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The Loma Seca Tuff and the Calabozos Caldera: a Major Ash-Flow and Caldera Complex in the Southern Andes of Central Chile

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The Loma Seca Tuff and the Calabozos Caldera: a Major Ash-Flow and Caldera Complex in the Southern Andes of Central Chile The Loma Seca Tuff and the Calabozos caldera: A major ash-flow and caldera complex in the southern Andes of central Chile 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 Miocene agglutinates that erupted for the most part from to Quaternary 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 basalt exceeds the inter- dera of late Pleistocene 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 dacite and andesite have persisted into Holocene 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 fault 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 lavas 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 Colombia 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- mafic 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 basalts 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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/95/1/45/3434580/i0016-7606-95-1-45.pdf by guest on 24 September 2021 IO km 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) Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/95/1/45/3434580/i0016-7606-95-1-45.pdf by guest on 24 September 2021 ASH-FLOW AND CALDERA COMPLEX, SOUTHERN ANDES, CHILE 47 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 pumice 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.
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