Exhumation of the Saualpe eclogite unit, Eastern Alps: constraints from 40Ar/39Ar ages and structural investigations |
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| Authors: | M Wiesinger F Neubauer R Handler |
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| Institution: | (1) Division General Geology and Geodynamics, University of Salzburg, Salzburg, Austria |
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| Abstract: | Summary The Cretaceous Eclogite-Gneiss unit and its tectonic overburden (Micaschist, Phyllite and Lower Magdalensberg units) and the
underlying Preims subunit of the Saualpe, Eastern Alps, have been investigated in order to constrain the mode of exhumation
of the type locality of eclogites. 40Ar/39Ar ages of white mica from the eclogite-bearing unit suggest rapid, uniform cooling and exhumation between 86 and 78 Ma (Santonian-Campanian).
Overlying units show upwards increasingly older ages with an age of 261.7 ± 1.4 Ma in the uppermost, low-grade metamorphic
unit (Lower Magdalensberg unit). We consider this Permian age as geologically significant and to record a Permian tectonic
event. Rocks of phyllite and micaschist units along western margins of the Saualpe block yield amphibole and white mica ages
ranging from 123 to 130 Ma. These are considered to closely date the age of nappe stacking, whereas a single biotite age of
66–68 Ma from a shear zone is interpreted to date retrogression during normal faulting. Biotite and amphibole of Micaschist
and Eclogite-Gneiss units show variable contents of extraneous argon. Consequently, their ages are in part geologically meaningless
whereas other samples yield meaningful ages. The white mica ages from the Eclogite-Gneiss unit range from 78 to 85 Ma and
argue for cooling through ca. 400 °C during the time as the westerly adjacent Upper Cretaceous Krappfeld collapse basin formed.
The Preims subunit with paragneiss and marbles is considered to represent a large synmetamorphic shear zone at the base of
the overthrusting Eclogite-Gneiss unit. The unit comprises a flat-lying foliation and a SE-trending lineation. This zone is
interpreted to represent a zone of top-NW thrusting. A major ductile low-angle normal fault with top to ESE shear has been
detected between the Eclogite-Gneiss and overlying units, and between the Micaschist and Phyllite units. The ductile thrust
at the base and the low-angle normal fault at the top are considered to confine a NW-ward extruding high-pressure wedge. The
new observations argue for rapid exhumation of a subducted high-pressure wedge within a subduction channel. Rapid surface
erosion of the exhuming wedge might have facilitated exhumation. Eroded sedimentary rocks are preserved within adjacent Gosau
basins, although only pebbles of low-grade metamorphic rocks of the uppermost tectonic unit can be found in these basins. |
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