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

The (micro)structural record of high pressure-low temperature (HP-LT) metam orphic rocks (T = 400 +/- 50 degrees C, P = 10 +/- 2 kbar) of the Phyllite- Quartzite Unit in western Crete, Greece, is interpreted in terms of deforma tion mechanisms and flow stress. Phyllites were deformed at low stress by d issolution precipitation creep, governed by strongly enhanced dissolution a long quartz-mica (001) phase boundaries. Quartzites and quartz veins were d eformed by dislocation creep at higher flow stress. The contrasting effecti ve viscosities caused stress concentration in the quartzites and quartz vei ns. Also, microstructures from aragonite marbles indicate that dissolution precipitation creep was the dominant deformation mechanism and that disloca tion creep has not been activated in these rocks. Pervasive ductile deforma tion was restricted to HP-LT metamorphic conditions and the microstructural record of deformation at maximum depth has been preserved, with all subseq uent deformation localized and confined to the brittle field. Constraints o n the timing of deformation allow an estimation of strain rates. Experiment ally determined flow laws for dislocation creep are used to pose upper boun ds on flow stress and bulk viscosities of rocks deformed by dissolution pre cipitation 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. Th is compares well to the viscosity contrast of 1-2 orders of magnitude refle cted by the mesoscopic structures. Since phyllites and carbonate rocks form large portions of the subducted sedimentary pile, the low flow stress duri ng rapid deformation of these rocks at HP-LT metamorphic conditions, and th e lack of deformation along the burial and exhumation path, imply very low strength of the plate boundary shear zones and negligible shear heating. (C ) 1999 Elsevier Science B.V. All rights reserved.