Interpretation of gravity surveys in grass and buena vista valleys, nevada

Abstract

The complete Bouguer gravity data from surveys in Grass and Buena Vista Valleys were interpreted by means of an iterative two-layer inversion process to obtain the depth to “basement” beneath the valley-fill material. Basement in this sense is an interface which, for a density contrast that must be determined by trial and error, has zero depth at the range fronts and the correct value at points where there is subsurface information, e.g. drill results. Supported by evidence from a deep U.S. Geological Survey drill hole in Grass Valley, a density contrast of 0–06 g/cm 3 was found to give a reasonable fit to the Paleozoic rock subcrop and the Paleozoic rock outcrop at the Sonoma Range. The depths to Paleozoic rocks were also found to agree very well with the seismic interpretation obtained by means of a finite element model of seismic reflection and refraction data along part of line E-E, Grass Valley. Gravity and seismic interpretations also give very close agreement on the location and dip of the Hot Springs fault. The gravity interpretation was compared to resistivity models obtained from dipole-dipole measurements on two lines. In general, the gravity interpretation agrees reasonably well with the configuration of the 150Ω m basement, although the electrical basement is usually 200–400m deeper than the density and velocity interface. This difference has not been explained, but could be due in part to resolution and nonuniqueness problems in the resistivity interpretation. One might modify the resistivity model to determine whether a better fit to the density-velocity model is possible. However, this has not been done. The Grass Valley gravity inversion reveals a complex basement picture which was interpreted in terms of numerous inferred normal faults, some of which have ssurface expressions. Inferred faults trend mainly north-south and northwest-southeast, but there is also a set of northeast-southwest trending faults. The latter are interesting because they not only conform to the regional trend of hot springs in northwestern Nevada, but two of them are associated with local thermal anomalies. The Hot Springs fault passes through Leach Hot Springs and there is an unexplained 6 HFU thermal anomaly near the Panther Canyon fault, the heat flow anomaly paralleling the Tobin Range. We can speculate that the intersection of older northeasttrending faults and the younger Basin and Range faults may provide the fracture permeability for ascending thermal waters. Two local gravity highs, both associated with heat flow anomalies were analyzed. Excess mass calculations for a 5 mgal residual anomaly centered at Leach Hot Springs gave an excess mass of 2.5 × 10 8 tonnes, believed due mainly to precipitated silica and an excess density of about 0.17 g/cm 8. This is equivalent to a silicified pipe 1 km in diameter and 1–9 km in depth extent. This vertical dimension seems sufficient to explain the 100 ms P-wave advance at Leach Hot Springs, but joint modeling of gravity and seismic delay data has not been done. The smaller Section 14 gravity anomaly was not studied in detail as it was believed to be caused by a shallow basement high, possibly fault controlled, extending northward from the Goldbanks Hills. The coincident 4 HFU anomaly could be quantitatively explained by high thermal conductivity of the shallow basement rocks. Gravity data from Buena Vista Valley were processed in a similar manner; a density contrast of 0.06 g/cm 3 gave a zero depth-to-basement that conforms closely with outcropping Mesozoic-Paleozoic rocks of the East Range. Over the valley, the depth to basement agrees well with 50 Ωm electrical basement determined from a model study of dipole-dipole resistivity data. A bulge in the gravity contours 2 km south of Kyle Hot Springs suggested a densification from precipitating CaCO 3, the principal deposit found at Kyle Hot Springs. A careful subtraction of the background gravity produced a broad 2 mgal local residual anomaly. The anomaly shape suggests it may be localized by northeast and northwest-trending faults, similar to the fault directions at Leach Hot Springs. The excess mass calculated for the anomaly is approximately 7.0 × 10 8 tonnes, nearly three times larger than the excess mass for the Leach anomaly. On the other hand, the age of the Kyle system, determined from radioelement abundances (Wollenberg et al., 1977), is approximately 78,000 yr, considerably less than the age determined for the Leach system. This implies that the average flow rate within the Kyle system must be considerably greater. Part of the difference in mass can be attributed to the fact that the total dissolved solids in the Kyle discharge waters is five times greater than at Leach (Wollenberg et al., 1977). Nevertheless, the evidence suggests that the Kyle system may be more active and more promising as a geothermal resource than Leach.