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Book Title Copahue VolcanoSeries Title

Chapter Title Active Tectonics and Its Interactions with Copahue Volcano

Copyright Year 2015

Copyright HolderName Springer-Verlag Berlin Heidelberg

Author Family Name BonaliParticle

Given Name F. L.Prefix

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Division Dipartimento di Scienze dell’Ambiente e del Territorio e di Scienze dellaTerra

Organization Università di Milano-Bicocca

Address Piazza della Scienza 4, 20126, Milan, Italy

Division Istituto Nazionale di Geofisica e Vulcanologia

Organization Sezione di Milano

Address Via E. Bassini 15, 20133, Milan, Italy

Email

Author Family Name CorazzatoParticle

Given Name C.Prefix

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Division Dipartimento di Scienze della Terra “A. Desio”

Organization Università di Milano

Address Via Mangiagalli 34, 20133, Milan, Italy

Email

Author Family Name BellottiParticle

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Organization Tele-Rilevamento Europa T.R.E. s.r.l

Address Ripa di Porta Ticinese 79, 21149, Milan, Italy

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Corresponding Author Family Name GroppelliParticle

Given Name G.Prefix

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Division Istituto per la Dinamica dei Processi Ambientali—Sezione di Milano

Organization Consiglio Nazionale delle Ricerche

Address via Mangiagalli 34, 20133, Milan, Italy

Email [email protected]

Abstract The aim of this review is to describe the state-of-art of the neotectonic setting of this area as well as topresent new data resulting from a recent structural field survey. The integrated analysis of literature andnew structural data shows incongruities mainly in some regional structures and in the ENE-WSW strikingfault system that affects and controls the feeding system of Copahue volcano. In addition, taking intoaccount the very recent volcanic activity, the structural constraints and the earthquakes occurred in the areaclose to the volcano, a static stress numerical model was applied to simulate the variations of the localstress perturbing the normal activity of the volcanic plumbing system favouring magma ascent andconsequent eruptions. At present, a comprehensive structural model is lacking and more in-depth studiescan furnish a complete tectonic framework of the area, which can provide fundamental information toassess volcanic hazard, forecast future volcanic activity, and to enhance the development of the associatedgeothermal field.

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2 23 Active Tectonics and Its Interactions4 with Copahue Volcano

5 F.L. Bonali, C. Corazzato, F. Bellotti and G. Groppelli

7 Abstract8

9 The aim of this review is to describe the state-of-art of the neotectonic10 setting of this area as well as to present new data resulting from a recent11 structural field survey. The integrated analysis of literature and new12 structural data shows incongruities mainly in some regional structures and13 in the ENE-WSW striking fault system that affects and controls the14 feeding system of Copahue volcano. In addition, taking into account the15 very recent volcanic activity, the structural constraints and the earthquakes16 occurred in the area close to the volcano, a static stress numerical model17 was applied to simulate the variations of the local stress perturbing the18 normal activity of the volcanic plumbing system favouring magma ascent19 and consequent eruptions. At present, a comprehensive structural model is20 lacking and more in-depth studies can furnish a complete tectonic21 framework of the area, which can provide fundamental information to22 assess volcanic hazard, forecast future volcanic activity, and to enhance23 the development of the associated geothermal field.

2425�2.1 Introduction

26�Active tectonics is capable of influencing activity27�at volcanoes both at local and regional scale,28�controlling the feeding system geometry, the29�magma rising, and the growth-dismantlement

F.L. BonaliDipartimento di Scienze dell’Ambiente e delTerritorio e di Scienze della Terra, Università diMilano-Bicocca, Piazza della Scienza 4, 20126Milan, Italy

F.L. BonaliIstituto Nazionale di Geofisica e Vulcanologia,Sezione di Milano, Via E. Bassini 15, 20133 Milan,Italy

C. CorazzatoDipartimento di Scienze della Terra “A. Desio”,Università di Milano, Via Mangiagalli 34, 20133Milan, Italy

F. BellottiTele-Rilevamento Europa T.R.E. s.r.l, Ripa di PortaTicinese 79, 21149 Milan, Italy

G. Groppelli (&)Istituto per la Dinamica dei Processi Ambientali—Sezione di Milano, Consiglio Nazionale delleRicerche, via Mangiagalli 34, 20133 Milan, Italye-mail: [email protected]

