Geology and Geochemistry of Volcanic Centers Within the Eastern Half of the Sonoma Volcanic Field, Northern San Francisco Bay Region, California

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

Donald S. Sweetkind1, James J. Rytuba2, Victoria E. Langenheim3, and Robert J. Fleck4 1U.S. Geological Survey, Denver Federal Center, Mail Stop 973, Denver, Colorado 80225, USA 2U.S. Geological Survey, 345 Middlefield Road, Mail Stop 901, Menlo Park, California 94025, USA 3U.S. Geological Survey, 345 Middlefield Road, Mail Stop 989, Menlo Park, California 94025, USA 4U.S. Geological Survey, 345 Middlefield Road, Mail Stop 937, Menlo Park, California 94025, USA

ABSTRACT INTRODUCTION centers and to estimates of offset along the West Napa-Carneros fault zones. Volcanic rocks in the Sonoma volcanic field The rocks of the Sonoma volcanic field (Fig. 1) This study defines minor- and major-element in the northern California Coast Ranges con- are part of a linear belt of exposures of Ceno geochemical trends within each volcanic tain heterogeneous assemblages of a variety zoic volcanic rocks that progressively young to center, building upon previous analyses of a of compositionally diverse volcanic rocks. We the northwest (Fox et al., 1985a) and have been relatively small number of samples from the have used field mapping, new and existing disrupted by dextral faults of the San Andreas Sonoma volcanic field (Johnson and O’Neil, age determinations, and 343 new major and fault system from their original depositional 1984; Whitlock, 2002). We define geochemi trace element analyses of whole-rock samples locations (Fox, 1983; McLaughlin et al., 1996; cal trends of intracaldera and outflow tuffs and from lavas and tuff to define for the first time Wakabayashi, 1999; Graymer et al., 2002a, distal fall equivalents from individual volcanic volcanic source areas for many parts of the 2002b). In the San Francisco Bay region, these centers. We use major-, minor-, and trace-ele Sonoma volcanic field. Geophysical data and volcanic rocks include the Quien Sabe Vol ment geochemical data to assist in correlation models have helped to define the thickness of canics, volcanic rocks in the Berkeley Hills, of volcanic units and to define caldera-related the volcanic pile and the location of caldera the Sonoma Volcanics, the Tolay Volcanics, the sources for regionally important tuffs, such structures. Volcanic rocks of the Sonoma Burdell Mountain Volcanics, and the Clear Lake as the Pinole and Lawlor Tuffs and the tuff of volcanic field show a broad range in erup- Volcanics, using the nomenclature of Fox et al. Napa, whose source areas have previously been tive style that is spatially variable and spe- (1985a) and Graymer et al. (2002b) (Fig. 1). only generally outlined. cific to an individual eruptive center. Major, Like other exposures of volcanic rocks in the The volcanic fields in the California Coast minor, and trace-element geochemical data northern California Coast Ranges, the Sonoma Ranges north of San Francisco Bay are tempo for intracaldera and outflow tuffs and their volcanic field contains heterogeneous assem rally and spatially associated with the northward distal fall equivalents suggest caldera-related blages of compositionally diverse lava flows, migration of the Mendocino triple junction sources for the Pinole and Lawlor Tuffs pyroclastic deposits, and local ash-flow tuffs (Dickinson and Snyder, 1979; Furlong, 1984; in southern Napa Valley and for the tuff of (Fig. 2) (Fox, 1983; Graymer et al., 2002a). Fox et al., 1985a; Dickinson, 1997). The north Franz Valley in northern Napa Valley. Strati- Although the Sonoma volcanic field is the ward younging of volcanism has been attrib graphic correlations based on similarity in largest of the volcanic fields in the northern uted to the transition from subduction and asso eruptive sequence and style coupled with California Coast Ranges, developing an under ciated arc volcanism to a slab window tectonic geochemical data allow an estimate of 30 km standing of the stratigraphy, volcanology, and environment (or “slabless window,” using the of right-lateral offset across the West Napa- geochemical evolution of this complex has terminology of Liu and Furlong, 1992) along Carneros fault zones since ~5 Ma. been difficult because of the limited erupted the western margin of the North American plate The volcanic fields in the California Coast volumes from individual volcanic centers and (Dickinson and Snyder, 1979; Johnson and Ranges north of San Francisco Bay are tem- consequent lack of lithologic continuity of O’Neil, 1984; Fox et al., 1985a). Recent work porally and spatially associated with the many of the units. Extensive structural disrup (Cousens et al., 2008) has defined a Miocene– northward migration of the Mendocino triple tion throughout the western half of the volcanic Pliocene Ancestral Cascades arc that was active junction and the transition from subduction field also contributes to the complexity of the at about the same time and roughly the same and associated arc volcanism to a slab win- regional correlation of units (Fox, 1983). This latitude as the volcanic centers of the Sonoma dow tectonic environment. Our geochemi- study delineates for the first time volcanic cen volcanic field. In this paper, we discuss the geo cal analyses from the Sonoma volcanic field ters primarily within the eastern, less deformed chemical and tectonic setting of volcanic rocks highlight the geochemical diversity of these half of the Sonoma volcanic field (Fig. 3) of the Sonoma volcanic field in the context volcanic rocks, allowing us to clearly distin- based on a combination of geologic mapping, of the northward migration of the Mendocino guish these volcanic rocks from those of the geophysical signature, and geochemical and Triple Junction (Johnson and O’Neil, 1984; roughly coeval ancestral Cascades magmatic petrographic criteria to tie eruptive products Dickinson, 1997) and also compare these rocks arc to the west, and also to compare rocks of to specific source areas. These combined tech to the generally contemporaneous rocks of the Sonoma volcanic field to rocks from other niques have led to our recognition of distinct the Ancestral Cascades volcanic arc (Cousens slab window settings. eruptive styles that vary between the different et al., 2008).

Geosphere; June 2011; v. 7; no. 3; p. 629–657; doi:10.1130/GES00625.1; 18 figures; 3 tables; 1 supplemental table.

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123°W 122°30′W 122°W Clear WA Lake 39°N OR ID

Clear Lake CSZ Maacama faul volcanic field

MFZ t MTJ NV

Pacific Sonoma Ocean QSV CA SA volcanic field Area shown F on map 38°30′N Napa

Sonoma V alle y RCF

San Valle

TF Area shown y Andreas fault on Figs. 2–7 Pacific Ocean TV BMV San Pablo V Bay WP R 38°N EXPLANATION HaywardP faul

Quaternary sedimentary deposits San Fr Cenozoic sedimentary rocks BHV ancisco Bay Quaternary volcanic rocks t CF Neogene volcanic rocks pre-Cenozoic rocks 036121824k0 m

Figure 1. Simplified geology of the northern San Francisco Bay area showing the Sonoma and Clear Lake volcanic fields (modified from Saucedo et al., 2002; Ludington et al., 2006). Black lines are faults, sense of displacement not shown (modified from Bryant, 2005; Graymer et al., 2006b); CF—Calaveras fault; RCF—Rodgers Creek fault; TF—Tolay fault. BHV—volcanic rocks in the Berkeley Hills; BMV—Burdell Mountain Volcanics; TV—Tolay Volcanics; P—Pinole; R—Rodeo; V—Vallejo; WP—Wilson Point. Inset tectonic map shows the Cascadia subduction zone (CSZ), Mendocino fracture zone (MFZ), Mendocino Triple Junction (MTJ), and San Andreas fault (SAF). Extent of modern Cascades arc volcanics shown in gray pattern (from Luedke and Smith, 1981); QSV— Quien Sabe Volcanics.

GEOLOGIC, GEOCHEMICAL, AND fieldwork supplemented and augmented geo paleomagnetic (Mankinen, 1972), geochrono GEOPHYSICAL METHODS logic mapping being conducted concurrently by logic (Fox et al., 1985a; McLaughlin et al., 2004, the California Geological Survey (Bezore et al., 2008), and tephrochronologic (Sarna-Wojcicki, The regional volcanic stratigraphy, identifica 2004, 2005; Clahan et al., 2004, 2005; Wagner 1976; Sarna-Wojcicki et al., 1979, 1984) data tion of the location of volcanic centers (Fig. 3), et al., 2003, 2004, 2006) and by other investi to synthesize the regional volcanic stratigraphy. and delineation of the spatial distributions of indi gators at the U.S. Geological Survey (Graymer Stratigraphic correlation of volcanic units was vidual volcanic units were accomplished through et al., 2002a, 2007; McLaughlin et al., 2004, accomplished through comparison of phenocryst extensive field observation by the authors. Our 2008). Where possible, we used new and existing assemblage and mineralogy, pumice and lithic

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/7/3/629/3339746/629.pdf by guest on 26 September 2021 Geology and geochemistry, Sonoma volcanic field, California - k TION k k k k k k k k olcanic center ent k W V k Dome V Calder a ′ k CS CS olcanic feature k X EXPLANA 122°15 V Geologic units as show n on Figure 2. MG MG SL SL k AH AH k X X X X k SON X BM BM W k ′ X WL WL X X 122°30 X XX SG SG X X k k CD k k k m k AD AD k k 0k k k MS H MS H 0246 81 N N N Figure 3. Simplified geologic map of the Sonoma volcanic field showing location of of the Sonoma volcanic field 3. Simplified geologic map Figure centers: Volcanic 2. geology and faults as shown on Figure Volcanic volcanic centers. Knob; CD—Calistoga Dome field; BM—Bismarck AH—Arrowhead; AD—Annadel; CS—Cup and Saucer; MG—Mount George; MSH—Mount St. Helena; SG—Sugar Locations of volcanic WL—Wildlake. loaf Ridge; SL—Stags Leap; SON—Sonoma; defined on the basis of geophysical vents and domes beneath Quaternary deposits are data. 38°4 5 ′ 38°3 0 ′ 38°1 5 ′ GV TION f MG lley Sequence W ′ Va CS ff elded tuf 122°15 Quaternary deposits Cenozoic sedimentary rocks Diatomite Basalt and andesite flows Rhyolite and dacite flow s Tu W Intracaldera megabreccia Intrusive rocks Francsican Comple x Great Serpentinite wate r " EXPLANA SC Napa

SL ey

Vall

CENOZOIC (NEOGENE) CENOZOIC MESOZOIC RR CV " Carneros fault ountville LV Y Napa We st Napa fault AM BK lley

" Va " MV SH Angwin Sonoma "

St. Helena s GM W ′ Sonoma Mountain 122°30 AC K Sonoma MH P " DM KC km Calistoga 0 " BM PV " MSH Rodgers Creek fault TM FV Middletow n YJFZ a 46 81

" lley Mayacamas Santa Va Ros Rosa 02 Mountains Sant a N N N Figure 2. Simplified geologic map of the Sonoma volcanic field showing place names Figure et al. Saucedo et al. (2002), Graymer geology modified from Volcanic cited in text. faults, sense of displacement not shown (modified from (2006a, 2007). Black lines are fault zone. Place name YJFZ—Yellowjacket et al., 2006b). Bryant, 2005; Graymer Mountain; BK—Bismarck AM—Arrowhead AC—Adobe Canyon; abbreviations: DM— Valley; CV—Carneros Saucer; and CS—Cup Mountain; BM—Bennett Knob; GM—Napa Glass Mountain; GV—Green Valley; Diamond Mountain; FV—Franz MG—Mount Valley; K—Kenwood; KC—Kortum Canyon; LV—Lovall Valley; P— Veeder; George; MH—Mount Hood; MSH—Mount St. Helena; MV—Mount Reservoir; SC—Soda Canyon; SH— RR—Rector Valley; Palisades; PV—Petaluma Mountain. TM—Taylor Schocken Hill; SL—Stags Leap; 38°45 ′ 38°30 ′ 38°15 ′

