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Home , Farallon Plate, Rivera Triple Junction, Tanya Atwater

Geochemistry and Origin of Middle Volcanic Rocks from Santa Cruz and Anacapa Islands, Southern Borderland Peter W. Weigand Department of Geological Sciences California State University Northridge, CA 91330

Cenozoic volcanism that began in the eastern Abstract - Major-oxide and trace-element Mojave Desert about 30 m.y. ago and swept compositions of middle Miocene volcanic rocks irregularly west and north. This extensive from north Santa Cruz and Anacapa Islands are extrusive activity was related closely to complex very similar. In contrast, they are geochemically tectonic activity that included of the distinct from the volcanic clasts from the Blanca whose subduction angle was Formation, of similar age but located south of steepening, and interaction of the Pacific and the Santa Cruz Island fault, which implies North American plates along a lengthening significant strike-slip movement on this fault. transform boundary; this activity additionally The island lavas are also compositionally involved rotation and possible northward distinct from the Conejo Volcanics located translation of crustal blocks. onshore in the Santa Monica Mountains. The The origin of the volcanic rocks in this area island lavas are part of a larger group of about has been variously ascribed to subduction of the 12 similar-aged volcanic suites from the Farallon plate (Weigand 1982; Crowe et al. California Borderland and onshore southern 1976; Higgins 1976), subduction of the Pacific- California that all belong to the calc-alkaline Farallon spreading center (Dixon & Ferrar series. This group is interpreted to have 1980), mantle diapirisrn into a slab-free window originated in a subduction environment in one (Dickinson & Snyder 1980), extensional of two possible scenarios: 1) their were melting at a migrating (Hurst produced from the subduction of the Cocos 1982), upper mantle and crustal dilation related plate south of the Rivera triple junction, which to the (Hawkins 1970) and would imply that the Baja-Borderland extension related to the formation of coastal allochthon onto which they were emplaced California Cenozoic basins (i.e., Crowell 1987). experienced northwestward translation A knowledge of the chemical composition of subsequent to eruption or 2) their magmas were the volcanic rocks may provide a better produced from warm, young subducted slab understanding of the specific tectonic located north of the Rivera triple junction, environment of their magma genesis. The which would imply that slab persisted in the chemical characterization of these lavas will aid area after the triple junction had also in the correlation between different migrated to tire south. volcanic areas. This information would be useful in placing constraints on amounts of northward translation, strike-slip offset and rotation experienced by adjacent crustal blocks. Introduction A better knowledge of the correlation between The volcanic rocks on Santa Cruz and volcanic units also might be useful in helping to Anacapa Islands (Fig. 1) are part of a larger direct exploration efforts for oil and gas in group of volcanic centers that erupted in the offshore southern California. Los Angeles and California Borderland areas For this project, I determined the major- about 13-18 m.y. ago. These areas represent oxide and trace-element concentrations in the culmination in southern California of samples from north Santa Cruz and Anacapa

