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.
Results from two deep seismic reflection lines are presented. When combined, these lines span the rifted continental margin off Nova Scotia, from crust unaltered by rifting to the ocean basin. These data provide crustal and upper mantle reflection geometry to depths of over 50 km and elucidate the rifting process on this margin which occurred during the Mesozoic breakup of Pangaea. The continental crust below the continental shelf and slope becomes progressively thinner toward the ocean–continent boundary. In the upper crust, normal faults accommodated Mesozoic extension, and these flatten and terminate at 5–6 s (two-way time). In the lower crust and upper mantle Mesozoic rifting may be reflected in dipping events, which are interpreted to be normal faults. All Mesozoic extensional faulting could be controlled by the preexisting fabric of the crust, which in this region would be related to Appalachian compression within the Meguma Terrane. Below the continental rise, there is some evidence for magmatic underplating of the thinned continental crust, but the presence of synrift diapiric salt prevents clear definition of deeper structure. The extreme seaward end of the profile lies in a region interpreted in most other, earlier studies to be oceanic in nature. However, the seismic profile described here shows that relief on basement is associated with listric normal faults, which flatten in decollement, and that linear, landward-dipping intrabasement reflections characterize the area. These features can be explained in either a continental or an oceanic context.
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