Combined onshore-offshore seismic refraction/wide-angle reflection
data have been acquired across Newfoundland, eastern Canada, to investigate
the structural architecture of the northern Appalachians, particularly
of distinct crustal zones recognized from earlier Lithoprobe vertical
incidence studies. A western crustal unit, correlated with the Grenville
province of the Laurentian plate margin thins from 44 to 40 km and
a portion of the lower crust becomes highly reflective with velocities
of 7.2 km/s. In central Newfoundland, beneath the central mobile
belt, the crust thins to 35 km or less and is marked by average continental
velocities, not exceeding 7.0 km/s in the lower crust. Further east,
in a crustal unit underlying the Avalon zone and associated with
the Gondwanan plate margin, the crust is 40 km thick, and has velocities
of 6.8 km/s in the lower crust.
Explanations for the thin crust beneath the central mobile belt include
(1) post-orogenic isostatic readjustment associated with a density
in the mantle which is lower beneath this part of the orogen than
beneath the margin, (2) mechanical thinning at the base of the crust
during orogenic collapse perhaps caused by delamination, and (3)
transformation by phase change of a gabbroic lower crust to eclogite
which seismologically would be difficult to distinguish from mantle.
Except for a single profile in western Newfoundland, velocities in
the crust are of typical continental affinity with lower-crustal
velocities less than 7.0 km/s. This indicates that there was no significant
magmatic underplating under the Newfoundland Appalachians during
Mesozoic rifting of the Atlantic Ocean as proposed elsewhere for
the New England Appalachians. A mid-crustal velocity discontinuity
observed in the Newfoundland region does not coincide with any consistent
reflection pattern on vertical incidence profiles. However, we suggest
that localized velocity heterogeneities at mid-crustal depths correspond
to organized vertical incidence reflections.