EVIDENCE OF DETACHMENT WITHIN PERI-GONDWANAN ARC TERRANES OF MARITIME CANADA

Low-grade platformal metasedimentary rocks juxtaposed against high-grade gneisses along late Neoproterozoic ductile shear zones are characteristic features of parts of the peri-Gondwanan terranes of Maritime Canada. In southern New Brunswick, the MacKay Highway shear zone brings the low-grade Green Head Group into contact with the high-grade Brookville Gneiss, whereas in the Cobequid Highlands of mainland Nova Scotia and the Creignish Hills of Cape Breton Island, ductile shear zones bring the low-grade Gamble Brook Formation and George River Suite into contact with the high-grade Great Village River Gneiss and Bras d'Or Gneiss, respectively. Kinematic indicators within the mylonites that define these shear zones give shear senses towards the NE in southern New Brunswick, the ESE in the Cobequid Highlands, and the SE in the Creignish Hills. Synkinematic granitoids and metamorphism (low-P/high-T in southern New Brunswick and Cape Breton Island) date these three events at ca. 565-540 Ma, ca. 605 Ma, and ca. 550Ma. In each case, deformation placed low-grade over high-grade rocks in a manner consistent with extensional detachment, while rotation of associated Carboniferous strata to the horizontal suggests the shear zones were initially low-angle structures, each with an easterly component of shear. In the Cobequid Highlands, detachment was synchronous with arc magmatism and has been attributed to pull-apart basin development in response to oblique subduction. In southern New Brunswick and Cape Breton Island detachment broadly coincides with the termination of arc magmatism and may reflect diachronous ridge-trench collision.