Aspects of the three-dimensional structure of the Alberta Foothills and Front Ranges

Abstract

Ever more detailed subsurface images of the Alberta Foothills and Front Ranges obtained from improving seismic techniques, coupled with an ever expanding base of information from deep wells and improved structural analysis techniques, have produced an understanding of fold-thrust deformation that is probably applicable to other fold-thrust belts. Comprehensive subsurface data obtained by the petroleum industry are used to outline and analyze the three-dimensional configuration of fold-thrust deformation of Paleozoic carbonate strata. These data indicate that displacement changes smoothly along individual thrusts, but displacement changes along strike may be large, marked by considerable horizontal rotation (20°–30° or more) of hanging-wall stratigraphic cutoffs relative to the footwalls. Stacking of thrusts with varying displacements has produced summed horizontal rotations of hanging-wall cutoffs of as much as 45°–50°. The three-dimensional trajectories of the thrusts through the stratigraphy are smooth for large distances, interrupted by abrupt offsets at lateral and oblique ramps. These are more common than previously realized. The intersections of these ramps with other thrusts produce complex three-dimensional links between thrusts with concomitant abrupt displacement changes. Some simple examples of these linkages are examined with a three-dimensional geometric analysis. The predictions of the analysis are consistent with structural geometries observed in two case studies of subsurface structures. The links at lateral ramps imply simultaneous displacement on parts of the thrusts connected by the ramps. Since the ramps extend downdip 20 km or more, displacement occurred simultaneously on structures occupying relatively interior and exterior positions in the thrust belt. The smooth and long radius-of-curvature folding of many upper detachments indicated by reflection seismic data also is consistent with simultaneous or overlapping periods of motion on groups of thrusts distributed along regional dip. Most major Alberta Foothills thrust sheets terminate southward at lateral ramps at the Paleozoic level. The abrupt southwest-northeast offsets at these features are most clearly apparent on a palinspastic reconstruction. These offsets cause the Paleozoic deformation front to step out east and northeast around the southern and eastern margins of the main arc of the Foothills. In contrast, northward thrust terminations at the Paleozoic level commonly occur at both tip lines and lateral ramps. Subsurface data reveal that there have been some misconceptions regarding large-scale structural relationships. For example, the McConnell and Livingstone sheets are actually portions of the same thrust sheet, as are the Bighorn and Nikanassin sheets. When corrected for these misconceptions, the data from the Alberta Foothills suggest that maximum displacement has a nonlinear relationship to thrust length, although a linear relationship cannot be ruled out.