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SAN ANDREAS FAULT [c4, p85-88]

The northern section of the San Andreas fault system, as defined here, extends from Punta Gorda, 40 km southeast of the Mendocino triple junction, to the northern Gabilan Range (Figure. 4.3). This belt of Quaternary- and earlier-faulting and folding, about 120 km wide by 500 km long, includes the San Andreas, at least four major related faults, and several smaller faults, 5 to 20 km long. All of these faults displace Quaternary deposits or landforms, and all are seismically active. From west to east, major faults within the system are the San Gregorio, San Andreas, Hayward (and its northern, right-stepping extensions, the Rodgers Creek-Healdsburg and Maacama faults), Calaveras (and its northern, rightstepping extensions, the Concord and Green Valley faults), and an ill-defined, possibly discontinuous fault zone along the eastern margin of the Coast Ranges that includes, from north to south, the Stony Creek, Greenville, and Ortigalita faults.

The offshore counterpart of the main San Andreas fault, which trends seaward and westward from Punta Gorda to the Mendocino triple junction, represents the latest segment of the northward-growing transform. This east-west-trending, offshore segment of the fault is located chiefly on seismic and bathymetric evidence; it follows along the north face of the Gorda Escarpment (along lat 40� 23' N. and west of long 124� 39' W.) and the south wall of Mattole Canyon (from the east end of the Gorda Escarpment southeast to lat 40� 17' N., long 124� 27' W.), but water depths of as much as 2,000 m, high submarine relief, and sea-floor deposits of young sediment (Silver, 1971; McCulloch and others, 1985) obscure details of its Quaternary history.

From Punta Gorda southward to Point Arena the fault is almost entirely off shore; its location is known from sea-floor features, coastal geomorphology, and relations at Point Delgada, where it locally intersects the coastline. A line of coastal bluffs, 35� to 85� m high and sloping 33�-43�, trends N. 50� W. for 20 km northwest of Point Delgada. At the base of the bluffline, Lajoie and others (1982) measured Holocene uplift rates of about 0.5 cm/yr from dated molluscan fossils in beach ridges 30 m above present sea level. Seaward of the bluffs, the Spanish and Delgada Submarine Canyons head near the present coast, 2 to 3 km landward of the 100-m bathymetric contour, which approximately defines the late Wisconsin sea level. Shoreward of this contour, both canyons are linear, and neither they nor the intervening sea floor exhibits evidence of vertical or horizontal displacement. These relations are consistent with a near-shore San Andreas fault, located in or near the surf zone and close to the base of the coastal bluffs. The unusually large component of vertical slip (0.5 cm/yr, northeast side up) for this segment of the fault may result from its more westerly trend than that of segments farther south, which exhibit almost pure strike slip; or, as suggested by Merritts and Bull (1989), it may result from the late Quaternary passage of the Mendocino triple junction along this part of the coast.

The fault intersects the coastline about 6 km northwest of Point Delgada, cuts across the point, and locally separates upper Pleistocene marine terrace gravel and younger alluvium (McLaughlin and others, 1983) along the coast from intensely deformed bedrock inland. Other locations have been suggested (Curry and Nason, 1967; Beutner and others, 1980; McLaughlin and others, 1983) for the fault here, but several lines of independent evidence support this trend, including fault slip during the 1906 earthquake (Lawson and others, 1908, p. 54-58), the alignment of scarps, sags, and saddles along the 1906 fault trend, bedrock faulting colinear with 1906 faulting (Brown and Wolfe, 1972), and continuity to the north and south with offshore and nearshore evidence of recent faulting.

From Point Delgada southward to Point Arena, a distance of 125 km, the fault is defined by aligned, chiefly west-facing scarps on the sea floor, the right-lateral deflection of Noyo Submarine Canyon, straight boundaries of irregular submarine topography, and a 2- to 5-km-wide zone of disturbed reflective layers evident in continuous seismic-reflection profiles (Curray and Nason, 1967). The shapes and dimensions of sea-floor features, as shown by seismic-reflection profiles and modern bathymetric maps, resemble fault-formed features on land and provide similar evidence for right-lateral strike slip. The offshore data, however, are insufficient to determine the amount of Quaternary displacement.

