MULTIVARIATE STATISTICAL-ANALYSIS OF GEOPHYSICAL-DATA IN NEVADA

Regional magnetic, gravity, and gamma-ray data in Nevada were analyzed using a multivariate statistical method. The algorithms used are part of the public-domain Geographical Resource Analysis Support System (GRASS). Twenty-seven statistical groups (clusters) were found to be the optimum number that best represents the geophysical data. The resulting map of the distribution of these groups shows linear features which define a geophysical trend not previously recognized. This trend is clearly related to the northern Nevada rift and extends from near the northern border of Nevada to the southern part of the State with a northwest-southeast trend to about 37-degrees-N latitude, where a change of direction occurs to a northeast-southwest trend. Although not spatially coincident, the Roberts Mountains Thrust, the Golconda Thrust, and the 0.706 contour of the Sr-87/Sr-86 ratio exhibit similar trends over part of their lengths. The significance of this geophysical trend is uncertain. One possible interpretation is that it corresponds to the central graben of a rift system larger than previously postulated. The classified data were also compared to a digital geologic map, and the results show that, for some areas of the state, mapped geology and specific groups do have a clear relationship. In general, however, mapped geologic units and the groups have no unique relationships.