Abstract

Gravity data are now available for about 90 percent of the Sierra Nevada batholith, magnetic data for about 30 percent. About 1,000 density and 250 magnetic property measurements have been made on selected samples. Bouguer gravity anomalies decrease about 200 mgal, from −40 to −240 mgal, eastward across the batholith and gradually rise about 50 mgal on its east side. Gravity calculations show that these Bouguer anomalies are consistent with the general form of Bateman and Eaton's seismic model of the central Sierra Nevada and are inconsistent with the model of Carder and others. The calculations confirm Eaton's velocity-density relation for crystalline rocks. The major gravity step across the western Sierra Nevada is caused primarily by a mafic mountain root, not the batholith. Local anomalies of −5 to −40 mgal are associated with the most leucocratic felsic plutons. Correlation with available isotope ages shows the gravity anomalies to be largest and broadest over the youngest Cretaceous plutons, which suggests that they extend to greater depths than the older, exposed plutons.

Magnetic contrasts between plutons are generally much greater than density contrasts, and they provide a method for mapping some of the more magnetic plutons under some types of cover. The depth extent of magnetic plutons in Yosemite National Park is at least 7 km and probably greater. A regional magnetic low centered near the western edge of the Sierra Nevada correlates with a regional depression in the Curie isothermal surface, as evidenced by low surface heat flux. Because the depression in the isotherm allows for a greater thickness of material to be magnetic, the magnetic low is unexpected, and the relation is a noteworthy paradox.

Bouguer gravity anomalies correlate fairly well with initial Sr87/Sr86 ratios, and the magnetic field correlates with oxidation ratios of sampled granitic rocks. These data, together with physical property measurements of gneissic and peridotitic xenoliths discovered in volcanic pipes by J. P. Lockwood, form a foundation on which new models of the petrogenesis and structure of the batholith must be based.

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