Nd isotope mapping of allochthonous Grenvillian klippen: Evidence for widespread ‘ramp-flat’ thrust geometry in the SW Grenville Province

Over sixty new Nd isotope analyses are used to constrain the extent of several small Grenvillian lithotectonic domains that form erosional outliers of the Grenvillian Allochthonous Polycyclic Belt. The identification of these tectonic klippen, when combined with previous mapping of large nappes, shows that the Allochthonous Polycyclic Belt forms a mid-upper crustal sub-horizontal thrust sheet that was transported to the NW over the Archean-Proterozoic parautochthon. This thrust sheet was corrugated by folding and dissected by erosion, leaving the klippen as synformal remnants. Nd isotope mapping of these remnants, when coupled with seismic reflection data from the Lithoprobe programme, suggests that these thrust sheets were exhumed on relatively steep crustal-scale ramps before transportation to the NW on sub-horizontal shear zones. This type of thrust geometry, previously observed in the central—eastern Grenville Province was termed ‘ramp-flat geometry’ by Rivers et al. (2012). Re-examination of Lithoprobe seismic data for the Georgian Bay transect shows that reflectors under the Parry Sound klippe have a synformal character in the mid crust, suggesting that this is also part of an undulose sub-horizontal thrust sheet exhumed on a steep crustal-scale ramp, rather than the previous model of a very long ramp with low inclination. Hence we argue that the Georgian Bay section has a geometry resembling the ramp-flat trajectory of the Allochthon Boundary Thrust previously discerned elsewhere in the Grenville Province.