A regional structural model of the Labrador Trough, northern Quebec, from gravity studies, and its relevance to continent collision in the Precambrian

Abstract

The Labrador Trough is a linear fold-belt marking the junction of the Superior and Churchill structural provinces in northern Quebec. Gravity profiles across the trough are characterized by gently decreasing anomalies over the Superior Province reaching a minimum beneath the trough and thence increasing abruptly over the Churchill Province to a level some 15 mgal higher than the Superior. Superimposed on this higher level are several broad maxima parallel to the trough, one of which corresponds to an extensive outcrop of migmatites. The gravity profiles may be interpreted in terms of a relatively elevated Conrad discontinuity beneath the Churchill Province isostatically compensated by a thickened lower crust. Such a model is consistent with basement reactivation following collision of the Superior and Churchill continental plates. Collision results in crustal thickening by ductile flow in the upper mantle and consequent partial melting in the lower crust leads to differentiation of the crust into a refractory lower part and potash-rich upper part separated by a zone of migmatites. The geological history of the Labrador Trough, and its present-day structure as deduced from gravity studies, is consistent with a sequence of events involving the gradual closure of a small ocean dividing the Superior and proto-Churchill crustal plates in Aphebian times. The closure was effected by subduction beneath the Churchill culminating in collision during the Hudsonian Orogeny and the formation of the present structural configuration after deep erosion.