More than 2300 gravity observations were made in the northern end of the Salton trough, including an underwater gravity survey of the Salton Sea. 400 gravity observations by Kovach are used to extend the gravity map southward and 700 gravity observations from oil companies and Woollard are used for part of the regional control. A complete Bouguer anomaly map of the California portion of the Salton trough area shows that the general trend of the isogal contours is parallel to the over-all northwest trend of the tectonic pattern. The contours northeast of the Coachella Valley trend east parallel to the Transverse Range structure. The Coachella Valley and Borrego sink are associated with gravity lows, the Salton volcanic domes with a gravity maximum, and the Peninsular Ranges with a gravity minimum. The anomalous mass of the Salton volcanic domes is 6 to 7 km deep with a radius of 3.5 to 4.5 km based on the "half-width" interpretation of a sphere. Due to uncertainties arising from contemporaneous metamorphism of the sediments and the ambiguity in the regional gravity field a detailed interpretation was not attempted.
All of the major fault zones are associated with small gravity lows. A series of these small lows southeast along the projected trace of the Banning-Mission Creek fault may indicate continuation of faulting toward Yuma, Arizona. The steep gravity gradient across this fault in the Coachella Valley can be explained by a steep contact between crystalline rock and sediments which exceed 4 km thickness in the Indio-Mecca area.
Seismic refraction profiles were established at Thousand Palms, Truckhaven, Frink, and Westmorland. These give depths to basement of 4350, 5540, 7340, and 18,300 ft respectively. The Westmorland profile establishes the depth to basement near the center of the trough.
Regional gravity studies indicate that much of the gravity low over the Peninsular Ranges can be explained by a thickening of the crust from 29 to 33 km. The Imperial Valley, with over 5.5 km of sediments, is anomalously associated with a broad gravity high. This is interpreted in terms of a thinning crust under the valley possibly to a depth of 21 km, relative to 29 km at San Diego. The crustal structure of the Imperial Valley is probably the northward continuation of the structure of the Gulf of California and may represent the initial stages of an alteration from continental to oceanic type section by rifting and northwest movement of the Baja California peninsula and western California relative to the stable area northeast of the San Andreas fault system.</p
