Abstract

The Labrador Trough is the best preserved and exposed of several Aphebian (lower Proterozoic) fold belts which surround the Archaean Ungava Craton of northern Quebec and is characterised by three longitudinal facies zones: predominantly meta-sedimentary rocks in the west and east and predominantly basic meta-igneous rocks in the centre. The results of a detailed gravity survey of the central part of the Labrador Trough between latitudes 55° 45′ and 57° 30′ and longitudes 66° 30′ and 70° are presented. Over 1500 rock samples provide density control for the interpretation of four residual gravity anomaly profiles in terms of the surface geology.In the eastern part of the Labrador Trough positive gravity anomalies correlate with outcrops of basic meta-igneous rocks. Their causative bodies extend subsurface to the east and reach depths of up to 9 km in the central part of the area, but are considerably thinner to the north and south. This interpreted depth is considerably less than the 15–20 km that has been inferred by other workers from surface geological investigations. Small positive gravity anomalies are associated with iron formation. A persistent depression in the observed gravity field over the centre of the trough in the south coincides with deposits of the basal sedimentary unit. Uncertainty in the location of the regional level prohibits accurate thickness determinations of the causative bodies of negative anomalies in this area, but the approximate values of 2–3 km obtained for the basal unit are of the same order as estimates based on geological investigations. In the northern part of the area the causative bodies of the negative anomalies are probably elevated areas of granitic basement. These elevated basement features may be related to a ridge that controlled sedimentation during much of the trough's history.

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