ASSESSING THE KINEMATIC SIGNIFICANCE OF THE PLATTENGNEIS, A MAJOR INTRACRUSTAL TRANSPORT HORIZON IN THE KORALPE REGION, EASTERN ALPS

EBSD analysis of quartz fabrics has been used to test hypotheses for the origin of the Plattengneis, a km-thick near-horizontal mylonitic transport horizon exposed sporadically over ~1000 km² in the Koralpe region of the eastern Austrian Alps.

The kinematic significance of the Plattengneis to the regional tectonometamorphic evolution of the eastern Alps is controversial. Proposed interpretations include: (1) formation during collision, based on a preponderance of top-to-the-north shear-sense indicators; (2) formation during exhumation, based on suggestions that shear-sense indicators in the south are top-to-the-south, whereas those in the north are top-to-the-north; and (3) formation by extrusion during flattening of the shear zone, based on GASP barometry that suggests an original thickness of ~10 km, which would imply that the shear sense within the zone reverses from top to bottom.

Testing of these hypotheses requires knowledge of the vertical position of samples within the shear zone, which is rarely determinable from field observations alone. Following Putz (2006, Tectonophysics, 412:82), this study combines prior field mapping and prior and new structural observations with a digital elevation model (DEM) to locate each sample vertically within the shear zone. Fifty-three oriented samples were collected with the aim of creating four vertical transects from top to bottom of the shear zone in the northern, north-central, south-central and southern parts of the exposure.

Fabric interpretation of initial EBSD pole figures suggests a top-to-the-south shear sense in the south, a top-to-the-north shear sense in the north, and a possible shear-sense reversal from top to bottom in the central, high-temperature, portion of the shear zone. The data capture greater complexity than previous interpretations accommodate, and may represent superposition of fabric-development events associated with more than one of the hypothesized origins.