Gravity and crustal structure in the western Snake River Plain, Idaho

A gravity survey was made over the western Snake River Plain, Idaho, during the summers of 1959 and 1960. The data were reduced to sea level and expressed as simple Bouguer gravity anomalies for an assumed density of 2.67 g/cm3. Three elongated, northwest-trending, en echelon gravity highs were defined by the survey. The largest high is about 150 km long and 40 km wide; its maximum amplitude is 70 mgal. Application of limiting-depth methods implies that the tops of the disturbing bodies can be no deeper than 5 km below sea level. Two-dimensional analyses, based on a density contrast of 0.3 g/cm3, indicate that the anomaly-causing bodies extend from near sea level to at least 6 km below sea level and possibly to as much as 18 km below sea level. The total mass excess of the disturbing bodies is estimated to be 1×1019 g, and the corresponding volume for material 0.3 g/cm3 more dense than the surrounding material is approximately 33,000 km3. If the highlands on either side of the plain are regionally compensated, the crust under the plain is probably undercompensated by an amount proportional to the local anomalous bodies.