Roots of the Labradorian orogen in the Grenville Province in southeast Labrador: Evidence from marine, deep-seismic reflection data

Abstract

Marine, deep-seismic reflection data obtained as part of the Eastern Canadian Shield Onshore-Offshore Transect (ECSOOT) Lithoprobe project from offshore southeast Labrador are evaluated using geodynamic and gravity models, and a tectonic interpretation is developed. The geodynamic model explains the seismic reflection data in terms of doubly vergent tectonism resulting from southward directed underthrusting of manfle and lowermost crust. Middle and upper crustal seismic reflectors are considered to be proshears and retroshears and correlated through potential field data with major mylonite zones at terrane boundaries mapped onshore. As there is little evidence of subsequent severe structural modification (apart from strike-parallel Grenvillian dextral transpression, which does not alter the crustal geometry along the line of the seismic transect), the seismic reflectors are considered to represent Labradorian structures, inferred from onshore geochronological data to be mostly related to a ∼ 1665 Ma (Labradorian) accretionary event. The southward directed underthrusting of the mantle and lowermost crust is consistent with existing models for southward Labradorian subduction. Coast-parallel gravity and magnetic anomalies are interpreted as an expression of a Neoproterozoic to early Phanerozoic extensional basin, the existence of which had little effect on die underlying crystalline basement, other than to downfault it relative to that exposed onshore.