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The Loma Seca and the Calabozos : A major ash-flow and caldera complex in the southern of central

WES HILDRETH U.S. Geological Survey, Menlo Park, California 94025 ANITA L. GRÜNDER Department of Geology, Stanford University, Stanford, California 94305 ROBERT E. DRAKE Department of Geology and Geophysics, University of California, Berkeley, California 94720

ABSTRACT Voluminous ash-flow sheets of early agglutinates that erupted for the most part from to age are extensive and well- long-lived composite centers. In contrast, south A 26 x 14-km composite ring-structure cal- preserved in the arid central segment (Guest, of 37°S, high-alumina exceeds the inter- dera of late age has been discovered 1969; Pichler and Zeil, 1972; Kussmaul and mediate rocks (Moreno, 1974) and to the north and mapped near the Andean crest in central others, 1977; Baker, 1981), where such sheets (28°S-33°S), an extensive segment of the Andes Chile (35°30'S). Rhyodacitic to dacitic zoned may constitute 10% to 40% of the total volume apparently has lacked post-Miocene volcanic ac- ash-flow sheets, each representing 150 to 300 erupted during that time interval (Francis and tivity entirely. km3 of magma, were emplaced 0.8, 0.3, and Rundle, 1976; Baker and Francis, 1978). Ash- Between 33°S and 36°S, the belt of active 0.15 m.y. ago; the youngest of the associated flow eruptions have also been important in the stratovolcanoes lies —260 km east of the axis of collapses was closely followed by resurgent basalt-poor part (33°S-36°S) of the southern the Chile Trench and ~90 km above the top of a doming of the caldera floor and development of segment, and one contribution of this study is to Wadati-Benioff zone that dips ~20°E (Stauder, a longitudinal graben. Postcaldera eruptions of correct the widespread misconception that ash- 1973; Barazangi and Isacks, 1976); the present and have persisted into flow tuffs are rare or absent in the southern relative convergence rate between the Nazca time, and active hot springs are abundant along Andes (Klerkx and others, 1977; Thorpe and and South American plates is — 9 to 11 cm/yr at caldera-marginal and resurgent systems, Francis, 1979). Our data from 35°S to 36°S and 30°S (Minster and others, 1974; Minster and suggesting a significant geothermal-energy re- many Argentine reports (Polanski, 1962, 1964; Jordan, 1978). The crust beneath the Andean source. The Pleistocene eruption rate of this dis- González Díaz, 1972; González Bonorino, 1944; Cordillera is thought to thin southward from trict and the abundance of older Quaternary to Volkheimer, 1968) concerning the eastern flanks >45 km at 33°S to -30 km at 41°S (Lomnitz, Miocene ash-flow remnants in the 33°S to 36°S of the range suggest that, of the total volume of 1962; Moreno, 1974, and references therein). segment of the glaciated southern Andes indi- material erupted in late Cenozoic time between This thinning coincides with (1) the change from cate that ash-flow magmatism has been no less 33°S and 36°S, the proportion of ash-flow tuff intermediate to dominantly basaltic volcanism, important here than in the arid central Andes was at least as great as that in the central Andes (2) a decrease in the number of earthquakes (16°S-28°S), where ash-flow sheets are far bet- of northern Chile and Bolivia. In that bet- deeper than 70 km (Stauder, 1973), and (3) a ter preserved. ter resist erosion, however, the silicic pyroclas- decline in basal elevations of the volcanic edifi- tics actually erupted tend to be severely under- ces from -4,000 m to <1,000 m. Our study area INTRODUCTION represented in the surviving geologic record of lies at 35°30'S, intermediate along these trends, icy Cordilleran arcs like the southern Andes. where the late Cenozoic volcanoes rest upon a Quaternary volcanism in the Andean Cordil- basement at elevations of 1,500 to 2,500 m and lera has been largely restricted to three discrete REGIONAL SETTING upon a crust that is about 40 km thick. segments, each with active composite volcanoes. The area we have mapped (Fig. 1) has re- The northern volcanic zone extends from 5°N to The area investigated (Fig. 1) lies on the crest ceived little previous attention. Gonzalez Ferran 2°S (in and Ecuador); the central and western slope of the Andes at about 35°30 'S and Vergara (1962) established some broad zone runs from 16°S to 28°S (in southern Peru, in the Chilean provinces of Curicó and Talca. outlines in the course of a regional geologic re- Bolivia, northern Chile, and northwestern Ar- Central Chile is here only 170 to 210 km wide connaissance, and Drake (1976) mapped a large gentina); and the southern segment, which con- and is physiographically divided into a Coastal area to the south and west of Figure 1. Near the tains the area discussed in this report (Fig. 1), Cordillera, Central Valley, and Andean Cordil- eastern limit of his field area, Drake identified at extends from 33°S to 46°S (in central Chile and lera. Post-Miocene volcanism has been largely Loma Seca (Fig. 1) a 700-m-thick, gorge-filling Argentina) and discontinuously to 52°S. All (but not entirely) restricted to the Andean Cor- tongue of welded tuff, the top of which yielded a three segments contain prominent stratocones of dillera, which at this latitude is ~ 140 km wide, K-Ar age of 0.14 m.y. (Table 1, no. 11). Air- andesite to dacite (Thorpe and others, its eastern half in Argentina. The Pliocene and photo study suggested a source for the tuff on 1980, 1981), but Quaternary within the Quaternary volcanic rocks of this part of the the Andean crest and led to our expeditions into Cordilleran arc itself are common only south of Andes (33°S-36°S) are essentially undeformed that remote region in 1980,1981, and 1982. We 37°S, where the crust thins to —30 km (Moreno, but include complex intracanyon assemblages, have subsequently identified several ash-flow 1974). chiefly of andesitic to dacitic lavas, tuffs, and sheets of great volume, discovered and mapped

Geological Society of America Bulletin, v. 95, p. 45-54, 6 figs., 2 tables, January 1984.

45

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f ¡Cambre JmLas Mu/as

Cerro San Francisquito

35045' _

Holocene andesitic and dacitic lavas and tuffs jtjX Holocene volcanic vent Glaciated andesitic and dacitic lavas OH younger than Unit S Loma Seca Tuff ir Hot-spring dusters • Unit S (0.15 my.) A Pleistocene eruptive centers Lavas intercalated between Units V and S

Unit f (0.3my.) -—-—" contact

Unit L (0.8 my) " fault (bar on downthrown side) Undifferentiated pre- Loma Seca rocks 12 Location of K-Ar sample Plutonic rocks (Table I)

