FABRIC ANALYSIS AND STRESS-STRAIN IN HIGH P-T VARISCAN CALCSILICATE TECTONITES TO THE SOUTH OF THE TAUERN WINDOW, AUSTRIAN ALPS

Alpine uplift has exposed the Austroalpine Variscan continental crust to the south of the Tauern Window, Austrian Alps. Variscan ductile deformational events in garnet- staurolite- and kyanite-bearing biotite-plagioclase paragneisses are a mainly non-coaxial progressive shearing D1-D2 with isoclinal folds, and a subsequent folding (D3) around the lineation L2. Both deformation events are older than mica cooling ages of 300 Ma. The paragneisses bear dm-scaled sheath-like calcsilicate-gneiss bodies which are elongated parallel to lineation L2. Quartz-c-axis fabrics signal deformation partitioning with non-coaxial shearing parallel to lineation and elongation axis along the margin of the body and coaxial deformation (flattening and constriction) in the center. Porphyroblastic garnets and local S-C offsets in the paragneiss host rock indicate the same sense of shear. Calcite is intergrown with all the main metamorphic mineral phases, including as tail growths on garnet porphyroblasts, and is thus interpreted as a primary recorder of the Variscan stress-strain history. Examination of mechanical twins in the calcite provided stress/strain orientations and magnitudes. Each sample had a high percentage of negative expected values (30-35%) such that two strain events were separated (PEV and NEV). The two events are: shortening parallel to the axis of the sheath fold (PEV), followed by vertical shortening (NEV). The second event also records extension parallel to the long axes of the bodies, corroborating the quartz data. The average shortening strain was -4.62%, and the average differential stress associated with twinning was -339 bars.