History and tectonic implications of low‐angle detachment faults and orogen parallel extension, Picentini Mountains, Southern Apennines fold and thrust belt, Italy

Abstract

Late Miocene to Pliocene movement on low‐angle extensional faults within the internal Southern Apennines orogenic belt was superposed on an earlier, Miocene imbricate thrust stack. The low‐angle faults formed within the interior of the belt during orogen parallel extension as thrust imbrication continued in the foreland. Extreme tectonic thinning defines discrete structural domains of hyperextension which are linked by a complex system of extensional and transcurrent faults. Some of the best examples of hyperextension structures in the Southern Apennines are exposed in the Picentini Mountains. In this area, detailed mapping, fault‐kinematic analysis, and excellent stratigraphie control contributed to the construction of restorable cross sections and forward models of deformation. With these constraints, it is possible to document extensional displacement on shallowly dipping supercrustal faults whose orientation is a primary feature and is not due to later tilting. During movement, upper plate rocks were disarticulated by listric and planar normal faults that soled into a ramp‐flat detachment system. The depth of the basal detachment increased in the direction of upper‐plate motion and ranged from 5 to 10 km. Displacement on the low‐angle detachment was accompanied by block tilting in the upper plate assemblage, incisement and excisement of the upper and lower plate rocks as fault trajectories changed through time, and the progressive cataclasis of hanging wall and footwall assemblages. Preexisting thrusts were only locally reactivated during extension, and faults emanating from the underlying decollement systems cross the imbricated thrust sheets at moderate to high angles. Longitudinal extension resulted in thinning of the thrust stack to less than half the original thickness and had a cumulative magnitude of between 200 and 250% (beta>2).