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a 26 x 14-km resurgent caldera complex that is annual rainfall at Laguna de la Invernada (1,300 Figure 1. Generalized map of the Calabo- the source of at least the two youngest sheets, m; Fig. 1) is 134 cm, and extremes for the period zos caldera complex and adjacent terrains. and initiated petrochemical studies on the 1964-1980 were 50 cm and 225 cm. Normal Inset shows map location, active stratovolca- caldera-related system. What we have named monthly precipitation from May to August is 20 noes (triangles) in central Chile, and belt of the Calabozos caldera (Hildreth and others, to 35 cm, largely snowfall, but the four austral silicic centers (circles) behind the arc. Out- 1981) was not previously recognized on air pho- summer months averaged <1 cm/month during flow sheets of Loma Seca Tuff extend an un- tographs because of its structural complexity, se- the 17-yr period of record. Temperatures often known distance into Argentina. Six diorite to vere postcaldera glacial erosion, and the mant- reach 25 °C on midsummer afternoons, but granodiorite plutons are shown; the one at ling of its western margin by younger lavas above 2,500 m they commonly dip below freez- Laguna de la Invernada is 7 m.y. old (Drake, (Fig. 1). ing at night. Vegetation is exceedingly sparse 1976). Abbreviations for key locations: CA, The mapped area (Fig. 1) covers ~ 1,800 km2 above 1,200 m, and the southern half of the area Cerro Azul, and VDG, Volcán Descabezado of ruggedly glaciated mountains, which are en- shown in Figure 1 has been a desert Grande, both major stratocones; CB, Cerro tirely roadless and uninhabited except by a few since the Plinian eruption of Volcán Quizapu in Bravo; CC, Cajón Los Calabozos; CD, Cor- summer herdsmen. Access is on foot or horse- 1932 (Kittl, 1933; Fuenzalida, 1943; Hildreth dón El Despalmado; CDM, Cerro del Medio; back from roadheads in the Andean foothills. and Drake, 1983). CR, Cerro Rajaduras; CT, Cerro Troncos, The caldera floor lies at -2,600 m, and its east the resurgent uplift; DZ, Cuesta del Durazno; wall and central uplift are at -3,000 m; several PRECALDERA ROCKS LC, Cerro La Chepa; LR, Loma Los Robles; volcanic edifices nearby are a few hundred MP, Cerro Manantial Pelado; RC, headwa- metres higher (Fig. 1), and at 4,090 m Volcán Drake (1976) and González Ferrán and Ver- ters of Rio Colorado; TM, Cerro Tetas de Peteroa (7 km north of Fig. 1) is the highest gara (1962) summarized what was known of the Monsalve; VDC, Volcán Descabezado Chico; peak in the district. Drainage is roughly radial geologic history of the district. A north-south VG, Valle Grande. Elevations in metres. from the caldera, the east wall of which is the belt of marine clastic, gypsiferous, and carbonate continental divide. Glacial gorges having 500 to rocks of age is truncated by the north- 1,000 m of local relief drop steeply toward the east margin of the Calabozos caldera, and parts Central Valley, which begins -60 km west of of the canyon walls west of the caldera consist of the caldera at -300 m elevation. Such gorges metavolcanic and plutonic rocks thought to be provide spectacular exposure, tedious transit, Late Cretaceous and in age. The Cam- and rivers that are a challenge to ford. Mean panario Formation, a Miocene pyroclastic se-

TABLE 1. K-Ar DATES AND ANALYTICAL DATA

40. 40. Map Ka Rock Unit Material Sample Wt % K atm AGE (m.y.)t Comments -n'rad no. no. dated* wt (g) (±1%) x '° (mol/g) ±2o

1 4107 Andesitic WR 6.285 1.968 0.691 79.9 2.02 ± 0.1 Base of thick subhorizontal stack of preglacial precaldera lavas of west wall of Cerro Mora, 4 km northeast of Paso Yeso 2 2620 Andesitic lava WR 13.612 1.609 0.303 81.0 1.09 ± 0.03 Subhorizontal stack of lavas. Predates stratocone and cutting of deep Pleistocene canyons; overlain by cooling unit V

3 2546 Mafic andesitic lava WR 13.490 1.033 0.194 80.3 1.08 ±0.02 Same as no. 2; 200 m higher, near top of stack at Cordon El Despalmado, Rio Claro del Maule 4 2614 Mafic andesitic lava WR 15.434 1.127 0.068 91.5 0.35 ±0.03 Subhorizontal stack of lava (lows 5 km northeast of Quizapu. Basal to strato- cone Descabezado Grande

5 4109 Mafic andesitic lava WR 11.377 1.063 0.101 88.6 0.55 ± O.OS Lava flow directly under cooling unit V at northwest end of Cuesta del Durazno (near Los Retamos)

6 2608 Rhyodacitic lava bi 1.074 6.617 0.300 97.7 0.26 ± 0.04 Silicic lava flow 5 km south of Cerro San Francisquito; part of extensive flow-dome complex

7 4054 Loma Seca Tuff, L gl 4.991 3.415 0.469 98.1 0.79 ± 0.15 Remnant of cooling unit L west of Cajon Grande; >95% removed by 8 3975 Loma Seca Tuff, V WRF 11.659 4.553 0.243 91.7 0.31 ± 0.03 Cooling unit V at southwest snout of Loma Seca 3975 R Loma Seca Tuff, V WRF 3.134 4.553 0.215 94.3 0.27 ± 0.04

9 4057 Loma Seca Tuff, V Pl 9.807 0.884 0.046 87.4 0.30 ± 0.02 Intracaldera welded tuff, 1 km south of Llolli. Overlain by cooling unit S and intercalated lavas

10 4116 Dacitic lava Pl 13.013 0.968 0.046 94.3 0.275 ± 0.02 -rich lava directly over cooling unit V and under cooling unit S; 7 km south of Llolli

4116R Pl 8.757 0.968 0.048 92.8 0.285 ± 0.02 11 2613 Loma Seca Tuff, S WR 12.063 3.320 0.080 94.1 0.14 ± 0.04 Cooling unit S; southwest rim of Loma Seca 12 3897 Loma Seca Tuff, S WRF 10.358 3.537 0.099 97.3 0.16 ± 0.03 Cooling unit S; northwesterly outflow lobe 13 4113 Loma Seca Tuff, S WRF 8.833 3.304 0.095 96.5 0.16 ± 0.02 Cooling unit S; top of main sheet in upper Cajon Los Calabozos

14 4053 Preresurgence gl 5.922 3.918 0.094 92.5 0.14 ± 0.01 Flow rests on Unit S at southwest side of the Cerro Troncos dome: both are dacitic lava upwarped and cut by antithetic faults 15 3974 Postresurgence WR 9.127 1.880 0.031 98.0 0.096 ± .036 Moat lava banked against southeast side of the Cerro Troncos dome; overlies andesitic lava Unit S. Not apparently deformed 16 3862 Postcaldera dacitic gm 10.403 3.819 0.057 93.3 0.08< ) ± .01 Flow caps plateau 4 km southwest of Llolli. Younger than one or more lava post-S glaciations

Note: Analytical mors calculated following Mahood and Drake (1982).

4 40 10 40 10 1 36 ^Constants used: '"'k/R = 1.167 x 10" ; KX^ - 4.962 x 10~ KXt - 0.581 x 1(T y" ; "°Ar/ Ar = 295.5. 'Abbreviations: WR, whole rock; pi, ; bi, biotite; gl. glass; gm, groundmass (less plagioclase, , and oxide ); WRF, single welded fiamme.