Layout: T4 Unicode Book ID: 309019_1_En Book ISBN: 978-3-662-48004-5

Chapter No.: 2 Date: 30-7-2015 Time: 1:51 pm Page: 1/23

© Springer-Verlag Berlin Heidelberg 2015F. Tassi et al. (eds.), Copahue Volcano, Active Volcanoes of the World,DOI 10.1007/978-3-662-48005-2_2

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30 phases of volcanic edifices (Marzocchi et al.31 1993; Barrientos 1994; Decker et al. 1995;32 Bautista et al. 1996; Nostro et al. 1998; Bellotti33 et al. 2006; Walter and Amelung 2006; Norini34 et al. 2008, 2010, 2013; Groppelli and Norini35 2011; Bonali et al. 2013; Bonali 2013). Active36 tectonics also impacts the hydrothermal circula-37 tion and consequently geothermal fields in vol-38 canic areas, by developing faults and fractures in39 the upper crust (e.g. Cameli et al. 1993; Brogi40 et al. 2010; Giordano et al. 2013; Norini et al.41 2013).42 The purpose of this chapter is to present the43 state of the art on Copahue volcano, an active44 Andean volcano located in the eastern Andean45 Southern Volcanic Zone, where a close connec-46 tion between tectonic control, volcanic activity47 and the associated geothermal field is well doc-48 umented in previous works (e.g. Velez et al.49 2011, and references therein). The resolution of50 the neotectonic setting around Copahue volcano51 has been improved in this chapter by means of a52 detailed structural field survey performed in53 2007, mainly devoted to recognising the evi-54 dence of active tectonics and the relationship55 among the fault pattern, the volcanic feeding56 system and the eruptive history.57 Moreover, for the reason that large earth-58 quakes could trigger volcanic activity at both59 close and large distances from the epicentre at60 regional scale (Linde and Sacks 1998; Hill et al.61 2002; Marzocchi 2002a, b; Marzocchi et al.62 2002; Manga and Brodsky 2006; Walter and63 Amelung 2007; Eggert and Walter 2009; Bebb-64 ington and Marzocchi 2011) in a time-frame of a65 few days after the earthquake event (Linde and66 Sacks 1998; Manga and Brodsky 2006; Eggert67 and Walter 2009; Moreno and Petit-Breuilh68 1999), a static stress numerical model was also69 applied to simulate the variations in the local70 stress induced by earthquakes on the plumbing71 system.72 Structural data in combination with numerical73 modelling can provide fundamental information74 to forecast future volcanic activity and to assess75 volcanic hazard. Finally, the in-depth knowledge76 of the tectonic setting of Copahue volcano can77 enhance the development and exploitation of the

78�geothermal field, because of the structural control79�on hydrothermal circulation (Giordano et al.80�2013; Invernizzi et al. 2014).

8182�2.2 Volcanotectonic Setting

83�As highlighted by Folguera et al. (this book), the84�2997 m-high Copahue active stratovolcano is85�located in the eastern part of the Southern Vol-86�canic Zone (SVZ) (Fig. 2.1). The deformation in87�the study area is partitioned in such a way that88�large thrust earthquakes dominantly occur in the89�subduction zone, whereas intra-arc transcurrent90�faulting events are observed (Barrientos and91�Ward 1990; Cisternas et al. 2005; Watt et al.92�2009). Dextral strike-slip moment tensor solu-93�tions dominate between 34° and 46°S in the main94�cordillera (e.g. Chinn and Isacks 1983; Lange95�et al. 2008), although it is only south of 38°S that96�long-term strike-slip faulting shows surface evi-97�dence, represented by the 1200 km-long98�Liquiñe-Ofqui major intra-arc fault zone99�(LOFZ, Figs. 2.1 and 2.2; Cembrano et al. 1996;100�Folguera et al. 2002; Adriasola et al. 2006;101�Rosenau et al. 2006; Cembrano and Lara 2009).102�This dextral transpressional fault zone plays a103�fundamental role in controlling the magmatic104�activity along the volcanic front between 37° and105�47°S (Lavenu and Cembrano 1999; Rosenau106�2004). López-Escobar et al. (1995) and Cembr-107�ano and Lara (2009) suggested that, with respect108�to the more contractional setting further north in109�the SVZ, the LOFZ allows a more rapid magma110�ascent with lesser occurrence of crustal assimi-111�lation or magma mixing (Collini et al. 2013).112�Folguera et al. (2006, this book) recognise a113�transition from the strike-slip fault zone to a114�thrust zone at approximately 37°S, testified by115�the merging of the LOFZ into the Agrio and the116�Malargue fold-and-thrust belts (Fig. 2.1). This117�transition also corresponds to a thickening and118�ageing of the continental crust (Rojas Vera et al.119�2014), to longer crustal magma residence times,120�and to greater differentiation, crustal assimilation121�and magma mixing (Tormey et al. 1991)122�(Table 2.1). AQ1

