Holocene tephrochronology of the Hualaihue region (Andean southern volcanic zone, ∼42° S), southern Chile

Quaternary International 246 (2011) 324e343

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Holocene tephrochronology of the Hualaihue region (Andean southern volcanic zone, w42
S), southern Chile Sebastian F.L. Watt a, b, *, David M. Pyle a, José A. Naranjo c, Gunhild Rosqvist d, Mauricio Mella e, Tamsin A. Mather a, Hugo Moreno c a

Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, UK School of Ocean and Earth Science, National Oceanography Centre Southampton, University of Southampton, European Way, Southampton SO14 3ZH, UK Servicio Nacional de Geología y Minería, Av. Santa María, 0104 Santiago, Chile d Department of Physical Geography and Quaternary Geology, Stockholm University, Stockholm S-106 91, Sweden e Servicio Nacional de Geología y Minería, La Paz 406, Puerto Varas, Chile b c

a r t i c l e i n f o

a b s t r a c t

Article history: Available online 12 June 2011

Late Glacial and Holocene soils and sediments in southern Chile contain an important record of explosive volcanic activity since the end of the last glaciation, and have considerable potential for the development of a regional tephrostratigraphical framework. This paper reports the discovery of several new tephra deposits from the Hualaihue region (w42
S) of southern Chile. Eruption sizes, constrained from field observations, and ages, constrained by 25 new radiocarbon dates, show that the volcanoes of the Hualaihue peninsula have had relatively few explosive, tephra-generating eruptions during the Holocene. An eruption of Apagado deposited w1 km3 of bedded basaltic scoria at w2.6 calibrated (cal) ka BP, and Hornopirén produced a similar, but volumetrically-smaller unit at w5.7 cal ka BP. Activity at Yate over the same time period has been predominantly characterised by lava production, although small explosive eruptions, the products of which span a range of compositions, have also occurred, including one at w0.9 cal ka BP. The northern part of the regional tephra sequence is dominated by andesitic pumice fall deposits derived from Calbuco volcano. These include deposits from several eruptions during a 3500-year-long period at the start of the Holocene, as well as two large explosive eruptions in the past 2000 years. A distinctive rhyolitic tephra layer that is interbedded with the locally derived tephra sequence is the Cha1 unit, from Chaitén volcano, 108 km south of Hornopirén. This rhyolitic pumice deposit, dated at w9.75 cal ka BP, is the largest volumetrically of those described here, with a volume of 3.5 km3. This new tephrostratigraphy covers a region whose volcanic history was previously very little known, and contributes to a regional record of large explosive eruptions that now spans a 500 km-long segment of the southern Andean arc, between Calbuco and Hudson volcanoes. Ó 2011 Elsevier Ltd and INQUA. All rights reserved.

1. Introduction The Andean southern volcanic zone (SVZ), which extends from Tupungatito (33.4
S) to Hudson (45.9
S), is the source of a rich, but incompletely studied, tephra record of Late Glacial to Holocene volcanism (Stern, 2004, 2008). Volcanoes in the southern part of the SVZ all lie in Chile (Fig. 1). Many of these edifices are little known, with historical records stretching back fewer than 100e200

* Corresponding author. School of Ocean and Earth Science, National Oceanography Centre Southampton, University of Southampton, European Way, Southampton SO14 3ZH, UK. Fax: þ44 (0)2380 593052. E-mail address: [email protected] (S.F.L. Watt). 1040-6182/$ e see front matter Ó 2011 Elsevier Ltd and INQUA. All rights reserved. doi:10.1016/j.quaint.2011.05.029

years (cf. Siebert and Simkin, 2002). The prevailing westerly winds and continental setting of the arc provide good potential for the preservation of tephra deposits, both within the Andes (e.g., Naranjo and Stern, 2004) and downwind in the lakes and basins of southern Chile and Argentina (e.g., Haberle and Lumley, 1998; Heusser, 2003; Corbella and Lara, 2008; Bertrand et al., 2008). Studies of such tephra sequences provide the primary evidence for the timing, composition and size of pre-historic explosive eruptions. When multiple tephra units are preserved in sequences containing datable material, this combination allows the development of a tephrostratigraphical framework, with the potential to link (using tephrochronology) marine and terrestrial records of changing climates during the late Quaternary. In addition, when tephra units have a sufficient spatial distribution, tephra deposition

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Fig. 1. Left: Volcanoes of the southern segment of the Andean southern volcanic zone (SSVZ), with the location of the main map outlined. Right: Hualaihue regional map showing all field sites used in compiling the regional tephrostratigraphy. Labelled sites refer to the stratigraphic columns in Figs. 3 and 4. The main lineament of the Liquiñe-Ofqui fault zone (LOFZ), coincident with fjords to the north and south of Hualaihue, is also indicated.

models (cf. Pyle, 1989, 1995) may be used to estimate eruption parameters (e.g., Carey and Sparks, 1986; Sparks et al., 1992). The importance of regional tephrostratigraphical studies is threefold: (1) they form the basis for inferring regional-scale explosive eruptive histories, essential for hazard assessments and for estimating long-term volcanic mass fluxes (White et al., 2006); (2) they provide valuable time markers (isochrons) in natural archives of environmental change (e.g., de Fontaine et al., 2007; Bertrand et al., 2008); and (3) they ultimately allow investigation of climatic and other controls on eruption frequency (e.g., McGuire et al., 1997; Nowell et al., 2006). The Late Glacial to Holocene tephra stratigraphy of the southernmost SVZ (Chaitén to Hudson; Fig. 1) has been previously studied, although only in limited detail, with the aim of identifying the major tephra deposits of the region (Haberle and Lumley, 1998; Naranjo and Stern, 1998, 2004). Eruption histories of several individual volcanoes in the central part of the SVZ have also been investigated (e.g., Clavero and Moreno, 2004; Lara et al., 2004; Singer et al., 2008). However, the volcanoes between Chaitén and Calbuco (Fig. 1) are little known. This paper describes a stratigraphy and chronology for Holocene explosive eruptions from the Hualaihue region (41.3e42.3
S), including tephra deposits from the arc segment between Chaitén and Calbuco. It recognises for the first time explosive eruptions from Yate, Hornopirén, Apagado and

Calbuco, and provides age estimates for these events. This work fills a major gap in the regional tephra stratigraphy, which is now nearly complete for large explosive eruptions between Calbuco and Hudson, a distance of over 500 km. This dataset opens possibilities for future work investigating regional patterns in volcanic activity since the retreat of the Patagonian ice sheet at the end of the last glaciation. 2. Study region Volcanoes of the southern SVZ (SSVZ) have edifices built both of lava flows, often showing evidence of sub-glacial or ice-contact volcanism, and interbedded pyroclastic and epiclastic deposits. Most of these volcanoes are hard to access and poorly studied, but pose a continuing threat to the local and regional infrastructure, from both eruptive activity and landslides (e.g., Watt et al., 2009a, 2009b). This study focuses on the remote and forested Hualaihue peninsula (Fig. 1), at the northern end of the SSVZ, and the region immediately surrounding it. Three late Quaternary volcanoes lie on the peninsula itself: Yate, a sprawling and heavily eroded edifice, is by far the largest; Hornopirén forms a youthful symmetrical cone; while Apagado consists of a monogenetic scoria cone nested within an eroded Pleistocene volcanic complex. Fjords to the north and

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south of Hualaihue mark the main lineament of the regional-scale Liquiñe-Ofqui fault zone (LOFZ; Cembrano et al., 1996). The nearest volcanoes to the north of Yate are a cluster of monogenetic cones around the head of the Reloncaví fjord, 45 km away, known as the Cayutué-La Viguería group (López-Escobar et al., 1995). At the same latitude, a little to the west, lies the Calbuco stratovolcano (41.33
S), similar in size to Yate. Several tephra deposits from Calbuco are documented here. Osorno, 30 km NNE of Calbuco, has also been highly active in the Holocene (Siebert and Simkin, 2002), but has been dominated by lava production, with relatively low levels of explosive eruption. If present within the study region, tephra units from Osorno would be likely to have a composition clearly distinguishable from the Calbuco deposits (cf. López-Escobar et al., 1995); to date, no such units have been recognised. South of Hualaihue, Huequi (42.38
S) is known to have had Holocene explosive activity (Watt et al., in press), but no widespread tephra deposits from the volcano have yet been recognised. Further south, tephra deposits from Chaitén and Minchinmávida (w42.8
S) are described by Naranjo and Stern (2004). Unlike the eastward dispersal of most regional tephra deposits, the Cha1 unit from Chaitén (Naranjo and Stern, 2004), was transported northwards. This distinctive rhyolitic pumice deposit has the potential of overlapping with the tephra stratigraphy in Hualaihue, thus tying the stratigraphy with that further south. Within the arc segment under study (Chaitén to Calbuco), historical eruptions (Siebert and Simkin, 2002) are known from Calbuco, as recently as 1973, Huequi, between 1890 and 1907, and Chaitén, in 2008. This last event produced a measurable tephra deposit that reached the Atlantic coast of Argentina (Watt et al., 2009b). Near F

3. Methods The SSVZ is characterised by a thick (1e5 m) Late Glacial to Holocene soil and vegetation cover (Fig. 2). Given the lack of previous tephra studies in the region, our focus was on visible tephra horizons preserved in soil sequences (here referred to as terrestrial settings). Consequently, the units described here are macroscopic, with thicknesses on the order of centimetres (Fig. 2), and represent the proximal to medial deposits of relatively large explosive eruptions. This study provides a framework for more specific and detailed investigations based on lake and marine sediment cores, for which many suitable sites exist both east of the Andes in Argentina, and west of the Andes in the Chilean fjords (e.g., Whitlock et al., 2006; Daga et al., 2008; G. Siani, personal communication, 2010). Field observations were used to develop stratigraphic correlations, the consistency of which were confirmed by deposit chemistry, as described below (Section 3.2). Radiocarbon dating of interbedded carbon samples enabled completion of a refined chronology for the region’s Late Glacial to Holocene explosive eruptions.

3.1. Field sampling The Hualaihue peninsula is densely vegetated and poorly accessible. The best soil profiles were provided by road cuttings along the relatively limited local road network. A single main road runs around the coast, continuing northwards to join a more extensive network near Calbuco. To the south, the road stops at Pichanco (Fig. 1). Grey pumice lapilli (Ca13)

O

30 cm

White ash (Cha1) Bedded scoria (Ap1)

1m

326

Orange pumiceous ash (Ca1)

Glacially eroded granitic bedrock

Soil

Hornopirén lava K

U

Grey lapilli (Ca12 & 13)

Ca7 Cha1

1m

? (Ca3-5) thin ash units

Yate lava

Soil

Angular lithics and soil

Lithified lahar deposits

White coarse ash (Cha1)

2m

Stony soil

Lenticular yellow pumice (Ca2) Orange pumice (Ca1)

Granitic bedrock, glacially eroded

Fig. 2. Photographs of field sites showing selected tephra deposits and sequences. Site labels refer to locations in Fig. 1 and to the stratigraphic columns in Figs. 3 and 4.