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content, and geochemical “fingerprinting” using Aeromagnetic anomalies are produced by a heterogeneous sequence of rocks (Weaver, 1949; major-, minor-, and trace-element geochemistry variety of sources of variable size and depth, Kunkel and Upson, 1960). Subsequent geologic augmented by petrography. typically related to the presence of Mesozoic mapping (Fox et al., 1973) and supporting Whole-rock samples of various volcanic rock basement rocks (ophiolitic rocks including geochronology (Fox et al., 1985b) allowed the types were collected at outcrops throughout the serpentinite, gabbro, and basalt) and magnetic volcanic rocks to be generally subdivided into Sonoma volcanic field (SVF) for geochemical rocks within the Tertiary volcanic section. upper and lower members largely on the basis analysis. Care was taken to sample unaltered To separate those short-wavelength anoma of age, rather than rock type—a considerable outcrops; hydrothermal alteration could gen lies caused by shallow sources (e.g., Tertiary stratigraphic improvement. Both the upper and erally be identified in the geochemical results volcanic rock) from long-wavelength anoma lower members were subdivided into a series of as a loss of alkalis. Samples were trimmed of lies (e.g., serpentinite or ophiolite) caused by informal units (Fox et al., 1985b), each of these obvious lithic inclusions, but individual fiamme deeply buried pre-Cenozoic rocks, a match fil units still included considerable lithologic varia or pumice were not sampled or analyzed. All ter was applied (Phillips, 2001) to the aeromag bility and did not explicitly link volcanic units whole-rock samples were analyzed for 10 major netic data (Langenheim et al., 2010). Match to eruptive vents or spatial distribution. More oxides, determined by wavelength dispersive filtering separates the data into different wave recently, the U.S. Geological Survey (USGS) X-ray fluorescence spectrometry (WDXRF). length components by modeling the observed has produced digital geologic compilations Techniques and standards used for WDXRF anomalies as a sum of anomalies from distinct that include the SVF at a regional scale (Blake analysis of major elements have been given by equivalent source layers at increasing depths et al., 2002; Graymer et al., 2002a, 2007), Taggart and Siems (2002). The detection limit (see Phillips, 2001). In order to assist in the while the California Geological Survey (CGS) for all elements including loss of ignition (LOI) interpretation of magnetic anomalies poten conducted 1:24,000-scale mapping within the is 0.01%. When compared with replicate analy tially caused by volcanic rocks or subvolcanic volcanic field (e.g., Bezore et al., 2004, 2005; sis of internal reference materials, determined intrusions, we created two maps that portray Clahan et al., 2004, 2005; Wagner et al., 2003, values are within 1%–5% of proposed or cer the resulting separated fields produced by 2004, 2006). The USGS and CGS maps provide tified values for elements at >1 wt% and are the dipole equivalent-source layers at 0.4141- greater delineation of the variability of volcanic within 5%–10% of proposed or certified values (Fig. 6) and 1.546-km (Fig. 7) depths. rock type, but do not identify volcanic source for elements at <1 wt% abundance (Taggart and areas or correlate specific volcanic units. Siems, 2002). All whole-rock samples were DEFINITION OF VOLCANIC CENTERS Geochemical characterization of vitric pyro analyzed for 55 trace elements using inductively clastic rocks from within and surrounding the coupled plasma–atomic emission spectrometry We define for the first time volcanic source volcanic field has been employed to facilitate (ICP-AES). Techniques and standards for the areas for many parts of the Sonoma volcanic stratigraphic correlation of volcanic units whose ICP-AES method for a smaller suite of elements field based on a combination of geologic map source is presumed to be the SVF (Sarna- have been described by Briggs (2002). ICP- ping, geophysical signature, and geochemi Wojcicki, 1976; Sarna-Wojcicki et al., 1979, AES precision and accuracy based on replicate cal and petrographic criteria. Table 2 presents 1984). Analyses are of volcanic glass from analysis of internal basalt standards, duplicate synoptic data from many of the volcanic cen tephra layers, which include ash fall, pumice analyses, and method blanks are ±5%–10% for ters within the SVF, describing eruptive style, fall, ash flow, and water-reworked tuff deposits. most elements, ±10%–15% for Nb. Chemical spatial distribution of erupted rocks, volcanic These tephrochronologic analyses have been analyses of a representative set of volcanic rocks facies changes with distance from an interpreted conducted at sites where the correlation of ash from the Sonoma volcanic field are presented in center, geochemical data, and interpretation of units would assist stratigraphic mapping and Table 1, and all geochemical analyses are listed geophysical data. Within the paper we describe structural interpretation (McLaughlin et al., in Supplemental Table 11. in detail selected volcanic centers where we 2004, 2008), but have not been used globally Gravity and aeromagnetic data reflect density have concentrated our mapping and geochemi within the field to establish stratigraphy nor and magnetization contrasts within the upper and cal sampling within the eastern, less deformed have the results been tied to major-element middle crust; we use these data to help define the half of the field. The list of volcanic centers pre whole-rock analyses of nonglassy rocks within spatial extent and the three-dimensional geom sented in Table 2 is not comprehensive; there are the field. These analyses have established that etry of the volcanic rocks and define the location a number of other known vents and sources for certain distal fall deposits that appeared to have of structures. The regional gravity data (Langen local flows that are not specifically described. their source within the Sonoma volcanic field heim et al., 2006a; Langenheim et al., 2010) In addition, there are volcanic units for which a were regionally important (Sarna-Wojcicki, were gridded to produce an isostatic residual specific source area has not been identified. Our 1976; Sarna-Wojcicki et al., 1979, 1984), but gravity map of the study region (Fig. 4). We used geochemical database and field mapping is not individual source vents were only generally the method of Jachens and Moring (1990) to sep exhaustive; we did not map nor analyze samples defined (McLaughlin et al., 2005). arate the isostatic gravity field into that compo from several regionally significant volcanic nent produced by variations in basement density units, including the Huichica Tuff and the Putah General Description of Volcanic Centers (“basement gravity field”) and that caused by Tuff, and thus cannot comment on their possible thick sedimentary and volcanic deposits, which correlation or source areas. We define five major volcanic centers adja is then inverted for basin thickness (Fig. 5). cent to Napa Valley (Fig. 3) including, from Previous Work in Defining north to south, the Mount St. Helena (MSH), 1Supplemental Table 1. Excel file of geochemical Volcanic Centers Calistoga Domes (CD), Wildlake (WL), Stags analyses, Sonoma volcanic field. If you are viewing Leap (SL), and Cup and Saucer (CS) centers the PDF of this paper or reading it offline, please (Table 2 and Fig. 3). The Mount St. Helena vol visit http://dx.doi.org/10.1130/GES00625.S1 or the In the vicinity of Napa Valley, early mapping full-text article on www.gsapubs.org to view Supple generally subdivided the volcanic rocks into an canic center (MSH, Table 2 and Fig. 3) includes mental Table 1. upper rhyolitic member and an underlying more the caldera source of the 2.85 Ma tuff of Franz

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TABLE 1. REPRESENTATIVE MAJOR AND TRACE ELEMENT COMPOSITIONS Sample 05CQR2 06DSN6904DSN6204DR06 04MSH7NUN04 05DSN2205SL1007KW7 07DR7 Volcanic Calistoga Cup and Cup and Mount Mount Stags Stags Sonoma SugarloafWildlake center Domes Saucer Saucer St. Helena St. Helena Leap Leap Basaltic Fine-grained Peralkaline Rock Rhyolite, Andesitic Lithic tuff, Lithic tuff, Basalt Porphyritic Andesite andesite phase, Stags rhyolite type glass tuff intracaldera outflow flow andesite flow flow Leap intrusive banded

SiO2 74.32 54.9962.87 71.54 74.21 51.4359.99 67.9273.53 66.35 Al2O3 13.37 16.3017.56 15.58 14.15 15.4916.80 15.4410.56 16.31 Fe2O3t 2.10 10.238.114.421.9512.86 6.51 4.80 6.26 4.34 MgO 0.41 3.44 0.73 1.02 0.09 3.64 3.40 0.99 0.08 0.98 CaO 1.05 7.51 3.14 0.35 0.78 7.88 6.57 2.04 0.28 3.33

Na2O 4.43 3.85 3.90 2.82 4.94 4.10 3.07 5.26 4.79 4.64 K2O 4.05 1.15 2.16 3.79 3.44 0.79 2.25 2.68 4.05 2.71 TiO2 0.21 1.98 1.09 0.35 0.22 2.94 1.11 0.68 0.34 0.74 P2O5 0.03 0.24 0.25 0.07 0.01 0.65 0.18 0.15 0.02 0.16 MnO 0.03 0.29 0.19 0.05 0.20 0.23 0.10 0.06 0.09 0.44 LOI 0.3 1.45 4.24.090.750.751.651.351.280.80 Ba 574 266556.3 866913 250451.8 574141 900 Ce 54 26 51.8 37.1 86.333.233.362.3137 49.5 Co 1.7 29.5 12.1 4.7<0.532.117.66 0.68.2 Cr 10 90 12 80 <10<10 72 <10<10 <10 Cs 12.2 2.74.9 5.42.7 1.55.2 1.50.8 8.1 Cu 52829101529165 610 Dy 6.21 4.62 8.75 8.18 9.71 7.36 5.27 7.06 27.6 6.12 Er 4.46 2.96 5.41 6.36.154.2 3.41 4.45 20.5 3.74 Eu 0.41 1.48 2.04 0.66 1.48 2.37 1.21 0.96 3.13 1.17 Ga 17 17 22 23 22 17 16 20 39 19 Gd 6.02 4.47 8.08 5.46 8.83 7.22 5.46 7.24 20.9 6.31 Hf 637 8834 4287 Ho 1.35 0.96 1.92 1.98 1.95 1.47 1.15 1.44 5.79 1.32 La 27.3 12.9 25.1 13.4 31.3 15.3 17 29.4 48.3 24.1 Li 70 <1013110 50 <1014207020 Lu 0.65 0.40.790.930.820.520.510.553.080.6 Nb 9712 13 14 91010649 Nd 23.6 16.5 30.9 15.1 35.7 24.8 19.7 30 80.1 25.2 Ni 10 30 <5 62 <5 <5 8<59<5 Pb 39 21 24 18 22 612172414 Pr 6.4 3.77 7.39 3.74 9.14 5.33 4.78 7.75 19 6.15 Rb 155 31.1 58 129118 18.6 72 80.7 127102 Sb 1.1 0.61.4 1.30.2 3.111.1 Sc <5 26 16 5<5271810<511 Sm 5.1 4.17.2 3.796.64.7 6.820.95.6 Sn 523 4522 7155 Sr 47 305222.4 57.3 59.4257 273.2164 4177 Ta 0.6 0.51 0.80.8 0.70.6 <0.5 3.80.7 Tb 0.93 0.85 1.51 1.05 1.47 1.24 0.93 1.08 4.21 1.1 Th 13 3.56.7 8.7112.9 6.27.9 14.5 9.3 Tl 0.8 <0.5 0.60.8 2.8<0.5<0.50.6 <0.5 0.8 Tm 0.64 0.39 0.79 0.93 0.88 0.55 0.45 0.62.940.55 U 6.18 1.41 3.12 3.75 4.26 1.04 2.62 3.31 5.68 3.87 V11288 45 18 <5 2609920<574 W2<1 <1 <1 <1 <1 11<1 1 Y 34.8 24.8 46.6 46.9 41.136.22935.9119 36 Yb 42.4 4.96 5.83.4 2.93.8 21.4 3.7 Zn 56 79 114100 106917237224 58 Zr 230 114240 313346 130190.1 1711230284 Northing 4,268,298 4,233,1534,239,421 4,278,2064,278,528 4,249,4654,250,729 4,250,4364,254,790 4,272,752 Easting 535,023 563,773570,122 533,937 529,523 542,379561,284560,005540,191543,660 Note: Major elements by wavelength dispersive X-ray ﬂuorescence spectrometry (WDXRF), in weight precent oxides. Reported major oxides have been recalculated to 100% volatile-free. Trace elements by inductively coupled plasma–atomic emission spectrometry (ICP-AES) and inductively coupled plasma–atomic emission spectrometry (ICP-AES), reported in weight parts per million. See text for analytical details. Northing and easting are in Universal Transverse Mercator projection. Zone 10 north, North American Datum of 1927 (NAD27).

Valley (Table 2), and is the youngest and last and Fig. 3) and a thick section of andesitic volcanic agglomerate, and lithic tuff is overlain active center in the Sonoma volcanic field (Fox flows and lahars of the Wildlake volcanic center by a 150-m-thick section of 5.4 to <4.70 Ma et al., 1985a; McLaughlin et al., 2005). This (WL, Table 2 and Fig. 3). In the south-central ash-flow tuffs, tephra, and dacite lava flows that center contrasts with many of the other erup part of Napa Valley we define the Stags Leap are associated with two nested calderas (Fig. 3). tive centers within the Sonoma volcanic field volcanic center (SL, Table 2 and Fig. 3) as a Rocks of the Mount George center (MG, Table in having volumetrically more abundant, thick large constructional composite volcano with 2 and Fig. 3) unconformably overlie rocks of siliceous ash deposits that include air fall, ash a more than 350-m-thick sequence of basal the Cup and Saucer center and include local flow, lahar, and reworked water-transported tic andesite flows and lahars. At the south end late-stage rhyolite flows and a welded tuff. On ash deposits. This center was constructed on of Napa Valley we define the Cup and Saucer the geochemical diagrams, rocks from this cen top of older domes and associated flows and volcanic center (CS, Table 2 and Fig. 3) where ter are combined with those from the Cup and tuffs of the Calistoga dome field (CD, Table 2 a lower sequence of interlayered mafic flows, Saucer volcanic center.

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/7/3/629/3339746/629.pdf by guest on 26 September 2021 Sweetkind et al. k k k k TION k k k k k k W k k m m m m m m m CS CS m lcanic center nt 122°15 ′ 500 1,000 1,500 2,000 2,500 3,000 3,500 4,000 EXPLANA pre-Cenozoic rocks Vo MG MG Modeled thickness of Cenozoic rocks, in m Dome Calder a Ve k X olcanic feature SL SL V k AH AH k X X X X k SON X W BM BM k X WL WL X X 122°30 ′ X XX SG SG X X k k CD k k k m k AD AD k k 0k k k MS H MS H 0246 81 N N N 38°4 5 ′ 38°3 0 ′ 38°1 5 ′ Figure 5. Modeled thickness of Cenozoic sedimentary and volcanic deposits from 5. Modeled thickness of Cenozoic sedimentary and volcanic deposits from Figure of volcanic centers as on inversion of the gravity data. Outlines and abbreviations 3. Figure

. k k k k k k k TION k k k W k k CS CS

122°15 ′ MG MG olcanic center ent Isostatic residual gravity Contour interval 2 mGal. Hachures indicate gravity lo w. with additional new gravity data) (after Langenheim et al., 2006; EXPLANA V Dome Caldera V SL SL X k olcanic feature V Geologic units as shown on Figure 2. k AH AH k X X X X k X W SON BM BM k X WL WL X X 122°3 0 ′ X XX X SG SG X k k CD k k k m k AD AD k k k 0k k MSH MSH 46 81 02 N N N Figure 4. Isostatic gravity contours (from Langenheim et al., 2006a) superposed on 4. Isostatic gravity contours (from Figure indicate gravity low. interval, 2 mGal; hachures Contour simplified volcanic geology. 3. of volcanic centers as on Figure Outlines and abbreviations 38°45 ′ 38°30 ′ 38°15 ′

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/7/3/629/3339746/629.pdf by guest on 26 September 2021 Geology and geochemistry, Sonoma volcanic field, California - k TION k k k k k k k k k W ′ k k lcanic center nt CS CS EXPLANA Vo Dome Calder a Ve 20 0 15 0 10 0 50 25 0 -2 5 -5 0 -100 122°15 MG MG k X Aeromagnetic data, in nT olcanic feature V SL SL k AH AH k X X X X SON k X BM BM W ′ k X WL WL X 122°30 X X SG SG XX X X k CD k k k k m AD AD k k k 0k k k MS H MS H 0246 81 N N N 38°45 ′ 38°30 ′ 38°15 ′ Figure 7. Aeromagnetic map filtered to emphasize medium-depth sources of magnetic to emphasize medium-depth sources map filtered Aeromagnetic 7. Figure intru - shallow (e.g., sources medium-depth by caused anomalies Magnetic anomalies. separated using match filter ophiolite) are shallowly buried serpentinite or sives or by the ing (Phillips, 2001) of the observed anomalies; map shows field produced of volcanic layers at 1.546 km. Outlines and abbreviations dipole equivalent-source 3. centers as on Figure k IO N k k k k k k k k k W k ′ k CS CS 122°15 4 4 olcanic center 20 12 ent - MG MG -12 -20 EXPLAN AT V Dom e Caldera V k X olcanic feature Aeromagnetic data, in nT SL SL V k AH AH k X X X X k X W BM BM k SON ′ X WL WL X X 122°30 X XX X SG SG X k k CD k k k m k AD AD k k k 0k k MSH MSH 46 81 02 N N N Figure 6. Aeromagnetic map filtered to emphasize shallow-source magnetic anoma - to emphasize shallow-source map filtered Aeromagnetic 6. Figure volcanic Tertiary (e.g., lies. Short-wavelength anomalies caused by shallow sources separated using match filtering (Phillips, 2001) of the observed anomalies; are rock) layers at 0.4141 km. by the dipole equivalent-source map shows the field produced 3. of volcanic centers as on Figure Outlines and abbreviations 38°45 ′ 38°30 ′ 38°15 ′

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# ) e d -

- lley

continue ( Anomalies Anomalies . and in Franz may correspond to intrusive rocks. anomalies probably associated with pre-Cenozoic rocks. Smaller positive anomalies at Diamond Mountain anomalies have a discontinuous, short wavelength pattern consistent with small-volume volcanic centers. interpreted to result from medium-depth sources are long, linear northwest southeast–trending produced by shallow sources show localized highs that generally correspond to post-caldera rhyolite domes. interpreted to result from medium-depth sources show a distinct positive anomaly centered on the resurgent intracaldera pile that may represent a subcaldera felsic pluton Aeromagnetic signatur Shallow-source Magnetic anomalies

fects of s and ff Modeled This result y. . D Gravity signature subdued relative to regional gradient; localized gravity lows associated with low-density volcanic rocks present near Diamond Mountain and Napa Glass Mountain. Modeled thickness of volcanic rocks, generally greater than 1 km and up to 3 km. Irregular pattern of modeled volcanic-rock thickness interpreted to represent the ef local rhyolite domes. residual gravity low centered on Mount St. Helena caused by low-density tu volcaniclastic deposits that ﬁll the caldera and by relatively less-dense pre-Cenozoic rocks in this vicinit thickness of volcanic rocks is generally 500 m or less within the caldera; only locally is the thickness greater than 1 km. is consistent with the resurgence of the caldera and the local exposure of the caldera ﬂoor Gravity signature 2–4 mGal isostatic /

