Compressional and extensional tectonics in an arc system: example of the Southern Apennines

Abstract

Adopting the Southern Apennines as a case example, we determine the relationships between extensional and compressional structures within an arc system, based on tectonic analysis in the field. We show that, in addition to the well-known occurrence of inner extensional structures during frontal accretion, two other types of extensional structures are present in the Southern Apennines. We classify the three types of extensional structures depending on their chronologies with the accretion of the thrust unit, and we characterize them in terms of palaeostresses. Some normal faults of the mountain chain are old extensional structures formed in a foreland basin. In the Matese Mountains, such faults with offsets ranging several hundred metres are sealed by a late Neogene thrust sheet. These foredeep structures were created by extension trending almost perpendicular to the axis of the present-day foreland basin. Other normal faults, located closer to the Tyrrhenian sea, developed simultaneously with the Pliocene-Quaternary accretion of thrust sheets at the front of the belt. In the Salerne graben (a Tyrrhenian Basin structure), extension occurred perpendicular to the compression that prevailed at the front of the mountain belt. A stress gradient existed between the outer part of the arc (with σ 1 trending ENE-WSW) and the inner part (with σ 2 trending ENE-WSW). In the Southern Apennines, middle Pleistocene-Holocene normal faults post-date all compression and thrusting. The corresponding recent stress field (NE-SW extension) results from uplift of the previously subsiding Adriatic lithosphere. We conclude that a given thrust unit of this NW-SE-trending mountain chain may have successively undergone NE-SW extension in the foreland basin, NE-SW compression during the accretion, NW-SE extension in the back-arc region and NE-SW extension during the recent evolution of the prism. The succession of these tectonic regimes has induced complex structures, as commonly observed in an arc system.