Middle Triassic subsidence in the Dolomites (Italy): control of tectonics.

In the Dolomites region, Middle Triassic tectonics was associated with fast subsidence rates and a Late Ladinian magmatic event that induced epicrustal intrusions (Monzoni, Predazzo, Cima Pape) and a significant shoshonitic-basaltic volcanism. Contrasting scenarios (from transpressional/transtensional tectonics to subduction backarc extension) were proposed to explain these tectonics and magmatism (Lustrino et al., 2019 and reference therein). In order to contribute to unravel of this issue, we built subsidence curves and maps for different Triassic periods, based on cross sections and well data. In particular, we analysed the thicknesses of four Triassic carbonate systems (represented by Contrin Fm., Sciliar Fm., Cassian Dolomite and Heiligkreuz Fm.) in 21 areas spread on the central-eastern Southern Alps. Subsidence calculations, covering a time span of about 12 Ma (from Illyrian to Tuvalian), provided us the opportunity to quantify local and regional subsidence rates, and to constrain and reconstruct the time-space evolution of subsidence at a regional scale. From preliminary results, we can observe that: 1) a depocenter of subsidence was persistent in the eastern Dolomites from late Anisian to middle-late Carnian; 2) subsidence migrated toward east and north-east in Julian time; 3) the time-space evolution and migration of the subsidence was controlled by the principal Triassic (strike-slip and extensional) tectonic lineaments; 4) subsidence rates were faster both in the eastern Dolomites and in areas characterized by Ladinian magmatism (central Dolomites), reinforcing the idea of a control of tectonics (i.e. crustal thinning). Based on our observations, we speculate the occurrence, during Middle Triassic time, of a wide (crustal scale) strike-slip system, associated with magmatism, in which the Dolomites represented a pull apart basin. Within this scenario, in the Dolomites, N70°-90° strike slip tectonics generated transtensional-transpressional faults (associated with local extra subsidence and uplift, respectively).