TRANSPRESSIONAL COLLISION STRUCTURES IN THE UPPER CRUST - THE FOLD-THRUST BELT OF THE NORTHERN CALCAREOUS ALPS

Two major structural events characterize the tectonic evolution of the Northern Calcareous Alps (NCA): (1) late-Early Cretaceous to Late Eoc ene NW-directed, dextral-transpressional stacking of nappes as an expr ession of the formation of the Austroalpine orogenic wedge; and (2) Mi ocene sinistral wrenching due to eastward lateral extrusion of crustal wedges along the central Eastern Alps. Three first-order detachment h orizons are defined by differences of competence within areas of facie s transition. Preexisting normal faults control the thrust architectur e. Transpressional contraction in the NCA is indicated by: (a) NW-dire cted thrusting, oblique to both the long axis of the NCA and the edge of the orogenic foreland; (b) the occurrence of en-echelon arrays of t hrusts and folds laterally displaced by dextral strike-slip faults; th e thrusts and strike-slip faults are kinematically connected to each o ther and dissect the NCA into rhomboidal blocks; (c) NE-directed exten sion parallel to fold and ramp axes and the internal strike of the NCA ; and (d) clockwise rotation (greater than or equal to 30 degrees) of the entire NCA around a vertical axis. Kinematic and dynamic analysis of mesoscale fault-striae data related to transpressional stacking ind icates a NW-trend of contraction directions and, in general, a NE-tren d of the extension directions, parallel to fold axes and branch lines of thrusts. Before rotation, the average contraction direction was pro bably parallel to the generally W-directed shear direction recorded in the crystal-plastically deformed central part of the Eastern Alps. Th ree balanced cross-sections across the NCA yield a minimum of 54-65% t otal shortening.