The question how extremely dense rocks like eclogites are exhumed in orogens is still a lively debated question in tectonics. In this case lattice preferred orientation in omphacite of the deeply exhumed Eclogite Zone (Tauern Window, Eastern Alps) were analysed and compared with kinematic indicators of the Eclogite Zone and its surrounding rock units. For testing the hypothesis that the Eclogite Zone represents an extrusion wedge characterized by a thrust fault at its base and a normal fault at its top, deformed eclogites from the bounding faults of the Eclogite Zone were collected assuming that LPO and kinematics should reflect the kinematic change during the exhumation process on both sides of the Eclogite Zone. The omphacite textures show strongly symmetric patterns across the entire Eclogite Zone, only in a few case a slight asymmetry is observed. The patterns are characterized by an alignment of {110} with the foliation and a maximum of <001> close to the lineation.
These patterns are indicative of deformation by intracrystalline glide on the (010) [001] and (110) [001] slip systems and seem to be not affected in any way during the exhumation. Otherwise the Eclogite Zone shows top-N kinematic indicators of mid-crustal levels at the base and strike-slip with a slight top-S indication at the top. According to this, the LPO patterns are in contradiction with the kinematics and do not fit an extrusion wedge hypothesis. Because of very rapid exhumation as calculated by Glodny et al. (2005) the kinematic indicators are interpreted to reflect the exhumation process. Therefore the omphacite textures are suggested to represent a prograde texture that was preserved during the exhumation process. This would not be in contradiction to an extrusion wedge suggestion. To confirm the data the textures were compared with omphacite patterns along a profile of the Western Gneiss Region (Norway) which is suggested to be a comparable tectonic setting than the Eclogite Zone. LPO patterns of the WGR show asymmetries that might indicate an extrusion wedge model. The textural difference between Eclogite Zone and Western Gneiss Region can be explained with different depths of eclogite formation (70 km for the EZ in comparison to 120 km for the WGR) and correspondingly different residence times under eclogite facies conditions. The residence time of the Eclogite Zone was too short to affect LPO in omphacite what enables the Eclogites Zone to keep the prograde signature during the exhumation process.
Glodny , J., Ring, U., Kühn, A., Gleissner, P. & Franz, G., 2005. Crystallization and very rapid exhumation of the youngest Alpine eclogites (Tauern Window, Eastern Alps) from Rb/Sr mineral assemblage analysis. Contributions to Mineralogy and Petrology 149, 699-712.
The Eclogite Zone in the central Tauern Window, Eastern Alps (Overview).
Symmetric omphacite LPO pattern from the Eclogite Zone, not affected during the exhumation process and representing a prograde texture.
Asymmetric omphacite LPO pattern from the western Gneiss Region, affected by deformation during the exhumation process.
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