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quence dominated by ash-flow tuffs, crops out in a broad belt from Laguna de la Invernada (Fig. 1) to at least 36°S and has been dated at 15 to 6 m.y. (Drake, 1976). A large diorite to gran- odiorite pluton only 7 m.y. old (Drake, 1976) is exposed from its roof to a depth of 1,400 m at Laguna de la Invernada and probably was part of the Campanario magmatism. The present phase of andesitic volcanism began here ~4 m.y. ago (Drake, 1976), lava flows from numerous composite centers coalesc- ing to build an extensive "andesitic ser- ies" (Vergara and Munizaga, 1974; López Escobar and others, 1976; González Ferrán and Vergara, 1962) that predates the onset of canyon-cutting glaciations. Stacks of subhori- zontal lavas, for the most part -bearing mafic andesite, accumulated to thicknesses of 300 to 500 m in much of the area of Figure 1, and such stacks form parts of the east and north Figure 2. Three main units of the Loma Seca Tuff on the southeast wall of Cajón Grande. walls of the Calabozos caldera. They have been Lower half of the 400-m-high exposure consists of tilted pyroclastic rocks of the Miocene K-Ar dated at 2.0 m.y. near both Cerro San Campanario Formation. Right of center, remnant 100-m cliff of unit L (0.8 m.y.) is draped Francisquito (Drake, 1976) and Paso Yeso and unconfonnably by unit V (0.3 m.y.), which forms the main 100-m cliff at the left. A bench- at 1.1 m.y. on Cordón El Despalmado (Table 1, forming nonwelded zone underlies the dark rim-forming part of unit S (0.15 m.y.) at the upper nos. 1-3). Near the snout of Cuesta del Durazno left. Unit S forms much of the plateau surface, thinly mantling V and in turn blanketed by 1932 (Fig. 1), the 0.3-m.y.-old outflow member of the pumice from Volcán Quizapu. All three units have prominent, dark, lower vitrophyres. Skyline Loma Seca Tuff rests directly upon intracanyon peak is pre-Loma Seca andesitic lava, south of Cumbre Las Mulas. andesitic lava dated at 0.55 m.y. (Table 1, no. 5). A younger pile of dated at 0.35 m.y. (Table 1, no. 4) appears to represent lavas basal to the active , Descabezado Grande. All of the Pliocene and Quaternary lavas are essentially undeformed. A north-south belt of silicic centers east of the line of active andesitic stratovolcanoes (Fig. 1, inset) includes the Holocene rhyolitic lavas at Laguna del Maule (36°S), a late Pleistocene flow-and-dome complex south of Cerro San Francisquito (Table 1, no. 6), and the Calabozos caldera itself.

LOMA SECA TUFF

We assign to the Loma Seca Tuff (Hildreth and others, 1981) two voluminous composite ash-flow sheets that erupted 0.15 and 0.3 m.y. ago from the Calabozos caldera complex, and we tentatively include the remnants of a third sheet (0.8 m.y. old) that also may have had its source there. From oldest to youngest, the sheets are designated units L, V, and S (Fig. 1; Table 1). Each consists of quartz-free, two-pyroxene- plagioclase dacitic to rhyodacitic tuff that for the most part is densely welded. Although basal ex- Figure 3. Units V and S along southeast wall of Cajón Los Calabozos. Plateau atop 500-m posures are common to the east, west, and south ash-flow cliff is named Loma Seca (Fig. 1). Basal vitrophyre of unit S and underlying 50-m- of the caldera, we have nowhere identified air- thick vapor-phase zone of unit V can be seen at right. Tall, thick columns are characteristic of fall deposits related to the ash-flow units. Partial V, and multistriped cooling zonation is typical of S. Mantle of 1932 pumice from Volcán confinement of some outflow lobes to glacially Quizapu is in the foreground. incised gorges permitted accumulation of indi-

