Structural synthesis of the Northern Calcareous Alps, TRANSALP segment

The Northern Calcareous Alps (NCA) are the site of very large top-to-north convergent movements during Cretaceous-Tertiary Alpine mountain building. To determine the amount of shortening, the depth of detachment and the style of deformation, we retro-deformed an approximately 40 × 40 km area comprising the Lechtal and Allgäu Nappes. On the basis of all available geological data and processed sections of the TRANSALP reflection seismic experiment, coherent 3D models were constructed by splining lines from N-S cross-sections. Integration of 3D kinematic modeling and field data shows the following. The structure of the Lechtal Nappe is controlled by the Triassic Hauptdolomit. Four main thrusts link to a detachment at 2-6 km depth below sea level. Shortening estimates vary, from 25% (east) to 42% (west). Additional contraction is accommodated by folding. In the east the subjacent Allgäu Nappe can be traced about 10 km down-plunge, and is shortened by about one third. In the western part the downplunge width is at least 15-20 km, with restorable shortening of one third. The triple (Inntal, Lechtal, Allgäu Nappes) NCA nappe system was moved uniformly N-S to produce laterally heterogeneus shortening of 40-90 km or 50-67%. We suggest that the NCA are underlain by substantial amounts of buried Molasse sediments and/or overthrust units of Helvetic and Rheno-Danubian Flysch, indicating post-Eocene N-S shortening of at least 55 km. Restored to an initial configuration, the basin topography of the NCA reveals strong E-W thickness variations of the Triassic Wettersteinkalk and Hauptdolomit platform carbonates. Such variations may pertain to N-S trending growth faults, which were important precursors to later Jurassic extension of the Austroalpine passive margin. Such structures are unlikely to be seen in the conventional N-S cross-sections, but form an essential geometrical and mechanical element in later, convergent mountain building.