2 F.L. Bonali et al.



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123 The name “Caviahue-Agrio caldera” where124 Copahue volcano is located (Figs. 2.2a and 2.3),125 encompasses both names that this structure was126 given in the literature, Caviahue caldera (Mel-127 nick et al. 2006; Velez et al. 2011) and Agrio128 caldera (Melnick and Folguera 2001; Rosenau129 et al. 2006; Rojas Vera et al. 2009) respectively.

130�It is described as a pull-apart basin, a transten-131�sional structural system developed in the north-132�eastern end of the LOFZ by dextral movements133�in the late Pliocene (Melnick and Folguera 2001;134�Folguera et al. this book) and part of the135�abovementioned transition zone between the136�Andean segments to the north and south (Lavenu

Fig. 2.1 Tectonic setting of the Southern VolcanicZone (SVZ). Triangles locate active Holocenic volca-noes. The white arrow indicates the approximateconvergence direction of the plates with estimatedvelocity. Main intra-arc faults with reverse motion(thrust faults) in the north, and right lateral strike-slipmotion in the south are reported (redrawn after Melnicket al. 2006; Cembrano and Lara 2009; Bonali 2013).LOFZ-Liquiñe-Ofqui Fault Zone; TZ-Transition Zone;CCM-Callaqui-Copahue-Mandolegüe lineament,ACFZ-Antinir-Copahue regional thrust-fold system.

NVZ, CVZ, SVZ and AVZ correspond to Northern,Central, Southern and Austral Volcanic Zones.TU-Tupungatito; SJ-San José; MA-Maipo; PA-Palomo;TI-Tinguiririca; PP-Planchón-Peteroa; DG-DescabezadoGrande; CZ-Cerro Azul; NL-Nevado de Longavi,NC-Nevados de Chillán; AN-Antuco; CA-Callaqui;TH-Tolhuaca; LO-Lonquimay; LL-Llaima; VI-Villarrica;QU-Quetrupillan; LA-Lanin; MC-Mocho-Choshuenco;PC-Puyehue-Cordón Caulle. Yellow stars locate earth-quakes with Mw ≥ 8 occurred since 1900 AD, focalmechanisms of the modelled earthquakes are reported

2 Active Tectonics and Its Interactions with Copahue Volcano 3



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137 and Cembrano 1999). The LOFZ shows clear138 evidence of Plio-Quaternary dextral kinematics,139 and the main active fault of this area is the140 NE-striking Lomìn fault (LF in Fig. 2.2), ending141 at the southwestern border of the Caviahue-Agrio142 caldera and representing the most relevant143 expression of a horsetail-like array and affecting144 Holocenic lavas of Copahue volcano (Melnick145 2000; Melnick et al. 2006).

146147 2.3 Active Tectonics

148 Several authors described the tectonics of the149 Caviahue-Agrio caldera—Copahue volcano150 complex, further on named as CAC as in Mel-151 nick et al. (2006), while in Velez et al. (2011) it152 is reported as CCVC (Caviahue caldera—Copa-153 hue stratovolcano). A recent structural synthesis154 is provided by Folguera et al. (this issue) and155 references therein. Here, the focus is on the156 evidence of active tectonics in the CAC area as157 reported in the literature, and further original158 field data are provided to understand the tectonic

159�structures presently controlling the volcanic160�activity of Copahue and the geothermal resources161�of the area.

