DEFORMATION PROCESSES IN ECLOGITIC ROCKS - EVIDENCE FOR THE RHEOLOGICAL DELAMINATION OF THE OCEANIC-CRUST IN DEEPER LEVELS OF SUBDUCTION ZONES

The deformation mechanisms and associated growth processes operating in omphacite during foliation development and veining have been studied in a km-scale eclogitic ductile shear zone from the Monviso meta-ophiolitic complex, Italian Western Alps. Dislocation creep due to multiple slip and/or climb is the dominant deformation mechanism, whereas (100) and (001) twinning, fracturing and intergranular fluid-phase diffusional creep are subordinate. Active slip systems include {110}[001], 1/2{110}[110BAR], (100)[001] and (1/2)(010)[100]. Fracturing is interpreted in terms of brittle instabilities arising from strain and pore fluid-pressure partitioning between adjacent omphacite domains. The transition from plastic to brittle behaviour of rocks is cyclic in nature as the internally generated fluids can be released and trapped repetitively during progressive deformation. The significance of the eclogitic shear zone is discussed in terms of a mechanically weak zone that developed during burial or at least prior to the peak of temperature at a minimum depth of 40 km in a subduction zone, and which subsequently acted as a zone of detachment during a late stage of the Alpine orogeny. A rheological model for oceanic crust delamination in deeper levels of subduction zones is presented.