Pressure solution in siliciclastic HP-LT metamorphic rocks — constraints on the state of stress in deep levels of accretionary complexes

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The magnitude of differential stress in deep levels of accretionary complexes along convergent plate margins is poorly constrained by theoretical models and cannot be directly measured. The only direct evidence to infer rheology and state of stress for such crustal settings is the microstructural memory of high-pressure/low-temperature metamorphic rocks recorded during their crustal evolution. Microfabrics in HP-LT metamorphic ( T = 400 ± 50°C, P = 10 ± 2 kbar) phyllites and quartzites of the Phyllite-Quartzite Unit on the island of Crete, southern Aegean, reveal (1) that progressive deformation was by pressure solution (dissolution precipitation creep), (2) that clastic quartz grains in the phyllites show no evidence for crystal plastic deformation during burial and exhumation, (3) that the unilaterally rational (001) mica quartz phase boundaries were sites of strongly enhanced dissolution, and (4) dislocation creep was restricted to a minor role in quartzites poor in mica. Consequently, the magnitude of differential stress in relation to temperature in the phyllites has remained below the level required to drive dislocation creep throughout the history of burial, to a depth of more than 30 km, and subsequent exhumation. Currently available flow laws for quartzite indicate that the differential stress in the phyllites remained well below 15 MPa at temperatures around 400°C at a depth of 30 km. This implies that the effective viscosity in deep crustal levels in forearc settings is much lower than that predicted by conventional models based on flow laws for dislocation creep.