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Home , Bandelier Tuff, Caldera, Valles Caldera

Is the Valles entering a new cycle of activity?

J. A. Wolff Department of Geology, University of Texas at Arlington, UTA Box 19049, Arlington, Texas 76019 J. N. Gardner Earth and Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, New 87545

ABSTRACT The formed during two major rhyolitic eruptive episodes (the Bandelier ) at 1.61 and 1.22 Ma, after some 12 m.y. of activity in the Jemez volcanic field, . Several subsequent eruptions between 1.22 and 0.52 Ma pro- duced dominantly high-silica domes and within the caldera. These were followed by a dormancy of 0.46 m.y. prior to the most recent intracaldera activity, the longest hiatus since the inception of the Bandelier system at ϳ1.8 Ma. The youngest volcanic activity at ϳ 60 ka produced the SW moat , a series of and tuffs that display abundant petrologic evidence of being newly generated melts. Petrographic textures conform closely to published predictions for silicic generated by intrusion of basaltic magma into continental crust. The Valles caldera may currently be the site of renewed silicic magma generation, induced by intrusion of mafic magma at depth. Recent seismic investigations revealed the presence of a large low-velocity anomaly in the lower crust beneath the caldera. The generally aseismic character of the caldera, despite abun- dant regional seismicity, may be attributed to a heated crustal column, the local effect of 13 m.y. of magmatism and emplacement of mid-crustal plutons. Seismic signals of magma movement in the deep to mid-crust may therefore be masked, and clear seismic indications of intrusion may only be generated within a few kilometres of the surface. We therefore encourage the establishment of a local dedicated volcanic monitoring system.

INTRODUCTION 11 m.y. of activity built a large volcanic ridge chemically consanguinous group. Spell et al. The Valles caldera is the type example of on the margin of the Rio Grande (1993) showed that the postresurgence a resurgent caldera (Smith and Bailey, rift. The isotopic compositions of these Valles Rhyolite units (Fig. 1) are themselves 1968). It has long been recognized that the lavas record extensive interaction between the products of four magma batches sepa- caldera complex went through at least two mantle-derived magmas and local Protero- rated by significant time intervals; the major ignimbrite cycles (Smith and Bailey, zoic crust throughout this period (DePaolo youngest batch produced the southwestern 1966). Little is known or understood about et al., 1992). At around 1.8 Ma, central ac- moat rhyolites, a series of lavas and pyro- the onset of identifiable eruptive cycles as- tivity became exclusively rhyolitic, and there clastic deposits (Fig. 1). These rocks are sociated with , because the early was eruption of high-silica rhyolitic ignim- petrologically and geochemically dissimi- products of an active cycle may be obscured brites, having strong chemical affinities to lar to all earlier caldera-related units. A or destroyed by climactic events. How such the , from the future site of hiatus of 0.46 m.y. preceded this youngest cycles begin bears on the questions of how the Valles caldera (Spell et al., 1990). The activity, by far the longest since the in- bodies of caldera-forming ash-flow magma first major episode of caldera formation at ception of exclusively rhyolitic activity at are generated, and identification of areas at 1.61 Ma accompanied the eruption of the ϳ1.8 Ma. long-term risk of major eruption. In this pa- Otowi Member of the Bandelier Tuff. Sub- per, we present data suggesting that the sequent activity produced intracaldera high- youngest eruptive products of the Valles silica rhyolite domes and associated tuffs SOUTHWESTERN MOAT RHYOLITES caldera represent a new magma batch, in- (Smith et al., 1970). At 1.22 Ma (Izett and The term SW moat rhyolites is used to dependent of earlier systems developed on Obradovich, 1994), the Tshirege Member of collectively denote the El Cajete, Battleship the same site. We take the position that the the Bandelier Tuff erupted to form the Rock, and Banco Bonito Members of the timing and character of the most recent vol- Valles caldera on approximately the same Valles Rhyolite Formation (Smith et al., canic products from the Valles caldera, in site as the earlier structure (Self et al., 1986). 1970), and the VC-1 rhyolite lava of Goff the light of geophysical considerations, may Resurgent uplift of the central part of the et al. (1986). These units and their vents signal the onset of a new active cycle. Valles was accompanied and followed by the are found in the southwestern sector of the eruption of rhyolitic lavas onto the caldera Valles caldera moat, between the caldera VOLCANIC HISTORY floor up to the late (the Valles wall and the (Fig. 1). They The volcanic field is a Rhyolite; Smith et al., 1970). Chemically are petrologically distinct from the pre- sprawling edifice of 2000 km3 with indica- and temporally, the caldera-related rhyo- ceeding high-silica rhyolite domes of the tions of activity stretching back to at least lites fall into several groups. Each of the two Valle Grande Member (Fig. 1) in consist-

