60th Annual Report on Research 2015

Research activities

The latest major tectonic event in the central Rocky Mountain (Rockies) orogenic plateau is the thick-skinned Laramide orogeny, which caused localized exhumation of basement-cored mountains and subsidence of intermontane basins in Wyoming and nearby areas during the latest Cretaceous-early Eocene. Although it is generally agreed that the Laramide orogeny was caused by the low-angle subduction of the Farallon oceanic plate underneath western U.S.A., we are still not clear about how deep Earth processes induced by low-angle subduction resulted in surface deformation. Because mantle dynamic process can influence lithosphere stiffness, this study aims at determining the spatiotemporal variations of lithosphere flexural rigidity and uplift of mountains by conducting two-dimensional basin subsidence modeling.

During the first year of the project, my PhD student, Min Gao, and I focused on collecting isopach data and reconstructing lithosphere stiffness in the Powder River, Wind River, and Green River basins. A minority undergraduate student, Patricia Garay, and a MS student, Jenna West, has assisted lab work and modeling. To date, this ongoing research has partially supported two graduate students and one undergraduate.

Preliminary results

By assuming that the Wyoming lithosphere behaved as an infinite elastic plate subject to tectonic loading of the basin-bounding mountain ranges, and by conducting 2D flexural subsidence modeling to major Laramide basins, we simulate the temporospatial variations of mountain uplift and lithosphere stiffness (Fig. 1). Our results show that flexural rigidity (D) or effective elastic thickness (T_e) varied slightly in each basin during the entire Laramide deformation, and decreased from northeastern Wyoming (D = 10^23.3-23.8 Nm, T_e = 31-45 km) to southwestern Wyoming (D = 10^21.1-21.7 Nm, T_e = 6-9 km). Our results also show that the height gain of major Laramide ranges accelerated during the early Eocene. We think that the results may suggest that the Farallon flat slab and/or the eclogitized oceanic plateau broke beneath southwestern Wyoming during the Maastrichtian–early Paleocene, which weakened the lithosphere initially by changing lithosphere thermal structure and possibly enhancing crust–mantle decoupling, and the combined dynamic and isostatic effects associated with the subsequent removal of the broken slab may have caused the uplift acceleration during the early Eocene.

Presentation of results

Our preliminary results were presented at the AAPG Annual Meeting in Denver, CO by Min Gao on June 2^nd, 2015, and the presentation was entitled Influence of Mantle Processes on the Formation of Petroleum-Bearing Basins in the Central Rocky Mountains, Western USA.

Future plans

In the second year of the project, we will refine the interpretations by linking the modeled results to major geodynamic models of the Laramide deformation, which include basal traction associated with low-angle subduction, oceanic plateau removal, or slab rollback. We will discuss the results with geodynamic modeler working in the area in order to refine the geodynamic models as necessary. Following completion of the data collection and interpretation, Min Gao is preparing a manuscript for a peer-reviewed journal in the spring and summer of 2016, and Jenna West is working on the landscape evolution during the Laramide deformation.

Impact of research

Our project reconstructs lithosphere stiffness and links surface deformation to deep mantle processes during geologic time by creatively using basin modeling approach. Preliminary results show that mantle dynamic processed associated with flat slab subduction can influence subsidence of sedimentary basins, thus maturation of hydrocarbons. This approach may be applied to other basins on the globe to understand lithosphere rheology and mantle influences.

This project opens a new research direction for the junior PI, who used to work on reconstructing exhumation and surface uplift histories of mountain belts. This project supported graduate student Min Gao partially during the last year.

Figure 1. Spatial variations of lithosphere stiffness in Wyoming during the Laramide deformation in latest Cretaceous-early Eocene.