Geophysical framework of the northern San Francisco Bay region, California

Abstract

We use geophysical data to examine the structural framework of the northern San Francisco Bay region, an area that hosts the northward continuation of the East Bay fault system. Although this fault system has accommodated ~175 km of right-lateral offset since 12 Ma, how this offset is partitioned north of the bay is controversial and important for understanding where and how strain is accommodated along this stretch of the broader San Andreas transform margin. Using gravity and magnetic data, we map these faults, many of which infl uenced basin formation and volcanism. Continuity of magnetic anomalies in certain areas, such as Napa and Sonoma Valleys, the region north of Napa Valley, and the region south of the Santa Rosa Plain, preclude signifi cant (>10 km) offset. Much of the slip is partitioned around Sonoma and Napa Valleys and onto the Carneros, Rodgers Creek, and Green Valley faults. The absence of correlative magnetic anomalies across the Hayward-Rodgers Creek-Maacama fault system suggests that this system reactivated older basement structures, which appear to infl uence seismicity patterns in the region.

Geology and geochemistry of volcanic centers within the eastern half of the Sonoma volcanic field, northern San Francisco Bay region, California

“…To separate those shortwavelength anoma lies caused by shallow sources (e.g., Tertiary volcanic rock) from longwavelength anoma lies (e.g., serpentinite or ophiolite) caused by deeply buried preCenozoic rocks, a match fil ter was applied (Phillips, 2001) to the aeromag netic data (Langenheim et al, 2010). Match filtering separates the data into different wave length components by modeling the observed anomalies as a sum of anomalies from distinct equivalent source layers at increasing depths (see Phillips, 2001).…”

“…Chemical analyses of a representative set of volcanic rocks from the Sonoma volcanic field are presented in Table 1, and all geochemical analyses are listed in Supplemental Table 1 1 . Gravity and aeromagnetic data reflect density and magnetization contrasts within the upper and middle crust; we use these data to help define the spatial extent and the threedimensional geom etry of the volcanic rocks and define the location of structures. The regional gravity data (Langen heim et al, 2006a;Langenheim et al, 2010) were gridded to produce an isostatic residual gravity map of the study region (Fig. 4).…”

“…15). Langenheim et al (2010) correlate deepsource gravity and magnetic anomalies across the West Napa fault to suggest as much as 40 km of rightlateral off set of Mesozoic features, potentially indicating a slip history that predated eruptions from the Cup and Saucer and Sugarloaf volcanic centers. Restoration of the Sugarloaf Ridge volcanic center is an example of how the Sonoma vol canic field may be reconstructed using a vari ety of geologic criteria.…”

“…3 and 13) that appear to be the principal eruptive vents for three regionally distributed ashflow tuff and tephra deposits (see stratigraphic columns in McLaughlin and SarnaWojcicki [2003] and McLaughlin et al [2005]). We define these calderas based on the on June 10, 2015 geosphere.gsapubs.org Downloaded from presence of a semicircular topographic low with an interior topographic high, the presence in this vicinity of the coarsest pumice breccia and greatest abundance of lithic clasts in out flow tuffs (SarnaWojcicki, 1976), the presence within the interpreted calderas of two composi tionally distinct types of megabreccia supported by a tuff matrix, and geophysical evidence for multiple arcuate structural margins (Table 2; see also Langenheim et al, 2010). A portion of one of the calderas was portrayed by Bezore et al (2004), but the relative lack of outflow facies ashflow tuff in the vicinity of the "Cup and Saucer" (the central topographic high, Fig.…”

“…The style of volcanism is somewhat similar in both fields; although we cannot document any caldera structures in the vicinity of the Sugarloaf Ridge volcanic center, we do recognize smallvolume fragmental volcanic material associated with mafic vents. Langenheim et al (2010) use offset gravity and magnetic anomalies to infer a com bined 25-30km rightlateral displacement on the Carneros and West Napa faults. Restoring the fault offset would place the Sugarloaf Ridge volcanic center to the southwest of the Cup and Saucer volcanic center, putting these two simi lar centers in proximity (Fig.…”