Transpressional collision structures in the upper crust: the fold-thrust belt of the Northern Calcareous Alps

Abstract

Two major structural events characterize the tectonic evolution of the Northern Calcareous Alps (NCA): (1) late-Early Cretaceous to Late Eocene NW-directed, dextral-transpressional stacking of nappes as an expression of the formation of the Austroalpine orogenic wedge; and (2) Miocene sinistral wrenching due to eastward lateral extrusion of crustal wedges along the central Eastern Alps. Three first-order detachment horizons are defined by differences of competence within areas of facies transition. Preexisting normal faults control the thrust architecture. Transpressional contraction in the NCA is indicated by: (a) NW-directed thrusting, oblique to both the long axis of the NCA and the edge of the orogenic foreland; (b) the occurrence of en-échelon arrays of thrusts and folds laterally displaced by dextral strike-slip faults; the thrusts and strike-slip faults are kinematically connected to each other and dissect the NCA into rhomboidal blocks; (c) NE-directed extension parallel to fold and ramp axes and the internal strike of the NCA; and (d) clockwise rotation (⩾ 30°) of the entire NCA around a vertical axis. Kinematic and dynamic analysis of mesoscale fault-striae data related to transpressional stacking indicates a NW-trend of contraction directions and, in general, a NE-trend of the extension directions, parallel to fold axes and branch lines of thrusts. Before rotation, the average contraction direction was probably parallel to the generally W-directed shear direction recorded in the crystal-plastically deformed central part of the Eastern Alps. Three balanced cross-sections across the NCA yield a minimum of 54–65% total shortening.