Re-interpretation of the TRANSALP seismic section in the light of new tectonic subdivisions of the western Northern Calcareous Alps

In this study we present a reprocessed part of the TRANSALP seismic section between the Austrian-German border and the Ziller valley. It offers new insights into deep tectonic structures around the lower Inn valley, which are interpreted with the use of recently published tectonic classifications of the area. The lateral extension of the observed major structures is discussed with another deep cross section across the Kaisergebirge. This study demonstrates that Paleogene out-of-sequence thrusting locally controls the relationship between different tectonic units. Paleogene thrusts overprint pre-existing nappe boundaries, and were erroneously interpreted as Cretaceous nappe boundary. This is the case for the large, laterally persistent unit of the Tirolic Staufen-Höllengebirge- and Bajuvaric Lechtal nappes, which are separated by a Paleogene out-of-sequence thrust and the Oligo-Miocene Inntal Shear Zone. Consequently, this means to eliminate the concept of a tectonic subdivision solely based on in-sequence nappe stacking, and Triassic sedimentary facies distribution. We suggest to use a nomenclature also based on geometry of the Permo-Mesozoic cover nappes. Our proposed nappe classification leads to a newly defined Karwendel-Höllengebirge Nappe, occupying most of the middle to western Northern Calcareous Alps, incorporating the crystalline basement, which is in primary contact with the Permo-Mesozoic cover. The partly existing sedimentary contact to the basement further allows us to propose a new terminology on cover nappe systems in the Northern Calcareous Alps, getting rid of the terms Bajuvaric, Tirolic and Juvavic. Oligocene-Miocene thrusting and eastward extrusion along the Subtauern Ramp and steep strike-slip structures - like the Inntal shear zone – have further complicated the structure of the region, off-setting older in- and out-of-sequence structures, and finally exhuming basement units underneath the cover nappe stack south of the Inn valley.