Thrust Loads and Foreland Basin Evolution, Cretaceous, Western United States

ABSTRACT Two-dimensional modeling of loading during the formation of the Idaho-Wyoming thrust belt shows that regional isostatic compensation by flexure of an elastic lithosphere is sufficient to control the formation of a foreland basin. The flexural rigidity of the lithosphere is inferred to have been approximately 1023 Nm (1030 dyne cm), on the basis of palinspastic comparison of predicted downwarping, due to the thrust plate loads, to the shape of the sedimentary wedge on the west side of the Cretaceous Western Interior seaway. Erosion of part of the uplifted thrust plates redistributed the load, depositing it farther to the east, thereby causing subsidence over a much wider area than could have been accomplished only by the loading by thrust plates. Paleotopography after major Cretaceous thrust events was calculated. The resulting mountainous terrain, gentle alluvial plain, and flat sea floor correspond well with the topography of the modern foreland thrust belt and basin system in the Andes of South America and to paleogeographic reconstructions in the western United States thrust belt. Topography is controlled by the subsurface geometry of the thrust faults, particularly the positions of ramp zones, and by isostatic subsidence.