Vibroseis seismic reflection data have been recorded to 18 s two-way
traveltime along three transects across the island of Newfoundland.
The upper crust has both steep and subhorizontal reflectors consistent
with a ramp - flat style of deformation, whereas the middle and lower
crust are largely free of regional flats. Reflectors descend through
ca. 20 km of vertical section in the middle and lower crust to flatten
into the Moho or perhaps cut through it in places. The Moho is interpreted
to be no younger than the dipping reflectors. Reflection fabrics,
interpreted to be indicators of dominantly Mid-Ordovician to Mid-Silurian
strain, show consistent orientations among the transects and divide
the crust into two blocks. A northwestern block is characterized
by upper and middle crustal reflectors dipping mostly southeast at
variable angles. This block is underlain to the southeast by supposedly
younger and dominantly northwesterly dipping reflectors that define
a northwest-tapering, wedge-shaped block floored by the Moho. This
latter block is cut by isolated southeast-dipping, upper crustal
reflectors near the southeast ends of the seismic transects. One
of these reflectors is spatially correlated with the Bay d'Est Fault,
on which the last ductile motion was south over north thrusting of
Mid-Silurian age. The two crustal blocks are proposed to represent
the Laurentian and Gondwanan plates juxtaposed during closure of
the Iapetus Ocean. The Gondwanan plate appears to be underthrust
westward beneath the Laurentian plate, perhaps by as much as 200
km.