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vidual cooling units as thick as 400 m. Subse- abundant vugs and vapor-phase seams; scattered occurrence of hornblende at the top of both, and quent ice advances excavated spectacular expo- hydrothermally altered lithic fragments that by the following stratigraphic reasoning. Both sures (Fig. 2) in the course of removing as much have generated thin alteration rinds in the en- facies of unit V directly underlie unit S, although as 75% of extracaldera units S and V and proba- closing tuff matrix; and widely spaced columnar agglutinates and lava flows are locally interca- bly >95% of unit L. joints, which occur in one to four tiers that are lated between them both inside and outside the each vertically continuous for as much as 100 caldera. Furthermore, the extensive outflow Unit L m (Fig. 3). Lithic inclusions are primarily 1- to sheets of unit V must have an intracaldera equiv- 5-cm fragments of intermediate lavas and nor- alent, and the intracaldera tuff beneath unit S is The three sheets of the Loma Seca Tuff are mally amount to 1% to 10% of the rock. In the the only permissive correlative. lithologically distinctive and readily distinguish- basal part of the sheet, however, poorly welded The dissimilarity of the intracaldera and out- able at most outcrops. Unit L differs from both lenses up to a few tens of centimetres thick con- flow facies of unit V are attributable to the fol- younger tuffs in being poor in both lithic inclu- tain as much as 50% lithics, which evidently lowing. (1) Intracaldera V is a relatively late sions and flattened lenticles (fxamme). Its pheno- provided a heat sink adequate to suppress weld- emplacement unit that tapped deeper and more cryst content ranges from <5% to -15%, chiefly ing. Throughout the outflow sheets of unit V the crystal-rich magma. (2) The intracaldera tuff 0.5 to 2 mm plagioclase, and it forms grussy- to fiamme contain less than 5% phenocrysts and banked against cold caldera walls during subsi- platy-weathering, dark gray-brown cliffs as high generally constitute -10% of the rock (but range dence, engulfed floods of lithics and coarse lithic as 100 m. Like the younger sheets, unit L was from 0% to 20%); the matrix seldom contains breccias, and may have accumulated slowly or deposited in Pleistocene canyons that were sub- >10% phenocrysts. The dominant phenocryst is fitfully during the waning stages of eruption. sequently reglaciated, leaving gorge-rimming plagioclase; hypersthene and titanomagnetite are Any of these factors could have inhibited dense shelves and wall veneers of welded tuff as the common throughout, but clinopyroxene, biotite, welding. (3) Hydrothermal activity was largely principal types of remaining outcrop. Restriction ilmenite, and apatite occur only sparsely and restricted to the caldera and preferentially al- of its remnants to two areas (Fig. 1) only a few may not all be present throughout the entire tered the relatively permeable intracaldera tuff. kilometres south and east of the Calabozos cal- outflow volume. Hornblende has been found only at the very top of unit V, both in the Loma dera is the main basis of our inference that unit L Units erupted from the same system as units V and S. Seca outflow lobe and inside the caldera. After an interval of -150,000 yr marked by Extracaldera Unit V Intracaldera Unit V glaciation and by eruption of a few andesitic lavas that overlie V (Fig. 1), another ash-flow Unit V is the thickest and was probably the In the northern half of the caldera complex sheet, unit S, was erupted from approximately most voluminous of the 3 sheets, with a recon- (Fig. 1), >500 m of intracaldera unit V is ex- the same source area about 0.15 m.y. ago. This structed volume of 250 to 300 km3 of predomi- posed, but its base is nowhere seen. Owing to unit is essentially coextensive with V but gener- nantly densely welded tuff. Prior to erosion, glacial erosion, outcrops of the intracaldera unit ally thinner, and its volume is estimated to have both V and S probably extended down drain- and of the inferred correlative outflow are no- been -175 to 250 km3, chiefly densely welded ages at least as far as the Andean foothills both where closer to each other than ~3 km. Al- tuff. Its present maximum thickness is -300 m east and west; but even several such 50-km though mineralogically similar, the intracaldera at Loma Seca (Fig. 3), where its top is eroded. tongues of tuff would add no more than 10% to facies differs from the outflow in being generally Intracanyon sections are generally strongly the respective volume estimates. poorly welded and richer in phenocrysts. welded except in the basal 10 cm or so, but The thickest extracaldera outcrops of unit V The intracaldera tuff is extensively altered, where unit S thins to -50 m on some , are 350 m near Cuesta del Durazno and 400 m largely by acid leaching, and disintegration of its large parts of it are only partly welded or are at Loma Seca (Fig. 1); the intracaldera tuff is rotten pumice produces pitted to cavernous nonwelded (Fig. 2). Thick welded exposures thicker yet, but its base is not exposed. Outflow weathering. It is usually buff or red-brown but typically exhibit six or more alternating light and V is predominantly devitrified, except for a 2- to locally chalky white or, especially near its top, dark gray bands (Fig. 3), which are laterally 20-m-thick basal vitrophyre, and it is densely green. Although everywhere devitrified and continuous for hundreds of metres and differ welded except in (1) lithic-rich zones of poor or generally well-lithified, most of intracaldera V is principally in degrees of devitrification and partial welding, generally <1 m thick, at or near poorly welded, except on the plateau south- vapor alteration. Although flow partings are not the base; (2) devitrified zones of partial welding southeast of Llolli (Fig. 1), where welding is generally prominent, such zones probably reflect as much as a few metres thick, which represent dense. Despite generally similar suites of pheno- the variable cooling zonation imposed by suc- contacts between successive emplacement pulses crysts, the intracaldera tuff contrasts with the cessive emplacement of several flow units. and locally promote development of cliff- outflow sheets in its higher total phenocryst con- Within 3 km of the caldera, this main com- interrupting ledges; and (3) an upper partly tent, which ranges from -5% to >30%. Strati- pound cooling unit is locally overlain conform- welded vapor-phase zone, which has been ex- graphically high parts of the intracaldera tuff are ably by remnants of two or three 5- to tensively eroded but is ~50 m thick near the richest in clinopyroxene and in total crystals, 25-m-thick sheets that have basal nonwelded snout of Loma Seca (Fig. 3). The sheet is com- contain a little hornblende, and have plagioclase zones and appear to represent waning stages in posite, but the apparent absence of erosion or as large as 3 to 5 mm. Much of the tuff has only the pulsatory emplacement of unit S. In upper reworking between subunits suggests emplace- 1% to 3% lithics, but in the west part of the Cajón Los Calabozos (Fig. 1, near location 13), ment within hours or days. caldera, a 100-m-thick zone near the top con- a 20-m-thick nonwelded zone between the main Outflow V is predominantly phenocryst-poor tains -20%. unit and these late sheets is uneroded, suggesting a pause no longer than a few days between em- and lithic-rich, contrasting in both respects with Correlation of intracaldera and outflow unit placement events. unit S. Also characteristic of V are locally V is supported by K-Ar dates (Table 1), by the

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such lavas are better preserved in the west part of the caldera and on its east rim (Fig. 1). A shallow porphyritic intrusion ~ 1 km across (not shown in Fig. 1) incised by the Rio Colorado northwest of Cerro Negro is apparently the root of a post-V dome emplaced in the fault zone. Following the eruption of unit S about 0.15 m.y. ago, the tuff was partly covered by several lava flows, some of them inside the caldera. Two such intracaldera lavas, one a dacite cut by the resurgent fault system and the other an andesite banked against the resurgent uplift (Fig. 1, nos. 14 and 15), have yielded respective K-Ar ages of 0.14 and 0.10 m.y. (Table 1) that bracket the structural resurgence. In addition, several rem- nants of stratified tuffaceous sediments that rest directly upon unit S on the slopes and crest of Cerro Troncos (Fig. 6) now occur as much as 300 m above the caldera floor and were clearly tilted and faulted along with the welded tuff during uplift. Several undated lavas that appear to postdate both unit S and the caldera faulting have survived as ice-scoured remnants along the eastern moat (Fig. 1). Figure 4. Welded-tuff lithology typical of devitrified zones of unit S outflow sheets. Flat- Between Cerro Negro and Llolli, the caldera tened lenticles commonly reach 50 to 150 cm in length. Hammer is 33 cm long. Prior to moat is a deep canyon that has been a persistent welding, some lenticles (fiamme) are thought to have been spatter blobs rather than well- trap for a wide variety of fill, including alluvium, vesiculated pumice; locally, some were angular fragments of vitrophyre. peat, till, and tuffaceous sediments; debris-flow, talus, and landslide deposits; sinter- and traver- Unit S is characterized by conspicuous fi- Los Calabozos, (2) a 15-m-thick cooling unit tine-cemented breccias; a large number of lava amme, many very large (Fig. 4); they constitute rich in lithics and lithophysae that caps the sec- flows, generally brecciated; and a few minor 5% to 10% of the rock, except in a 5- to 8-m tion around the head of Cajón Grande (Fig. 1), pyroclastic-flow and -fall units. Despite repeated basal zone where they are absent and in some and (3) swarms of vitrophyre blocks, presumed glacial scouring, such deposits form ubiquitous intracaldera exposures where their abundance cognate, in the top of the main sheet in many veneers on the lower walls of the canyon, and reaches 20%. The fiamme contain 5% to 15% areas in and around the caldera. Inasmuch as all the modern river gorge cuts >100 m into such phenocrysts, for the most part small (<1 mm) such concentrations of angular inclusions, both fill, much of it badly altered by hot-spring and plagioclase, and the matrix generally has 15% to accidental and cognate, formed late in the em- fumarolic activity. 20% crystals. However, upper parts of the tuff in placement sequence, their origin is probably re- Holocene eruptive activity has been common and near the caldera contain slightly larger and lated to structural collapse of the source after and voluminous within and just west of the Ca- more abundant phenocrysts, locally as much as partial evacuation of the magma reservoir. Pro- labozos caldera. Cerro del Medio (Fig. 1) exhib- 25% to 30%. As in unit V, the assemblage is gressive deformation of the angular chunks of its several overlapping and nested craters that plagioclase-hypersthene-clinopyroxene-titano- vitrophyre into flattened lenticles is evident in produced ~6 km3 of sheeted dacite agglutinate, magnetite with accessory biotite, ilmenite, and several welded sections, indicating that not all andesitic cinders, and one lava dome. The large- apatite, but it has not yet been confirmed that all fiamme passed through a well-vesiculated pum- ly Holocene complex is built upon a platform of of the accessory phases are present throughout iceous stage. glaciated dacitic lavas and rises to -3,500 m, the sheet. Clinopyroxene is more conspicuous in some 900 m higher than the caldera floor. Just the crystal-rich, later-emplaced parts of the tuff, POSTCALDERA VOLCANIC AND west of the caldera, another cluster of cinder and consistent with the progressive increases in both SEDIMENTARY ROCKS sheeted-agglutinate cones, including Volcán mafic and total phenocryst content observed in Descabezado Chico (Fig. 1), has remained ac- most major pyroclastic eruptive sequences (Hil- After eruption of unit V and of tive into postglacial time; a late Holocene dacite dreth, 1981). Preliminary chemical data from part of the present caldera complex, a consider- lava that vented there flowed >30 km down the Loma Seca sector of unit S indicate that the able volume of tuffaceous sand and gravel, ap- Cajón Los Calabozos. A third but smaller cen- minimum extent of its zonation is from >12% to parently derived in large part from V, accumu- ter, near the northwest margin of the caldera, <66% SiC)2 (Fig. 5). lated inside the depression. These sediments also produced a thick accumulation of dacitic Unit S contrasts with unit V in its greater have survived along the structurally complex pumice-agglutinate in Holocene time. A veneer phenocryst content and its generally much lower and glacially incised Rio Colorado fault zone as of white biotite- pumice occurs on content of lithics, lithophysae, and vapor-phase discontinuous remnants, the largest of which lie the plateau east of Cerro Negro and is probably seams. Exceptions are (1) a 20-m-thick zone 6 to 8 km southeast of Llolli. Dacitic and ande- also of Holocene age, but its source, presumably ' rich in angular blocks of older andesite within sitic lavas intermediate in age between units V local, remains uncertain. the top of the main cooling unit in upper Cajón and S also accumulated along this fault zone, but The stratovolcanoes Cerro Azul and Volcán

