The gravity field of the Central Labrador Trough, northern Quebec

The Labrador Trough is the best preserved and exposed of a series of Aphebian (lower Proterozoic) fold belts which surround the Archaean Un gava Craton of northern Quebec and mark the junction of the Superior and Churchill Structural Provinces in this region. The results of a detailed gravity survey of the central Labrador Trough are presented. Regional gravity profiles across the Labrador Trough are characterized by a field which decreases gradually to the east over the Superior Province, reaches a minimum beneath the Trough and thence increases rapidly over the Churchill Province to attain a mean level some 15 mgal higher than observed over the Superior Province. This regional field may be interpreted in terms of a relatively elevated Conrad discontinuity beneath the Churchill Province isostatically compensated by a thickened lower crust. The model is consistent with basement reactivation of the Churchill Province following collision with the Superior Province during the Hudsonian Orogeny. Subtraction of the regional field from the observed gravity data reveals that positive residual anomalies over the eastern part of the Labrador Trough correlate with outcrops of basic meta-igneous rocks. Their causative bodies probably dip to the east and extend to maximum depths of 9 km in the central part of the area. These interpreted depths are considerably less than previous estimates based on geological analysis. Small positive anomalies correlate with iron formation. A persistent depression in the observed gravity field over the centre of the Labrador Trough in the south coincides with thick deposits of the basal continental sedimentary unit. 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. The geological history and present structure of the Labrador Trough may be related to a sequence of events involving the closure of a small ocean dividing the Superior and Churchill Provinces in Aphebian times culminating in their collision during the Hudsonian Orogeny.