Three-Dimensional Analysis of Sedimentary Basins Using Regional Gravity and Magnetic Data- A Case Study from the Southwestern United States

A determination of the thickness and nature of Cenozoic deposits throughout the Basin and Range geologic province of the southwestern United States, estimated from digital gravity, magnetic, geologic, and topographic information, is proving useful in understanding the extensional evolution and mineral resource potential of the region. Most known mineralization in the state of Nevada, for example, is located in exposures of pre-Tertiary rocks which comprise only about 20 percent of the total area. A province-wide gravity inversion shows that 75 percent of the remaining area is covered by Cenozoic deposits less than 1 km thick, thus indicating a vast target area for mineral exploration. Depth-to-source calculations applied to low-altitude aeromagnetic anomalies show that 55 percent of the state of Nevada is underlain by magnetic sources within 1 km of the surface. These magnetic sources are in most cases Cretaceous and younger plutonic rocks or Tertiary and Quaternary volcanic rocks. Forty percent of these shallow magnetic sources are concealed by Cenozoic deposits and thus are possible targets for volcanic- or plutonassociated mineralization. In most basins, the gravity inversion predicts a much more complex basementthan might be assumed from surface topography. Death Valley in southeastern California, for example, is to first order a two-dimensional feature as reflected by its topography, but this apparent uniformity belies four deep (3 to 5 km), steep-sided depressions that may have formed as relatively small pull-apart structures superimposed on the more uniform extension that created the two-dimensional aspects of Death Valley.