Inversion of Gravity Data to Define the Pre-Cenozoic Surface and Regional Structures Possibly Influencing Groundwater Flow in the Rainier Mesa Region, Nye County, Nevada

A three-dimensional inversion of gravity data from the Rainier Mesa area and
surrounding regions reveals a topographically complex pre-Cenozoic basement
surface. This model of the depth to pre-Cenozoic basement rocks is intended for
use in a 3D hydrogeologic model being constructed for the Rainier Mesa area.
Prior to this study, our knowledge of the depth to pre-Cenozoic basement rocks
was based on a regional model, applicable to general studies of the greater
Nevada Test Site area but inappropriate for higher resolution modeling of
ground-water flow across the Rainier Mesa area. The new model incorporates
several changes that lead to significant improvements over the previous
regional view. First, the addition of constraining wells, encountering old
volcanic rocks lying above but near pre-Cenozoic basement, prevents modeled
basement from being too shallow. Second, an extensive literature and well data
search has led to an increased understanding of the change of rock density with
depth in the vicinity of Rainier Mesa. The third, and most important change,
relates to the application of several depth-density relationships in the study
area instead of a single generalized relationship, thereby improving the
overall model fit. In general, the pre-Cenozoic basement surface deepens in the
western part of the study area, delineating collapses within the Silent Canyon
and Timber Mountain caldera complexes, and shallows in the east in the Eleana
Range and Yucca Flat regions, where basement crops out. In the Rainier Mesa
study area, basement is generally shallow (< 1 km). The new model identifies
previously unrecognized structures within the pre-Cenozoic basement that may
influence ground-water flow, such as a shallow basement ridge related to an
inferred fault extending northward from Rainier Mesa into Kawich Valley.

[Summary provided by the USGS.]