Gravity survey in the western Snake River Plain, Idaho-a progress report

From June 24 through August 16, 1960, a regional gravity survey was made in 3,600 square miles of the Snake River Plain in southwestern Idaho. Six hundred and seventy-two gravity stations were established between latitudes 42?30N and 43?30N and between longitudes 115?15W and 116?15W at an average density of one station per 5.4 square miles. The data were reduced to simple-Bouguer gravity by standard methods and plotted as a gravity contour map.

Three major anomalies were defined by the survey; two 30-mile long, en echelon gravity highs with amplitudes of 15 and 50 milligals, and one elongated gravity low with an amplitude of -10 milligals. A two-dimensional graticule analysis suggests that the highs are caused by vertical slabs of dense rock (probably basalt), the larges about 4 miles wide, about 30 miles long, and extending from about 1,000 feet (0.3 kilometer) below sea level to about 66,000 feet (20 kilometers) below sea level. A possible geological interpretation is that the vertical slabs are large, en echelon, basalt-filled fissures or zones of fissures in the earths crust. The gravity low is attributed to an alluvium-filled trough about 5,000 feet (1.5 kilometers) deep.