Thrust imbrication and subsequent normal faulting in the western Austroalpine: The Arlui Fault and the Jaggl Mesozoic sediments (Ötztal Nappe, Vinschgau, Italy)

Jaggl is a mountain in the Upper Vinschgau, built up of Mesozoic sedimentary rocks and surrounded by crystalline basement rocks of the Ötztal Nappe. The Jaggl sedimentary rocks were originally interpreted as a remnant of the sedimentary cover of the Ötztal basement (“cover hypothesis”). Later, a different interpretation became prevalent, according to which the sedimentary rocks and a portion of the adjoining basement rocks represent a tectonic window into a deeper unit underlying the Ötztal Nappe (“window hypothesis”). A key role is played by the steeply southeast-dipping Arlui Fault along which the Jaggl Mesozoic rocks to the Southeast are in contact with Ötztal basement rocks to the Northwest. In the “window hypothesis”, the displacement along the Arlui Fault should be southeast-side-up, whereas in the “cover hypothesis”, it should be southeast-side-down. Our study of ductile (greenschist facies) to brittle fault rocks along the Arlui Fault yielded exclusively evidence for southeast-side-down, i.e. normal faulting. In addition, we identified the prolongation of the Arlui Fault towards northeast, represented by a top-southeast, greenschist-facies mylonite zone within the amphibolite-facies Ötztal basement. The Arlui Normal Fault belongs to the system of normal faults that overprinted the Austroalpine nappe stack during the Late Cretaceous, the Jaggl rocks are a downthrown part of the cover of the Ötztal Nappe, and the Jaggl Window does not exist.