13 Ma; most of its history is dominated by members of the Bandelier Tuff and the tem- ing of 72%–73% SiO2 rhyolite, with very mafic to intermediate volcanism (Smith et porally associated precursor and successor distinctive disequilibrium relations among al., 1970; Gardner et al., 1986). The first minor lavas and tuffs forms a distinct but phenocrysts.

Geology; May 1995; v. 23; no. 5; p. 411–414; 3 figures. 411

Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/23/5/411/3515857/i0091-7613-23-5-411.pdf by Rice University user on 23 March 2020 Eruption Chronology lava, which occupies much of the southwest- itic. The resorption relations and the re- Self et al. (1991) considered the three SW ern moat area. placement of biotite by hornblende suggest moat rhyolite members to be the related strongly that much of the SW moat rhyolite products of two eruptions. Our work on Petrology and Origin of the SW Moat magma was being heated up to the time of newly available exposures suggests that the Rhyolite Magma eruption. whole sequence is the result of a lengthy ep- The SW moat rhyolites display clear evi- Scarce small euhedral grains of plagio- isodic eruption, which consisted of at least dence of melt generation very shortly prior clase and hornblende, lacking reaction or re- three phases of lava extrusion, each pre- to eruption. All units are petrographically sorption textures, are scattered through the ceded by pyroclastic activity. identical, apart from those differences im- high-silica rhyolite glass, and probably crys- Initial Plinian falls were dispersed south- posed by eruptive style. Resorption and tallized from the hot, high-silica melt. The east of the vent, and were accompanied by overgrowth textures predominate among compositions of the small plagioclase crys- minor pyroclastic surges and flows. This crystals and crystal aggregates (Fig. 2), and tals (An41–54) overlap with the most calcic phase was followed by a hiatus in explosive compositions are highly varied. The zones in the large restite crystals. The small mineral assemblage is dominated by strong- activity during which a grew in hornblende crystals have similar composi- ly resorbed grains of plagioclase (An ) tions to the hornblende overgrowths. the vent; this dome was destroyed during 16–51 up to 3 mm in length, with lesser amounts of Banded mingled are found subsequent explosions and was the source , hornblende, biotite, hypersthene, au- throughout the SW moat rhyolites. The for abundant cognate vitrophyric fragments gite, olivine, , ilmenite, and acces- brown glass component in these is less silicic during the second phase. Fallout was dis- sory apatite and zircon in high-silica rhyolite (72.5%–76% SiO ) than the prevalent col- persed to the south during this phase, ac- 2 glass (76.0%–77.8% SiO2). Quartz grains orless glass. Associated with this brown glass companied by numerous and relatively vo- are invariably resorbed. Mafic typ- is a crystal population characterized by Mg- luminous pyroclastic flows. Several of these ically show complex overgrowth relations in- rich pyroxenes (Mg# up to 78), titanomag- flows ponded in San Diego Canyon to form stead of resorption. Biotite is commonly netite with up to 2.3 wt% Cr2O3, and olivine the Battleship Rock ignimbrite. This phase overgrown by hornblende (Fig. 2A). Crystal (Fo74). We interpret these crystals and glass ended with the extrusion of a lava flow, the aggregates (Fig. 2B) containing plagioclase, as representatives of a mafic magma, of VC-1 rhyolite, which is known only from the biotite, and pyroxene Ϯ quartz, are also re- probable basaltic composition, that VC-1 corehole (Goff et al., 1986; Fig. 1). placed by hornblende to varying degrees. has mixed and largely hybridized with the After an indeterminate period, during The internal textures of these aggregates rhyolitic magma. which some erosion of the partly welded top (Fig. 2B) are more suggestive of completely The chief features of the petrology of the of the Battleship Rock ignimbrite occurred, solidified than of partly crys- SW moat rhyolites, as outlined above, are the third eruptive phase commenced with tallized, glass-bearing cognate ‘‘cumulate’’ predicted in almost every detail by the dy- emplacement of a pyroclastic flow in the fragments of the type described by Tait namic model of Huppert and Sparks (1988) caldera moat. This was followed by effusive (1988). We regard all of these inclusions, for the generation of silicic magma by fusion activity, production of the Banco Bonito both single crystals and aggregates, as rest- of crustal rocks at the upper boundary of a

