LATERAL VARIATIONS IN THRUST DEVELOPMENTS ALONG THE CENTRAL ALBERTA FOOTHILLS, CANADA: TRIANGLE ZONE VS. IMBRICATE FAN

Fold-thrust belts in general exhibit significant lateral variations in structural styles and deformation histories, such as differences in thrust geometries, frequency of imbrications and variations in displacement fields etc. The Central Alberta Foothills of the Canadian Rockies preserve natural laboratory where thrust geometries show lateral variations in terms of triangle zone vs. imbricate fan. We have studied the structural geometries of the major thrusts and variations in stratigraphic thickness and mechanical stratigraphy to seek the factors that make the differences in thrust geometries in the Central Alberta Foothills. Structural interpretations are conducted by balancing admissible cross-sections. In northern region, development of a backthrust for forming triangle zone might be related to pinch out of the shale layer at the foreland side. Backthrust is also developed in the southern region, where strength of a strata is increased toward the foreland. In the central region, however, imbricate fan is developed along the weak continuous decollement layer. Incompetent layers also show lateral thickness variations, so that detachment horizons are different in each regions. These results indicate that backthrusts for forming triangle zones are developed by the stratigraphic pinch-out in the northern region and at the layer having strength variations along the transport direction in the southern region. They are not developed in the area of continuous stratigraphy in case of the central region. This suggests that initial stratigraphic variations of the sedimentary basin play a role for the development of the lateral variations in structural styles along the Central Alberta Foothills.