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

Abstract

Remnants of the early-Ottawan thrust-sheet stack are exposed in the Central Gneiss Belt (CGB, lower portion of stack) and the Composite Arc Belt (upper portion of stack). Post-collisional vertical thinning and associated horizontal extension of the stack produced structures ranging over eight orders of magnitude in horizontal length, and both orogen-parallel and orogen-perpendicular in orientation. At the 100 km scale, the fold-induced constriction in the northern Parry Sound domain appears to have been enhanced, and lineation trend lines in its footwall locally deflected, by a component of NW–SE (i.e., orogen-perpendicular) flattening and a component of NE–SW (i.e., orogen-parallel) ductile extension. At the 10 km scale, four non-cylindrical lenticular bodies of gabbro–anorthosite gneiss within the domain, inferred to be triaxial mega-boudins or heterogeneously strained plutons, are separated by large extensional bending folds, the complementary structures attesting to a component of NW–SE flattening and a component of NE–SW extension. Non-cylindrical lenticular structures in other domains of the CGB, interpreted as triaxial foliation mega-boudins, exceed 30 km in length. Their moderately strained granulite-facies interiors give way to highly strained amphibolite-facies margins, thus documenting subvertical ductile flattening and multi-lateral extension during retrogression. Well-layered, highly strained gneiss is commonly deformed by steep NE–SW-trending extensional faults and associated monoclinal fault-propagation folds (FPFs). The short limbs of the FPFs bend the regional elongation lineation and host a set of fault-parallel, unstrained to slightly deformed, granite–pegmatite dikes. Dilation vectors of most dikes are oblique to the granite–pegmatite contacts, and the sense of their tangential components attests to orogen-perpendicular extension. The fault-parallel dikes and associated FPFs are cut by a set of unstrained dikes. Collectively these observations document a prolonged history of post-collisional extension of the mid crust, from ductile structures indicative of a significant component of orogen-parallel extension shortly after the metamorphic peak at mid-crustal depths, to brittle–ductile structures indicative of a component of orogen-perpendicular extension and associated magmatic dilation following its exhumation and cooling in the upper crust.