The eruptive chronology of the Ampato–Sabancaya volcanic complex (Southern Peru)
Introduction
Reconstructing the eruptive chronology of active volcanic systems represents a key step for any hazard assessment initiative. However, the recent eruptions of Chaitén (2008, Major and Lara, 2013) and Reventador volcanoes (2002, Hall et al., 2004) showed that the eruptive chronology of many active volcanic complexes remains poorly known. In the Andean cordillera, the Peruvian segment of the Central Volcanic Zone (CVZ) results from the subduction of the oceanic Nazca plate below the South American continental lithosphere. As a result, the volcanic front includes at least twelve volcanic centres of Pleistocene age (Fig. 1a) of which seven have experienced historical eruptive activity (i.e. since the arrival of the Spanish conquistadors in the 16th century). These volcanoes include El Misti (Thouret et al., 2001, Harpel et al., 2011), which threatens the city of Arequipa, the active volcanoes of Ubinas (Thouret et al., 2005, Rivera et al., 2014) and Sabancaya (Gerbe and Thouret, 2004), and Huaynaputina volcano (Thouret et al., 1999, Adams et al., 2001), which has had the biggest historical eruption in the Andes. However, little is still known about the eruptive chronology of some of these volcanic centres, such as the Sabancaya volcano, and its neighbouring Ampato edifice. Rare historical accounts mention eruptive activity that occurred in AD 1750 and 1784 (Siebert et al., 2010, Travada y Córdova, 1752, Zamácola y Jaúregui, 1888). More recently, Sabancaya entered a new eruptive phase in 1988, which lasted until at least 1997 (Global Volcanism Program, 1988, Global Volcanism Program, 1997). During this period, Sabancaya experienced low to moderate explosive eruptions (VEI 1–2) that were characterised by violent vulcanian explosions accompanied by small (up to 5–7 km height) eruption columns with a local ash fallout impact. The most significant activity was observed between April–May 1990 and April 1991 (Global Volcanism Program, 1990, Global Volcanism Program, 1991). Since March–April 2013, Sabancaya has shown increased fumarolic activity, accompanied by frequent seismic swarms (Global Volcanism Program, 2013, Jay et al., 2015).
Following its reactivation in 1988, several studies have been carried out on Sabancaya. These works include an initial geological reconnaissance, comprising a hazard assessment (Thouret et al., 1994), a regional tephro-chronological survey (Juvigné et al., 1998, Juvigné et al., 2008) and a petrological description of the last eruption products (Gerbe and Thouret, 2004). Based on detailed field work and geochronological and petrological studies, we reconstruct the structure and the volcanic and magmatic history of the Ampato–Sabancaya volcanic complex from the Pleistocene to the present day.
Section snippets
Geological setting
The Ampato–Sabancaya Volcanic Complex (ASVC, 15° 49.3′S, 71° 52.7′W) is located 70–75 km NW of Arequipa (Fig. 1). It is constructed upon the Western Cordillera of the Peruvian Andes, which is composed of Mesozoic and Cenozoic volcanic and sedimentary formations (Klinck et al., 1993, Sébrier and Soler, 1991). To the north, the ASVC borders the older and highly eroded Hualca Hualca volcano (6025 m above sea level – m asl), located at the southern margin of Colca canyon. An 40Ar–39Ar age of 0.80 ± 0.04
Methodology
Fieldwork was carried out during several field campaigns between 2009 and 2012, which included geological mapping and sampling of most volcanic units. At high altitude (above 5000 m asl), fieldwork was complicated by the presence of a large icecap as well as voluminous glacial deposits. However, the presence of numerous deep glacial valleys allowed sampling of almost all volcanic units, resulting in a broad sample array for petrographic and geochemical studies (Fig. 2). Major and trace element
Morphology and structure
The Ampato–Sabancaya volcanic complex has a roughly elliptical basal outline (16–20 km NE–SW by 12–14 km NW-SE; Fig. 2, Fig. 3) and is composed of two main edifices: The older Ampato compound volcano (6280 m asl) and the younger Sabancaya edifice (5967 m asl). The lower flanks of the Ampato edifice have gentle slopes (5–10°), and are strongly glacially eroded. As a result, the flanks are coated by a thick layer of moraine deposits, especially on the southern and western sides. At higher altitude,
The eruptive chronology of the Ampato volcano
Our geomorphologic, stratigraphic, and geochronological data show that Ampato is a compound volcano comprising (Fig. 4, Table 4): (1) The Basal edifice, which is an old, highly eroded volcano; (2) the Upper edifice, which started with the Yanajaja stage and continued with the successive construction of the Northern, Southern, and Central peaks.
The eruptive chronology of the Sabancaya volcano
Geomorphologic, stratigraphic, and geochronological data point toward a two-stage development of Sabancaya edifice (Fig. 4, Table 4): (1) A Basal edifice, and (2) a Young terminal cone.
Main petrological characteristics
The Ampato–Sabancaya samples display a high-K magmatic trend, ranging from andesites to dacites (57 to 69 wt.% SiO2, Fig. 10, Table 5), with rare rhyolitic compositions (74–77 wt.% SiO2). Four different groups were identified:
- (1)
The first group, composed mainly of andesites (57–60 wt.% SiO2), corresponds primarily to lavas from the Former andesitic stage (Ampato Basal edifice), as well as to the scoriaceous tephra fallout and pyroclastic flow deposits of Ampato Upper edifice. These samples are
Edifice volumes and eruptive rates
Using a 40-m digital elevation model (obtained from 1:50,000 topographic maps), we obtained the volcano's morphometric parameters (volcano basal area, height, and volume), following the methodology of Grosse et al. (2014). This consists of computing the volcano basal area (the edifice outline) and then fitting a 3D volcano basal surface corresponding to the substratum topography. Using this surface, it is possible to compute the volcano's maximum height and volume. Here, the edifice outline was
Late Holocene eruptive activity and hazards
Our new data confirms previous tephrochronological studies (Juvigné et al., 1998, Juvigné et al., 2008) and enables better constraints to be placed on the Holocene explosive activity of Sabancaya. Several ash layers dated between 11 and 8 ka point to Early Holocene explosive activity associated with this complex. Given that the younger ages of the Ampato edifice fall in the range 20–15 ka and that the younger volcanic unit of this edifice (the NE dome) lacks glacial erosion, the early Holocene
Conclusions
The Ampato–Sabancaya volcanic complex (ASVC) comprises two successive edifices. The oldest one, the Ampato Basal volcano, was built during at least two cone-building stages: The Former andesitic cone, and the Moldepampa stage dated at 450–400 ka and 230–200 ka, respectively. After a period of quiescence, the Ampato Upper edifice developed on the remnants of the Basal edifice about 80 to 70 ka ago with the lava sequence of the Yanajaja stage. This edifice comprises several cone-building stages that
Acknowledgements
This work is part of a Peruvian–French cooperation programme carried out between the Instituto Geológico, Minero y Metalúrgico (INGEMMET, Peru) and the Institut de Recherche pour le Développement (IRD, France). It was partially founded by a “Jeunes Equipes Associées à l'IRD” (JEAI) project that is an initiative designed to promote and strengthen new research teams in developing countries. Several radiocarbon dates were obtained thanks to IRD support to Laboratoire de Mesure du Carbone 14
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