DISPLACEMENT OF PRE-QUATERNARY ROCKS BY THE SAN ANDREAS FAULT
[c3, p67-68]

The largest offset on the San Andreas fault postulated by Hill and Dibblee (1953) is 560 km, on the basis of the speculation that a contact between Sierran basement and Franciscan rocks near the south end of the Great Valley was cut by the fault and that the contact on the west side of the fault was offset northward to a point at sea north of Point Arena. This concept, however, is questionable not only because of the uncertain correlation of the Salinian block with rocks of the Sierra Nevada but also because the northernmost exposure of granitic rocks along the west side of the fault is at Bodega Head, about 90 km southeast of Point Arena, and neither the presence nor the type of a contact between Sierran-type basement and Franciscan rocks is known off shore north of Point Arena (see McCulloch, 1987). Nor is the significance clearly known of the possibly substantial lateral offset along the San Gregorio fault, which intersects the San Andreas at a low angle just west of the entrance to the San Francisco Bay. However, Upper Cretaceous strata that crop out near Gualala, on the west side of the San Andreas fault between Bodega Head and Point Arena, were thought by Ross and others (1973) also to have been offset 560 km, consistent with Hill and Dibblee's concept. The strata near Gualala include quartz-plagioclase arkose and a conglomerate characterized by unusual quartz-bearing mafic clasts, including volcanic rocks, diabase, and diorite to gabbro. Paleocurrent features indicate a source to the east of the fault. The source of the unusual gabbroic clasts is thought to be the Eagle Rest Peak area in the San Emigdio Mountains, just east of the San Andreas fault in the Transverse Ranges (Figure. 3.5). This area may also be the source of relatively small fault slivers of similar mafic rocks that now lie 160 km northwest along the San Andreas fault at Gold Hill and 320 km northwest at Logan (Ross and others, 1973).

Other features that suggested substantial right-lateral offset to Hill and Dibblee (1953) include (1) the south limit of Cretaceous strata near Fort Ross, offset 512 km from their south limit in the Temblor Range; (2) Eocene formations in the Santa Cruz Mountains, offset 360 km from lithologically and faunally similar formations in the Temblor-San Emigdio Mountains; (3) sequences of lower Miocene volcanic rocks, red beds, and Oligocene and lower Miocene marine deposits of the Gabilan Range, offset 280 km from similar sequences in the San Emigdio Mountains; (4) a facies transition from marine to continental middle and upper Miocene beds of the Carrizo Plain, offset 104 km from a similar transition projected from the south end of the Great Valley; and (5) 16 km of offset based on the juxtaposition of two facies of Pleistocene gravel south of the Temblor Range.

Offset of the Eocene formations was described in more detail by Clarke and Nilsen (1973), who considered the lower to upper Eocene sedimentary sequences to represent parts of a single deep-sea fan that have been offset 305 km along the San Andreas fault. These sequences are (1) the Twobar Shale Member of the San Lorenzo Formation and the Butano Sandstone, exposed in the Santa Cruz Mountains on the west side of the fault; and (2) the shale member and Point of Rocks Sandstone Member of the Ereyenhagen Formation, exposed along the southwest boundary of the Great Valley on the east side of the fault (Figure. 3.5). Both sandstone units, which consist chiefly of detritus from the Salinian block, are thought to have been deposited as a submarine fan on the west side of a deep, northwest-trending offshore basin. The Eocene fan deposits on the east side of the fault are in a structurally complex area at the south end of the Diablo antiform. If they overlie Franciscan basement there and Salinian basement on the west side of the fault, they indicate that the Franciscan and Salinian basements had already been juxtaposed by Eocene time when the fan was deposited, as noted by Page and Engebretson (1984).

The strongest evidence for measuring the large amount of right-lateral displacement along the trace of the San Andreas fault through the Coast Ranges may be the correlation of the Pinnacles and Neenach Volcanic Formations (Matthews, 1976), which are exposed on opposite sides of the fault about 315 km apart (Figure. 3.5). The Pinnacles Volcanic Formation is in the central Coast Ranges, on the west side of the Chalone fault, a parallel strand that is several kilometers west of the San Andreas. The Neenach Volcanic Formation is in the Mojave Desert, adjacent to the San Andreas fault on the northeast side, about 20 km southeast of its intersection with the Garlock fault. The volcanic rocks at both exposures rest on granitic basement rocks that are petrographically and chemically similar (Ross, 1984). As described by Matthews (1976), the volcanic rocks form stratigraphic sections that are remarkably similar in composition, lithologic sequence, and age; they consist of calc-alkaline andesite, dacite, and rhyolite flows interbedded with pyroclastic and volcaniclastic rocks ( Figure. 3.6 ). K-Arisotopic analyses of the volcanic rocks indicate an early Miocene age (23.5 Ma; Turner, 1970). The similarity of the 315-km offset of the volcanic formations and the 305-km offset ascribed to the previously mentioned Eocene fan deposits suggests that little movement occurred along that section of the San Andreas fault during intervening Oligocene time (Clarke and Nilsen, 1973).

Paleobathymetric contour maps of middle Tertiary (late Oligocene to early Miocene) topographic features of the San Joaquin sedimentary basin match similar features across the fault in the La Honda Basin (Figure. 3.5), according to Stanley (1987). The paleobathymetry is based on studies of the distribution of fossil foraminifers. In both basins, (1) the southwestern margins were bounded by shelf areas and steep north-facing slopes, (2) maximum water depth was about 2,000 m, (3) the deepest part was truncated by the San Andreas fault, and (4) the paleobathymetric contours generally trend at large angles to the fault and are truncated by it. These features indicate a post-late Zemorrian (late Oligocene to early Miocene) displacement of 325 to 330 km and a post-Saucesian (early Miocene) displacement of 320 to 325 km.

Large offset along the San Andreas fault is also indicated by an anomalous distribution of upper Miocene sedimentary deposits that occur on both sides of the fault for more than 300 km in the central Coast Ranges. According to Huffman (1972), the clast composition of upper Miocene sedimentary rocks of the Temblor Range lying east of the fault in the southern Coast Ranges indicates that they were deposited adjacent to the Gabilan Range, which lies to the northwest on the opposite side of the fault, and that they subsequently have been displaced approximately 240 km (Figure. 3.5).

The various aforementioned features that have been used to measure lateral offsets along the San Andreas fault are all situated near the present-day trace of the fault. They do not measure the substantial displacements that probably occurred along presently inactive older faults of the system, nor do they measure the large lateral movement (described below) that is thought to have occurred along the interface between continental and oceanic crust during the early plate-tectonic development of the fault system.