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Fieldwork was conducted in January and February, in 2007 and 2008. Sampling and measurement sites are shown in Figs. 1 and 2. Road cuttings were cleared of vegetation and debris using a steel spade. Measurements were made of unit thicknesses and, where possible, of the maximum grain-size, estimated from the five coarsest grains. Bulk samples of tephra fall deposits or of individual pumice and scoria grains were taken at several sites, providing a comprehensive set of over 100 samples. Additional samples were taken by soil auger (column C, Fig. 3), and by Russian corer at bogs near Apagado and Yate, providing samples of organic mud encapsulating tephra deposits. Penetration with both methods was very poor, and in general there were very few suitable sites for core sampling. However, additional data are included from two short (w1 m) lake sediment cores, collected in 1993 from two small intermontane lakes SE of Hornopirén (Fig. 1), using a 90-mm diameter modified Livingstone corer. These cores preserve the upper part of the tephra sequences identified elsewhere in Hualaihue. Initial correlations between identified terrestrial tephra fall deposits were made based on stratigraphic order and the physical characteristics of the deposits, such as colour, grain-size, sorting, lithology and texture. Samples were cleaned after collection by soaking in 50% molar volume hydrogen peroxide for 48 h, washing in 6.5% molar volume sodium hypochlorite, and multiple rinsing by deionised water, before being dried at 50
C. Coarser-grained samples were then sieved and measured for maximum grain-size. 3.2. Chemical analyses The macroscopic tephra deposits identified within the study region were principally correlated through their stratigraphic relationships and physical characteristics. Chemical analyses were undertaken to check the consistency of this correlation. The purpose of these measurements was also to characterise these newly identified tephra deposits and to confirm the identification of source volcanoes. Thus, for units which were easily correlated stratigraphically across multiple sites, and whose source was identifiable on this basis, only a single fresh, coarse-grained sample was analysed for its bulk chemical composition, simply to provide a more comprehensive characterisation and allow comparison with other units. Regional geochemical trends (e.g., López-Escobar et al., 1993; Naranjo and Stern, 2004) of young volcanic rocks show distinct compositional differences between individual volcanoes within this arc segment. Bulk chemical composition, combined with deposit distribution, was therefore sufficient for the identification of source volcanoes. In several of the pumiceous deposits along the Reloncaví fjord, predominantly from Calbuco, the matrix glass is deeply weathered. Weathering potentially affects bulk compositions (Dugmore et al., 1992; Gislason et al., 1996; Aiuppa et al., 2000), particularly for highly mobile elements, but the trace element fingerprint (Section 4.1.2) of these samples was nevertheless adequate to confirm the source volcano of these deposits. However, this approach was inadequate to discriminate between some of the Calbuco fall deposits, which have highly similar bulk chemical compositions. Consequently, there is some ambiguity in the correlation of less-widely preserved and volumetrically-smaller Calbuco fall deposits (see Section 4.1.2.2). Bulk chemical compositions were also used to correlate distal deposits of a tephra from Apagado to its proximal equivalent, an ambiguity that could not be resolved stratigraphically. For particularly fresh, coarse-grained samples from Apagado, Yate and Hornopirén, bulk tephra compositions were measured by X-ray fluorescence spectrometry (XRF). For more weathered units, commonly finer-grained and with smaller sample sizes, trace

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element measurements were made on bulk tephra samples using inductively-coupled plasma mass spectrometry (ICP-MS). For ICPMS analyses, either picked scoria or pumice grains (>1 mm diameter) or bulk splits of samples for finer ash deposits, were powdered in an agate ball mill or agate pestle and mortar, and digested in 1 ml HF and 4 ml HNO3 in pressure bombs at 150
C for 24 h. Samples were dried and redissolved in 2 ml HCl and 0.6 ml HNO3, dried, and twice redissolved and dried in 2 ml HCl. Final dissolution was in 1 ml HNO3 and 3 ml MilliQ water, diluted to a mass of 50 g with MilliQ water for short term storage. U.S. Geological Survey standards RGM-1, G-2, BHVO-2 and BCR-2 were used for correlation. Some of the more silica-rich samples were digested at lower pressures (on a hot plate) to avoid formation of insoluble fluorides. Prior to analysis, 50 ml of each digestion solution was prepared with an indium spike to give a dilution of 104. Mass spectrometer (Thermo-Finnegan Element 2 ICP-MS) measurements were calibrated with synthetic standards. BHVO-4 was used as a drift standard, and results were corrected against digestion blanks. For the basaltic samples, nearly all elements showed good recoveries based on reference standard compositions. Due to digestion problems for some more silica-rich samples, resulting from fluoride formation, a restricted range of elements was selected for sample correlation, including Ti, K, V, Cr, Co, Zr, Nb, Ba, Hf and Ta. These elements provided sufficient compositional variation to confirm the stratigraphic correlations. Where there was analytical overlap, ICP-MS results were consistent with XRF measurements. In addition to the above, fine ash samples within the two lake sediment cores were measured for particle grain-size using a laser granulometer. These samples were also analysed for the major element compositions of matrix glass, measured by electron microprobe at the University of Edinburgh. Measurements with analytical totals below 97 wt% or anomalous compositions, likely to arise from analyses of microlite mineral phases within the glass, were rejected. These data were compared with matrix glass major element compositions of mounted scoria grains from Hornopirén and Apagado, measured by electron microprobe at the University of Oxford. Similar analyses were also made on the matrix glass of mounted pumice grains from distal and proximal samples of the Cha1 tephra fall deposit, from Chaitén, to confirm the deposit correlation and further characterise this important regional marker horizon. Plotted glass compositions in this paper have been normalised to 100% anhydrous compositions. All glass compositional data, as well as bulk tephra deposit compositions, are provided as Supplementary Data. 3.3. Dating Radiocarbon (14C) dating of organic material sampled from as near as possible to the bases or tops of tephra deposits was used to fix ages to the stratigraphy. From over 50 samples of suitable organic material collected in 2007 and 2008, most of which were charcoal, 17 were analysed. Samples were collected from several centimetres beneath exposed surfaces to reduce the potential for modern contamination, transferred to sealed polythene bags, dried at room temperature if necessary, and then refrigerated until analysis. In addition, five dates from samples collected by J.A. Naranjo around the north end of the Reloncavi fjord in 1998, and three dates from the two lake sediment cores (Fig. 1; collected by G. Rosqvist) are included. Two published radiocarbon dates (Naranjo and Stern, 2004) from locations near Chaitén volcano also help constrain the age of the Cha1 unit. Radiocarbon ages for the 2007e2008 samples were determined by accelerator mass spectrometry (AMS) at the NERC Radiocarbon Facility (SUERC, East Kilbride, U.K.). Carbon samples (except for samples 24-4A and 803-4A) were digested in 2 M HCl (80
C, 8 h),

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Fig. 3. Hualaihue tephrostratigraphy for sections south of Hornopirén (top) and between Puelche and Lago Tagua Tagua (bottom; see inset map and Fig. 1 for site locations). Original field site identifiers are marked at the top of each column, below the column letter. Some columns represent a composite of information gathered from more than one field site (all within <1 km; marked as measurement sites on Fig. 1). Sample names and positions (which relate to original field site names) are shown on the right of each column in stratigraphic position. Red triangles mark tephra samples analysed for bulk chemical composition (ICP-MS), while blue diamonds show sampling positions for radiocarbon dating. Coloured bands link correlated units, named in bold (Table 1). The horizontal scale showsapproximated mean grain-size, with the basal increment representing a phi grade of 3, and subsequent increments at 0, 3 and 6 phi (1, 8 and 64 mm respectively, representing coarsening from medium-coarse ash to coarse lapilli). (For interpretation of the colour indicated in the caption, please refer to the online version of this paper.)

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washed free of acid with deionised water, then dried and homogenised. Sample 24-4A (wood) was broken into small pieces, digested in 4 M HCl (80
C for 8 h), washed free of acid with deionised water and digested in 2 M KOH. The digestion was repeated using deionised water until no further humics were extracted. The residue was washed free of alkali with deionised water and digested in 1 M HCl (80
C, 5 h) and rinsed free of acid with deionised water. The dehumified wood was digested at 70
C in acidified sodium chlorite solution (13 g NaClO2 þ 2 ml concentrated HCl in 500 ml deionised water) until the entire sample had been oxidised to cellulose. The cellulose was filtered through glass fibre filter paper (Whatman GF/ A), washed acid free with hot deionised water and dried in a box oven. Sample 803-4A was digested in 2 M HCl (80
C, 8 h), washed free from acid with deionised water then digested in 1 M KOH (80
C, 2 h). The digestion was repeated using deionised water until no further humics were extracted. The residue was rinsed free of alkali, digested in 1 M HCl (80
C, 2 h) then rinsed free of acid, dried and homogenised. The total carbon in a known weight of the pre-treated sample was recovered as CO2 by heating with CuO in a sealed quartz tube. The gas was converted to graphite by Fe/Zn reduction. Results are reported as conventional radiocarbon years BP (relative to AD 1950), normalised to d13CVPDB&-25 using the d13C values measured on a dual inlet stable isotope mass spectrometer (VG OPTIMA). The quoted precision is the uncertainty of repeated measurements of the same CO2 aliquot. The Reloncaví fjord samples were dated at the Beta Analytic Inc. Laboratories (Miami, U.S.A.), and are reported in the same way. Four of these samples (charcoal) were analysed using a standard radiometric method of converting the carbon to benzene and measuring 14C by liquid scintillation counting and normalising to d13CVPDB&-25; a further organic sediment sample was analysed by AMS. The lake sediment dates are for samples of terrestrial macrofossils dated by AMS methods. All radiocarbon dates were calibrated with OxCal4.0 (Bronk Ramsey, 2009) using the Southern Hemisphere curve SHCal04 (McCormac et al., 2004). This calibration method produces a probabilistic result defined by a distribution curve across an age range. 4. Results 4.1. Stratigraphy of tephras Field measurements were used to construct detailed stratigraphic logs across the Hualaihue region (Figs. 3 and 4), which have been divided into three sections (Fig. 1). The section south of Hornopirén and Apagado (sites AeI) offers several short, simple sequences, dominated by bedded scoria deposits. North of Yate, a section runs from west to east, from Puelche to Puelo to Lago Tagua Tagua (JeQ). Profiles are shallow, usually <2 m, but the stratigraphy is more complex than that within the southern section. This complexity increases along the section running north from Puelo to Cochamó and the head of the Reloncaví fjord (ReZ). Here, numerous tephra deposits occur in deep profiles of several metres thickness. Throughout the region these profiles directly overlie glacial till deposits or scoured granitoid bedrock. Because the focus of this study is on macroscopic tephra deposits, from (presumably) relatively large explosive eruptions, the intervening soil material is not discussed in detail. These soil materials are dominantly andic in character, and form Andisols. The volcanic ash component within these upbuilding but pedogeneticallyaltered materials is likely to include contributions from numerous small eruptions as described, for example, by Alloway et al. (1995) and McDaniel et al. (in press). Such upbuilding soil sequences are difficult to unravel stratigraphically because the thin and finegrained constituent tephras are intermixed. Consequently, examination of lake sediment cores is likely to provide a better prospect for

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developing records of these deposits from smaller explosive eruptions. In general, the thickness of the regional soil cover increases northwards, in line with the number of identified tephra horizons, and this pattern is consistent with an overall more rapid accumulation of fine-grained volcanic material along the Reloncaví section. The andic material within these soils is dominated by highlyweathered, fine-grained volcanic ash. These soils are also rich in angular lithics, indicative of slope reworking, and a minor pumiceous component, likely to originate from larger Calbuco eruptions. Further south, along the Puelche to Lago Tagua Tagua section, the soils are similar in nature, with much of the volcanic material derived from lahar deposits around the north flank of Yate. Throughout this area, the organic component of the soils is low, but it is higher to the south in the shallower soil profiles around Apagado and Hornopirén. These soils are darker and less dominated by primary volcanic material, consistent with relatively low levels of Holocene explosive volcanism on the Hualaihue peninsula. The two lake sediment cores are from separate but nearby lakes (Fig. 1). They preserve stratified fine ash deposits interbedded with sediments comprising glaciogenic clays, mixed fluvial lithics and organic detritus, as well as reworked tephra (Fig. 5). Core LN2 is from a more stable environment, and is dominated by laminated clays, whereas LV is from a lake with a steeper catchment and contains more reworked sediment with coarser silts and sands. Both cores have a base within an indurated ash deposit. Correlation of the cores with the mapped terrestrial sequence has been made on the basis of stratigraphic order, distributional constraints, age and matrix glass chemistry. The correlated terrestrial tephra stratigraphy is shown in Figs. 3 and 4, and the lake core stratigraphy in Fig. 5. Locations of samples used for dating and chemical analyses are also shown. Many previously unknown tephras have been identified (Table 1) and the distribution of Cha1 has been extended and refined. 4.1.1. Unit descriptions Stratigraphic overlap constrains the relative age sequence of the >20 tephra deposits recognised in the Hualaihue region. For each unit, descriptions based on defined reference localities are given in Table 1, in postulated stratigraphic order. All units have a broadly eastward dispersal, except for a northward directed white pumice deposit, which is correlated both in terms of composition and dispersal with the Cha1 tephra (Naranjo and Stern, 2004). This tephra deposit erupted from Chaitén, 108 km south of Hornopirén volcano, and is the only unit documented here that has previously been named. Since Cha1 crosses the entire study region, it is useful for constraining the ages of disparate stratigraphic sections. The Hualaihue regional tephras may be split into two broad types. The first are dark, stratified scoria deposits, which thin rapidly and have dispersal distances on the order of tens of kilometres. Single deposits of this type are identified from Apagado, Hornopirén (Ap1 and Ho1; Fig. 3) and from one of the La Viguería cones (LaVig; Fig. 4). These bedded deposits are a result of vigorous but pulsed explosions. The second type are more widely dispersed pumice deposits, generally with more evolved bulk compositions, produced by Yate, Calbuco and Chaitén. A very young pumice fall deposit has been recognised from Yate (Ya2), with poorly constrained thickness and distribution. It was identified as low-density pumices several centimetres in diameter scattered on the SE flanks of Yate, and as discrete lenses of reworked, rounded pumice grains in stream gullies SW of Yate. No primary fall deposit of Ya2 was found within the exposed terrestrial stratigraphy, but the unit has been correlated with an evolved ash preserved in the upper parts of the LN2 and LV lake sediment cores (Fig. 5). This ash is compositionally bimodal: the matrix glass

