Low effective viscosity during high pressure metamorphism due to dissolution precipitation creep: the record of HP–LT metamorphic carbonates and siliciclastic rocks from Crete

Abstract

The (micro)structural record of high pressure–low temperature (HP–LT) metamorphic rocks ( T=400±50°C, P=10±2 kbar) of the Phyllite-Quartzite Unit in western Crete, Greece, is interpreted in terms of deformation mechanisms and flow stress. Phyllites were deformed at low stress by dissolution precipitation creep, governed by strongly enhanced dissolution along quartz–mica (001) phase boundaries. Quartzites and quartz veins were deformed by dislocation creep at higher flow stress. The contrasting effective viscosities caused stress concentration in the quartzites and quartz veins. Also, microstructures from aragonite marbles indicate that dissolution precipitation creep was the dominant deformation mechanism and that dislocation creep has not been activated in these rocks. Pervasive ductile deformation was restricted to HP–LT metamorphic conditions and the microstructural record of deformation at maximum depth has been preserved, with all subsequent deformation localized and confined to the brittle field. Constraints on the timing of deformation allow an estimation of strain rates. Experimentally determined flow laws for dislocation creep are used to pose upper bounds on flow stress and bulk viscosities of rocks deformed by dissolution precipitation creep. For the phyllites, a conservative estimate is about 10 19 Pa s, or below, in contrast to 10 20 Pa s derived for the quartzites. This compares well to the viscosity contrast of 1–2 orders of magnitude reflected by the mesoscopic structures. Since phyllites and carbonate rocks form large portions of the subducted sedimentary pile, the low flow stress during rapid deformation of these rocks at HP–LT metamorphic conditions, and the lack of deformation along the burial and exhumation path, imply very low strength of the plate boundary shear zones and negligible shear heating.