Figure 1. Maps showing location of Jemez Moun- tains volcanic field (JMVF) (solid square) at edge of in northern New Mexico (in circle) and location of JMVF and Valles caldera with re- spect to major population centers (upper right). Random dash pattern is present extent of Bande- lier Tuff; details of caldera geology are on main map on left (after Smith et al., 1970). Heavy line is topo- graphic rim of caldera; di- agonal shading is resur- gent dome; small crosses are postresurgent high- silica rhyolite lava domes and flows (Valle Grande Member of Valles Rhyo- lite); random pattern is Battleship Rock Member; heavy stipple is El Cajete Member; circle pattern is Banco Bonito Member. Numbers on Valle Grande domes are ages (in ka), from Spell and Harrison (1993). Note location of SW moat rhyolites vent area (hachured ovoid) and VC-1 CSDP core hole.

412 GEOLOGY, May 1995

Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/23/5/411/3515857/i0091-7613-23-5-411.pdf by Rice University user on 23 March 2020 convecting mafic magma body emplaced Grande high-silica rhyolite eruption of to ϳ60 Ϯ 15 ka, and the 14C and TL data into the crust. In this model, a proportion of South . The SW moat rhyolites are suggest an age within a few thousand the melting rock may survive in the anatectic essentially undateable by U-Th disequilib- of 60 ka. This young age of the SW moat liquid as restite, and is accompanied by crys- rium methods (Self et al., 1991). Spell and rhyolites is very significant. These eruptions tallization of the same assemblage as heat is Harrison (1993) showed that SW moat rhyo- represent the first activity to have occurred lost to the melting roof (Fig. 3). Eruption of lite biotites contain excess Ar and for the for almost half a million years in the Valles the magma at this stage would preserve the most part give geologically unreasonable old caldera. restite textures, and cause some mingling ages, but they argued that the youngest ap- with the subjacent mafic magma through parent 40Ar/39Ar age of 205 ka provides an GEOPHYSICAL CONSIDERATIONS drawdown effects. Thus, the SW moat rhyo- older age limit for the SW moat rhyolites. If silicic melt were currently being gener- lite magma erupted while it was still being Toyoda et al. (1995) obtained eruptive ated beneath the Valles caldera, as hap- generated through melting of an intrusive— ages of 45 to 73 ka by electron spin reso- pened with the SW moat rhyolites, high tem- probably granodioritic—rock body by basal- nance (ESR) dating of quartz grains from peratures in the subcaldera crust may tic andesite magma emplaced into the sub- the El Cajete and Battleship Rock units of actually serve to mask seismic events that caldera crust (Fig. 3). the SW moat rhyolites. Consistent with this may be heralding impending volcanic are 14C determinations on carbonized logs eruptions. A number of studies have iden- Age of the SW Moat Rhyolites that indicate an age greater than 58 ka, and tified seismic anomalies, including low-ve- The SW moat rhyolites rocks have proven thermoluminescence (TL) dating of pa- locity zones and/or attenuation of seismic extraordinarily difficult to date, due largely leosols beneath the El Cajete fall deposits waves, at shallow to mid-crustal depths be- to the unequilibrated nature of the crystal that constrain soil burial to Ͻ60 ka (S. L. neath Valles caldera (Ankeny et al., 1986; assemblage. Published fission-track ages Reneau, 1994, personal commun.). Roberts et al., 1991, 1995; Lutter et al., (Marvin and Dobson, 1979; Miyachi et al., The ESR results (Toyoda et al., 1995) 1995). A 5-km-thick low-velocity zone was 1985) have large errors and do not constrain constrain the age of the SW moat rhyolites identified recently beneath the caldera at the time of the SW moat rhyolite eruptions within the 0.52 m.y. since the latest Valle