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Fig. 4. Hualaihue tephrostratigraphy for sections north of Puelo. See Fig. 1 for locations. Caption as for Fig. 3.

extends between dacitic (corresponding to the Ya2 pumices found on Yate’s flanks) and basaltic compositions. A bimodal composition is characteristic of another fall deposit, Ya1, identified by Mella (2008) and associated with a scoria cone on the south flank of Yate. Ya1 is dominated by basaltic andesite scoria but also contains intermingled rhyolitic pumice. The rhyolite is rich in plagioclase and contains quartz and biotite phenocrysts, and the basaltic andesite contains olivine and plagioclase phenocrysts. Ya1 is Holocene in age, but otherwise unconstrained chronologically. The Ya1 and Ya2 deposits suggest that mixing of primitive and evolved magma has resulted in explosive volcanism at Yate on more than one occasion in the Holocene, producing relatively small eruptions

with limited tephra dispersal and preservation. In addition to these fall deposits, a minor scoria fall deposit is associated with a lava flow on the north flank of Yate (YaSc). At the base of the lake cores (Fig. 5) is a black, stratified ash, coarser than any of the tephra deposits above. The glass composition of this ash could correlate with Ho1 or Ap1, since the glass composition at the top of Ap1, in spite of a more mafic bulk deposit composition, overlaps with that of Ho1 (Fig. 5). However, given the age and thickness of this basal ash, it is correlated with Ap1. Measurements of Ap1 based on the terrestrial stratigraphy suggest a thickness at the core sites of w0.5 m. Allowing for some over-accumulation within the lake, this thickness estimate is consistent with that in the lake cores. Between Ap1 and

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Fig. 5. Stratigraphy of lake sediment cores collected SE of Hornopirén (location in Fig. 1). Lake water depths were 8 m for LN2 and 40 m for LV. Identified tephra units (Table 1) correlated between the cores are shown. HoSc is a scoria unit, suggested to be from Hornopirén. Red triangles mark samples for tephra glass analyses (electron microprobe), and blue diamonds show sampling positions for radiocarbon dating. To the right of core LV grain-size histograms for selected horizons are shown (phi intervals against relative frequency). The graphs on the right show glass compositions (normalised to 100% anhydrous) for selected major elements. Bulk compositions of Yate tephra units are plotted for comparison. (For interpretation of the colour indicated in the caption, please refer to the online version of this paper.)

Ya2 in the lake cores, a much thinner (<10 cm), finer-grained tephra occurs (Fig. 5). This is a basaltic ash, similar in composition to Ap1 and Ho1, but with a much smaller deposit volume (correlations suggest that Ho1 at this site is w0.5 m thick). Local source possibilities are Apagado and Hornopirén, which cannot categorically be distinguished on the basis of glass chemistry (Fig. 5). However, since the Apagado cone appears from its morphology to be monogenetic, and is correlated with Ap1, it is suggested that this fine ash represents a minor explosive eruption, or perhaps a period of small eruptions, from Hornopirén, and hence is named HoSc. The relatively thin, finegrained nature of this ash means that it is unlikely to have been widely preserved in terrestrial settings. The occurrence of minor explosive activity at Hornopirén, younger and much less vigorous than Ho1 but accompanying the extensive lava flows on the volcano’s flanks, is corroborated by the presence of small cinder cones around the summit of Hornopirén. It is possible that other minor scoria deposits from Hornopirén have contributed to the mixed and reworked sediment within the lower half of the lake cores. Thirteen of the newly identified tephra units originate from Calbuco (Ca1e13), several of which cannot yet be widely correlated.

All these deposits weather brown or yellow-orange, and contain rounded, crystal-rich and relatively dense pumice grains. The basal unit, Ca1, overlies a very thin clay-rich soil on glacially scoured granitoid bedrock. Ca2 lies directly above. The mixed nature and variable thickness of both units suggests a possible flow-type emplacement, although the sample region is topographically remote from the immediate vicinity of Calbuco. Several more minor pumice deposits (Ca3e6) lie below Cha1, but only appear as thick deposits at the northern limit of the study area. Here (Column Z; Fig. 4), correlation with the stratigraphy to the south was difficult and uncertainties remain over the precise sequence and relative timing of some events (see Section 4.2.1). Ca7 lies, in many places, immediately above Cha1, and at least two further units occur below the LaVig scoria. Additional units again appear at the north end of the Reloncaví fjord. One of these, Ca8, is the coarsest in the whole sequence. Intervening soils are rich in coarse lithics and a pumice component, presumed to be reworked, potentially disrupting the preservation of finer-grained tephra layers. Above LaVig are two final coarse-grained units, Ca12 and Ca13, separated by an ash-rich soil. These appear widely, and are difficult to distinguish in places,

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Table 1 Unit descriptions and reference localities for tephra deposits in the Hualaihue region. Unita

Type location (
N;
E)

Columnb

Ca13

72.2834; 41.4093

Y

7

25

Ca12

72.2834; 41.4093

Y

35

50

Tagua

72.0260; 41.7504

Q

Ya2

72.3450; 41.8510

e

Ap1

72.6104; 41.8790

I

238

22

LaVig

72.2732; 41.3850

e

130

4

Ho1

72.3738; 41.8871

E

143

15

Ya1

72.3537; 41.8133

e

203

500

Ca11

72.3052; 41.4611

W

30

8

Ca10

72.3052; 41.4611

W

15

4

Ca9

72.3051; 41.3821

Z

10

1

Ca8

72.3051; 41.3821

Z

50

45

Ca7

72.2958; 41.4446

X

19

15

Cha1c

72.4565; 42.0763

A

16

7

Ca6

72.3051; 41.3821

Z

30

15

Ca5 Ca4

72.3051; 41.3821 72.3051; 41.3821

Z Z

20 50

15 15

Ca3

72.3051; 41.3821

Z

50

25

YaSc

72.4382; 41.7148

M

12

17

Ca2

72.3193; 41.5819

S

0e35

8

Ca1

72.3193; 41.5819

S

25

10

Thickness (cm)

4.5 e

Max. grain-size (mm)

<0.5? 60

Type description Yellow-brown, dense, crystal-rich, rounded lapilli pumice. Upper bound at root level, lower bound above soil mixed with Ca12 coarse ash. Dispersed SE. Yellow, dense, crystal-rich, rounded lapilli pumice. Coarse base, sharp soil boundary, grading upwards to fine lapilli; ash-rich soil above. On occasion merges with Ca13. Dispersed E. Yellow, very fine ash, weathered, sharply bounded by clean brown-grey soil. Unknown source and dispersal. Yellowewhite and grey pumice, highly vesicular, sporadically scattered near soil surface. Dispersal from south flank of Yate, probably E. Associated with basaltic tephra, inferred from ash preserved in lake sediment cores. Dark grey and orange-brown, glassy, elongate scoria, extremely well-stratified on cm-scale, each bed fining up. Upper third finer. Extensive thick deposit, sharply bounded by clean dark soil, above and below. Dispersed ESE. Dark grey and yellow-brown, angular, flattened, moderately vesicular scoria, well stratified on sub-cm scale, beds fining up. Predominantly coarse ash. Sharply bounded by clean soil, above and below. Dispersed E. Black-brown and yellow, elongate, open, glassy scoria, very well stratified on cm-scale. Upper third finer. Dense, clean, dark clay above and below at sharp boundary. Dispersed E to SE. Red and black-brown, open, moderately vesicular scoria, with mingled white pumice as bombs and coarse lapilli. Dispersed SE. Yellow, rounded, crystal-rich dense pumice. Occasionally coarse, predominantly fine lapilli. Sharply bounded above and below by sandy soil. Dispersed SE. Yellow, rounded, crystal-rich lapilli pumice to coarse ash, bounded by sandy brown soil. Dispersed SE. Fine yellow ash, weathered. Bounded by sandy, lithic-rich soil. Continuous unit, uncorrelated elsewhere. Yellow, dense, rounded, crystal-rich coarse lapilli. Prominent bed, correlated locally, fining upwards, sharply bounded by sandy, lithic-rich soil. Dispersed E. Orange-weathering, dense, crystal-rich, rounded lapilli pumice, moderately weathered. Sandy brown soil above; sharp boundary below with white coarse ash of Cha1. Occasionally a thin sandy soil between. Dispersed ESE. Yellow, occasionally grey, aphyric pumice, elongate or flattened grains. Scattered in brown clay-rich soil above, clean soil below. Finer at sites further north, forming pink-white coarse ash. Dispersed N to NNE. See Naranjo and Stern, 2004. Orange weathered, rounded, crystal-rich pumice, bounded above and below by lithic-rich sandy soil. Not correlated elsewhere. Appearance as for Ca6. Not widely correlated. Rusty orange, weathered, rounded, crystal-rich pumice, bounded by lithic-rich soils, faintly divided into two halves. Not widely correlated. Yelloweorange, weathered, rounded, crystal-rich pumice, bounded by lithic-rich soils, faintly divided into two halves. Not widely correlated. Lead-grey rounded dense scoria, rich in feldspar phenocrysts, structureless, lying directly over associated flank lava flow. Thin and localised. Weathered, orange lapilli pumice, rounded, crystal-rich, mixed with granitoid lithics. Widely variable thickness, with unclear relationship to Ca1. Deeply-weathered orange lapilli pumice, often soft, rounded, dense, crystal-rich, with lithics. Black organic rich matrix. Widely correlated. Ca2 above, granitoid bedrock directly below, occasionally with thin soil. Dispersed SE.

a Unit codes refer to volcano of origin: Ca, Calbuco; Ya, Yate; Ap, Apagado; Ho, Hornopirén; Cha, Chaitén; LaVig, La Viguería cone; with 1 representing the oldest eruption. Tagua has an unidentified source; YaSc is a minor scoria deposit from Yate. b Columns refer to Fig. 3. c Cha1 previously described, in a proximal setting, by Naranjo and Stern, 2004.

having identical compositions. However, they have slightly different distributional patterns, and dating confirms that two separate eruptions occurred in relatively close succession. Source volcanoes for all units were assigned primarily on the basis of mapping deposit thickness variations. This approach was generally unambiguous, given the limited number of possible volcanic sources in the region. The several units inferred to be derived from Calbuco suggest that the volcano has been extremely active during the Holocene when compared to the volcanoes on the Hualaihue peninsula. A thick sequence of proximal Calbuco tephras was also documented by Moreno (2004), some of which are likely to correlate with this stratigraphy. One major factor assisting with the

development of a long stratigraphic sequence from Calbuco is the presence of a road transect due east, and downwind, of the volcano (along the Reloncaví fjord). No similar transects are possible near Yate, and some explosive eruptions from here may therefore not have been recognised due to a lack of suitable field sites. Along the road section north of Yate, a number of indurated lahar deposits and lava flows occur interbedded with the tephra sequence (Fig. 2), showing that the volcano was active throughout much of the Holocene. 4.1.2. Unit chemistry Selected representative bulk chemical compositions for the identified fall deposits are given in Table 2. The stratigraphic

