BUILDUP AND DISMEMBERING OF THE EASTERN NORTHERN CALCAREOUS ALPS

The structural geometry and kinematics of a segment of the classical f old-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-Ma riazell-Puchberg)-lineament, which extends 400 km WSW-ENE from the nor thern Tauern window in the west to the Vienna basin in the east. It fo rms an orogen-parallel strike-slip zone, constitutes the major left-la teral wrench corridor accommodating eastward lateral extrusion of crus tal wedges along the central Eastern Alps, and crosses the NCA as an a nastomosing zone of distributed shear. The SEMP-line and related struc tures dismember a fold-and-thrust belt formed during early transpressi onal and subsequent orthogonal contraction. Most Early Miocene structu res are reversed in the Late Miocene accommodating little strain. Dist inct strain fields are calculated based on fault-slip and calcite defo rmation-twin data. They are regionally consistent in orientation and r elative age and correlate with the following deformation stages: (1) L ate Cretaceous to early Tertiary top-to-NW nappe stacking and right-la teral wrenching along NW-trending tear/transfer faults as an expressio n of the build-up of the Austroalpine orogenic wedge (e(3) = 317 +/- 1 7 degrees; e(1) = 058 +/- 28 degrees; e(1) greater than or equal to e( 2) greater than or equal to e(3), principal strains). (2) Early Tertia ry top-to-N stacking and conjugate strike-slip faulting reflecting the change from transpressional to frontal contraction within the NCA (e( 3) = 356 +/- 12 degrees; e(1) = 088 +/- 10 degrees). (3) Early to Midd le Miocene large-scale, left-lateral wrenching, including early transp ression and late transtension, as expression of eastward displacement of the southern part of the NCA (e(3) = 019 +/- 22 degrees; e(1) = 109 +/- 27 degrees). (4) Post-Middle Miocene E-W contraction, reactivatin g strike-slip and normal faults (e(3) = 092 +/- 10 degrees; e(1) = 004 +/- 12 degrees). The distributed nature of the lateral extrusion defo rmation and the anastomosing faulting along the SEMP-line reflect the rheological heterogeneity inherited by the lithology and multiple defo rmations of the NCA. A combined extension and stress-deflection model possibly accounts best for the pattern of northward concave faults bra nching off the SEMP-line.