Longitudinal Strain in the Forearc of a Rollback-Subduction System Forced to Change Length: Structural evolution of the Crotone Basin in NE Calabria, Southern Italy

Calabria is a continental fragment incorporated into a forearc overriding the WNW directed subduction system. This system rolled back toward ESE across the central Mediterranean during the Neogene to form the Tyrrhenian Basin. Riding above the megathrust, forearcs seek a dynamic equilibrium between boundary stresses (drag below and lateral containments) with body stress (gravity acting on the shape of the forearc). Changes in boundary conditions are balanced by changes in the shape. The internal deformation history of the forearc, therefore, is expected to reflect changes in subduction tectonics during the evolution of the arc. We analyzed the structure of the Crotone Basin, located in northeastern Calabria, which is located in the exposed part of the forearc closest to the deformation front and to the Apennines. The main purpose was to compare the successive phases of deformation in the basin to the known evolution of the arc. We found four distinct events from the late Tortonian to the present. A widespread unconformity correlated with the onset of rollback marks a regional foundering with multidirectional normal growth faults. Following this pervasive and deeply rooted extension, the Crotone Basin experiences a period of parallel and distal sedimentation (Ponda clay). These sediments mark a relative long period (~5ma) of remarkable tectonic quiescence, even though subduction-rollback is moving the arc rapidly (3-5cm/yr) to the ESE. In addition, the forearc is shortening by progressive collision with Apulia (the Apennines) and Africa (the Maghrebides) during this time, but our study area is still far from the oblique collisions occurring at the ends of the forearc. The Messinian Salinity Crisis (5.3-6Ma) causes major instabilities in the accretion by loading it with evaporite deposits first and then removing the water load. Landward (westward) thrusting of the accretionary complex correlates with the Messinian in the Crotone basin and elsewhere along eastern Calabria. A characteristic fluvial conglomerate that locally caps the evaporite sequence records this thrusting by a systematic fracturing of the cobbles. After a well-known mid-Pliocene basin-forming extensional event, we find evidence of a basin-wide contractional event affecting the entire Neogene sequence up to the mid-to-late Pliocene. The data show a north-south compression with vergence to the north. This arc-longitudinal shortening may correlate with mid-Pliocene N-S shortening reported in the southern Apennines. Finally, many of these shortening structures are cut or reactivated by a recent (mid-Pleistocene?) faults, that accommodate extension also directed N-S to NW-SE. Our data show a shift from radial to longitudinal tectonics in the Pliocene as the Crotone basin nears the oblique collision with Apulia. Longitudinal forearc shortening may lead to extension in the Pleistocene, as the forearc squeezes through the narrow between Africa (Sicily) and Apulia, and begins lengthening as rollback consumes progressively wider Ionian lithosphere.