162�2.3.1 Literature Data

163�The CAC represents an arc volcanic complex in164�an oblique subduction setting, and it is located165�in an important morphotectonic zone, transi-166�tional from strike-slip to thrust-dominated167�(Melnick et al. 2006). It represents the central168�sector of the NE-trending, 90-km-long169�Callaqui-Copahue-Mandolegue (CCM) volcanic170�alignment (Fig. 2.1), the longest Plio–Quater-171�nary volcanic alignment of the SVZ (Melnick172�et al. 2006) and regarded as a transfer zone173�accommodating the step-wise configuration of174�the strike-slip LOFZ, controlled at the arc front,175�and the Antinir-Copahue regional thrust-fold176�system (ACFZ in Fig. 2.1, as defined in Fol-177�guera et al. 2004; Folguera and Ramos 2009;178�Velez et al. 2011, also named CAFS in Melnick179�et al. 2006), running through the inner retroarc180�(Radic et al. 2002; Melnick et al. 2004; Folguera181�et al. 2004; Melnick et al. 2006), with persistent

Table 2.1 Characteristics for the four Mw ≥ 8 earthquakes and finite fault models: date of the event, magnitude, searchradius, fault rupture length (Comte et al. 1986; Lomnitz 1970; Kelleher 1972; Lomnitz 1985; Okal 2005; Watt et al.2009; Lin et al. 2013; Servicio Sismológico de la Universidad de Chile, http://ssn.dgf.uchile.cl; USGS earthquakehazards program), fault-plane strike and dip angle, number of patches, averaged fault slip and rake angle, depth of topand bottom of the fault plane, trend and plunge of σ1, σ2, σ3 (based on Barrientos and Ward 1990; Lin and Stein 2004;USGS earthquake hazards program, http://earthquake.usgs.gov; Okal (2011), written communication; Kanamori (2011),written communication)

Name Valparaiso Valdivia Santiago Maule

Date 8/17/2006 5/22/1960 3/3/1985 2/27/2010

Mw 8.2 9.5 8.0 8.8

Rupture length (km) 330 940 170 460

Fault geometry (strike/dip, °) 8/17 7/20 11/26 8–2°/12–14

N. of patches 1 864 153 292

Average slip (m) 2.6 3.8 1.6 2.5

Average rake angle (°) 117.0 105.0 105.0 105.0

Fault top (km) 20.5 0.0 21.8 2.9

Fault bottom (km) 47.5 153.9 61.3 50.1

σ1 (azimuth/plunge, °) 71/31 100/35 86/20 92/28

σ2 (azimuth/plunge, °) 157/7 10/0 174/9 178/5

σ3 (azimuth/plunge, °) 56/58 80/55 61/68 80/61




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182 activity in the Holocene. Melnick and Folguera183 (2001) described the Quaternary184 volcano-tectonic phase of activity of the area as185 mixed transtensional and transpressive, in close186 spatial relation with the CCM volcanic align-187 ment. The following subsections describe in188 detail the state of the art on the active structures189 in the CAC, in particular the Caviahue-Agrio190 caldera, the structures located south of Copahue

191�volcano, the WNW-striking system, the192�Chancho-Co structures, and the Copahue Fault.

193�2.3.2 The Caviahue-Agrio Caldera

194�The Caviahue-Agrio caldera (Figs. 2.2 and 2.3),195�15-km-long and 10-km-wide, formed as a vol-196�canic feature with a strong structural control.197�Volcanic activity in the Caviahue-Agrio caldera