Third California Islands Symposium 21 andesite) and more felsic near the top (andesite differences in the stress regime at a triple and dacite). junction largely control the petrologic Crowe & co-authors (1976) studied the processes that operate. Locally variable and petrology and chemical composition of the SCI mild extension and compression at the Volcanics. They noted a slight iron-enrichment Mendocino triple junction resulted in magmas trend and a similarity in major-oxide dominated by crustal anatexis, whereas composition to island-arc tholeiites, and substantial extension at the Rivera triple suggested that they may have formed part of an junction produced rocks derived primarily by island-arc sequence that recorded Miocene crystal fractionation of mantle-derived basalt subduction off southern California. They with only limited crustal contamination. For speculated that the East Pacific Rise played a samples of SCI volcanics, they reported significant but unknown role in the generation 818O(magma) values of 6.7 to 7.3%o and of these magmas. Higgins (1976) included the 87Sr/86Sr(initial) ratios of 0.70253 to 0.70361. SCI Volcanics in his study of Miocene Figure 1. Sketch map showing some of the Miocene volcanic areas (lined) mentioned in the text. The Santa Cruz Island The SCI Volcanics are classified subalkaline Volcanics and Blanca Formation crop out respectively north and south of the Santa Cruz Island fault (SCIF), volcanic rocks volcanism in the greater Los Angeles Basin area on a SiO2-alkalis plot and calc-alkaline on the comprise all of Anacapa Island, and the Conejo Volcanics and Zuma Volcanics crop out respectively north and south of the and supported the conclusion that this basis of AFM ratios (Crowe et a!, 1976, Fig.2; Malibu Coast fault (MCF). volcanism was probably related to active Higgins 1976, Fig. 3). They also are classified Islands in order to 1) evaluate possible subduction. Higgins also noted that the SCI Canyon Member consists of flows and as calc-alkaline according to the Si02- Volcanics 1) lack a distinctive iron-enrichment correlations between the two lava sequences epiclastic volcanic breccias of basaltic and FeO/MgO criterion of Miyashiro (1974) and and other nearby volcanic units of similar age trend on an AFM diagram, 2) are generally andesitic composition that were deposited in a the Al203-normative plagioclase criterion of and 2) evaluate the origin of the lavas and the higher in K20 man are the Conejo Volcanics subaerial environment. The overlying Stanton Irvine & Baragar (1979). Silica-K20 SCI Volcanics and 3) are both calc-alkaline and tectonic setting of their emplacement. Ranch Member is composed of andesite flows, relationships show that these rocks are tholeiitic on the basis of Si02-K20 flow breccias and subordinate tuff breccias relationships. predominantly medium-potassium andesite Previous Work erupted on the flanks of a volcanic edifice. Next using the scheme of Gill (1981, Fig. 1.2), which Hurst & co-authors (1982) and Hurst (1983) Santa Cruz Island: The volcanic sequence younger is the Devil's Peak Member is in agreement with petrographic estimates containing a variety of scoriaceous andesitic included the SCI Volcanics in their studies of (Nolf & Nolf 1969; Crowe et al. 1976). exposed north of the Santa Cruz Island fault Miocene volcanism in the Southern California was first correlated with the Conejo Formation and dacitic flows, flow breccias, and reworked Turner (1970) reported K/Ar age dates of pyroclastic rocks. These rocks were apparently Borderland. They reviewed the available 16.1 ± 0.9 Ma on a sample probably from the (now the Conejo Volcanics) of the western geochemical data and presented Sr-isotopic emplaced on the slopes of, and adjacent to, a upper part of the Devil's Peak Member, and Santa Monica Mountains on the basis of data for 10 samples of SCI Volcanics. Initial volcanic center. The uppermost Prisoner's 16.5 ± 0.8 Ma on a sample probably from the lithologic similarities and stratigraphic position 87Sr/86Sr ratios range from 0.70253 to 0.70318, Harbor Member is composed of andesitic and middle of the Prisoner's Harbor Member (Kleinpell 1938). The volcanic section was overlap the range of Leg 63 DSDP samples renamed the Santa Cruz Island (SCI) Volcanics dacitic flows, flow breccias and tuffaceous (these and subsequent dates have been from the California Borderland (Pal Verma corrected to new IUGS constants). Crowe & and subdivided into four members by Nolf & volcaniclastic beds, probably deposited in a 1981), and are generally lower than those from co-authors (1976) reported dates of 16.0 ± 0.7 Nolf (1969). It forms a north-dipping submarine environment. Basalt, andesite and Conejo or Santa Catalina Island volcanic rocks. Ma and 19.9 ± 0.9 Ma on the same sample from homocline composed of interbedded flows, dacite are most commonly porphyritic and all Hurst & co-authors (1982) concluded on the a dike cutting the lower part of the Devil's flow breccias and volcaniclastic rocks that have contain phenocrysts of plagioclase, opaque basis of a variety of petrographic, chemical and Peak Member. The lower part of the sequence a cumulative thickness of 2,400 m and that are oxides and variable amounts of augite, isotopic evidence that the SCI Volcanics has not been dated. San Onofre Breccia of cut by numerous shallow intrusions at an hypersthene and pigeonite. An-contents of exhibit a mixture of tholeiitic and calc-alkaline probable Saucesian age was encountered inferred eruptive center near Devil's Peak. The plagioclase decrease from An65 to An28 in the characteristics, and resulted from mantle-ridge beneath the volcanics in the Union Oil Gemini formation overlies inferred San Onofre Breccia three rock types. Basalt contains areas of melting due to complex ridge-trench-transform No. 1 well drilled at the east end of the island in the subsurface and is overlain by the alteration (iddingsite) presumed to be interactions in the vicinity of the Rivera triple (Weaver & Meyer 1969), which probably limits Monterey Formation. pseudomorphic after olivine, and dacite junction. contains variable amounts of highly altered the onset of volcanism to less than 23-25 Ma The following brief summary of stratigraphy Johnson & O'Neil (1984) measured trace- (Crouch & Bukry 1979). hornblende. The sequence is mafic near the and petrography is from Nolf & Nolf (1969), element abundances and isotopic ratios of Sr Anacapa: Being nearly entirely volcanic, the base (basalt and andesite), intermediate in with additional information from Crowe & co- and O in numerous Neogene volcanic centers of Anacapa Island is much simpler than composition in the middle members (largely authors (1976). The lowermost Griffith in western California. They concluded that that of Santa Cruz Island and has

22 23 Figure 2. Rare-earth element patterns normalized to chondritic values. Circles are SCI Volcanics, crosses are Anacapa samples. Lined area represents field of available Conejo data (Weigand 1982, unpubl.).

Figure 3. Silica-variation diagrams for selected trace elements. Filled circles are SCI Volcanics, stars are Anacapa samples, triangles and crosses are respectively Conejo and Zuma samples (Weigand 1982, unpubl.), and boxes are Blanca samples (Savage & Despard 1990; H. McLean & D. Howell unpubl).

