Lawsonite vorticity and subduction kinematics

The exceptional preservation of lawsonite in eclogite and blueschist in a subduction complex (Sivrihisar, Turkey) allows evaluation of high-pressure (HP) deformation and exhumation using vorticity analysis of a HP index mineral. The Sivrihisar HP rocks were deformed during HP metamorphism and initial decompression by oblique (normal and/or dextral) shearing beneath serpentinized peridotite. We evaluated kinematic vorticity (W_m pure shear = 0, W_m simple shear = 1) using the shape fabric of lawsonite crystals in blueschist layers and meter-scale eclogite pods. Pods record dominant simple shear (W_m > 0.8) in their eclogitic core and at pod margins partially transformed to blueschist; map-scale blueschist layers record a large component of pure shear (W_m = 0.4-0.6), consistent with oblique extrusion at depth in the subduction zone, and local simple shear (W_m = 0.8) at the fault contact with serpentinite. A combination of oblique unroofing from beneath a weak serpentinized hanging wall and pure shear extrusion in the subduction channel accounts for rapid exhumation along a low geothermal gradient and the rare preservation of lawsonite eclogite during exhumation to the Earth's surface.