Figure 2. Photomicrographs of restite tex- ,plagioclase ؍ tures in SW moat rhyolites. P ؍ glass, O ؍ biotite, G ؍ hornblende, B ؍ H pyroxene. A: Ragged biotite ؍ opaque, and PX with hornblende jacket, Banco Bonito Mem- ber. Note optical continuity of biotite patches. Figure 3. Diagram illustrating thermal and dynamic structure of SW moat rhyolite magma, at mm. B: Clot consisting of mid-crustal levels prior to eruption, after Figure 7 of Huppert and Sparks (1988); see text. Note 1.1 ؍ Field width plagioclase, hornblende partly replacing bi- mixing of relatively cool new melt containing restite with hotter rhyolite produced at earlier otite, and minor pyroxene and opaque miner- stage during melting event. Eruptive withdrawal further mixes liquids and their crystal popu- .mm. lations, including subjacent mafic magma, which provides heat source 2.2 ؍ als. Field width

GEOLOGY, May 1995 413

Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/23/5/411/3515857/i0091-7613-23-5-411.pdf by Rice University user on 23 March 2020 depths around 30 km, presumably at the with potential for future volcanism, includ- Mullineaux, D.R., 1976, Preliminary overview base of the crust (Lutter et al., 1995). The ing large explosive eruptions. On the basis of map of volcanic hazards in the 48 contermi- mid-crustal anomalies are attributed to high the past record, any future eruption would nous : U.S. Geological Survey Miscellaneous Field Studies Map MF786, temperatures and possible pockets of resid- probably be, in part, explosive. We encour- scale 1:7,500,000. ual melt associated with the cooling pluton age establishment of a dedicated monitoring Roberts, P. M., Aki, K., and Fehler, M. C., 1991, that produced the Bandelier Tuff. system, including seismographs, tilt meters, A low-velocity zone in the basement be- Also attributed to the effects of the large, and periodic leveling surveys. neath the Valles caldera, New Mexico: Journal of Geophysical Research, v. 96, cooling pluton is a notably aseismic zone co- ACKNOWLEDGMENTS p. 21,583–21,596. incident with the caldera, in contrast to This work was supported by the U.S. Depart- Roberts, P. M., Aki, K., and Fehler, M. C., 1995, abundant surrounding seismic activity (Cash ment of Energy Office of Basic Energy Sciences, A shallow attenuating anomaly inside the and Wolff, 1984). The combined thermal ef- the University of Texas at Arlington, the Univer- ring fracture of the Valles caldera, New Mex- fects of long-lived precaldera magmatism sity of , and Associated Western Uni- ico: Journal of Volcanology and Geothermal versities, Inc. We thank the Baca Land and Research (in press). and the emplacement of mid-crustal plutons Co. and Copar Mining for allowing access to crit- Self, S., Goff, F., Gardner, J. N., Wright, J. V., and have resulted in elevation of the brittle-duc- ical areas. Steve Self, Steve Reneau, Terry Spell, Kite, W. M., 1986, Explosive rhyolitic volcan- tile transition to very shallow levels beneath Fraser Goff, Tom Kolbe, John Carney, Mike ism in the Jemez Mountains: Vent locations, the caldera. Thus, with such a thermal struc- Fehler, Leigh House, Peter Roberts, Lee Steck, caldera, development and relation to re- gional structure: Journal of Geophysical Re- ture of the crust, deep intrusion of new and Scott Baldridge provided discussion and help. Constructive reviews of the manuscript were pro- search, v. 91, p. 1779–1798. magma batches, such as may be represented vided by Bob Christiansen, Steve Reneau, Terry Self, S., Wolff, J. A., Skuba, C. E., Morrisey, by the low-velocity zone 30 km beneath