24 correspondingly received less attention. Scholl Table 1. Methods, detection limit (DL) and comparison of values reported by XRAL (plus errors estimated for trace (1960) correlated the lava flows and various elements) with values from Flanagan (1973) for reference volcanic and volcaniclastic breccias with the basalt BCR 1; Flanagan's data are recommended, averages Conejo Volcanics. As on Santa Cruz, the or magnitudes volcanic sequence forms a north-dipping BCR1 homocline. A maximum of 1,700 m is exposed Method DL XRAL Flanagan

although neither the base nor top are seen. Si02 XRF 0.01 54.60 54.50 XRF 0.01 2.20 2.20 Two strata of San Onofre Breccia are Ti02 XRF 0.01 13.50 13.61 interbedded with the volcanic rocks near their Al2O3 FeOT XRF 0.01 12.06 12.06 exposed base. The rock type is vesicular and M11O XRF 0.01 0.19 0.18 MgO XRF 0.01 3.38 3.46 porphyritic andesite containing phenocrysts of CaO XRF 0.01 6.94 6.92

plagioclase, augite, hypersthene and opaque Na20 XRF 0.01 3.18 3.27 XRF 0.01 oxides. No previous geochemical or isotopic K20 1.80 1.70 P2O5 XRF 0.01 0.36 0.36 analyses or age determinations have been V DCP 2 nd — published. Cr NA 0.1 22+1 17.6 Ni DCP 1 nd — Co NA 0.1 44±2 38 Methods Cu DCP 0.5 nd — Zn DCP 0.5 nd — Six samples of SCI Volcanics were chosen Ge DCP 10 nd — As NA 1 1±1 0.7 from numerous cores available from a Se NA 0.5 <.5 0.1 paleomagnetic study by Kamerling & Br NA 0.5 0.8±.6 0.15 Luyendyk (1985) on the basis of freshness and Mo NA 2 3 2 1.1 Ag DCP 0.5 nd — representative geographic and stratigraphic Cd DCP 0.2 nd — location. Sites SB and SF are located in the W NA 1 <2 0.4 Griffin Canyon Member, SCVA and SCFA in Au (ppb) NA 5 <5 0.7 Pb DCP 2 nd — the Devil's Peak Member, and SP in the Bi DCP 0.1 0.1±.l 0.05 Prisoner's Harbor Member. Site SS is located Li AA 1 nd — Be DCP 1 nd — in rocks mapped as undifferentiated Santa Cruz B DCP 10 nd — Island Volcanics (Weaver 1969). The Stanton Sc NA 0.01 42.9±2 33 Ranch Member is not represented. The three Rb XRF 10 50+17 46.6 Sr XRF 10 310±25 330 Anacapa samples were chosen from a collection Cs NA 0.2 1.0±.4 0.95 of cores drilled at the east end of the island as Ba XRF 10 670±34 675 Y XRF 10 20±3 37.1 part of an earlier paleomagnetic study by Zr XRF 10 200±24 190 Kamerling & Luyendyk (1979). Nb XRF 10 20±6 13.5 Sb NA 0.1 0.9±.2 0.69 The Channel Islands samples and a Hf NA 0.2 5.2±.5 4.7 geochemical standard were analyzed by X-Ray Ta NA 0.5 1.1+.7 0.91 Th NA 0.2 7.2±.5 6.0 Assay Laboratories (XRAL) for 10 major oxides, U NA 0.1 4.8±.3 1.74 44 trace elements and loss on ignition (LOI). La NA 0.1 33.1±1.6 26 Methods included x-ray fluorescence (XRF), Ce NA 1 66±4 53.9 Nd NA 3 32±6 29 direct coupled plasma (DC), atomic absorption Sm NA 0.01 7.52±.4 6.6 (AA), neutron activation (NA) and delayed Eu NA 0.05 1.95±1.4 1.94 neutron count (DNC). Methods and detection Gd NA 0.5 7.6±1.1 6.6 Tb NA 0.1 1.1±.2 1.0 limits are included in Table 1. Precision is Dy NA 0.5 7.0±1.1 6.3 estimated to be ± 100% at detection limit (DL), Yb NA 0.05 4.31±.31 3.36 0.01 ± 20% at 20 x DL, and ± 5% at 100 x DL. Lu NA 0.63±.03 0.55 To assess the accuracy of these analyses, U.S. Major oxides in wt. %, trace elements in ppm except Au, total iron expressed as FeOT, nd not determined; see text Geological Survey geochemical reference for methods.

