LITHOPROBE EAST ONSHORE OFFSHORE SEISMIC-REFRACTION SURVEY - CONSTRAINTS ON INTERPRETATION OF REFLECTION DATA IN THE NEWFOUNDLAND APPALACHIANS

Combined onshore-offshore seismic refraction/wide-angle reflection dat a have been acquired across Newfoundland, eastern Canada, to investiga te the structural architecture of the northern Appalachians, particula rly of distinct crustal zones recognized from earlier Lithoprobe verti cal incidence studies. A western crustal unit, correlated with the Gre nville province of the Laurentian plate margin thins from 44 to 40 km and a portion of the lower crust becomes highly reflective with veloci ties 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 contin ental velocities, not exceeding 7.0 km/s in the lower crust. Further e ast, in a crustal unit underlying the Avalon zone and associated with the Gondwanan plate margin, the crust is 40 km thick, and has velociti es of 6.8 km/s in the lower crust. Explanations for the thin crust ben eath the central mobile belt include (1) post-orogenic isostatic readj ustment associated with a density in the mantle which is lower beneath this part of the orogen than beneath the margin, (2) mechanical thinn ing at the base of the crust during orogenic collapse perhaps caused b y delamination, and (3) transformation by phase change of a gabbroic l ower crust to eclogite which seismologically would be difficult to dis tinguish from mantle. Except for a single profile in western Newfoundl and, 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 Appal achians during Mesozoic rifting of the Atlantic Ocean as proposed else where for the New England Appalachians. A mid-crustal velocity discont inuity 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 dep ths correspond to organized vertical incidence reflections.