Constraints on the Distribution of Slip and the Evolution of the Northern San Andreas Fault System
John Wakabayashi, Consultant, and Scott Dickerman, Cal State Hayward ― June 24, 2000
This account by Dan Day was published in the September 2000 NCGS newsletter.
On June 24th, 2000, consultant John Wakabayashi and recent Cal State Hayward graduate student Scott Dickerman led an informative eight-stop field trip along the San Andreas fault system from Berkeley to Point Reyes. This was a one-day excursion to examine several key outcrops that place limitations on the complex geological history of the San Andreas fault system in the Greater San Francisco Bay Area.
At the first stop at an outcrop of the Orinda Forrnation on Berkeley’s Claremont Avenue, John and Scott gave an introduction to the tectonic evolution of the northem San Andreas fault system. They pointed out that although these strike-slip faults collectively mark the boundary between the Pacific and North American Plates, researchers must also consider motion in the Walker Lane/Basin and Range Province in Nevada–Idaho–eastern California when partitioning slip between the two plates, Dextral slip along the San Andreas fault system totals 40 mm/yr and accommodates about 80% of the inter-plate motion. Near Monterey Bay the San Andreas registers ~35 mm/yr slip, the remainder occurs on the San Gregorio fault. In the Bay Area, the San Andreas splays into several branches distributing the annual 35 mm/yr slip on the Hayward (9 mm/yr), Calaveras (6 mm/yr), Greenville (2 mm/yr), and Peninsular San Andreas (17 mm/yr) faults. The San Gregorio fault merges with the San Andreas proper near Point Reyes and accommodates ~24 mm/yr displacement that has not been reconciled with Bay Area San Andreas system slip rates due to uncertainties in determining offset rates on its mostly submarine fault trace. Radiometric dating and petrological matching of the 20 to 24 m.y. old Neenach and Pinnacles volcanic fields, and displacement of Tertiary strata exposed in the Santa Cruz Mountains and southern San Joaquin Valley has provided the 310 to 320 km displacement along the central San Andreas fault. The San Gregorio has been assigned 180 km of offset, and submarine faults to the west 45 to 95 km of displacement by subtracting continental displacements from the total Pacific/North American plate offset calculated from migration of the Mendocino triple junction. An important piece of information is provided by the Salinian granite presumed transported from Southern California to points west of the Franciscan complex in Monterey and Point Reyes. Field evidence can assign only 25 to 30 km of motion on the Peninsula San Andreas fault, compelling geologists to place the ~280 km of residual slip on the nearby Pilarcitos fault that juxtaposes Salinian and Franciscan basement. John also clarified the usage of “piercing blobs” in reconstructing fault displacement, emphasizing that these correlations use offset planes or three-dimensional bodies rather than the more desirable linear structures. Uncertainties with using these structures to measure fault offsets can result in errors of tens of kilometers, but are the best that can be done when only this data is available.
Scott’s thesis work on clasts in the Upper Miocene Contra Costa Group provides important evidence that constrains movement along the San Gregorio fault in the last ~10 m.y. The uppermost Orinda Formation is overlain by andesitic basalts of the Berkeley Hills volcanics, the base of which is dated by the 40Ar/39Ar method at 10 m.y. old. Very minor but significant granitic clasts in the Orinda Formation bear closer resemblance to the Salinian than to Sierran plutonics, and the expected Miocene Mehrten detritus that would accompany the latter is also absent. Restoration of the last 10 m.y. of slip along all faults to the west of the Orinda outcrops, excluding the San Gregorio fault, permits less than 50 km of movement on it if a Salinian source is to be made available at Orinda times. Between 5 and 12 m.y. ago the Franciscan terrane uplifted and early on became the major source of Orinda clasts, mostly greywackes and metagreywackes, which are ubiquitous in the Franciscan. The Salinian-type granite and rare marble clasts provide a restricted source terrane that constrains post-Orinda motion on the San Andreas system, specifically on the nearby Hayward fault. The next stop about, a mile uphill on Grizzly Peak Road, exposed the upper Orinda Formation and the overlying Berkeley Hills volcanics radiometrically dated at 9.2 m.y. (top) and 10.0 m.y. (bottom). The upper Orinda contains abundant glaucophane-lawsonite and glaucophane-epidote blueschist clasts [thought] to have been derived from an intact blueschist unit rather than from a volumetrically insignificant melange block source. This constraint limits potential epidote-bearing blueschist sources to the Ward Creek area (stop 6), just south of there in the Bohemian Grove (Scaggs Springs schist), and the eastern border of the northern Coast Range. The post-10 m.y. displacement on the Orinda Formation from the epidote blueschist source is 105 to 110 km, depending on the location of the southernmost epidote blueschist outcrop, offset fault geometry, and the geometry of the Orinda conglomerate fluvial system. The recent revision of the Berkeley Hills volcanics age range by Ar/Ar dating points to potential inaccuracies in the K/Ar method when dating young rocks. The Berkeley Hills volcanics are correlated with the Tolay volcanics located north of San Pablo Bay and west of the Rodgers Creek fault (the northern continuation of the Hayward fault). The dextral post ~9 m.y. offset along the Hayward fault using these two volcanic fields is ~35 to 60 km. The 40Ar/39Ar [method] has improved dating of key volcanic units, constrained eruption intervals, and clarifies the relationship between igneous activity and displacement on transform fault systems. It should be pointed out that the Holocene offset on the Hayward fault is only ~l to 2 km, illustrating how displacement has shifted across faults on the San Andreas system throughout its history, and periodically across the entire Pacific–North American plate boundary.