25 standard basalt BCR-1 was included as an enclose numerous rounded inclusions of glass. unidentified sample in the set sent to XRAL. The other phenocryst, less abundant, smaller Table 1 compares values reported by XRAL and less well formed than the plagioclase with those reported by Flanagan (1973). Eleven phenocrysts, is pyroxene, both augite and elements were not determined due to hypersthene. A few grains exhibit insufficient sample. Major oxides show compositional zoning, and others enclose excellent agreement, as do the majority of the rounded glass inclusions. The SCI basaltic- trace elements. Four elements (Ce, Lu, Nb and andesite samples have a fine-grained Th) are about 20% higher than the values groundmass of plagioclase, pyroxene, opaque reported to have the highest reliability by oxides and minor glass. The SCI andesite and Flanagan (1973). Only U shows a gross dacite samples exhibit a dark, very fine-grained dissimilarity. None of these elements forms the matrix; the dacite matrix is cut by thin quartz main basis of the conclusions in this study. veins. The andesite samples from Anacapa are Because many of the comparisons are made variably vesicular and exhibit a merocrystalline between samples analyzed by XRAL, precision groundmass with plagioclase and pyroxene set is a more important parameter than accuracy. in dark glass. Nevertheless, accurate results give confidence Major-oxide analyses of the nine samples are in the analytical procedures, and for the listed in Table 2, trace-element analyses are comparison of these analyses with those from listed in Table 3, and previously unpublished other labs. analyses of an additional 5 samples (H. McLean & D. Howell, priv. comm.) are listed Results in Table 4. The classification of Le Bas & co- The petrography of the analyzed samples, authors (1986) can be used to identify the rock including those from Anacapa, agrees closely type of the samples; these are indicated in with the descriptions of Crowe & co-authors Tables 2 and 4). The observation by Nolf & (1976). Plagioclase, the most abundant Nolf (1969) and Crowe & co-authors (1976) phenocryst, occurs as small to large, euhedral that the SCI Volcanics become less mafic to subhedral crystals, many of which are upsection is reflected in the major-oxide concentrically zoned and many of which analyses (Tables 2 and 4); Griffin Canyon

Table 2. Major-oxide analyses of lavas from north Santa Cruz and Anacapa Islands; SCI samples listed in ascending stratigraphic order.

Major oxides in wt. %, total iron expressed as FeOT, LOI is loss on ignition.

26 samples are basaltic-andesite and andesite, alkaline classification of SCI Volcanics on the those from the Stanton Ranch and Devil's basis of major-oxide abundances. Peak members are andesite and andesite On the basis of trace-element abundances, transitional to dacite and the Prisoner's the Channel Islands rocks are classified as calcalkaline according to the MgO-Ni criterion of Harbor samples are dacite transitional to Gill (1981) and to the Ti-Zr-Y-Sr criteria of rhyolite. Unfortunately, the stratigraphic Pearce & Cann (1973). On the other hand, position of the 24 samples analyzed by Crowe principal component values derived by the & co-authors (1976) are unknown. recalculation of these latter trace elements The Santa Cruz samples fall within the range (Butler & Woronow 1986) suggest that these of previous analyses (Crowe et al. 1976) and are rocks are more similar to within-plate basalts valid representative samples from the island. than to calc-alkaline basalts. However, The Anacapa samples fall within the range evaluating these andesites using criteria derived defined by available analyses of SCI Volcanics from basalts is probably not appropriate. It is (Table 2; Crowe et al. 1976) for every major primarily the relatively high Zr contents that oxide, and agree in all cases with the calc-

27 Table 4. Additional chemical analyses of SCI Volcanics light REE enrichment (La ~74 x chondrite), from H. McLean & D. Howell (priv. comm.); listed in ascending stratigraphic order. moderate light REE fractionation (LaN/LuN averages 5.2) and a relatively flat heavy REE

enrichment (TbN/YbN averages 1.3). All of the samples exhibit small Eu-anomalies; Eu*/Eu averages 1.4. The Ni contents of the samples are relatively low (<63 ppm for Anacapa, <3 7 for Santa Cruz). In addition, the Mg-numbers [100 x molar ratio Mg/(Mg+Fe2+)] also are relatively low (<62); none of the samples from Crowe & co-authors (1976) exceed 56. Both of these parameters preclude any of the samples so far analyzed from Santa Cruz or Anacapa Islands reflect inadequate sampling. Petrographic and fault (Shelton 1988; Savage & Despard 1990; from being direct melts from the mantle (Gill geochemical similarities suggest that the lavas H. McLean & D. Howell unpubl. data) 1981). Although the basaltic andesite and on Anacapa Island are genetically similar to, confirm the high proportion of dacite and andesite samples exhibit only a small and stratigraphically correlative with the SCI rhyolite suggested by petrography (Weaver et compositional range, parabolic relationships Volcanics. al, 1969). Of the 42 available samples, about

exhibited between Ni and Rb are consistent With respect to major oxides, northern 2/3 are higher in SiO2 than any sample of SCI with these rocks being the result of 10-15% Channel Islands lavas are similar to those from Volcanics so far analyzed. Compared to SCI partial melting of a primitive basalt or eclogite the Conejo Volcanics located in the Santa volcanic rocks, the silicic Blanca samples as a

(Gill 1981). The higher contents of Ni in Monica Mountains. Previous studies have whole are generally lower in Ti02, FeOT,