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LOMA SECA TUFF UNIT S 300 A _ o^ Rb - 200 A \ 100 S r 0 V —y 2 ALK

- _ A * AI20,-7 § 7 £—

- 0 FeO**3 • KzO

Figure 6. View from the west side of Cerro del Medio northward across graben system that • CaO cuts the resurgent uplift, Cerro Troncos. Visible surface of the uplift is largely unit S, but a ~¡» MgO 1 0.14-m.y. glaciated dacitic lava flow atop unit S is also faulted at the left. The lake and a Holocene lava flow rest on the glaciated caldera floor, which here consists of both unit S and Si02 postcaldera andesitic lavas. Several clusters of hot springs mark the hingeline of the dome Figure 5. Compositional variations in unit along the far lakeshore and farther to the right. In the distance rises the ice-clad slope of the S of the Loma Seca Tuff. U.S. Geological active 4,000-m Peteroa-Planchón-Azufre andesitic complex (just north of Fig. 1). Survey analyses by XRF; all data normalized

to 100%, excluding H20 but including an es- timated 0.4% trace oxides and halogens. For dense agglutinates. In April 1932, Volcán Qui- moat of the caldera was deeply excavated by comparison, open symbols represent a single zapu, a flank vent of Cerro Azul, produced a 15 valley glaciers. 3 analysis of basal vitrophyre of unit V at to 20 km Plinian tephra eruption (Kittl, 1933; southwest snout of Loma Seca, and dashed Fuenzalida, 1943), which was zoned from 70% Unit-V Caldera to 55% Si02 (Hildreth and Drake, 1983). line is the K20-Si02 trend of the 1932 com- positionally zoned eruption of Volcán Qui- Shortly thereafter, in June 1932, Volcán Des- The subsidence structure related to eruption zapu (Fig. 1), 20 km southwest of the cabezado Grande underwent a minor scoria of Unit V is well displayed in the northern half Calabozos caldera (Hildreth and Drake, eruption that created a small crater on its north- of the caldera complex outlined in Figure 1. east flank. Along the left bank of the Rio Colorado be- 1983). In order of decreasing Si02, the 5 samples of unit S are (1) basal vitrophyre at tween Cerro Negro and Llolli (Fig. 1), >500 m southwest snout of Loma Seca; (2) welded STRUCTURAL EVOLUTION of intracaldera V is exposed and abruptly termi- lenticle at top of main 170-m-thick unit on rim nated against precaldera rocks along an arcuate of Cajón Los Calabozos west of Cerro del The caldera is a composite structure that re- fault zone. Just southwest of Llolli, intracaldera Medio; (3) welded lenticle from 25-m-thick sulted from at least two episodes of subsidence V and coarse andesitic breccias are juxtaposed subunit conformably overlying 2; (4) welded in response to eruption of units V and S, ~0.3 against fossiliferous Mesozoic sandstones on a pumice block from top of resurgent uplift, and 0.15 m.y. ago. We infer that unit L also fault that dips steeply toward the caldera. Cerro Troncos; (5) vitrophyric top of 5-m- erupted here because of its distribution, age, Most of the V-age displacement was concen- thick topmost subunit, conformably overlying lithology, and the absence of any other known trated in a 1 -km-wide zone close to the present 3. The thin upper subunits are preserved only source for this thick welded tuff; however, no course of the Rio Colorado, where unit V, de- within 3 km of the caldera. Unit-S data pro- conclusive structural evidence linking unit L to rivative tuffaceous sediments, and subsidence- ject to an alkali-lime index of 58.5 (Peacock, this caldera has survived eruption of the younger related mesobreccias (Lipman, 1976) are cut by 1931), whereas that for Quizapu is 59.5. units. several near-vertical faults. Offset of unit V The composite caldera is -26 km long and across the Rio Colorado fault zone appears to ~10 to 14 km wide, its southern half a broad have been at least 500 m and may well have basin floored by unit S and younger units, and been much greater, as the base of the intracal- Descabezado Grande, 15 to 20 km southwest of its northern half a high plateau and structural dera tuff is nowhere exposed. Caldera collapse the caldera, could be as old as 0.35 m.y. (Table uplift consisting of units V and S. Mesozoic was at least in part syneruptive, as shown by the 1, no. 4), and their activity may thus have been rocks crop out along the well-defined eastern great contrast in thickness between the 500-m contemporaneous with much of the history of wall, where steep inward-dipping faults promote intracaldera exposure of unit V and the 80- to the Calabozos system. Cursory examination has erosional recession of the caldera's topographic 100-m-thick outflow sheet that is well-exposed revealed that each consists of 40 to 70 km3 of rim. During late Pleistocene time, a range-crest on the caldera's erosionally receding eastern mafic andesite, andesite, and dacite and is domi- icecap modified the southern part only moder- scarp only 3 to 5 km away (Fig. 1). Addition- nantly pyroclastic, including an abundance of ately, but, like the outflow sheets, the northern ally, densely welded intracaldera V grades into