The third stop was a unique outcrop of quartz keratophyre (soda rhyolite) in north Berkeley called Indian Rock. A late igneous differentiate associated with the Upper Jurassic Coast Range Ophiolite, there is a similar outcrop exposed west of the Hayward fault to the north of this location. This displacement is only a few kilometers and indicates only a very slight Holocene movement occurred along the northern portion of the Hayward fault compared with the 40 to 60 km registered along the entire Hayward fault zone. This recently activated fault zone represents a 2 to 4 km lateral step-over that mimics broader scale slip-shifting between the various splays of the northern San Andreas fault system. After a 27 mile drive across the Richmond–San Rafael bridge to Novato, the caravan exited Highway 101 at Atherton Avenue. There John discussed an exposure of blueschist grade jadeite/ lawsonite-bearing metagreywacke. This outcrop is an example of blueschist that is not blue-colored. It was erroneously recognized as the source of blueschist clasts in the Orinda Forrnation near the Berkeley Caldecott Tunnel. The Orinda clasts, however, are from a volumetrically minor and spatially limited blueschist mineral assemblage which increases the slip offset between the Caldecott Tunnel outcrop and its proposed source area from less than 30 km (Novato) to over 100 km (Ward Creek/Bohemian Grove area, stops 6 and 7). This stop illustrates some of the mineral assemblages that make up the blueschist metamorphic facies, and how they can be used to determine sedimentary provenance and to constrain major tectonic displacements.
A residential section of Petaluma was the last stop before lunch. Its focus was an outcrop of the 12.6 m.y. old Burdell Mountain Volcanics, the oldest Cenozoic volcanics in this part of the Coast Ranges. These rocks and the 9.2 to 10 m.y. old Berkeley Volcanics followed behind the northward-migrating Mendocino triple junction. The eruptions occurred about 100 km from the temporal location of the triple junction. Reconstructing the triple junction to its location 12.6 m.y. ago requires 200±50 km of right lateral slip along the San Andreas fault system east of Petaluma.
The lunch break included a brief beer tasting. John enjoys brewing his own beer and generously brings samples of his latest concoctions on field trips for others to taste. The group hit three stops after lunch. The first was along the Cazadero–Fort Ross road near Cazadero, at the Ward Creek blueschists. These rocks are the southernmost coherent outcrop of epidote blueschist in the northern Coast Ranges. The outcrop includes very high grade garnet amphibolite and is thought to be a likely source for the blueschist clasts in the Orinda Formation. The blueschist at this locale, unlike the Novato stop, is a blue color and foliated, making it both mineralogically and texturally a better source candidate for the Orinda clasts. Two miles up the road the group stopped to examine the Skaggs Springs schist, one of the most distinctive basement outcrops in coastal Califomia. It is a completely recrystallized quartz-rich greywacke with a noticeable schistosity, and contains blue glaucophane, lawsonite, and white mica. It weathers into topographic lows and can be correlated with similar rocks near Mt. Hamilton east of San Jose, and with rocks near the Quien Sabe Volcanics. These piercing blobs yield an aggregate offset on the San Andreas fault system of 230 to 250 km. Like limestone in the Permanente terrane, this unit provides a fairly accurate means of measuring large scale displacernent on the fault system.
The final stop took the vehicles to an outcrop of granitic rock on the Point Reyes peninsula west of the San Andreas fault. South of here the San Gregorio and San Andreas faults meet near Bolinas Bay. Scott explained that the Salinian block exposed here is the likely source of granite clasts observed in the Orinda Formation at the first stop. The actual source rock is probably north of Point Reyes and now below sea level. The Marshall Beach outcrop on the northern tip of the peninsula exposes a marble roof pendant in contact with Salinian granite and overlain by Cretaceous sediments. Here the group ended the field trip standing on a small sliver of California that resides on the Pacific Plate. During the trip, John and Scott emphasized that these reconstructions of displacements on the greater San Andreas system in northern California are testable by geochronology and simple field work. The NCGS wishes to thank John Wakabayashi and Scott Dickerman for an excellent trip that covered quite a distance in a single day. Their careful outcrop selection was used to illustrate the data that they use for correlating and modifying offset estimates on the San Andreas fault system based on correlating key piercing units across the fault segments. Research using improved radiometric dating techniques and detailed petrographic studies of outcrops will help to unravel the complex tectonic history of the San Andreas fault system.