Preliminary Gravity Inversion Model of Basins East of Yucca Flat, Nevada Test Site, Nevada

The Yucca Flat eastern extension study area, a 14-kilometer-by-45-kilometer region contiguous to Yucca Flat on the west and Frenchman Flat on the south, is being studied to expand the boundary of the Yucca Flat hydrogeologic model. The isostatic residual gravity anomaly was inverted to create a model of the depth of the geologic basins within the study area. Such basins typically are floored by dense pre-Tertiary basement rocks and filled with less-dense Tertiary volcanic and sedimentary rocks and Quaternary alluvium, a necessary condition for the use of gravity modeling to predict the depth to the pre-Tertiary basement rocks within the basins. Three models were created: a preferred model to represent the best estimate of depth to pre-Tertiary basement rocks in the study area, and two end-member models to demonstrate the possible range of solutions. The preferred model predicts shallow basins, generally less than 1,000 m depth, throughout the study area, with only Emigrant Valley reaching a depth of 1,100 m. Plutonium valley and West Fork Scarp Canyon have maximum depths of 800 m and 1,000 m, respectively. The endmember models indicate that the uncertainty in the preferred model is less than 200 m for most of the study area.