OFFSETS OF ANOMALIES

Strike-slip movement on the faults of the San Andreas system has produced offsets in formerly continuous eophysical anomalies. As might be expected, on those faults where the geologic offset is at most a few tens of kilometers, it is generally easy to identify corresponding magnetic or gravity features that are offset by similar distances. Examples of such faults are the Elsinore fault and the rectilinear system of minor strike-slip faults in the Mojave Desert block northeast of the San Andreas fault. In figure 9.4, the two or more piercing points of an offset geophysical anomaly are labeled with the same letter, and the specific points being described are designated with subscripts, numbered consecutively from northeast to southwest across the fault system.

Offset along the San Andreas fault system in southern California is believed to be approximately 300 km in a right-lateral sense, based on offset of the Pelona-Orocopia schist belts, together with associated characteristic Precambrian and Triassic rock assemblages of the thrust plate overlying the schist belts (Crowell, 1962; Clarke and Nilsen, 1973; summarized in Hamilton, 1978). Isostatic gravity highs are associated with the Orocopia Schist (point A, at lat 33°35' N., northeast side of fault), with the Pelona Schist of the Sierra Pelona/Soledad area (points A2, A3 at lat 34°35' N., between the San Andreas and San Gabriel faults), and adjacent to the south side of the Pelona Schist of the Tejon/Garlock area (point A4 at lat 34°50' N., south of the San Andreas fault and west of the San Gabriel fault). Point A4 is not well determined. The offsets of the gravity highs are, respectively, 240 km along the San Andreas between the first two highs (A1 and A2) and 60 km along the San Gabriel between the second two highs (A3 and A4), for a total of 300 km along the San Andreas fault system, in agreement with Crowell (1962). The source of the gravity highs is not obvious and may not be any of the rocks exposed at the surface (Griscom, 1980b), both because the density of the schist coring the antiforms is probably similar to or slightly lower than that of the surrounding Precambrian crystalline rocks and because other large areas of Pelona/Orocopia schist do not display associated gravity highs. The schist is marine in origin, predominantly metagraywacke of low metamorphic grade (Haxel and Dillon, 1978), and may be underlain by subducted oceanic crust. The gravity highs may indicate relatively uplifted oceanic crust beneath these specific antiformal exposures of the schist, or else the proportion of greenstone interbedded with schist may increase here with depth.

As mentioned above in the subsection entitled "Plan View," a linear magnetic high that extends along the San Andreas fault from long 116° to 118° W., a distance of about 200 km, indicates that a large area north of the fault in this region is composed of magnetic rocks, predominantly Mesozoic granitic plutons; the northwest limit of this magnetic area (J1) is shown in figure 9.4. A similar large area of magnetic basement, also predominantly Mesozoic granitic rocks, that extends along the southwest side of the San Andreas fault is displaced from the former area right-laterally approximately 300 km; the northwest limit of this correlative area (J2) is also shown in figure 9.4. This second area of magnetic rocks does not produce a significant magnetic high directly at the fault because the fault is on the northeast side of the magnetic mass and a magnetic low should occur for this geometry.

Several significant geophysical anomalies are found along the central section of the San Andreas fault north of its junction with the Garlock fault. A pronounced gravity high is located on the northeast side of the fault at lat 34°55' N., where the southern "tail" of the Sierra Nevada is exposed. The associated rocks are hornblendequartz gabbro and anorthositic gabbro (Ross, 1970, 1984) that also produce a substantial aeromagnetic high (point B1, fig. 9.4). Similar rocks (Ross, 1970) are found within the San Andreas fault zone at Gold Hill (point B2 at lat 35°50' N., too small to show at this scale) and at Logan (point B3 at lat 36°52' N.), where magnetic anomalies (U.S. Department of Energy, 1981; U.S. Geological Survey, 1987) indicate that the gabbro bodies are thin slivers within the fault zone. The Logan outcrops are offset about 290 km from the gabbro of the Sierran "tail." A major northwest-trending magnetic anomaly extends northwestward of Logan near the coast (from point B4 at lat 37°08' N.). The source rocks for this magnetic anomaly are interpreted to be gabbro, similar to that exposed near Logan (Hanna and others, 1972a), because the anomaly requires a source body several kilometers thick. These corresponding offset geophysical anomalies support the geologic correlations implying about 300 to 320 km of offset.

The additional 100 km of granitic rocks extending northward from Logan to Montara Mountain (lat 37°35' N.) along the southwest side of the San Andreas fault does not have any correlative rocks exposed on the northeast side of the fault north of the gabbro of Sierran "tail" but the concealed crystalline basement rocks beneath the sedimentary rocks of the Great Valley may be correlative. Indeed, recent work on the Tertiary sedimentary rocks that overlie this additional 100 km of granitic rocks on the San Francisco peninsula suggests a lithologic and paleogeographic correlation with similar sedimentary rocks of the Great Valley (San Joaquin Basin) that are relatively offset 320 to 330 km to the southeast (see fig. 3.4; Stanley, 1987).