Onland segments of the fault between Point Arena and the Gabilan Range exhibit abundant geomorphic and geologic evidence of Quaternary displacement, some of which can be translated into slip rates. Trunk streams, systematically displaced from their headwaters by at least 10 km, record cumulative strike-slip faulting that must span much of Quaternary time. For example, the fault-deflected, northwest-trending courses of the Garcia and Gualala Rivers follow the fault zone for 50 km between Point Arena and Fort Ross; their present alignment (Figure. 4.4) documents a long and complex history of channel extension and stream piracy. Three streams east of the fault - the Garcia River and the Wheatfield and South Forks of the Gualala River - exhibit similar offsets of about 13.5 km from possible former outlets to the sea west of the fault: the Garcia River from its present mouth near Point Arena, the Wheatfield Fork from the present mouth of the Gualala River near Gualala, and the South Fork from a low gap (110-m elevation) in the coastal ridge near the Sea Ranch. The average rate of post-Pliocene slip across the boundary between the North American and Pacific plates, derived from spreading rates at the mouth of the Gulf of California (DeMets and others, 1987, p. 912), is estimated at 5.1 cm/yr. In northern California, this slip is distributed chiefly on the San Andreas and other active faults to the east. On the San Andreas fault at Point Arena, Prentice (1989) obtained a maximum slip rate of 2.6+/-0.3 cm/yr from a faulted stream channel 14C dated at 2,350 to 2,710 yr. Together with the assumed 13.5-km offsets, this rate implies that the postulated drainage diversions required about 519 ka. Other lines of evidence indicate much greater antiquity for this and nearby parallel strands of the fault.

Despite abundant evidence of Quaternary faulting along the northern section of the San Andreas fault, other examples of measurable fault displacement of dated upper Pliocene and Quaternary deposits are sparse. Much of the fault lies off shore, and most onshore segments cut older, highly deformed rocks of the Franciscan complex, from which the upper Cenozoic deposits have been eroded. Some reliable indicators of Quaternary slip, however, come from geologic and geomorphic relations observed from San Francisco southward.

On the San Francisco peninsula and farther south in the Santa Cruz Mountains, estimates of Quaternary slip on the San Andreas fault have been derived from offset biofacies and lithofacies relations. The fault offsets are in discontinuously exposed upper Cenozoic strata, which have been assigned by different investigators to the marine Merced, or Merced(?), Formation and the nonmarine Santa Clara Formation. East of the San Andreas fault, both the Merced and Santa Clara Formations contain, in their upper parts, the 400-ka-old Rockland ash bed of Sarna-Wojcicki and others (1985), thus establishing at least a partial age equivalence of these two units.

The upper, Rockland ash-bearing part of the Merced Formation (Clifton and others, 1988) is fault bounded on the southwest and juxtaposed against a ridge of older rock for 11 km southeast of San Francisco ( Figure. 4.5). The minimum offset of 11 km from any western counterpart of the Merced outcrop belt and the 0.4- to 2.0-m.y. age of the faulted Merced strata require a minimum slip rate of 1.7+/-1.0 cm/yr for this section of the San Andreas fault.

A similar rate can be derived from the present distribution of upper Pleistocene marine deposits, which Addicott (1969) interpreted as the faulted remnants of a narrow marine embayment that once extended southeastward across the San Andreas. The fault separation of 35 to 40 km (Addicott, 1969, Figure. 2) and an assumed 2- to 3-m.y. age for the embayment yield a geologic slip rate of 1.6+/-0.4 cm/yr.

Additional evidence for Quaternary strike slip (Dibblee, 1966; Cummings, 1968) comes from the southern Santa Cruz Mountains south and west of San Jose. Distinctive reworked conglomerate clasts in the Corte Madera facies of the Santa Clara Formation west of the San Andreas fault are displaced 28 km from their source, an Upper Cretaceous pebble conglomerate lens on the east side of the fault. The age of the Corte Madera facies is poorly constrained, but Cummings (1968) assumed a probable age range of 1 to 3 m.y., which yields slip rates of about 1 to 3 cm/yr.

These slip rates carry uncertainties as great as +/- 1 cm/yr, but because they integrate slip over millions of years of geologic time, short-term changes in fault activity influence them only slightly. For comparison, a late Holocene slip rate is available from a site on the San Francisco peninsula, 11 km south of San Francisco. There, displaced and abandoned stream channels and relict alluvial deposits ( Figure. 4.6) were explored and described by Hall (1984; N.T. Hall, written commun., 1986). The alluvium, 14C dated at 1,130+/-160 yr, accumulated along a northwest-flowing stream and behind a northwest-trending shutter ridge along the main fault trace. Alluviation ceased when fault slip, unusually high seasonal streamflow, or stream piracy caused the stream to abandon its fault-extended course and to carve a shorter, steeper, southwest-flowing channel across the fault and directly down the slope. This younger channel now exhibits 13.5 m of right-lateral offset where it is crossed by the fault; about 2.7 m of this total offset can be attributed to fault displacement during the 1906 earthquake. Hall's (1984) analysis of this sequence of depositional, erosional, and faulting events yields a minimum slip rate for the past 1,200 yr of about 1.25 cm/yr.