Abstract

New gravity data were collected in the Nemiscau and La Grande subprovinces of the Superior Province. This ∼350 km gravity profile follows the Matagami–Radisson road and extends northward the gravity transect along the ∼260 km long Lithoprobe seismic line 48, across the northern Abitibi and Opatica subprovinces. For the Abitibi–Opatica segment, the interpretation is consistent with the Lithoprobe seismic profile. It calls for crustal thickening near the boundary between the Abitibi and Opatica belts, where the Moho is ∼5 km deeper than in the Abitibi subprovince and ∼8 km deeper than in the northern Opatica subprovince. The gravity model complements the seismic reflection data and provides information on the uppermost supracrustal sequences poorly imaged in the seismic profile. Most of the intrusive rocks in the Opatica Belt appear as thin (<5 km) bodies. Across the Nemiscau and La Grande subprovinces, the Bouguer anomalies are of short wavelengths and their sources lie in the upper crust. The crustal thickness is constant from the northern Opatica Belt throughout the southern part of the Nemiscau subprovince. Density measurements indicate that the upper crustal density is higher in the Nemiscau and La Grande subprovinces than in the Abitibi and Opatica belts. There is some crustal thickening beneath the La Grande subprovince, and a gravity high at the northern end of the subprovince is related to the occurrence of mafic supracrustal sequences. The gravity anomaly signature associated with the lateral density variation and field evidence indicate that the main tectonic boundaries dip to the north.

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