BRUNSWICK SUBDUCTION COMPLEX IN THE CANADIAN APPALACHIANS - RECORD OFTHE LATE ORDOVICIAN TO LATE SILURIAN COLLISION BETWEEN LAURENTIA AND THE GANDER MARGIN OF AVALON

The Brunswick subduction complex in the New Brunswick part of the Cana dian Appalachians records the Late Ordovician to Late Silurian collisi on between Laurentia and the Gander margin of Avalon. The Brunswick co mplex is anomalously well preserved compared with equivalent rocks and structures elsewhere owing to its unique position in the deepest part of the Quebec reentrant of the Laurentian margin. This part of the ma rgin experienced less underthrusting and exhumation and overprinting b y orogen-parallel faulting than the adjacent promontories where collis ion started earlier. The early, southeast to east vergent thrust-relat ed structures represent a progressive D1 deformation that formed in re sponse to northwestward subduction of the previously extended Gander m argin and subsequent tectonic unroofing of the subduction complex. The original, shallow northwestward dipping envelope to S1 was deformed i n the Late Silurian by D2 upright folds and associated shear zones int o a steep belt during terminal collision. The D2 structures probably f ormed in response to sinistral transpression. Together, D1 and D2 indi cate that convergence was oblique and sinistral. Most rocks incorporat ed in the subduction complex formed in the Tetagouche back arc basin t hat evolved from rifting of an Arenig magmatic arc built on the Gander margin into a wide marginal basin. Subduction was initiated in the La te Ordovician (almost-equal-to 455 Ma) in the back arc basin following collision of the Middle Ordovician Popelogan arc with Laurentia in th e Caradoc, shortly after its opening in the late Arenig (almost-equal- to 473 Ma). The closure history of the Iapetus Ocean involved more tha n one subduction zone and arc-continent collision and rivals the south western Pacific Ocean in its complexities.