Divergent fold overturning and regional tectonics,southern Slave Province,Northwest Territories

Map analysis of metasedimentary belts within the southern part of the Archean Slave Province reveals that folds are inclined or overturned in a pattern which can be related to regional compression during emplacement of granitic plutons. Tight folds of variable trend are overturned away from divergent fan axes, which are zones where the folds are upright. The fan axes transgress trend lines and curve in general conformity with bordering plutons. Overturning is preferentially westward for 400 km across five sedimentary belts and is pronounced near eastern margins of plutons within the belts, where axial surfaces commonly dip < 45°. A secondary regional cleavage or schistosity (a plane of flattening) strikes consistently northward; though oblique to axial surfaces, it tends to dip and fan in the same sense.The structural relations suggest that initially upright folds were tilted and overturned during a phase of regional E-W compression similar to that which formed the cleavage. Strain modifications leading to the overturning were apparently due to upper-level movement outward from fan axes accompanied by uplift of granitic plutons. Intracrustal underthrusting during the compression could account for the predominance of westerly overturning.