Evidence for Triassic contractional tectonics in the Northern Dolomites (Southern Alps, Italy)

Abstract

The occurrence of Middle Triassic extensional and strike-slip tectonics has been widely recognized in the Dolomites and is supported by stratigraphic evidence. Contrarily, the recognition of Triassic contractional tectonic structures is less straightforward because such structures were heavily superimposed by Neogene Alpine shortening.In this work we provide a detailed analysis of a top-class outcrop displaying a syn-sedimentary Middle Triassic extensional fault that displaces a coeval sedimentary mélange containing an olistholith of Lower Triassic rocks, characterized by Triassic contractional structures (folds, axial plane cleavage, joints, local thrust faults ornamented with slickenfibers and associated with forced folds).In particular, integrating geological field mapping and structural analyses, structural analyses on a 3D virtual outcrop model and microstructural analyses, we demonstrate that: 1) the orientation of tectonic structures in the olistholith is at odd with that of regional structures, implying that deformation occurred prior to deposition of the melange; 2) folds and small-scale thrusts in the olistholith developed in lithified rocks, pointing to the existence of Middle Triassic fold and thrust tectonics. In summary, the tectonic structures in the analyzed olistholith represent an outstanding, possibly unique, example of fully preserved, practically frozen, Middle Triassic contractional structures.