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nonwelded tuff against a steep wall at the The S-age collapse took the form of a warped lava banked against the southeastern base of the northwestern margin of the caldera. trap door, maximum fault displacement being dome at -0.10 m.y. (Table 1, nos. 14 and 15). It is not known whether the V-age caldera along the east-central margin and much of the Considering the analytical errors, the data indi- was resurgent or whether a thick accumulation apparent subsidence elsewhere accommodated cate that resurgence followed eruption of unit S of unit V is present in the southern half of the by marginal warping and minor step-faulting. by no more than 50,000 yr, probably by less caldera complex (Fig. 1), but intracaldera V Trap-door subsidence has also been described than 20,000, and possibly by much less. shows no indication of thinning southward for the Platoro, Silverton, and other in Apart from the patchy sediments and the where it: disappears beneath the cover of intra- the San Juan , Colorado (Lipman, faulted lava, the entire surface of the dome con- caldera unit S. Glacial stripping, younger fault- 1975; Steven and Lipman, 1976). sists of welded unit S, except for exposures of ing related to S-age subsidence, subsequent The S-age caldera is morphologically best unit V low on the cliffs of the east-side fault resurgent uplift, and the cover of younger units preserved in the southern half of the complex, zone. Atop the dome, unit S is nearly as high as have combined to complicate and obscure the and intracaldera unit S is probably thickest the caldera's east rim, 200 to 400 m higher than V-age structural picture. there, beneath the floor of the upper Rio Colo- its buried western margin, and >300 m higher rado basin. On the resurgent dome, where unit S than outcrops of unit S in the south and east Unit S Caldera mantles a rugged erosion surface cut on unit V, S parts of the moat. Maximum apparent uplift was is generally only 100 to 200 m thick, but an at Cerro Troncos, not far from the area of max- Eruption of > 150 km3 of magma to produce unknown thickness has been glacially stripped. imum apparent subsidence adjacent to the east- unit S —0.15 m.y. ago led to further subsidence central wall of the caldera. of all but the northwesternmost part of the cal- Post-S Resurgence The 2-km-wide graben is not apical to the dera, where lavas intercalated between units V uplift but lies just west of its crest and is bounded and S apparently are unfaulted (Fig. 1). Along Evidence for structural resurgence of the cal- by at least eight subparallel faults (Fig. 6; simpli- the eastern wall near Cerro Negro, offset of unit dera floor following the 0.15-m.y. collapse is fied to four faults in Fig. 1). Displacements on S from the rim to the caldera floor is 400 to 500 well-preserved and includes uplifted postcaldera individual faults, within the graben and within m. The offset diminishes southeastward to sediments, radial dips in sediments and welded the sets of antithetic faults that bound the dome -200 m near Paso Yeso, and on the flank of tuff, a longitudinal graben, and sets of faults anti- on its west, south, and east flanks, are generally Cumbre Las Mulas the caldera margin is a thetic to the uplift on three sides. Although 10 to 30 m, but the easternmost fault of the broad, step-faulted downwarp that is entirely Cerro Troncos is neatly domical from many graben has an apparent offset of si00 m, west mantled by an inward-dipping sheet of unit S. perspectives (Fig. 6), the resurgent structure is side down (Fig. 6). Antithetic faults dip toward At the head of Cajón Grande, unit S thickens not a very symmetrical dome; uplift was greatest the dome, graben faults dip toward the graben's abruptly at the caldera margin from — 50 m in on its east side and its gently sloping northwest axis, major faults along the Rio Colorado are the proximal outflow to >150 m inside, where flank is stepped down by an array of normal nearly vertical, and second-order subsidence- the base is not exposed. The last-emplaced sub- faults transverse to its long axis (Fig. 1). Its east related faults along the caldera wall and outer units of unit S are banked against and over an side is a faulted, arcuate monocline, as unit S is part of the moat are either near-vertical or inward-facing fault scarp that cuts most of S subhorizontal both atop the dome and in the inward-dipping. The graben and antithetic faults itself as well as the underlying precaldera lavas. east moat but dips outward at 15° to 60° on the do not extend beyond the limits of the resurgent Much of S is strongly welded here but is only multiply faulted, steep, eastern slope of the up- dome, which terminates in a faulted hingeline partly welded where banked against and draped lift. The western and southern slopes (Fig. 6) are marked by hot springs and zones of hydrother- over this evidently syneruptive caldera-marginal structurally simpler, and both compaction folia- mal alteration. A few patches of hydrothermally fault. Additional evidence that subsidence was tion in the tuff and patches of overlying sedi- altered tuff and sediments have also been found in part concurrent with eruption is the apparent ments exhibit 8° to 35° dips that are radial to the high on the dome, but the hydrothermal activity restriction of the tuff richest in mafic and total uplift. there appears to predate at least the last phenocrysts and poorest in Si0 (Fig. 5) to in- glaciation. 2 The sediments that have survived glaciation tracaldera and uppermost proximal-outflow are preserved as patches as much as 150 m In summary, we note that ring-structure cal- exposures. across and 2 to 10 m thick in the downthrown deras like the Calabozos are uncommon in The southwestern side of the caldera is en- troughs of antithetic faults on the flanks of the strictly andesite-dacite systems and that resur- tirely concealed by younger lavas and tuffs, but dome and in a network of minor conjugate gent doming seldom occurs where postcaldera the distribution of several postcaldera eruptive faults on its crest. They consist of tuffaceous silt, volcanism is andesitic (Smith and Bailey, 1968). centers and the major cluster of hot springs in sand, and gravel, mostly reworked from unit S Given that units V and S each represent eruption Cajón Los Calabozos is consistent with the in- prior to the resurgent deformation; generally of large volumes of silicic magma that had

ferred margin shown in Figure 1. Topography well-cemented by silica and Fe-oxides, they evolved as far as 12% Si02 rhyodacite (Fig. 5), does not suggest a major buried scarp on the have locally been dragged to near-vertical atti- and because the postcaldera lavas included high- west side but rather a downwarped hinge zone, tudes against faults atop and around the dome, K (Table 1, no. 14), there is really no as along the southeastern margin of the caldera. as well as tilted radially by the doming. anomaly here. By referring to the Calabozos as a For -2 km along upper Cajón Los Calabozos, Also faulted (antithetically) during resurgence "ring-structure caldera," we draw attention to the most proximal exposure of outflow unit S, was a dacite lava flow that rests directly upon the quasi-coherent subsidence of a cauldron here -150 m thick, dips back toward the cal- unit S at the southwestern foot of the dome block larger than any single precaldera volcanic dera at 6° to 8°, an unusual feature most likely (Fig. 6). This preresurgent unit has been K-Ar edifice, and we emphasize the structural distinc- caused by subsidence-induced warping. dated at 0.14 m.y. and a postresurgent andesitic tion between such a system and the chaotic col-

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TABLE 2. ANALYSIS OF HOT-SPRING WATER AT BAÑOS DE LLOLL1 of local partial melts—and penetrates to the shallow crust in a plexus of dike-like, tabular — 2,100 m Chemical geothermometets and podiform conduits. At a thermally more Orifice T 98.5 °C TSIO2 (conductive cooling; no steam loss) 267 °C mature stage, crustal reservoirs develop, permit- PH TSIO2 (adiabatic cooling; maximum steam loss) 237 °C TNaKCa ting zonation of andesite to dacite by fractiona- 235 °C 582 A"O (S04-H20) geothermometer tion in magma chambers (Hildreth, 1981, Figs.

