Geophysical studies of the structure of the Appalachian orogen in the Atlantic borderlands of Canada

Results of 6000 km of crustal seismic profiling are presented with gravity and aeromagnetic maps for the Appalachian orogen in eastern Canada. Wide-an gle seismic surveys show that the central mobile belt of the orogen has a t hinner crust than its margins. High-velocity lower crust, attributed to und erplating, is found below the former Laurentian continental margin in Newfo undland and below the Magdalen basin. Potential field data are used to trac e the surface zones of the orogen from the northeast Newfoundland shelf to Cape Breton, but extrapolation to New Brunswick and Quebec is unclear becau se of late Paleozoic basin development. The central mobile belt of the orog en is only a few tens of kilometres wide in southwest Newfoundland and Cape Breton, but broadens substantially to around 200 km elsewhere. Reflection images show a strong deep-crustal fabric that runs along the orogen, with a margin that crosses into the Avalon zone in southern Newfoundland but coin cides with the Avalon-Gander zone boundary elsewhere. The fabric formed dur ing mid-Silurian continental collision and (or) during postorogenic collaps e. Variation in fabric pattern and metamorphic grade, tightening of structu res towards southwest Newfoundland and Cape Breton, and voluminous plutonis m in southern Newfoundland are all in accord with maximal crustal thickenin g followed by erosion and isostatic readjustment in southwest Newfoundland and Cape Breton, and relatively little in northeast Newfoundland and its ad jacent shelf. Reflection fabrics in the upper crust appear to be detached f rom those in the deeper crust; this is attributed to rheological contrast a cross the base of a quartz-rich upper crust.