Mechanisms of thrust faulting in the gran sasso chain, central apennines, italy

Abstract

The Gran Sasso chain in Central Italy is made up of an imbricate stack of eight thrust sheets, which were emplaced over the Upper Miocene—Lower Pliocene Laga Flysch. The thrust sheets are numbered from 1 to 8 in order of their decreasing elevation in the tectonic stack, and their basal thrusts are numbered from T 1 to T 8, accordingly. On the basis of their different deformation features, the major thrust faults fall into three groups: (1) thrust faults marked by thick belts of incoherent gouges and breccia zones (T 1, T 2, T 3); (2) thrust faults characterized by a sharp plane which truncates folds that had developed in the footwall rocks (T 5, T 6); and (3) thrust faults truncating folds developed in both the hangingwall and footwall units, and bordered by foliated fault rocks (T 7). The deformation features observed for the different faults seem to vary because of two combined factors: (1) lithologic changes in the footwall and hangingwall units separated by the thrust faults; and (2) increasing amounts of deformation in the deepest portions of the imbricate stack. The upper thrust sheets (from 1 to 6) are characterized by massive calcareous and dolomitic rocks, they maintain a homoclinal setting and are truncated up-section by the cataclastic thrust faults. The lowermost thrust sheets (7 and 8) are characterized by a multilayer with competence contrasts, which undergoes shear-induced folding prior to the final emplacement of the thrust sheets. Bedding and axial planes of folds rotate progressively towards the T 5, T 6, T 7 and T 8 thrust boundaries, and are subsequently truncated by propagation of the brittle thrust faults. The maximum deformation is observed along the T 7 thrust fault, consistent with horizontal displacement that increases progressively from the uppermost to the lowermost thrust sheet in the tectonic stack. The axial planes of the folds developed in the hangingwall and footwall units are parallel to the T 7 thrust fault, and foliated fault rocks have developed. Field data and petrographic analysis indicate that cleavage fabrics in the fault rocks form by a combination of cataclasis, cataclastic flow and pressure-solution slip, associated with pervasive shearing along subtly distributed slip zones parallel to the T 7 thrust fault. The development of such fabrics at upper crustal levels creates easy-slip conditions in progressively thinner domains, which are regions of localized flow during the thrust sheet emplacement.