FLUID-MEDIATED DEFORMATION LEADS TO WEAKENING, STRENGTHENING AND BLOCK-IN-MATRIX STRUCTURES DURING PROGRADE SUBDUCTION AND MELANGE FORMATION

Mélange, or block-in-matrix structures, influence mechanical and chemical processes along the subduction plate interface. However, the timing and mechanisms that form these nearly ubiquitous units remain controversial. Field observations and microstructural analyses from a largely metasedimentary mélange unit in the lawsonite blueschist rocks of the Catalina Schist, CA reveal a set of syn-subduction fluid-mediated deformation processes leading to mélange formation at the plate interface. Initially deposited as turbidites, these rocks accommodated some shear parallel to compositional layers (S1) during early prograde deformation. At near peak subduction conditions (~1.0 GPa, 320°C) the rocks were intensely folded as top to the SW recumbent near isoclinal folds (F2) with axial planar cleavages (S2). Fracturing, fluid flow, and quartz precipitation is preserved as extensional veins in fold noses creating a vein mesh network resulting in a net strengthening of these areas. Continued shearing led to beheading of these strengthened noses which progressively appear as strong blocks within the weaker less veined matrix composed of high strain fold limb surfaces (S1-2). Importantly, microstructures reveal fluids accommodated viscous deformation in the high strain fold limbs by pressure-solution creep of phyllosilicate-pinned fine-grained quartz and albite. In contrast, fold noses/blocks contain coarse-grained quartz veins with little evidence of dynamic recrystallization, preservation of coarse grained albite, and kinked strain hardened phyllosilicates. Rheological modeling shows that initial and evolving quartz grain size controls strength, leading to strong coarse grained, vein-rich blocks and weak, fine-grained, highly sheared fold limbs that became a mélange matrix. This reveals a set of concomitant deformational and fluid-mediated processes that result in a classic block-in-matrix mélange structure occurring during prograde subduction, and the strengthening and weakening effects of mineral precipitation from water.