Structure of Austroalpine and Penninic units in the Tilisuna area (Eastern Rätikon, Austria): Implications for the paleogeographic position of the Allgäu and Lechtal nappes

Abstract.

This study presents new data related to the architecture and evolution of the Penninic-Austroalpine plate boundary in the vicinity of the Prättigau halfwindow (eastern Switzerland). The Tilisuna area in the northeastern corner of the Prättigau halfwindow provides a structural section from the Middle Penninic Sulzfluh nappe through various South Penninic and Lower Austroalpine units up to the Upper Austroalpine Phyllitgneis zone. The Tertiary age basal thrust of the units derived from the Apulian margin over the Penninic domain is located at the top of the lowermost subunit of the Arosa zone, which represents the remnants of the South Penninc ocean. The hanging wall of this basal thrust is formed by a nappe pile of higher South Penninic, Lower Austroalpine and Upper Austroalpine units that was already assembled in Late Cretaceous times. It is shown that the Cretaceous nappe stack beneath the Phyllitgneis zone in the study area is identical to that found below the Silvretta nappe southeast of the Prättigau halfwindow. In this scheme the Arosa Dolomites nappe represents the southern prolongation of the Allgäu nappe of the Northern Calcareous Alps. The results confirm earlier petrographic studies proposing that the Phyllitgneis zone and the Silvretta nappe are not separated by an important Alpine fault. Instead, the Phyllitgneis zone and the Silvretta nappe represent one and the same unit. Accordingly, the deposition area of the western Calcareous Alps (Lechtal- and Allgäu nappes) was located north of the Silvretta nappe.