A pattern of crustal seismic reflectivity tentatively dated as Ordovician- Silurian recurs along strike in the Newfoundland Appalachians. Comparable fault patterns are produced in sandbox models of asymmetric compressional systems where oppositely dipping step-up shear zones are localized above a discontinuity in basal velocity. The reflectivity pattern may be produced by a mechanically analogous process in which a stress discontinuity results from detachment and underthrusting of mantle and possibly lower crustal lithosphere during con- tinent-continent collision. Similar seismic reflectivity patterns exist elsewhere in the Appalachian-Caledonide mobile belt as well as in older and younger compressional orogens, suggesting that detachment and underthrusting leading to asymmetric crustal deformation may be a common feature of compressional orogens.

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