A footwall system of faults associated with a foreland thrust in Montana

Abstract

Some recent structural geology models of faulting have promoted the idea of a rigid footwall behaviour or response under the main thrust fault, especially for fault ramps or fault-bend folds. However, a very well-exposed thrust fault in the Montana fold and thrust belt shows an intricate but well-ordered system of subsidiary minor faults in the footwall position with respect to the main thrust fault plane. Considerable shortening has occurred off the main fault in this footwall collapse zone and the distribution and style of the minor faults accord well with published patterns of aftershock foci associated with thrust faults. In detail, there appear to be geometrically self-similar fault systems from metre length down to a few centimetres. The smallest sets show both slip and dilation. The slickensides show essentially two-dimensional displacements, and three slip systems were operative—one parallel to the bedding, and two conjugate and symmetric about the bedding (acute angle of 45–50°). A reconstruction using physical analogue models suggests one possible model for the evolution and sequencing of slip of the thrust fault system.