Gravity measurements between Hazen and Austin, Nevada: A study of basin-range structure

From the pendulum base stations at Mystic and Truckee, California, a line of gravimeter stations, with side loops, was extended eastward through Hazen, Fallon, Eastgate, and Austin, Nevada, crossing the area displaced by faults in 1954. Regionally, the Bouguer anomaly is about −160 mgal from Hazen eastward through Fallon to the Stillwater Range. Farther east the values decrease to −215 mgal near Austin, a change that can be accounted for by isostatic compensation for the increase in average elevation of about 1700 ft. Low-density sedimentary and volcanic debris is abundant enough in the intermontane basins and in parts of the mountains to make the average density of the whole elevated land mass abnormally low with respect to the conventional value, 2.7 g/cc. Consequently, 10 to 15 pct of the compensation for this high region is in the superficial materials themselves. The Basin Ranges show no evidence of being individually compensated. Locally, each of the basins has a negative Bouguer anomaly relative to the adjacent ranges, reflecting thick sedimentary and volcanic fill. Dixie and Fairview Valleys, in the area displaced by faults in 1954, are characterized by local negative anomalies of about 30 mgal, indicating that these valleys contain several thousand feet of lightweight Cenozic sediments and that their bedrock floors lie below sea level. The 1954 faulting is thus only the latest of many displacements that produced not only the visible topographic relief of 5000 ft but also a buried relief of comparable magnitude. Romney's [1957] seismic studies of the 1954 faulting and Whitten's [1957] geodetic measurements agree with direct observations of fault surfaces to indicate a horizontal extension of about 5 ft normal to the trace of the fault. Independent of a strike-slip component of faulting, the region is expanding in area. If the total structural relief was produced by displacements comparable to that of 1954, the extension across Dixie and Fairview Valleys amounts to about 1½ mi in roughly 15 million years, and if this is taken as a fair sample of the Basin and Range Province, the rate of distension in the Province is about 1 ft/century, a rate well within that of historical fault displacements. At the time of the 1954 faulting, the Bouguer gravity anomaly either did not change or decreased algebraically by an amount no greater than 1.0 mgal.