The eruptive chronology of the Ampato–Sabancaya volcanic complex(Southern Peru)

Pablo Samaniego a,⁎, Marco Rivera b, Jersy Mariño b, Hervé Guillou c, Céline Liorzou d, Swann Zerathe e,Rosmery Delgado b, Patricio Valderrama a,b, Vincent Scao c

a Laboratoire Magmas et Volcans, Université Blaise Pascal - CNRS - IRD, 6 Avenue Blaise Pascal, TSA 60026 - CS 60026, 63178 Aubière, Franceb Observatorio Vulcanológico del INGEMMET, Dirección de Geología Ambiental y Riesgo Geológico, Urb. Magisterial B-16, Umacollo, Arequipa, Peruc Laboratoire des Sciences du Climat et de l'Environnement, LSCE/IPSL, CEA-CNRS-UVSQ, Université Paris-Saclay, F-91198 Gif-sur-Yvette, Franced Laboratoire Domaines Océaniques, Institut Universitaire Européen de la Mer, Université de Bretagne Occidentale, Rue Dumont d'Urville, 29280 Plouzané, Francee Institut des Sciences de la Terre, Université Grenoble Alpes – CNRS - IRD, 1381 rue de la piscine, 38400 Saint Martin d'Hères, France

a b s t r a c ta r t i c l e i n f o

Article history:

Received 14 January 2016

Received in revised form 1 April 2016

Accepted 29 April 2016

Available online 07 May 2016

Wehave reconstructed the eruptive chronology of the Ampato–Sabancaya volcanic complex (Southern Peru) on

the basis of extensive fieldwork, and a large dataset of geochronological (40K–40Ar, 14C and 3He) and geochemical

(major and trace element) data. This volcanic complex is composed of two successive edifices that have experi-

enced discontinuous volcanic activity from Middle Pleistocene to Holocene times. The Ampato compound

volcano consists of a basal edifice constructed over at least two cone-building stages dated at 450–400 ka and

230–200 ka. After a period of quiescence, the Ampato Upper edifice was constructed firstly during an effusive

stage (80–70 ka), and then by the formation of three successive peaks: the Northern, Southern (40–20 ka) and

Central cones (20–10 ka). The Southern peak, which is the biggest, experienced large explosive phases, resulting

in deposits such as the Corinta plinian fallout. During theHolocene, eruptive activitymigrated to the NE and con-

structed themostly effusive Sabancaya edifice. This cone comprisedmany andesitic and dacitic blocky lava flows

and a young terminal cone, mostly composed of pyroclasticmaterial. Most samples from the Ampato–Sabancaya

define a broad high-K magmatic trend composed of andesites and dacites with a mineral assemblage of plagio-

clase, amphibole, biotite, ortho- and clino-pyroxene, and Fe–Ti oxides. A secondary trend also exists, correspond-

ing to rare dacitic explosive eruptions (i.e. Corinta fallout and flow deposits). Both magmatic trends are derived

by fractional crystallisation involving an amphibole-rich cumulate with variable amounts of upper crustal

assimilation.

A marked change in the overall eruptive rate has been identified between Ampato (~0.1 km3/ka) and Sabancaya

(0.6–1.7 km3/ka). This abrupt change demonstrates that eruptive rates have not been homogeneous throughout

the volcano's history. Based on tephrochronologic studies, the Late Holocene Sabancaya activity is characterised

by strong vulcanian events, although its erupted volume remained low and only produced a local impact through

ash fallout. We have identified at least 6 eruptions during the last 4–5 ka, including the historical AD 1750–1784

and 1987–1998 events. On the basis of this recurrent low-to-moderate explosive activity, Sabancaya must be

considered active and a potentially threatening volcano.

© 2016 Elsevier B.V. All rights reserved.

Keywords:

Ampato

Sabancaya

Central Andes

Eruptive chronology

Eruptive rates

Volcanic hazards

1. 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 vol-

canoes 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

Journal of Volcanology and Geothermal Research 323 (2016) 110–128

⁎ Corresponding author.

E-mail address: pablo.samaniego@ird.fr (P. Samaniego).

http://dx.doi.org/10.1016/j.jvolgeores.2016.04.038

0377-0273/© 2016 Elsevier B.V. All rights reserved.

Contents lists available at ScienceDirect

Journal of Volcanology and Geothermal Research

j ourna l homepage: www.e lsev ie r .com/ locate / jvo lgeores

180000 190000 200000 210000

180000 190000 200000 210000

82

40

000

82

50

000

82

60

000

82

70

000

AMPATO

SABANCAYA

HUALCA

HUALCA

RIO COLCA

CabanacondePinchollo

Madrigal

Lari

Maca

Río

Sep

ina

Achoma

Y

Colihuiri

Cajamarcana

Sallalli

Japo

Baylillas

Corinta

Collpa

c

Mucurca

lake

Río Parcomayo

Qda. H

uayura

y

70º

16º

15º

18º

CHILE

BOLIVIA

TACNA

TIT

ICACA L

AKE

JULIACA

PUNO

SOLIMANASARA SARA

COROPUNA

ANDAHUA

HUAYNAPUTINA

TUTUPACA

TICSANI

YUCAMANE

CASIRI

AMPATO-

SABANCAYA

0 50

74º

5-6 cm/y

18ºS

16º

72º100 km

MOLLENDO

ILO

CAMANA

CHACHANI

MISTI

MOQUEGUA

CHALA

70º74º 72º

AREQUIPA

UBINAS

Historically active

Potentially activePlio-Quaternary

volcanic fronta

72º

Ampato

Sabancaya

Hualca

Hualca

Normal fault

Strike slip

Lineaments

0 5 km

16º

Huambo

Chivay

Maca

CabanacondeRío Colca

Ichupamba

Huanca

Trigal

Sepina

b

Solarpampa

72º

Fig. 1. (a) The Peruvian volcanic arc. (b) Structural context of the Ampato–Sabancaya region, including the Colca river valley.Main structures fromMering et al. (1996) andGerbe and Thouret (2004)

Sabancaya and Hualca Hualca complexes and the nearby Colca canyon.

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, 1997). During this period, Sabancaya experienced low to moder-

ate 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, 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,

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.

2. 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 construct-

ed upon the Western Cordillera of the Peruvian Andes, which is

composed of Mesozoic and Cenozoic volcanic and sedimentary for-

mations (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 Ma has been deter-

mined by Gerbe and Thouret (2004) for an andesitic lava flow from

this edifice, which represents the base of the Ampato–Sabancaya

volcanic complex. Southwards, the ASVC dominates a high plateau

with an overall southward slope, which is composed of Mio-

Pliocene volcanic formations comprising lava flows and large

ignimbritic deposits that overlie the Western Cordillera basement

(Klinck et al., 1993; Mamani et al., 2010). Thouret et al. (2007)

obtained a 40Ar–39Ar age of 2.20 ± 0.15 Ma for a dacitic ignimbrite

deposit on top of the Patapampa plateau, located to the east of the

volcanic complex.

Regional tectonic investigations have identified three main fault

systems in this part of the Andes (Fig. 1c; Sébrier and Soler, 1991;

Mering et al., 1996). The first one corresponds to NW-SE striking faults

that are oblique-slip extensional structures, with a minor left-lateral

component. These faults correspond to regional structures such as

the Huanca and Ichupampa faults (located to the SW and NE of the

ASVC, respectively). A second system is composed of E-W striking

faults with a southward normal, dip-slip component, notably the

Trigal and Solarpampa faults. These structures are almost parallel to

the Colca river valley, and located to the NW of the complex. A recent

Mw5.9 earthquakewas associatedwith these faults (Jay et al., 2015).

The third, NE-SW striking faults represent local extensional

structures, such as those crossing the Patapampa plateau, the most

prominent being the Sepina fault. This latter structure seems to

be the focus of several seismic events (Mw 4.5–5) that have

occurred during the last few decades (Antayhua et al., 2001; Jay

et al., 2015). The Ampato and Sabancaya vents, as well as several

glacial valleys in the western part of Ampato, are roughly aligned

in a NE-SW direction. This observation suggests that the NE-SW

Sepina fault probably controlled the structural development of the

ASVC.

3. 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 aswell 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 whole-rock analyses were obtained from

agate-crushed powders of 133 samples spanning the entire volcanic

complex, at the Institut Universitaire Européen de la Mer, Université

de Bretagne Occidentale (Brest, France), using an Inductive Coupled

Plasma-Atomic Emission Spectrometer (ICP-AES) and following the

analytical procedure described by Cotten et al. (1995). These data,

together with petrographic descriptions, have been used to characterise

and correlate the different volcanic units.

We constrained the Pleistocene eruptive chronology via the

unspiked 40K–40Ar dating method at the Laboratoire des Sciences du

Climat et de l'Environnement (LSCE/IPSL, Gif-sur-Yvette, France). We

obtained 10 ages covering the entire history of this volcanic complex

(Table 1). The Holocene chronology is based on 14 new radiocarbon

ages mainly obtained from peat and soil samples from several peatbogs

around the volcanic complex. Most samples (8) were analysed at the

Laboratoire de Mesure du Carbone 14 (LMC14, Gif-sur-Yvette, France)

and an additional group (6) were analysed at the Centre for Isotope

Research (CIO), Groningen University (Netherlands). Table 2 shows

the conventional 14C ages (±1σ), the 13δ values, and the calibrated

ages are given at 2σ confidence levels. Conversion from conventional14C ages to calendar ages was carried out using the Calib 7.1 code

(Stuiver and Reimer, 1993; Stuiver et al., 2005) and the recently

updated Southern Hemisphere calibration curve (SHcal13, Hogg et al.,

2013), which is available back to 50 ka cal BP. Finally, additional

constraints on the eruptive chronology of Sabancaya volcano were

obtained by cosmic ray exposure dating of lava flows using the couple3He/pyroxene. Three samples (Table 3) were collected from the tops

of undisturbed lava boulders and were processed at the CRPG noble

gas lab (Centre de Recherches Pétrographiques et Géochimiques,

Nancy, France). Additional information concerning the geochemical

and geochronological methods is included in the Supplementary

material.

