Beveling the Colorado Plateau: Early Mesozoic Rift-Related Flexure Explains Erosion and Anomalous Deposition in the Southern Cordilleran Foreland Basin

Deposition of the Late Jurassic Morrison Formation in a back-bulge depozone and formation of the overlying sub-Cretaceous unconformity above a forebulge mark the birth of the foreland basin system in the central U.S. Cordillera. In the southern U.S. Cordillera, the Morrison Formation is either anomalously thick or absent and the sub-Cretaceous unconformity cuts out progressively older stratigraphy toward the south on the Colorado Plateau. Based on results of 2D and 3D flexural modeling, we suggest that flexural uplift of the northern rift flank of the Bisbee segment of the Borderland Rift Belt can explain these observations. Structural restoration of the sub-Cretaceous unconformity indicates a minimum of 1.5 km of uplift and flexural models with an effective elastic thickness of 55 ± 5 km can reproduce the geometry of the unconformity and rift flank. This implies that effective elastic thickness has decreased between the Jurassic and the present, consistent with hypotheses for uplift and modification of the Colorado Plateau lithosphere during the Late Mesozoic to Cenozoic. Modeling results also predict the presence of a rift-related flexural trough in the Four Corners region of the Colorado Plateau, which may explain above-average thickness of the Morrison Formation. Constructive interference between a flexural back-bulge depozone and a flexural rift-flank trough may help explain anomalously high Late Jurassic subsidence.