Initiation and development of the Twelve Mile Bay shear zone: the low viscosity sole of a granulite nappe

Abstract

Structural, microstructural and petrological data have enabled determination of the mechanical and geochemical processes involved in dynamic weakening and fabric transposition along the margins of a granulite nappe (the Parry Sound domain (PSD)) during transport to mid-crustal levels of the Grenville Orogen. The data establish a genetic link between outcrop-scale structures in the southern PSD and the development of the underlying Twelve Mile Bay shear zone (TMBSZ). Following granulite facies metamorphism (� 11 kbar ⁄ � 850 � C) in the southern PSD, the emplacement of pegmatite dykes resulted in hydration reactions within adjacent wall rocks and the development of thin (<1 m) amphibolite facies (� 6.5 kbar ⁄ � 700 � C) shear zones. The shear zones exhibit bulk H2O and K2O enrichment and oxygen isotope values similar to the adjacent pegmatites, suggesting metasomatic alteration by pegmatite-derived fluids. Phase-equilibrium models indicate that the destabilization of the pre-existing pyroxene and garnet-bearing assemblages, as observed within discrete shear zones in the southern PSD and the TMBSZ, requires H2O-saturated conditions at these (amphibolite facies) P-T conditions. The spacing between discrete shear zones and the depth of hydration into the adjacent wall rock are of comparable length-scales (� metres), suggesting that this type of reworking process can be an effective means of hydrating kilometre-scale areas of crust relatively rapidly. Furthermore, considering the well- established effects of hydrous fluids on the creep strength of anhydrous minerals, a fracture-initiated, localized hydration-and-shearing process may be an efficient mechanism for weakening strong, dry rocks (e.g. granulites) in the middle to lower orogenic crust.