Anticline-syncline Pair Research Articles

Geometry and Mechanical Relationship of Folds to Thrust Fault Propagation Using a Minor Thrust in the Front Ranges of the Canadian Rocky Mountains

ABSTRACT A west-dipping minor thrust fault and an associated NW-plunging anticline-syncline pair found in the hanging wall of the Lac des Arcs Thrust fault were mapped for the purpose of analyzing the structure's geometry and deformation mechanisms. The surface trace of the thrust intersects the Lac des Arcs Thrust to the southeast, and is rooted in the core of an anticline 5 km to the northwest. A syncline to the northeast parallels the anticline for 2.5 km to the northwest, where the surface traces of the axial surfaces intersect. Major deformational mechanisms include thrusting, folding, and bedding-plane slip (implied by style of folding and by the magnitude of the intragranular strains). Minor deformation mechanisms include carbonate twinning and fracturing involving no offset. The minor thrust is concave upward and joins the Lac des Arcs Thrust at depth along a line of intersection apparently plunging gently to the northwest. An orthogonal fracture system exists with a definite relationship to the macroscopic structural geometry. One set intersects bedding yielding a lineation approximating the structural trend and plunge of the area. The poles to the second set represent a similar lineation. Three structural levels have been defined in the structure, all three sloping gently to the northwest. The lower level consists of thrusting and associated drag folding, the upper predominantly of buckle folding. The middle level is a transitional zone between the upper and the lower, having characteristics of both. Three mechanical relationships between thrusting and folding were recognized in the hanging wall of the thrust. The first relationship is that in which the fault acts as a decollement surface. Above the fault a fold pair has developed, thereby resulting in shortening of the hanging wall. The second relationship involves the propagation of the thrust at its leading edge. Shortening of the hanging wall is accomplished as the hanging wall is crumpled toward the leading edge. The third relationship appears to involve propagation of the thrust fracture along the axial surface of an anticline. The leading edge of the thrust fault is mapped in the proximity of a mesoscopic kink-band (or chevron) fold core. The fold's axial surface is in an orientation of high resolved shear stress with respect to the regional stress field. The location of the leading edge close to the axial surface, and the orientation of the axial surface, suggest that propagation of the thrust fracture could be along this surface.

STRUCTURAL ANALYSIS OF THE QUEENSWAY FOLDS, OTTAWA, CANADA

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.