S.F.L. Watt et al. / Quaternary International 246 (2011) 324e343

positions of analysed samples are indicated in Figs. 3 and 4, while selected element plots are provided in Fig. 6 to illustrate compositional differences between the units. The basaltic scoria deposits (Ap1, Ho1, LaVig) are easily discriminated from each other on the basis of bulk chemical composition, accurately determined from their relatively fresh, coarse-grained deposits. The more evolved tephra deposits, predominantly from Calbuco, accumulated rapidly along the Reloncaví fjord, forming thick, free-draining Andisols, and are more extensively weathered. The multiple units from Calbuco are also chemically similar, but by using a wide range of elements, including several that are among the least mobile (e.g., Ti, Nb, Ta, Hf; Aiuppa et al., 2000; Kurtz et al., 2000), some differences between units were observable and fully consistent with correlations based on stratigraphic and distributional patterns. In general, the variability of repeat chemical measurements did not increase with sample age, and there was no clear evidence of a strong weathering effect on bulk deposit chemical compositions. However, the potential effects of weathering on the absolute accuracy of bulk compositions measured on fine-grained fall deposits, particularly for the more evolved units, should be borne in mind. It was clear in the field, for example, that grains in some Calbuco pumice deposits had been affected by substantial alteration to secondary minerals. 4.1.2.1. Scoria fall deposit compositions. Ap1, Ho1 and LaVig form prominent bedded proximal deposits, commonly with a total thickness in excess of 1 m. More distally, these units can be distinguished on the basis of bulk chemical compositions. In particular, overlap of the Ap1 and Ho1 deposits required bulk chemical analyses of ash samples to clarify their distributional extent. The incompatible element plot in Fig. 6A shows most clearly how a range of trace elements may be used to distinguish these units, and the analytical consistency between multiple samples of, for example, Ap1. A plot of the highly compatible element, Cr, against the incompatible element, Rb (Fig. 6F) also shows the different bulk compositional space occupied by the scoria units. Cr is highly sensitive to fractional crystallisation, and this plot indicates the unusually primitive composition of Ap1. The scatter in Cr content between the Ap1 analyses is a result of variable proportions of olivine and Cr-spinel crystals within the bulk tephra samples, with fine-grained or distal samples having lower concentrations as a result of transport related sorting. The coarsest, most proximal samples have the highest bulk Cr-contents. Even between separate beds of the Ap1 deposit, olivine crystal content was observed to vary slightly. In spite of these variations, the overall bulk compositional pattern of distal Ap1 samples is consistent with proximal Ap1 samples (Fig. 6A) and corroborates the mapped stratigraphy. The bulk composition of the Ho1 tephra deposit is similar to that of Holocene lavas from Hornopirén (Table 2; Fig. 6A), supporting the deposit’s source identification. Incompatible trace element levels are systematically higher in Ho1 than in Ap1 (Fig. 6A), indicating a more evolved magma. LaVig lies at intermediate compositions. The patterns of enriched Sr and Ba and relatively depleted Nb for Ap1 and LaVig are typical subduction-related basalt signatures, indicating fluid contributions to the magma source (e.g., Pearce and Peate, 1995). The absence of a Sr spike in Ho1 suggests that plagioclase fractionation affected this magma, whereas only olivine and Cr-spinel fractionation appear to have affected LaVig (based on decreasing bulk Cr and Ni contents; Fig. 6F). Finally, the bulk composition of YaSc, a thin, structureless and volumetrically minor scoria deposit associated with Yate flank lavas, has a remarkably similar trace element pattern to Ap1 (Fig. 6A). However, its unusually high levels of alumina (20.2 wt%)

333

and relatively low levels of compatible elements such as Cr and Ni (Table 2), as well as its appearance and distribution, distinguish it. 4.1.2.2. Pumice deposit compositions. The Calbuco and Cha1 tephra samples were characterised using a relatively limited range of trace elements found from bulk analyses of pumice grains (Fig. 6CeE). Examples of full incompatible element trends for selected pumiceous units are given in Fig. 6B, although concentrations of more mobile elements within these trends (e.g., for Ca1) may have been affected by weathering. Cha1 forms a distinctive marker both in appearance and distribution across the study region, and its rhyolitic bulk composition (Table 2) clearly distinguishes it from the Calbuco tephras with which it is interbedded. Rhyolitic magmas are unusual within the SVZ (Naranjo and Stern, 2004). The bulk composition of Cha1 is extremely similar to the magma erupted by Chaitén in May 2008 (Watt et al., 2009b; Fig. 7), suggesting that Chaitén has repeatedly erupted magmas of rhyolitic composition. Cha1 has notably high levels of high-field-strength elements (HFSE; e.g., Nb and Ta), low levels of the compatible elements Co and Cr, low Ti and Zr (suggesting zircon fractionation), and high Ba (Naranjo and Stern, 2004). These characteristics are shared by bulk compositional analyses of pumice grains from seven samples of Cha1 throughout Hualaihue. To confirm the correlation of the unit in Hualaihue with the proximal deposit identified by Naranjo and Stern (2004), glass compositions (Fig. 7) from mounted pumice grains of three distal samples were also analysed and compared with analyses from a single coarse pumice grain from the proximal Cha1 deposit (sample 825-2A). Full analytical results are given as Supplementary Data. There is a very good overlap in glass compositions between samples, with almost all points falling in the range 75.3e76.2 wt% SiO2 (normalised to 100% anhydrous compositions). Although the Chaitén 2008 glass is similar and also highly homogeneous, there are subtle compositional differences, such as the slightly higher CaO content in the 2008 glass (Fig. 7). The dacitic Ya2 deposit has lower bulk-rock HFSE concentrations than Cha1 (Table 2; Fig. 6C), but distinctively high Ba, Zr and Hf. The eruption appears to have been small, and the correlation with the uppermost ash in the lake sediment cores (Fig. 5) suggests a broadly SE dispersal direction. It may also correspond with a young pale coarse-ash to fine-lapilli deposit, 10 cm in thickness, noted on the east flanks of Yate by Mella (2008). Ya1 shares similar characteristics to Ya2; the high Ba concentrations in the rhyolitic end member distinguish it clearly from the similarly silica-rich Cha1. Along the Puelche-Lago Tagua Tagua section (Fig. 3), finegrained Calbuco ash deposits are preserved in places. At the easternmost site (Q) a fine-grained andesitic ash above Ap1, named the Tagua unit, has a distinctive bulk composition (Table 2; Fig. 6G,H). Its fine grain-size suggests a relatively distal deposit, and it was only found at this one location. The alkaline composition of the Tagua unit, with high concentrations of Ba, Ta, Hf, Zr and Nb, is more typical of intraplate magmatism. The only similar documented SSVZ rocks are basalts from the monogenetic cones at Puyuhuapi (Gonzalez Ferran et al., 1996), which have been explained by melting in extensional conditions along the LOFZ (Gutiérrez et al., 2005). There are no known comparable monogenetic centres along the LOFZ at this latitude (w42
S) (LópezEscobar et al., 1995). The ash composition suggests a back-arc source, and Holocene cinder cones associated with the Gastre fault system, 150 km east of this site (Massaferro et al., 2006; named the “crater basalt volcanic field”), may be related. However, the known rocks from this volcanic field are basaltic, and the centres may be too distant to provide a plausible source for the Tagua ash. Thus, the Tagua source remains somewhat enigmatic,

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Table 2 Whole rock compositions of selected Hualaihue volcanic samples.a Columnd

SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 l.o.i. Total Sc V Cr Co Ni Cu Zn Ga Rb Sr Y Zr Nb Ba Sn La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Pb Th U

Ap1

Ho1

LaVig

YaSc

Cha1

Ya1b

Ya1b

Ya2

Ca1

Ca7

Ca13

Tagua

Ho lava

Cha1 glass

21-5C

26-8A

803-5C

23-9A

26-8B

YLP-12b

YLP-12a

26-5A

25-5C

25-5N

23-2A

14-5A

15-5A

26-8Bc

I

E

Y

M

E

e

e

e

S

S

P

Q

e

E

49.82 0.77 15.49 9.31 0.15 12.24 9.13 2.18 0.47 0.13 0.21 99.9 33 228 803 36 249 58 72 17 3.2 427 18 70 4.6 81 2.3 6.7 15.0 2.3 10.9 2.8 0.9 3.0 0.5 3.1 0.6 1.8 0.3 1.7 0.3 1.8 0.1 4.7 0.8 0.3

50.72 1.06 18.53 9.79 0.15 6.17 7.96 2.66 0.74 0.29 1.68 99.75 32 236 219 25 80 62 71 19 13 404 28 133 6.4 226 10.4 15.9 33.9 4.7 20.5 4.7 1.4 5.0 0.7 4.6 0.9 2.5 0.4 2.3 0.4 3.0 0.2 10.5 1.8 0.4

49.25 0.85 18.13 9.45 0.15 7.94 9.12 2.64 0.59 0.2 1.41 99.72 36 255 210 29 84 69 68 18 7.5 521 18 85 3.4 149 1.6 9.5 22.5 3.1 14.3 3.5 1.1 3.5 0.5 3.4 0.7 1.8 0.3 1.8 0.3 2.2 0.1 8.6 1.3 0.5

50.34 0.81 20.24 8.32 0.14 5.60 10.71 2.42 0.44 0.11 0.3 99.44 28 211 78 21 34 37 67 18 11 404 19 63 3.7 146 e 6.2 14.8 2.1 9.6 2.6 0.9 2.8 0.5 3.0 0.7 1.9 0.3 1.8 0.3 1.7 0.1 4.7 e 0.3

w73 0.2* 13.7* 1.9* 0.1* 0.3* 1.4* 5.3* 2.7* e N/A N/A 3.3* 12.8* 6.6* 2.3* e 7.5* 50.3* 14.2* 102.7* 139.6* 11.9* 75.8* 8.0* 571.3* 14.5 24.1 46.8 4.9 16.6 2.9 e 2.9 0.4 2.2 0.4 1.3 0.2 1.4 0.2 2.3 0.9 20.8 e 3.5

56.33 0.79 16.24 7.65 0.14 5.63 8.21 2.62 1.52 0.17 0.01 99.30 e 208 e 25.9 e 23.7 19.0 16.9 47.3 530.2 23.0 116 3.0 404 e 19.1 40.3 4.9 21.5 4.5 1.1 4.4 0.7 3.8 0.8 2.4 0.4 2.2 0.4 3.3 0.3 e 9.1 2.2

70.84 0.29 14.66 2.66 0.06 0.58 2.84 3.11 3.19 0.10 0.74 99.07 e 28 e 9.8 e 14.1 15.0 14.7 108.8 251.3 22.3 167 5.5 831 e 27.4 55.8 6.1 21.9 4.2 0.8 3.9 0.7 3.5 0.7 2.2 0.3 2.3 0.3 5.0 0.6 e 10.1 2.2

65.54 0.60 14.95 4.44 0.09 1.54 3.68 4.18 2.66 0.07 1.42 99.16 9.7 61 14 4 6.1 9.6 49 18 96 277 28 224 7.9 676 e e e e e e e e e e e e e e e 5.0 0.4 e e e

w50 1.7* 23.3* 12.8* 0.1* 5.3* 3.2* 2.8* 0.2* e N/A N/A 32.4* 375.3* 19.4* 29.7* e 27.4* 102.7* 29.4* 1.2* 137.6* 9.4* 114.7* 3.3* 36.4* 26.7 2.9 6.6 1.1 5.0 1.5 e 1.7 0.3 1.9 0.4 1.3 0.2 1.3 0.2 3.2 0.3 11.8 e 0.8

w65 0.9* 18.0* 7.3* 0.1* 2.4* 2.7* 2.0* 0.7* e N/A N/A 29.4* 224.2* 14.2* 16.9* e 26.0* 78.1* 23.1* 23.1* 130.2* 17.3* 81.2* 3.7* 176.8* 6.6 10.1 20.9 2.7 11.7 2.7 e 3.1 0.5 3.0 0.6 1.9 0.3 1.9 0.3 2.2 0.3 9.3 e 1.2

w57 1.0* 17.7* 8.2* 0.2* 3.3* 6.8* 3.9* 0.9* e N/A N/A 20.4* 191.5* 7.2* 22.5* e 33.1* 93.4* 21.3* 15.7* 329.8* 18.6* 96.9* 2.9* 173.8* 33.2 6.9 17.7 2.5 11.9 3.1 e 3.4 0.6 3.5 0.7 2.1 0.3 2.1 0.3 2.5 0.2 8.8 e 0.4

w60 1.3* 17.7* 7.6* 0.2* 2.0* 4.4* 3.8* 2.0* e N/A N/A 24.7* 161.0* 27.7* 15.2* e 31.1* 112.3* 23.3* 39.6* 342.2* 38.6* 367.8* 15.1* 555.7* 12.7 39.3 95.5 11.4 46.0 9.3 e 9.5 1.4 7.9 1.5 4.5 0.7 4.5 0.7 8.4 0.9 16.5 e 1.6

53.46 0.95 17.38 8.88 0.15 5.44 8.77 2.90 0.95 0.30 0.09 99.09 24 207 103 22 45 61 83 18 20 447 26 142 7.0 287 1.3 19.5 43.0 5.7 24.3 5.2 1.5 5.4 0.8 4.8 1.0 2.7 0.4 2.5 0.4 3.5 0.3 8.6 2.4 0.5