. /

fs and

VOLCANIC FIEL from a ff A f ranges alley ash- r versus Ba eochemistry /S composition and trace-element ratios are similar to those for the ash-flow tuf associated lavas derived from the Mount St. Helena caldera. Samples from this center of the domes define a broad spread of Rb Zr composition. The obsidian of Glass Mountain has the highest Rb Sr from this center indicating that it is the most evolved rhyolite within the dome field. flow tuf from high-silica rhyolite in the outflow facies to low-silica rhyolite in the intracaldera facies with REE, and LILE indicating eruption of the tu chemically zoned magma chamber decreasing alkalis, Range in silica Franz V

THE SONOM

- - sG OF

. the east RT To PA Rock type ejecta are dacite to rhyolite in composition siliceous ash deposits that include air falls, ash ﬂows, lahars, and reworked water-transported ash deposits. Post- caldera eruption of high-silica rhyolite occurred outside the south margin of the caldera. Eruption of post collapse andesite was widespread outside and to the southwest of the caldera. and northeast of Mount St. Helena, widespread late stage basalt ﬂows cap the stratigraphic sections. Flows and ash Abundant, thick

f of THE EASTERN

alley and

apron ruptive style ff fs erupted during lcanic rocks aprons, flows, and small-volume ash ejecta. Domes commonly have a tu consisting of lithic rich fall deposits that represent the vent-clearing nonwelded ash-flow domes that have associated tu phase of the eruption, overlain by thinly bedded fall deposits. Some domes are associated with tuf emplacement of the domes. associated with the formation of the Mount St. Helena caldera. Older rhyolitic fall deposits and ash-flow tuf were erupted from the vicinity of Mount St. Helena caldera but the source region for these eruptions was destroyed during the eruption of the younger tuf Franz V formation of the Mount St. Helena caldera. Local rhyolitic Vo

ﬂow ff

. ash ff and formation of Franz ff welded ff . lley tu lley consists of a f. Present primarily with highly variable volcanic stratigraph Stratigraphic relations suggest that the dome field formed prior to eruption of the Franz Va of the Mount St. Helena caldera Va single cooling unit up to tuf to the southwest of the caldera; mapped distribution extends 20 km from the caldera source. Intracaldera facies is over 900 m thick. Base of the intracaldera sequence is nonwelded and consists of collapse breccia composed of rocks derived from the caldera wall in a matrix of ash flow tu 30 m thick comprised of both lithic- and pumice rich nonwelded to partially The upper part of the intracaldera section consists of a single cooling unit of partly to densely welded, lithic- rich ash flow tu Stratigraphic descriptio Localized accumulations Outflow tu

fs Ages †

of of Petriﬁed ff of Franz ff ff ABLE 2. DESCRIPTION OF VOLCANIC CENTERS WITHIN

T Age (Ma) (3.27 Ma). lley; older tuf ff Glass Mountain (McLaughlin et al., 2005). Va probably associated with this center include tu Forest (3.34–3.35 Ma), the tu Pepperwood Ranch (3.19 Ma) and Putah Tu from McLaughlin et al. (2004, 2005). 2.78 Ma, obsidian of 2.85 Ma, tu

* lley and Va . Location alley of the northern part of Napa in hills that rise from the valley ﬂoor south of Calistoga east and west side of Calistoga, at and near Mount St. Helena, and in the vicinity of Franz V Exposed on both the North and northwest

Calistoga Dome ﬁeld (CD) Name (MSH) Mount St. Helena

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) e d

continue ( Anomalies Anomalies lley Sequence and near the stock by strong argillic alteration. and reduction of magnetization in character of the sedimentary rocks, sedimentary rocks that the stock intrudes that reflects the weakly magnetic from medium-depth sources show a broad area of generally low magnetization in the vicinity of outcrops of the Stags Leap stock and the Great Va anomalies are long and arcuate can be correlated to the outcrop of individual flows, corroborating the continuity of individual flows. interpreted to result produced by shallow sources follow outcrop pattern of the subhorizontal surficial volcanic rocks. interpreted to result from medium-depth sources are linear and northwest- trending, probably associated with the pre-Cenozoic rocks. Small superimposed positive anomalies may represent shallow intrusive bodies. Aeromagnetic signatur Shallow-source Magnetic anomalies

. .

basins

lley ﬂoor ) Va d semicircular Gravity signature o arcuate lows in center is relatively in the vicinity of Stags Leap volcanic the result of gravitational signature of a large constructional apron of lava ﬂows. Modeled thickness of volcanic rocks varies from 250 m to about 1 km, consistent with the nearly 500 m of volcanic section exposed from the top of Stags Leap to the Napa featureless, probably isostatic residual gravity are 2–4 mGal below the regional trend due to thick accumulations of low-density volcanic rocks. Gravity values are relatively higher near volcanic necks and may indicate the presence of subvolcanic intrusive rocks. Modeled thickness of volcanic rocks shows irregular pattern with several local about 2 km in diameter and up to 3 km deep continue (

Isostatic residual gravity Tw

;

Air fall 2

O . 2 . Maﬁc to O 2 (<1.3%). 2 2 O O O . Basaltic Ti eochemistry Ti 2 s high in section VOLCANIC FIELD ff that comprises most of the volcanic section has 60%–67% Si range from 54.9% to 66% Si section. Rocks show enrichments in LILE such as K and Ba. (0.66%–0.83%) to the intermediate part of the volcanic section and indicate that the last phase of volcanism in the volcanic center was bimodal. intrude the upper part of the volcanic from 69% to 72.3% SiO tu are low-silica rhyolite with silica content that ranges dikes, with 51.5% intermediate rocks have distinctly low to 52.3% SiO capping rhyolite ﬂows and rhyolite dikes range up to 75% Si Associated intrusive rocks have similar silica (65%–67%) and Andesite to dacite Flows and lahars

sG THE SONOMA fs of OF RT Rock type PA lahar breccias comprise 40%–50% of the volcanic section close to inferred volcanic gentle slopes. which predominate in the volcanic section range in composition from basaltic andesite to medium K-andesites. Small volume fall deposits in the upper part of the volcanic section, local basalt dike vent. Farther away from the inferred vent, andesite ﬂows predominate and generally form Prominent clif Flows and lahars

, and

THE EASTERN

Eruptive style nts consist of the vent area for the volcanic center and may be time-equivalent to rhyolite dikes that intrude the andesitic inferred to occupy pile and to the extrusive rhyolite flows that cap the volcanic sequence formed from repeated eruptions of andesitic pyroclastic material and lava flows. Sequence intruded by north-striking rhyolite dikes. Granodiorite is complexes that erupted basalt to basaltic-andesite lava flows, tu associated lahars. Ve small diatremes less than 0.5 km in diameter filled with breccia clasts up to several across. Composite volcano Several vent

lley

Andesitic capped by two ff basaltic andesite flows and lahars, overlain by nonwelded rhyolitic tu rhyolite lava flows. Andesitic flows and lahars thin to the north and south away from the core of volcano. Underlying Great Sequence rocks and andesite flows low in the volcanic section are intruded by two phases of granodiorite, a fine-grained phase that has been intruded by a coarser equigranular phase. lahars comprise the thickest volcanic units and were deposited in paleocanyons developed within the volcanic field. Northeast- and northwest-striking dikes extend from source volcanic vents and are typically columnar jointed. dipping volcanic rocks. horizontal to gently Stratigraphic descriptio 350-m-thick sequence of 600-m-thick section of

† 8 Age (Ma) ) ABLE 2. DESCRIPTION OF VOLCANIC CENTERS WITHIN eiss and Livermore, ﬂows, respectively (this paper) for the lowest exposed andesite ﬂow and one of the stratigraphically highest andesite 1996 T 4.35 Ma and 4.3 3.2 to 2. (W

* lley untville. Va Yo Location fs of Stags Leap Angwin. Includes alley near Rector V Reservoir and Soda Canyon, west side of Napa west of Include prominent clif of the Palisades. the prominent clif of of Calistoga, southward to town East side of Napa North and northeast

Stags Leap (SL) Name (WL) Wildlake

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erently -

continue ( Anomalies uncation of magnetic anomalies show an arcuate pattern near the interpreted calderas, but are dominated by short-wavelength anomalies related to exposed volcanic rocks outside of the caldera. interpreted to result from medium-depth sources display an annular magnetic low surrounding a magnetic high beneath exposed intracaldera megabreccia. Tr these medium depth anomalies, presumably near the structural margin of a caldera, indicates that the ring structure penetrates to at least moderate depth (about 1.5 km) and juxtaposes di magnetized rock packages. from medium- depth sources display medium wavelength magnetic highs beneath the domes which may correspond to intrusive rocks at depth beneath the dome ﬁeld or serpentinite that survived alteration. magnetic anomalies are related to outcrops of the rhyolite domes. Anomalies interpreted to result Aeromagnetic signatur Shallow-source Shallow-source

) d . Gravity signature to 2 km, but as little 0.25–0.5 km near the intracaldera breccia where a subcaldera intrusion may be present at shallow depths isostatic residual gravity low within which is a domical 2 mGal relative gravity high. Negative anomaly interpreted to be the result of a thick accumulation of low-density intracaldera volcanic rocks; domical high is interpreted to be related to caldera resurgence and may be related to a subcaldera intrusion. Modeled volcanic-rock thickness outside the caldera is between 1 and 1.5 km, consistent with mapped thickness of volcanic rocks. Within the interpreted calderas, modeled volcanic-rock thickness is locally up the location of rhyolite domes. is a relative gravity low over the thickest accumulations of rhyolite lava ﬂows. Local areas of thick volcanic rocks as modeled from the gravity data also correspond to continue ( Semicircular 4 mGal The Mount George area

. .

basaltic 2

K as (<1.3%), 2 These O (andesite predominate eochemistry (2%–2.5%) Ti 2 2 O. 2 VOLCANIC FIELD 2 few samples are O SiO volcanic center on geochemical plots. Flows with >70% Cup and Saucer 53%–75%; samples from lower part of section range silica content from from 60%–65% SiO Ti content, distinctly low samples have concentrations of and dacite). Most alkalis and distinctly higher and low K Ba samples are medium-K calc alkaline series with relatively low total and enrichments in LILE such as Ba. A enriched in alkalis and would be classiﬁed trachyandesites and trachydacites; alkali enrichment is primarily the result of elevated Na Combined with Rocks range in

The of

THE SONOMA ff OF RT Rock type is densely of andesite and s, tephra, and ff ff ff PA an upper densely welded vitrophyre zone. the unit is vitric with dacite to rhyolite. Rhyolitic tu Monticello Road is crystal-rich with 10%–15% feldspar phenocrysts. tu welded and most of includes basalt, basaltic andesite, and andesitic flows interbedded with tu dacite composition. Overlying the mafic section is a of andesitic to rhyolitic ash-flow tu dacite lava flows Lava flows are Lower section

of ff

unger THE EASTERN Yo ; source ff and tephra Eruptive style lcanic field. fs in the Sonoma ff from Mount George are geomorphically young, preserving steep flow margins and pressure ridges along the flow tops. 10-m-thick tu aprons. Lava flows extending west Monticello Road one of the few densely welded tuf Vo and associated tuf series of mafic lava flows and breccias interbedded with volcanic agglomerate and yellow nonwelded lithic tu areas for these units are not defined. nested calderas within the Cup and Saucer volcanic center appear to be the principal eruptive vents for two regionally distributed ash-flow tu deposits. Constructional domes Lower section is a

s ff .

s with a ff that is at least 300 and a 10-m-thick , both of which ff ff ff at least three rhyolitic lava ﬂows separated by Saucer volcanic center intervals of nonwelded tu the city of Napa rhyolitic welded tu unconformably overlie the tilted older volcanic units of the Cup and tu m thick overlain by a 150-m-thick section of andesite, dacite, and rhyolite ash-ﬂow tu the Stags Leap center on the north and Mount George center in the hills to east of and minor interbedded that are interbedded with generally coeval small-volume extrusive domes and lithic nonwelded tu similar compositional range. unconformably overlain by volcanic rocks from Stratigraphic descriptio 125-m-thick sequence of Lower section of andesite Rocks from this center are

Tu ff ff /A † . ojcicki (5.2–5.4 ojcicki et al., ff , Evernden et . Tu Age (Ma) ABLE 2. DESCRIPTION OF VOLCANIC CENTERS WITHIN on rhyolite lava ﬂows (Fox et al., 1985b); 4.50 Ma for the tu of Monticello Road (Sarna-W in press) low in the stratigraphic section. Overlying section includes Pinole Ma; K-Ar al., 1964), Lawlor (4.84 Ma), and tu Napa (<4.70 to 4.71 Ma) (Sarna-W et al., in press) T 3.73 and 3.4 Ma (K 5.02 Ma (K-Ar) from

. alley on . Location alley on the west George in highlands to the east of city of Napa Napa and in the surrounding hills; volcanic rocks associated with this center extend from near Carneros V to Green V the east At and near Mount Near the city of

Mount Georg e (MG) Name Saucer (CS) Cup and

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lley may continue ( Arrowhead 2-km Va A Anomalies . . magnetic anomalies mimic outcrop pattern of maﬁc rocks; local highs may coincide with vent areas. interpreted to result from medium-depth sources display broad positive anomalies that may represent shallow intrusive bodies. wide, north-striking magnetic anomaly present beneath Sonoma be a buried, fault- controlled volcanic center is coincident with the northwest trend of the rhyolite of Mountain. superimposed on the overall pattern of rocks of the Sonoma center Aeromagnetic signatur Shallow-source An aeromagnetic high Magnetic pattern is .

