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CENTRAL SECTION OF
THE SAN ANDREAS FAULT SYSTEM

SAN ANDREAS FAULT [c4, p92-95]

From the Gabilan Range southward to the Transverse Ranges, the San Andreas fault itself dominates the fault system. Except for the San Gregorio-Hosgri set of coastal faults, branch faults are few and short, and exhibit less evidence of Quaternary activity than those farther north. This relatively simple pattern changes at the northern margin of the Transverse Ranges, where major folds and faults trend more east-westerly and incorporate large components of reverse and thrust motion. In this chapter, faults in the western Transverse Ranges and southern Coast Ranges are grouped together as components of the central section of the San Andreas fault system. This combination of two different structural and physiographic provinces stresses that the transition from compressional structures in the Transverse Ranges to strike-slip structures in the Coast Ranges is more gradual and less sharply defined than the physiographic boundary.

This section of the fault system, 340 km long by 140 km wide, extends from the northern Gabilan Range to the southern Transverse Ranges, near lat 340 N. (Figure 4.10). Besides the San Andreas, faults with Quaternary slip include the Hosgri, San Simeon, and Rinconada in the north; the Big Pine, Ozena, South Cuyama, and Morales to the south and in the Coast Ranges; and the eastwest-trending reverse and thrust faults in the Transverse Ranges. Several faults in the Transverse Ranges continue offshore into the Santa Barbara Basin.

The San Andreas fault trends S. 40� E. from the northern Gabilan Range to the Carrizo Plain; there, it bends eastward and ultimately attains a S. 750 E. trend in the Transverse Ranges. North of the bend, it follows a series of nearly aligned stream valleys, which separate the Diablo Range and parts of the Temblor Range on the northeast from the Gabilan Range, Cholame Hills (lat 35�27' N., long 120�18' W.), and Caliente Range (lat 35�02' N., long 119�46' W.) on the southwest (Brown, 1970; Vedder and Wallace, 1970). An actively creeping length of the fault, with historical slip rates as high as 3.4 cm/yr, extends 160 km southward from the northern Gabilan Range to the south end of the Diablo Range.

To the south, along the boundary between the Transverse Ranges and the Mojave Desert, the fault continues as a series of closely aligned echelon breaks, in a narrow (0.5 km wide) linear trench. This segment, which also displays abundant geomorphic evidence of Quaternary 3" slip (Ross, 1969), is currently locked and seismically quiet.

Hill and Dibblee (1953, p. 446), in proposing large sustained strike slip on the San Andreas fault, described fault-juxtaposed gravels of similar age but different lithologies in the area between the Temblor Range and the Transverse Ranges (Figure. 4.11). Pebble gravel in the Paso Robles Formation, of Pleistocene and Pliocene age, consists chiefly of porcellanite and siliceous shale clasts derived from the Miocene Monterey Formation of the western Coast Ranges and the Temblor Range. These distinctive Paso Robles gravel beds are most extensive southwest of the San Andreas fault, but northeast of the fault they extend as a narrow band to about lat 35� N. For several kilometers near this latitude, they are faulted against Pleistocene gravel containing clasts of plutonic and metamorphic rock and indurated sandstone, derived from the San Emigdio Mountains to the south. According to Hill and Dibblee (1953, Figure. 2), the depositional contact between the two gravel lithologies is displaced about 16 km from its prefaulting location.

Farther north, between lat 35�30' and 35�45' N., Galehouse (1967, p. 974-976) described different clast compositions and heavy-mineral assemblages in Paso Robles strata on opposite sides of the fault. These differences are reasonably and consistently resolved if 40 km of right-lateral fault slip is removed, restoring the offset facies to their original positions.

These displaced stratigraphic relations imply Quaternary slip of 16 to 40 km, but the uncertain age of the faulted strata makes estimates of the slip rate unreliable. More accurate measurements of fault slip have been obtained at three localities north of the Transverse Ranges: two in the creeping segment and one in the more southerly, currently locked segment that produced the 1857 earthquake.

