Three deep-penetration seismic reflection profiles were collected off southwest Nova Scotia to determine the crustal structure and geometry beneath the Avalon and Meguma zones of the Appalachian Orogen in Canada. Onshore geological features have been traced seawards using new gravity and magnetic anomaly maps. The seismic data can also be correlated with the previous United States Geological Survey profile in the central Gulf of Maine.Two seismically distinct lower crustal blocks are identified: the Avalon and Sable lower crustal blocks, separated by a major north-dipping reflection zone that cuts the entire crust. The recognition of the Sable block adds a fourth block to the three already identified in the Canadian Appalachians. The Sable block is overlain by the Meguma Zone. The Avalon Zone overlies at least the northern part of the Avalon lower crustal block. Although offshore extension of geological features is not unequivocal, it appears that a north-dipping reflection zone southwest of Nova Scotia marks the site of Devonian thrusting of Avalon Zone over Meguma Zone. In the Bay of Fundy to the north, two south-dipping reflection zones are interpreted as major thrusts, possibly placing Avalon lower crust over a unit with different tectonic affinities. The Fundy Fault is a Carboniferous thrust within the Avalon block along the coast of New Brunswick; this was reactivated during Mesozoic extension as a transtensional fault. Extensional displacement farther southwest was probably accommodated along east-west-trending faults and small rift basins associated with them.