The structural, metamorphic and thermal history of the Sonnblick Dome, southeast Tauern Window, Austria

Abstract

Within the southeastern Tauern Window, the Sonnblick Dome is a large, NE-verging, antiformal structure composed of orthogneisses of the Zentralgneis Complex. This unit represents part of the European crystalline basement, or Penninic domain, over which the Adriatic microplate was thrust during Alpine continental collision. The igneous precursors to the gneisses formed as granitoids above a subduction zone during the Hercynian. During Alpine continental collision, overthrusting of the African-derived Austroalpine units toward the northwest produced a foliation that becomes more intense towards the tectonic contact of the gneiss and the overlying Peripheral Schieferhülle. This foliation was folded during the formation of the Sonnblick Dome, which is interpreted to have developed during progressive top-to-NW shearing in the hangingwall of an oblique ramp. Shear zones also developed oblique to the northwest transport direction and led to imbrication in the basement. These shear zones are commonly marked by retrogression of the primary mineralogy and the development of mica-schists. Although this alteration is associated with syn-deformational fluid infiltration, a spatial relationship between reaction site and deformation suggests that the energy associated with deformation contributed to reactions during shear zone formation. As a response to tectonic thickening, pressures and temperatures in the Pennine basement increased. Peak Alpine metamorphic conditions are estimated to be 540±50°C and 8±lkbar and probably represent conditions developed during uplift from initially greater depths. White mica isotopic ages suggest that the peak of metamorphism took place at 25-28Ma, with older ages being observed towards the southeastern end of the Dome. Post-metamorphic cooling rates appear to be variable throughout the Dome, with faster rates being found for the southeastern end of the Dome. After 20Ma ago, cooling rates around the Dome became more uniform (17-27°C/Ma). Rapid cooling rates in the area are associated with rapid, post-metamorphic uplift rates. These were probably accomodated by gravity-driven extension of the tectonically thickened crust. Evidence for post-metamorphic extension is represented by ductile shear bands, which are associated with thinning of the more micaceous units found at higher levels in the Dome