Movement on the San Andreas fault north of San Francisco (Griscom and Jachens, 1989) is complicated by right-lateral displacement added by the presently active San Gregorio-Hosgri fault, which intersects the San Andreas fault at San Francisco and provides an additional 115 km (Graham and Dickinson, 1978) or 150 km (Clark and others, 1984; Ross, 1984) of offset. The total offset on the San Andreas fault system here is further complicated by movement on branch faults to the east (Calaveras and Hayward fault systems of unknown offset) and, more importantly, by past movement along the Pilarcitos fault, the presently inactive fault strand branching westward from the San Andreas fault on the San Francisco peninsula. Probably most of the 300 to 320 km of displacement on the San Andreas fault has taken place along this strand because the presently active strand of the San Andreas fault that lies directly east of the Pilarcitos fault demonstrates only about 26 km of offset of a characteristic limestone belt within the Permanente terrane of the Franciscan assemblage (Bailey and others, 1964, p. 69; M.C. Blake, Jr., oral commun., 1987). The Pilarcitos fault (now truncated to the northwest by the San Gregorio fault) may have its former extension on the ocean side of the San Gregorio-San Andreas fault at about lat 38°30' N. (see fig. 9.4), as proposed by Graham and Dickinson (1978). This proposed extension may have granitic rocks on the southwest side (more than the additional 100 km already discussed) that have no correlatives northeast of the San Andreas fault, unless the total offset on the San Andreas system substantially exceeds 300 km or unless granitic rocks underlie thrust blocks of Franciscan assemblage near the south end of the Great Valley (see preceding paragraph). There may be other, unidentified faults within the Salinian block that allow for this additional offset.

Offsets of geophysical anomalies along the San Gregorio-Hosgri fault support a total right-lateral movement of about 100 to 130 km that has been added to the total offset on the San Andreas fault system north of its junction with the San Gregorio fault. An offset gravity high (Silver, 1974) is located on the northeast side near Point Sur (point C1 at lat 36°30' N.) and on the southwest side at Ano Nuevo (point C2 at lat 37°15' N.), with an offset of 105 to 130 km as remeasured by Graham and Dickinson (1978). We prefer an offset of 105 km (max 115 km) because any larger displacement will place the offset extension of the Pilarcitos fault on land north of lat 38°30' N., where no such fault is known.

Displacements along the San Andreas fault north of lat 38°30' N. have proved difficult to measure, both because the rocks exposed southwest of the fault near Point Arena have no obvious correlatives on the opposite side of the fault and because most of the fault trace is concealed beneath the Pacific Ocean (Griscom and Jachens, 1989). The rocks cropping out southwest of the fault near Point Arena are Upper Cretaceous and Tertiary marine sedimentary rocks, together with some older spilitic volcanic rocks that may be part of the Franciscan assemblage (Wentworth, 1968). Little basement information from rock samples is available in the shelf areas west of the San Ardreas fault between Point Arena and the ???gr Mendocino fault. An important well 20 km west of Point Arena (fig. 9.4) recovered quartz-mica schist and slate basement cuttings (Hoskins and Griffiths, 1971) at a depth of about 1.43 km. This description resembles rocks either from the eastern metamorphosed Franciscan assemblage south of San Francisco or from roof pendants in the Salinian block, implying that a major strike-slip fault is located between the well and Point Arena. The proposed Pilarcitos fault extension is thus interpreted to lie here between the well and the coastline on a major fault shown by McCulloch (1987). Location of this proposed Pilarcitos fault extension farther northwest than Point Arena is uncertain, although the fault presumably continues to the former triple junction. McCulloch (1986; 1987, fig. 2b) described a boundary, termed the "Navarro discontinuity," trending east-west from the Point Arena area to the lower continental slope, on the basis of regional differences in magnetic pattern and physiography; this boundary may be the fault extension or an earlier strike-slip fault of this system. Griscom and Jachens (1989) also hypothesized a more northwestward extension, approximately colinear with the fault segment south of Point Arena, following a fault trace interpreted from seismic-reflection profiles (McCulloch, 1987, fig. 14).

Distinctive gravity and magnetic anomalies characterize the poorly known shelf area lying north of Point Arena and between the San Andreas fault and the proposed Pilarcitos extension (figs. 9.2, 9.3). The sources of these anomalies lie in the basement, with their upper surfaces at the basement interface below Tertiary sedimentary rocks, according to geophysical models and basement-depth calculations. A major gravity high (+20 mGal) is located near lat 40° N. (point D2, fig. 9.4). We believe that the high-density basement rocks which cause this high extend southward along the west side of the San Andreas fault at least as far as at Point Arena (E2, fig. 9.4), even though the gravity values on the map fall below 0 mGal along the southern part of this reach. The basement along the postulated southern part of the high is mantled by 1 to 3 km of Tertiary sedimentary rocks (Hoskins and Griffiths, 1971), which probably cause gravity lows (-15 to -30 mGal) that here mask the gravity high caused by the basement. Two magnetic anomalies on the shelf are truncated by the San Andreas fault at point F2 and at a place a few kilometers south of point G2 (fig. 9.4), which is located where the steepest gradient on the northeast side of the second anomaly is truncated by the fault (see McCulloch, 1987, fig. 17).

