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The Peninsular Ranges are underlain in the west by supracrustal rocks, including, from top to bottom, Cenozoic marine sedimentary rocks, Cretaceous forearc sedimentary rocks, Lower(?) Cretaceous and Upper Jurassic andesite (Santiago Peak Volcanics), and Middle Jurassic flysch (Bedford Canyon Formation) that was disrupted and overturned before the Late Jurassic (Larsen, 1948; Jennings, 1977; Criscione and others, 1978). These rocks are intruded by Early Cretaceous plutons of the Peninsular Ranges batholith that include chiefly tonalite and gabbro and show no special age trends (static magmatic arc; Silver and others, 1979). About 80 km east of the coastline, both prebatholithic and batholithic rocks change (Figure. 8.6A): To the east, the prebatholithic rocks are dominantly metamorphosed clastic rocks of amphibolite grade, and the batholithic rocks are chiefly tonalite and granodiorite whose ages decrease progressively eastward (from 105 to 80-90 Ma; migrating magmatic arc; Silver and others, 1979). Major-element chemistry and oxygen isotopes indicate that deep crustal rocks in the west half of the batholith are dominantly primitive and tholeiitic but, in the east, more aluminous and oxidized (Figure. 8.6A). Older crust that was once at the Earth's surface is inferred at depth in the east (Silver and others, 1979).

Seismic constraints for the deep structure of the Peninsular Ranges are sparse. Using blasts at the Corona Quarry in the northernmost Peninsular Ranges, Gutenberg (1951) and Shor (1954) obtained an unreversed refraction profile, extending southward to the United States-Mexican border, along with a reflection record at the blast site. Interpretation of these data by Shor and Raitt (1958) indicated velocities of 5.9 km/s to 8-km depth, 6.8 km/s to 26-km depth (with a possible low-velocity zone in this interval), and 7.0 km/s to the Moho at 30- to 32-km depth (Figure. 8.6A). In contrast, a study by Nava and Brune (1982) using a blast at the same quarry, reversed by an earthquake in Baja, Mexico, indicated a Moho depth of 42 km. Hearn and Clayton (1986a, b) used as many as 600,000 arrivals from local earthquakes in southern California to map the velocity of the crust and upper mantle, using tomography. Their map indicates that the west half of the Peninsular Ranges has a higher average upper-crustal velocity and a lower average mantle velocity in comparison with the east half. Their map of Pn delays for the Peninsular Ranges suggests no crustal root and an average crustal thickness of nearly 30 km. Gravity modeling of the Peninsular Ranges (Fuis and others, 1984) and isostatic calculations also indicate a maximum crustal thickness of 30 to 33 km. In our cross section (Figure. 8.6B), we adopt a maximum crustal thickness of 33 km.

An additional constraint on crustal structure is the modeling by Jachens and others (1986; R.C. Jachens, written commun., 1988) of strong magnetic and gravity steps (500 nT and 40 mGal, respectively) in the central Peninsular Ranges: A moderately east dipping boundary is modeled between more magnetic, dense rocks on the west and less magnetic, lighter rocks on the east. This boundary is poorly defined at the latitude of our transect; it correlates approximately (within 15 km or so) with the boundary between the east and west halves of the Peninsular Ranges batholith, as discussed above (Figure. 8.6A). In the cross section (Figure. 8.6B), we interpret an eastward deepening of mafic rocks, including prebatholithic and (or) batholithic mafic rocks (gabbro, diabase, and metamorphic rocks), along this magnetic/gravity boundary. R.C. Jachens (oral commun., 1989) indicated that, in some places, this boundary is so planar as to be interpretable as a fault. As beneath the borderland, the mafic rocks beneath the Peninsular Ranges may have reached their current thickness by thrust imbrication, tectonic underplating, or magmatic underplating. We speculatively show some tectonic underplating on the west side.