Build-up and dismembering of the eastern Northern Calcareous Alps

Abstract

The structural geometry and kinematics of a segment of the classical fold-and-thrust belt of the Northern Calcareous Alps (NCA) is analyzed by map, mesoscale fault-slip, and calcite-twin data. The most distinct feature of the eastern NCA is the Miocene SEMP (Salzachtal-Ennstal-Mariazell-Puchberg)-lineament, which extends 400 km WSW-ENE from the northern Tauern window in the west to the Vienna basin in the east. It forms an orogen-parallel strike-slip zone, constitutes the major left-lateral wrench corridor accommodating eastward lateral extrusion of crustal wedges along the central Eastern Alps, and crosses the NCA as an anastomosing zone of distributed shear. The SEMP-line and related structures dismember a fold-and-thrust belt formed during early transpressional and subsequent orthogonal contraction. Most Early Miocene structures are reversed in the Late Miocene accommodating little strain. Distinct strain fields are calculated based on fault-slip and calcite deformation-twin data. They are regionally consistent in orientation and relative age and correlate with the following deformation stages: (1) Late Cretaceous to early Tertiary top-to-NW nappe stacking and right-lateral wrenching along NW-trending tear/transfer faults as an expression of the build-up of the Austroalpine orogenic wedge ( e 3 = 317 ± 17°; e 1 = 058 ± 28°; e 1 ≥ e 2 ≥ e 3, principal strains). (2) Early Tertiary top-to-N stacking and conjugate strike-slip faulting reflecting the change from transpressional to frontal contraction within the NCA ( e 3 = 356 ± 12°; e 1 = 088 ± 10°). (3) Early to Middle Miocene large-scale, left-lateral wrenching, including early transpression and late transtension, as expression of eastward displacement of the southern part of the NCA ( e 3 = 019 ± 22°; e 1 = 109 ± 27°). (4) Post-Middle Miocene E-W contraction, reactivating strike-slip and normal faults ( e 3 = 092 ± 10°; e 1 = 004 ± 12°). The distributed nature of the lateral extrusion deformation and the anastomosing faulting along the SEMP-line reflect the rheological heterogeneity inherited by the lithology and multiple deformations of the NCA. A combined extension and stress-deflection model possibly accounts best for the pattern of northward concave faults branching off the SEMP-line.