4. 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; Figs. 2, 3) and

is composed of twomain edifices: The older Ampato compound volcano

(6280masl) and the younger Sabancaya edifice (5967masl). The lower

flanks of the Ampato edifice have gentle slopes (5–10°), and are strong-

ly 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, slopes are steeper (10–30°), locally up to 45°. The

upper part of the Ampato edifice is composed of three major cones

oriented NE-SW (Figs. 2 and 3), hereafter termed the Southern, Central,

and Northern cones, respectively located 1.2 and 2.5 km from the

summit (Southern cone). The Ampato edifice reaches an elevation of

~1800 m on the west and east sides, and slightly higher (~2250 m) on

the southern side. The summit area is characterised by these three

peaks and several unconformities that delimit the different cones. The

summit zone of the Southern cone displays a 1-km-long scar open to

the east testifying to an explosive phase associated with the upper

part of Ampato. It also marks the transition from the older Southern

cone to the younger Central cone.

The Sabancaya edifice is located 4–5 km to the NE of Ampato's

Southern peak. It is built on the remnants of Ampato and Hualca Hualca

(Figs. 2 and 3), and reaches 1300–1500 m in elevation on the west and

112 P. Samaniego et al. / Journal of Volcanology and Geothermal Research 323 (2016) 110–128

east sides. Its lower part is characterised by gentle slopes (10–15°), free

of glacial erosion. Two peaks compose the upper part of this edifice: A

dome-like structure characterised by steep slopes (30–45°) and a

younger main cone with moderate slopes (20–30°), located to the

northeast, which comprises an active crater (300–400 m in diameter)

with continuous fumarolic activity.

The morphology of the Ampato–Sabancaya volcanic complex was

shaped by the late Pleistocene glaciations. With the exception of the

north and north-eastern flanks, covered by younger Sabancaya lavas,

medium-sized glacial valleys have been carved all around the edifice.

Alcalá et al. (2011) identified at least two well-defined groups of

moraines radially oriented around the volcano. The oldest and biggest

moraines, which are morphologically well-defined, bear witness to the

Last Glacial Maximum (LGM) and extend down to 4250–4450 m asl.

In contrast, the younger moraines reach higher elevations, in the

range of 4400–4650 m asl, and are interpreted as resulting from late

glacier re-advances.

Based on cosmogenic 3He surface-exposure dating of Coropuna

volcano moraine deposits, Bromley et al. (2009) propose that the

LGM in this part of the Andean Cordillera was synchronous with

that at other latitudes (i.e. 17–25 ka). These authors also observe a

younger glacier re-advance in the range of 10–13 ka. These ages

agree with some 36Cl cosmogenic dates on samples from the

Huayuray valley on the northern flank of the Hualca Hualca edifice

that give ages of around 17–18 ka (Alcalá et al., 2011). These authors

also propose that a younger re-advance occurred at Hualca Hualca

around 11–12 ka. On the basis of these data, we consider that the

old moraines around the ASVC (M1 in Fig. 4) are associated with

the LGM period (i.e. 17–25 ka) whereas the younger moraines

(M2) are probably associated with a late glacier re-advance just

after the Holocene–Pleistocene boundary (10–13 ka). Other small

moraines (M3) observed at higher altitudes (above 4800 m asl) at

Ampato are clearly associated with younger Holocene fluctuations

(cf. Jomelli et al., 2011).

5. The eruptive chronology of the Ampato volcano

Our geomorphologic, stratigraphic, and geochronological data show

that Ampato is a compoundvolcano 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.

5.1. The Ampato Basal edifice

5.1.1. A former andesitic stage

The older remnants of the Ampato Basal edifice correspond to a 600-

m-thick volcanic pile that crops out on the south-western and western

flanks of Ampato. This sequence is composed of 40–60 m-thick lava

flows interlayered with proximal scoria-flow deposits and spatter ag-

glutinates (Figs. 3, 5a). Volcanic units corresponding to the subsequent

volcanic stages discordantly cover this unit. Another remnant of this

180000 190000 200000

180000 190000 200000

8240

00

08250

00

0

8240

00

08250

00

0

10 km

11-54

09-17

11-71A

09-30

11-28

11-15

11-37

11-38

11-03

11-64

11-68

11-20

10-18

10-13

10-1911-79

11-67

11-42

10-33

11-11

11-19

11-44

10-14/16/17

10-24

10-20

11-33

Japo

Sallalli

Qda. B

aylil

las

oy

am

arc

ah

C.

ad

Q

Cajamarcana

Collpa

Colihuiri

Corinta

Yanajaja

MoldepampaJatún

Pampa

Qda

. Hua

raya

Río

Par

com

ayo

Qda

. Sah

uanc

aya

Mucurca

Lake

Qda. Huaycumayo

To Taya

To Chivay

To Huambo

Qda

. Viz

cach

ane

Fig. 2. Digital elevation model of the Ampato–Sabancaya volcanic complex showing locations of rock samples (open circles) and dated rock samples and/or other studied sections (solid

circles).

113P. Samaniego et al. / Journal of Volcanology and Geothermal Research 323 (2016) 110–128

stage consists of a 200–400-m-thick sequence of sub-horizontal and

highly eroded lava flows that crop out in the Jatún Pampa plain, at the

south-eastern foot of the volcano (Figs. 2, 4). Lavas and tephras from

both units are dark-grey, aphanitic, olivine-pyroxene andesites (57.2–

59.8 wt.% SiO2). Two 40K–40Ar ages (Table 1, Fig 3, 4), indicate that

these two units are almost contemporaneous. A lava sample from the

proximal unit is 440 ± 8 ka old (SA-11-64), and a second sample from

the Jatún Pampa sequence is 410 ± 10 ka old (SA-11-03). Thus, the

oldest part of the Ampato Basal edifice was possibly built up between

400 and 450 ka.

5.1.2. Moldepampa stage

This unit consists of a 200–300-m-thick sequence of lava flows

representing the main cone-building stage of the Ampato Basal edifice

(Figs. 2, 4, 5). To the south, lavas rest discordantly on the lavas of the

previous stage. Based on the radial distribution of these lavas, and its

average extension (8–10 km) and slope (7–8°), we infer that the

summit of the Ampato Basal edifice was located at roughly the same

location as the current summit, at an elevation of 5200–5400 m asl.

These lavas are porphyritic dacites (62.9–65.1 wt.% SiO2), with plagio-

clase, amphibole, biotite, Fe–Ti oxides and minor pyroxenes. Thus, a

sharp mineralogical and geochemical contrast (see below) exists

between these lavas and those of the previous andesitic stage. Two

samples from the south-western side of the cone yielded coherent

ages of 217 ± 5 ka and 226 ± 4 ka, which correspond to the upper

part of this lava sequence (SA-11-37 and SA-11-38, respectively,

Table 1). A third sample, from the southern flank yields a similar

age (210 ± 3 ka, SA-11-15, Table 1). On the basis of these dates we

constrained this cone-building stage to 230–200 ka.

5.1.3. Rhyolitic fallout deposits

The eroded plateau located to the west of ASVC is partially covered

by thick rhyolitic tephra fallout deposits. At 10 km to the southwest of

the summit, in the Chacramayo valley, crops out a 4–5-m-thick

sequence of tephra fallout deposits. This sequence consists of four

pumice and lithic lapilli horizons of metre-sized thickness, interlayered

with altered and indurated ash layers. Pumice clasts reaching a maxi-

mumsize of 3–4 cm(at 10–12 km from theAmpato summit) are fibrous

and rhyolitic in composition (74.2–76.9 wt.% SiO2) and contain

plagioclase and biotite. We also observed accidental andesitic lithics

(≤2 cm). This sequence is covered by 20–25 m of dm-sized layers of

andesitic gravel and sand. At a distal location (20–25 km west-

southwest of the summit), this sequence is thinner and the pumice

smaller (≤2–2.5 cm). Here, the fallout deposits are interlayered with

layers of reworked ash. Unfortunately we could not obtain an age for

this unit because pumice clasts are altered. However, these distal

deposits are probably not related to this volcanic complex, because

these deposits have no proximal counterparts around Ampato. In

addition, the lack of amphibole and the different geochemical signature

of these samples do not correlate with the Ampato magma series (see

below).

5.2. The Ampato Upper edifice

5.2.1. The Yanajaja stage

This unit consists of a sequence of lava flows reaching 200–300m in

thickness that crops out on the south and south-western flanks of

Ampato (Figs. 2, 4). These lavas rest on top of the remnants of

the Basal edifice (i.e. Moldepampa stage) defining a marked erosional

discontinuity (Fig. 5c). The lava sequence includes porphyritic andesites

and dacites (61.8–63.5 wt.% SiO2) of similar mineralogy to the

Moldepampa stage. A sample from this unit (SA-11-28) was dated at

77 ± 4 ka.Table

140K–40Arag

esforrocksfrom

Ampato–Saban

cayavolcan

icco

mplex.