Ca 8.28 T( (conductive cooling) 280 °C 12, 15). The largest of these chambers produce Mg <0.01 T|( (one-step steam loss) 238 °C dacitic to rhyolitic ash flows and associated ring- Na 374 T|m (continuous steam loss) 250 °C K 61.2 -4.53 structure calderas in the heart of an otherwise so4 2.39 -9.81 andesitic arc. Characteristically at this stage, an-

Rb 0.68 HCOJ 111 desitic eruptions are suppressed within the silicic Cs 0.81 so4 120 focus until the more buoyant silicic magma is CI 548 *S 0.23 F 3.2 either crystallized or flushed, although andesite *NH. 1.3 13.5 (and basalt) may continue to erupt around the periphery of such a focus. Note: Analyses (in mg/1 by J. M. Thompson, U.S. Geological Survey, Menlo Park, except starred species by spectrophotometer in the field. Isotopic analyses by C. J. Janik. Estimated geothermal reservoir temperatures calculated following Fournier (1981) and McKenzie and Truesdell (1977). Prior to aggregation of the voluminous silicic magma reservoir that produced the Loma Seca Tuff, the surface expression of Calabozos mag- lapse thought to be typical of smaller calderas combine to suggest that the Calabozos caldera matism was probably a cluster of andesite-dacite such as Krakatau and Mazama and summit cal- contains a significant resource for geothermal stratovolcanoes, such as those in the nearby deras on stratovolcanoes such as Katmai and energy. Peteroa and Descabezado areas today. After the Descabezado Grande. Calabozos system solidifies, its subterranean ex- DISCUSSION pression will be a composite diorite to grano- HOT SPRINGS AND diorite batholith similar to the 7-m.y.-old pluton GEOTHERMAL RESOURCES Development of a large, multicyclic, ring- now exposed at Laguna de la Invernada (Fig. 1). structure caldera in a province that is otherwise Empirically, such an evolution appears to re- Several clusters of hot springs (Fig. 1) are dominated by andesitic shields, cinder cones, quire a crust of some minimum thickness and a distributed along the arcuate fault zone border- and composite volcanoes can be accounted for stress distribution that favors an intrusion/extru- ing the resurgent uplift, and another large cluster within the framework of the following model. sion ratio high enough to partially melt and mo- is located at the southwestern (concealed) mar- Where mantle-derived magmas invade conti- bilize large crustal domains. If the crust is too gin of the caldera in Cajón Los Calabozos. At nental crust, in sufficient volume and for thin and crustal stresses are appropriate, a high the Calabozos locality (elevation -2,475 m), sufficient time, belts of intermediate-composi- proportion of the mafic magma erupts, causing hundreds of springs, many of them emerging at tion stratovolcanoes commonly develop (with only modest thermal and chemical perturbation 65 to 78 °C and pH 5.9, give a combined dis- or without halos or alignments of more mafic of the crust traversed. Too thin a continental charge of-7,500 1/min. Around the base of the shields and cinder cones around the andesitic crust accounts, accordingly, for the dominance resurgent uplift, orifice temperatures are 39 to foci) and can evolve with time to a stage charac- of mafic volcanism and apparent absence of 68 °C, estimated discharges range from 20 to terized by eruption of voluminous silicic ash ring-structure calderas in the Andes south of 8,000 1/min, and pH is 6.2 to 7.1; considerable flows. It has been argued elsewhere (Hildreth, 37°S. Where the crust is thicker, however, as dilution by snow melt seems likely. 1981) that Cordilleran batholiths, of which large from 33°S to 36°S (Lomnitz, 1962) and in the Table 2 gives analyses of water collected from ash-flow sheets are a surficial expression, result central Andes (16°S-28°S; James, 1971), vo- a boiling spring at Baños de Llolli (Fig. 1) in the from prolonged diffuse injection of the litho- luminous ash-flow eruptions are important, cal- north moat at an elevation of—2,100 m. As this sphere by basalt. This basalt hybridizes, frac- deras are common, andesite to magma- spring is of modest discharge (15-20 1/min) tionates, and preheats the crust with pervasive tism is dominant, and penetration of basalt to and emerges through clay-altered moat-filling mafic to intermediate forerunners, culminating the surface is virtually unknown. This concep- debris of low permeability, it may be the least in large-scale diapiric mobilization of partially tual framework also accounts well for the abun- diluted spring we have encountered. Reservoir molten injection zones from which granodioritic dance of large ash-flow sheets in Japan temperatures calculated (assuming steam loss) magmas separate. The "early intermediate" vol- (Aramaki and Yamasaki, 1963) but not the 18 canism of many ash-flow terrains (Lipman, Kuriles, on the Alaska Peninsula (Miller and by the Si02, Na-K-Ca, and sulfate-water 0 geothermometers (Fournier, 1981; McKenzie 1975) corresponds to the ubiquitous "mafic Smith, 1977) but not the Aleutian Islands, and and Truesdell, 1977) are in good mutual agree- forerunners" that are intruded by granitoid plu- in New Zealand but not Tonga. ment at 235 to 238 °C. This spring is part of a tons at the levels exposed in older Cordilleran zone of fumaroles, springs, and pervasively al- batholiths (Bateman and others, 1963). Central versus Southern Andes tered postglacial moat-fill that extends eastward As feedback between crustal melting and the along the walls of the Rio Colorado gorge for interception of mantle-derived intrusions focuses For a 375-km-long segment (19.5°S to several kilometres. The size and youth of the and amplifies crustal magmatic anomalies, mafic 22.5°S) of the central Andes, Baker and Francis magmatic system, the abundant recharge, the volcanism is suppressed; thereafter, andesite (1978) estimated minimum erupted volumes to permeability associated with the caldera fracture forms over a large depth range—by concurrent be 104 km3 of intermediate lavas and 2 x 103 system, and these preliminary analytical data fractionation, internal remixing, and assimilation km3 of ignimbrite (ash-flow tuff) during the past

