Paper No. 9
Presentation Time: 3:45 PM
ISOSTATIC STATE OF THE DENVER BASIN AND ROCKY MOUNTAIN FRONT RANGE UPLIFT
The Denver Basin is a Laramide (Late Cretaceous through Eocene) basin located in the Rocky Mountain foreland of eastern Colorado, northwestern Kansas, southwestern Nebraska, and southeastern Wyoming, USA. It is bordered on the west by the Front Range Uplift, a basement cored Laramide anticline bounded by thrust faults, and on the east by the Great Plains and stable North American craton. A large negative Bouguer gravity anomaly reaches its minimum of ~400 mGal 100 km west of the basin, over the highest topography of the uplift. 2D flexural modeling of subsidence in the Denver Basin shows that the elastic strength of the lithosphere supports <15% of the weight of the Front Range Uplift topography. The best fit flexural model requires a flexural rigidity of 1.7-4.6 x 1024 Nm and a requisite line load of 2-5 x 1012 N/m, compared to an estimated topography weight of 3.63-4.65 x 1013 N/m. A compensating mass deficiency of ~3-4 x 1013 N/m is required at depth to support the modern topography. Seismic constraints prohibit a sufficiently thick crustal root to account for the required subsurface mass deficiency. We constructed three gravity models along profiles crossing the northern, central, and southern parts of the Denver Basin to examine four end-member scenarios for placement of the required density anomaly: the upper crust, the lower crust, the lithospheric mantle, and the asthenosphere. The models are based on a stratified density model derived from the Bouguer gravity power spectra, which reveal major density interfaces at the base of the lithosphere (132 to 153 km average depths), base of the crust (45-55 km), a mid-crustal boundary (20 km), the top of Precambrian basement (1-2 km), and an interface within the pre-Laramide sedimentary section (~1 km). Crustal structure in the models is constrained by well data and regional seismic data. The models show that the requisite subsurface mass deficiency can’t lie entirely within any of the depth intervals investigated without invoking unrealistically low densities. Tomographic evidence supports placement of at least part of the mass deficiency in the asthenosphere. Our models suggest that additionally, post Late Miocene density modification of either the lithospheric mantle or crust is required to account for the modern Rocky Mountain topography.