Garnets recording deep crustal earthquakes

Syn-eclogite facies pseudotachylytes of the Bergen Arcs, western Norway, were formed during co-seismic faulting at a depth of 60 km during Caledonian continent collision, concurrent with fluid-induced partial eclogitization of Precambrian anorthositic granulites. The pseudotachylytes contain the high-pressure assemblage omphacite, garnet and kyanite and formed in the metastable granulites at the border of eclogitized areas. Rock volumes corresponding to outcrops of 100 m(2) in area were shattered by pseudotachylytes associated with ultramylonites, cataclasites and shear zones. The granulite facies garnets, unzoned and inclusion-free away from the pseudotachylyte veins, display planar and curved fractures, brecciation and melting with decreasing distance from the veins. Sieve-textured garnet developed by the disintegration of granulite facies garnet, crystallization of eclogite facies mineral inclusions and welding together of garnet fragments. Fractured granulite facies garnets trend to more almandine-rich compositions, with variable grossular contents. In the pseudotachylyte veins, a second generation of minute (10-200 mu m) euhedral or dendritic garnets has a higher Fe/Mg ratio than the granulite facies garnet and a wide range in grossular content (Grs(11)-Grs(89)), suggesting rapid growth and cooling. Fracturing of garnet and its preferential disappearance in the pseudotachylyte show that garnet has a relatively low fracture toughness. Seismic faulting caused enhanced reaction rates during ongoing fluid-induced eclogitization of the deep crust by increasing the surface to volume ratio of garnet and its dislocation density. The sudden volume reduction associated with eclogitization of metastable crust may have nucleated further faulting and pseudotachylyte formation.