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.