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The usefulness of potential-field data along the San Andreas fault system is maximized where rock masses with differing physical properties are juxtaposed. Under these conditions, geophysical anomalies arise from which p the location and attitude of the fault may be calculated (Blakely and Simpson, 1986). In general, the fault is expected to be situated at or near the steepest gradient of the anomaly. These sites are particularly helpful in areas where the fault trace or zone is concealed by young sedimentary deposits or by the Pacific Ocean. In addition, we have found that some of these data are useful in identifying the main strand of the fault zone where the presently active fault trace may not, in fact, be the original plate boundary. Some areas where the potential-field data define the locations of faults are shown on figure 9.4 and are discussed below.

Although the location of the San Andreas fault between Point Arena and Cape Mendocino is concealed by the Pacific Ocean, the aeromagnetic data show a linear magnetic anomaly, striking northwest within the Pacific plate, that is inferred to be obliquely cut off by the fault about 20 km northwest of Point Arena. Farther north, the fault trace just south of Cape Mendocino has proved particularly difficult to locate because it may be too close to shore to be resolved by marine geophysical surveys. A recent aeromagnetic map of this problematic area has, indeed, displayed a magnetic boundary trending close to and along the shore, thus representing the likely location of the San Andreas fault (Griscom, 1980a).

Aeromagnetic surveys over the Pacific Ocean at the entrance to the San Francisco Bay (Brabb and Hanna, 1981) show that the offshore extension of the Pilarcitos fault (an inactive fault branching westward from the San Andreas fault) is cut off by the offshore northward extension of the San Gregorio fault. The San Gregorio fault can be traced northward by using a detailed aeromagnetic map to the point where it intersects the San Andreas fault at Bolinas Lagoon, about 20 km northwest of the bay mouth (see McCulloch, 1987, fig. 15).

From San Francisco southward to lat 35�15' N., the detailed gravity and magnetic data indicate that, in general, the westernmost strand of the main San Andreas fault zone is the major plate boundary. The layered Franciscan assemblage to the east may be less competent than the granitic basement of the Salinian block to the west, and new strands may be more likely to appear in the less competent rocks. An exception to this generalization is found at lat 36� N., where a thin fault sliver of hornblende-quartz gabbro occurs at Gold Hill (Ross, 1970) that has been used to estimate offset on the San Andreas fault. The magnetic anomaly associated with this gabbro body indicates that it is at most 10 km long by 2 km wide (U.S. Geological Survey, 1987).

Farther south along the San Andreas fault, a linear magnetic high extends along the fault approximately between long 116� and 118� W. (fig. 9.3). On the basis of local model studies of this anomaly, Simpson and others (in press) show that this feature probably reflects the edge of an extensive block of magnetic rocks on the northeast side of the San Andreas fault, where the magnetic material is Precambrian igneous and metamorphic rocks, as well as Mesozoic plutonic rocks. Using detailed magnetic data (U.S. Geological Survey, 1979), the south border or magnetic boundary of this magnetic block (fig. 9.4) can be traced from west to east along a series of fault segments; from long 117�15' W., the boundary follows the southern fault trace to long 116�15' W., then crosses over to the northern trace along the short, east-west-trending fault segment, and finally continues eastward along the northern trace. These faults thus may represent the original fault boundary (now somewhat kinked) between the two plates. The geologic observation that rocks on the north side of these fault segments are native to the San Bernardino Mountains and contrast with compositionally different rocks on the south side (Matti and others, 1985) agrees with the magnetic interpretation. The magnetic boundary continues southeastward along the San Andreas fault in Coachella Valley to long 116�08' W. A possible farther continuation of this linear magnetic high extends southeastward at a lower amplitude and diverges eastward from the present San Andreas fault, generally following and lying northeast of the Clemens Wells fault, a possible earlier strand of the San Andreas fault.

In Coachella Valley, the San Andreas fault (North Branch or Coachella segment) is situated along the northeast side of a substantial linear gravity low caused by at least 4.7 km of low-density sedimentary rocks (Biehler, 1964) filling the valley. Gradient studies on the relatively detailed gravity data in this valley by one of us (Griscom) identify numerous fault strands, including the northern and southern branches of the San Andreas fault (the latter, the Banning fault), as well as several possible fault segments on the southwest side of the valley.

The Garlock fault at long 118� W. changes direction and forms a zone as much as 8 km wide. Models of both the magnetic and gravity fields calculated normal to the fault indicate that here the main lithologic boundary is the most northerly fault (fig. 9.4); the granitic rocks farther north are more magnetic and less dense than those to the south.