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.