Our search for geophysical anomalies or features matching points D2, E2, F2, and G2 on the opposite (northeast) side of the San Andreas fault (see Griscom and Jachens, 1989) began with the observation that gravity highs are not characteristic of much of the Franciscan assemblage and are observed only extending along the San Andreas fault between approximately lat 37° and 38° N. (fig. 9.2). We have selected points D1 and E1 (fig. 9.4) as the approximate limits of the gravity highs on the northeast side and propose to correlate these points and their connecting strip of high gravity with the corresponding points D2 and E2 and associated gravity high discussed in the previous paragraph. The positions of these points (D and E) along the fault vary in reliability but are probably no more accurate than +/-20 km; point E2 is the most uncertain. The total offset of the gravity high by the San Andreas fault is thus about 250+/-40 km. We have used the gravity results to explore our magnetic-anomaly map (fig. 9.3) for additional correlations. Only one correlation was found within an offset range of 200-300 km. We suggest that point F1, marking the end of a truncated magnetic high passing through San Francisco, correlates with point F2 and that point G1, the truncated end of a magnetic gradient more than 50 km long, correlates with the other truncated gradient at point G2. The locations of points F1, F2, and G2 along the fault are accurate to within about +/- 5 km. Point G1 is located a few kilometers too far to the southeast because a short northwestward extension of the feature was recently cut off by the young segment of the San Andreas fault in the San Francisco peninsula area and now lies between the San Andreas and Pilarcitos faults. The offset of points F1 and F2 is 250 km; the offset of points G1 and G2 is 263 km. The magnetic anomalies truncated at points F1 and G1 are associated with northwest-striking belts of mafic and ultramafic rocks within the Franciscan assemblage and are best shown on the more detailed maps by Brabb and Hanna (1981) and Griscom and Jachens (1989).

We conclude that the total offset of the pairs of corresponding magnetic features is approximately 250+/-10 km. Of this offset, about 105 km is attributable to the San Gregorio-Hosgri fault, leaving only 145 km for the San Andreas fault south of its junction with the San Gregorio fault. Because the total San Andreas offset south of the San Francisco peninsula is considered to be much larger, namely, about 300 km, we suggest that the missing 155 km is predominantly accounted for by former movement on the Pilarcitos fault and its proposed north-westward extension, which is thought to intersect the San Andreas at about lat 38°30' N., as described above. This early Pilarcitos fault was thus formerly the main strand of an earlier San Andreas fault system that lay to the west of both magnetic features F and G (that is, before they were offset by faulting). Note that this interpreted fault-movement history and the subsequent plate-tectonic analysis all depend on the correctness of the correlation between the pairs of offset magnetic and gravity anomalies on the San Francisco peninsula and northwest of Point Arena. The magnetic and gravity anomalies northwest of Point Arena and west of the San Andreas fault are such conspicuous features and so obviously truncated by the San Andreas fault that we would expect to find their counterparts somewhere on the opposite side of the fault. Although we can find no alternative correlations for these anomalies other than those indicated in figure 9.4, we are aware that they may not correlate with any anomalies on the opposite side of the fault, although we consider this noncorrelation to be unlikely.

Additional information on offset along the proposed Pilarcitos fault extension is provided by interpretation of two strong magnetic anomalies on the northeast side of the San Andreas fault in central California (H1 at lat 35°30'-35°40' N. and lat 36°00'-36°15' N., respectively). The source bodies for both anomalies appear to be truncated by the fault, and interpretations of the gravity and magnetic fields over both bodies suggest that they are composed of serpentinite (Hanna and others, 1972; Griscom and Jachens, in press). The most likely candidates for corresponding magnetic features on the southwest side of the fault system are the poorly defined anomalies at points H2 and 12 west of Point Arena. The magnetic field is poorly known in this area, and so anomaly locations and shapes may not be accurate, but the offset is approximately 435 km from points H1 and I1. This distance can be obtained by summing an assumed 320 km for offset on the San Andreas fault south of San Francisco plus 115 km offset on the San Gregorio-Hosgri fault. The location of point 12 supports the Navarro discontinuity as a possible continuation of the proposed Pilarcitos fault extension, or some earlier continuation. We suggest that the large magnetic-high area bounded by the 500-nT contour and located 25 km south of point 12 (fig. 9.3) may represent a southerly extension of anomaly I which north of point I1 extends for 400 km along the east side of the Coast Ranges. There appear to be no satisfactory alternative anomalies for correlation with points H1 and I1 along the southwest side of the present San Andreas fault near Point Arena.