FACIES CONTROL OF REGIONAL DISHARMONIC STRUCTURAL PATTERNS, WESTERN ROCKY MOUNTAINS, BRITISH COLUMBIA

Regional structural patterns in orogenic belts, determined by field-based mapping of vergence and structural geometries, is used commonly as a means of structural correlation and establishment of regional deformation scenarios (e.g. “phases” of deformation). We present the results of 1;50,000 scale mapping which demonstrates the presence of regional-scale disharmonic structural patterns in the western Rocky Mountains that were controlled by large scale facies patterns in the sedimentary sequence that hosts the deformation (Windermere Supergroup). The Croydon 1;50,000-scale map sheet straddles the Southern Rocky Mountain Trench at the latitude of ca. 54^oN in British Columbia and includes part of the Robson synclinorium, a regional structural depression in the western Main Ranges. Regional northeast-verging folds, typical of the western Rockies, are overprinted by large-scale southwest-verging folds and associated axial planar cleavage. The southwest-verging deformation produced a composite anticlinorium (Nevin anticlinorium), folds that range from tight to isoclinal, and a pervasive crenulation cleavage. The southwest-verging fabrics, which are atypical of the western Rocky Mountains, extend along strike for ca 15 km and die out along strike to the northwest and southeast. The controlling factor on the localization of the southwest-verging structures appears to be the presence of thick (ca. 200 m) conglomeratic channels within the predominantly mud-rich slopes facies of deep marine Miette Group (Neoproterozoic). The mud-rich units contain only a northeast-verging simple slatey cleavage which leads us to suggest that the conglomerates acted as rigid beams that served to localize buckling instabilities and represents a field-based example that compares closely with centrifuge analog models of facies controlled structural patterns. The possibility of scale independence of the this style of structural evolution during orogenesis cautions against regional correlation of structural fabrics based solely on vergence and geometry.