Structural geology of the northern termination of the Lewis Thrust, Front Ranges, southern Canadian Rocky Mountains
Creators: name: Stockmal, Glen Stanley
Publisher: name: University of Calgary
Year: 1979
Subjects: subject: QE 606.5 C2 S85 1979
subject: Folds (Geology)
subject: Geology - Alberta - Rocky Mountains
subject: Geology, Structural

description: Bibliography: p. 127-137.
description: Detailed field mapping of the northern termination of the Lewis Thrust in the Front Ranges, Southern Canadian Rocky Mountains, was conducted at a scale of 1:12,000 to study the geometrical accommodation germane to the progressive loss of displacement at a major thrust termination. In addition to geometry, the interdependence, kinematics, and strain histories of structures related to, and in the vicinity of, the northern termination of the Lewis Thrust are of interest. The position of the northern surface termination of the Lewis Thrust within the Rundle Thrust sheet must be inferred as the trace of the Lewis Thrust is obscured by glacial drift on the floor of the Kananaskis Valley, 70 km SW of Calgary, Alberta. However, inferences regarding the subsurface continuation of the leading edge of the Lewis Thrust may be made with confidence based upon an understanding of exposed structures and structural relationships between lithologic units. Immediately along strike north of the inferred surface termination of the Lewis Thrust is a major fold pair best exposed on the spectacular south face of Mt. Kidd. This major fold pair is a structurally complex yet indirect reflection of displacement on the Lewis Thrust at depth. Complications related to this major fold pair (eg. anticlinal room problem solved via minor folding) are likely a function of lithology. This major fold pair rapidly dies out within 3 km north of the south face of Mt. Kidd. The rapid disappearance is seen to be due to the geometrical influence of a conical flanking anticline, immediately east of the main syncline. The orientation of the best-fit small circle to bedding pole measurements obtained from this flanking anticline plus a nonparametric statistic to aid in selecting a conical fold model over a cylindrical fold model were obtained from a new computerized mathematical/statistical method. The termination of the major Mt. Kidd folds heralds the juncture of the leading edge of the Lewis Thrust with the Rundle Thrust surface at depth. The geometries of mesostructural features (primarily fractures and cleavage) indicate that these features were likely developed due to both regional and local stress fields. Fractures related to regional stresses were created with 01 bedding parallel and trending 057°, which is very close to the estimated regional trend of 063°. Fractures related to local stresses appear to be linked with folds and/or unusual fault plane geometries such as abrupt lateral thrust ramps. Microstructural analysis of calcite and dolomite twin lamellae indicates that intragranular strain was also a result of both regional and local stress fields. Twin events related to regional stresses were imposed early in the buckling history when o1 was layer parallel normal to fold hinges and in some cases were imposed again, late in the buckling history when folds began to lock up, limb dips were high and 01 was at a high angle to bedding normal to fold hinges. Intragranular strains related to local stress fields during folding indicate lateral confinement parallel to fold hinges and the existence of instantaneous neutral surfaces within buckles. Comparison between 01 orientations inferred from the mesostructural analysis and microstructural analysis is very good, except in areas that possess a structural geometry which suggests a complex strain history.