. ) d Gravity signature felsic pluton underlying residual gravity low caused by thick section of volcanic rocks north of the town Sonoma. Modeled volcanic-rock thickness is between 1.5 and 2 km, generally consistent with mapped thickness of volcanic rocks . the dome complex, not surprising given the relatively small volume of erupted material no volumetrically large low associated with the volcanic center indicates that most of the rhyolite was erupted and there is residual gravity low encompasses the vent on Bismarck Knob but the center of low is located to the southeast of the rhyolite vent area continue (

2–4 mGal isostatic The absence of a gravity Broad isostatic

O 2

. Rhyolite of eochemistry 2 VOLCANIC FIELD Sonoma volcanic center on geochemical plots. Si O Earliest basalt ﬂows are low-K basalts with 50% 70%–74% Si alkaline series. composition, most samples are medium-K calc Bismarck Knob has 71.2%–75% Si Combined with Rhyolite ﬂows have Basalt to andesite

. s, THE SONOMA ff OF were derived y, RT Rock type . lle ff PA flows overlain by late-stage rhyolite. volume nonwelded similar to the rhyolite dome ash-flow tu exposed in Lovall Va from the dome complex and are compositionally andesite lava flows with interbedded tu Basalt and andesite Rhyolite flows. Small- Basaltic andesite and

. THE EASTERN . Arrowhead Eruptive style fs that appear to area and domes. Flows extend for up to 8 km from the vent andesite flows overlain by small volume welded tu and rhyolite flows andesite and complex and associated flows and welded tu with vents localized on Mountain flows and breccias with subordinate tuf have erupted from a number of volcanic vents Massive basaltic Rhyolite dome Sequence of lava n

s fs. Unconformably flows that is overlain by rhyolite flows and dome of basalt and andesite andesite and andesitic flows and breccias lava flows with local vitrophyre overlie a sequence of basaltic 300-m-thick volcanic section consists of basaltic andesite and andesite flows and interbedded tuf overlain by rocks of the Bismarck Knob center Stratigraphic descriptio 500-m-thick sequence Rhyolitic dome and Lower part of the

† Ar 39 / Ar agner et al., 40 ( Arrowhead Age (Ma) gner et al., in ABLE 2. DESCRIPTION OF VOLCANIC CENTERS WITHIN in press), obtained from the base of capping rhyolite. from rhyolite lava flow of Mountain near the top of the section (Fox et al., 1985b) of Schocken Hill near the base of section (fission track, Fox et al. 1985b) to 6.64 Ma for tilted dacites high in the section Wa press) T 6.14 Ma (W 7.5 Ma (fission track) 7.9 Ma for the andesite

Location the city of Sonoma graphically high area to the north of Knob in the topo Arrowhead Mountain to the east of the city Sonoma. volcanic highlands to the north of town of Sonoma. At and near Bismarck Localized on Occupies most of the

Bismarck Knob (BM) Arrowhea d (AH) Name Sonoma (S)

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ylor The Ta . Anomalies . superimposed on the overall pattern of rocks of the Sonoma Mountain- Mountain area. coincides with strong negative magnetic anomalies. negative anomalies are consistent with paleomagnetic data on rhyolite dated by K-Ar methods at 5.3 Ma near the base of the volcanic section indicating reversed directions (Mankinen, 1972) magnetic anomalies mimic outcrop pattern of maﬁc rocks. interpreted to result from medium-depth sources display broad positive anomalies that may represent shallow intrusive bodies Aeromagnetic signatur Magnetic pattern is The volcanic section Shallow-source

. . ) d Gravity signature signature from this localized center with a diameter of 4 km is coincident with the vents in Sugarloaf volcanic center and may reﬂect low density caldera-ﬁlling volcanic rocks or a near-surface felsic pluton most likely related to the presence of dense Mesozoic rocks, such as that associated with a small outcrop of serpentinite in the Rodgers Creek fault zone (McLaughlin et al., 2008). continue broad gravity high is 6 mGal gravity low (

A Negligible gravity A

) -

2 . O

lcanism Vo These rocks ) and dacite ). ). eochemistry 2 2 2 VOLCANIC FIELD andesite (49%–55% samples are basalt and basaltic Si O Si O basalt (49.3% SiO compositional range from low-potassium (<1.3%). repeatedly alternated from andesite to rhyolite. SiO very high Zr (up to 1300 ppm) and are enriched in LREE. This occurrence of to high-silica rhyolite (76.5%–77.2% peralkaline rhyolite is unique within the Sonoma volcanic field are medium-K calc alkaline rocks with generally low ltered to emphasize medium-depth (1.546 km) sources of magnetic Most of the analyzed Rhyolite (74%–75% Rocks span the Peralkaline flows have fi

sG ersion of the gravity data shown on Figure 5. v THE SONOMA , classified 2 OF s with minor ff RT Rock type PA Aeromagnetic map and tu basalt and rhyolite. rhyolite. Peralkaline rhyolite flows (70%– 73% SiO as pantellerite) containing sodic amphibole comprise a large part of the volcanic section at the north end of volcanic center age but relation to the rest of the volcanic section is uncertain. Andesitic lava flows Perlitic rhyolites Basalt, andesite, and -

that

lley aylor ff Va , younger THE EASTERN ” (Sonoma The arcuate T. Eruptive style any eruptive centers for these rocks as “S Mountain-T Mountain), but we have not defined in this area, we have chosen to classify our samples collected west of Sonoma limited sampling of localized vents. of basaltic andesite vents and dikes that erupted agglomerate and flows. basin is suggestive of a caldera. However volcanic rocks have obscured any structural evidence of caldera related collapse; identification of a regionally extensive ash-flow tu could be linked to rocks exposed at this center would lend further support to the presence of a caldera. Given our extremely Erupted from a series Arcuate alignment

fs, in s that ff . fs. flows and local domes that are flow banded and locally obsidian sedimentary rocks of the Petaluma Formation. Mafic lava flows cap the volcanic section bearing. a faulted anticline of a greater than 500-m-thick section of basalt and andesitic flows and tuf part interbedded with than 210 m. Basal rhyolite flows with overlying sequence of near-vent, nonwelded to welded lithic-bearing rhyolitic tu contain lithic clasts up to 1 m in diamete Andesite flows and breccias containing spindle bombs and clasts of andesite up to 2 m in diameter are interbedded with the tuf Stratigraphic descriptio otal thickness of more Massive rhyolite lava Sonoma Mountain is T

† agner et al., in . Age (Ma) ABLE 2. DESCRIPTION OF VOLCANIC CENTERS WITHIN al., 2008). press) Ma (Mankinen, 1972) on rhyolite flows near the base of section. Peralkaline rhyolite flows are 4.83 Ma (W flow (McLaughlin et Ar age of 5.3 ± 0.2 T K/ 4.5 Ma, rhyolite lava

Ar except as noted.

39 * /

. . 40

ylor , including Location Ta lley st of Sonoma Va Sonoma Mountain, Bennett Mountain, and Mountain of the town Kenwood Mountain southeast of the city Santa Rosa . We North and northeast Near Bennett

Age determinations by Isostatic residual gravity contours shown on Figure 4. Modeled thickness of Cenozoic sedimentary and volcanic deposits from in Aeromagnetic map ﬁltered to emphasize shallow-source (0.4141 km depth) magnetic anomalies shown on Figure 6. ylor *Boundaries of volcanic centers shown on Figure 3; locations described relative to place names 2. † § # Mountai n Sonoma Mountain- Ta anomalies shown on Figure 7. Name Sugarloaf Ridge (SG) Annade l (AD)

640 Geosphere, June 2011

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For the purposes of geochemical classification, Supplemental Table 1 [see footnote 1]). Volcanic with respect to LILE and strongly enriched we identify four volcanic centers in the vicinity rocks of the Sonoma volcanic field span the full with respect to HFSE (Fig. 10). For basalts and of the town of Sonoma and in the mountains to compositional range from basalt and basaltic basaltic andesites, fractionation between LREE the north: the Sonoma (SON), Arrowhead (AH), andesite through andesite, dacite, and rhyolite (La, Ce) and HREE (Y and Lu) is minimal Bismarck Knob (BM), and Sugarloaf (SG) (LeBas et al., 1986) (Fig. 8). Individual vol and the REE pattern is relatively flat (Fig. 10). centers ( Table 2 and Fig. 3). Additional local canic centers tend to have a broad compositional Incompatible element patterns normalized to the eruptive centers are described by Wagner et al. range even though they are dominated volumet primitive mantle values of Sun and McDonough (2011). In contrast to the large volcanic centers rically by a single rock type. For example, the (1989) (Fig. 10) are moderately steep with rela adjacent to Napa Valley, the mountains to the Stags Leap center is comprised predominantly tively flat patterns in the middle to heavy rare north of the town of Sonoma contain a com of andesitic composition flows and lahars, but earth elements. Andesites display clear negative plex assemblage of volcanic units that appear to contains rocks that nearly span the composi Eu anomalies, whereas this is less apparent in have erupted from numerous small volcanic cen tional range of the entire field (Fig. 8). Most of more mafic samples (Fig. 10). ters (Wagner et al., 2011). The volcanic centers the SVF rocks are subalkaline; however, a small For selected tuffs from the Sonoma volcanic become progressively younger northward from but important subset of samples from the Sugar field, we have major-, minor-, and trace-element the 7.5 Ma Arrowhead volcanic center (Table 2) loaf Ridge, Sonoma, and Cup and Saucer vol geochemistry for intracaldera and outflow tuffs at the southern end of the range to the 5.65–4.81 canic centers are compositionally distinct and and their distal fall equivalents. Intracaldera Ma volcanic rocks (Table 2) within the Sugarloaf trend toward alkaline compositions (Fig. 8A). and outflow tuff geochemistry are whole-rock Ridge volcanic center at the northern end of the SVF basalts, basaltic-andesites, and andesites results from this study; distal fall material is range. The largest-volume and most stratigraphi have medium-K calc-alkaline compositions, from glass analysis of tephra (Sarna-Wojcicki, cally complex center is the Sonoma center (SON, although its more silicic rocks have high-K calc- 1976; Sarna-Wojcicki et al., 1979). We recog Table 2 and Fig. 3), which consists of a deformed alkaline compositions (Fig. 8B). nize that the whole-rock analyses can be subject sequence of basaltic andesite and andesitic flows For silica contents between 45% and 70% to unintended contamination from lithic frag and breccias with subordinate tuffs that appear to (basalt to dacite), Ba is strongly positively cor ments, or subject to a variety of postdepositional

have erupted from a number of volcanic vents. related with SiO2 (Fig. 9), as are Rb and Th alteration processes that can induce scatter in The Sugarloaf Ridge and Arrowhead centers (not shown on Fig. 9). These correlations are the geochemical plots. Even given these uncer (SG, AH, Table 2 and Fig. 3) adjoin it to the north much weaker among SVF rhyolites, which tainties, the glass and whole-rock analyses and south, respectively, and form more localized have widely variable concentrations of these show a remarkable degree of consistency where volcanic accumulations. The Sugarloaf Ridge elements (Table 1). Zr and Ba abundances are samples from a single tuff, defined on the basis volcanic center features near-vent, nonwelded to positively correlated with silica content between of stratigraphic position and lithology, in addi

welded lithic rhyolitic tuffs that contain clasts up 50% and 70% SiO2, but are inversely correlated tion to geochemistry, are compared (Fig. 11).

to 1 m in diameter and an arcuate alignment of above 70% SiO2 (Fig. 9). Samples of the Cup We plot Zr against the high-field-strength ele

basaltic andesite vents. The Bismarck Knob cen and Saucer, Sugarloaf Ridge, and Sonoma vol ment Ti (as TiO2) and use the element ratios ter (BM, Table 2 and Fig. 3) is defined to include canic centers that trend toward alkaline compo Zr/Ba and Rb/Sr to geochemically differentiate local eruptive centers and late-stage rhyolite sitions have Zr abundance greater than 350 ppm, intracaldera, outflow, and distal fall deposits for flows that unconformably overlie rocks of the which are only typical among alkaline rocks. Sr selected ash-flow tuffs from the Sonoma vol

Sonoma center. increases slightly with silica up to ~60% SiO2 canic field (Fig. 11). Each eruptive unit tends to The elongated exposure of volcanic rocks to but shows a general decrease with increas define its own geochemical trend; intracaldera,

the west of Sonoma Valley, including Sonoma ing silica content above 60% SiO2 (Fig. 9). outflow, and fall deposits do not plot at the same

Mountain, Bennett Mountain, and Taylor Moun Most SVF rocks have TiO2 concentrations of place but tend to define somewhat linear trends tain (Fig. 2) is the part of the volcanic field first <1.5% (Table 1 and Fig. 9). Basalts and basaltic on the geochemical plots (Fig. 11). Specific

identified as “Sonoma Volcanics” (Weaver, 1949). andesites have highly variable TiO2 concentra tuffs are discussed below as part of the volcanic

Our sampling in this area is extremely limited, so tions (from 0.75% to 3%), whereas the TiO2 center they are inferred to be associated with. for the purposes of plotting geochemical data we concentrations of more silicic rocks decrease group the heterogeneous and structurally complex monotonically with silica content (Fig. 9). MgO VOLCANIC CENTERS ON THE volcanic rocks in the Sonoma Mountains and concentrations decrease monotonically with EAST SIDE OF NAPA VALLEY

Taylor Mountain as the Sonoma Mountain-Taylor silica content, whereas those of K2O increase Mountain group (ST, Table 2), but we do not with increasing silica (Table 1, Figs. 9 and 10). Mount St. Helena Volcanic Center define specific eruptive centers. Overlying this Incompatible element abundances vary by as sequence is the relatively minor Annadel center much as a factor of 10 among samples from the The Mount St. Helena volcanic center (Table 2) to the east of the City of Santa Rosa (AD, Table 2 various volcanic centers (Table 1 and Fig. 10). consists of volcanic rocks associated with the for and Fig. 3) (McLaughlin et al., 2008), consist SVF basalts, basaltic andesites, and andesites mation of the Mount St. Helena caldera (Fig. 3). ing predominantly of localized eruptions of late- from the Sonoma volcanic field are enriched in We interpret that the eruption of the tuff of Franz stage rhyolite lava flows. large-ion lithophile elements (LILE) such as K, Valley at 2.85 Ma (McLaughlin et al., 2004) Rb, Sr, Ba, and Cs and are distinctly depleted resulted in formation of the caldera. This outflow Descriptive Geochemistry of in high-field-strength elements (HFSE) such facies tuff consists of a single cooling unit com Volcanic Centers as Nb, Ta, and to a lesser extent Ti (Fig. 10). prised of both lithic- and pumice- (up to 30 cm) The magnitude of the Nb-Ta depletion is small rich, nonwelded to partially welded ash-flow tuff We present 343 new major- and trace-element est in the most mafic SVF rocks and increases (Table 2). Rocks from the Mount St. Helena vol analyses of whole-rock samples from lavas and in more felsic SVF rocks (Fig. 10). Rare earth canic center that are associated with the Mount tuff from the Sonoma volcanic field (Table 1 and elements (REE) (La through Lu) are depleted St. Helena caldera range from high silica rhyolite,

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10 primarily represented in the outflow facies, to low Trachyte silica rhyolite in the intracaldera tuff. Trachy- A andesite Trachy- The caldera collapse is defined by a dacite >900-m-thick sequence intracaldera facies ash- 8 flow tuff and collapse breccias that comprise the upper part of Mount St. Helena (Sweetkind et al., Basaltic trachy- 2005). The base of the intracaldera sequence is andesite Alkaline nonwelded and consists of beds of collapse brec cia composed of rocks derived from the caldera Subalkaline 6 Rhyolite wall in a matrix of ash-flow tuff. The upper part of Trachy- basalt the intracaldera section consists of a single cool

O, wt% ing unit composed of partly to densely welded 2 lithic-rich ash-flow tuff. The intracaldera tuff has a phenocryst assemblage of quartz and feld 4