Spell, D. M. Fountain, and an anonymous M. M., and Spell, T. L., 1991, Revisions to Valles caldera (Lutter et al., 1995), or pos- reviewer. the stratigraphy and volcanology of the post- sibly even mid-crustal intrusions, might have 0.45 Ma units and the volcanic section of REFERENCES CITED VC-1 corehole, Valles caldera, New Mexico: little or no seismic signature, and seismic Ankeny, L. A., Braile, L. W., and Olsen, K., 1986, Journal of Geophysical Research, v. 96, monitoring will reveal intrusive, prevolcanic Upper crustal structure beneath the Jemez p. 4107–4116. events only within a few kilometres of the Mountains volcanic field, New Mexico, de- Smith, R. L., 1979, Ash-flow magmatism, in surface. termined by three-dimensional simultaneous Chapin, C. E., and Elston, W. E., eds., Ash- inversion of seismic refraction and earth- flow tuffs: Geological Society of America quake data: Journal of Geophysical Re- Special Paper 180, p. 5–28. DISCUSSION search, v. 91, p. 6188–6198. Smith, R. L., and Bailey, R. A., 1966, The Ban- There are a number of possible interpre- Cash, D. J., and Wolff, J. J., 1984, Seismicity of the delier Tuff: A study in ash-flow eruption cy- tations of the significance of the SW moat Rio Grande rift in northern New Mexico, cles from zoned magma chambers: Bulletin rhyolites in terms of their age, petrology, 1973–1983: New Mexico Geological Society Volcanologique, v. 29, p. 83–104. Guidebook 35, p. 25–28. Smith, R. L., and Bailey, R. A., 1968, Resurgent and place within the behavior pattern of the DePaolo, D. J., Perry, F. V., and Baldridge, W. S., cauldrons, in Coats, R. 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J., Thurber, C. H., Roberts, P. M., caldera, New Mexico: Journal of Geophysi- scale of the mafic intrusive activity, which is Steck, L. K., Fehler, M. C., Stafford, D., cal Research, v. 98, p. 19,723–19,739. Baldridge, W. S., and Zeichert, T. A., 1995, Tait, S., 1988, Samples from the crystallising unconstrained, will dictate the longevity of Teleseismic P-wave image of crust and upper boundary layer of a zoned : this cycle. Whereas the SW moat rhyolite mantle structure beneath the Valles caldera, Contributions to Mineralogy and Petrology, eruption may represent an isolated mag- New Mexico: Initial results from the 1993 v. 100, p. 470–483. matic event, it is equally possible that the JTEX passive array: Geophysical Research Toyoda, S., Goff, F., Ikeda, S., and Ikeya, M., lower crustal low-velocity zone detected by Letters, v. 22 (in press). 1995, ESR dating of quartz phenocrysts in Marvin, R. F., and Dobson, S. W., 1979, Radio- the El Cajete and Battleship Rock members Lutter et al. (1995) may represent the metric ages: Compilation B, U.S. Geological of the Valles Rhyolite, Valles caldera, New present-day expression of renewed magma- Survey: Isochron/West, no. 26, p. 25. Mexico: Journal of Volcanology and Geo- tism. If so, there is the possibility of a sig- Miyachi, M., Izett, G. A., Naeser, C. W., Naeser, thermal Research (in press). nificant volcanic hazard to communities in N. D., and Andriessen, P. A. M., 1985, Zircon fission-track ages on some volcanic rocks and Manuscript received September 30, 1994 and around the Jemez Mountains, including pyroclastic flow deposits of the Jemez Moun- Revised manuscript received January 25, 1995 Los Alamos National Laboratory. Mul- tains, New Mexico: Volcanological Society of Manuscript accepted February 6, 1995 lineaux (1976) identified the area as one Bulletin, v. 30, p. 90.

414 Printed in U.S.A. GEOLOGY, May 1995

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