Fig. 2.2 a Tectonic map of theCallaqui-Copahue-Mandolegüe transfer zone (CCM).CAC Caviahue-Agrio Caldera; LOFZ Liquiñe-Ofqui faultzone; LF Lomìn fault; CA Callaqui volcano; CO Copahuevolcano. Thick black lines represent the strike of volcanofeeding systems, while black arrows represent the

regional σHmax. Shaded-relief digital elevation model isfrom SRTM90 data (datum WGS84) (http://srtm.csi.cgiar.org/). Boxes locate Figs. 2.2b, 2.3 and 2.4a.b Structural model of the Copahue volcanic complexcharacterized by a N60 °E trending post-glacial ventalignment. Redrawn from Melnick et al. (2006)

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198 area has been controlled by both the regional199 stress field imposed by plate convergence and by200 local structures formed at the intersection of201 regional fault systems (Melnick et al. 2006). The202 main structures of the Caviahue-Agrio caldera203 area are represented by (1) the normal faults of204 the caldera border, building up-to-400-m high205 scarps affecting Lower Pliocene units, (2) the206 right-lateral strike-slip faults affecting Copahue207 postglacial lava flows in the Lomín valley to the208 south of the volcano, (3) the NE-trending fissure209 system controlling the effusive products of Co-210 pahue volcano and the hydrothermal manifesta-211 tions inside the caldera, and (4) the NE-trending212 imbricated Chancho-Co fault system (Folguera213 and Ramos 2000, Mazzoni and Licitra 2000),214 (5) the WNW-trending Caviahue graben, hosting215 the Agrio lake, and (6) the NE-striking reverse216 fault affecting the Cola de Zorro formation to the217 south of the volcano (Melnick and Folguera218 2001). Deflation processes at the intersection

219�between the Caviahue graben and the220�Chancho-Co antiform have been recently recog-221�nized by radar interferometry (Ibáñez et al.222�2008). Moreover, the CAC area includes several223�active geothermal fields, showing manifestations224�of boiling pools and bubbling pools (Velez et al.225�2011), namely Las Máquinas, Maquinitas, Co-226�pahue, Anfiteatro and Chancho-Co (Fig. 2.3),227�which are aligned through the Chancho-Co228�frontal structures (Varekamp et al. 2001; Velez229�et al. 2011).230�In the northern and central part of the231�Caviahue-Agrio caldera, Folguera et al. (2004)232�and Rojas Vera et al. (2009) recognise reverse233�faults with subordinate righ-lateral components,234�affecting post-glacial products and displacing235�Quaternary morphologies, and producing small236�pull-apart basins. These structures are associated237�with the ENE-trending Mandolegue fault system,238�which represents a transfer zone between the239�LOFZ and the ACFZ, and whose western edge is

Fig. 2.3 Main structural features of theCaviahue-Copahue complex mapped on Aster DTM

and Image (redrawn from Melnick et al. 2006). Geother-mal fields are indicated after Velez et al. (2011)




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240 represented by the Chancho-Co uplift. Based on241 fault activity in unconsolidated deposits, Folguera242 et al. (2004) suggests ongoing deformation for the243 ACFZ, an E-vergent arrangement of high-angle244 dextral transpressive and transtensive faults.

245 2.3.3 The Structures South246 of Copahue

247 To the south of Copahue and SW of the248 Caviahue-Agrio caldera, the NNE-striking249 Lomín Fault corresponding to the northern edge250 of the LOFZ, is another Quaternary-active strike251 slip structure with minor normal component (LF252 in Figs. 2.2 and 2.3; Folguera et al. 2004), for253 which Melnick et al. (2000) and Melnick et al.254 (2006) recognise neotectonic activity evidenced255 by 300–1000 m long and up to 5–10 m high fault256 scarps cutting Holocenic lavas erupted from the257 volcano, close to the Pucón-Mahuida Pass258 (Fig. 2.3). An at least Pleistocenic activity is259 recognized for the NE-striking structures affect-260 ing the southern flank of Copahue (Rojas Vera261 et al. 2009).

