Three seismically defined lower crustal blocks (LCBs) have been recognized
to underlie the familiar tectonic-stratigraphic zones of the Canadian
Appalachians. Tectonic development of the orogen is inferred to have
been strongly influenced by collision of the two outboard LCBs (Central
and Avalon LCBs) against the sharply irregular North American craton
(Grenville LCB), as evidenced by an abrupt offset in the trend of
the orogen localized at this irregular edge. The LCBs are restored
to a precollisional configuration by (1) removing Carboniferous strike-slip
motion on major faults linking through the Magdalen pull-apart basin;
and (2) removing Devonian(?) offset along a hypothesized fault through
the present-day Strait of Canso (Canso fault) to realign the Central
and Avalon LCBs, presumed to have been continuous prior to accretion
to North America. The restoration leads to (1) a qualitative explanation
for deep seismic reflection observations across the Magdalen basin
which suggest that portions of the underlying crust do not correspond
to any of the three principal LCBs; and (2) a plate tectonic model
for the orogen which suggests the backarc basin separating the Taconian
(Ordovician) arc from the eastern margin of the early Paleozoic Iapetus
ocean was at least as wide (>450 km) as the offset in the irregular
Grenville LCB, inherited from rifting of Iapetus. The present-day
Southwest Newfoundland Transform Margin may be a segment of the Canso
fault reactivated during Mesozoic opening of the Atlantic ocean.