Anacapa samples imply a source with a higher noted that SCI Volcanics have higher K20, P205, Rb, Ba and Cu, and higher in MgO

Major oxides in wt. %, trace elements in ppm; Ti is initial abundance of this element. The Anacapa abundances of Ti02 and K20 relative to the and Ni (Figs. 3 and 4). These differences in Tertiary intrusion. samples have Ba/Ta ratios (542-900) that Conejo Volcanics (Crowe et al, 1976; Higgins chemical composition agree with the exceed 450, satisfying the single most 1976; Hurst et al. 1982). These comparisons conclusion of McLean & co-authors (1976) cause the similarity to within-plate basalts. diagnostic geochemical characteristic of arc persist (Fig. 4) when a larger Conejo data set is that the Blanca Formation was not derived by These high Zr contents are likely caused not by magmas (Gill 1981); SCI andesite samples used (Weigand 1982; Blackerby 1965, unpubl. erosion of the SCI volcanics. This conclusion is tectonic setting, but rather by crystal range from 277 to 391. Ratios of La/Th data). Anacapa samples mimic those from SCI. consistent with the ~30 km separation of the fractionation, since Zr generally increases between 7.5 and 5.1, and of La/Ba between Isotopic ratios of Sr and O overlap between the island along the Santa Cruz Island fault during this process. 13.1 and 22.7 for the seven basaltic andesites SCI and Conejo volcanics (Hurst 1983; required by the restoration of ~80° of Neogene and andesites are within the ranges shown by clockwise rotation evidenced by paleomagnetic Although the trace-element averages Johnson & O'Neil 1984). These general most orogenic andesites (Gill 1981). data (Hornafius et al, 1986). reported in Jakes & White's (1971) comparison similarities do not extend to trace-element Conversely, La/Nb ratios of 1.5 to 2.2 for these comparisons. Unfortunately, data available for of calc-alkaline and tholeiitic rock series in Fractionation modelling: Crystal samples compare most closely to normal-type the Conejo Volcanics (Weigand 1982, unpubl. orogenic areas are somewhat obsolete, a fractionation models were calculated using mid-ocean ridge basalt (Gill 1981). data) are somewhat restricted. For the great comparison of these averages with the results of methods described by Bryan & co-workers Comparison: The three samples from majority of trace elements, abundances are this study is instructive. Abundances of nine (1969). Five major-oxide compositions were Anacapa fall within the range exhibited by significantly lower in Conejo rocks than they trace elements in the Channel Islands andesites chosen that span the range of available analyses available analyses of SCI Volcanics (Crowe et are in SCI and Anacapa rocks (Rb, Zr, Nb, compare more closely with the average calc- (Crowe et al, 1976; this study). Because no al. 1976; this report) for every major oxide, and REE, Y Hf, Th, Sc and Co; Figs. 2 and 3). The mineral analyses are available from SCI alkaline andesite - Rb, Ba, Sr, La, Th, U, Cr, for many trace elements (Tables 2, 3 and 4). two exceptions are Ni, for which Conejo rocks volcanic rocks, selected mineral analyses were Zr and Hf. Only Yb contents compare more Abundances of several trace elements differ, are intermediate between those from Anacapa compiled from the Conejo Volcanics (Weigand closely with the average tholeiitic andesite. however, between the two sets of samples. (higher) and SCI (lower), and Cr, for which 1982, unpubl. data) and Central American Rare-earth element (REE) patterns (Fig. 2) Anacapa samples are generally higher in Y, Ba, SCI abundances are lower than those of volcanic rocks (M. J. Carr, pers. comm. 1985), are similar for the nine analyzed samples, Ni, Cr and Br, and are generally lower in Co Conejo or Anacapa. as well as from Deer & co-authors (1961, especially for the seven samples with Si02 and Ta compared to the SCI andesites (Fig. 3). Available analyses of volcanic rocks from the 1963). Pigeonite, which is a minor phase in all <60%. The rocks as a whole exhibit moderate These differences are minor and probably Blanca Formation located south of the SCI SCI Volcanic rocks and hornblende, which is

28 29 abundant in some dacites (Crowe et al, 1976), Rosarito Beach. In addition, presumably similar were not considered in the models. mid-Miocene volcanic rocks floor the center of Results of the calculations are summarized in the Los Angeles basin (Yerkes et al. 1965) and Table 5. The low sums of squares of residuals are present throughout much of the Southern (sosor) indicate a relatively good fit for each California Borderland (Vedder et. al. 1981). As a case. The calculations infer that olivine was not group, these rocks are characterized by being an important fractionating phase in the calc-alkaline, low- to medium-K basalt and differentiation of basalt to basaltic andesite. andesite that erupted primarily around 15±1 Fresh olivine has not been observed in SCI Ma (Weigand & Savage 1991). Available basalts, although its presence was inferred by isotopic ratios of Sr and O are low, that of Nd Crowe & co-authors (1976) by the shape of relatively high (Weigand 1982; Hurst 1983; pseudomorphic iddingsite and by jackets of Johnson & O'Neil 1984). altered pyroxene around these alteration The bulk of the major-oxide and trace- masses in most basalt samples. The calculations element evidence, presented in detail for SCI indicate that augite (WoEnFs = 42:46:12) was and Anacapa volcanic rocks above, demon• more important in the earlier stages of strates that all of these volcanic centers are fractionation and hypersthene (WoEnFs = predominantly calc-alkaline and were erupted 4:77:19) was more important in later stages. in an orogenic environment. Thus, according This is consistent with the observation by to the preponderance of data, the SCI Hurst & co-authors (1982) that augite Volcanics are calc-alkaline in spite of relatively