262 2.3.4 WNW-Striking Structures263 and Grabens

264 The WNW-striking normal fault system affects265 the eastern and central, as well as the northern266 and southern sectors of the caldera, cutting the267 Las Mellizas sequence and andesites from the268 base of Copahue volcano (Fig. 2.3). These faults269 have a strong topographic expression, and from270 N to S they form the Trolope and Caviahue271 grabens, controlling the elongated shape of Lake272 Caviahue (Melnick et al. 2006). The general273 strike of the Trolope and Caviahue grabens is274 parallel to the long axis of the rectangular275 Caviahue-Agrio caldera, and their formation is276 related to late Pliocene-Pleistocene reactivation277 of the caldera-pull-apart structure in response to278 dextral shear along the LOFZ (Melnick et al.279 2006). The Quaternary extensional activity of280 these grabens represents the latest pulse of the281 caldera activity, and the Caviahue graben reveals282 a differential amount of extension from Copahue

283�volcano to the east (Folguera et al. 2004). The284�Trolope graben affects the Las Mellizas forma-285�tion, and its most recent control is over singlacial286�monogenetic cones (Rojas Vera et al. 2009).287�Copahue volcano is located at the intersection288�between two main structural systems, the289�WNW-trending Caviahue graben and the290�NE-trending Chancho-Co antiform (cfr. 3.1.4),291�the second main positive feature in the area292�(Fig. 2.3).

293�2.3.5 The Chancho-Co Structure

294�The Chancho-Co structure (Fig. 2.3) is located in295�the central to northern part of the CAC and is296�considered to have been active during the Pleis-297�tocene and Holocene, controlling the main active298�geothermal systems in the area that form an299�en-echélon arrangement (Melnick et al. 2006).300�The Chancho-Co is a N60 °E trending elongated301�ridge (Folguera et al. 2004), representing a302�transpressive imbricated structure whose devel-303�opment is linked to a series of SE-vergent304�NE-trending thrusts and associated305�hanging-wall anticlines (Folguera and Ramos306�2000), affecting Late Pliocene successions gath-307�ered in Las Mellizas Formation (Melnick et al.308�2006). Its uplift would have preceded the309�extension that led to the Caviahue graben for-310�mation (Rojas Vera et al. 2009), and its front is in311�correspondence of the inhabited Copahue village.312�There is poor evidence of the potential Quater-313�nary activity of these structures and of their314�relations with Copahue volcano, or of their deep315�control (Rojas Vera et al. 2009). The control on316�the Quaternary and present uplift of the317�Chancho-Co structure is exerted by a main318�NE-striking fault system and a subordinate319�WNW-striking system, coinciding with the320�geometry of the southernmost part of a gravi-321�metric low (Rojas Vera et al. 2009).322�Regarding neotectonic activity along this323�structure, several authors noticed that: (i) some324�reverse scarps, a few metres high, affect a glacial325�surface, together with open cracks (Folguera and326�Ramos 2000; Folguera et al. 2004); (ii) the327�southernmost of these reverse faults (defined as




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328 the “Copahue Fault” by Rojas Vera et al. 2009,329 cfr. 3.1.5 and Fig. 2.3) marks the main topo-330 graphic break, thrusting ignimbrites over Qua-331 ternary till-like deposits near the Copahue village332 (Melnick et al. 2006); (iii) Rojas Vera et al.333 (2009) describe south-facing reverse fault scarps334 that uplift the Chancho-Co antiform and affect335 less than 1 Ma-old volcanics and postglacial336 Copahue products, as well as Pleistocene glacial337 morphologies; (iv) extensional faults and open338 cracks (extended for tens of meters and reaching339 depths of 15 m) along the axis of the Chancho-Co340 anticlines, with scarps up to 3-m high, were341 described by Melnick et al. (2006). They define342 small crestal grabens that trend slightly oblique to343 the reverse faults and the axis of the anticlines.344 The Chancho-Co features do not fit with ground345 deformation contours determined from InSAR346 (Interferometric Synthetic Aperture Radar) stud-347 ies observed by Velez et al. (2011), who describe348 an oval-shaped deflation feature whose location is349 partially coincident with Copahue volcano and350 also extends north-eastward over the Chancho-Co351 elevation. Their conceptual model is compatible352 with the present geochemical, geological and353 geophysical evidence, and deflation is regarded as354 a possible trigger mechanism for phreatic erup-355 tions as those observed in the last eruptive cycles356 of Copahue in 1992, 1995 and 2000. The few357 fault-slip data measured along the reverse fault358 scarps indicate oblique dextral transpressional359 motion, consistent with the en-echélon arrange-360 ment formed by the Anfiteatro, Copahue, and Las361 Maquinitas depressions as well as the slightly362 oblique trend of the crestal grabens and open363 cracks with respect to the main reverse faults364 (Figs. 2.3 and 2.4). Melnick et al. (2006) explain365 the Chanco-Co kinematics due to a local rotation366 of the regional stress field, and they do not pro-367 vide any justification for the hydrothermal activ-368 ity and hot springs occurring along planes369 perpendicular to the greatest horizontal stress370 (σHmax). In fact, Melnick et al. (2006) state that371 magma ascent has occurred along planes per-372 pendicular to the least principal horizontal stress,373 whereas hydrothermal activity and hot springs

