GEOGRAPHIC EXTENT AND NATURE OF STRUCTURES ASSOCIATED WITH THE SAGUENAY GRABEN, QUEBEC

The Saguenay Graben is a major crustal structure in the Grenville Province that runs perpendicular to the St. Lawrence Rift System (SLRS) and to which the Saint-Honoré (540 Ma) and Crevier (896 Ma) carbonatites are spatially related. Relatively little work has been done on this structure; only a few studies exist on the interpretation of Landsat and magnetic lineaments in the Saguenay and Charlevoix regions. The extent and nature of fracturing possibly related to the Saguenay Graben towards the Superior Province are also poorly documented.

Federal topographic data and a digital elevation model from the NASA were used to identify more than 500 linear lineaments between the Grenville Front to the west and the SLRS to the east. The majority of lineaments correspond to clear breaks in magnetic anomalies and/or to narrow and linear topographic valleys. Two dominant orientations were established: NE-SW, and E-W to WNW-ESE. NE-SW lineaments are subparallel to the SLRS and the Grenville Front, and are concentrated along three zones that define horst-and-graben structures. E-W to WNW-ESE lineaments occur along a corridor parallel to the Saguenay River. West of Lac Saint-Jean, this structural corridor widens and changes direction toward the Grenville Front, which can be explained by the morphology of the underlying lithospheric mantle. The Saint-Honoré and Crevier carbonatites in the Grenville Province, and the Lac Shortt and Montviel carbonatites in the Superior Province, are possibly associated with this corridor, suggesting that a better understanding of the Saguenay Graben may have major implications for niobium and rare earth element exploration. A field reconnaissance study along the Saguenay River was conducted in order to more precisely define the geological significance of some lineaments. In this area, most lineaments appear to correspond to (1) variably spaced subvertical joints and fractures, (2) more rarely to cataclastic breccia zones along normal or strike-slip faults, and (3) a network of veins and breccias with quartz-calcite cement. The orientation of such structures commonly reflects the orientation of the different sets of lineaments. On some outcrops, several faults show apparent displacement directions compatible with those recognized along certain geophysical lineaments.