A high-resolution gravity profile, with station elevations and locations determined by a differential Global Positioning System, shows that the Saganash Lake fault that defines the northwestern boundary of the southern Kapuskasing Structural Zone (KSZ) is southeast dipping and of reverse type. Previous interpretations of a normal fault, caused by crustal collapse following east-verging thrusting along the Ivanhoe Lake fault that forms the southeastern boundary of the KSZ are no longer tenable. Instead, the reverse nature of the Saganash Lake fault is more in harmony with a model of the Kapuskasing zone, whereby its fault-bounded, uplifted blocks are positive flower structures formed by dextral shear along a zone of left-stepping en echelon faults.