Gravity, isostasy, and crustal structure in the southern Cascade Range

Gravity measurements taken in northern California reveal an eastward decrease in the Bouguer anomaly of 100 mgal in about 30 km near Mount Shasta. In places the observed gradient approaches 8 mgal/km. Removal of the regional gravity field reveals two major gravity minimums in the southern Cascade Range. Their maximum amplitudes are both close to −50 mgal. These anomalies suggest a striking similarity between the upper crust beneath the Mount Shasta area and the upper crust beneath the Lassen Peak area. Both are regions with large volumes of low-density rocks at relatively shallow depths. The following conclusions are made regarding the anomalies under study: (1) A large volume of low-density material exists at a shallow depth beneath the High Cascades. In places the top surface of this anomalous mass must be within the upper 4 km, and in general it must be within the upper 10 km of the crust. (2) The free-air anomaly in the Interior Platform region averages nearly zero; it follows from Gauss's theorem that this broad region must be in a state of isostatic compensation. (3) The application of Gauss's theorem to the negative gravity anomaly associated with the High Cascades near and including Mount Shasta indicates a mass deficiency of about 4.0×1018 grams. This estimate corresponds well with the mass excess of the range and suggests that the High Cascades are compensated. (4) The application of Gauss's theorem to the negative gravity anomaly at Mount Shasta indicates a mass deficiency beneath Mount Shasta of about 5.4×1017 grams. This estimate corresponds well with the mass excess of the mountain and suggests isostatic compensation, of which a large part must be local.