75.88 0.10 14.16 1.32c 0.07 0.20 1.24 3.88 3.14 e N/A 100 e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e e

a Major element oxides (wt%) and Sc to Ba (ppm) measured by XRF (analyses at Open University, Milton Keynes, U.K. and Edinburgh University, U.K.), except values marked with an asterisk, which were measured by solution ICP-MS at Oxford University. For these, silica estimates found by subtraction are given. Total iron quoted as Fe2O3. Sn to U (ppm) measured by ICP-MS for all samples. Missing data result from poor recoveries. For XRF, standard recoveries were within 1% of reference values for major elements (other than 3% for Ca and Mn), and 5% for trace elements. ICP-MS standard recoveries were generally within 5% (other than 15% for Y, Nb, Gd, Hf, Ta, Pb, Th, U). b Ya1 measured by XRF (major elements; analyses at Geochronological Research Centre, Sao Paulo University, Brazil) and ICP-MS (trace elements; analyses by AcmeLabs, Vancouver, Canada). Compositions given for both basaltic andesite and rhyolitic endmembers. c Average of 15 analyses measured by electron microprobe (University of Oxford). Major element oxide analytical totals all >95 wt%, normalised to 100% anhydrous compositions. Total iron given as FeO. d Refers to sample location in stratigraphic columns in Figs. 3 and 4.

perhaps related to an unknown back-arc volcanic centre west of the crater basalt volcanic field. Several pumice deposits of similar composition (Table 2) occur along the Reloncaví section, all assigned to Calbuco. They are less evolved than Ya1, Ya2 and Cha1 (Fig. 6CeE), and distinctive due to their relatively low bulk-rock K and Ba contents, consistent with analyses of Calbuco lavas by López-Escobar et al. (1995). Several of these deposits are deeply weathered, a likely cause of some of the compositional spread between analyses of the same unit (e.g., Ca11, Fig. 6E). This, along with the relative chemical homogeneity of all the Calbuco tephras, prevents chemical discrimination between some of these deposits.

The Calbuco pumices are predominantly andesitic. Ca1 has a distinctive bulk composition, with low levels of Ba and relatively elevated levels of some less incompatible elements, such as Ti and Hf (Table 2). ICP-MS measurements of major elements in bulk deposit samples, although not highly precise, suggest elevated Al and Fe and a relatively silica-poor composition, and these compositional patterns may reflect alteration to secondary clay minerals. Nevertheless, there are consistent trace element differences between bulk analyses of Ca1 and Ca2e11, which point to a less evolved magma source for Ca1. Ca12 and Ca13 have high levels of Ba, but are otherwise similar to older Calbuco units (Fig. 6D). Of these, Ca7 is distinctive for its high Ba and comparatively high Nb

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335

Fig. 6. Bulk chemical compositions of the Hualaihue tephra deposits. Volcano abbreviations are Apagado (Ap), Hornopirén (Ho), Calbuco (Ca), Chaitén (Cha), Yate (Ya) and Crater Basalt Volcanic Field (CBVF). For Ya1 the basaltic andesite (b) and rhyolite (r) are distinguished. Sample numbers correspond to those in Figs. 3 and 4. All analyses were made by ICPMS on bulk samples, except for the Calbuco lavas, which were measured by instrumental neutron activation analysis. A, B: Incompatible trace element plots normalised to primitive mantle (data from Sun and McDonough, 1989) for basaltic (A) and more evolved (B) units. C, D, E: Selected incompatible trace element compositions of intermediate and silicic units, used to confirm correlation of samples between sites. Several Calbuco units (E) are not easily distinguished on the basis of composition, but are stratigraphically correlated (as indicated by different line symbols). F: Cr against Rb for basaltic units, indicating increasingly evolved magmas from Apagado to Hornopirén. Hornopirén lavas are shown for comparison. G, H: Selected ratio plots, showing clear distinctions between the regional units. Compositional fields are shown for Hornopirén lavas (3 points, authors’ unpublished data), Calbuco lavas (12 points, López-Escobar et al., 1995) and CBVF (21 points, Massaferro et al., 2006).

and Ta. The remainder of the Calbuco tephra deposits are chemically indistinguishable, and stratigraphic relationships are the basis of the correlation. Inevitably, a degree of ambiguity remains for the identification of some the less-widely preserved Calbuco units in Fig. 4. As a brief aside, the mineralogy of well-preserved pumice grains in Ca12 and Ca13 may be compared with the distinctive Calbuco lava petrology described by López-Escobar et al. (1995). Unlike some of the lavas, amphibole is absent from Ca12 and Ca13; plagioclase (core An72e86) and orthopyroxene (core En64e68) are the dominant phenocryst phases, with less common augite and coarse Ti-magnetite. Rare xenolithic clots preserve olivine, replaced by orthopyroxene. Uncommon enclaves of fine-grained quartz,

magnetite and more albitic plagioclase may represent the remains of crustal granitoid xenoliths. Similar features were used by LópezEscobar et al. (1995) to suggest significant crustal assimilation within the Calbuco parental magma. 4.1.2.3. Summary of chemical differences. To summarise, all the regional tephra units may be compared using a few bulk-rock trace element concentrations. A plot of Ba/Zr against Nb (all relatively immobile elements on weathering) separates units well (Fig. 6G). The compositions lie along a closely defined linear field, correlating with silica content, as may be expected for arc lavas. Nb content increases as the arc rocks become more evolved, with the lowest overall levels in Ap1. In contrast, similarly primitive rocks from the

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3.5

1.6 Distal 13-1C (n=38) 20-2C (n=13) 26-8B (n=15) Proximal 825-2A (n=25) Chaitén 2008

1.3

K O (wt%)

CaO (wt %)

3.0

1.0 2.5 72

74 SiO (wt%) 76

78

1.0

FeO (wt%)

1.4

1.8

Fig. 7. Major element data for Cha1 glass. All analytical totals below 95 wt% were rejected. Remaining analyses were normalised to 100% anhydrous compositions before plotting. The compositional field of glass from the 2008 Chaitén eruption (Watt et al., 2009b) is shown for comparison.

crater basalt volcanic field (Massaferro et al., 2006) have high Nb, without Ba enrichment. The Tagua unit has an affinity with these back-arc compositions. Lava compositions for Hornopirén and Calbuco both lie near to their respective tephras, particularly the most recent pumice deposits (Ca12 and Ca13) in the case of Calbuco. Deviation from lava compositions for older Calbuco tephras may in part reflect postdepositional alteration. Overall, Calbuco’s andesites are distinctive in plotting close to basalts from elsewhere in the region, a result of an unusually low Ba content. Ba/Nb, with higher values indicating a stronger subduction signature, against Ti/Nb shows the full compositional range of the Calbuco tephras (Fig. 6H). Since Ti and Nb are particularly immobile, the deviation from Calbuco lavas is likely to reflect real compositional differences. Although scatter between samples from any individual volcano can be relatively large, it is clear that units from different centres, such as Yate and Calbuco, occupy a different compositional space. The high Ba content of Ya1 and Ya2 is notable, and relatively depleted Nb means that these and Cha1 are clearly separated from the less evolved units. The same plot also separates the primitive Ap1, Ho1 and LaVig units, with particularly low Ba and Nb concentrations in Ap1. 4.2. Dating The order of tephra deposits given in Table 1 is derived from the stratigraphy presented in Figs. 3 and 4. However, the relative ages of deposits that do not overlap cannot be determined from the stratigraphy alone. Radiocarbon dating and age modelling were used to resolve uncertainties and to constrain eruption ages. Results and sample details are given in Table 3, with sample positions shown in Figs. 3e5. The age modelling also used published ages for Cha1 (Naranjo and Stern, 2004). Radiocarbon dates are reported for some Calbuco tephra deposits by Moreno (2004), based on proximal sections north of the volcano. At present, there is insufficient information to correlate these data with the results presented here, and this is an area for future work. The new radiocarbon age data constrain tephra deposit ages with variable degrees of precision, due both to stratigraphic separations between datable material and tephra deposits, and to the number of dates relating to individual units. It is difficult to produce objective eruption ages from a set of dated samples that have variable stratigraphic relationships with eruption deposits. This problem applies particularly to soil sequences, where accumulation rates between volcanic events cannot easily be approximated and

sedimentation cannot be assumed to be continuous. Furthermore, even when a dated sample is in direct contact with a tephra deposit, it does not necessarily provide a conclusive eruption age. This is clear from the span of calibrated ages for multiple samples at the base of Cha1 and Ap1 (Table 3). In some respects this type of problem has been ignored in the wider volcanological literature in order to produce single eruption ages (with errors), by simply using the age of the stratigraphically-closest carbon sample as the age of the eruption. Here, a Bayesian modelling approach is adopted to combine formally the available information and produce calibrated age ranges for each deposit. It is assumed that a sample from within a tephra deposit constrains its age most accurately. Samples directly at the base of a deposit are also likely to provide good constraints, while those at the top of a unit may differ substantially in age if soils are only slowly re-established after a large eruption. At the top of a stratigraphic sequence there is the additional problem of recent contamination (e.g., likely for the wood of sample 24-4A, and for the date above Ya2; Table 3). 4.2.1. Bayesian analysis Bayesian methods are used increasingly widely in palaeoclimatic and archaeological applications (e.g., Buck et al., 1991; Blockley et al., 2004, 2008; Lowe et al., 2008) to provide age models for sequences constrained by radiocarbon dates. The key aspects of such analyses involve a prior model, which in this case is the field stratigraphy, and a likelihood, which is the information arising from the set of dating measurements that have been made. The Bayesian analysis combines these data, utilising all available information, to produce a posterior model. Although some choices are inevitably necessary when selecting how information is incorporated into the model, as a whole the approach provides a coherent framework for the interpretation of chronological data. The Bayesian analysis in the OxCal program (Bronk Ramsey, 2009) was used, which combines stratigraphic information, dates, and radiocarbon calibration with a Markov chain Monte Carlo sampling method to produce a set of posterior probability density functions defining the age of each event. For this terrestrial soil sequence, with widely variable inter-deposit thicknesses, a conservative model that simply incorporates the temporal sequence of units (cf. Bronk Ramsey, 2008) was applied. To improve overall constraints, if there was not a sample within a tephra deposit, samples that lay directly at the base of a unit (the youngest, if there were more than one) were considered to represent the deposit age. Using this assumption, Ap1, Ca1, 7, 11 and 13, Cha1, LaVig, Ya2, Ho1

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Table 3 Radiocarbon dates for the Hualaihue tephra stratigraphy. Sample name

AMS pub. codea

Sample description

Column (Figs. 3 and 4)

Sample age

Conventional age (error), BPd

Calibrated age range, cal yr BPd,e,f

d13CVPDB&  0.1

03-1E LV-1b 23-2B 23-7A 130398-2Gc 25-5K 25-5J 130398-2Ec 260298-2Ac LV-2b LN-2b 20-6B 24-4A 21-8B 24-6E 801-1F 25-5F 25-5Of 803-4A DHP-1c 25-3A 13-1B 23-11D T13Dc,g T13Ec,g 130398-2Bc 25-5B

21509 e 16170 16171 e 21507 16172 e e e e 21498 21493 21499 21501 21510 21506 e 22349 e 21502 21497 21500 e e e 21503

Organic mud Organic macrofossil Charcoal Charcoal Charcoal Charcoal Charcoal Charcoal Charcoal Organic macrofossil Organic macrofossil Organic mud Wood Organic mud Charcoal Plant stem Charcoal Charcoal Charcoal Charcoal Charcoal Charcoal Charcoal Charcoal Charcoal Organic sediment Charcoal

e Fig. 5 P N Z S S Z Z Fig. 5 Fig. 5 I F I F E S S X K R A L e e Z S

>Ya2 Ya2 Ca13 Ca13 >Ca12; Ca12; Ap1 >Ap1 >Ap1 >Ap1 Ap1 Ap1; >>Ho1 Ho1 Ca11 ¼ Ca7¼ Ca7; Cha1 Ya lava; Cha1 Cha1 ¼ Cha1
319(35) 980(20) 1378(37) 1469(37) 1730(80) 1887(44) 2075(37) 1910(70) 2200(70) 1755(80) 2185(70) 1094(37) 266(37) 2876(36) 2538(39) 4977(46) 6166(52) 8047(68) 8559(39) 6730(60) 8849(74) 9235(78) 9233(78) 9370(60) 9810(90) 7510(40) 9246(78)

