STRUCTURAL ANALYSIS OF THE QUEENSWAY FOLDS, OTTAWA, CANADA

Abstract

The Queensway folds are an anticline–syncline pair in layered limestone and shale of the Ottawa Formation in the Ottawa – St. Lawrence Lowlands. They are parallel, flexural-slip folds with horizontal axes trending northwest, parallel to the surface trace of the Gloucester fault.Five principal fracture subpatterns were recognized in the fold-pair, caused by at least four geometrically distinct stress fields. The principal stress directions at failure for all five subpatterns coincided, moreover, with the three orthogonal fabric axes, and the maximum principal stress was either parallel or perpendicular to the fold axes and to the Gloucester fault.Slickenside striae on bedding and on fractures at an angle to bedding indicate two principal kinematic patterns in the fold-pair; the one arises from motion in the deformation plane as a consequence of the folding and the other from strike-slip motion perpendicular to that plane as a consequence of displacement on the Gloucester fault.Slickensides indicate that each bed was free to move relative to adjacent ones during folding and that the fundamental structural unit in flexural-slip folding is the bed. Model studies support the field data and indicate that the sense and magnitude of interbed slip in any structural position is dependent upon an integral of conditions throughout the fold-pair and that the fundamental fold unit is the anticline–syncline pair.