Compressional tectonics overprinting extensional structures in the Abruzzo Periadriatic Foredeep (central Italy) during Pliocene times

The external part of the Central Apennines represents a Plio-Quaternary ENE verging thrust system that affects the foredeep deposits (Lower Pliocene Cellino Formation and more recent clastic sediments). Seismic analyses have been used to define the geometry of the deformational structures within the Periadriatic Foredeep. The main detachment level of the outermost synsedimentary buried growth structure (the Coastal Structure) is located along the top of the Messinian Gessoso-Solfifera Formation, which also represents the foredeep substratum. The Gessoso-Solfifera Formation is affected by an array of Upper Messinian-basal Pliocene extensional faults striking N170 degrees -175 degrees . The faults are conjugate, arranged in en echelon pattern, and form a horst and graben system, 10 km wide. The throw of a single fault does not exceed some hundreds of metres, to a maximum of about 1,000 m. The structures do not involve the siliciclastic Plio-Quaternary sequence. These tectonics influence the position, geometry and development of the successive thrust faults, forcing the formation of an imbricate fan which is parallel to the extensional faults and lies above them. This fan (Coastal Structure) is characterized by a complex geometry and kinematic development. Analogue experiments performed in sandboxes have confirmed the close connections between the morphology of the detachment level and the structural features of the thrust system. The parallelism between the Apennine compressional front and the Messinian-basal Pliocene extensional structures suggests that this extensional event may be in response to the flexural folding of the W-dipping underthrust of the Adriatic Plate under the Apennine chain.