EXPERIMENTAL MODELING OF THE CONTROLS OF BASEMENT FAULTS ON STRUCTURAL GEOMETRY: 1. SINGLE AND OPPOSITE VERGENT STRUCTURES

Foreland basement-involved structures commonly occur in front of major fold-thrust belts as irregular chains of uplifts, in contrast to the more continuous geometry of most fold-thrust belts. The varying map and three-dimensional geometries of individual structures, and their mutual spatial and angular relationships are strongly suggestive of the influence of pre-existing basement discontinuities. Three dimensional experimental models were conducted to determine the role of preexisting frontal and lateral faults in determining the geometry of the structures. The models consisted of two layers, with stiff clay representing basement and soft clay representing the sedimentary cover. Laser scanning and three-dimensional surface modeling were used to determine the map geometry to compare the models with examples of natural structures. In all cases, the primary basement fault loses its slip upward into multiple splays within a deformation zone in the sedimentary cover. Planar discontinuities result in asymmetric 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), with variations in the trend of the fold axis. Opposite-dipping faults result in uplifted blocks with varying relief and trends of the two structures, depending on the relative orientations of the two fault trends. Examples of structural pairs exhibiting these geometries are the Sheep Mountain-Cate Creek anticline and Rattlesnake Mountain-Oregon Basin pairs. Three sets of experiments were conducted with the frontal fault trending 15, 30, and 45 degrees to that of the back fault. In each case, uplift related to contractional deformation was concentrated where the two faults converged, with the frontal structure plunging out in the direction of divergence. Significant strike-slip faulting along the frontal fault only occurred with a 45 degree angle between the two trends. The mapped geometries of the experimental structures, and the orientations of secondary faults provide predictive analogs that can be used in the interpretation of surface and subsurface structures.