Sam

ple

number

Experiment

number

Edifice

Volcan

icstag

eUnitan

dlocation

UTM

Easting

UTM

Northing

Altitude

(masl)

SplitK

(wt.%)±

2σ

Massmolten

(g)

40Ar*

%40Ar*

10−

12

(mol./g)±

1σ

Weightedmean

40Ar*

10−

12

(mol./g)±

1σ

Age

(ka)

±2σ

SA-09-17

8448

Sab

ancaya

Basal

edifice

Lavaflow,N

Eflan

k196,910

8,255,870

5010

2.457±

0.025

0.47434

0.017

0.007±

0.016

0.012±

0.011

3±

5

8464

0.51325

0.064

0.017±

0.017

SA-11-54

8521

UpperAmpato

Central

Cone

Lavaflow,E

flan

k193,676

8,249,484

5760

2.632±

0.026

0.41030

0.323

0.053±

0.034

0.078±

0.014

17±

6

8545

1.99591

0.665

0.083±

0.015

SA-11-71A

8522

UpperAmpato

South

cone

Lavaflow,S

Eflan

k192,823

8,248,561

5680

2.789±

0.028

0.39318

0.556

0.168±

0.031

0.165±

0.019

34±

8

8546

1.05451

1.108

0.163±

0.026

SA-09-30

8430

UpperAmpato

South

cone

Lavaflow,S

flan

k193,200

8,247,108

5155

3.055±

0.031

0.51211

1.159

0.214±

0.012

0.211±

0.007

40±

3

8446

1.18860

0.824

0.209±

0.008

SA-11–28

8720

UpperAmpato

Yan

ajaja

Lavaflow,S

Wflan

k189,043

8,244,402

4682

2.806±

0.028

0.42555

1.095

0.386±

0.021

0.373±

0.014

77±

4

8729

1.27769

2.024

0.363±

0.019

SA-11-15

8801

Basal

Ampato

Moldepam

pa

Lavaflow,S

flan

k192,405

8,243,491

4589

3.512±

0.035

1.51905

9.803

1.296±

0.010

1.281±

0.007

210±

3

8830

1.56919

5.43

1.263±

0.011

SA-11-37

8411

Basal

Ampato

Moldepam

pa

Lavaflow,S

Wflan

k187,567

8,246,350

4710

3.512±

0.035

0.54128

6.336

1.310±

0.010

1.327±

0.009

217±

5

8420

1.01345

6.498

1.333±

0.010

SA-11-38

8706

Basal

Ampato

Moldepam

pa

Lavaflow,S

Wflan

k188,471

8,247,137

4854

3.188±

0.032

1.00949

5.136

1.241±

0.018

1.248±

0.007

226±

4

8721

1.03340

8.182

1.275±

0.012

8722

1.01938

12.503

1.233±

0.010

SA-11-03

8561

Basal

Ampato

Form

eran

desiticstag

eLavaflow,S

Eflan

k194,252

8,244,656

4542

2.067±

0.021

1.03014

2.312

1.446±

0.020

1.471±

0.013

410±

10

8593

1.03500

8.666

1.490±

0.018

SA-11-64

8692

Basal

Ampato

Form

eran

desiticstag

e189,617

8,249,025

5392

3.221±

0.032

0.51816

4.371

2.506±

0.030

2.456±

0.019

440±

8

8707

0.54099

11.581

2.425±

0.024

114 P. Samaniego et al. / Journal of Volcanology and Geothermal Research 323 (2016) 110–128

5.2.2. North cone

This small cone is constructed on the northern remnants of the Basal

edifice. It has an almost elliptical basal outline (1.5 × 3 km) located at

5200–5400 m asl, steep slopes (30–40°), and consists of viscous,

andesitic (61–62 wt.% SiO2) lava flows and breccias with plagioclase,

amphibole, pyroxene, biotite, and Fe–Ti oxides. Co-magmatic, cm-

sized aphanitic enclaves of andesitic composition (57–58 wt.% SiO2)

are common. Given the degree of erosion of these lavas, we consider

this cone to be the oldest of the three peaks comprising the Ampato

Upper edifice (Figs. 3, 5).

5.2.3. South cone

This edifice is constructed on the remnants of the Yanajaja

stage and the Basal edifice. It includes several volcanic units with a

maximum thickness of 800–1000 m. We identified several effusive,

dome-forming phases as well as explosive activity, represented by

frequent, albeit highly eroded, pyroclastic sequences. The presence

of angular unconformities between lava flows, allows to reconstruct

the structure of this cone. The western flank of Ampato, around

5600–6000 m asl, is marked by an unconformity separating

the older volcanic sequences of the Basal edifice from a 150–200-

m-thick subhorizontal sequence of lavas. This sequence corresponds

to the infilling of a depression, probably associated with major

explosive activity of the Basal edifice (Fig. 5).

5.2.3.1. First cone-building stage. Lavas overlying the previously

mentioned subhorizontal lavas form the bulk of the Southern cone.

They originate from the Ampato summit and descend to 5100 m asl

Table 2

New 14C data for Ampato–Sabancaya volcanic complex. We also include additional radiocarbon data from the literature.

Lab code Sample no. Locality UTM

Easting

UTM

Northing

Type of

sample

14C age (aBP) δ13C

(o/oo)

Calendar age range

(2σ)§Relative area

(%)

Lab⁎ Reference

GrA 57971 SA-11-68C Qda. Huaraya 186,942 8,251,723 Peat 85 ± 35 −26.39 1697–1725 cal AD 12 CIO This study

1807–1870 cal AD 35

1875–1954⁎ cal AD 53

SacA 27951 SA-11-68C (bis) Qda. Huaraya 186,942 8,251,723 Peat 265 ± 30 −25.20 1627–1680 cal AD 52 LMC14 This study

1731–1801 cal AD 47

GrA 50536 SA-10-13D Colihuiri 201,497 8,248,928 Peat 730 ± 35 −25.23 1269–1326 cal AD 58 CIO This study

1341–1390 cal AD 42

SacA 27943 SA-10-13B Colihuiri 201,497 8,248,928 Peat 810 ± 30 −24.40 1218–1286 cal AD 100 LMC14 This study

GrA 57885 SA-11-79C Sallalli (III) 202,341 8,246,831 Peat 2925 ± 45 – 2876–3159 cal BP 100 CIO this study

SacA 27952 SA-11-79C (bis) Sallalli (III) 202,341 8,246,831 Peat 3080 ± 30 −25.80 3144–3358 cal BP 99 LMC14 This study

GrA 50535 SA-10-13F Colihuiri 201,497 8,248,929 Peat 3105 ± 40 −23.80 3156–3380 cal BP 100 CIO This study

GrA 56327 SA-10-19C Sallalli (II) 201,980 8,246,726 Peat 3815 ± 35 −26.50 3986–4051 cal BP 13 CIO This study

4062–4258 cal BP 86

GrA 50533 SA-10-19F Sallalli (II) 201,980 8,246,726 Peat 4150 ± 40 −24.53 4450–4464 cal BP 2 CIO This study

4518–4821 cal BP 98

SacA 27946 SA-10-19C Sallalli (II) 201,980 8,246,726 Peat 5050 ± 30 −26.30 5651–5773 cal BP 63 LMC14 This study

5779–5795 cal BP 2

5804–5892 cal BP 35

SacA 27947 SA-10-19D Sallalli (II) 201,980 8,246,726 Peat 5830 ± 35 −22.20 6477–6676 cal BP 100 LMC14 This study

SacA 27948 SA-11-20A Japo 194,357 8,240,560 Soil 9480 ± 40 −26.90 10,553–10,786 cal BP 98 LMC14 This study

11,036–11,057 cal BP 1

SacA 27944 SA-10-18C Sallalli (I) 202,016 8,246,444 Soil 9705 ± 35 −26.60 10,792–10,964 cal BP 38 LMC14 This study

11,004–11,024 cal BP 2

11,065–11,201 cal BP 60

SacA 27945 SA-10-18G Sallalli (I) 202,016 8,246,444 Peat 11,165 ± 45 −24.90 12,831–13,089 cal BP 100 LMC14 This study

Beta-126965 – Sallalli – – Peat 300 ± 50 – 1485–1678 cal AD 83 Juvigné et al. (2008)

1733–1800 cal AD 17

Lv-2184 – Sallalli (S2) – – Peat 2050 ± 70 – 1813–2154 cal BP 99 Juvigné et al. (2008)

GrN-25586 – Sallalli (S2) – – Peat 2370 ± 90 – 2150–2710 cal BP 100 Juvigné et al. (2008)

Hv-24660 – Sallalli (S3) – – Peat 1790 ± 110 – 1409–1919 cal BP 100 Juvigné et al. (2008)

Hv-24662 – Sallalli (S3) – – Peat 2955 ± 80 – 2849–3258 cal BP 97 Juvigné et al. (2008)

3290–3333 cal BP 3

Hv-24661 – Sallalli (S3) – – Peat 4500 ± 125 – 4823–5469 cal BP 100 Juvigné et al. (2008)

– – Qda. Huaraya – – Peat 5440 ± 40 – 6014–6081 cal BP 12 Gerbe and Thouret

(2004)

6104–6158 cal BP 14

6171–6292 cal BP 74

Lv-2107 – Sallalli (A) – – Peat 8520 ± 80 – 9281–9597 cal BP 100 Juvigné et al. (1998)

Lv-2110 – Sallalli (A) – – Peat 9650 ± 170 – 10,420–11,358 cal BP 100 Juvigné et al. (1998)

§ Calendar ages were obtained using the updated SHcal13 calibration curve (Hogg et al., 2013) except for SA-11-68C (GrA57971) for which we used the former SHcal04 calibration

curve (McCormac et al., 2004).⁎ CIO, Centre for Isotope Research, Groningen University (Netherlands).LMC14, Laboratoire de Mesure du Carbone 14, Gif-sur-Yvette (France).

Table 3

Cosmogenic 3He data and exposure ages.

