Implications for the North American Lower Crustal 7.xx km/s Layer from an Exhumed Crustal Root, Western Canadian Shield

Studies of the Thorsby magnetic low, the southern extent of the 2,800 km-long Snowbird Tectonic Zone (STZ) in Alberta, Canada, show pronounced lower crustal seismic reflectivity and preservation of a crustal root. These features may be correlative to the well-exposed central STZ, where two lower crustal tectonic events have been identified at 2.6-2.55 Ga and 1.92-1.89 Ga. High-resolution cross-sections of the central STZ reveal the late-stage interaction of Paleoproterozoic intracontinental thrust and strike-slip shear zones responsible for juxtaposing ca. \u3e20,000 km2 of lower continental crust (0.9-1.5 GPa) with middle continental crust (0.5 GPa) (Mahan and Williams, 2005; Dumond et al., 2013). Exhumed crust within the Athabasca granulite terrane (AGT), in northeastern Saskatchewan, revealed that garnet-producing reactions in felsic to mafic granulites resulted in significant densification during long-term lower crustal residence. These reactions provide a viable mechanism for the preservation of crustal roots long after orogeny (Fischer, 2002). This study utilized surface outcrops of mafic granulite and felsic granulite in northern Saskatchewan to test the hypothesis that they represent an exposed record of crustal root development that is presently imaged in the subsurface of Alberta. A georeferenced map and database was compiled that includes topography, aeromagnetic anomalies, and gravity anomalies coincident with the Upper Deck domain of the Athabasca granulite terrane. These data were coupled to phase equilibria modeling of lower crustal bulk compositions to derive a 2-dimensional density and velocity structure of the hypothesized crustal root. The results informed our understanding of how garnet production in lower continental crust influences the stability and isostatic response of continental crust that was thickened and modified during orogeny