Deflection of older structures (stratification, schistosities, fold axes, and thrust faults) defines a major left-lateral transcurrent shear belt associated with the Gwillim Lake fault in the Chibougamau area. Stratigraphic and sedimentological work in the Waconichi syncline permitted determination of vertical and lateral movement components of the Gwillim Lake fault and its associated second-order faults. Comparison with experimental and theoretical studies suggests an interpretation as follows: north to east-northeast trending minor faults are Riedel (R) shears, northwest trending minor faults are conjugate Riedel (R′) shears, and east-trending minor faults are thrust (or restraint) shears (P). The Gwillim Lake fault is the principal displacement (D) shear. The main regional compressive stress had a N10°E direction. The shear belt formed during late Archean time and has been slightly reactivated after the deposition of the Early Proterozoic Chibougamau Formation.