Sample Altitude

(m)

Latitude

(°S)

Longitude

(°W)

Sampling

thickness (cm)

Sampling thickness

correction

Topographic

shielding

Scaling

factor⁎Mass

(mg)

4He

(at/g)

3He

(106 at/g)

Li

(ppm)

Exposure

age (ka)

SA1 4905 15.8222 71.827667 3 0.954 0.997 11.89 43.2 b.d.l 9.37 ± 0.47 9.5 6.30 ± 0.31

SA4 4754 15.823417 71.820067 4 0.94 0.995 11.15 42 b.d.l 9.11 ± 0.44 15.1 6.65 ± 0.32

SA5 4740 15.82505 71.817383 3.5 0.947 0.999 11.05 31.6 b.d.l 17.0 ± 0.76 14.1 12.34 ± 0.55

b.d.l.: below detection limit, indistinguishable with the 4He blank at 1 sigma of uncertainty.⁎ Time-dependent scaling of Stone (2000) considering a specific Andes atmosphere model (Farber et al., 2005). Calculated using CosmoCalc (Vermeesch, 2007).

115P. Samaniego et al. / Journal of Volcanology and Geothermal Research 323 (2016) 110–128

on the southern flank, whereas they reach down to only 6000 m asl on

thewestern flank. Two lavas from the south and south-eastflanks of the

cone yielded ages of 40±3 and 34±8 ka (SA-09-30 and SA-11-71A re-

spectively, Table 1).

5.2.3.2. Block-and-ash flow deposits. A N20-m-thick sequence crops out

5–6 km from the present summit to the east and west of the volcano

(Fig. 4), and lies on lava flows from the Basal edifice. These deposits

are massive, indurated, and matrix-supported, with 20–40 vol.% of

Southerncone

Centralcone

Northerncone

AMPATO SABANCAYA

Terminal cone& active crater

Sallallipeatbog

Fig. 3. Panoramic viewof the Ampato–Sabancaya volcanic complex. View from the SE showing theAmpato and Sabancaya edifices. Note the discordances between the Northern, Southern

and Central cones. Sallalli peatbog is in the foreground.

AMI-1

AMI-1

AMI-1

AMI-2

AMI-2

AMI-3

AMI-3

AMI-3

AMI-3

M1

M1

M1

M1

M1

M1M1

M1

M1

M1

M2

M2

M2M2

M2

M2

M2

M2

M3

M3

M3

FGS

FGS

FGS

FGS

FGS

FGS

FGS

FGS

SA-1

SA-1

SA-1SA-2

SA-2

SA-2

SA-2

SA-4

SA-4

SA-4

SA-3

SA-3

AMII-1

AMII-1

AMII-2

AMII-3

HH

HH

HH

HH

AMII-6

184000 188000 192000 196000 200000 204000

184000 188000 192000 196000 200000 204000

8240

00

08244

00

08248

00

08252

00

08256

00

0

8240

00

08244

00

08248

00

08252

00

08256

00

0

C2

C1

77±4 ka

210±3 ka

217±5 ka

226±4 ka

440±8 ka

40±3 ka

410±10 ka

34±8 ka

17±6 ka

3±5 ka

2925±45aBP

5440±40aBP

6.30±0.31ka

6.65±0.32ka

12.34±0.55ka

vent

crater

caldera

South cone lavas (AMII-3)

Thick lavas (AMII-6)

Block-and-ash sequences (AMII-5)

Sub-horizontal lavas (AMII-4)

Pyroclastic deposits (AMII-7)

Lower cone lavas (AMII-1)

Northern cone lavas (AMII-2)

Central cone lavas (AMII-8)

Young dome (AMII-9)

Lower lavas (SA-1)

Upper lavas (SA-2)

Satellite vent lavas (SA-3)

Central cone

Terminal cone

Basal edifice

South cone

Yanajaja stage

Lower cone lavas (AMI-3)

Moldepampa stage

North cone

Distal lavas (AMI-2)

Proximal lavas and

pyroclastics (AMI-1)

Former andesitic cone

Hualca Hualca lavas (HH)

Undifferentiated volcanics

Older formations

BA

SA

L A

MP

AT

O

(AM

I)

UP

PE

R A

MP

AT

O

(AM

II)

SA

BA

NC

AY

A

(SA

)

Lavas and pyroclastics (SA-4)

GLA

CIA

L

DE

PO

SIT

S Younger moraines (M3)

Intermediate moraines (M2)

Older moraines (M1)Undifferentiated fluvio-glacial

sediments (FGS)

D

D

10 km

Fig. 4. Geological map of the Ampato–Sabancaya volcanic complex.

116 P. Samaniego et al. / Journal of Volcanology and Geothermal Research 323 (2016) 110–128

subangular blocks (Fig. 6a-d). The matrix is composed of pink

ash enriched in lithic material and free crystals. Two main lithologies

are present: The dominant group consists of dark-grey porphyritic

amphibole-rich andesites, whereas the second group consists of

porphyritic amphibole-biotite dacitic blocks. This pyroclastic sequence

testifies to a large dome-forming stage during the AmpatoUpper edifice

development.

The above sequence is overlain by thick lava flows related to the

upper part of the Southern cone. On the eastern flank, these lavas

consist of a 150–200-m-thick sequence of at least three to four

heterogeneous lava flows, with individual thicknesses of 30–50 m, and

that are characterised by the ubiquitous presence of two end-

members: A more abundant porphyritic, dark grey andesite (62.9–

63.6 wt.% SiO2), and a subordinate, highly porphyritic, light grey dacite

(66.9–67.3 wt.% SiO2). These two contrasting compositions, similar to

those observed in the previous block-and-ash flow deposits, are distrib-

uted as both dm-sized bands and magmatic inclusions. The amount of

these inclusions is variable, but it increases towards the base of the

lava sequence until it resembles a breccia in the lower part of the

sequence. On the western flank, a conspicuous lava sequence with a

similar stratigraphic position and degree of erosion is present. There,

the andesitic (61–63 wt.% SiO2) lava flows reach 4800 m asl at 5 km

from the present summit and have a thickness of 40–60 m. These

lavas spilled into an ancient glacial valley and overlie the older and

larger moraines, but they also show marked relief inversion that

suggests intense glacial erosion. Based on these field constraints, we

infer that this lava sequence was contemporaneous with the LGM (i.e.

17–25 ka).

5.2.3.3. Baylillas scoria fallout and flowdeposits.A5–10-m-thick sequence

of tephra fallout deposits crops out in the southern part of Ampato, near

the Quebrada Baylillas, at 10–12 km from the summit (Figs. 7a, 8).

Across this section, at least 6 scoria-lapilli fallout deposits are

interlayered with ash-rich horizons. Their thickness ranges from ~10–

20 cm, although a 50-cm-thick scoria fallout layer marks the middle

part of the sequence (Figs. 7a, 8). The 80-cm-thick pumice fallout depos-

it at the top is described below. This sequence crops out southwards,

along the road towards Taya village. Despite of the high magnitude of

the eruptions that originated these deposits (up to 50-cm-thick at 10–

15 km from the vent), outcrops are scarce and it is not possible to

trace their extension and dispersion axes, because these layers were

eroded probably during the Pleistocene glaciations.

Petrographic and chemical characteristics of the juvenile material

link these scoria fallout deposits to the scoria flow deposits that crop

out at the south and south-western foot of Ampato. For instance, at

Japo,we found at least four scoria-flowdeposits, each up to 3m in thick-

ness (Figs. 7d, 8). These deposits arematrix-supported (around 30 vol.%

bombs), with 10–40-cm-sized andesitic blocks and bombs (59–60 wt.%

SiO2) and a reddish-grey consolidated matrix. These andesitic deposits

testify to an intense explosive activity associated with this edifice.

Because these deposits are highly eroded, we infer that they pre-date

the LGM period roughly dated at 17–25 ka (Bromley et al., 2009;

Alcalá et al., 2011).

5.2.3.4. Corinta pumice fallout and flow deposits. A thick pumice-and-

lithics fallout deposit crops out in the south-western sector of

Ampato, from the Collpa and Quebrada Vizcachane in the north to the

Quebrada Baylillas in the south. In the Corinta area, 10–12 km from

the present volcano's summit (Figs. 7b, 8), the tephra deposit is 3–4-

m-thick and is composed of two layers separated by a distinctive

lithic-rich coarse ash horizon 3–4 cm in thickness. In this section, the

lower layer displays a minimum thickness of 2.5 m whereas the upper

layer is 0.8–1.0-m-thick. Pumice fragments, reaching a maximum size

of 7–8 cm, are white, dacitic in composition (65–68 wt.% SiO2) with

plagioclase, amphibole, biotite, and minor of pyroxene. These deposits

are covered by a 1–1.5 m ash-rich horizon with disseminated pumice

lapilli. Above an erosional contact, we observed a 1–2 m-thick, roughly

stratified, matrix-supported deposit, bearing subrounded lithic and

pumice fragments (b20 cm in diameter) in an indurated ash-rich

matrix. This deposit is interpreted as being associated with fluvio-

glacial activity. This suggests a major explosive eruption prior to or

during the LGM period (i.e. 17–25 ka).

Encircling the south and south-western base of Ampato we

identified the vestiges of a thick, reddish-orange pumice flow deposit.

In the Collpa and Quebrada Vizcachane zone, this deposit displays a

roughly metric stratification, with an overall thickness of N10 m. Other

outcrops occur along the Quebrada Chacramayo (south-western flank)

and around the Japo zone (south flank). In this latter location, this unit

rests subhorizontally on top of the pre-Ampato substratum and is

covered by a sequence of younger scoria-flow deposits (Fig. 8). Other

outcrops of this unit are scarce, probably because they have been eroded

by LGM glaciers. It is composed of pumice fragments (up to 50 cm in

diameter) in an ash-rich unconsolidated matrix. The deposit also

includes lithics and scoria fragments that suggest magma mixing

Table 4

Generalized chronostratigraphy showing the main eruptive stage at Ampato–Sabancaya volcanic complex.

