TECTONIC JUXTAPOSITION AND TRANSCURRENT FAULTING IN THE DEEP CRUST, THE STRIDING-ATHABASCA AREA, NORTHERN SASKATCHEWAN

Data from geophysical, geochemical, and xenolith studies all provide information about the state of the deep crust, but the nature of deformation processes are less clearly revealed. Isobarically cooled high-P terrains, regions that existed in the deep crust for an extended period of time before exhumation, are an important source of information on crustal processes. The East Athabasca area of northern Saskatchewan is an example that may preserve a major 40 km wide deep crustal fault/shear zone. The shear zone has been divided into three subdomains, each with a distinct tectono-metamorphic history. The southeastern domain consists of mylonitized tonalite gneiss intruded by mafic dikes which experienced syntectonic dehydration melting at 800C, 1.0 GPa. Rocks in the northwestern domain document all stages in the transformation of Opx-bearing igneous rocks into Gt-Cpx-Hbl-Pl granulite gneisses (at 0.9 GPa, 750C). The southern, structurally highest, domain contains gneisses and eclogite bodies that preserve evidence of an early eclogite facies (1.5-2.0 GPa, 1000C) history overprinted by granulite grade (1.0 GPa, 750C) deformation.

The three domains were juxtaposed during transcurrent shearing at 2.6 Ga, and exhumation occurred at 1.8-1.9 Ga. Thus, the East Athabasca region provides a view of the deep crust from 2.6-1.9 Ga, and this view includes a major transcurrent shear zone and its wall rocks. The picture presented is not one of a simple layered lower crust, but of a lithologically complex region with heterogeneous steeply and shallowly dipping compositional boundaries. Mass and heat transfer, in the form of mafic and felsic plutonic rocks controlled many aspects of deformation and metamorphism. Although anhydrous rocks are present, water is an important component, and served to localize deformation, metamorphism, and migmatization. Finally, deformation process within the lower crustal shear zone were apparently able to transport and juxtapose diverse tectonic components from distinct depths and tectonic settings. This emphasizes the heterogeneity that may characterize parts of the deep crust, and that must be incorporated into models of unexposed crustal sections.