predominates over hypersthene in mafic rocks; high Ti02 contents similar to some tholeiites; however, it is inconsistent with the observed similarly, the Conejo Volcanics are calc-

predominance of augite in the intermediate alkaline in spite of relatively low K20 contents rocks as well. The range in plagioclase similar to some tholeiitic rocks (Fig. 5). To my compostions falls within the ranges measured knowledge, all modern calc-alkaline volcanic by Crowe & co-authors (1976). The small rocks are being erupted in subduction settings. amounts of Fe-Ti oxides suggested by these Available Sr, O and Nd isotopic data for these calculations are supported by the report of lavas from coastal southern California are all minor or sparse Fe-Ti oxide microphenocrysts consistent with generation from a mantle in rocks of all composition (Crowe et al. 1976). source in a subduction environment. Several theories of origin for the coastal Discussion volcanic rocks involve dilation or extension. Volcanic rocks in modern and ancient rifted An understanding of the origin of the SCI areas, whether oceanic or continental, almost and Anacapa lavas cannot be attempted without invariably belong to the tholeiitic or alkalic rock also considering the other mid-Miocene series; with two known exceptions, calc-alkaline volcanic rocks found throughout coastal rocks are non-existent. "Triple-junction" southern California. Gross geochemical and magmatism described by Johnson & O'Neil age similarities led the author to recognize (1985) produced a string of calc-alkaline these as a group of volcanic rocks distinct from volcanic centers adjacent to the San Andreas those generally located to the north and fault that were associated with the northward- adjacent to the (Weigand migrating Mendocino triple junction during the 1982). Besides Santa Cruz and Anacapa, this Neogene. There is no similar geochemical or group includes the following volcanic centers: age-location evidence to suggest that the coastal Figure 4. Silica-vanation diagrams comparing SCI Volcanics (boxes) and fields of the Conejo Volcanics (C) and Blanca Santa Catalina, San Clemente and Santa California volcanic rocks were associated with a Formation (B). Data for fields from Weigand (1982, unpubl.), Crowe & co-authors (1976), Higgins (1976) Shelton (1988), Barbara Islands, Conejo, Zuma, Glendora, El Savage & Despard (1990), B. Blackerby (unpubl.) and H. McLean & D. Howell (unpubl). non-stable migrating triple junction. Calc- Modeno, Palos Verdes, Laguna Beach and alkaline volcanic rocks of middle Cenozoic age

30 31 also have been reported from the Rio Grande been tied to the passage of the Mendocino rift in New Mexico by Thompson & co-authors triple junction (Weigand & Frizzell 1986). (1986). The origin of these volcanic rocks, The alternative is that the coastal volcanics however, actually may be related to the were produced by subduction of the Cocos subducted Farallon plate. Until the igneous plate, yet this interpretation is not without processes that gave rise to these types of difficulties. Primarily, it seems geometrically volcanic rocks are better described and impossible to produce the Vasquez Volcanics at understood, the more conventional the site of the northward-migrating Mendocino interpretation that the generation of calc- triple junction 25 m.y. ago, and subsequently alkaline rocks is related in some manner to produce Conejo Volcanics less than 20 km away subduction environments seems the more 10 m.y. later at a site that was south of the prudent explanation. Rivera triple junction. Furthermore, several The fundamental plate-tectonic framework reconstruction models place the location of the of California during the Cenozoic has been Rivera triple junction far south of the Los understood since the work of Atwater (1970). A Angeles area by 16-14 m.y. ago (Atwater & critical question that must be addressed Molnar 1973; Engebretson et al. 1985). concerns whether or not southern California Restoration of offset of the Los Angeles area was the site of active subduction during the produced by movement on the southern San volcanism that occurred in the middle Miocene. Andreas fault and shear in the southern Sierran Relevant aspects include: 1) the Farallon plate orocline and the only was being subducted under the North American moves these volcanic centers 450 ± 85 km nearer plate until middle to late time; 2) to the Rivera triple junction (Atwater 1989). when the impinged on the North There are four considerations that may yield a American plate, the Farallon split into the solution to this dilemma. 1) Uncertainties in northern Juan de Fuca and the southern reconstructions are large, leaving both the plates; 3) a transform boundary (the proto-San timing of onset of interaction between the Andreas) was established between the Pacific North American and Pacific plates, and its and North American plates; 4) two triple position of incidence, only modestly constrained junctions formed the ends of this transform (Stock & Molnar 1983). Thus, the Rivera triple boundary - the northern transform-transform- junction may have been farther north than some trench Mendocino triple junction and the models postulate (see for example Crowell southern spreading-transform-trench Rivera 1987). 2) Subduction of the Cocos plate was not triple junction; and 5) inboard of this normal to the subduction zone, but was oblique. lengthening transform boundary, a no-slab For instance, Engebretson & co-authors (1985) window incapable of generating calc-alkaline suggest that its motion was oriented about magmas began enlarging beneath the North N30°E3) Movement of the Rivera triple American plate. junction did not mirror the motion of its sibling The coastal volcanics could riot have been and migrate constantly southward; its position produced by subduction of the Juan de Fuca was more or less stationary until the middle plate because the Mendocino triple junction Miocene (see Crowell 1987). 4) All of the coastal was in the Los Angeles area about 25 m.y. ago volcanic rocks are located on the Baja- and had migrated 400-500 km to the northwest Borderland allochthon (Blake et al. 1982). by the time most coastal volcanism was Although the uncertainties are large, this occurring (Atwater & Molnar 1973; allochthon may have been about 2.5° farther Engebretson et al. 1985). The origin of the south relative to the North American craton and Vasquez Volcanics, dated at 25 Ma and located the Santa Lucia-Orocopia allochthon (Howell et <20 km northeast of the Conejo Volcanics, has al. 1987); the latter is the site of the Vasquez

