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CSPG Bulletin

Abstract


Bulletin of Canadian Petroleum Geology
Vol. 32 (1984), No. 2. (June), Pages 147-161

Origin of Transverse Faulting, Rocky Mountain Front Ranges, Canmore, Alberta

Ian W. Moffat,, John H. Spang,

ABSTRACT

A system of transverse faults, oriented at high angles to predominant northwest-southeast trending structures, is contained within the major Rundle and Sulphur Mountain Thrust Sheets of the Canadian Rocky Mountain Front Ranges near Canmore, Alberta. These transverse faults have been mapped in detail at a scale of 1:12,000. Structural observations at the megascopic, mesoscopic and microscopic levels have been used to develop a comprehensive kinematic model for the origin of transverse faults in this area.

Shortening within the structurally lower Rundle Thrust Sheet was accomplished by folding and imbricate thrusting, whereas the overlying Sulphur Mountain Thrust Sheet was deformed solely by thrusting in the study area. Analysis of field data suggests that progressively eastward-advancing deformation, characterized by early thrusting in the Sulphur Mountain Sheet, was followed by folding and then thrusting in the Rundle Sheet.

The transverse fault system consists of several minor transverse faults apparently restricted to the individual thrust sheets, as well as a major fault that offsets the Sulphur Mountain Thrust and terminates at a major hanging-wall imbricate in the Rundle Thrust Sheet. Interpretation of palinspastic cross sections through the Rundle Thrust Sheet, constructed for the time interval between folding and thrusting, suggests that the aforementioned imbricate thrust, against which the major transverse fault terminates, steepens rapidly northward from the transverse fault. This geometry determined that strata north of the transverse fault would be forced higher than their southern counterparts during thrusting, thus resulting in a transverse fold and, with further displacement, a transverse fault. Following the major faulting event, the combined thrust sheets were essentially separated into two distinct composite structural domains. As lock-up occurred along the lower bounding imbricate thrust, further shortening within the combined thrust sheets proceeded by sliding along pre-existing hanging-wall imbricate thrusts. This additional shortening contributes to the net slip observed across the major transverse fault in both the Sulphur Mountain and Rundle Thrust Sheets.

ORIGINE DU FAILLAGE TRANSVERSAL, CHAINES FRONTALES DES MONTAGNES ROCHEUSE, CANMORE, ALBERTA

RESUME

Un systeme de failles transversales, recoupant la structure predominante orientee Nord-Ouest - Sud-Est selon une direction Est-Ouest, est contenu a l'interieur des zones chevauchantes des Monts Rundle et Sulphur situes dans les chaines frontales des Montagnes Rocheuse candiennes pres de Canmore (Alberta). Ces failles transversales ont ete cartographiees en detail a l'echelle 1:12 000. Un modele cinetique de l'origine des failles transversales dans cette region a ete developpe a partir d'observations structurales mega-, meso- et micro-scopiques.

Le raccourcissement de la zone chevauchante inferieure du Mont Rundle s'est opere par plissement et ecaillage, alors que la zone chevauchante superieure du Mont Sulphur a ete deformee par ecaillage seulement. L'analyse des donnees de terrain suggere que la deformation progressant vers l'Est, caracterisee par le debut du chevauchement de la zone du Mont Sulphur, a ete suivie par une phase de plissement et ensuite de faillage dans la zone chevauchante du Rundle.

Le systeme de failles est constitue de plusieurs failles transversales mineures, apparemment restreintes a chaque zone chevauchante individuelle ainsi que d'une faille transversale majeure qui decale le chevauchement du Mont Sulphur et qui se termine du toit du plan de decollement de la zone du Rundle. L'interpretation des coupes palinspastiques a travers la zone du Rundle, construites pour la periode suivant le plissement, mais precedant le chevauchement, suggere que le plan de chevauchement susmentionne, contre lequel se termine la faille transversale majeure, s'incline davantage au Nord de la faille transversale. Cette geometrie indique que les series au Nord de la faille transversale aurait ete soulevees plus haut que dans la contrepartie Sud durant la phase de chevauchement, aboutissant ainsi a un plis transversal, ou avec un deplacement plus important, a une faille transversale. A la suite du faillage majeur, les zones chevauchantes combinees etaient essentiellement separees en deux domaines structuraux distincts. Malgre le blocage du charriage, le raccourcissement, a l'interieur des deux zones chevauchantes combinees, continue de s'effectuer par glissement le long des plans de decollement preexistants. Ce raccourcissement additionnel contribue au deplacement observe de part et d'autre de la faille transversale majeure dans les deux zones chevauchantes du Mont Rundle et du Mont Sulphur.

Traduit par F. Monnier


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