Northwest- or east-west-trending
folds deform much of the San Andreas fault system except for the relatively
undeformed Sebastopol block (Fox, 1983) between the San Andreas fault and the
fault trend defined by the Hayward, Rodgers Creek, and Maacama faults. The relative
structural simplicity of this block is most evident between San Francisco Bay
and about lat 39°30' N. (Figure. 4.3), where
flat-lying upper Miocene and Pliocene strata of the Wilson Grove (Fox, 1983)
and Ohlson Ranch (Higgins, 1960) Formations extend across much of its surface.
The erosional surface of this part of the block, the Mendocino Plateau (Lawson,
1894; Wahrhaftig and Birman, 1965, p. 323; Fox, 1983, p. 22-24), is a planar
or gently warped upland with accordant ridge crests at elevations of 300 to
600 m. Fox (1983) contrasted the apparent stability of his Sebastopol block
with a more typical, highly deformed region to the east, his Santa Rosa block.
In the Santa Rosa block and in other more easterly parts of the Coast Ranges, Quaternary folding and faulting have left uplifted or anticlinal ridges and downwarped or downfaulted basins. Some of the larger basins, now filled with locally derived sediment of Pliocene and Quaternary age, are the southern arm of the San Francisco Bay, Napa and Livermore Valleys (lat 38°20' N., long 122°20' W., and lat 37°40' N., long 121°50' W.), and the topographic basin surrounding Clear Lake (lat 39°01' N., long 122°30' W.). Smaller aligned or linear basins that follow northwest-trending faults or synclinal folds (Figure. 4.3) clearly are structurally controlled.
Rates of folding and uplift are best known for coastal regions. Marine terraces between San Francisco and Monterey Bay indicate general uplift of 120 to 180 m during the past 0.5 to 1 m.y. (Helley and others, 1979, p. 18) - an uplift rate of about 0.02 cm/yr. Local variation in uplift is shown by the deformed shoreline angle and wave-cut platform of the youngest (82 ka) terrace ( Figure. 4.8); near Half Moon Bay, the wave-cut platform has been warped by northwest-trending folds into a surface that exhibits 60 m of structural relief over a distance of about 11 km (Lajoie, 1986, Figure. 6.21). About 10 km west of Santa Cruz, this terrace is about 14 m above modern sea level (Bradley, 1957; Bradley and Griggs, 1976; Hanks and others, 1984, p. 5776-5777), indicating a local uplift rate of 0.04 cm/yr.
Coastal uplift south of San Francisco contrasts with Quaternary crustal subsidence 25 to 30 km inland and east of the San Andreas fault (Figure. 4.3) and Figure. 4.5. Estuarine, stream-laid, and freshwater-swamp deposits of Quaternary age underlie the northwest-trending structural basin containing the San Francisco Bay and the Santa Clara Valley (lat 37°10' N., long 121°40' W.) to depths greater than 200 m below sea level. This prolonged Quaternary downwarping (Wahrhaftig and Birman, 1965) locally attained subsidence rates of 0.02 to 0.04 cm/yr (Atwater and others, 1977) during post-Sangamon time.
Despite such local variations in the amount and orientation of crustal processes, Quaternary uplift ( Figure. 4.9) prevails throughout the northern Coast Ranges. Regional uplift, at least partly of Quaternary age, is greatest and most evident east of the Hayward-Rodgers Creek-Maacama fault trend. Summit elevations north of Clear Lake exceed 1,500 m (Wahrhaftig and Birman, 1965, Figure. 9), and those in the Diablo Range, south of the Livermore Valley, range from 600 to 1,200 m (Christensen, 1965, pl. 1). Both of these colinear uplands exhibit high relief, evidence of rapid downcutting, and cores of the emergent Franciscan complex flanked by outward-dipping strata of Pliocene and Quaternary age.
Volcanic rocks of late Cenozoic age, widely distributed between the San Pablo Bay and Clear Lake, define a triangular, northwest-trending outcrop area, 120 km long by 35 km wide, with its most acute, north apex at Clear Lake. These rocks are assigned to two geographically separated extrusive sequences: the late Miocene and Pliocene Sonoma Volcanics to the south, and the younger, more areally restricted Clear Lake Volcanics (Figure. 4.3) to the north. The Clear Lake Volcanics is almost wholly of Quaternary age. Flow surfaces, volcanoes, cinder cones, obsidian domes, and craters retain their original constructional form, and the Geysers Hot Springs area on the west side of the main volcanic field attests to continuing geothermal activity. Lava flows range in composition from olivine basalt to rhyolitic obsidian, and pyroclastic deposits are nearly as varied. The oldest well-dated rocks in the Clear Lake Volcanics yield K-Ar ages near 2.1 Ma; the youngest - ash beds in lake sediment beneath Clear Lake - yield 14C ages of about 11 ka (Hearn and others, 1976; Sims and Rymer, 1975).
The sustained late Cenozoic volcanic episode recorded by the Sonoma and Clear Lake Volcanics is unique within the San Andreas fault system, although similar but less extensive Quaternary volcanism marks the northern end of the Gulf of California ridge-transform system in the Salton Trough (lat 33°19' N., long 115°50' W.).