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Constriction during exhumation: Evidence from eclogite microstructures

¹ Institut für Angewandte Geowissenschaften (Technische Geologie und Angewandte Mineralogie), Technische Universität Graz, Rechbauerstrasse 12, A-8010 Graz, Austria

The Eclogite zone (Tauern Window, Eastern Alps, Austria) represents one of only a few examples of high-pressure units providing both the prograde and the retrograde metamorphic evolution. The eclogites are associated with rocks originating from continental and transitional crust that also have been affected by high-pressure metamorphism. Eclogites that formed along the prograde path indicate metamorphic conditions of 17–20 kbar (1700–2000 MPa) at 550–580 °C; the peak assemblages formed at 21–25 kbar (2100–2500 MPa) at 600–620 °C. Omphacite microstructures, in particular shape fabrics and crystallographic preferred orientations, indicate that the final phases of the prograde evolution were characterized by flattening strain. The evolution at the pressure peak and along the exhumation path shows a constrictional strain geometry. This change is interpreted to have been controlled by the force balance between slab pull (related to an oceanic slab) and the buoyancy of subducted adjacent continental crustal material, including the eclogites. At a certain lithospheric level, where the subducted continental rocks were entirely surrounded by high-density lower-crustal and upper-mantle material, the buoyancy forces exceeded the slab-pull forces. This force change resulted in the buoyancy-driven extrusion of continental material between two lithospheric plates. The part of the subducted slab that remained in buoyant equilibrium was therefore affected by constriction. Moreover, this process may have resulted in breakoff of the subducted oceanic slab. If so, the cessation of slab pull resulted in accelerated buoyancy-driven extrusion of the continental material. Extrusion of these high-pressure sheets would have been accompanied by constriction subparallel to the subduction channel as well.

Key Words: eclogites • microstructures • crystallographic preferred orientations • flattening • constriction • Eastern Alps

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