Active and passive seismic experiments show that the southern Sierra, despite standing 1.8 to 2.8 kilometers above its surroundings, is underlain by crust of similar seismic thickness, about 30 to 40 kilometers. Thermobarometry of xenolith suites and magnetotelluric profiles indicate that the upper mantle is eclogitic to depths of 60 kilometers beneath the western and central parts of the range, but little subcrustal lithosphere is present beneath the eastern High Sierra and adjacent Basin and Range. These and other data imply the crust of both the High Sierra and Basin and Range thinned by a factor of 2 since 20 million years ago, at odds with purported late Cenozoic regional uplift of some 2 kilometers.

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A mountain-type calibration loop has been established in southern Nevada to test the performance and check the calibration of gravity meters used in the Nevada Nuclear Waste Storage Investigations. The observed gravity values are on the IGSN 71 datum and range from about 979,265 mGal at Charleston Park (elevation 7,540 ft) to 979,541 mGal at Indian Springs (elevation 3,115 ft), a difference of 275 mGal. Five stations have been established between Indian Springs and Charleston Park at gravity intervals of about 50 mGal. Geomorphic evidence indicates the area is reasonably stable with regard to possible short-term (<50 years) gravity changes.

This report describes pastand planned geophysical activities associated i with the Yucca Mountain Project and is intended to serve as a starting point for integration of geophysical activities. Geophysical surveys were conducted at Yucca Mountain as early as 1978, when repository siting investigations in the Nevada Test Site (NTS) area were begun. This report I relates results to site characterization plans, as presented in the past Yucca Mountain Site Characterization Plan (SCP). As indicated in the $CP, many geophysical activities have not been planned explicitly or in detail I because of uncertainty as to the applicability of various methods. A charscterization activity was incorporated in the SCP to structure the evaluation and planning of geophysical activities during site I characterization (SCP Section 8.3.1.4.1.2). This integration activity is tasked with reducing the uncertainty attendant to the application of geophysical methods. This report ("white paper") is a preparation for that i activity. Whereas this report identifies some new exploration concepts and elaborates on some activity descriptions in the SCP, if changes are made to the scope of work described by the SCP, they will be made in accordance with change-control procedures. I Importantly, this report does not present geophysical data or interpretation. Rather, only survey coverage, data quality, and applicability of i results to site characterization are discussed, as a means to relate past and planned activities. Extensive references to data and interpretive reports are provided, including many not directly cited in this report. Several such i reports are currently in preparation and could not be referenced, including i one summarizing regional geophysics, one summarizing geophysical logging at Yucca Mountain, and one describing teleseismic tomography based on data collected in 1982. I

About 6,000 specific‐gravity (SG) measurements of samples collected from nearly 200 granitic plutons comprising the central Sierra Nevada batholith yield a SG contour map across the batholith from 36.25° to 38° north latitude. With notable exceptions, SG decreases from values generally greater than 2.7 in the west to less than 2.6 over a few small areas of high‐silica, high‐potassium granites near the east edge. A good correlation between measured SG and analyzed weight percent SiO2 enables estimation of average silica variations across the batholith. The average SG is 2.69 corresponding to an average of 68 wt. % SiO2 for the 18,000 km² central part of the batholith. A 1‐km gridded version of the SG measurements has been used to generate a series of synthetic gravity maps, assuming that the SG of rocks at the surface extends unchanged to various depths. The synthetic map computed for a depth of 10 km shows the best correspondence with the isostatic residual gravity map indicating that variations in the observed gravity residuals are largely caused by SG variations of the plutonic rocks exposed at the surface that apparently extend downward to an average depth of about 10 km. Although the 10‐km synthetic gravity map gives the best overall fit to the observed gravity data, comparison of individual anomalies indicates that the bottoms of the plutons as defined by SG variations at the surface are generally shallower along the west edge of the Sierra Nevada (7±2 km) and deeper in the younger and more felsic eastern part (12±3 km). These depths do not necessarily represent a distinct base of the Sierra Nevada batholith. They may indicate the depth below which density homogenization occurs, either by igneous, or possibly, structural processes.