Abstract

Results of 6000 km of crustal seismic profiling are presented with gravity and aeromagnetic maps for the Appalachian orogen in eastern Canada. Wide-angle seismic surveys show that the central mobile belt of the orogen has a thinner crust than its margins. High-velocity lower crust, attributed to underplating, is found below the former Laurentian continental margin in Newfoundland and below the Magdalen basin. Potential field data are used to trace the surface zones of the orogen from the northeast Newfoundland shelf to Cape Breton, but extrapolation to New Brunswick and Quebec is unclear because of late Paleozoic basin development. The central mobile belt of the orogen 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 coincides with the Avalon-Gander zone boundary elsewhere. The fabric formed during mid-Silurian continental collision and (or) during postorogenic collapse. Variation in fabric pattern and metamorphic grade, tightening of structures towards southwest Newfoundland and Cape Breton, and voluminous plutonism in southern Newfoundland are all in accord with maximal crustal thickening followed by erosion and isostatic readjustment in southwest Newfoundland and Cape Breton, and relatively little in northeast Newfoundland and its adjacent shelf. Reflection fabrics in the upper crust appear to be detached from those in the deeper crust; this is attributed to rheological contrast across the base of a quartz-rich upper crust.

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