The tectonic evolution of the Friuli Alps (NE Italy) in the context of the Alpine-Dinaridic orogenic system

Abstract

The Friuli Alps in NE Italy are located directly at the intersection of the south west vergent Dinarides and the south vergent Alps and is therefore a key area to study interference tectonics caused by the two mountain ranges. In order to understand the structural and paleo-geographical evolution of the geologically complex area, a clear separation of Dinaridic and Alpine structures had to be made. For this purpose field kinematic data have been collected, which serve together with the geological map to construct a NNE-SSW trending crosssection. This cross-section provides important insights into the subsurface structure of the area and is used to estimate the amount of shortening that affected the Friuli Alps since the beginning of the Paleocene, utilizing balancing software MOVE. The stratigraphic formations occurring in the area are an alternation of soft evaporates and basinal deposits and rigid platform carbonates; this alternation most likely influences the tectonic style of the area. The field data and balanced cross section define four stages of deformation: (D1) N-S extension, creating E-W striking normal faults, that have been dated as Late Cretaceous or older. (D2) NNE-SSW shortening, characterized by (S)SW vergent thrust faults and NW or SE plunging fold axes, related to the Dinaridic orogeny (60 - 30 Ma). The D2 phase caused typical ramp-flat structures in the subsurface, which accommodated thinskinned tectonics along the "soft" Bellerophon and Raibl formations. (D3) N-S shortening, related to the Alpine phase (23 Ma), which is defined by north and south vergent folds and faults. The large fault offsets suggest thick-skinned tectonics. (4) Dextral E-W to ENE-WSW oriented transpression due to NW-SE reactivation of preexisting thrusts, starting at 8 Ma. The youngest two phases (D3 and D4) are still active, as they showed recent seismic activity. The D2 thin-skinned thrusting had the biggest influence on the total amount of shortening in the Friuli Alps; it facilitated 57 km of the overall shortening. The subsequent D3 thick-skinned thrusts that displaced the D2 faults, added an additional amount of 11 km to this shortening, resulting in a total 68 km of shortening. However, the amount of D2 shortening is considered to be a minimal amount, due to the lack of shortening constraints on this phase at the surface. More shortening was expected, which could for example be obtained by introducing more shallow ramps or longer décollements into the balanced cross-section. The latter would require a substantial amount of subduction of the Adriatic plate underneath the European plate, which favours the subduction polarity switch theory in this area.