Nd isotope mapping of a tectonic window near Chalk River, Ontario: Implications for large-scale Grenvillian tectonics

Over forty new Nd isotope determinations are presented for granitoid orthogneisses from the Chalk River area, on the border between Ontario and Quebec in the SW Grenville Province. TDM model ages provide evidence for a tectonic window through the allochthonous belt into the Grenvillian parautochthon in the Chalk River area. This tectonic window constrains the location of the main ramp of the Allochthon Boundary Thrust southeast of the Chalk River area. However, the structure of the tectonic window has been complicated by down-faulting on the Ontario side of the Ottawa River, due to reactivation of late Precambrian normal faulting associated with the Ottawa-Bonnechere graben. Thus, on the Ontario side of the border, the surface expression of the tectonic window is represented by a strongly sheared tectonic duplex that underlies the main allochthon. However, the lower section of a 1 km-deep borehole drilled by Canadian Nuclear Laboratories has TDM model ages indicating penetration into the underlying parautochthon. The new data, combined with previous Nd isotope mapping and Lithoprobe seismic reflection results, point to a relatively constant thickness for the exhumed Muskoka allochthon. By showing that this exhumation occurred immediately below the Central Metasedimentary Belt boundary, this evidence strongly supports a ramp-flat thrusting model for the SW Grenville province. This model greatly reduces the likelihood of major crustal exhumation to the NW of Parry Sound domain, thus removing the theoretical basis for pervasive southeast directed orogenic collapse across the Ontario gneiss belt.