374�also occur along parallel planes. The geometries375�of the Chanco-Co indicate that the direction of376�shortening was oblique to the trend of the main377�contractional structures (Burbank and Anderson378�2001; Melnick et al. 2006). Finally, Melnick et al.379�(2006) suggest a tectonic control for the devel-380�opment of the caldera-related Chancho-Co block381�and of the N60 °E-trending compressive struc-382�tures located on its upper part, whereas they383�advance the hypothesis of gravitational instability384�being the cause for the normal faults affecting the385�resurgent block.

386�2.3.6 The Copahue Fault

387�Rojas Vera et al. (2009) describe the main388�topographic break affecting the Chancho-Co389�system of reverse faults as the “Copahue390�Fault”, a system of reverse (thrust) scarps to the391�E of Las Mellizas lakes, some of them reaching392�the Trolope graben to the E (Fig. 2.3), and cut-393�ting the Copahue lavas to the SW. They also394�highlight a parallelism between the uplift struc-395�tures of the Chancho-Co and the alignment of396�post-glacial monogenetic centres and the active397�crater of Copahue, giving rise, as a whole, to a398�fault-and-scarp system extending more than399�15 km. The study of uplifted Late Pliocene400�sequences over younger unconsolidated fluvial401�and colluvial deposits enabled Rojas Vera et al.402�(2009) to identify at least two periods of activity403�for the NE-striking Copahue Fault.

404�2.3.7 New Field Data

405�For a better understanding of the tectonic struc-406�tures that control the volcanic activity of Copa-407�hue a structural survey in December 2007 mainly408�devoted to the neotectonic features was carried409�out. The frequent eruptions of Copahue volcano,410�the sharp scarps along its flanks and in the sur-411�roundings, as well as the presence of abundant412�springs, spa and geothermal wells and power413�plants (Fig. 2.4a) suggest the presence of recent414�and active tectonics in the area. During the field415�survey, the strike of fractures, fault scarps, dikes,




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416 eruptive fissures were measured, at the same417 time, the abundant evidence of a structural con-418 trol linked to geothermal manifestations and419 alteration zones were considered. By plotting420 direction of all measured faults, fractures, erup-421 tive fissures and dikes (Fig. 2.5), two main tec-422 tonic trends were recognized, a dominant one423 oriented ENE-WSW, and a secondary424 ESE-WNW trend (see also Fig. 2.4a). These two425 trends correspond to the well known structures

426�affecting the CAC, as described in the previous427�paragraph (e.g. Folguera et al. 2004; Melnick428�et al. 2006; Rojas Vera et al. 2009).429�The ESE-WNW-oriented structures mainly430�crop out along the lower northeastern flank of431�Copahue volcano, where the springs are aligned432�along this trend (Fig. 2.4b) as well as the alter-433�ation zones due to hydrothermal circulation.434�Along the same structure a geothermal well is in435�operation (Las Mellizas lake, Fig. 2.4b), and a

Fig. 2.4 a Morphostructural map showing the mainlineaments of the area and the studied outcrops. The mapcontains the location of the photos illustrated in Fig. 2.7.Box locates Fig. 2.4b. b The Copahue Village Fault

System: it affects the NE flank of Copahue volcano fromLas Mellizas—Las Maquinas to Copahue Village. Boxeslocate Fig. 2.6a, b