At the northernmost locality (lat 36�34.9' N., long 121�10.4' W.), along the San Benito River near the east boundary of the Gabilan Range, the fault displaces the risers of river terraces ( Figure. 4.12), exhibiting 64 m of right slip. Trenches on the terrace near the fault disclosed a fluvial scarp and a distinctive sand bed, both offset about 17.7 m by faulting during the past 790 + 144 14C yr (Perkins and others, 1989). The slip rate calculated from these data, about 2.2 cm/yr, agrees closely with creep rates currently observed along this section of the fault.

In Bitterwater Valley (lat 36�23.9' N., long 120�58.9' W.), 27 km southeast of the San Benito River locality, fault-displaced buried channels in an alluvial fan of Quaternary age record a slip rate over the past 1 ka of about 2.8 cm/yr (Cotton and others, 1986), but the slip rate, as well as its distribution within the fault zone, has varied over time. The historical slip rate, 3.4 cm/yr, represents chiefly gradual fault creep and is based on offsets observed in fences built in 1908 and in channels that presumably date from about 1885. According to Cotton and others (1986), at least one episode of slower movement is evident, and, during the past 1 ka, slip across the 20-m-wide fault zone has followed different breaks.

Sieh and Jahns (1984) measured and dated late Quaternary displacement on the San Andreas fault at Wallace Creek (lat 35�16.3' N., long 119�49.7' W.), near the southwestern margin of the Temblor Range. "Wallace Creek," an informal name (Sieh and Jahns, 1984, p. 896) for a previously unnamed drainage, does not appear on published U.S. Geological Survey topographic maps. At this locality, dated fan and channel deposits, stratigraphic and geomorphic relations, and fault offsets of both abandoned and existing stream channels document 128+/-1 m of strike-slip displacement ( Figure. 4.13). This record of cumulative fault slip began with incision of the existing channel of Wallace Creek sometime after 3680+/-155 yr ago and ended with the nearly 10 m of fault slip that accompanied the 1857 earthquake. Because the 3,680-yr 14C age is measured relative to A.D. 1950, the appropriate timespan for the measured slip is 3,680-(1,950- 1,857), or 3,587 yr. The minimum rate is thus 128 m/3,587 yr, or 3.57+/-0.2 cm/yr; the maximum rate based on the stratigraphy and geometry of the abandoned channel, is 3.53+/-0.2 cm/yr. When these maximum and minimum values are combined and adjusted to incorporate uncertainty in the state of strain at the beginning of the time interval, a preferred slip rate (Sieh and Jahns, 1984, p. 891) of 3.39 +/- 0.3 cm/yr is obtained. A similar but more complex analysis of 13,250-yr-old fan deposits displaced 475 m from their source gullies yields a slip rate of 3.58+/-0.5 cm/yr (Sieh and Jahns, 1984, p. 891-892) for the past 13,250 yr. The close agreement of these rates suggests that the average rate of late Quaternary fault slip has been relative uniform at this locality.

Horizontal slip rates on the San Andreas fault are less well constrained in the western Transverse Ranges. Davis (1983, p. 368) and Rust (1982) independently estimated slip rates in the range 2-5 cm/yr for areas 70 km apart and on opposite sides of the junction of the Garlock fault with the San Andreas. These estimates involve substantial uncertainty in the amount and (or) timing of slip; they also apply to different parts of the Quaternary-no more than the past 400 ka for displaced gravel deposits in the San Emigdio Mountains (Davis, 1983), and no more than the past 2 ka for offset landslide deposits 32 km southeast of the Garlock-San Andreas fault junction (Rust, 1982). These rates, however, are broadly consistent with those measured to the north. Moreover, they indicate that much Quaternary strike slip in the western Transverse Ranges has followed the main trace of the San Andreas-a departure from pre-Quaternary history, when the San Gabriel fault to the southwest accommodated much of the fault slip (Crowell, 1975).