32 33 Volcanics. Paleomagnetic evidence from the considerably in trace-element contents from coastal volcanic rocks is consistent with the and P. Molnar. 1973. Relative motion of the Dickinson, W. R. and W. S. Snyder. 1980. the Conejo Volcanics, implying differences in Pacific and North American plates deduced from interpretation that since their eruption, the Geometry of subducted slabs related to San source area or petrologic process, or both. sea-floor spreading in the Atlantic, Indian, and Andreas Transform. J. Geophys. Res. 84:561-572. Conejo and northern Channel Island volcanic Significant dissimilarities in petrography and south Pacific oceans. Stanford Univ. Publ. Geol. Dixon, J. M. and E. Farrar. 1980. Ridge subduction rocks experienced northward translation relative major-oxide and trace-element abundances Sci. 13:136-148. education, and the Neogene tectonics of to the North American craton, perhaps up to 8° between the SCI Volcanics and the Blanca Blackerby, B. A. 1965. The Conejo volcanics in the southwestern America. 67:81-99. or ~860 km (Kamerling & Luyendyk 1985; see Formation imply strike-slip movement along Malibu Mountains, Los Angeles County, Engebretson, D. C, A. Cox and R. G. Gordon. also Champion et al. 1986). Movement may have the Santa Cruz Island fault. California. Ph.D. dissertation, University of 1985. Relative motions between oceanic and been fairly rapid because correlations of middle Lavas on Santa Cruz and Anacapa are part of California, Los Angeles, CA. 157 pp. continental plates in the Pacific basin. Geol. Soc. to late Miocene sedimentary rocks across the a larger group of volcanic centers located on Blake, M. C, D. G. Howell and D. L. Jones. 1982. Amer. Spec. Pap. 206: 59 pp. Gulf of California suggest little movement since the Baja-Borderland allochthon that share Preliminary tectonostratigraphic map of Flanagan, F. 1973. 1972 values for international California. U. S. Geol. Survey Open File Rep. 82- the middle Miocene (Hausback 1984). gross age and geochemical characteristics. geochemical reference standards. Geochem. 593. 10 pp. Cosmochem. Acta 37:1189-1200. On the other hand, recent quantitative plate Several lines of compositional evidence indicate Bryan, W. B., L. W. Finger and F. Chayes. 1969. Gill, J. B. 1981. Orogenic andesites and plate tecton• reconstructions based on plate circuit or hot- that these rocks were produced from a mantle Estimating proportions in petrographic mixing ics. Springer-Verlag: Berlin, GERMANY. 391 pp. spot circuit methods offer little support for this source in a subduction environment. Volcanism equations by least-squares approximation. Science Hauseback, B. P. 1984. Cenozoic volcanic and amount of translation of the Baja-Borderland may have occurred above the subducting Cocos 163:926-927. tectonic evolution of Baja California Sur, Mexico. allochthon since the Pacific plate first impinged plate south of the Rivera triple junction; the Butler, J. C. and A. Woronow. 1986. Discrimination Pp. 219-236. In: V. A. Frizzell, Jr. (ed.), Geology on the about 25 m.y. ago Baja-Borderland allochthon may then have among tectonic settings using trace element of the Baja California Peninsula. Society of (Atwater 1989). It seems significant that undergone northwestward translation sub• abundances of basalts. J. Geophys. Res. 91:10289- Economic Paleontologists & Mineralogists, volcanism in the Los Angeles area post-dated sequent to the wind-down of volcanic activity 10300. Pacific Section 39. the initial formation of the Los Angeles and about 14 m.y. ago. Alternatively, volcanism Champion, D. E., D. G. Howell and M. M. Hawkins, J. W. 1970. Petrology and possible Ventura basins (ca, 22 Ma; Crowell 1986) and may have occurred north of the Rivera triple Marshall. 