Structural Geology of the Knowlton-Richmond Area, Quebec
The geometric relationships of intersecting foliation and minor folds exposed in numerous outcrops in the Knowlton-Richmond area suggest two and possibly three phases of deformation. Cleavage and axial surfaces that formed during the late phase of deformation lie in a plane that strikes about N. 27° E. and dips steeply. Plunges of folds within this plane, however, are variable. The early formed foliation and axial surfaces do not have uniform orientation, but the poles of many lie in a girdle whose pole is consistent with the late folding. The axes of early folds lie near two steeply dipping planes, one striking about N. 52° E. and the other about N. 60° W. The plunges within these planes are variable. The gross distribution of lithologic units, from east to west, delineates the major St. Etienne anticline, the major Valcourt syncline, and the major Enosburg Falls anticline. All three have axial traces close to N. 30° E., and consequently they are believed to be related to the late phase of deformation. Where outcrops are abundant, the map-pattern suggests that early folds have been preserved in the flanks of the late folds. The axial traces of these early folds in the flanks of the late folds are approximately N. 40° E. to N. 50° E. In the west limb of the St. Etienne anticline the early folds are overturned toward the southeast, in its crest they are nearly recumbent, and in its east limb they are inverted. In contrast to those in the St. Etienne anticline, the early folds in both limbs of the Enosburg Falls anticline are overturned toward the southeast. The amplitudes of these folds decrease toward the crest of the Enosburg Falls anticline; thus, the effect of the late deformation has been to “fold out” the earlier folds in the crest of the superimposed fold. No major folds with axial traces near N. 60° W. were identified. The diversity of plunges of minor late folds is consistent with a unique direction of maximum compressive stress for the whole area. Its direction is 5° N. 61° W. The relationship between early lineations and late minor folds provides ambiguous evidence as to whether deformation was by concentric or shear folding. However, the orientations of axes of early minor folds in the late major folds suggest that to a large extent the major late folds were produced by shear folding. The geologic age of the earliest phase of folding may be post-Ordovician and pre-Silurian. The age of the two later phases may be Upper Devonian.