Evolution of an eclogitized continental fragment in the Eastern Alps (Sieggraben, Austria)

The sequence of tectonometamorphic events that developed in continental basement rocks during the Alpine orogeny was studied in the eclogitic Sieggraben structural complex (SSC) in the Austrian Eastern Alps. The eclogitization of a pre-Alpine basement fragment occurred during Early Cretaceous when this was collisionally buried at the depth of ca. 50–55 km with an estimated mean temperature of 720°C at about 15 kbar (D1). Thermal relaxation under increasing temperatures to ca. 770°C and slightly decreasing pressures resulted in rapid exhumation recorded by symplectitic intergrowth of Cpx2Jd3-22 and Pl2 in eclogites and HP amphibolites. The exhumation was enhanced by the dynamic strain softening of prograde metamorphic OmJd38 and Zo1 into an aggregate of minor Cpx2Jd18-31 and Zo2. Dynamic recrystallization of Hbl1, feldspars and Qtz ribbons at high- to medium-temperatures was followed by medium- to low-temperature Qtz recrystallization and Cal twinning. The textures indicate dislocation creep as the principal micromechanism of ductile deformation of Cpx, Am, Pl and Qtz, or dominated mechanical twinning in Cal. Textural patterns are mainly related to high–medium temperature mesoscopic fabrics (D2). Compressional exhumation along a detachment fault initiated top-to-SSE extensional unroofing of the eclogitized SSC now overlying the low-grade MP/HP Lower Austro–Alpine structural complexes.