Deformation Processes of Subduction and Exhumation in Alpine Eclogites with Focus on the Tauern Window

Abstract:

Deformation processes inside subduction channels or during subduction and exhumation of HP and UHP nappes in collisional orogens are a matter of debate. Dehydration embrittlement, localized faulting and dissolution-precipitation creep have been suggested as major deformation processes. These processes, however, do not correspond to the field-based deformation record of penetrative foliations and stretching lineations in most HP and UHP nappes of the Central Alps, indicative of dislocation and diffusion creep processes including metamorphic reaction transfer. The Eclogite Zone of the Tauern Window (Eastern Alps) consists of fresh and retrogressed eclogites in a matrix of metasediments. Peak metamorphism was at 600°C and 2.0-2.5 GPa in the Oligocene followed by fast exhumation within a few million years. Eclogites and blueschists display a pronounced foliation and lineation fabric. Fresh and retrogressed samples have been investigated by neutron diffraction texture, electron backscatter diffraction and microprobe analysis. All investigated eclogites exhibit a pronounced crystallographic preferred orientation (CPO) of omphacite. In retrogressed eclogites the glaucophane CPO is strong as well and matches topologically always that of omphacite. Omphacite microstructures are characteristic of subgrain rotation recrystallization, identifying high-stress dislocation creep as main deformation mechanism. Diffusion and reaction transfer can be derived from the variable jadeite contents in omphacite porphyroclasts and recrystallized grains. The corresponding CPO of omphacite and glaucophane indicates that progressive CPO development spanned the whole high-pressure part of metamorphism and lasted until retograde blueschist grade. We propose that deformation in the Eclogite Zone was mostly distributed and continuous during subduction and exhumation and that dislocation and diffusion creep processes accommodated most of the strain below the frictional-viscous transition.