Thrust sequences and evolution of the external sector of a fold and thrust belt: An example from the Southern Apennines (Italy)

The Southern Apenninic chain was built up from the late Oligocene-early Miocene to the middle Pleistocene. It consists of an upper wedge composed of strongly deformed deep marine and carbonate platform successions and a buried thrust and fold belt in the carbonate rocks of the Apulian Platform. This paper analyzes the frontal part of the Southern Apennines, where contractional structures display complex geometries related to progressive incorporation of accreted material, internal shortening and gravitational processes, Late normal faults, which hinder the interpretation of contractional structures in other sectors of the chain, are scarcely represented in the frontal part of the Southern Apennines. This allows a detailed analysis of thrust and fold geometry. In particular it is possible to recognize the relationships between gravity-driven nappes, foreland-vergent thrusts and backthrusts. New field observations allow us to depict a detailed structural framework of this area and to construct regional cross-sections, which illustrate the inferred evolution of the frontal sector of the Apenninic chain. The chronological constraints are given by the age of thrust-top basins and foredeep deposits of middle-late Miocene to Pleistocene age. Deepening of the decollement level during thrusting creates complicate overprinting relationships connected to the deformation of the older thrust surfaces. The inferred evolution can be summarized in two stages. During the first stage shortening of the wedge was associated to emplacement of thin thrust sheets, interpreted as gravity-driven nappes, before sedimentation of middle-late Miocene thrust top basins. The second stage, of middle Pliocene-middle Pleistocene age, was characterized by folding and breaching of the pre-existing structures, related to deep thrusting in the Apulian Platform. The examples shown suggest the presence of transfer zones and lateral ramps to explain lateral variations in thrust geometry and shortening amount. The along strike geometry of non-cylindrical structures provides a valuable tool to decipher the tectonic evolution of the thrust belt. The proposed scheme is well suited to interpret the structural setting of thin-skinned thrust and fold belts characterised by change in the geometry of the thrust systems through time. (c) 2005 Elsevier Ltd. All rights reserved.