Edifice Volcanic stage Units Structure Age Magma composition

Sabancaya

(SA)

Terminal cone Lavas and pyroclastics (SA-4) Explosive activity ~3 ka Amphibole-biotite, silicic andesites and

dacites (60–66 wt. SiO2)Satellite vent Lavas (SA-2) Lava flow emission - Cone building

- No glacial erosion

3–10 ka

Basal edifice Lavas (SA-1,3)

Upper Ampato

(AM-II)

Central cone Young dome (AMII-10) and Central

cone lavas (AMII-9)

Lava flows and dome growth 10–20 ka Amphibole-biotite, silicic andesites and

dacites (61–66 wt. SiO2)

South cone Upper lavas (AMII-6, 7, 8) Mainly lavas 20–40 ka Amphibole-biotite, siliceous andesites and

dacites (61–67 wt. SiO2)Block-and-ash flow deposits (AMII-5) Dome growth

Corinta pyroclastic deposits

(AMII-4) - Scoriaceous tephra

Explosive activity - caldera

formation (C2)

Amphibole-biotite dacites (65–69 wt. SiO2)

and andesitic scoria (57–59 wt. SiO2)

Lower lavas (AMII-3) Cone-building Amphibole-biotite dacites (63–64 wt. SiO2)

North cone Lavas (AMII-2) Cone-building ~50 ka (?) Amphibole-biotite silicic andesites (61–62

wt. SiO2)

Yanajaja Thick lava flows (AMII-1) Cone-building 70–80 ka Amphibole-biotite, siliceous andesites and

dacites (62–64 wt. SiO2)

Basal Ampato

(AM-I)

Terminal

explosive

activity (?)

Rhyolitic tephra (AMI-4) Explosive activity b200 ka (?) Biotite-bearing rhyolites (74–77 wt. SiO2)

Moldepampa Thick, viscous lava flows (AMI-3) Two main cone-building stages

separated by quiescence period

200–230 ka Amphibole-rich dacites (63–65 wt. SiO2)

Former andesitic

cone

Proximal lavas and pyroclastics

(AMI-1) and distal lavas (AMI-2)

400–450 ka Two-pyroxenes andesites (57–60 wt. SiO2)

117P. Samaniego et al. / Journal of Volcanology and Geothermal Research 323 (2016) 110–128

processes. The juvenile pumice fragments are fibrous and dacitic in

composition (67.1–69.3 wt.% SiO2), with plagioclase, amphibole, and

scarce biotite. These characteristics suggest a genetic relation with the

Corinta plinian fallout deposit. This explosive phase probably developed

a crater-like structure (C2 in Fig. 4) marking the transition between the

Southern and Central cones.

5.2.4. Central cone

This edifice was discordantly constructed over the remnants of

the Northern and Southern cones of the Ampato Upper edifice (Figs. 3,

5). It is composed of a sequence of andesitic and dacitic (61.3–

66.0 wt.% SiO2) lava flows and interlayered breccias (Fig. 4), with a

maximum thickness of 400–600 m. These lavas fill a depression

between the Southern and Northern peaks, forming a steep (25–30°)

cone that extends down to 5300 m on the eastern flank. This cone

shows evidence of glacial erosion, although to a lesser extent than the

others. A lava sample from this unit was dated at 17 ± 6 ka (SA-11-

54, Table 1). We also included a young dacitic lava dome (63–64 wt.%

SiO2) in this eruptive stage, which extruded on top of the Northern

cone, between 5700 and 6000 m asl. This structure has an oblate

morphology and shows no evidence of glacial erosion. This lava dome

represents the youngest volcanic structure of the Ampato compound

edifice.

6. 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.

Southern Cone

Basal

Edifice

a

440±8 ka

226±4 ka

CentralCone

SouthernCone

NorthernCone

b

34±8 ka

40±3 ka

17±6 ka

c

Basal Edifice

Basal edifice

Southern Cone

77±4 ka

40±3 ka

210±3 kaYanajaja

Upper Edifice

Fig. 5. Panoramic views of theAmpato–Sabancaya volcanic complex, showing some of themain structures. (a) View from theWshowing the former andesitic cone (Ampato Basal Edifice),

the Ampato Upper Edifice (subhorizontal sequence and the upper part of the South cone). (b) View from the SE showing the discordance between the South and Central cones of the

Ampato Upper Edifice. (c) View from the S showing the Ampato Basal Edifice with its former andesitic cone and the Upper edifice. Note the thick package of Pleistocene moraines in

the foreground.

118 P. Samaniego et al. / Journal of Volcanology and Geothermal Research 323 (2016) 110–128

6.1. The Basal edifice

This stage is represented by a sequence of blocky lava flows that

spilled out onto the older rocks of the Ampato and Hualca Hualca

edifices. These flows extend 6–8 km from the vent and have individual

thicknesses of 40–80m, forming a 300–400m thick lava pile (Fig. 4). On

the basis of satellite images, Bulmer et al. (1999) identified more than

42 distinct lava flows at Sabancaya. Here, using geomorphological data

and stratigraphic information, we group these lava flows into two

successive units (SA-1 and SA-2, Fig. 4). All lava flows show an uneven

surface morphology, often with flow structures and levees, and no

evidence of glacial erosion. A lava block, corresponding to SA-1 unit,

was sampled for 3He surface-exposure dating and yielded an age of

6.30± 0.31 ka.We include a 400-m-high lava dome in this unit, located

in the south-west part of Sabancaya, which is considered to be older

than most of the lava flows. These lavas are porphyritic andesites and

dacites (60.6–65.6 wt.% SiO2) with plagioclase, hornblende, biotite,

and minor ortho- and clinopyroxene and Fe–Ti oxides.

Associated with this unit, we identified a thick lava sequence

that crops out in the Sallalli plain (unit SA-2, Fig. 4). Following

Bulmer et al. (1999), we correlate this sequence to a satellite vent

located 4 km to the southwest of the current active crater (Fig. 4). This

sequence comprises at least 3 subhorizontal, andesitic (60.9–62.5 wt.%

SiO2) lava flows, with their respective lobes overlying the sedimentary

deposits and the Sallalli peatbog. Two lava blocks were sampled for3He surface-exposure dating and yielded ages of 6.65 ± 0.32 ka and

12.34 ± 0.55 ka. These age determinations are consistent, and suggest

that Sabancaya Basal edifice started its construction probably as early

as the Holocene–Pleistocene boundary. The ages at 6–7 ka are in agree-

mentwith a peat sample that underlies a distal lava flow (unit SA-1, Fig.

4) located at 7–8 km to the west of Sabancaya's crater that has been

radiocarbon dated at 5440 ± 40 aBP (Gerbe and Thouret, 2004). In

addition, a new 40K–40Ar age of a lava sample from the SA-3 unit yielded

a younger age of 3 ± 5 ka (SA-09-17, Table 1). Based on these

constraints and the lack of glacial erosion, we propose that the

Sabancaya Basal edifice has a lower to middle Holocene age (10–

6 Ma) and that units SA-1 to SA-2 are almost synchronous.

6.2. The Young cone

This cone is represented by a sequence of lava flows that lie discor-

dantly on the lavas of the previous stage. This unit also includes a

young cone, covered by pyroclastic material, with an active summit

crater (unit SA-4, Fig. 4). These lava flows reach 4–5 km from the vent,

have a thickness of 40–60 m and consist of porphyritic andesites and

dacites (61.7–65.7 wt.% SiO2) with a mineral assemblage similar to the

Basal Sabancaya lavas.

6.3. Holocene tephra deposits

In order to better constrain Sabancaya's Holocene eruptive activity

as first established by Juvigné et al. (1998, 2008), we carried out three

new excavations in the Sallalli peatbog, and further excavations in

other peatbogs on the south-eastern (Colihuiri), southern (Japo), and

western (Quebrada Huaraya) flanks. We also obtained 14 additional14C ages on peat and paleosoil samples. The oldest ages come from the

a b

c d

Fig. 6. (a) Sequence of indurated block-and-ash flow deposits associated with the Ampato Upper Edifice (site SA-10-17). (b) Block of the deposit in (a). (c) Consolidated block-and-ash

flow deposit (SA-10-14). (d) Fragments of blocks with mixing/mingling textures (SA-10-16).

119P. Samaniego et al. / Journal of Volcanology and Geothermal Research 323 (2016) 110–128

Sallalli - Section A (Juvigné et al., 1998) and Sallalli I and Japo sections

(this work). These sections are characterised by peat and/or soil

horizons interlayered with reworked gravel and sand layers (Fig. 9). In

these sections, we found medium-to-coarse ash layers ranging from 5

to 30 cm in thickness. On the basis of granulometry and microscope

examination, Juvigné et al. (1998) concluded that some of these

deposits represent pristine ash fallout layers. Thus, we consider that

there are at least 4–5 ash fallout layers in the period between 11 and

8 ka. Younger ages (b4.5 ka) have been obtained for the Sallalli S2

(not shown) and S3 (Juvigné et al., 1998), as well as the Sallalli II and

III, Colihuiri, and Quebrada Huaraya I and II sections (Fig. 9). The period

between 4.5 and 3 ka shows two well-preserved ash layers, whereas

stratigraphic correlations are difficult for the period younger than

3 ka, and reveal thick layers that represent either sequences of ash

fallout deposits or reworking of individual layers. At least 6 ash fallout

deposits were observed in the Sallalli II section. In the upper part of

the sequence, we found a 5–10-cm-thick, biotite-bearing, fine yellow

ash layer that represents a key marker horizon. We interpret this layer

as being associated with the 1600 AD Huaynaputina eruption

(Thouret et al., 1999). Above this layer, and only at the Quebrada

Huaraya section, we found a 10–30-cm-thick coarse ash layer, whose

underlying peat has been dated at 265 ± 30 aBP (Fig. 9). This age is in

agreement with the 300 ± 50 aBP date obtained by Juvigné et al.

(2008) and confirms the occurrence of explosive eruptions during the

17th or 18th century (Travada y Córdova, 1752; Zamácola y Jaúregui,

1888). Finally, in the upper part of most sections (and sometimes

mixed with the recent peat layer), we found a 4–8-cm-thick coarse

layer that corresponds to the AD 1987–1998 eruption.