O + K spar similar to that of the Franz Valley tuff and 2 Dacite Mount St Helena an 40Ar/39Ar age of 2.83 ± 0.08 Ma (Weiss and Na Stags Leap Livermore, 1996) that is identical with the tuff of Basalt Basaltic Andesite Annadel Sonoma Franz Valley, from which we infer that the Franz 2 andesite Valley tuff represents the outflow facies of the Arrowhead Sonoma-Taylor intracaldera tuff deposited within the Mount St. Cup and Saucer Sugarloaf Helena caldera. Using the areal extent and the thickness of volcanic rocks, we estimate that Calistoga Domes Wildlake 0 the volume of material erupted from the Mount St. Helena center may be at least 40 km3. 50 60 70 80 The geomorphic expression of the caldera has SiO2, wt% been highly modified as a result of caldera resur gence, uplift of the eastern side of Napa Valley, and erosion of the older rocks that formed the 6 walls of the caldera (Sweetkind et al., 2005). B Hydrothermal alteration of both wall rocks and intracaldera tuffs on the west and southwest side s of the caldera has contributed to erosion and destruction of the topographic expression of the caldera (Sweetkind et al., 2005). As a result intracaldera rocks form a prominent topographic high. The floor of the caldera is exposed on the 4 northeast side of the caldera where intracaldera Shoshonitic Serie tuff rests on serpentinite (Fig. 3). Older rhyolitic air fall and ash-flow tuffs High-K Calc-alkaline Series were erupted from the vicinity of the Mount St. Helena caldera but the source region for these eruptions was destroyed during the eruption O, wt% 2 of the younger tuff of Franz Valley and forma K 2 tion of the Mount St. Helena caldera. These older tuffs include the ash-flow tuff of Petrified Calc-alkaline Series Forest (3.34–3.35 Ma, K-Ar, Sarna-Wojcicki, 1976; Sarna-Wojcicki et al., 1979), and the tuff of the Pepperwood Ranch (3.19 Ma, 40Ar/39Ar, McLaughlin et al., 2004). Also underlying the Arc Tholeiite Series Mount St. Helena volcanic center is the Cali Dacite Rhyolite stoga Dome center, which consists of broadly Basalt Andesite 0 distributed, mostly silicic tuff apron and rhyolite 50 Basaltic 60 70 80 andesite dome complexes that form a coalescing dome SiO2, wt% field (Table 2). Assuming that the deep, linear trough in the modeled depth-to-basement sur Figure 8. (A) Total alkalies-silica diagram (Le Bas et al., 1986) for the various volcanic cen- face (Fig. 5) beneath Diamond Mountain (to the ters within the Sonoma volcanic field. Alkaline-subalkaline series boundary from Macdonald south of Calistoga, Fig. 2) is filled with silicic and Katsura (1964). (B) Potassium oxide-silica diagram for the various volcanic centers volcanic rocks of the Calistoga Dome field, we within the Sonoma volcanic field. Boundaries between arc tholeiite, calc-alkaline, high-K estimate a total erupted volume of material from calc-alkaline, and shoshonitic series rocks after Peccerillo and Taylor (1976). the Calistoga Dome center to be ~100 km3.

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EXPLANATION OF SYMBOLS Annadel Calistoga Domes Cup and Saucer Stags Leap Sonoma

Arrowhead Mount St. Helena Sugarloaf Ridge Wildlake Sonoma Mtn-Taylor Mtn

10 3

MgO TiO2 (wt%) (wt%)

1500 80 Ba, ppm La, ppm

60 1000

500 20

800 500 Sr, ppm Zr, ppm 400 600

300

400

200

200 100

0 0 50 60 70 80 50 60 70 80

SiO2, wt%SiO2, wt%

Figure 9. Selected major-element and trace-element Harker plots for the various volcanic centers within the Sonoma volcanic field.

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Incompatible Elements Rare Earth Elements 1000 Basalts Basalts

le 4 centers 4 centers nt (MSH, SG, SON, ST) 100 (MSH, SG, SON, ST)

Ma NEB 100 ve ti mi Basalts Chondrites ri

ck/ 10 ck /P 10 Ro Ro

1 1 1000 Basaltic Andesites Basaltic Andesites

le MgAND 6 centers 6 centers nt (CS, MSH, SG, SL, SON, ST) 100 (CS, MSH, SG, SL, SON, ST) Ma

100 ve Andesites ti mi Chondrites ri

CA ck/ 10 ck /P

10 Ro Basaltic Ro

Thol

1 1 1000 Andesites Andesites

le 6 centers 6 centers

nt (CD, CS, MSH, SL, ST, WL) 100 (CD, CS, MSH, SL, ST, WL) Ma

100 ve ti mi Chondrites Andesites ri

ck/ 10 ck /P

10 Ro AD Ro

1 1 Cs Rb Ba Th UNbTaKLa Ce Pb Pr Sr PNdZrSmEuTiDyYYb Lu La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Figure 10. Primitive mantle-normalized incompatible-element and chondrite-normalized rare earth element diagrams for selected basalt

(<52% SiO2), basaltic-andesite (52%–57% SiO2), and andesite (57%–63% SiO2) samples from the Sonoma volcanic field. Sequence of incompatible elements represents relative degree of element incompatibility, decreasing from Cs to Lu; normalizing values are from Sun and McDonough (1989). Not all elements plotted for all samples; in certain cases element analyses were not reported or were reported as less than a specified value and were not plotted. In these cases, the plotted line connects adjoining elements. For comparison, incom patible element trends from five types of lavas from southern Baja, California (Benoit et al., 2002) are shown: NEB—niobium-rich basalt; MgAND—magnesian basaltic andesite; CA—calc-alkaline basaltic andesite; Thol—tholeiitic basaltic andesite; AD—adakite.

Post–Mount St. Helena caldera eruption of Springs quadrangle (McLaughlin et al., 2004). form a constructional composite volcano. Proxi high silica rhyolite occurred outside the south To the east and northeast of Mount St. Helena, mal to the inferred center of the volcano, lahar margin of the Mount St. Helena caldera. Erup widespread late-stage basalts cap the strati breccias comprise 40%–50% of the volcanic tion of postcollapse andesite was widespread graphic sections. section (Figs. 12A and 12B); the volcanic sec outside and to the southwest of the Mount tion thins and becomes dominated by andesitic St. Helena caldera. Numerous flows of ande Stags Leap Volcanic Center flows to the north and south of the inferred site cap the outflow facies of the Franz Val vent area (Fig. 12C). The volcanic sequence ley tuff in the Mark West Springs quadrangle The Stags Leap Volcanic Center (SL, Table is cut by north-striking rhyolite dikes and is (McLaughlin et al., 2004). The vent areas for 2 and Fig. 3) is well exposed to the east of capped by nonwelded rhyolitic tuff overlain these flows are not well exposed except for one Napa Valley, where a more than 350-m-thick by two rhyolite lava flows (Fig. 12A) (Fox on the east central part of the Mark West sequence of basaltic andesite flows and lahars et al., 1973).

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1.40 A EXPLANATION Franz Valley Tuff (2.85 Ma, Ar/Ar)

1.20 Outflow tuff Intracaldera tuff (Mount St. Helena)

1.00 tuff of Monticello Road (4.5 Ma, Ar/Ar)

Outflow tuff 0.80 , wt%

2 tuff of Napa (<4.70–4.71, Ar/Ar) 0.60 Fall deposit TiO Outflow tuff 0.40 Intracaldera tuff (capping rhyolite phase, Cup and Saucer)

0.20 Lawlor Tuff (4.84, Ar/Ar) Fall deposit 0.00 050 100 150 200 250 300 350400 450 500 Outflow tuff Zr (ppm) Outflow tuff? (tuff of Syar Quarry) Intracaldera tuff (main phase, Cup and Saucer) 7

B Pinole Tuff (5.2 Ma, K/Ar) 6 Fall deposit Outflow tuff 5 Intracaldera tuff (andesitic phase, Cup and Saucer) r 4 Roblar Tuff (6.26 Ma, Ar/Ar) /S Fall deposit Rb 3 All tuff samples collected for this study

2 Circled samples previously mapped as Pinole Tuff. Whole-rock geochemistry, this study; glass geochemistry, 1 Sarna-Wojcicki, 1976. Reported Ti concentration (in ppm) from glass

geochemistry converted to TiO2 (wt%) by dividing by a conversion factor 0.5995 (Le Maitre, 2002). 0 00.2 0.40.6 0.811.2 Zr/Ba

Figure 11. Descriptive geochemistry of intracaldera tuff, outflow tuff, and distal fall deposits for some major tuff units from the Sonoma

volcanic field. (A) Variation in TiO2 content with Zr concentration. (B) Ratio of incompatible elements showing variation in Rb/Sr with Zr/Ba. Samples of fall deposits whose symbols are circled were previously mapped as Pinole Tuff and come from stratigraphically complex sections south of San Pablo Bay that appear to include multiple tuffs, including the three main units shown here. Small gray rectangles represent analyses from other tuffs from the volcanic field that are not specifically correlated to one of the named units.

The age of the volcanic center ranges from to the extrusive rhyolite flows that cap the vol lake center to be ~100 km3. For comparison, 4.35 Ma to 4.3 Ma (40Ar/39Ar, Table 3), based on canic sequence (Fig. 12). these volumes are similar to those of smaller samples of the stratigraphically lowest and high To the north of the Stags Leap center on the stratovolcanoes in the modern Cascades arc, est exposed andesite flows, respectively (Fig. east side of Napa Valley, the Wildlake center but smaller than the largest of the Cascade vol 12). The uncertainty associated with the dates (Fig. 3) is also dominated by andesites but is canoes such as Medicine Lake (600 km3) or the means that these two dates are essentially identi much less flow-dominated than the Stags Leap Newberry volcano (450 km3). cal; it is clear that the rocks from this volcanic center. The Wildlake center is mostly composed center were erupted over a very short period of of small-volume andesitic pyroclastic eruptions Cup and Saucer Volcanic Center time. Granitic rocks associated with the volcanic with a smaller component of andesitic flows center (Fig. 12A) are some of the few outcrops (Table 2). Using the areal extent of the volcanic The Cup and Saucer volcanic center (CS, of intrusive rocks associated with the Sonoma center, the exposed thickness of the volcanic sec Table 2 and Fig. 3) includes volcanic rocks volcanic field. We suggest that these granitic tion, and the estimated total thickness from the exposed near the city of Napa and in the sur rocks occupy the vent area for the Stags Leap depth-to-basement map (Fig. 5), we have esti rounding hills; volcanic rocks associated with volcanic center and may be time-equivalent to mated the erupted volume of material of the this center extend from near Carneros Valley rhyolite dikes that intrude the andesitic pile and Stags Leap center to be ~150 km3 and the Wild on the west to Green Valley on the east (Figs. 2

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A lava flow forms upland surface 4.3 Ma

y Valle oss ws F lava flo capping rhyolite flow Soda Canyon

Fig. 12c p Stags Lea

Fig. 12b y breccia Valle 4.35 Ma Napa Ti

KJgv KJgv ws lava flo N

Base image created from high-resolution orthophoto imagery from Napa County, CA draped on View is looking approximately towards due east, from about 30 degrees airborne LIDAR data from the National Center of Airborne Laser Mapping at the University of California, above the horizon. Berkeley. LIDAR data digitally resampled to a 10-m grid. Geology modified from Bezore et al., 2005. Illumination is from the southwest at about 50 degrees above the horizon.

B C

Figure 12. Perspective image (A) and outcrop photographs of breccia-dominated (B) and flow-dominated (C) stratigraphic sections from the Stags Leap volcanic center. (A) In the perspective view, nearly all upland areas shown in this image are underlain by volcanic rocks associated with the center, except for Mesozoic Great Valley Sequence rocks (KJgv) that underlie the volcanic sequence, and granitic rocks of the Stags Leap intrusive (Ti). Quaternary alluvium in Napa Valley and Foss Valley is not labeled. Red lines indicate faults, ball and bar on downthrown side where offset is known. Solid lines are geologic contacts and bound breccia bodies within the volcanic sequence and also bound the base of the capping rhyolite flow. Dashed lines indicate the general outcrop trace of individual lava flows. The locations and ages of two samples collected for40 Ar/39Ar age determinations are shown from near the base (4.35 Ma) and the top (4.3 Ma) of the volcanic section associated with this center. (B) Stags Leap escarpment, showing stratigraphic section composed of 40%–50% pyroclastic breccia. Total relief shown ~300 m. (C) Eastern side of Soda Canyon, showing stratigraphic section composed almost entirely of andesite and dacite lava flows. Total relief ~250 m.

and 3). Volcanic rocks from this center uncon lain by a 150-m-thick section of andesite, dacite, Calderas and Intracaldera Rocks formably overlie Eocene sedimentary rocks and and rhyolite ash-flow tuffs that are interbedded On the basis of geologic, geochemical, and Great Valley sequence to the south of the city of with generally coeval small-volume extrusive geophysical data, we interpret at least two Napa (Graymer et al., 2002a). Rocks from this domes and lithic nonwelded tuffs with a simi nested calderas within the Cup and Saucer vol center are unconformably overlain by volcanic lar compositional range (Table 2). The erupted canic center (Figs. 3 and 13) that appear to be rocks from the Stags Leap center on the north volume from the Cup and Saucer center is diffi the principal eruptive vents for three regionally and the Mount George center in the hills to the cult to estimate given the great uncertainty in the distributed ash-flow tuff and tephra deposits east of Napa (Fig. 13). Rocks from this center thickness of the intracaldera pile and the rela (see stratigraphic columns in McLaughlin and include a lower section of andesite and minor tive lack of map data concerning distribution of Sarna-Wojcicki [2003] and McLaughlin et al. interbedded tuff that is at least 300 m thick over outflow tuff. [2005]). We define these calderas based on the

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TABLE 3. 40Ar/39Ar AGE DETERMINATIONS FROM THE STAGS LEAP VOLCANIC CENTER Sample 058-4C PlagioclaseIRR231-20 J = 0.003457569 Step Age 39 40 40 39 37 39 36 39 (°C) % ArK Rel% Ar* Ar/ Ar Ar/ Ar Ar/ Ar K/Ca Cl/K (Ma) 550 0.7894.2826.40755 16.854630.09016 0.0310.02429 7.118 ± 1.146 700 9.33744.44 1.31342 23.384040.00887 0.0220.00294 3.694 ± 0.243 775 11.854 73.540.87247 29.812680.00895 0.0170.00165 4.078 ± 0.240 850 15.28983.01 0.81714 29.234930.00848 0.0180.00175 4.309 ± 0.087 950 20.95069.06 1.00698 27.931550.00871 0.0180.00223 4.414 ± 0.080 1050 10.25634.78 1.87531 27.230500.01160 0.0190.13151 4.139 ± 0.124 1150 7.43529.21 2.20798 25.620870.01231 0.020 0.00115 4.088 ± 0.155 1250 5.07249.06 1.43717 23.303480.00886 0.0220.00231 4.462 ± 0.164 1400 19.01837.53 2.12551 23.497970.01093 0.0220.00135 5.047 ± 0.099 Weighted Mean Plateau age (Ma)MSWD = 1.5 4.304 ± 0.047 Intercept = 288.3±8.0 Isochron age (Ma)MSWD = 1.34.37 ± 0.06 Integrated age (Ma) 4.383 ± 0.054 Low Cl/K age (Ma)4.28 ± 0.08 Sample 058-4A Andesite groundmass IRR231-18 J = 0.003476687 Step Age (°C) % 39ArK Rel % 40Ar* 40Ar/39Ar 37Ar/39Ar 36Ar/39Ar K/Ca Cl/K (Ma) 650 25.38452.70 1.27762 0.42273 0.00213 1.241 0.002634.219 ± 0.041 1300 74.61651.13 1.37355 1.12216 0.00255 0.467 0.00734 4.403 ± 0.032 Weighted Mean Plateau age (Ma)none Isochron age (Ma)none Integrated age (Ma) 4.356 ± 0.052 39 39 40 Note: MSWD, mean square of weighted deviates. % ArK Rel is the amount of ArK (moles) released for each single analysis (heating step). % Ar* is the radiogenic 40Ar (40Ar*) yield for each single analysis (heating step). All ages were determined in the U.S. Geological Survey radiometric dating laboratory, Menlo Park, Calif. by R.J. Fleck. Samples were irradiated in the U.S. Geological Survey TRIGA Reactor Facility in Denver, Colo. The neutron ﬂux standard (monitor mineral) used in all irradiations was Taylor Creek Rhyolite sanidine, 85G003, with an age of 27.92 Ma as reported by Dufﬁeld and Dalrymple (1990). Age in bold is best estimate of emplacement age. Sample 058-4A gave a disturbed spectrum for which neither plateau or isochron ages are possible.

