Drainage pattern evolution during continental indentation in Central Alps: Constraints from the sedimentary record of associated deep-water clastic offshoots

Abstract

This work reconstructs the evolution of a source-to-sink system in a transpressional setting from compositional changes within its deep-water clastic offshoots. Field-based clast counts on 10 conglomerate beds of three transects, combined with petrographical and mineralogical (XRD) analyses on 14 samples, have been used to integrate the already-existing large dataset developed during more than 60 years of study on the Como Conglomerate (the base of the Gonfolite Lombarda Group, Chattian to Aquitanian – Northern Italy). The results shed a new light on the terranes involved in the production of sediments and further constrain the evolution of the fluvial drainage in response to the Cenozoic Adria indentation during the post-collisional phases of Alpine orogeny. In particular, it has been possible to track-back the steps during which paleorivers were connected, and later disconnected, with the basin due to the Oligo-Miocene activity of the right-lateral Periadriatic Fault System (PFS). During its transpressive movement, accommodating the Adria westward indentation, the PFS cumulated a total displacement increasing from ca. 20 km to the west, up to ca. 60 km, to the east. The results of this study also constrain the post-depositional kinematics of the clastic wedge, which was progressively rotated clockwise and thrust to its present-day position, onto the Southalpine Mesozoic sequence. This model represents a fundamental step forward in the comprehension of how tectonic indentation interacts with surface source-to-sink systems, and demonstrates that tectonic induced connections/disconnections of fluvial systems are as fundamental as exhumation processes in the transfer of sediments from source to sink areas.