Near-vertical incidence and refraction -wide-angle reflection seismic data, recorded as part of Lithoprobe studies in the Paleoproterozoic-Archean domains of Canada's Northwest Territories, show remarkable reflections from within the upper mantle. A parallel pair of reflectors imaged by the near-vertical data can be traced from Moho levels (*33 km) down to *70 km depth. In a previous study, the reflectors were interpreted as the top and bottom of an *1.8 Ga subducted oceanic crust beneath the Hottah terrane. Further inboard, where the seismic line changes its direction from eastwest to nearly north-south, another pair of reflectors extends subhorizontally for about 100 km at *70 km depth before dipping downward. The subhorizontal reflectors were not correlated with the dipping slab; instead they were interpreted as a separate feature. However, they roughly coincide with a horizontal interface modeled from wide-angle data by an earlier study. Considering the crooked line acquisition geometry, we re-examined both near-vertical incidence and wide-angle reflection data using 2-dimensional (2-D) and 3-D forward and inverse modeling algorithms. Our results demonstrate that the subhorizontal reflectors are the continuation of the relict subducted slab, which now extends laterally for 300 km. Its base is the source of the wide-angle data. The apparent flattening for the near-vertical data is most likely an artifact of projecting a 3-D geometry onto a 2-D cross section. The shallowly subducted slab probably contributed to the thickening and stabilization of the subcrustal lithosphere below the Wopmay orogen.Résumé : Des données de réflexion sismique à incidence quasi-verticale et de réfraction -réflexion grand angle, enregistrées dans le cadre des études Lithoprobe dans des domaines datant du Paléoprotérozoïque-Archéen des Territoires du Nord-Ouest, au Canada, présentent de remarquables réflexions à partir de l'intérieur du manteau supérieur. Une paire de réflecteurs parallèles imagée par les données d'incidence quasi-verticale peut être retracée à partir du Moho (*33 km) jusqu'à une profondeur d'environ 70 km. Dans une étude antérieure, les réflecteurs ont été interprétés comme étant le sommet et la base d'une croûte océanique subductée, *1,8 Ga, sous le terrane de Hottah. Plus vers l'intérieur, où la ligne sismique change de direction, d'est-ouest à presque nord-sud, une autre paire de réflecteurs s'étend subhorizontalement sur une distance d'environ 100 km à une profondeur *70 km avant de plonger. Les réflecteurs subhorizontaux n'ont pas été reliés à la dalle plongeante; ils ont plutôt été interprétés en tant qu'une caractéristique séparée. Toutefois, ils concordent de façon générale avec une interface horizontale modélisée à partir de données sismiques de réflexion grand angle provenant d'une étude antérieure. En analysant le profil géométrique en ligne brisée, nous avons examiné de nouveau les données de sismique réflexion à incidence quasi-verticale ainsi que les données de réflexion grand angle au moyen d'algorithm...
Although the year 2009 marked one century since the discovery of the Moho, or crust-mantle boundary, the exact nature of that boundary and the manner in which it formed remain major uncertainties in lithospheric studies. In northwestern Canada, sharp Moho refl ections at both near-vertical and wide-angle incidence have been imaged beneath the Great Bear magmatic arc. They show a remarkably fl at Moho and do not refl ect the complex tectonic history of the Wopmay orogen, of which the Great Bear arc is one part. In order to understand the origin of these refl ections and the nature of the Moho, we calculated near-vertical and wide-angle synthetic seismograms for a number of crust-mantle transition models using one-and two-dimensional wave propagation algorithms. Only laterally and vertically heterogeneous models can properly simulate the observed seismic signature recorded on both near-vertical and wide-angle refl ection data. The heterogeneity is achieved by either laterally discontinuous layering or a lamellae structure with randomly distributed ellipses. These models suggest that the Moho represents a thermal/metamorphic front, a regional décollement, or both.
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