PRESSURE SOLUTION IN SILICICLASTIC HP-LT METAMORPHIC ROCKS - CONSTRAINTS ON THE STATE OF STRESS IN DEEP LEVELS OF ACCRETIONARY COMPLEXES

The magnitude of differential stress in deep levels of accretionary co mplexes along convergent plate margins is poorly constrained by theore tical models and cannot be directly measured. The only direct evidence to infer rheology and state of stress for such crustal settings is th e microstructural memory of high-pressure/low-temperature metamorphic rocks recorded during their crustal evolution. Microfabrics in HP-LT m etamorphic (T = 400 +/- 50 degrees C, P = 10 +/- 2 kbar) phyllites and quartzites of the Phyllite-Quartzite Unit on the island of Crete, sou thern Aegean, reveal (1) that progressive deformation was by pressure solution (dissolution precipitation creep), (2) that elastic quartz gr ains 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 dissolu tion, and (4) dislocation creep was restricted to a minor role in quar tzites poor in mica. Consequently, the magnitude of differential stres s in relation to temperature in the phyllites has remained below the l evel required to drive dislocation creep throughout the history of bur ial, to a depth of more than 30 km, and subsequent exhumation. Current ly available flow laws for quartzite indicate that the differential st ress in the phyllites remained well below 15 MPa at temperatures aroun d 400 degrees C at a depth of 30 km. This implies that the effective v iscosity in deep crustal levels in forearc settings is much lower than that predicted by conventional models based on flow laws for dislocat ion creep.