Shear Zone Development and Rheology in the Deep Orogenic Crust

Within the Central Gneiss Belt (CGB) of the southwestern Grenville Province, Ontario, Canada, a number of allocthonous lithotectonic domains are juxtaposed along crustal-scale shear zones. Extensive exposure of variably reworked granulites of the interior Parry Sound domain (iPSD) has enabled investigation of the structural and petrologic character of domain-bounding shear zones within the deep orogenic crust. Recent detailed mapping and structural data collected along the southwestern margin of the iPSD is consistent with the suggestion of Culshaw et al. (in prep) that spaced outcrop-scale shear zones have coalesced and progressively reworked layered granulites into a transposed amphibolite-facies tectonite. The tectonites comprise the Twelve Mile Bay Shear Zone (TMBSZ), which separates the iPSD from para-autocthonous rocks to the south. This study investigates the grain- and outcrop-scale mechanisms involved in shear zone development and attempts to quantify the associated changes in rock rheology. Northwest of TMBSZ, samples collected across individual outcrop-scale shear zones (i.e., across large strain gradients) have distinct differences in mineralogy and microstructure. In mafic layers the original granulite texture and cpx + opx + pl + hbl +/- grt assemblage is commonly retained away from the shear zones within unsheared "panels". With proximity to the shear zones pyroxenes and garnet are progressively consumed in hydration reactions producing hornblende and biotite, which define a new planar foliation within the highly attenuated and deflected layering. Felsic layers generally have only minor mineralogical changes across the zones, but develop an increasingly intense and recrystallized structural fabric into the sheared margin. The shear zones are commonly cored by variably deformed pegmatite dikes that were emplaced prior to, or during the early stages of shearing. Evidence for incipient shear zone formation along mineralized fracture sets that cut unsheared granulites, often with clear centimeter-decimeter wide alteration halos, is preserved in adjacent rocks closer to the domain interior. Approaching the TMBSZ, the proportion of undeformed panel is decreased considerably and a finer-grained tectonite fabric becomes dominant. Panels in this area are more podiform, and relict layering is often at a much lower angle to the transposed fabric that wraps it. Large feldspar porphyroclasts, often with extensive tails parallel to layering, are commonly observed in the tectonite suggesting that these rocks represent widened and strongly attenuated pegmatite-cored shear zones. Thus, a six-stage conceptual model is proposed in which iPSD granulites are reworked into amphibolite facies tectonites (of the TMBSZ) through growth of spaced shear zones that initiated on fluid-filled fractures, and progressive consumption and rotation of relict granulite panels. Previous experimental studies have shown that changes in rock mineralogy and microstructure can have a substantial effect on bulk rheology and the partitioning of strain. For reactions where relatively rigid phases such as garnet and pyroxene are replaced by well-aligned biotite and amphibole, orders of magnitude strength drops may be possible. Structural data and observations are used here to constrain grain- and outcrop-scale numerical modeling exercises that evaluate the 1) strength contrasts between the shear zones and the granulite protolith, 2) bulk strength of crustal blocks with variable proportions of shear zones, and 3) sensitivity to various physical parameters.