Structure of Ahmic domain and its vicinity, southwestern Central Gneiss Belt, Grenville Province of Ontario (Canada)

Post-convergent structures in lower parts of the 1090–1050 Ma (early-Ottawan) thrust-sheet stack, Grenville Province of Ontario, southern Canadian Shield

“…3), was previously interpreted as a product of superimposed deformation (Schwerdtner 1987(Schwerdtner , 1988Gower 1992), but an alternative possibility, not considered in the earlier studies, is that it developed after the metamorphic peak during regional buckling under conditions of ductile transtension (e.g., Dewey et al 1998). Such an interpretation is attractive because it has been known for some time that buckling played a major role in the formation of some km-scale cross-folds, especially those that deform the domain boundaries in the lower parts of the CGB thrust-sheet stack (Schwerdtner 1988;Schwerdtner and van Berkel 1991;Schwerdtner and Klemens 2008). Irrespective of their origin, it is important to note that km-scale cross-folds are absent from the walls of the CAB boundary (formerly Central Metasedimentary Belt boundary; Ontario Geological Survey 1992; Schwerdtner et al 2010b), suggesting that cross-folding was confined to the lower portion of the earlyOttawan thrust-sheet stack.…”

“…3), in which mesoscopic boudinage of amphibolite dikes was associated with migmatization and occurred under peak metamorphic conditions at ca. 1080 Ma, implying boudinage took place during thrusting at mid-crustal levels (Bussy et al 1995;Klemens 1996;Schwerdtner and Klemens 2008). On this basis, mesoscopic boudins in peak assemblages elsewhere in the CGB, locally with wide gaps attesting to >500% bulk extension of competent layers, are also inferred to have formed at mid-crustal levels.…”

“…Other mesoscopic boudins in the western CGB occur within cross-folds that developed by late-orogenic buckling of lithostructural contacts and the main foliation (Schwerdtner and van Berkel 1991;Schwerdtner and Klemens 2008). The presence of biotite-rich veinlets and strained plagioclase pseudomorphs after garnet attests to their formation in the upper crust, but some mesoscopic cross-folds bend, and therefore postdate, boudinaged layers (Schwerdtner 1987 , his fig.…”

“…8b, 8c), for which an origin by gravitational sinking of the dense Parry Sound allochthon has been proposed (Lindia et al 1983;Schwerdtner et al 2010a). Unfortunately, this hypothesis is difficult to assess rigorously due to superposed deformation (Davidson et al 1982;Schwerdtner and Klemens 2008). It is feasible that superposed km-scale NW-SE-trending cross-folding was responsible for the southerly plunge and hinge-line curvature of the Parry Sound synform (Schwerdtner et al 2010a, their fig.…”

“…The basal Parry Sound domain furnishes two km-scale examples of type-2 extensional-bending folds, both of which are defined by the curved form-lines of the main foliation and the traces of transposed marble-breccia units in the structural gap between the lenticular Whitestone body and its northern neighbour ( Fig. 7; Schwerdtner and Klemens (2008), their fig. 2).…”