CHRONOSTRATIGRAPHIC FRAMEWORK FOR THE SONOMA VOLCANICS AND ASSOCIATED SEDIMENTS, LONG-TERM FAULT DISPLACEMENT RATES, AND AREAL DISPERSION OF TEPHRA FROM THE SOURCE AREA, CENTRAL COAST RANGES, CALIFORNIA

Detailed geologic mapping, tephrochronology, and isotopic dating of volcanic units are providing a spatial and temporal framework for geologic studies north of San Francisco Bay. Studies include timing of propagation of the San Andreas Fault system, fault movement histories, and long-term slip rates on faults, as well as regional identification of widespread tephra layers. Isotopic ages of the Sonoma Volcanics, both new Ar⁴⁰/Ar³⁹ and redating of previous K-Ar ages, are (in Ma; previous K-Ar ages in parens.):

Youngest Sonoma Volcanics dacite flow: 2.50±0.09; Obsidian of Napa Glass Mountain: 2.78±0.02; Tuff of Franz Valley: 2.85±0.02; Tuff of Riebli Road: 3.12±0.03; Tuff of Pepperwood Ranch: 3.19±0.02; Putah Tuff: 3.27±0.03 (3.4); Obsidian of Annadell Park: 4.50±0.08; St Helena Rhyolite: 4.45±0.02 (3.8); Tuff of Napa ("Healdsburg tuff"): 4.65±0.03; Huichica tuff: 4.71±0.03 (4.0); Lawlor Tuff : 4.83±0.02 (4.1); Roblar tuff : 6.26±0.03 (5.7, 6.1); Rhyolite tuff of Zammoroni Quarry: 7.26±0.04.

Apparent dextral offsets and rates are: the Roblar tuff by the Hayward-Rodgers Creek Fault (~64 km,10.2 mm/yr); the Lawlor Tuff by the combined West Napa-Carneros-Bennett Valley faults (~21 km, 4.3 mm/yr), and by the Green Valley Fault (~20 km, 4.1 mm/yr); and the Putah tuff by the Maacama Fault (~21 km, 6.3 mm/yr). Apparent offsets were determined for the Roblar tuff on water-deposited tuff and SE to NW fining of associated sediment facies; for the Lawlor and Putah tuffs, offsets were obtained mostly on coarse ash-flows. Dispersal of chemically "fingerprinted" obsidian clasts across the Rodgers Creek-Healdsburg Fault from the Annadel source, and across the Maacama Fault from the Napa Glass Mountain source, yields long-term displacement rates of 5-6 mm/yr from both sources. In the latter case, the age of the unit containing the obsidian clasts, the Glen Ellen Formation, is obtained from the correlated age of an exotic tephra, a Bishop-Glass Mt. tephra type ~0.8-1.2 Ma. Plinian dispersal of several tephra units extends considerably beyond the central Coast Ranges: the Lawlor Tuff is present in east-central, northern, and southern California, and in western Nevada. The Roblar, Huichica, and Putah tuffs are also distributed widely.