Tectonic implications of flexural modeling of the Uinta Mountains and surrounding basins since early Eocene time
Abstract
Basin subsidence patterns provide a record of the evolution of regional loading during orogenesis. As such, flexural analysis provides insight on the impact of topographic growth in adjacent ranges, as well as documenting lithospheric behavior and timing of deformation. Flexural analysis of a north-south transect across the Uinta Mountains and associated basins shows much of the topographic load of the Uinta Mts developed long after initiation of Uinta deformation in latest Cretaceous time, and that sharply contrasting rigidities are necessary to explain regional subsidence patterns. Two paleohorizontal datums, c. 48 Ma, exist in the Green River Formation. The Mahogany bed is found in the Uinta Basin of northeastern Utah, and the Laney Member is in the Green River Basin of southwestern Wyoming. Deflections of these beds result from regional loading since early middle Eocene time due to shortening and topographic development of the Uinta Mts. Downward deflection of the Mahogany bed is at least 2800 m over a distance of 90 km with respect to an inflection point at 2250 m above sea level, while the Laney Member is deflected only 600 m over a horizontal distance of 140 km with an inflection point at 1850 m above sea level. Two-dimensional flexural modeling along a transect at 110.66° W using the present topography of the Uinta Mts and densities for the mountain load, basin fill and mantle lid of 2800, 2400 and 3300 kg/m3, respectively, indicates strongly varying rigidity between the two basins. Best-fit rigidities are 1022 N●m to the south, and 1024 N●m to the north. The contrast in rigidities is coincident with the projected position of the Archean-Proterozoic suture of the Cheyenne Belt — the older crust to the north being more rigid than younger crust to the south. To achieve the best fit, all of the present topography of the Uinta Mts, as much as 1.7 km above the basin floor, must have developed after deposition of the 48 Ma datum. The estimated rigidity across southern Wyoming is an order of magnitude higher, and across Utah is an order of magnitude lower, than previous estimates based on analysis of Cretaceous deflections, suggesting possible changes in rigidity through time. In addition, assuming lakes in the Uinta and Green River basins were connected at the time of deposition of the paleodatum, the difference in present elevation of the inflection point of the subsidence profiles suggests that there has been a net isostatic offset between the two basins. The origins of this offset might be due to regional isostatic differences from lithospheric density contrasts. Future work will include 3-D flexural modeling across all of the basins surrounding the Uinta Mountains, including the Piceance Creek, Sand Wash, and Washakie Basins.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.T33B2655R
- Keywords:
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- 8102 TECTONOPHYSICS / Continental contractional orogenic belts and inversion tectonics;
- 8110 TECTONOPHYSICS / Continental tectonics: general;
- 8138 TECTONOPHYSICS / Lithospheric flexure;
- 9350 GEOGRAPHIC LOCATION / North America