454e283 915e794 1303e1176 1385e1280 1810e1395 1879e1625 2111e1881 1967e1606 2334e1951 1814e1415 2322e1949 1057e845 440e146 3065e2799 2720e2364 5856e5584 7164e6800 9024e8629 9544e9449 7656e7436 10155e9561 10561e10220 10559e10217 10685e10294 11609e10808 8367e8185 10566e10226

27.3 e 27.4 27.4 e 29.1 27.1 e e e e 27.0 20.7 27.0 25.3 28.1 30.0 27.0 27.2 e 26.6 24.9 24.9 e e 28.3 28.3

a

Samples analysed at the NERC Accelerator Mass Spectrometry (AMS) radiocarbon facility in East Kilbride. Publication codes are SUERC- followed by the listed number. Samples analysed at the Svedberg Laboratory, Uppsala University, Sweden, by AMS, and using an estimated d13CVPDB& of 25.0. c Samples analysed at Beta Analytic Inc. Laboratories, Miami, by standard radiometric methods, using an estimated d13CVPDB& of 25.0 (except 130398-2B, by AMS). d BP refers to years before 1950 A.D. e Calibrated using OxCal4.0 (Bronk Ramsey, 2009) and the Southern Hemisphere calibration curve SHCal04 (McCormac et al., 2004), except T13E, which was calibrated using IntCal09 (Reimer et al., 2009), due to its age. Results are quoted at 95.4% confidence intervals. f Inconsistent results produced during analysis; results quoted for reference only. g Data from Naranjo and Stern (2004). b

and HoSc are assigned direct dates, while the remaining radiocarbon dates constrain the rest of the stratigraphy. Modelled eruption ages are shown in Fig. 8 as probability density functions, with the age ranges at the 95.4% highest probability density given in Table 4. The directly constrained units have complex probability density function shapes in Fig. 8, as a result of calibrating the raw radiocarbon ages against the complex fluctuations of the terrestrial radiocarbon curve (e.g., McCormac et al., 2004). Units which have an age that is simply constrained stratigraphically have a simple curve shape. The final Bayesian model included all stratigraphic columns with a dated radiocarbon sample (Figs. 3e5). The dated sample 130398-2B was not included in the final model because its age is inconsistent with the stratigraphy. The uncertainty at this northern limit of the study area revolves around the position of Cha1 within Column Z (Fig. 4). Cha1 is well constrained by both its age and chemistry from sites further south, and has been correlated to an ash bed in Column Z (Fig. 4). However, the date below Ca3 in Column Z is inconsistent with this, and suggests that the lower package of Calbuco units at this site (Ca3e6), as well as Cha1 and Ca7 above (Fig. 4), may be incorrectly assigned, and in fact could all be younger in age than Cha1. At present, however, this possibility is suggested only by a single dated sample, and the presented sequence is otherwise consistent. The date, from a small organic sediment sample, may be inaccurate, and further fieldwork and dating are required to resolve this ambiguity. This part of the Calbuco sequence, where wide correlation of units was not possible, is the part of the stratigraphy that is the least well constrained, and sampling in the proximal area around Calbuco may improve the record of its explosive eruption history. YaSc, a minor deposit associated with lava flows on the north flank of Yate, was not included in the age model but is known to be

older than Cha1. Similarly, Tagua is only known to be younger than Ap1, while the age of Ya1 is unconstrained within the Holocene. 4.2.2. Summary of eruption sequence The dated eruption sequence, as far as it can be constrained by current data, is summarised in Fig. 8. The date obtained for Ca1 is surprisingly close to that of Cha1, given the relatively thick pyroclastic deposits separating the units (Ca2e6). This finding suggests that Calbuco was highly active in the early Holocene, with several large explosive eruptions occurring over less than 1000 calibrated (cal) years. The sequence of mixed pumice and lithic material may correspond to the period following the Late Glacial or earliest Holocene cone collapse (Calbuco 2 unit) noted by Moreno (2004). At around the same time, lavas erupted on the north flank of Yate, where lithified lahar deposits, rich in weathered material, are also present. This sequence at Yate lies above glacial deposits and is interbedded with Cha1, suggesting a period of slope instability and effusive eruption following glacial retreat. This activity continued until at least 7.5 cal ka BP (Table 3; Fig. 8). Evidence of later activity at Yate is limited, although two relatively small explosive eruptions of evolved magma occurred, the latter w900 cal years ago (Ya2), making it the most recent explosive eruption documented within the regional tephra stratigraphy. At Calbuco, Ca11 marks the end of a 3500-year-long period of explosive activity. Following this, there is little evidence of tephra fall deposits in the study area. This paucity contrasts with the stratigraphy of Moreno (2004), who identified pyroclastic sequences from Calbuco which span the past few thousand years up until the present day. This latter sequence may be a more proximal succession, which is not preserved further afield. Geochemical data support the idea of a time gap with magma replenishment, because

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Ya2 Ca13 Ca12 LaVig Tagua (younger than Ap1)

Ap1

HoSc

Ho1

Ho1 associated with extensive effusive volcanism at Hornopirén and minor explosive activity, including HoSc (total time period poorly constrained)

Ca11

Ya1 (age unconstrained: Mid-Holocene?)

Ca10 Ca9 Ca8 Ca7 Cha1

Sequence of lavas and lahar deposits on N flank of Yate, including YaSc (older than Cha1)

Ca6 Ca5 Ca4 Ca3 Ca2 Ca1 10000

8000

6000 Modelled date (BP)

4000

2000

Fig. 8. Bayesian analysis of the Hualaihue tephrostratigraphy, using the OxCal (v4.1.3) program (Bronk Ramsey, 2009) and the Southern Hemisphere calibration curve (McCormac et al., 2004). The modelled probability density functions show the tephra age, with bars beneath representing the 95.4% range of highest probability density. The model has an overall A index of 95.1 (indicating a good model fit; Bronk Ramsey, 2009). Complex probability density function shapes arise for tephra units that are directly constrained by radiocarbon dates, and result from calibration of the raw ages against the complex fluctuations of the terrestrial radiocarbon curve (McCormac et al., 2004). Units whose ages were simply constrained stratigraphically have a simple probability density function. Additional recognised volcanic events, not constrained by the regional stratigraphy, are noted in grey.

the early Holocene Calbuco tephras (Ca1e11) are compositionally very similar, whereas Ca12 and Ca13 are of identical composition, but have notably different trace element signatures when compared to the older units (Fig. 6). The morphology of the symmetrical cone of Hornopirén suggests that it is young, with Holocene activity dominated by lava Table 4 Summary of modelled ages for the Hualaihue tephra units. Constrained by datable sample

Stratigraphically constrained

Tephra

Age (95.4% range; cal yr BP)

Tephra

Age (95.4% range; cal yr BP)

Ya2 Ca13 HoSc LaVig Ap1 Ho1 Ca11 Ca7 Cha1 Ca1

917e799 1304e1180 1812e1415 2333e1953 2712e2360 5846e5493 7162e6800 9545e9452 9885e9543 10561e10254

Tagua Ca12 Ca10 Ca9 Ca8 Ca6 Ca5 Ca4 Ca3 Ca2 YaSc Ya1

Younger than Ap1 1919e1654 8652e6892 9285e7214 9529e7881 10157e9605 10222e9643 10345e9745 10445e9870 10526e10023 Older than Cha1 Unconstrained (mid-Holocene?)

Table 5 Summary of eruption parameters for the Hualaihue tephra units. Tephra

Field sitesa

11 Ca13d 8 Ca12d c 3 LaVig Ap1 34 Ho1 5 7 Ca11d 5 Ca10d,e d Ca8 4 d 8 Ca7 Cha1 20 8 Ca1d

Isopach area (km2) Volume (km3) Massb [thickness (cm)] (  1012 kg)

Magnitude

1360 [10] 700 [30] 50 [100] 1020 [20] 420 [20] 770 [30] 480 [20] 730 [40] 590 [20] 3050 [20] 1090 [30]

>4.5 5 4.4 5 4.5 w5 >4.5 >5 >4.5 5.5 >5

0.5 0.8 0.3 1.01 0.34 0.9 0.4 1.1 0.4 3.54 1.2

0.4 0.7 0.28 0.94 0.32 0.7 0.3 0.9 0.4 3.01 1

a Number of sites on which parameter estimations are based, providing an indication of uncertainty. b Calculated using assumed densities of 930 kg m3 for Ap1, Ho1 and LaVig and 850 kg m3 for the remaining tephra units. c Assuming the same deposit thinning rate as Ho1. Although sparsely sampled, the smaller spatial extent of both these deposits allows better constraints than for the larger Calbuco deposits. d Estimated parameters are minimum values, based on a single isopach (Pyle, 1999). Values quoted to 1 d.p. to indicate relative uncertainty. e The unlisted Calbuco tephras are considered to represent smaller eruptions, but are very poorly constrained.

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production. The single significant explosive episode, Ho1, is dated to the mid-Holocene. However, basaltic scoria deposits within the sampled lake sediment cores (HoSc) suggest that smaller explosive eruptions accompanied effusive activity more recently, at w1.5 cal ka BP. The La Viguería eruption (1953e2333 cal BP) is slightly younger than the Ap1 eruption (2360e2712 cal BP). The latter is likely to correspond to an eruption documented in a treering study as extensively damaging to the forests around Apagado (Wolodarsky-Franke et al., 2005), at w2.3 14C ka BP. The same study reports that the causal tephra occurs as a 2 cm ash in Chiloé, 100 km to the west. If correct, this extends significantly the distribution of Ap1 and indicates westward tephra transport at some stages of the eruption.

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4.3. Eruption parameters Measurements of unit thickness and maximum grain-size for the identified tephra units allow estimation of eruption parameters such as magnitude, column height, eruption volume and transport direction (summarised in Table 5). Levels of uncertainty are variable, as discussed below. 4.3.1. Calculation methods Plots of ln(thickness) against the square-root of isopach area can be used to estimate deposit volume, following Pyle (1989). On such a plot, a straight line indicates exponential decay of thickness with distance, described by:

Fig. 9. A, B: Isopach (A) and isopleth (B) maps for Apagado, Hornopirén and Yate. Sampling points are shown for each unit. Maximum grain-size data (mm) are also shown on B for Ap1 and Ho1. Lines have been drawn by hand. C: Isopach and isopleth map for Cha1. Isopleths are shown as dashed lines with italic labels. Sampling points are indicated by small dots, including some field sites north of Chaitén, where measurements were taken and used in addition to the data described by Naranjo and Stern (2004). D, E: plots of the squareroot of isopach area against thickness for Ap1, Ho1, LaVig and Cha1 units (D) and isopleth area against maximum clast-size for Cha1 and Ap1 (E). Straight lines indicate exponential decay. LaVig is only constrained by a single isopach, and a thinning rate equal to that of Ho1 has been assumed to estimate deposit volume.

