Cataclastic deformation of garnet: a record of synseismic loading and postseismic creep

Microstructural evidence for episodic deformation of the uppermost plastosphere related to failure of the schizosphere in major seismic events has been previously reported from the Sesia Zone, Western Alps, with inferred extraordinarily high stresses and strain rates for the early stages of creep during stress relaxation at temperatures of about 300–350 °C. In some of these rocks, crystals of the high strength mineral garnet embedded in a quartz matrix are intensely fragmented. Detailed optical, electron microprobe and scanning electron microscopic study, combined with electron backscatter diffraction techniques, suggests that fragmentation of garnet took place quasi-instantaneously at very high peak stresses. Subsequent deformation during stress relaxation caused displacement of the garnet fragments by frictional sliding, controlled by dislocation creep in the surrounding quartz matrix. In a final stage, after restoration of a near-lithostatic pore fluid pressure, the garnet fragments were pulled apart and the opening cracks were filled by quartz and feldspar precipitated from the pore fluid. This record appears to be consistent with the proposed model of synseismic loading and postseismic creep, and the related pore pressure changes.