presence of a semicircular topographic low Andesite to Dacite Breccia vitrophyre strongly suggests that the tuff and with an interior topographic high, the presence The andesitic to dacitic intracaldera breccia breccia were emplaced as a hot mass during

in this vicinity of the coarsest pumice breccia (63% SiO2) is a monolithologic breccia consist the caldera-forming eruption, rather than as a and greatest abundance of lithic clasts in out ing of dark andesitic fragments up to 0.3 m in cold “mega-landslide” block as envisioned by flow tuffs (Sarna-Wojcicki, 1976), the presence a matrix of yellow nonwelded to perlitic tuff. some previous workers (Howell and Swinchatt, within the interpreted calderas of two composi Matrix and clasts have nearly identical major- 2003). We interpret breccia bodies containing tionally distinct types of megabreccia supported and trace-element chemistry, with distinctly flow-banded dacite clasts to be collapse-related by a tuff matrix, and geophysical evidence for low Zr concentrations and Rb/Sr ratios and deposits within the caldera; as such dacite is

multiple arcuate structural margins (Table 2; high concentrations of TiO2 relative to the other common outside of the inferred structural mar see also Langenheim et al., 2010). A portion phases of intracaldera breccia at the Cup and gin of the caldera. of one of the calderas was portrayed by Bezore Saucer (Fig. 11). This breccia was mapped as et al. (2004), but the relative lack of outflow a distinct unit called “the Andesite of Tulocay Capping Rhyolite Breccia facies ash-flow tuff in the vicinity of the “Cup Creek” by Fox et al. (1985b). The third breccia phase is a texturally distinct and Saucer” (the central topographic high, Fig. rhyolite breccia that overlies the main breccia 13A) and lack of definitive ties between region Main Rhyolitic Breccia phase (Fig. 13A). This is also a rhyolite brec

ally distributed fall material and ash-flow tuffs The main rhyolitic type of intracaldera brec cia (71%–73% SiO2) that is distinguished by at the volcanic center have hindered previous cia (Fig. 13A) forms the bulk of the highland the high Zr concentrations, high Zr/Ba, and

interpretations of this volcanic center. within the town of Napa and was mapped as low concentration of TiO2 relative to the other Intracaldera rocks lie within the arcuate the “tuff breccia of Napa” (Fox et al., 1985b). phases of intracaldera breccia at the Cup and topographic depression formed by the hills that In places this type of intracaldera breccia is a Saucer (Fig. 11). ring the town of Napa, and are well-exposed in monolithologic breccia with angular, aphyric the “Cup and Saucer,” which we interpret as a rhyolite clasts up to 0.4 m within a rhyolitic tuff Correlative Outflow Tuffs resurgent dome (Fig. 13A). Intracaldera rocks matrix. Elsewhere, the breccia clasts are flow- The three phases of intracaldera breccia from are a complex assemblage of rhyolite breccia, banded dacite commonly 20–40 cm but up to the Cup and Saucer volcanic center are simi generally consisting of clasts of welded rhyolite 3 m in diameter within an altered tuff matrix. lar in whole-rock major- and minor-element in a nonwelded tuffaceous matrix. The intra This main breccia phase is rhyolitic (71%–76% geochemistry to three known outflow tuffs, the

caldera breccia is crudely stratified and consists SiO2) with low concentration of TiO2, low Zr/Ba, Pinole Tuff (5.2–5.4 Ma, K/Ar, Sarna-Wojcicki, of three phases that are spatially, lithologically, and high Rb/Sr relative to the other phases of 1976), the Lawlor Tuff (4.84 ± 0.02 Ma, and geochemically distinct: an andesite to dacite intracaldera breccia at the Cup and Saucer (Fig. 40Ar/39Ar, McLaughlin et al., 2005), and the tuff breccia that is exposed to the south of the main 11). In one location, vitrophyre grades down of Napa (<4.70–4.71 Ma, 40Ar/39Ar, McLaughlin topographic prominence, a rhyolitic phase that ward into partly vitric, densely welded tuff and et al., 2005) (Fig. 11; see also stratigraphic comprises most of the breccia outcrops, and a then downward into intracaldera breccia with columns in McLaughlin and Sarna-Wojcicki separate capping rhyolite breccia (Fig. 13A). a densely welded tuff matrix. The presence of [2003] and McLaughlin et al. [2005]).

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Figure 13. Perspective image of Cup and Saucer volcanic center (A) and detail of rhyolite lava flows near Mount George (B). A L Mount George N Mount George Small inset map shows approxi- P volcanic Stags Leap center mate extent of both figures; gray volcanic shaded areas denote exposed center Milliken Canyon P volcanic rocks. (A) In the per- N P spective view, parts of the Cup N Cup and Saucer and Saucer, Mount George, and volcanic center Stags Leap volcanic centers are shown; boundaries between Napa Main Valley centers shown by solid orange RC lines. Dashed line portrays Cup and Saucer Main inferred caldera boundary. Solid white lines denote geo- And logic contacts between phases Sy of intracaldera breccia; And— andesitic phase; Main—main rhyolitic phase; RC—rhyolitic West Napa fault capping phase. Locations of outflow tuff inferred to correlate to caldera-forming eruptions are shown by: P—Pinole Tuff; L— 122°15′W122°14′W Lawlor Tuff; N—tuff of Napa;

Sy—tuff of Syar Quarry. N B (B) Image showing rhyolite lava CA 121 flows of the Mount George vol- 38°21 ′ canic center overlying older N tuffs of the Cup and Saucer vol- Monticello Road canic center. Rhyolite lava flow lobes are outlined by solid lines, dashed where inferred. Flows N are outlined on the basis of sur- rhyolite face morphology and cliff-form- lava flow ing habit, but no specific age N

Area of Fig. 13ACA correlations are suggested in apa Va

12 the mapping as shown. Dashed CA lines within flows are pressure rhyolite lava flow 29 1

ridges preserved on the sur- lley face of the flows. P—outcrop of x Mount Pinole Tuff; N—historic pumice George Napa quarries that expose the tuff of Area of Napa. Image combines digital Fig. 13B orthophoto with LIDAR data (LIDAR courtesy of National P Center of Airborne Laser Map- 00.5 1km ping, University of California, Berkeley).

Pinole Tuff can be mapped as a reasonably continuous Pablo Bay (Fig. 1), the Pinole Tuff consists of a In the vicinity of southern Napa Valley, band along the lower eastern slopes of Mount series of lapilli and vitric-crystal-lithic tuff beds the Pinole Tuff is a lithic-rich tuff with dis George, to the east of the city of Napa (Fig. with an aggregate thickness of ~50 m, includ tinctive scoriaceous black rounded pumices 13A). Whole-rock major- and minor-element ing a basal white, massive tuff, succeeded by 2–4 cm in size set in a dark brown andesitic geochemistry of this tuff bears a distinct simi lithic pumiceous ashy tuff in layers up to a few partly welded tuff matrix (Fig. 14C). Angular larity to the andesitic phase of intracaldera meters thick (Lawson, 1914; Clark, 1912; Vitt, lithic blocks up to 10 cm constitute 5%–10% breccia (Fig. 11). 1936; Chesterman, 1956). Parts of this sec of the rock mass; the scoriaceous pumice As originally defined in exposures near tion were identified by these early workers as typically makes up ~20% of the rock. The tuff Pinole and Rodeo along the south side of San having an andesitic composition on the basis

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pumiceous fall deposit Figure 14. Photographs of tuff AABB inferred to have erupted from the Cup and Saucer volcanic lithic center. (A) Sequence of pumi- tuff ceous fall deposits, lithic- and reworked pumice-rich tuff, and reworked interval tuff historically mapped as Pinole Tuff, near Wilson Point on the south side of San Pablo Bay (location shown on Fig. 1). Height of outcrop shown ~3 m. Pinole Tuff (B) Interval within the Wilson Point section lithologically and geochemically identifiable as C D Pinole Tuff; scoriaceous pumice forms dark spots, such as imme- diately below hammer shaft. Length of hammer 40 cm. (C) Partly welded Pinole Tuff below Mount George. Lower scale bar is 10 cm long. (D) Typi cal exposure of pumice breccia from the tuff of Napa, below Mount George. Upper scale bar is 10 cm long.

of lack of quartz and presence of augite and Lawlor Tuff that this tuff, which directly overlies the lower, hypersthene. The volcanic section originally Regionally distributed samples of fall depos dominantly andesitic part of the volcanic section defined as Pinole Tuff along the south side of its and reworked tuff identified as the Lawlor within the Cup and Saucer volcanic center, may San Pablo Bay is really a stack of distal fall Tuff (Sarna-Wojcicki, 1976) are similar in be a proximal outflow facies of the Lawlor Tuff. and tephra related to many units in the Sonoma major- and minor-element geochemistry to volcanic field. The exposed sections are geo the main rhyolitic phase of intracaldera brec Tuff of Napa chemically heterogeneous and stratigraphically cia, including the exposures of vitrophyre, in The tuff of Napa is a clast-supported, dacitic to complex, clearly belonging to several distinct the Cup and Saucer area (Fig. 11). Geochemi rhyolitic pumice lapilli tuff breccia and pumice eruptive and/or fall units (Fig. 14A). Outcrops cal data from glass analysis of tephra identified block breccia that has been extensively mined are stratigraphically layered consisting of falls as Lawlor Tuff (Sarna-Wojcicki, 1976) fall in for pumice to the east of the Cup and Saucer and tephra of varying pumice and lithic con a very tight cluster (Fig. 11). The Lawlor Tuff area (Fig. 13) (Chesterman, 1956). Typical out tent (Fig. 14A). Samples collected from mul is interpreted to have been erupted from the crops consist of massive beds (0.6–6 m thick) of tiple units in this section have a relatively wide southern part of the Sonoma volcanic field on unsorted, angular pumice fragments with minor geochemical spread, only some of which cor the basis of coarsening and thickening of plinian lithic clasts of andesite, rhyolite, and dacite respond to the outflow Pinole Tuff (Fig. 11). tephra deposits toward the volcanic field (Sarna- (Fig. 14D). Pumice clasts are coarse, averag Whole-rock analyses of samples from single Wojcicki, 1976). Coarse plinian tephra of the ing 1.3–5 cm in diameter, with blocks as much beds of pumiceous andesitic tuff with distinc Lawlor Tuff crops out along Monticello Road, as 40 cm long; the size of pumice blocks and tive black scoriaceous pumice in exposures near just to the east of the Cup and Saucer complex thickness of deposit are evidence that this unit is Pinole and Rodeo (Figs. 1 and 14B) correspond (Fig. 13A) (Sarna-Wojcicki, 1976). Few expo proximal to the eruptive vent (Sarna-Wojcicki, to the whole-rock chemistry of outflow Pinole sures of welded outflow are known for the Law 1976). The pumice deposits are broadly distrib Tuff near Mount George (Fig. 14C) and to the lor Tuff. One possible correlative outflow tuff uted in the hills to the east of the town of Napa whole-rock chemistry of the andesitic phase of is a thick rhyolitic ash-flow tuff exposed in the (Fig. 13B); correlative pumiceous tuff is com the intracaldera megabreccia in the Cup and Syar Industries quarry to the south of the town mon in the vicinity of Glen Ellen and Bennett Saucer (Fig. 11). We infer that the Pinole Tuff of Napa (Fig. 13A); this tuff is at least 60 m Valley, south and west of the town of Kenwood was erupted from a caldera within the Cup and thick with a well-developed basal vitrophyre. (Delattre et al., 2007). Above Milliken Canyon Saucer volcanic center, that the tuffs at Mount Although we analyzed only a single sample, it (Fig. 13A) there is a partly welded tuff that has George represent the proximal outflow facies, is a good geochemical match for the rhyolitic similar chemistry and may represent outflow and the scoria-bearing unit in the Rodeo and phase of the intracaldera breccia and to the tuff from the same pumice-producing erup Pinole sections represents a more distal, pre fall deposit and reworked tuff identified as the tion. The capping rhyolitic part of intracaldera dominantly fall, part of the unit. Lawlor Tuff (Fig. 11). We tentatively suggest megabreccia in the Cup and Saucer is similar in

