Isostasy and Flexural Subsidence of the Denver Basin and Rocky Mountain Front Range Uplift

The Denver Basin is an asymmetric Laramide (Late Cretaceous through Eocene) foreland basin covering portions of eastern Colorado, northwestern Kansas, southwestern Nebraska, and southeastern Wyoming, USA. It is bordered on the west by the Rocky Mountain 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 ~400 mGal negative Bouguer gravity anomaly exists over the Denver Basin and Front Range Uplift, with its minimum located over the highest topography of the uplift, approximately 100 km west of the Denver Basin. This study examines three hypotheses concerning the isostatic state of the basin and adjacent Front Range Uplift. These hypotheses are that the modern shape of the basin is due to 1) flexure of the lithosphere under the surface load of the current Rocky Mountain topography; 2) flexure under a subsurface load beneath the Rocky Mountains; or 3) a combination of both surface and subsurface loads. To test these hypotheses, spectral analysis and forward gravity modeling was conducted along three profiles located in the northern, central, and southern parts of the basin. Bouguer gravity power spectra along the profiles reveal 5 major density interfaces interpreted to represent the base of the lithosphere (at depths of 132 to 153 km), base of the crust (45-55 km), a mid-crustal boundary (about 20 km), the top of Precambrian basement (1-2 km), and a boundary between the Pierre Shale and Niobrara Formations within the pre-Laramide sedimentary section (-1- 0 km). Flexural modeling shows that the shape of the basin can be fit with an elastic plate model having a line load of magnitude 2-5 x 10¹² N/m and a flexural rigidity of 1.73-4.55 x 10²⁴ Nm. The location of the load is 90-115 km west of the Bouguer gravity minimum on each profile. The gravity anomaly associated with flexural subsidence of the basin, assuming the layered density structure derived from the spectral analysis, is calculated to reach a minimum of -60 mGal, only 15 % of the observed Bouguer gravity anomaly. The magnitude of the flexural load is less than the present topography weight of 3.63-4.65 x 10¹³ N/m, indicating that the weight of the Rocky Mountain Front Range is only partially compensated by flexural isostasy. Since seismic data indicate a lack of a pronounced crustal root, a buoyant subsurface load is required (hypothesis 3). Forward gravity models, supplemented with available well and seismic refraction data, are developed to test four end-member hypotheses as to the location of the required buoyant subsurface load. We consider in turn that the load lies entirely within the: (1) asthenosphere, (2) shallow lithosphere mantle, (3) lower crust, or (4) upper crust. The models show that the subsurface load can't lie entirely within in any of the depth intervals investigated without needing unrealistically low densities. The simplest model invokes anomalously low density material within both the asthenospheric mantle and lithospheric mantle. In all of the gravity models, the crust thickens abruptly at the boundary between the Rocky Mountains and Great Plains, from about 43 km beneath the Denver Basin to about 50 km beneath the Front Range Uplift.