Influence of Mantle Processes on the Formation of Petroleum-Bearing Basins in the Central Rocky Mountains, Western USA
The central Rocky Mountain region in the western interior of U.S.A. consists of a sequence of basement-cored mountain ranges and intervening petroleum-bearing basins. The area was situated within the foreland of the Cordilleran retroarc thrust belt during the Cretaceous, and was partitioned by the Laramide deformation during the latest Cretaceous-early Eocene. Although it is generally agreed that the Laramide deformation was caused by the low-angle subduction of the Farallon oceanic plate underneath western U.S.A., recent studies suggest that hot asthenospheric upwelling associated with slab-removal may be the major driver of the deformation. Here we test whether mantle processes induced by slab-removal may have influenced the formation of these intermontane basins in the state of Wyoming. We assume that the Wyoming lithosphere behaved as an infinite elastic beam under the tectonic loading of the Laramide ranges, and conduct 2D flexural subsidence modeling to the intermontane basins in order to document the temporal and spatial variations of mountain size and lithosphere flexural rigidity (D). The decompacted profile of basin fill along a transect perpendicular to the basin-bounding mountain range represents the observed amount of flexural subsidence. We derive the mountain size and D by matching modeled subsidence profile to the observed subsidence profile. Our results show that the Laramide ranges gained elevations equal to or higher than their present mean elevations during the Laramide deformation, and the uplift of most of the ranges accelerated during the early Eocene. D remained relatively constant in each basin, but decreased from northeastern Wyoming (~1023 Nm) to southwestern Wyoming (~1020 Nm) during the Laramide deformation. When compared to the Wyoming lithosphere strength (D=1023–24 Nm) during the late Cretaceous and at present, the lithosphere in southwestern and central Wyoming was weakened during the Laramide deformation. We suggest that the uplift of the Laramide ranges and the weakening of the Wyoming lithosphere were caused by the breakoff and subsequent removal of the Farallon flat slab. The flat slab may break off underneath southwestern Wyoming, and the associated asthenospheric upwelling thermally weakened the lithosphere. The loading of Laramide ranges and weakening of lithosphere caused the fast subsidence of the intermontane basins, which contributed significantly to the burial and maturation of the Cretaceous source rocks.
AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015