The Effects of Anisotropy at the Leading Edge of a Thin Skinned Contractional Orogenic Wedge - Southern Canadian Rocky Mountains
Abstract
The development of an orogenic wedge is a complex process that reflects the interaction of a large number of components and rates. One such component, strength anisotropy, is present in all bedded sedimentary rocks, so understanding its influence on the development of the orogenic wedge is important. Many other components of the system such as surface processes, fluid pressure, and mechanical failure have been shown to influence the development of the orogen, however the influence of strength anisotropy has not been as thoroughly investigated. By examining the structures at the leading edge of an orogenic wedge with respect to the orientation of anisotropy relative to the principal stress directions, insight into the development of the wedge can be gained. Field data (fracture orientations, bedding orientations, bedrock maps, and hammer seismic data), were combined with subsurface data (well log, seismic and aeromagnetic data) to interpret the anisotropy and structures at the leading edge of the orogenic wedge in the Southern Canadian Rocky Mountains. The structures described indicate that strength anisotropy influences the development of the orogenic wedge. The layering of sedimentary rock creates a strength anisotropy parallel to bedding planes that is a significant control on rock failure at the scale of the thrust front. The change in the orientation of the strength anisotropy from foreland dipping (shallow) to hinterland dipping (deep) at the leading edge of the thrust front creates preferred failure directions that lead to wedge geometries. Although the shape of the leading edge of an orogenic wedge suggests a conjugate system, the upper and lower failure surfaces are not a conjugate pair. They are separate failure surfaces, one that ramps and steps forward and one sub-parallel to bedding that deflects upward. Current theory, which relies on wedge failure due to advancing conjugate failures, does not adequately accommodate the strength anisotropy inherent in sedimentary rock at the leading edge of a contractional orogen. As the orientation of the strength anisotropy is a significant control on rock failure, factors that control the strength anisotropy such as basement topography, subsidence (accommodation space), erosion rate and sedimentation rate will also affect rock failure and thus the geometry of the orogenic wedge is the result of a complex system.
Description
Keywords
Geology, Geophysics
Citation
Lamb, M. A. (2017). The Effects of Anisotropy at the Leading Edge of a Thin Skinned Contractional Orogenic Wedge - Southern Canadian Rocky Mountains (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca. doi:10.11575/PRISM/28096