Are there relict crustal fragments beneath the Moho?

Abstract

The application of new data enhancement techniques on a Lithoprobe seismic reflection profile in northwest Canada has produced images of upper mantle structures that are overlain with angular discordance by the reflection Moho and are underlain by a relict Mesoproterozoic subduction zone. The reflection Moho is interpreted to postdate the underlying fabric and may thus have been superimposed on it by thermal and/or structural processes. Reflections within the mantle that are subparallel to the east dipping subduction zone are interpreted as structural fabric that formed during, and perhaps as a consequence of, subduction. Interpretation of the lithology in the upper mantle layering is uncertain, as peridotites, eclogites, mafic granulites, and/or pelitic restites can have velocities and densities appropriate for seismic mantle (or a portion of it). Nevertheless, if some layers beneath the Moho were originally in the crust, they may retain a structural memory.