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T ¼ T0 exp kA0:5



and Ho1 eruptions. Isopachs for Ap1 are very well constrained (Fig. 9A). In contrast, Ho1 was more sparsely preserved due to burial by Ap1 and later Hornopirén lavas. The available data suggest Ho1 was smaller in volume than Ap1. The source vent of Ya2 is unknown, but given that it was not found to the north of Yate it may, like Ya1, be associated with activity on the south flank. The only primary thickness measurement of Ya2 is for the ash preserved in the lake sediment cores, where it is up to 8 cm, at an estimated distance of 20 km, but this accumulation is potentially reworked. Proximally, Ya2 occurs as coarse lapilli of up to several cm in diameter, suggesting a small but relatively powerful explosive eruption. Preservation higher on the slopes of Yate is likely to have been hindered by ice cover, and subsequent snowmelt explains the presence of riverbed deposits of rounded Ya2 dacitic pumice grains (accumulations up to 18 cm; maximum grain-size 40 mm) south of Yate. Associated denser scoria, whose presence is inferred from the lake core ash, would have been separated by this reworking. Ya1 was identified only at a single proximal site, although this was a primary fall deposit close to its scoria cone source, preserving mixed scoria and pumice. The small cone and lack of wide tephra dispersal suggests a relatively small and low-energy eruption. A volume of 1.0 km3 is estimated for Ap1 (Fig. 9). Using a bulk deposit density, measured from three fine-grained lapilli samples, of 930 kg m3, this indicates a mass of 9.4  1011 kg. There is also a change in bt from 2.7 km proximally to 6.9 km distally. These values indicate rapid thinning, as expected for a scoria deposit produced by cyclic explosive activity. The overall tephra volume is large for an eruption of this type. From Pyle (2000), the eruption has a magnitude (where magnitude ¼ log10 [erupted mass, kg]  7) of 5.0. Similarly, Ho1 has an estimated volume of 0.34 km3 and a magnitude of 4.5. Estimated eruption parameters are summarised in Table 5. For coarse-grained samples, maximum grain-size was taken by measuring the five coarsest grains and taking the arithmetic mean of their three axes. For finer-grained samples an estimate was made based on the coarsest sieved fraction. From these data, isopleth maps have been drawn (Fig. 9B). In the case of Ap1, given the multiple beds within the unit, the coarsest grains at different sites

where T is tephra thickness, T0 is the maximum deposit thickness (from graph), A is the isopach area and k is the line slope. By integration, tephra volume, V, is defined as (Pyle, 1995):

  V ¼ 2 T0 =k2 When more than one straight-line segment fits the data, a correction must be used to account for this (cf. Pyle, 1995; Watt et al., 2009b). From Pyle (1989), the thickness half distance, bt, may be derived from k. This parameter characterises the rate of deposit thinning. Isopleth maps, showing lines of constant maximum clast-size, can be used to estimate the neutral buoyancy height (e.g., Carey and Sparks, 1986) of an eruptive column. Using a similar method to that outlined above, isopleth area is plotted against maximum clast-size, and the slope used to define a maximum clast-size half distance, bc. Again, a straight line indicates exponential decay, and may be related to column height using the derivation in Pyle (1989). For several of the mapped tephra deposits, only a single isopach or segment of an isopach could be drawn. Since the volume calculation described above reaches a minimum at a particular T0, by assuming exponential decay, a minimum deposit volume Vmin can be estimated from a single isopach (Pyle, 1999):

Vmin z3:7Ai Ti where Ai and Ti are the area and thickness of the isopach. This approach was applied to several of the Calbuco units. Where isopachs could only be partially constructed, the full isopach was approximated as a simple elliptical form.

0

Calbuco

20 cm

La m Viguería

20 c

40 cm

20 c m

<1,45,14

30 cm

4,9,3

10

10

-72°15’

30,4

-72°

14 14,7,2

cm

8,0.5

cm

11

2

-41°45’

Ca10 Ca8 Ca7 Ca1

-41°45’

-72°30’

cm

10

4

Yate

8

20 cm

3,8

-72°45’

25,50 10

30 cm

6

Ca13 Ca12 LaVig Ca11

30 10

30,15

cm

km 20

10

La Viguería 1 m

-41°30’

30

0

Calbuco

4,10

cm 20

-41°30’

km 20

10

-41°15’

-41°15’

4.3.2. Eruption parameters: Apagado, Hornopirén and Yate Relatively little explosive volcanic activity occurred at the Hualaihue volcanoes during the Holocene. In terms of erupted volume, the largest events were the vigorous scoria-producing Ap1

0.25

Yate -72°45’

-72°30’

-72°15’

-72°

Fig. 10. Isopach maps (field sites as black dots) and maximum clast-size data (values shown where measured, in mm) for tephra units along the Reloncaví sections. Isopach segments are drawn for all correlated Calbuco eruptions. Clast-size data for individual units are differentiated by shade and italic/upright font, according to the legend. Data are shown on two maps for clarity.

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are not necessarily from equivalent beds. This results in an exaggerated spread of isopleths, particularly more distally, and explains the curve to the plot in Fig. 9E. Using just the four inner points to estimate bc suggests a maximum column height (Pyle, 1989) of 22 km. Although it is likely to be a slight overestimate, due to data from multiple beds, this column height is much higher than typical for explosive basaltic eruptions. 4.3.3. Eruption parameters: Cha1 The recognition of Cha1 in the Hualaihue region, 180 km north of the Chaitén volcano, significantly increases the known extent of

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this tephra deposit, previously mapped by Naranjo and Stern (2004) to 30 km north of the volcano. The study is thus able to provide better constraints on the eruption parameters, which is of particular interest in light of the 2008 Chaitén eruption (Watt et al., 2009b). Cha1 represents one of the largest Holocene explosive eruptions in the SSVZ. Although several data points constrain the deposit isopachs (Fig. 9C), the precise direction of transport and deposit width are not well constrained, due to a lack of cross-deposit transects. Therefore, a symmetrical distribution is assumed. The northward dispersal deviates from the common eastward direction

Fig. 11. Summary of currently known Late Glacial to Holocene explosive eruptions between Calbuco and Hudson. The chart on the left shows dated eruptions (calibrated years BP defined as before AD 1950), with magnitude. The map on the right shows 10 cm isopachs for some of the better mapped and larger volume tephra deposits. Additional data sources as follows: Hudson (Scasso et al., 1994; Haberle and Lumley, 1998; Naranjo and Stern, 1998; Stern, 2008); Cay to Chaitén (Naranjo and Stern, 2004); Chaitén (Watt et al., 2009b; Alfano et al., 2011); Huequi (Watt et al., in press); Palena and Yanteles (Heusser et al., 1992).

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of most regional tephras (Naranjo and Stern, 2004), and the linearity of the deposit suggests relatively strong winds at the time of eruption. Measured deposit thicknesses are inconsistent with simple thinning and, in spite of some variability likely to result from reworking, the thickness measurements fit best with a distal secondary thickness maximum, at a distance of w130 km. Here, the deposit thickens from 10 cm to values in excess of 20 cm, before thinning again. The thickening begins in the region where the maximum pumice clast-size is 2.5 mm. Such a feature has been documented for several large explosive eruptions (e.g., Carey and Sigurdsson, 1982; Scasso et al., 1994; Durant et al., 2009), although the distance of the observed maximum for Cha1 is relatively close to source. The formation of secondary maxima is likely to result from ash particle aggregation, generating a larger effective clast-size and early fine-particle fallout. As expected, the maximum clast-size shows a simple exponential decay with distance (Fig. 9E). The preservation of the unit is insufficient to assess whether a significantly increased proportion of fine particles occurs at the position of the secondary maximum. In the observed thickened region the maximum grain-size remains in excess of 1 mm, although the deposit does appear to be relatively poorly sorted. The total volume of the Cha1 tephra deposit is w3.5 km3. Using a bulk deposit density of 850 kg m3 gives an eruption magnitude (Pyle, 2000) of 5.5 (Table 5). Isopleth data suggest a column height of 32.5 km, but given the strong wind indicated by the dispersal pattern, this will be a slight overestimate (cf. Pyle, 1989), and the true value is likely to be closer to 30 km. These results indicate that Cha1 was the product of a Plinian eruption, several times larger in volume than the May 2008 Chaitén eruption (tephra volume of 0.2e0.5 km3; Watt et al., 2009b; Alfano et al., 2011). 4.3.4. Eruption parameters: Calbuco and the Reloncaví section The tephra units mapped to the north of the Hualaihue peninsula are relatively poorly constrained, because only a single transect, along the Reloncaví fjord, constrains the deposit distributions. Nevertheless, approximated isopachs (Fig. 10) and grain-size data may still be used to infer relative sizes of parental eruptions. In cases of inconsistent thickness measurements, likely to be due to reworking, maximum grain-sizes were also used to estimate transport direction. Volume estimates should not be considered to be precise, but they provide a minimum estimate (cf. Pyle, 1999). Several of the Calbuco tephras represent large explosive eruptions: Ca1 and Ca8 had an eruption magnitude (Pyle, 2000) >5 (Table 5), while Ca11 and Ca12 were also notably large. The particularly coarse-grained deposits of Ca8, Ca11 and Ca12 suggest Plinian column heights (>20 km). The remaining identified Calbuco eruptions had smaller volumes. Distributions of the identified tephra deposits were broadly eastwards, although units transported in other directions would not have been found here, because of limited sampling in this area. The LaVig scoria deposit, morphologically similar to Ap1 and Ho1, can only be constrained by a 1 m isopach (Figs. 9 and 10). This isopach suggests broadly eastward transport, and by comparison with the thinning rate of Ho1, a volume of 0.3 km3 is estimated.

eastward, reflecting prevailing wind directions. While some gaps and uncertainties remain, it is suggested that the majority of large explosive eruptions, which are those most likely to be preserved as widely distributed macroscopic units, easily identifiable in soil sequences, have now been recognised. In contrast, both preservation and access prevent the recognition of the majority of volumetrically-smaller eruption deposits in the region, although such records may potentially be sampled from lake sediments or peat mires, where suitable environments exist. For the Hualaihue volcanoes, there is a notable absence of large Holocene explosive eruptions of intermediate or evolved magma, compared to eruption records from volcanoes further south (e.g., Naranjo and Stern, 2004). Activity on Hualaihue has rather been dominated by effusive eruptions at Yate and Hornopirén, and explosive eruptions of basaltic scoria at Hornopirén and Apagado. All three volcanic centres on the Hualaihue peninsula were active in the Holocene. The Ap1 eruption represents an isolated reactivation of a deeply eroded volcanic centre, and is sited away from the main locus of Holocene activity on Hualaihue, which is along the LOFZ at Yate and Hornopirén. The primitive composition of Ap1 is unusual within a continental arc setting, and the eruption provides an interesting example of activity at a long-dormant volcanic centre, after a repose interval of several thousand, if not tens of thousands of years. The youthful edifice at Hornopirén, suggesting rapid recent growth, and the evidence for relatively low levels of Holocene volcanism at Yate, may indicate that the main focus of Holocene activity has shifted southwards, towards Hornopirén volcano. At Calbuco, tephra deposits indicate a highly active volcano, particularly in comparison to those on the Hualaihue peninsula. Holocene activity included an intense 3500-year-long eruptive period beginning around 10.5 cal ka BP, followed by a gap in large explosive eruptions before renewed high levels of activity in the past 2000 years. The mapped distribution of Cha1, now more precisely dated at w9.75 cal ka BP (this study; Naranjo and Stern, 2004), has been greatly extended. The distributional pattern, age and glass chemistry of the Cha1 tephra identified in Hualaihue are all consistent with the proximal deposit, confirming that this event was one of the larger Holocene eruptions in southern Chile (tephra volume of w3.5 km3), forming an important regional marker horizon due to its along-arc dispersal direction. Acknowledgements This work was funded by a NERC studentship to SFLW and NERC Radiocarbon Facility grants 1232.0407 and 1285.0408. JAN and HM acknowledge funding through FONDECYT Projects 1960186 and 1960885. GR acknowledges funding from SAREC (BILeCHIe-CHILM-02-K) and analytical assistance from Jane Boygle and Bob McCulloch. Additional fieldwork at Chaitén was supported by a NERC urgency grant NE/G001715/1. We are grateful to NERC, Steve Moreton and staff at the SUERC East Kilbride radiocarbon facility, and to José Luis Urrutia and CONAF staff at Parque Nacional Hornopirén for their assistance. Brent Alloway, Jennie Gilbert and David Lowe provided reviews and comments, which greatly improved the manuscript.

