STRAIN LOCALIZATION IN DEEP CONTINENTAL CRUST: EXAMPLES FROM AN EXHUMED SECTION IN THE WESTERN CANADIAN SHIELD

Heterogeneous behavior of deep crustal deformation is controlled by a variety of factors including lithologic variations, pre-existing structures and earlier metamorphism, the distribution of melt, and infiltration of externally derived fluids. Here, we review examples of multi-scale interactions between deformation and metamorphism that illustrate some of these changing intrinsic and extrinsic conditions during shear zone development. The Athabasca Granulite Terrane in the western Canadian shield records at least 700 m.y. of tectonism, but emphasis here is on structures that developed during the peak and exhumational stages of the youngest event (1.9-1.8 Ga). The 1.88 Ga Cora Lake shear zone is a dominantly sinistral strike-slip structure characterized by multiple strands of ultramylonite superimposed on ~5 km wide zone of mylonite. Rock types vary but all display evidence for deformation under relatively dry, high-pressure granulite-facies conditions (1.0-0.8 GPa, 850-750 °C). The ultramylonite zones also host pseudotachylyte networks and associated kinematically compatible shear fracture systems. Stable granulite-facies mineral assemblages, including grain-size reduced pyroxene, in the immediate vicinity of the shear fractures and local development of new garnet in the pseudotachylyte matrix suggest seismic slip under hot and deep crustal conditions. In contrast, deformation in a younger (1.85 Ga) set of km-scale amphibolite-facies shear zones reflects a conversion back to fully plastic and more widely distributed flow, with hydration and reaction-weakening via growth of mica and amphibole as facilitating mechanisms. Thus, the AGT may record portions of multiple cycles of distributed and focused deformation even within already localized high strain zones via a range of plastic to brittle modes. These examples are chosen to illustrate how localized strain and attendant metamorphic processes played key roles in the styles and scales of deformation that were operative in deep continental crust and in their preservation in the rock record.