Controls of basement faults on the geometry and evolution of compressional basement-involved structures

Abstract

Foreland basement-involved structures commonly occur in front of major fold-thrust belts as irregular chains of uplifts. The variation in the map and three-dimensional (3-D) geometries of individual structures, and their mutual spatial and angular relationships, are strongly suggestive of the influence of preexisting basement discontinuities. Three-dimensional experimental models were constructed to determine the function of preexisting frontal and lateral discontinuities in determining the geometry of the structures. The models consisted of two layers, with stiff clay representing the basement and soft clay representing the sedimentary cover. Laser scanning and 3-D surface modeling were used to determine the map geometry to compare the models with examples of natural structures from the Rocky Mountain foreland in Wyoming. Planar discontinuities result in doubly plunging structures terminating against a frontal fault with a linear trace (Sheep Mountain anticline), whereas intersecting fault sets with sharp or curved intersections result in trapdoor geometries (Grass Creek anticline). Opposite-dipping faults result in uplifted blocks with varying relief and orientations of the structures, depending on the relative orientations of the two fault trends. Significant strike-slip faulting along the frontal fault only occurs when the angle between the two fault trends exceeds 30°. Chains of structures show offsets or relay patterns, commonly accompanied by changes in the orientations of adjacent structures. The Sage Creek–Steamboat Butte chains, the Hamilton Dome–Wagonhound anticline chain, the Maverick Springs–Circle Ridge chain, and the Grass Creek–Walker Dome trend in the Rocky Mountain foreland are examples of these configurations. These patterns suggest the influence of preexisting frontal or lateral discontinuities on the trends and locations of structures. Lateral discontinuities are either preexisting, controlling the lateral extent of structures, or form during deformation to accommodate the formation of structures formed along offset frontal faults. Preexisting lateral faults that penetrate the entire basement commonly result in surface lateral faults bounding the surface structures. However, reverse faults in the basement connected by buried lateral faults result in surface structures with overlaps between them. A relay of reverse faults in the basement not connected by lateral faults may develop parallel or oblique structures with transfer zones between them. The mapped geometries of the experimental models, and the orientations of secondary faults, provide predictive analogs that can be used in the interpretation of surface and subsurface structures.