1986. Paleomagnetism of Cretaceous tectonic significance of late Cenozoic volcanic was largely coincident with the onset of crustal junction above warm, young subducted slab and strate, San Miguel Island, California rocks, southern California and Baja California. Borderland and the northward translation of Baja rotation (ca. 16 Ma; Kamerling & Luyendyk which was encouraged to melt by extension Geol. Soc. Amer. Bull. 81: 3323-3338. California. J. Geophys. Res. 91:11557-11570. Higgins, R. E. 1976. Major-element chemistry of 1985). Perhaps some sort of warm, young slab related to crustal rotation. Crouch, J. K. and D. Bukry. 1979. Comparison of the Cenozoic volcanic rocks in the Los Angeles persisted in the Los Angeles area after the Miocene provincial formaniferal stages to basin and vicinity. Pp. 216-227. In: D. G. Howell Rivera triple junction moved to the south and Acknowledgments coccolith zones in the California Continental (ed.), Aspects of the geologic history of the the extension attendent with crustal rotation Borderland. Geology 7:211-215. California Continental Borderland. American Bruce P. Luyendyk generously supplied the initiated the melting event. The area of Crowe, B. M., H. McLean, D. G. Howell and R. E. Association of Petroleum Geologists, Pacific volcanism would have been positioned within samples utilized in this study, and Mobil Oil Higgins. 1976. Petrography and major-element Section, Misc. Publ. 24. 561 pp. the no- of Dickenson & Snyder Corporation financed the chemical analyses. chemistry of the Santa Cruz Island Volcanics. Hornafius, J. S., B. P. Luyendyk, R. R. Terres and (1980) or the slab-free region of Severinghaus Hugh McLean and David Howell made Pp. 196-215. In: D. G. Howell (ed.), Aspects of M. J. Kamerling. 1986. Timing and extent of & Atwater (1990). However, the latter authors available the analyses in Table 4. I thank J. the geologic history of the California Neogene tectonic rotation in the western noted that exactly where and when subduction Scott Hornafius for his interest and support in Continental Borderland. American Association . Geol. Soc. Amer. Bull. ceased at the Rivera triple junction is poorly this project. I had informative discussions with of Petroleum Geologists, Pacific Section, Misc. 98:1476-1487. Publ. 24. 561 pp. constrained. Thus calc-alkaline volcanism in William Dickenson and . Howell, D. G., D. E. Champion and J. G. Vedder. Crowell, J. C. 1987. Late Cenozoic basins of 1987. Terrane , crustal kinematics, and the Los Angeles area, if indeed it was associated onshore southern California: complexity is the Literature Cited basin evolution, southern California. Pp. 242-258. with a subducted slab, may provide important hallmark of their tectonic history. Pp. 207-241. In: R. V. Ingersoll and W. G. Ernst (eds.), Atwater, T. 1970. Implications of for constraints on the middle Miocene tectonic In: R. V. Ingersoll and W. G. Ernst (eds.), Cenozoic basin development of coastal California. the Cenozoic tectonic evolution of western North history of this area. Cenozoic basin development of coastal California. Ruby Volume VI. Prentice Hall: Englewood America. Geol. Soc. Amer. Bull. 8:3513-3536. Ruby Volume VI. Prentice Hall: Englewood Cliffs, NJ. 496 pp. . 1989. Plate tectonic history of the northeast Cliffs, NJ. 496 pp. Hurst, B. PI. 1983. Volcanogenesis contempora• Summary Pacific and western North America. Pp. 21-72. In: Deer, W. A., R. A. Howie and J. Zussman. 1961. neous with mid-ocean ridge subduction and E. L. Winterer, D. M. Hussong and R. W. Rock Forming Minerals. Vol. 5 Non-silicates. translation. Pp. 197-213. In: S.S. Augustithis (ed.), With minor-exceptions, lavas from Santa Decker (eds.), The Eastern Pacific Ocean and John Wiley and Sons: New York, NY. 371 pp. The significance of trace elements in solving Cruz and Anacapa Islands are compositionally Hawaii. The Geology of North America, vol. N. , and . 1963. Rock Forming petrogenetic problems and controversies. similar and are probably stratigraphically Geological Society of America: Boulder, CO. 577 Minerals. Vol. 4 Framework Silicates. John Wiley Theophrastus Publications: Athens, GREECE. correlative. In contrast, these rocks differ pp. and Sons: New York, NY. 435 pp. 910 pp.

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