Controls of structural style and development of the foreland fold and thrust belt, NE British Columbia, Northwest Territories and Yukon

A prominent step to the east of the fold and thrust belt occurs north of 60°N latitude in northeastern British Columbia, southeastern Yukon and southwestern Northwest Territories. Associated with this deflection is a change in structural trend from NW-SE south of 60°N to N-S north of this latitude. Folds are dominant in the area; they have low amplitude and box-types are common. Fold wavelengths increase north of 60°N latitude and west-verging structures occur.; The deflection and change in structural style may result from (1) original margin configuration, (2) strike-slip motion, (3) pre-existing structures, (4) facies variations, (5) variations in the regional stress field, and (6) variations in the local stress field. In the area of interest, variations in the local stress field would have occurred near uplifts and depressions and several are present here. Pre-existing structures are difficult to recognize but early deformation occurred during the Proterozoic and Lower and Late Paleozoic. These pre-Laramide structures localized and influenced later structural development and controlled many of the subsequent facies boundaries. These facies boundaries also influenced the local stress field, with reef to basinal transitions representing a significant lateral contrast in the competency of the rocks. Possible early strike-slip motion could have configured the margin so that Lower Paleozoic rocks were more widely preserved in the north than south of 60°N latitude. Later reactivation of these faults is possible, but is not required to create the observed map patterns.; Physical models, designed to reproduce structures similar to those observed on maps and cross-sections in the area, were deformed using the centrifuge modelling technique. Models with two differently oriented driving wedges were used to create deflections in the structural trend. Additional models included a piece of cardboard under a portion of the deformable sequence to simulate a rigid basement uplift and absence of Lower Paleozoic stratigraphy in the south. Facies variations were modelled by replacing competent with incompetent material to represent platformal to basinal transitions. The results of the rigid basement uplift and facies variation models indicate these features produce variations in the local stress field and nucleate later structures.