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major- and minor-element geochemistry to the from the Cup and Saucer volcanic center. The the Mendocino Triple Junction migrated north tuff of Napa (Fig. 11). Pumice-rich outflow tuff two centers are similar in age; volcanic rocks ward, the boundaries of the slab window were has a broad range in major- and minor-element within the Sugarloaf center are 5.65–4.81 Ma, defined at the north end by the southern edge geochemistry, perhaps in part due to loss of rocks within the Cup and Saucer center are 5.4 of the Gorda plate and to the west by the San alkalis from the porous material. We infer that to <4.70 Ma (Table 2). The centers share geo Andreas fault and the edge of the Pacific plate the tuff of Napa was erupted from a caldera chemical traits in having some rocks that are (Furlong and Schwartz, 2004) (Fig. 16B). The within the Cup and Saucer volcanic center, the alkaline to peralkaline (Fig. 10). The style of east-west width of the window was controlled principal eruptive products being pumice brec volcanism is somewhat similar in both fields; by the dip of the descending slab and the resul cias and flows deposited in the Mount George although we cannot document any caldera tant shape of the overlying lithosphere of the area and farther to the west. A number of pumi structures in the vicinity of the Sugarloaf Ridge North American plate (Thorkelson, 1996). In ceous tephra beds in exposures near Pinole and volcanic center, we do recognize small-volume general, the migratory volcanic centers of north- Rodeo along the south side of San Pablo Bay fragmental volcanic material associated with central California with clear northward-young (Fig. 1) are geochemically similar to the tuff of mafic vents. Langenheim et al. (2010) use offset ing trends follow the northward progression of Napa (Fig. 11) and may represent a more distal, gravity and magnetic anomalies to infer a com the Mendocino Triple Junction and are in some predominantly ash fall, part of the unit. bined 25–30-km right-lateral displacement on way associated with thermal perturbations asso the Carneros and West Napa faults. Restoring ciated with the transition from subduction set DISCUSSION OF VOLCANO-TECTONIC the fault offset would place the Sugarloaf Ridge ting to a strike-slip setting and the development ENVIRONMENT volcanic center to the southwest of the Cup and of a slab window tectonic environment (Fox Saucer volcanic center, putting these two simi et al., 1985a; Furlong, 1984; Liu and Furlong, Age Progression and Dextral Offset lar centers in proximity (Fig. 15). Langenheim 1992; Furlong and Schwartz, 2004; Dickinson, of Volcanic Centers et al. (2010) correlate deep-source gravity and 1997). Neogene volcanic rocks in north-central magnetic anomalies across the West Napa fault California are roughly coeval with the inactive Neogene volcanic rocks of the northern San to suggest as much as 40 km of right-lateral off subduction-related ancestral Cascade magmatic Francisco Bay region are part of a linear belt of set of Mesozoic features, potentially indicating arc in the Bodie-Lake Tahoe area (Cousens volcanic fields that are progressively younger a slip history that predated eruptions from the et al., 2008) (Fig. 16B), however, volcanic rocks to the northwest (Fox et al., 1985b). Mirroring Cup and Saucer and Sugarloaf volcanic centers. of the Sonoma and other volcanic fields erupted the regional trend, the volcanic centers herein Restoration of the Sugarloaf Ridge volcanic considerably to the west of the magmatic defined on the east side of Napa Valley become center is an example of how the Sonoma vol arc and therefore are unlikely to be related progressively younger to the northwest (Fig. 15) canic field may be reconstructed using a vari to arc magmatism. from 5.4 to <4.7 Ma within the Cup and Saucer ety of geologic criteria. We have not attempted Of critical importance are tectonic reconstruc volcanic center at the south end of the valley to to reconstruct fault blocks to the west of the tions that attempt to pinpoint the location of the the 4.35 to 4.3 Ma Stags Leap volcanic center, Rodgers Creek fault (Fig. 15) given our lim volcanic fields with respect to the paleoposition through the 3.2 to 2.8 Ma Wildlake and 2.85 Ma ited sampling and mapping and the necessity to of the Mendocino Triple Junction (Fox et al., Mount St. Helena volcanic centers (citations constrain offset on various fault strands. 1985b; Dickinson, 1997). Many reconstruc for ages given in Table 2). Farther to the north, tions of the northwestwardly younging volcanic 2.2–0.09 Ma volcanic rocks of the Clear Lake Tectonic Setting and Reconstruction rocks in the northern California Coast Ranges volcanic field (Donnelly-Nolan et al., 1981; are portrayed with a static North American ref Schmitt et al., 2003a, 2003b) are the youngest A slab window forms where the subducting erence frame such that all of the volcanic rocks Neogene rocks in the northern California Coast slab is removed from beneath the overriding are plotted as they exist today and the position Ranges (Fig. 15). Volcanic rocks of the SVF plate (Dickinson and Snyder, 1979; Thorkel of the triple junction is shown through time show a variety of eruptive styles that are spa son, 1996) (Fig. 16A). Removal of a subducting (Johnson and O’Neil, 1984; Fox et al., 1985b). tially variable and specific to an individual erup slab typically results in cessation or reduction In these diagrams, there is considerable uncer tive center, but do not follow the northward age of arc volcanism, which is often supplanted by tainty in the exact location of the Mendocino progression of the volcanic centers (Fig. 15). volcanism of different eruptive style and com Triple Junction with respect to the location of The volcanic centers on the east side of position (Johnson and O’Neil, 1984; Cole and a specific volcanic field, and thus, there exists Napa Valley are largely intact, existing within Basu, 1995). Slab window formation can result some uncertainty as to the relation of these vol a structural block bounded on the west by the in an increase in heat flow as hot asthenosphere canic rocks to the slab window forming in the Carneros and West Napa faults and on the east that previously existed beneath the slab (subslab wake of the triple junction. Recent kinematic by the Green Valley fault (Fig. 15). However, lithosphere) or in the lithospheric mantle wedge reconstructions of the western United States volcanic rocks in the western half of the vol above the subducting slab (supraslab) fills the (McQuarrie and Wernicke, 2005; Wilson et al., canic field have been displaced from their origi region previously occupied by the subduct 2005; McCrory et al., 2009) integrate seafloor nal depositional positions by dextral faults of ing slab (Liu and Furlong, 1992; Furlong and magnetic anomaly data with regional extension the San Andreas system (Fox, 1983; Fox et al., Schwartz, 2004) (Fig. 16A). and block rotations across the western United 1985b; McLaughlin et al., 1996; Dickinson, The transition from subduction and associ States in addition to correlations across fault 1997; Wakabayashi, 1999; Graymer et al., ated arc volcanism to a slab window tectonic blocks to arrive at dynamic tectonic reconstruc 2002b). Based on similarity in age, eruptive environment along the western margin of tions through time. We use the reconstruction style, and geochemical traits, we suggest that the American plate began in the Miocene as the of Wilson et al. (2005) because in the northern the Sugarloaf Ridge volcanic center, located triple junction first impinged on the North California Coast Ranges, in addition to the San to the north of Sonoma Valley (Fig. 15), is an American plate (Dickinson and Snyder, 1979; Andreas fault, the Rodgers Creek and the Cala example of such dextral offset, being displaced Johnson and O’Neil, 1984; Fox et al., 1985a). As veras faults (for our purpose, approximating the

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123°0′W 122°45′W 122°30′W 122°15′W Clear 39°0′N EXPLANATION Lake Explosive volcanism CL 2.2–0.09 Ma Volcanic domes

Flows

Volcanic rocks, undivided

Sedimentary rocks and Quaternary deposits

Mesozoic rocks 38°45′N MSH 3.35–2.85 Ma M Fault MSH Volcanic center name 2.85 Ma and age, ages from Table 2 WL 3.2–2.8 Ma CD ~2.8 Ma

RC-H

38°30′N

SL 4.35–4.3 Ma WN BV 30 km slip restored

CS C 5.4– 4.7 Ma

38°15′N

Pacific Ocean GV SAF SG 5.3– 4.83 Ma

San Pablo Bay

Base data from CA state digital data, 2009. Geology modified from Saucedo and others (2002); Graymer and 036912 15 km others (2006b); and Graymer and others (2007). Universal Transverse Mercator Projection Zone 10 North American Datum 1927

Figure 15. Volcanic centers discussed in text, showing age and predominant eruptive style. Age ranges are from Table 2. Volcanic center abbreviations: CD—Calistoga Domes; CL—Clear Lake volcanic field; CS—Cup and Saucer; MSH—Mount St. Helena; SL— Stags Leap; SG—Sugarloaf, WL—Wildlake. Sugarloaf volcanic center and other volcanic rocks in the vicinity of Sonoma are shown after restoration of 10 km right slip on the West Napa (WN) fault and 20 km right slip on the Carneros (C) fault. Slip on the Green Valley (GV), Bennett Valley (BV), Rodgers Creek-Healdsburg (RC-H), Maacama (M), and San Andreas (SAF) faults not restored.

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Figure 16. (A) Schematic east- A tion of the reconstructed edge of the slab. For west cross section across the older time steps, such as 6 Ma, Dickinson’s Pacific-North American plate Pacific plate North American plate Slab window (1997) reconstruction puts the Mendocino boundary south of the Men- inactive arc Triple Junction much farther to the south. As a docino Triple Junction, in the SVF result, Dickinson’s (1997) reconstruction results region of the slab window. Continental crust in the older fields having formed closer to San Andreas fault (SAF) is the the slab edge, with ever-increasing distances western boundary of the region Lithosphere Lithospheric to the slab edge for the younger fields. In con previously occupied by the mantle trast, the Atwater and Stock (1998) reconstruc subducting Gorda plate. Two pr tion results in a relatively constant 90–110 km subductingior locationGorda plat of e supraslab possible mantle sources to fill asthenosphere distance between the volcanic fields and the slab Asthenosphere the vacancy left by the removal edge (Fig. 17). This constant difference is con of the subducting Gorda slab sistent with modern heat-flow measurements and subslab are subslab asthenosphere and SAF asthenosphere thermal modeling (Liu and Furlong, 1992; Fur the supraslab asthenosphere. long and Schwartz, 2004) that suggests a time (B) Generalized plate configu- B (and thus distance) lag between the passage of the ration at ~6 Ma in the vicinity triple junction and the generation of the largest

of the Mendocino Triple Junc- Trench heat-flow anomalies associated with movement tion (MTJ) (after Dickinson North of asthenospheric material into the slab window. and Snyder, 1979; Furlong and active American magmatic plate Schwartz, 2004). Subduction arc DISCUSSION OF VOLCANIC-ROCK of the Gorda Plate beneath GEOCHEMISTRY the North American Plate is Ridge G G restricted to north of the triple southern edge Volcanic rocks from slab window settings MTJ Gorda slab junction. MTJ migrates to have a variety of geochemical signatures; the the northwest with the same P inactive chemistry of the erupted material depends on relative direction and veloc- magmatic the tectonic setting, thermal history, and chemi arc ity as the Pacific plate (arrow SAF SVF cal characteristics of the mantle source, as well Pacific plate labeled P). Gorda Plate motion as the degree of crustal interaction and the path is relatively to the northeast SLAB WINDOW way taken by the rising melt (Thorkelson, 1996). (arrows labeled G); southern Slab windows interpreted to be related to ridge- edge of the subducting Gorda slab also moves to the northeast, creating the slab window. trench collision may result in depleted, mantle- Active arc magmatism occurs only above subducting slab; Sonoma volcanic field (SVF) derived melts that ultimately produce tholeiitic - occurs within the slab window to the west of the inactive segment of the magmatic arc. to alkalic-composition volcanism (Johnson and O’Neil, 1984; Cole and Basu, 1995). Slab window settings where mid-oceanic-ridge Green Valley fault at the latitude of the Sonoma the southern edge of the Gorda Plate (north edge basalt–like rocks with alkaline affinities have volcanic field) are incorporated into the recon of the slab window) from 8 Ma to present, fol been identified include the central and southern structions, allowing for the volcanic rocks of lowing Atwater and Stock (1998). California coast (Johnson and O’Neil, 1984; the Sonoma volcanic field to be placed within Volcanism associated with the slab window Cole and Basu, 1995), Baja California, Mexico a discrete fault block (Fig. 17). In addition, migrated northward in response to northward (Bellon et al., 2006; Benoit et al., 2002), Costa this reconstruction used the volcanic fields as a migration of the triple junction while the south Rica and Panama (Johnston and Thorkelson, constraint on the location of fault blocks with end of Cascades arc volcanism also migrated 1997), and the Antarctic Peninsula (Hole, 1988, respect to the slab edge. northward to its present location at Lassen 1990). Slab window volcanism can also produce On Figure 17 we show the tectonic recon volcanic field (Cousens et al., 2008) (Fig. 17). rocks whose geochemical signatures are related struction of Wilson et al. (2005) at 5.9 Ma, Between 5 and 10 Ma, when the oldest rocks to enriched mantle sources similar to oceanic- which is roughly coincident with the early within the Sonoma volcanic field rocks were island basalts (OIB), such as in Patagonia phase of the Cup and Saucer volcanic center. forming, arc volcanism was ongoing along the (Gorring and Kay, 2001). With elevated heat The reconstruction is fixed with respect to the Ancestral Cascades magmatic arc at approxi flow the trailing edge of the slab at the margin of Pacific plate. Superimposed on the fault-block mately the same latitude. Arc volcanism the slab window may melt, resulting in the erup reconstruction are the volcanic centers that we migrated northward at a slightly faster rate than tion of sodic rocks including adakites (Defant have identified along the east side of Napa Val did volcanism within the Sonoma volcanic field, and Drummond, 1990; Martin et al., 2005). ley. We have restored 30 km of combined offset such that at the time of the Mount St. Helena Although we do not have trace-element on the Carneros and West Napa faults such that eruptions at 2.85 Ma, coeval ancestral Cascades modeling results or isotopic data to constrain the Sugarloaf Ridge volcanic center is adjacent volcanism is ~50 km farther north. This dispar petrogenetic interpretation, here we use trace- to the Cup and Saucer volcanic center at this ity increases for arc rocks younger than 2.6 Ma, element ratios to compare the volcanic rocks of time. All of these volcanic centers fall between which are situated well north of the generally the Sonoma volcanic field to rocks from other the Rodgers Creek fault to the west and the Cala coeval Clear Lake volcanic field. slab window settings and to generally coeval veras fault to the east (Fig. 1). We superimpose Dickinson (1997) and Atwater and Stock rocks of the ancestral Cascades. Chemical analy on this reconstruction the successive position of (1998) differ significantly concerning the loca ses of basalts and basaltic andesite are useful in

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124°W 123°W 122°W 121°W 120°W 119°W Shasta 020406080 100 km CA NV

Figure 17. Kinematic recon- <2.6 Ma 41°N struction of northern California Sierra Nevada-Great V at 5.9 Ma. Continental crustal 2.6–5 Ma blocks are reconstructed with Pacific coordinates fixed (after Wilson et al., 2005). In this (fixed) Lassen area, kinematic reconstruction is accomplished by motion along three faults: San Andreas, alley block Rodgers Creek-Healdsburg, and 40°N Calaveras. Modern-day outlines 0 Ma of Point Reyes (PR), San Fran- cisco Bay (SF), San Pablo Bay (SP), and Suisin Bay (SS) are 2 Ma 5–10 Ma shown (irregular dashed lines) 4 Ma Sutter Buttes in their 5.9 Ma position for ref- 1.59−1.36 Ma erence. Location of the southern 2.2–0.09 Ma; CLVF 39°N edge of the subducting Gorda 6 Ma plate at 8 Ma, 6 Ma, 4 Ma, Rodgers Creek-Healdsburg fault 2 Ma, and 0 Ma is after Atwater 3.35−2.8 Ma; CD and MSH and Stock (1998). The inferred San 8 Ma location of the Ancestral Cas- 4.35−4.3 Ma; SL cades arc through time is after Andreas fault 5.65−4.7 Ma; CS and SG Cousens et al. (2008). Sutter Buttes age from Hausback and SS Nilsen (1999). Volcanic centers 38°N 5.9 Ma reconstruction of the Sonoma volcanic field Calav lie between the Calaveras and Sierra Nevada-Great the Rodgers Creek-Healdsburg Fault used in kinematic Valley block is fixed with reconstruction (after Wilson SP eras faul faults; age range of erupted et al., 2005). respect to fault-bounded volcanic rocks for selected cen- Location of the southern blocks to the west. edge of Gorda Plate in past Modern-day outlines of ters is shown within the white 8 Ma (after Atwater and t Point Reyes (PR), San rectangles. Volcanic centers: Stock, 1998). San Gregorio block Francisco Bay (SF), and CS—Cup and Saucer; SG— 37°N Location of Ancestral San Pablo Bay (SP), Sugarloaf Ridge; SL—Stags Cascades arc through time SF (after Cousens et al., (dashed lines) are shown Leap; CD—Calistoga Domes; 2008). in their 5.9 Ma position and MSH—Mount St. Helena; Location of volcanic center must move to the CLVF—Clear Lake volcanic <2.6 Ma (after Cousens et northwest to merge with al., 2008). field. Suisin Bay (SS) and form 2.85 Ma Location of volcanic center the present-day coastline. within the Sonoma volcanic PR field, with age range of erupted volcanic rocks 36°N

defining melt sources and processes because classes (Fig. 18): (1) volcanic centers that range age (oldest to youngest). All other mafic rocks, these rocks are least likely to be affected by from mafic to silicic, are explosive in eruptive predominantly from the western half of the vol crustal contamination or fractionation. In Fig style, and include some rocks that trend toward canic field, are denoted on Figure 18 as “other.” ure 18 we compare our geochemical data from alkalic compositions (Cup and Saucer and Mafic rocks of the Sonoma and Clear Lake

