Spatially heterogeneous strength in thrust fault zones

Abstract

Deformation in the hanging wall of the Lewis thrust fault at Marias Pass, northwest Montana, is heterogeneous in space, with regions dominated by either large‐scale contraction or extension faults. The distribution of mesoscopic structures within the two regions also differs. The density of fractures is highest in the extensional region, whereas the density of solution seams is highest in the contractional regime. This spatial heterogeneity of structures at the base of the Lewis thrust sheet may result from spatially heterogeneous fault zone strength. In analytical models of an elastic thrust sheet overlying a fault represented by spatial heterogeneity in shear traction or displacement, stress magnitudes and principal directions vary with position within the model thrust sheet. With the application of a Mohr‐Coulomb criterion, contraction and extension faults initiate within localized regions at the base of the model thrust sheet. Spatial variations in mean stress at the base of the thrust sheet predicted by the models may explain both the greater density of solution seams in the contractional region than in the extensional region and the greater density of mesoscopic faults and veins in the extensional region than in the contractional region of the Marias Pass exposure. The spatial heterogeneity of pressure solution and cataclasis in response to variable mean stress within a fault zone of heterogeneous strength may facilitate thrust sheet movement relative to movement over a fault zone of uniform strength, because a decrease in mean stress enhances cataclasis and an increase in mean stress maintains a constant shear stress during pressure solution.