7. 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) Thefirst group, composedmainly of andesites (57–60wt.% 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 porphyritic lavas and tephraswith plagioclase,

ortho- and clinopyroxene, and Fe–Ti oxides.

(2) The second group corresponds to andesitic and dacitic composi-

tions (60–67wt.% SiO2) from theMoldepamba stage, the Ampato

Upper edifice and the Sabancaya edifice. No significant differ-

ences have been observed between these successive volcanoes.

All these samples are porphyritic with phenocryst of plagioclase,

amphibole, biotite, ortho- and clinopyroxene, and Fe-Ti oxides.

(3) The third group corresponds to dacitic compositions (65–69wt.%

SiO2) associated with the Corinta plinian fallout and pyroclastic

flowdeposits of theAmpatoUpper edifice. These samples display

a similar mineral composition to the second group.

(4) Lastly, the fourth group displays rhyolitic compositions (74–

77 wt.% SiO2) and corresponds to tephra fallout deposits

cropping out in the western plateau.

In general themajor oxides (excepting K2O, Fig. 10) show a negative

correlation with silica content, although some scattering is observed for

Na2O (not shown). Light Ion Lithophile Elements (LILE, e.g. Rb, Ba, Th),

Light Rare Earth Elements (LREE, e.g. La, Ce), and some High Field

Strength Elements (HFSE, e.g. Nb) show broad positive correlations

(Fig. 10). In contrast, Sr and the transition elements (e.g. Cr, Ni, V) are

inversely correlated with silica. Lastly, Zr, Y and Heavy Rare Earth

Elements (HREE, e.g. Dy, Yb) display contrasting behaviour with silica

increase. Samples from groups 1 and 2 have an inverse correlation

with silica, while group 3 dacites display higher HREE concentrations,

as well as elevated values of Al2O3, K2O, and LILE (e.g. Rb, Th). In

addition, this latter group has lower concentrations of CaO, MgO, Sr,

and transition elements. Lastly, the rhyolites of group 4 plot as an

extension of the trend defined by groups 1 and 2, except for LREE and

MREE.

Trace element variations clearly indicate mineral fractionation, for

instance a decrease in Ni and Cr with increasing silica suggests olivine

and/or clinopyroxene fractionation, whereas Sr and Eu decrease and

depletions in MREE and HREE indicate plagioclase and amphibole

crystallisation. Trace elements ratios plotted against a differentiation

index place additional constraints on the differentiation process. In a

Rb versus Rb/Sr diagram (Fig. 11), groups 1 and 2 samples define a

single linear trend (the ASVC main trend), probably controlled by

fractional crystallization processes. In contrast, samples from group 3

show a different slope, and those of group 4 have a completely different

trend. These differences suggest a prominent role of assimilation of

the local upper crust (represented by the Charcani gneiss; Rivera,

2010) in the petrogenesis of these magmas. On the basis of these trace

element constraints, we tested some first-order geochemical models

based on major and trace element concentrations. Major element

mass-balance calculationswere carried out betweenmafic and siliceous

end-members in order to estimate the modal compositions of the

fractionating cumulate. These results were comparedwith the observed

mineralogy and subsequently used in trace element modelling,

using fractional crystallisation and assimilation models. As a result, a

a

b

Fig. 7. Pyroclastic deposits of the Ampato Upper edifice. (a) Scoriaceous tephra fallout

deposits at Quebrada Baylillas (SA-10-33). (b) Corinta plinian fallout deposit (SA-11-12).

120 P. Samaniego et al. / Journal of Volcanology and Geothermal Research 323 (2016) 110–128

fractional crystallisationmodel (FC in Fig. 11) can be used to explain the

evolution of the main ASVC trend by fractionation of a cumulate

composed by pl + amph + bio + cpx + mag ± ol. However, this

model does not explain the composition of the silica-rich magmas

(namely the dacites of group 3). In contrast, an assimilation-fractional

crystallisation model (AFC1 and 2 in Fig. 11) can account for the

chemical composition of these silicic magmas.

Thus, the differences in magma chemistry of the ASVC reflect

complex crustal process involving fractional crystallisation of an

amphibole-rich cumulate, together with variable amounts of upper

crustal assimilation. In addition, the frequent banding textures observed

at Ampato Upper edifice rocks as well as in the 1990–1998 eruptive

products of Sabancaya (Gerbe and Thouret, 2004) clearly indicate that

magma mixing and mingling must be taken into consideration in the

interpretation of the diverse Ampato–Sabancaya magmas. A detailed

reconstruction of the magmatic processes in operation during the life

of this magmatic system is beyond the scope of this work, and will be

developed elsewhere.

8. 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 parame-

ters (volcano basal area, height, and volume), following themethodolo-

gy 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,

0

20

40

60

80

100

[cm]

Qda. Baylillas

0190776 - 8237067

40

40-120

>200

[cm]

50

39

2

66

30-40

50

80

10

15

15

25

12

116

20

20

15

50

128

10-30

1010

20

10

10-205

SA-

10-33A

SA-

10-33B

SA-

10-33C

SA-

10-33D

SA-

10-33E

Route to Taya

0185941 - 8233726

SA-

11-33A

SA-

11-33B

50

[cm]

10

30

12

14

10

15

30

5

1313

110

40

40

15

50

>20

0

1

2

3

4

5

[m]

Taya-Sallalli road

0195619 - 8240453

>3

[m]

0.4

>1

0.50.30.6

3

1

2

1.5SA-

11-45

5

1.7

>2

SA-

10-30

SA-

10-31

Collpa

0184842 - 8245165

>3-5

[m]

1.3

2.7

>1.5

SA-

11-39A

SA-

11-39B

SA-

11-39C

Corinta

0184069 - 8240343

>2.5

0.8

0.8-1.5

1-2

[m]>1

fine ash

medium ash

coarse ash

pumice and lithics

lapilli fallout

scoria lapilli fallout

reworked material

pumice flow

scoria flow

[cm]

Fig. 8. Stratigraphic sections of the tephra fallout deposits associated with the Ampato Upper Edifice. UTM Easting and Northing are included below the section's name.

121P. Samaniego et al. / Journal of Volcanology and Geothermal Research 323 (2016) 110–128

0

10

20

30

40

50

100

[cm]

Sallali (S3)

Juvigne et al. (2008)

[cm]

300

14

15

46

25

10

6

15

7

50

Colihuiri

SA-10-13

0201497 - 8248928

5

4-6

13

[cm]

5

7

5

5

15

20

6

5

31

6

5233

6-8

12

5

5

8

5

6

5

20

20

5

6

6

>15

HP?

Sallalli III

SA-11-79

0202341 - 8246831

2

[cm]

4-6

24

3-422

5-6

14

1-23-41-2

13

8-9

3-4

25-26

> 30

Q. Huaraya II

SA-11-68

0186942 - 8251723[cm]

20

6-7

50

30

8

40

1987-

1998 AD

Sallali II

SA-10-19

0201980 - 8246726

5

[cm]

2-4

14

4

3-5

6

3

15

5

7

3

18

7

10

30

15

6-7

30

5

25

Q. Huaraya I

SA-11-67

0187021 - 8251578

5

[cm]

20

10

20

2

13

15

4

4

5-8

8

5

5

22

5-6

12-13

1

7

1

6

2

2-3

7

> 8-10 m

SA-10-13D

730 ± 35

aBP

SA-11-68C

265 ± 30

aBP

HP?

SA-10-13F

3105 ± 40

aBP

SA-10-19C

3815 ± 35

aBP

SA-10-19F

4150 ± 40

aBP

SA-11-79C

2925 ± 45

aBP

3080 ± 30

aBP

1790 ± 110

aBP

2955 ± 80

aBP

4500 ± 125

aBP

peat

soil

fine ash

medium ash

coarse ash reworked gravel and ash

block-and-ash flowdiatomite

Sallali ISA-10-18

202016 - 8246444

12

[cm]

23

1-332

3-42

25

2-32

32

30

6

2

20

1-3

20

5

40

35

35

SA-10-18C

9705 ± 35

aBP

SA-10-18G

11165 ± 45

aBP

Late

Holocene

activity

Sallali (Section

Fig. 9. Stratigraphic sections of the Holocene explosive activity of the Sabancaya edifice. UTM Easting and Northing are included below the section's

the edifice outline was obtained from the geological map (Fig. 4)

taking into account that the edifice's base is defined by a concave

break in slope (Grosse et al., 2014). Then, we applied different

interpolation techniques (linear, cubic, inverse distance weighting

method) to delimit the substratum topography. This procedure yielded

a basal area of 170–180 km2, amaximumheight of 1.6 km, and a volume

1

2

3

4

5

6

wt.%

K2O

wt.%

Al 2

O3

12

14

16

18

20

0

1

2

3

wt.%

MgO

0

100

200

Rb p

pm

0

100

200

300

Zr

ppm

0

400

800

Sr

ppm

0

20

40

60

La p

pm

55 60 65 70 75 80

wt.% SiO2

0

1

Yb p

pm

55 60 65 70 75 80

wt.% SiO2

Former andesitic cone

Moldepamba stage

Rhyolitic tephra

Yanajaja stage

North, South and Central cones

Corinta tephra and related

scoriaceous depositsUndifferentiated lavas

BASAL AMPATOUPPER AMPATOSABANCAYA

A D R

MK

HK

G1

G2

G3

G4

ba

c d

e f

g h

Fig. 10. Selectedmajor and trace elements for theAmpato–Sabancaya samples, plotted against silica. (a) SiO2 vs. K2O classification diagram. A andesites, D dacites, R rhyolites,MKmedium-

potassium, HK high-potassium (after Peccerillo and Taylor, 1976). (b–h) Variation diagrams for Al2O3, MgO, Rb, Sr, Zr, La, and Yb respectively. G1–4 in (b) correspond to the four groups

identified (see text for more details).

