Contractional and transcurrent tectonics in the Marmolada Group (Dolomites, Italy)

The Marmolada Massif (Southern Alps) consists of sedimentary successions ranging in age from the Late Permian to the late Middle Triassic. These successions are involved in a complex polyphase Alpine deformation, which is apparently stronger here than in the surrounding areas. Integrated stratigraphic and structural analyses, based on a detailed 1:10,000 geological mapping, were performed. The Upper Permian shows evaporitic sulphates, that acted as the major detachment level, during the Alpine deformation. The Middle Triassic was characterised by tectonic uplift, followed by accelerated subsidence, that controlled the development of subtidal carbonate platforms (Contrin Fm) and of elongated basins, followed by the aggradation of three independent buildups (Marmolada, Vernel and Ombrettola), surrounded by deep water basins. A nice example of platform-top to slope section is visible at the Marmolada Glacier front. The platform-top is very rich in subtidal calcarenites, punctuated by emersion surfaces. The wave resistant reef was very rich in automicritic crusts and marine cements, while the huge slope deposits show nice clinostratifications. The Ladinian was characterised by a short lived magmatic episode, with high-K calc-alcaline to shoshonitic affinity. In the Marmolada area, at least two main Alpine tectonic phases have been recognised. The first phase, which is certainly the more important in terms of deformation, is characterised by low- to middle-angle contractional faults, associated with important shortening. The northern Marmolada and Vernel Platforms overthrusted the southern Ombrettola Platform and the slope clinostratifications were often activated as reverse faults. Kinematic indicators on the principal structures document a vergence variability between SSE and SSW, probably related to the mechanical anisotropies of the stratigraphic cover or to two possible subevents. North-verging back-thrusting planes have also been discovered. For comparison with other sectors of the Southern Alps, the age of the major shortening is probably Oligo-Miocene. The second phase is characterised by high-angle mainly strike-slip faults, which partly dismembered the older low-angle structures. Particularly important is the shear zone developed along the northern slope of the Contrin and Ombrettola Valleys, changing in direction from WNW-ESE to W-E. This phase is probably Late Messinian-Pliocene in age and strongly influences the present-day landscape.