Late Cretaceous exhumation history of an extensional extruding wedge (Graz Paleozoic Nappe Complex, Austria)

Abstract

Late Cretaceous structures within the eastern Graz Paleozoic Nappe Complex define an extruding wedge with north-eastward directed thrusting in eastern portions and strike-slip shear along the margins. Stacking structures are overprinted by south-westward directed extension with low-grade metamorphic rocks in the hangingwall and highgrade basement rocks in the footwall. Pressure-temperature and structural data are obtained from successively opening quartz veins that record various stages of progressive deformation and metamorphism. Fluid inclusion data and related structures show that during extension isothermal decompression from ca. 550°C and 8 kbar down to ca. 450°C and 2 kbar was related to exhumation of rocks from deep crustal levels. The data point to a high geothermal gradient and explain condensed paleo-isotherms due to ductile normal faulting in the eastern areas of the Graz Paleozoic Nappe Complex. The investigated Late Cretaceous structural elements suggest that the Graz Paleozoic Nappe Complex decoupled from the surrounding basement units and operated as a large-scale extensionextrusion corridor that evolved prior to Miocene extrusion tectonics in the Eastern Alps.

Polyphase metamorphism at the southeastern margin of the Graz Paleozoic and the underlying Austroalpine basement units

“…Based on Sm-Nd ages on garnet cores and U-Th-Pb chemical dating on monazite, the Rappold Complex experienced a Permian metamorphic imprint (Rög-gla, 2007;Gaidies et al, 2008) and an Eoalpine (Cretaceous) overprint. Calculated P-T-conditions of the Permian garnet cores from the ACU are about 580±20°C at about 0.8 GPa (Rög-gla, 2007;Krenn et al, 2008) and are in agreement with conditions of the Rappold Complex in the Niedere Tauern mountain range (Faryad and Hoinkes, 2003;Gaidies et al, 2006). Du-______________________ ____________ __________________ Table 8: The structural distribution of cations after Spear and Kimball (1984) for the same amphibole analyses (given in Tab.…”

“…The dotted square indicates the position of the map shown in Figure 1B. (B) Geological sketch map of the Graz Paleozoic and the underlying crystalline complexes (modified after Krenn et al 2008). The study area is indicated by the dotted square.…”

“…Metamorphic conditions in the Paleozoic of Graz were studied by Hasenhüttl (1994), Russegger (1996), Rantitsch et al (2005), and Krenn et al (2008). In general sub-greenschist to lower-___ Philip SCHANTL, Ralf SCHUSTER, Kurt KRENN & Georg HOINKES microscope was used at the Institute of Earth Sciences, University of Graz.…”

“…Based on different models the main imprint was attributed to the Variscan or Eoalpine (Cretaceous) tectonometamorphic event (Frank, 1981;Flügel et al, 1980;Esterlus, 1986;Fritz, 1988;Russegger 1996;Neubauer et al, 1999;Krenn et al, 2008). According to Fritz et al (1992) an Eoalpine thrusting event at about 125 Ma is the major event, and most of the deformation in the GP took place during the Cretaceous.…”

“…9C). No chloritoid occurs within the matrix of the mica schists from the Rossegg Complex but Krenn et al (2008) described the appearance of staurolite from the quarry near to Steg.…”