-thrust belt interaction recognised provide important information on the estimate of orogenic shortening, and on the burial-exhumation path of the Central Apennines.

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