GARNETS RECORDING DEEP-CRUSTAL EARTHQUAKES

Syn-eclogite facies pseudotachylytes of the Bergen Arcs, western Norwa y, were formed during co-seismic faulting at a depth of 60 km during C aledonian continent collision, concurrent with fluid-induced partial e clogitization of Precambrian anorthositic granulites. The pseudotachyl ytes contain the high-pressure assemblage omphacite, garnet and kyanit e and formed in the metastable granulites at the border of eclogitized areas. Rock volumes corresponding to outcrops of 100 m(2) in area wer e shattered by pseudotachylytes associated with ultramylonites, catacl asites and shear zones. The granulite facies garnets, unzoned and incl usion-free away from the pseudotachylyte veins, display planar and cur ved fractures, brecciation and melting with decreasing distance from t he veins. Sieve-textured garnet developed by the disintegration of gra nulite facies garnet, crystallization of eclogite facies mineral inclu sions and welding together of garnet fragments. Fractured granulite fa cies 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 grossula r content (Grs(11)-Grs(89)), suggesting rapid growth and cooling. Frac turing of garnet and its preferential disappearance in the pseudotachy lyte 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 r atio of garnet and its dislocation density. The sudden volume reductio n associated with eclogitization of metastable crust may have nucleate d further faulting and pseudotachylyte formation.