Comment on se-2021-19

Abstract

Neogene indentation of the Adriatic plate into Europe led to major modifications of the Alpine orogenic structures and style of deformation in the Eastern Alps. Especially, the offset of the Periadriatic Fault by the Northern Giudicarie Fault marks the initiation of strike-slip faulting and lateral extrusion of the Eastern Alps. Questions remain on the exact role of this fault zone in changes of the Alpine orogen at depth. This necessitates quantitative analysis of the 5 shortening, kinematics and depth of decoupling underneath the Northern Giudicarie Fault and associated fold-and thrust belt in the Southern Alps. Tectonic balancing of a network of seven cross sections through the Giudicarie Belt parallel to the local shortening direction reveals that it comprises two kinematic domains with different amounts and partly overlapping ages of shortening. These two domains are delimitated by the NW-SE oriented strike-slip Trento-Cles -Schio-Vicenza fault system, cross-cutting the Southern Alpine orogenic front in the south and merging with 10 the Northern Giudicarie Fault in the north. The SW kinematic domain (Val Trompia sector) accommodated at least ~18 km of Late Oligocene to Early Miocene shortening. Since the Middle Miocene, the SW kinematic domain experienced a minimum of ~12-22 km shortening, whereas the NE kinematic domain underwent at least ~25-35 km shortening. Together, these domains contributed to an estimated ~53-75 km of sinistral strike-slip motion along the Northern Giudicarie Fault, implying that (most of) the offset of the Periadriatic Fault is due to Late Oligocene to 15 Neogene indentation of the Adriatic plate into the Eastern Alps. Moreover, the faults linking the Giudicarie Belt with the Northern Giudicarie Fault reach ~15-20 km depth, indicating a thick-skinned tectonic style of deformation. These fault detachments may also connect at depth with a lower crustal Adriatic wedge that protruded north of the Periadriatic Fault and was responsible for N-S shortening and eastward escape of deeply exhumed units in the Tauern Window. Finally, the east-west lateral variation of shortening indicates internal deformation and lateral variation in 20 strength of the Adriatic indenter, related to Permian -Mesozoic tectonic structures and paleogeographic domains. 25