20 basaltic (<52% SiO2) lavas and dikes and 33 Sugarloaf Ridge centers); (2) dominantly basal volcanic fields generally have medium-K calc-

basaltic andesite lavas (52%–57% SiO2) with tic to andesitic fields (Stags Leap and Wildlake alkaline series compositions, with generally similar composition rocks from the Clear Lake centers); and (3) dominantly silicic volcanic increasing total alkali content with silica (Fig. volcanic field, the ancestral Cascade magmatic centers at the north end of the SVF (Mount St. 18A). In this sense only, SVF rocks are simi arc, and rocks from various slab window-related Helena and Calistoga Domes centers). These lar in composition to the ancestral Cascade volcanic fields. For the purposes of geochemical groupings allow the geochemical characteristics magmatic arc rocks (Fig. 18A). Some sam classification, volcanic centers from the eastern of the individual centers to be assessed rela ples from the Sugarloaf Ridge and Cup and half of the SVF are grouped into three general tive to geographic location (south to north) and Saucer volcanic centers trend toward alkaline

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8 200

180 7 160

6 140

120 5 b

/N 100 wt %

4 Ba 80 O, 2 K 3 60 O+ 2 Th 40 Th Na 2 20

1 0 45 50 55 60 00.501.001.502.002.503.003.50

SiO2, wt% TiO2, wt% 8 25

7 20 6

5 15 e r

/Z 4 /C Ba Ba 10 3

2 Th 5 1 Th 0 0 0510 15 20 010203040 La/Yb Ce/Yb

Sonoma and Clear Lake volcanic fields Casacdes magmatic arc Slab window-related volcanic rocks Mount St. Helena and Calistoga Domes northern Baja California (Luhr et al., 1995) ancestral Cascades Stags Leap and Wildlake (Cousens et al., 2008) Antarctic Peninsula (Hole, 1990)

Cup and Saucer and Sugarloaf Ridge modern Cascades, southern Patagonia, Argentina (Gorring and Kay, 2001) southern segment southern Baja California (Benoit et al., 2002) Other Sonoma volcanic field (Schmidt et al., 2008) Clear Lake volcanic field

Figure 18. Geochemistry of mafic rocks from selected volcanic centers within the Sonoma volcanic field and other volcanic rocks. (A) Total

alkali-silica diagram (Le Bas et al., 1986) for silica contents of less than 60%, including basalt (<52% SiO2), basaltic andesite (52%–

57% SiO2), and some andesites. No data are plotted for modern Cascades, southern segment (Schmidt et al., 2008), Na2O analyses not

included in published data. (B) Variation in TiO2 content with Ba/Ta (LILE/HFSE) for basalt (<52% SiO2) and basaltic andesite (52%–57%

SiO2). (C) Ratio of incompatible elements showing variation in Ba/Zr (LILE/HFSE) with La/Yb (LRRE/HREE) for basalt (<52% SiO2)

and basaltic andesite (52%–57% SiO2). (D) Ratio of incompatible elements showing variation in Ce/Yb with Ba/Ce for basalt (<52%

SiO2) and basaltic andesite (52%–57% SiO2). Data sources for volcanic rocks other than Sonoma volcanic field: ancestral Cascades (Cousens et al., 2008); Clear Lake volcanic field (Hammersley and DePaolo, 2006; Schmitt et al., 2006); modern Cascades, southern segment (Schmidt et al., 2008); slab window volcanic rocks (Luhr et al., 1995; Hole, 1990; Gorring and Kay, 2001; Benoit et al., 2002). Th—two samples from tholeiitic flows interpreted by Benoit et al. (2002) to be associated with slab window.

compositions, but in general mafic rocks of than younger, more northerly centers such as LILE/HFSE ratios (here, Ba/Nb) (Fig. 18B). the Sonoma volcanic field appear distinct from CD-MSH and the Clear Lake field (Fig. 18A). A small number of samples from the Sonoma intraplate alkalic rocks with OIB affinities Sonoma volcanic field basalts are characterized and Clear Lake volcanic fields display a “sub from some slab window settings (Hole, 1990; by primitive compositions, including moderate duction component” and overlap with the data Gorring and Kay, 2001) (Fig. 18A). Total alkali to high MgO (4.2%–13.1%) and Cr (80–400 from the ancestral and modern Cascades maga content varies subtly but systematically with ppm), and Mg# that ranges from 40 to 66. matic arc (Fig. 18B). Basalt and basaltic ande age and spatial position of the SVF volcanic Mafic rocks from the Sonoma volcanic site from the Sonoma volcanic field centers are centers (Fig. 18A). The older, more southerly field and other slab window-derived rocks are similar in composition to slab window tholeiites volcanic centers such as the Cup and Saucer clearly distinguished from rocks of the ances from southern Baja (Benoit et al., 2002), but are and related Sugarloaf Ridge volcanic center are tral and modern Cascades magamatic arc in enriched in Ba and have elevated Ba/Nb ratios

more sodic and have higher total alkali content having elevated TiO2 concentrations and low relative to mafic rocks from ridge collision slab

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window environments that have OIB affinities segment, similar to many of the Sonoma vol field have the greatest similarity to tholeiitic slab such as Patagonia (Gorring and Kay, 2001) and canic field rocks, was attributed to proximity window-derived rocks in southern Baja Califor the northern Baja Peninsula (Luhr et al., 1995) to the Gorda plate edge and the slab window nia (Benoit et al., 2002). However, some mafic (Fig. 18B). (Schmidt et al., 2008) (Fig. 18D). SVF rocks from the Cup and Saucer and Sugar The plot of LREE/HREE (La/Yb) versus loaf Ridge centers have alkaline compositions, LILE/HFSE (Ba/Zr) separates the rocks of the CONCLUSIONS very high Zr content, and relatively small LILE Sonoma volcanic field from rocks associated enrichment. with magmatic arc settings and from the more Volcanic rocks in the Sonoma volcanic field REFERENCES CITED alkali rocks from slab window environments contain heterogeneous assemblages of a vari (Fig. 18C). Basalt and basaltic andesite rocks ety of compositionally diverse volcanic rocks. Atwater, T., and Stock, J., 1998, Pacific-North America plate tectonics of the Negoene southwestern United States: from the ancestral Cascades magmatic arc have We have used field mapping and whole-rock An update, in Ernst, W.G., and Nelson, C.A., eds., high Ba/Zr ratios and moderately sloping REE geochemistry in combination with new and Integrated Earth and Environmental Evolution of the patterns indicated by moderate to low values existing age determinations to define for the Southwestern United States: The Clarence A. Hall, Jr. Volume: Bellwether Publishing, Ltd. for Geological of La/Yb (Fig. 18C). Rocks from OIB-related first time discrete eruptive centers. Along Society of America, p. 393–420. slab window environments are distinguished by the eastern side of Napa Valley, volcanic centers Bellon, H., Aguillón-Robles, A., Calmus, T., Maury, R.C., strong HREE depletions (high La/Yb) and lack become progressively younger northwards, Bourgois, J., and Cotten, J., 2006, La Purísima volcanic field, Baja California Sur (Mexico): Miocene to Qua incompatible element enrichment (low Ba/Zr) mirroring the regional age progression of vol ternary volcanism related to subduction and opening of (Fig. 18C). Mafic rocks from the Sonoma vol canic rocks in the northern California Coast an asthenospheric window: Journal of Volcanology and Geothermal Research, v. 152, p. 253–272, doi: 10.1016/ canic field have relatively flat REE patterns (low Ranges (Fox et al., 1985a) and the northward j.jvolgeores.2005.10.005. La/Yb) and low LILE/HFSE (low Ba/Zr) (Fig. migration of the Mendocino Triple Junction Benoit, M., Aguillón-Robles, A., Calmus, T., Maury, R.C., 18C). Mafic rocks from the Sonoma volcanic (Dickinson and Snyder, 1979; Johnson and Bellon, H., Cotton, J., Bourgois, J., and Michaud, F., 2002, Geochemical diversity of late Miocene vol field centers are perhaps most similar in com O’Neil, 1984; Fox et al., 1985a). Volcanic rocks canism in southern Baja California, Mexico; implica position to slab window tholeiites from south of the Sonoma volcanic field show a broad tion of mantle and crustal sources during the opening ern Baja (Benoit et al., 2002) (Fig. 18C, points range in eruptive style that is spatially vari of an asthenospheric window: The Journal of Geology, v. 110, p. 627–648, doi: 10.1086/342735. labeled “Th”); Sonoma volcanic field rocks have able, specific to an individual eruptive center, Bezore. S.P., Clahan, K.B., Sowers, J.M., and Witter, R.C., much flatter REE patterns (low La/Yb) than slab and does not correspond to the general north 2005, Geologic map of the Yountville 7.5′ quadrangle, Napa County, California: A digital database: California window-related rocks from other tectonic set ward age progression. Measured heat flow in Geological Survey Preliminary Geologic Map, scale tings in southern Patagonia (Gorring and Kay, the northern California Coast Ranges (Lachen 1:24,000, http://www.conservation.ca.gov/cgs/rghm/ 2001) and the Antarctic Peninsula (Hole, 1990). bruch and Sass, 1980) and numerical models of rgm/preliminary_geologic_maps.htm. Bezore. S.P., Sowers, J.M., and Witter, R.C., 2004, Geo Incompatible trace-element patterns of mafic the thermal effects of slab window migration logic map of the Mt. George 7.5′ quadrangle, Napa and rocks from the Sonoma volcanic field centers (Liu and Furlong, 1992; Furlong and Schwartz, Solano Counties, California: A digital database: Cali show some similarities to the various lavas from 2004; Groome and Thorkelson, 2009) indicate fornia Geological Survey Preliminary Geologic Map, scale 1:24,000, http://www.conservation.ca.gov/cgs/ southern Baja (Benoit et al., 2002) but are rela abrupt transient thermal effects associated with rghm/rgm/preliminary_geologic_maps.htm. tively depleted in Ba and have flatter REE pat the northward passage of the triple junction as Blake, M.C., Jr., Graymer, R.W., and Stamski, R.E., 2002, Geologic map and map database of western Sonoma, terns (Fig. 10). In general, mafic rocks from the upwelling asthenospheric material replaces the northernmost Marin, and southernmost Mendocino Sonoma volcanic field centers do not precisely Gorda slab beneath North America. The rapid Counties, California: U.S. Geological Survey Mis correspond to any of the general compositional temporal and spatial variation in eruptive style cellaneous Field Studies Map MF-2402, 42 p., scale 1:100,000. categories defined by Benoit et al. (2002) and and volcanic rock composition would thus Briggs, P.H., 2002, Chapter G—The determination of forty instead occupy their own space on the incom seem to indicate that slab window volcanism elements in geological and botanical samples by induc patible trace-element plots (Fig. 10) and ratio results from a transitory pulse of elevated heat tively coupled plasma-atomic emission spectrom etry, in Taggart, J.E., Jr., ed., Analytical methods for plots (Fig. 18). flow over a relatively small geographic area. chemical analysis of geologic and other materials, U.S. Sonoma volcanic field rocks are also distin Using tectonic reconstructions that locate the Geological Survey: U.S. Geological Survey Open-File Report 02–0223, 20 p. guished from magmatic arc rocks and rocks Sonoma volcanic field with respect to the paleo Bryant, W.A. (compiler), 2005, Digital database of Qua from OIB-associated slab window settings position of the Mendocino Triple Junction, we ternary and younger faults from the fault activity when the ratio Ce/Yb is plotted against Ba/Ce surmise that (1) Sonoma volcanic field rocks map of California, version 2.0: California Geological Survey Web Page, http://www.consrv.ca.gov/CGS/ (Fig. 18D). Again, the Sonoma volcanic field were erupted in a position some 90–110 km information/publications/QuaternaryFaults_ver2.htm centers are most similar in composition to slab south of the Gorda plate edge; and (2) the (August, 2008). window tholeiites from southern Baja (Benoit Miocene–Pliocene ancestral Cascades mag Cameron, B.I., Walker, J.A., Carr, M.J., Patino, L.C., Matías, O., and Feigenson, M.D., 2003, Flux versus decompres et al., 2002). Schmidt et al. (2008) defined four matic arc was active at about the same time and sion melting at stratovolcanoes in southeastern Guate main segments of the modern Cascade arc on latitude as the volcanic centers of the Sonoma mala: Journal of Volcanology and Geothermal Research, v. 119, p. 21–50, doi: 10.1016/S0377-0273(02)00304-9. the basis of tectonic setting and mantle melting volcanic field. Our trace-element analyses of Chesterman, C.W., 1956, Pumice, pumicite and volcanic regimes; the southernmost segment is directly whole-rock samples from lavas and tuff from cinders of California: California Division of Mines north of the Gorda slab edge. High values of the Sonoma volcanic field highlight the geo Bulletin 174, 119 p. Clahan, K.B., Wagner, D.L., Bezore, S.P., Sowers, J.M., and Ba/Ce in the southern segment rocks were chemical diversity of volcanic rocks from the Witter, R.C., 2005, Preliminary geologic map of the viewed by Schmidt et al. (2008) as indicative slab window setting, allow us to clearly dis Rutherford 7.5′ Quadrangle: A digital database: Cali of subducted slab–derived fluids and decreasing tinguish these volcanic rocks from those of the fornia Geological Survey Preliminary Geologic Map, scale 1:24,000, http://www.conservation.ca.gov/cgs/ Ce/Yb as indicative of increased degree of melt roughly coeval magmatic arc to the west, and rghm/rgm/preliminary_geologic_maps.htm. ing (Fig. 18D), using the trace-element ratios also to compare rocks of the Sonoma volcanic Clahan, K.B., Wagner, D.L., Saucedo, G.J., Randolph- Loar, C.E., and Sowers, J.M., 2004, Geologic Map of defined by Cameron et al. (2002). The distinct field to rocks from other slab window settings. the Napa 7.5′ Quadrangle Napa County, California: fluid-rich chemistry of the southern Cascades Most of the mafic rocks of the Sonoma volcanic A Digital Database: California Geological Survey

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