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20 m.y., an average of ~1.6 km3/m.y. per kilo- calderas of late Quaternary age in the 33°S to Guest, J. E., 1969, Upper Tertiary ignimbrites in the Andean Cordillera of part of the Antofagasta Province, northern Chile: Geological Society of meter of arc length. For the past 6 m.y., their 36°S segment of the Andean Cordillera have America Bulletin, v. 80, p. 337-362. Hildreth, W., 1981, Gradients in silicic magma chambers: Implications for data showed the minimum eruption rate to have important implications for (1) the safety of sev- lithospheric magmatism: Journal of Geophysical Research, v. 86, been -3.8 km3/km/m.y., of which only -10% eral major cities near the mouths of Andean p. 10153-10192. Hildreth, W„ and Drake, R. E., 1983, The 1932 eruption of Quizapu, central 2 was ignimbrite. In our 1,800-km map area canyons, and (2) the geothermal-energy re- Chilean Andes: Geological Society of America Abstracts with Pro- grams, v. 15. (Fig. 1), we estimate that >60% of the terrain sources potentially available from long-lived Hildreth, W., Drake, R. E., and Sharp, W. D., 1981, Voluminous late Pleisto- cene ash-flow and caldera complex in the Andes of central Chile: Geo- has been covered by nonignimbritic volcanic silicic centers that lie only 40 to 60 km from logical Society of America Abstracts with Programs, v. 13, p. 61. rocks younger than 1 m.y. If the thickness of major population centers. James, D. E., 1971, Andean crustal and upper mantle structure: Journal of Geophysical Research, v. 76, p. 3246-3271. these lavas had averaged 500 m before erosion, Kittl, E., 1933, Estudio sobre los fenómenos volcánicos y material caído du- 3 rante la erupción del grupo del "Descabezado" en el mes de Abril de this would give -540 km and a rate of 12 ACKNOWLEDGMENTS 1932: Anales del Museo Nacional de Historia Natural (Buenos Aires), 3 km /km/m.y. In that the three sheets of the v. 37, p. 321-364. Klerkx, J., Deutsch, S., Pichler, H., and Zeil, W., 1977, Strontium isotope Loma Seca Tuff alone were greater than this, The project was funded by National Science composition and trace element data bearing on the origin of Cenozoic volcanic rocks of the central and southern Andes: Journal of Volcanol- however, the total eruption rate has probably Foundation grant EAR-80-18354 to G. H. Cur- ogy and Geothermal Research, v. 2, p. 48-71. 3 been no smaller than 25 km /km for the past 1 tis and was supported in Chile by the Servicio de Kussmaul, S., Hórmann, P. K.. Ploskonka, E., and Subieta, T., 1977, Vol- canism and structure of southwestern Bolivia: Journal of Volcanology m.y., and at least half of the volume has been Desarrollo Científico, Artístico, y Cooperación and Geothermal Research, v. 2, p. 73-111. ignimbrite. Eruption rates and lava/tuff ratios Lipman, P. W., 1975, Evolution of the Platoro caldera complex and related Internacional, Universidad de Chile. Teresa Iri- volcanic rocks, southeastern San Juan Mountains, Colorado: U.S. Geo- obviously vary greatly along the length of the arte and Professors José Corvalán and Mario logical Survey Professional Paper 852, 128 p. 1976, Caldera-collapse breccias in the western San Juan Mountains, Andes. Vergara provided important logistical support; Colorado: Geological Society of America Bulletin, v. 87, p. 1397-1410. Lomnitz, C., 1962, On Andean structure: Journal of Geophysical Research, Although the Pleistocene eruption rate in the Professors H. Moreno, L. López Escobar, and F. v. 67, p. 351-363. Munizaga improved our understanding of Chil- López Escobar, L., Frey, F. A., and Vergara, M., 1976, Andesites from central- Descabezado-Calabozos region might be unrep- south Chile: Trace element abundances and pedogenesis, in Proceed- resentative of the southern Andes as a whole, ean volcanology; and, in successive seasons, ings, Symposium on Andean and Antarctic Volcanology Problems, Santiago, Chile, September 1974: Naples International Association of we do not think it is. Neither the active Peteroa- Warren Sharp, Ian Madin, and John Dilles con- Volcanology and Chemistry of the Earth's Interior Special Series, tributed substantially to the field work. Person- p. 725-761. Planchón-Azufre andesitic edifice just north of Mahood, G. A., and Drake, R. E., 1982, K-Ar dating young rhyolitic rocks: A ta Figure 1 nor the terrain southward as far as nel of the 4 Comisaría (Molina), Carabineros case study of the Sierra La Primavera, Jalisco, Mexico: Geological Society of America Bulletin, v. 93, p. 1232-1241. Laguna del Maule (Drake, 1976) suggests a de Chile, especially A. Gutiérrez Martinez, J. McKenzie, W. F., and Truesdell, A. H., 1977, Geothermal reservoir tempera- Caro Sáez, and J. Parra Guzmán, were exceed- tures estimated from the oxygen isotope compositions of dissolved sul- lesser rate of activity anywhere between 35°S fate and water from hot springs and shallow drillholes: Geothermics, and 36°S. From Santiago to Talca (Fig. 1), ingly helpful in the back country. The friendli- v. 5, p. 51-61. Miller, T. P., and Smith, R. L., 1977, Spectacular mobility of ash flows around welded to nonwelded piedmont lobes of An- ness and hospitality of these men and of the Aniakchak and Fisher calderas, Alaska: Geology, v. 5, p. 173 176. Narvaez, Concha, and Santander families, Ma- Minster, J. B., and Jordan, T. H.. 1978, Present-day plate motions: Journal of dean ash-flow tuff are commonly preserved Geophysical Research, v. 83, p. 5331-5354. where they debouch from canyons into the Cen- nuel Oyarce, Alejandro Farias, Octavio Medina, Minster, J. B„ Jordan, T. H., Molnar, P., and Haines, E., 1974, Numerical modelling of instantaneous plate tectonics: Royal Astronomical Society tral Valley. Similar distribution of many such and Juan Saavedra, seasonal herdsmen of the Geophysical Journal, v. 36, p. 541 - 576. Cordillera, are recalled with warmth and grati- Moreno, H., 1974, Airplane flight over active volcanoes of central-south Chile: sheets in the Andean foothills of Argentina (ref- International Association of Volcanology and Chemistry of the Earth's erences cited in Introduction) shows that the tude. We appreciate helpful reviews by N. S. 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The long record of voluminous silicic pyro- González Ferrán, O., and Vergara, M., 1962, Reconocimiento geológico de la MANUSCRIPT RECEIVED BY THE SOCIETY AUGLST 24, 1982 Cordillera de los Andes entre los paralelos 35° y 38° latitud sur: Univer- REVISED MANUSCRIPT RECEIVED JANUARY 4, 1983 clastic activity and the existence of ring-structure sidad de Chile, Instituto de Geología. Publicación 24. 121 p. MANUSCRIPT ACCEPTED JANUARY 14,1983

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