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STRUCTURE OF THE UPPER MANTLE

In addition to the Transverse Ranges and Salton Trough, other regions in California show mantle velocity anomalies that imply structure within the lithospheric mantle and even the asthenosphere. The seismic networks in California (see chap. 5) provide an abundant source of regional earthquake and teleseismic arrivals that have been used to determine this upper-mantle structure.

A detailed study of the compressional-wave velocity of the uppermost mantle in central California reveals a normal velocity of about 8.0 km/s and no evidence for velocity anisotropy (Oppenheimer and Eaton, 1984). A similar study in southern California finds nearly the same average velocity, 7.95 km/s, with evidence for 2-percent velocity anisotropy (Vetter and Minster, 1981; Hearn, 1984). The fast direction is N. 75� W., approximately parallel to the San Andreas fault in southern California. Seismic-velocity anisotropy in the upper mantle has been reported elsewhere, notably in oceanic crust, and is commonly attributed to alignment of olivine in the mantle along a shear-stress direction (Bamford and others, 1979). In southern California, this shear would presumably be that associated with the motion of crustal blocks above the lithospheric mantle.

Lithospheric thickness along the San Andreas fault has been investigated by using delay times of teleseismic arrivals and thermal models (Zandt and Furlong, 1982). These studies indicate a lithospheric thickness of only 30 to 60 km for much of western California, and as little as 20 km for northern California just south of Cape Mendocino. These lithospheric thicknesses contrast with averages of 60 to 80 km for the Western United States and 120 to 170 km for the Central and Eastern United States (Iyer and Hitchcock, 1989). The thinness of the lithosphere in northern California south of Cape Mendocino is due to the creation of the San Andreas fault system itself: The transform fault is lengthening as the Mendocino triple junction migrates northward. As this junction migrates northward, the west edge of North America is sliding off the edge of the northward-moving, subducting Gorda plate, thereby creating a "window" where no subducting lithospheric slab is present (Dickinson and Synder, 1979). In this slabless window, the North American crust is initially in direct contact with the asthenosphere that has welled upward to fill the hole left by the Gorda plate (Zandt and Furlong, 1982). This geometry produces the thinnest lithosphere in California and, probably, in North America. In contrast, the lithosphere is abnormally thick (250 km) in the Transverse Ranges, where "subduction" of lithospheric mantle is occurring, as discussed above.

Velocity anomalies appear to extend even into the asthenosphere beneath western California. Aki (1982) summarized the results of Cockerham and Ellsworth (1979) and Raikes (1980) in a combined velocity-anomaly model for a depth range of about 100-225 km in the mantle (Figure. 8.9). Aki suggested that the low-velocity region in central California is hot, mobile material associated with the slabless window. Such an association appears likely for the northwest-trending prong of this anomaly, as refined by the recent work of Benz and others (1990); however, the center of the anomaly, located near Long Valley Caldera (Figures. 8.2, 8.9), apparently has a different origin. Low-velocity regions are also associated with the Salton Trough, where asthenospheric upwelling is inferred, and the eastern Mojave Desert, where crustal extension has occurred. The high-velocity region that crosses the San Andreas fault in southern California is similar to the one discussed above (Figure. 8.7).