123P. Samaniego et al. / Journal of Volcanology and Geothermal Research 323 (2016) 110–128

Table 5

Geochemical analyses representative of main volcanic units of the Ampato–Sabancaya volcanic complex.

Edifice Ampato Basal edifice Ampato Upper edifice

Volcanic

stage

Former andesitic

cone

Moldepampa Riolitic

sequence

Yanajaja North

cone

South cone Central

cone

Sample no. SA-11-64 SA-11-03 SA-11-15 SA-11-38 SA-11-37 SA-11-84A SA-11-28 SA-11-75A SA-09-30 SA-11-71A SA-10-33C SA-11-27A SA-11-12A SA-11-54

UTM-North 8,249,025 8,244,656 8,243,491 8,247,137 8,246,350 8,234,467 8,244,402 8,251,299 8,247,108 8,248,561 8,237,095 8,242,318 8,243,120 8,249,484

UTM-East 189,617 195,706 192,405 188,471 187,567 809,521 189,043 193,121 193,200 192,823 190,769 183,658 184,012 193,676

Nature lava lava lava lava lava tephra lava lava lava lava fallout

tephra

fallout

tephra

PF bomb lava

SiO2 (wt.%) 60.28 57.37 64.22 62.78 61.90 70.62 61.06 60.27 61.79 62.82 56.71 63.23 65.82 60.79

TiO2 0.86 1.17 0.80 0.88 0.85 0.16 0.93 0.97 0.83 0.86 1.23 0.69 0.60 0.95

Al2O3 15.76 16.52 15.51 15.49 16.04 12.47 16.11 16.24 15.56 15.96 17.59 15.86 15.01 16.96

Fe2O3 * 5.46 7.16 4.54 4.95 4.83 0.98 5.42 5.61 4.98 5.26 7.44 3.83 3.24 5.22

MnO 0.07 0.09 0.05 0.06 0.06 0.08 0.07 0.07 0.07 0.07 0.10 0.05 0.04 0.07

MgO 2.20 3.59 1.83 1.99 2.00 0.21 2.66 2.60 2.21 2.25 2.83 1.01 0.82 2.49

CaO 4.57 6.08 3.72 3.91 3.84 0.89 4.81 5.08 4.41 4.61 5.87 2.51 2.30 5.14

Na2O 3.61 4.32 4.12 4.19 4.01 2.84 4.18 4.26 4.07 4.35 4.12 3.18 3.64 4.51

K2O 3.74 2.53 3.57 3.44 3.45 5.18 2.93 2.84 3.11 3.23 2.17 4.37 4.75 2.74

P2O5 0.28 0.44 0.28 0.32 0.30 0.03 0.32 0.33 0.26 0.33 0.34 0.17 0.16 0.33

LOI 1.81 0.00 0.64 0.26 0.74 4.71 1.22 0.31 1.23 −0.02 1.96 4.66 2.88 0.37

TOTAL 98.65 99.26 99.30 98.28 98.02 98.16 99.71 98.57 98.50 99.72 100.37 99.58 99.26 99.56

Sc (ppm) 9.8 13.5 7.5 7.5 7.8 2.6 10.6 10.1 9.2 9.1 13.0 6.1 5.3 9.0

V 124.2 169.0 97.9 109.1 103.4 5.6 124.6 128.6 113.2 115.0 181.1 69.6 58.6 117.3

Cr 19.0 104.9 34.3 39.8 39.7 1.7 46.1 49.3 34.2 42.3 7.5 1.3 0.0 37.7

Co 15.0 22.7 11.8 14.1 13.1 0.3 16.0 16.1 14.5 14.2 21.2 9.3 6.2 15.5

Ni 18.0 53.4 20.6 21.8 22.8 1.2 30.9 23.4 24.1 23.7 16.4 5.1 1.8 25.3

Rb 104 69 140 134 117 135 87 89 77 101 50 182 204 84

Sr 679 911 693 704 734 141 771 821 678 817 835 438 410 893

Y 16.5 15.1 13.1 13.1 13.2 12.0 12.8 12.6 12.5 12.3 16.8 18.2 17.3 12.4

Zr 214 238 146 143 152 91 133 129 134 148 207 342 333 120

Nb 9.6 9.2 9.3 9.3 9.4 12.8 7.8 8.2 8.0 9.7 7.2 12.6 12.4 7.4

Ba 1014 1168 1020 983 1065 1174 977 1036 1005 1154 949 1186 1137 1045

La 40.9 42.1 46.2 44.0 44.7 20.0 38.2 38.1 37.0 45.8 33.8 49.1 40.3 37.4

Ce 84.8 85.2 94.6 89.4 89.8 56.4 78.7 80.1 74.0 94.0 71.7 108.6 98.5 73.1

Nd 35.6 38.7 40.1 38.7 41.3 15.2 34.6 34.9 32.0 38.3 35.8 40.6 33.7 35.3

Sm 6.5 6.9 6.1 6.7 6.6 3.1 6.1 6.5 5.4 6.5 6.7 7.1 5.8 6.2

Eu 1.4 1.7 1.3 1.4 1.4 0.6 1.5 1.5 1.3 1.4 1.6 1.2 1.0 1.6

Gd 4.4 4.7 4.2 4.5 4.3 2.3 4.2 4.0 3.7 4.3 5.0 4.9 4.7 4.1

Dy 3.0 3.0 2.5 2.6 2.7 1.8 2.6 2.5 2.3 2.4 3.2 3.3 2.9 2.4

Er 1.7 1.0 0.7 1.2 1.0 0.8 1.0 1.2 1.0 0.9 1.5 1.5 0.9 0.9

Yb 1.4 1.2 1.0 1.0 1.1 1.2 1.0 1.0 1.0 0.9 1.3 1.6 1.6 0.9

Th 9.8 4.3 16.4 14.5 15.7 8.8 9.7 8.5 11.0 9.7 4.8 21.8 19.7 8.1

0.0

0.2

0.4

0.6

0.8

Rb/S

r

Rb0 100 200 300

10

30

50

70

La/Y

b

Rb0 100 200 300

0.0

0.2

0.4

0.6

0.8

Rb/S

r

Rb0 100 200 300

10

30

50

70

La/Y

b

Rb0 100 200 300

FC

FC

AFC2

AFC1

AFC1

AFC2

UC

UC

Rhyolithes:

Rb: 140-240 ppm

Rb/Sr: 0.6-2.9

ASVC

main trend

Corinta

dacites

a b

c d

Fig. 11. (a, c) Trace element ratios (Rb/Sr and La/Yb) plotted against Rb. Same symbols as Fig. 10. (b, d) Fields for Ampato–Sabancaya main trend (group 1 and 2, grey field) and Corinta

dacites (group 3, blue field). Note that rhyolites (group 4) display very high Rb/Sr values. FC: fractional crystallisation model from a parental composition (SA-11-62) and a amphibole-

bearing cumulate (62% pl + 30% amph +1% cpx + 7% mag). AFC: assimilation-fractional crystallisation models from the same parental composition and fractionating phases. AFC 1,

r = 0.5 and AFC2, r = 1.0, r = ratio between assimilated and fractionated mass. The assimilated upper crustal material (UC) is represented by the Charcani gneiss (Rivera, 2010).

Table 6

Magma eruptive rates at Ampato–Sabancaya Volcanic Complex, compared to eruptive rates from other dacitic volcanoes in active continental margins.

Volcano Compositional range Arc Volume

(km3)

Lifespan

(ka)

Average eruptive rate

(km3/ka)⁎Peak eruptive rates

(km3/ka)

Reference

Mt. Baker Andesitic to rhyolitic Cascades 161 1300 0.12 Hildreth et al. (2003)

Mt. Adams Basaltic to dacitic Cascades 230–400 940 0.24–0.42 1.5–2.5 Hildreth and Lanphere (1994)

Mt. Mazama Andesitic to dacitic Cascades 176 420 0.42 0.8–2.5 Bacon and Lanphere (2006)

Ceboruco Dacitic to rhyolitic Mexico 81 800 0.10 0.6 Frey et al. (2004)

Tancitaro Andesitic Mexico 97 556 0.17 Ownby et al. (2007)

Whole Pichincha Andesitic to dacitic Northern Andes 250 850 0.30 Robin et al. (2010)

Guagua Pichincha Andesitic to dacitic Northern Andes 30–32 50 0.60–0.64 0.7–2.2 Robin et al. (2010)

Chimborazo Andesitic to dacitic Northern Andes 63–100 120 0.53–0.84 1.2–1.6 Samaniego et al. (2012)

Parinacota Andesitic to rhyolitic Central Andes 46 163 0.25–0.31 0.5–1.2 Hora et al. (2007)

Ollague Dacitic Central Andes 85 ~1000 0.09 Feeley and Davidson (1994)

Lascar Dacitic Central Andes 35 ~400 0.08 Mathews et al. (1994)

Llullaillaco Dacitic Central Andes 50–60 ~1000 0.04–0.06 Richards and Villeneuve (2001)

Aucanquilcha Dacitic Central Andes 38 1000 0.04–0.16 Klemetti and Grunder (2008)

Ubinas Andesitic to rhyolitic Central Andes 56 235 0.17–0.22 Thouret et al. (2005)

El Misti Andesitic to rhyolitic Central Andes 70 112 0.63 2.1 Thouret et al. (2001)

Tatara - San Pedro Basaltic to rhyolitic Southern Andes 40 250 0.16 Singer et al. (1997)

Puyehue - Cordon Caulle Basaltic to rhyolitic Southern Andes 131 314 0.42 0.8–0.9 Singer et al. (2008)

Ampato–Sabancaya Volcanic complex

Sabancaya Andesitic to dacitic Central Andes 6–10 6–10 0.60–1.70 This work

Ampato Andesitic to dacitic Central Andes 38–42 440 0.08–0.09 This work

Whole complex Andesitic to dacitic Central Andes 44–54 450 0.10–0.12 This work

⁎ Deduced from estimated total volume and entire duration of the volcanic activity.

