Constraints on the structure of Brooks Range and Colville Basin, northern Alaska, from flexure and gravity analysis

Abstract

Tectonic thickening of the crust within the adjacent Brooks Range fold‐thrust mountain belt has been suggested as a probable cause for rapid subsidence of the Colville basin in the Early Cretaceous. A simple mechanical model in which the response of the lithosphere to loading was approximated by a semi‐infinite (broken) elastic plate overlying a weak fluid was used to analyze deflection of the basement beneath the Colville basin. Comparison between the calculated and observed depth to basement suggests that the emplacement of thrust sheets onto the lithosphere during mountain building cannot produce a load sufficient to create the observed basin geometry and observed topography regardless of the position of the slab end or flexural rigidity assumed for the lithosphere. An additional subsurface load capable of producing a deflection of the foreland basin roughly equal to the depression caused by the topographic load of the Brooks Range is required. Bouguer gravity data from the region are characterized by an asymmetric gravity low which spans the Brooks Range and Colville basin and a gravity high to the south. Gravity modeling is consistent with a subsurface load with a mass of approximately 2.5 × 1013 g/cm located beneath the southern Brooks and the Yukon‐Koyukuk Province to the south. The origin of the subsurface load is interpreted as obduction of crustal blocks onto an old rifted continental margin during attempted continental (A‐type) subduction. The Colville basin was also influenced by the contemporaneous development of the northern Arctic continental margin. Loading by subsurface and topographic loads to the south and loading on the continental margin to the north resulted in the formation of the Barrow arch just offshore of northern Alaska. A best fitting value for mechanical thickness of the lithosphere beneath the Colville basin is approximately 65 km.