5. Summary and conclusions Appendix. Supplementary material Tephra deposits produced by arc volcanoes in southern Chile provide a long record of explosive volcanism. The new data presented here extend this eruption record, and contribute further to a history of large explosive eruptions that now spans the arc between Hudson and Calbuco volcanoes. The current state of knowledge regarding explosive eruption history for this arc segment is summarised in Fig. 11. With the exception of Cha1, all recognised tephra units in the region were transported broadly

Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.quaint.2011.05.029. References Aiuppa, A., Allard, P., D’Alessandro, W., Michel, A., Parello, F., Treuil, M., Valenza, M., 2000. Mobility and fluxes of major, minor and trace metals during basalt

S.F.L. Watt et al. / Quaternary International 246 (2011) 324e343 weathering and groundwater transport at Mt. Etna volcano (Sicily). Geochimica et Cosmochimica Acta 64, 1827e1841. Alfano, F., Bonadonna, C., Volentik, A.C.M., Connor, C.B., Watt, S.F.L., Pyle, D.M., Connor, L.J., 2011. Tephra stratigraphy and eruptive volume of the May, 2008, Chaitén eruption, Chile. Bulletin of Volcanology. doi:10.1007/s00445-0100428-x. Alloway, B.V., Neall, V.E., Vucetich, C.G., 1995. Late Quaternary (post-28,000 year B.P.) tephrostratigraphy of northeast and central Taranaki, New Zealand. Journal of the Royal Society of New Zealand 25, 385e458. Bertrand, S., Charlet, F., Charlier, B., Renson, V., Fagel, N., 2008. Climate variability of southern Chile since the Last Glacial maximum: a continuous sedimentological record from Lago Puyehue (40 S). Journal of Paleolimnology 39, 179e195. Blockley, S.P.E., Lowe, J.J., Walker, M.J.C., Asioli, A., Trincardi, F., Coope, G.R., Donahue, R.E., 2004. Bayesian analysis of radiocarbon chronologies: examples from the European Late-glacial. Journal of Quaternary Science 19, 159e175. Blockley, S.P.E., Bronk Ramsey, C., Pyle, D.M., 2008. Improved age modelling and high-precision age estimates of late Quaternary tephras, for accurate palaeoclimate reconstruction. Journal of Volcanology and Geothermal Research 177, 251e262. Bronk Ramsey, C., 2008. Deposition models for chronological records. Quaternary Science Reviews 27, 42e60. Bronk Ramsey, C., 2009. Bayesian analysis of radiocarbon dates. Radiocarbon 51, 337e360. Buck, C.E., Kenworthy, J.B., Litton, C.D., Smith, A.F.M., 1991. Combining archaeological and radiocarbon information: a Bayesian approach to calibration. Antiquity 65, 808e821. Carey, S., Sparks, R.S.J., 1986. Quantitative models of the fallout and dispersal of tephra from volcanic eruption columns. Bulletin of Volcanology 48, 109e125. Carey, S.N., Sigurdsson, H., 1982. Influence of particle aggregation on deposition of distal tephra from the May 18, 1980, eruption of Mount St. Helens volcano. Journal of Geophysical Research 87 (B8), 7061e7072. Cembrano, J., Hervé, F., Lavenu, A., 1996. The Liquiñe Ofqui fault zone: a long-lived intra-arc fault system in southern Chile. Tectonophysics 259, 55e66. Clavero, J., Moreno, H., 2004. Evolution of Villarrica volcano. Boletín de Servicio Nacional de Geología y Minería, Gobierno de Chile 61, 17e27. Corbella, H., Lara, L., 2008. Late Cenozoic Quaternary volcanism in Patagonia and Tierra del Fuego. In: Rabassa, J. (Ed.), The late Cenozoic of Patagonia and Tierra del Fuego, Developments in Quaternary Sciences, vol. 11, pp. 95e119. Daga, R., Ribeiro Guevara, S., Sánchez, M.L., Arribére, M., 2008. Source identification of volcanic ashes by geochemical analysis of well preserved lacustrine tephras in Nahuel Huapi National Park. Applied Radiation and Isotopes 66, 1325e1336. de Fontaine, C.S., Kaufman, D.S., Scott Anderson, R., Werner, A., Waythomas, C.F., Brown, T.A., 2007. Late Quaternary distal tephra-fall deposits in lacustrine sediments, Kenai Peninsula, Alaska. Quaternary Research 68, 64e78. Dugmore, A.J., Newton, A.J., Sugden, D.E., Larsen, G., 1992. Geochemical stability of fine-grained silicic Holocene tephra in Iceland and Scotland. Journal of Quaternary Science 7, 173e183. Durant, A.J., Rose, W.I., Sarna-Wojcicki, A.M., Carey, S., Volentik, A.C.M., 2009. Hydrometeor enhanced tephra sedimentation: constraints from the 18 May 1980 eruption of Mount St. Helens. Journal of Geophysical Research 114, B03204. Gislason, S.R., Arnorsson, S., Armannsson, H., 1996. Chemical weathering of basalt in Southwest Iceland: effects of runoff, age of rocks and vegetative/glacial cover. American Journal of Science 296, 837. Gonzalez Ferran, O., Innocenti, F., Lahsen, A., Manetti, P., Mazzuoli, R., Omarini, R., Tamponi, M., 1996. Alkali basalt volcanism along a subduction related magmatic arc: the case of Puyuhuapi quaternary volcanic line, southern Andes (44
20’S). XIII Congreso Geológico Argentino y III Congreso de Exploración de Hidrocarburos. Actas 3, 549e565. Gutiérrez, F., Gioncada, A., González Ferran, O., Lahsen, A., Mazzuoli, R., 2005. The Hudson Volcano and surrounding monogenetic centres (Chilean Patagonia): an example of volcanism associated with ridge-trench collision environment. Journal of Volcanology and Geothermal Research 145, 207e233. Haberle, S.G., Lumley, S.H., 1998. Age and origin of tephras recorded in postglacial lake sediments to the west of the southern Andes, 44 S to 47 S. Journal of Volcanology and Geothermal Research 84, 239e256. Heusser, C.J., Heusser, L.E., Hauser, A., 1992. Paleoecology of Late Quaternary deposits in Chiloé continental, Chile. Revista Chilena de Historia Natural 65, 235e245. Heusser, C.J., 2003. Ice age southern Andes: a chronicle of paleoecological events. Developments in Quaternary Science 3, 240. Kurtz, A.C., Derry, L.A., Chadwick, O.A., Alfano, M.J., 2000. Refractory element mobility in volcanic soils. Geology 28, 683e686. Lara, L.E., Naranjo, J.A., Moreno, H., 2004. Lanín volcano (39.5
S), southern Andes: geology and morphostructural evolution. Revista geológica de Chile 31, 241e257. López-Escobar, L., Parada, M.A., Hickey-Vargas, R., Frey, F.A., Kempton, P.D., Moreno, H., 1995. Calbuco volcano and minor eruptive centers distributed along the Liquiñe-Ofqui fault zone, Chile (41
e42
S): contrasting origin of andesitic and basaltic magma in the southern volcanic zone of the Andes. Contributions to Mineralogy and Petrology 119, 345e361. López-Escobar, L., Kilian, R., Kempton, P.D., Tagiri, M., 1993. Petrography and geochemistry of Quaternary rocks from the southern volcanic zone between 41
300 and 46
000 S, Chile. Revista Geologica de Chile 20, 35e55.

343

Lowe, D.J., Shane, P.A.R., Alloway, B.V., Newnham, R.M., 2008. Fingerprints and age models for widespread New Zealand tephra marker beds erupted since 30,000 years ago: a framework for NZ-INTIMATE. Quaternary Science Reviews 27, 95e126. Massaferro, G.I., Haller, M.J., D’Orazio, M., Alric, V.I., 2006. Sub-recent volcanism in Northern Patagonia: a tectonomagmatic approach. Journal of Volcanology and Geothermal Research 155, 227e243. McCormac, F.G., Hogg, A.G., Blackwell, P.G., Buck, C.E., Higham, T.F.G., Reimer, P.J., 2004. SHCal04 Southern Hemisphere calibration, 0e11 cal. kyr BP. Radiocarbon 46, 1087e1092. McDaniel, P.A., Lowe, D.J., Arnalds, O., Ping, C.-L., 2011. Andisols. In: Huang, P.M., Li, Y., Sumner, M.E. (eds-in-chief), Handbook of Soil Sciences, second ed., CRC Press (Taylor and Francis), London, in press. McGuire, W.J., Howarth, R.J., Firth, C.R., Solow, A.R., Pullen, A.D., Saunders, S.J., Stewart, I.S., Vita-Finzi, C., 1997. Correlation between rate of sea-level change and frequency of explosive volcanism in the Mediterranean. Nature 389, 473e476. Mella, M. Petrogenesis of the Yate Volcanic Complex (42
30’S), Southern Andes, Chile. PhD Thesis, Institute of Geosciences, University of Sao Paolo, Brazil, 2008. Moreno, H., 2004. Osorno-Calbuco. IAVCEI General Assembly 2004, Pucon, Chile Field Trip Guide C4. Naranjo, J.A., Stern, C.R., 1998. Holocene explosive activity of Hudson Volcano, southern Andes. Bulletin of Volcanology 59, 291e306. Naranjo, J.A., Stern, C.R., 2004. Holocene tephrochronology of the southernmost part (42
30’e45
S) of the Andean southern volcanic zone. Revista Geológica de Chile 31, 224e240. Nowell, D.A., Jones, M.C., Pyle, D.M., 2006. Episodic Quaternary volcanism in France and Germany. Journal of Quaternary Science 21, 645e675. Pearce, J.A., Peate, D.W., 1995. Tectonic implications of the composition of volcanic arc magmas. Annual Review of Earth and Planetary Sciences 23, 251e285. Pyle, D.M., 1989. The thickness, volume and grainsize of tephra fall deposits. Bulletin of Volcanology 51, 1e15. Pyle, D.M., 1995. Assessment of the minimum volume of tephra fall deposits. Journal of Volcanology and Geothermal Research 69, 379e382. Pyle, D.M., 1999. Widely dispersed Quaternary tephra in Africa. Global and Planetary Change 21, 95e112. Pyle, D.M., 2000. Sizes of volcanic eruptions. In: Sigurdsson, H. (Ed.), Encyclopedia of Volcanoes. Academic Press, San Diego, pp. 463e475. Reimer, P.J., Baillie, M.G.L., Bard, E., Bayliss, A., Beck, J.W., Blackwell, P.G., Bronk Ramsey, C., Buck, C.E., Burr, G.S., Edwards, R.L., Friedrich, M., Grootes, P.M., Guilderson, T.P., Hajdas, I., Heaton, T.J., Hogg, A.G., Hughen, K.A., Kaiser, K.F., Kromer, B., McCormac, F.G., Manning, S.W., Reimer, R.W., Richards, D.A., Southon, J.R., Talamo, S., Turney, C.S.M., van der Plicht, J., Weyhenmeyer, C.E., 2009. IntCal09 and Marine09 radiocarbon age calibration curves, 0e50,000 years cal BP. Radiocarbon 51, 1111e1150. Scasso, R.A., Corbella, H., Tiberi, P., 1994. Sedimentological analysis of the tephra from the 12e15 August 1991 eruption of Hudson volcano. Bulletin of Volcanology 56, 121e132. Siebert, L., Simkin, T., 2002. Volcanoes of the World: An Illustrated Catalog of Holocene Volcanoes and Their Eruptions. Smithsonian Institution, Global Volcanism Program Digital Information Series, GVP-3. http://www.volcano.si. edu/world/. Singer, B.S., Jicha, B.R., Harper, M.A., Naranjo, J.A., Lara, L.E., Moreno-Roa, H., 2008. Eruptive history, geochronology, and magmatic evolution of the PuyehueeCordón Caulle volcanic complex, Chile. Bulletin of the Geological Society of America 120, 599e618. Sparks, R.S.J., Bursik, M.I., Ablay, G.J., Thomas, R.M.E., Carey, S.N., 1992. Sedimentation of tephra by volcanic plumes. Part 2: controls on thickness and grain-size variations of tephra fall deposits. Bulletin of Volcanology 54, 685e695. Stern, C.R., 2004. Active Andean volcanism: its geologic and tectonic setting. Revista Geologica de Chile 31, 161e206. Stern, C.R., 2008. Holocene tephrochronology record of large explosive eruptions in the southernmost Patagonian Andes. Bulletin of Volcanology 70, 435e454. Sun, S.S., McDonough, W.F., 1989. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. Geological Society London Special Publication 42, 313e345. Watt, S.F.L., Pyle, D.M., Naranjo, J.A., Mather, T.A., 2009a. Landslide and tsunami hazard at Yate volcano, Chile as an example of edifice destruction on strike-slip fault zones. Bulletin of Volcanology 71, 559e574. Watt, S.F.L., Pyle, D.M., Mather, T.A., Martin, R.S., Matthews, N.E., 2009b. Fallout and distribution of volcanic ash over Argentina following the May 2008 explosive eruption of Chaitén, Chile. Journal of Geophysical Research 114, B04207. Watt, S.F.L., Pyle, D.M., Mather, T.A. Geology, petrology and geochemistry of the dome complex of Huequi volcano, southern Chile. Andean Geology 38, in press. White, S.M., Crisp, J.A., Spera, F.J., 2006. Long-term volumetric eruption rates and magma budgets. Geochemistry, Geophysics, Geosystems 7, Q03010. Whitlock, C., Bianchi, M.M., Bartlein, P.J., Markgraf, V., Marlon, J., Walsh, M., McCoy, N., 2006. Postglacial vegetation, climate, and fire history along the east side of the Andes (lat 41e42.5
S), Argentina. Quaternary Research 66, 187e201. Wolodarsky-Franke, A., Moreno, P.I., Lara, A., Pino, M., Villarrosa, G., 2005. Tree-ring, stratigraphic, and palynological evidence for volcanic and climatic controls on Fitzroya cupressoides forests in southern Chile over the last 5700 years. In: Holocene Environmental Catastrophes in South America: From the Lowlands to the Andes, March 11e17, 2005, Laguna Mar Chiquita, Miramar, Córdoba Province, Argentina.