125P. Samaniego et al. / Journal of Volcanology and Geothermal Research 323 (2016) 110–128

of 44–54 km3 for the whole Ampato–Sabancaya volcanic complex. This

estimate is highly dependent on the substratum topography, and is

responsible for the uncertainty (10–20%) in the bulk volume assess-

ment. These bulk volumes take into account moraine deposits, whose

volume was estimated at 3–5 km3. Given that the older ages for the

volcanic complex are ~450 ka, we estimated an average eruptive rate

of ~0.10–0.12 km3/ka (Table 6). Using a similar approach, we also

estimated the volume for Ampato,which is 38–42km3. Given a duration

from ~450 ka until the end of the Pleistocene, its average eruptive rate

was calculated at 0.08–0.09 km3/ka (Table 6). The young Sabancaya

edifice has a smaller volume (6–10 km3). This large uncertainty is relat-

ed to imprecise constraints on the substratum topography because

Sabancaya is constructed on remnants of the older Ampato. Sabancaya's

construction lasted from 10 to 6 ka until ~3 ka, at an average eruptive

rate of around 0.6–1.7 km3/ka (Table 6), which is an order of magnitude

higher than for Ampato.

Estimating the volume of the main eruptive stages of the ASVC

is challenging given the uncertainties associated with the lateral extent

of the older units (Table 7, Fig. 12). We used a GIS-based procedure

to estimate the surface of each volcanic unit and its minimum and

maximum thickness and applied these parameters and simple

geometrical forms (cone, frustum, parallelepiped) to constrain the

bulk volume for these units. This highlights significant variations

between the long-term eruptive rates associatedwith thewhole edifice,

and the eruptive rates for individual eruptive stages. For instance, the

average eruptive rate for Ampato was estimated at (~0.1 km3/ka),

whereas we obtained higher eruptive rates for some volcanic

stages such as Moldepampa (0.38–0.45 km3/ka) or Yanajaja (0.35–

0.50 km3/ka, Table 7).

The bulk eruptive rates of the ASVC are comparable to those

from other silicic volcanic systems in continental arc settings (0.04–

0.2 km3/ka, Table 6), such as the Central Andes (i.e. Aucanquilcha;

Klemetti and Grunder, 2008), the Cascades (Mt Baker; Hildreth et al.,

2003) and the Trans-Mexican volcanic belt (i.e. Ceboruco, Tancítaro;

Frey et al., 2004; Ownby et al., 2007). In contrast, ASVC eruptive rates

are notably lower than those of other Andean volcanoes, particularly

the neighbouring El Misti volcano (0.63 km3/ka, Thouret et al., 2001)

and Parinacota (0.25–0.31 km3/ka, Hora et al., 2007) and others from

the Northern Andes, such as Guagua Pichincha (0.6 km3/ka, Robin

et al., 2010), Chimborazo (0.5–0.8 km3/ka, Samaniego et al., 2012),

and Puyehue-Cordón Caulle in the Southern Andes (0.42 km3/ka,

Singer et al., 2008).

We stress that these estimates suffer from an averaging effect, since

the long repose times are also taken into consideration. In fact, large

eruptive rate variations have been observed at several continental arc

volcanoes, such as Chimborazo (Samaniego et al., 2012), Puyehue-

Cordón Caule (Singer et al., 2008), Mt. Adams (Hildreth and Lanphere,

1994), and Mount Mazama (Bacon and Lanphere, 2006). Therefore,

we consider that the eruptive rate estimated for Sabancaya volcano

(0.6–1.7 km3/ka; Fig. 12) represents peak eruptive rates associated

with a main cone-building stage, and corroborates the hypothesis that

composite volcanoes grow in “spurts” with peak eruptive rates as high

as 1–2 km3/ka (Hildreth and Lanphere, 1994; Davidson and de Silva,

2000).

9. Late Holocene eruptive activity and hazards

Our new data confirms previous tephrochronological studies

(Juvigné et al., 1998, 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 ash layers preserved in the peatbogs of the southern flank

could be related to the last eruptive phases of Ampato or the beginning

of volcanic activity at Sabancaya. In contrast, the Late Holocene

tephrochronology reveals recurrent explosive activity, with at least 6–

8 eruptions during the last 4000–5000 years, including the historic

eruptive phase of the 17th–18th century and the eruptive episode at

the end of the 20th century (AD 1987–1998).

An eruption of Sabancaya potentially threatens the Colca valley

(located 23–26 km to the north), which is an important tourist destina-

tion in Southern Peru. Based on the tephrochronological and historical

eruptive records, as well as the geological information for both the

Ampato and Sabancaya edifices, themost probable scenario for an erup-

tion of Sabancaya would be a low-to-moderate magnitude (VEI 1–2)

vulcanian or phreatomagmatic activity, accompanied by notable tephra

emissions that would produce a local impact. In addition, due to the

large icecap on Ampato and Sabancaya volcanoes, secondary lahars

might be triggered, as during the AD 1987–1998 eruption (Global

Volcanism Program, 1991). Moreover, given the major explosive activi-

ty (subplinian to plinian) of the Ampato volcano, higher magnitude

scenarios (VEI ≥ 3) have also been taken into account for the recently

published volcanic hazard map (Mariño et al., 2013; Global Volcanism

Program, 2013).

Table 7

Minimum–maximum volumes and eruptive rates for the main eruptive stages of ASVC.

Sabancaya Min–max

volume (km3)

Time

(ka)

Lifespan

(ka)

Min–max

eruptive rate

(km3/ka)

Terminal cone (SA4) 1.4–2.2 3–0 3 0.5–0.7

Satellite vent (SA3) 0.3–0.7

Basal edifice (SA1–2) 5.0–8.1 (10–6)-3 3–7 1.5–2.7

Whole Sabancaya 6–10 (10–6)-0 10–6 0.6–1.7

Ampato Upper edifice

Central 0.2–0.5 20–10 10 0.02–0.05

South 1.5–5.2 40–20 20 0.08–0.26

North 0.1–0.7

Yanajaja 3.5–5.0 70–80 (?) 10 0.35–0.50

Ampato Basal edifice

Moldepampa 19–22 250–200 50 0.38–0.45

Former andesitic cone 12–16 450–400 50 0.24–0.32

Whole Ampato 38–42 450–10 440 0.08–0.09

0

10

20

30

40

50

60

70

80

0100200300400500

~0.1 km3/ka

~1 km3/ka

Age (ka)

Cum

ula

tive v

olu

me (

km

3)

Ampato-Sabancaya

average

min-max

range

Parinacota

El Misti

Puyehue

Tatara-San Pedro

Mt. Mazama

Fig. 12. Cumulative volume versus age diagram for the Ampato–Sabancaya volcanic

complex.

126 P. Samaniego et al. / Journal of Volcanology and Geothermal Research 323 (2016) 110–128

10. 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 successively shaped the

current peaks of Ampato: The Northern, Southern, and Central cones.

The Southern peak, which is the largest, was built over several effusive

episodes spanning the time period ka. The Baylillas tephra fallout se-

quence and the Corinta plinian fallout deposit testify to the intense ex-

plosive activity during this volcanic stage. The last cone-building stage

of Ampato constructed the Central cone from 20 to 10 ka, and probably

continued until Holocene times. During the Holocene, eruptive activity

migrated to the NE and built up the mostly effusive Sabancaya edifice,

between 6 and 3 ka. This cone comprises andesitic and dacitic blocky

lava flows which were emplaced during at least two eruptive stages.

During the Late Holocene (4.5–3 ka), Sabancaya's activity turnedmostly

explosive, although the erupted volume remained low.

Samples from the ASVC define a broad high-K magmatic trend

composed of andesites and dacites with a mineral assemblage of

plagioclase, amphibole, biotite, ortho- and clino-pyroxene, and Fe–

Ti oxides. A secondary trend is also identified, and corresponds to

rare dacitic explosive eruptions (Corinta fallout deposits). Both

magmatic trends are derived by fractional crystallisation involving

an amphibole-rich cumulate with variable amounts of upper crustal

assimilation.

The overall eruptive history of the Ampato–Sabancaya volcanic

complex reveals major changes in the bulk eruptive rates during the

volcano's development. Throughout Ampato's evolution, peak eruption

rates reached values of 0.2–0.5 km3/ka, although the greatest variation

has been identified between the overall eruptive rate for Ampato

(0.09 km3/ka) and that for Sabancaya (0.6–1.7 km3/ka). This abrupt

change highlights variable eruptive rates throughout the volcano's

history.

Based on tephrochronologic studies, the Late Holocene activity at

Sabancaya is characterised by vulcanian events with local ash fallout

impact. We identified at least 6 eruptions during the last 4–5 ka, includ-

ing the historical AD 1750–1784 and 1987–1998 events. On the basis of

this recurrent low-to-moderate explosive activity, Sabancaya must be

considered an active volcano, as corroborated by the frequent signs of

unrest since 2013.

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 radiocar-

bon dates were obtained thanks to IRD support to Laboratoire de Mesure

du Carbone 14 (LMC14), UMS 2572 (CEA-CNRS-IRD-IRSN-MCC), Saclay,

France.We thank B. Jicha, C. Siebe and J.L. Le Pennec for their constructive

reviews of a previous version of thismanuscript, and F. vanWyk des Vries

for improving the English. This is Laboratory of Excellence ClerVolc

contribution no 200.

Appendix A. Supplementary data

Supplementary data to this article can be found online at http://dx.

doi.org/10.1016/j.jvolgeores.2016.04.038.

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