Deformation processes in a transtentional mantle shear zone within fore-arc oceanic lithosphere: Mavrovouni shear zone, Pindos Mts, Greece

The Mavrovouni Shear Zone (MSZ), Greece, is a mantle shear zone in the Pindos ophiolite. The NW-SE trending Pindos peridotite is the remnant of a supra-subduction zone in the Jurassic Pindos Ocean. The MSZ has a width of 2 km, strikes NE-SW, dips steeply to the SE, and has a moderately to steeply S toplunging lineation. Kinematic analysis shows that the MSZ has an oblique normal sense of shear. The strain gradient across the MSZ involves lateral transition in the microstructural configuration across four structural domains: 1) Host spinel harzburgites; 2) A marginal zone with harzburgites characterized by two planar fabrics (one host rock-related, and another shear zone-related); 3) A domain of strongly deformed protomylonites; and 4) A mylonitic core. Within the mylonite zone, the presence of dm-scale dunite bands - alternating with the host harzburgites - suggest channelized melt migration facilitated by deformation. Ultramylonite zones of mm-scale are present in the MSZ but their formation is not associated with the large-scale strain gradient described above.

Three-dimensional shape fabric analyses of spinel and orthopyroxene grains from the different structural domains shows dominantly prolate fabric ellipsoids. Olivine crystallographic preferred orientations (CPOs) show well-developed axial-[100] and orthorhombic (010)[100] symmetries. The fabric geometry and CPO results agree with the transtentional deformation in the MSZ constrained independently from the kinematic analysis. Activation of the olivine (010)[001] slip system indicates that the MSZ may have served as a fluid pathway leading to upper mantle hydration. Using olivine recrystallized grain size and subgrain size paleopiezometers, we place constraints on the differential stresss within the MSZ. Our results constrain the mechanical properties of extensional/transtensional shear zones, and we hypothesize that the MSZ formed during subduction initiation in supra-subduction setting.