SALTON BUTTES SPREADING CENTER

The San Andreas fault terminates to the southeast in a buried spreading center at the south end of the Salton Sea, where a row of five small siliceous volcanic domes ("buttes") protrude above recent sedimentary deposits of the Salton Trough. These domes, in addition to being associated with a local northeast-striking magnetic high, are situated on the crest of a larger, northwest-trending magnetic high (outlined on fig. 9.4) that is interpreted (Griscom and Muffler, 1971) to be caused by a magnetic mass, 30 km long, 3 to 12 km wide, and about 4 km thick, with its top buried more than 2 km below the surface. This magnetic high is associated with a similarly shaped gravity high (Biehler and Rotstein, 1979), the source of which may partly be the magnetic mass but may also be the relatively high density metamorphosed sedimentary rocks associated with the geothermal area (Elders and others, 1972). The Salton Buttes spreading center probably strikes northeast because the row of domes, the local aeromagnetic and gravity anomalies, and the geothermal area all coincide and strike northeast; (2) this proposed position for the center bisects the larger northwest-trending magnetic high into approximately equal parts interpreted to be new "oceanic" crust; and (3), ideally, a spreading center should trend approximately normal to an associated transform fault. In apparent contradiction, the Brawley seismic zone strikes S. 20° E. from the Salton Sea (Johnson and Hill, 1982) and consists of shallow earthquakes (Severson and McEvilly, 1987) located mostly within the valley fill; this seemingly anomalous direction may be due to accommodation of these overlying, partly decoupled materials to a series of short northeast-trending spreading centers between the Salton Sea and Cerro Prieto, Mexico (see fig. 3.6), on strike S. 20° E. and 100 km distant (Fuis and Kohler, 1984; Sibson, 1987). The large, northwest-trending magnetic mass is interpreted to reflect about 30 km of northwestward spreading along its long axis, in which the spreading was associated with intrusive activity that built up a 30-km-long strip of magnetic mafic rocks and new crust in the lower section of and below the sedimentary fill. This magnetic feature may not be directly comparable to oceanic-crustal anomalies because slow cooling beneath the fill probably results in weak remanent magnetization, unlike the situation for oceanic crust. This anomaly thus may be predominantly caused by induced magnetization.

The gravity field of the Salton Trough, which is filled with great thicknesses of Cenozoic sedimentary rocks, varies systematically from north to south. An elongate gravity low of -30 to -40 mGal is associated with the sedimentary rocks northwest of the Salton Sea (beyond lat 33°20' N.). Southward along the axis of the trough, the gravity field increases rapidly until the south end of the Salton Sea, where maximum values of 0 mGal are obtained over the presumed spreading center described above. Farther southeast, to the United States-Mexico border, gravity values range from only -10 to -20 mGal, an initially surprising observation because the 3.5 km or more of young, unmetamorphosed sedimentary deposits in this area might be expected to produce anomalies lower than -10 mGal (Biehler, 1964; Griscom, 1980c, p. 20), similar to the gravity expression northwest of the Salton Sea. Biehler (1964) offered two explanations for the missing low: thinner crust or local high-density basement beneath the trough. Seismic-refraction studies (Fuis and others, 1982, fig. 17A) confirm the second explanation and show a deep "subbasement" (density, 3.1 g/cm3) in the trough that extends below about 12-km depth. Using this refraction model as a constraint, a gravity model (Fuis and others, 1982, fig. 20) indicates that the crust beneath the trough is no thinner than that of the bordering mountains a few kilometers to the northeast.