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436 recent lava flow shows open fractures (Fig. 2.4b).437 This trend results parallel to a regional one that is438 affecting the northern border of Copahue and439 shows its morphological expression also in the440 Caviahue lake and at the Copahue pass441 (Fig. 2.4a). In addition, close to the Copahue442 village, in the large crateric depression called443 Anfiteatro, two N120 °E-aligned lava domes are444 emplaced.445 The ENE-WSW-oriented structures show a446 more complex trend but, at the same time, they447 testify to the presence of more recent activity,448 with open fractures, soil displacement, etc449 (Figs. 2.4a, b, and 2.6a, b). They were defined as450 the Copahue Village Fault System (CVFS—451 Fig. 2.4a). Such fault system partially comprises452 the previous defined Chancho-Co structure and453 Copahue Fault (Melnik et al. 2006; Rojas Vera454 et al. 2009), but in this case it is characterised by455 a different kinematics. The system is affecting456 Copahue volcano (Fig. 2.4a), where the summit457 vents and craters are aligned along a N60–70 °458 E-striking trend. More specifically, in the summit459 area a dike strikes N60 °E (Fig. 2.7f, g). The460 southern rim of the summit crater lake (Fig. 2.7b)461 is affected by N55 °E-striking fractures, showing462 a left-step en-echelon arrangement. Following463 the same trend, on the NE flank of Copahue464 volcano numerous eruptive fissures are present,465 with a N65 °E–N78 °E-striking orientation. The466 Copahue village is located inside a graben with a467 general N60°–70 °E-striking trend and extending468 towards Copahue volcano (see Fig. 2.4a, b). The469 main graben, 3 km long and 2 km wide, is

470�bordered by two main normal faults, along with471�some minor structures that form smaller asym-472�metric grabens. In detail, these smaller grabens473�are made of several discontinuous normal faults,474�whose trace is usually bent, locally forming a475�step morphology (Fig. 2.6a, b). All the faults476�show normal kinematics, striking from N50 °E–477�N70 °E, with a vertical displacement ranging478�from 50 cm to 30 m, and usually of some meters,479�often associated with open fractures unfilled by480�sediments or soil. Most faults and fractures are481�also recognisable on aerial photos (Fig. 2.6a, b).482�Frequently, the ground is also affected by open483�fractures, with opening up to 20 cm, or small484�faults with a maximum vertical displacement of485�50 cm. This is a clear evidence of ongoing tec-486�tonic deformation, as shown also by interfero-487�metric data reported in Velez et al. (2011, this488�book).489�In addition to the normal component, the main490�structure bordering the northern side of the gra-491�ben (see arrow in Fig. 2.6a) also shows strike-slip492�kinematics. In particular, close to the scarp rim,493�this structure presents 30 m of vertical displace-494�ment associated with a dextral strike-slip com-495�ponent and an estimated offset of a few496�centimetres, not feasible to be measured in the497�field (Fig. 2.7c, d). 1.5 km eastward along the498�structure discussed above, a sub-vertical fault499�plane was measured, striking N75 °E and dipping500�88°, affecting a lava flow. On this fault plane501�slickenlines are present, and a pitch angle of 45 °502�W (Figs. 2.4b and 2.7e) confirms its transtensive503�kinematics.

Fig. 2.5 Rose diagram showing the strike of the measured faults, fractures, eruptive fissures and dikes




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504 In addition, two dikes belonging to the same505 trend were recognized, which were intruded into506 a fault plane along the southern wall of the507 Caviahue caldera, close to the Pucón Mahuida508 pass. They strike N47 °E–N52 °E, with a509 left-stepping en-echélon arrangement. One of510 these dikes is aphyric and 2-m-thick, the second511 is porphyritic and 4-m-thick, with abundant512 phenocrysts of plagioclase, pyroxene and olivine513 (Fig. 2.7f, g).

514�The kinematics of CVFS is transtensive,515�characterized by metric extensional fault scarps516�that border an ENE-WSW striking graben517�(Fig. 2.4b), associated with a minor dextral518�strike-slip component indicated by slickenlines,519�en-echélon arrangements of dikes and fractures,520�as shown by field and morphological observa-521�tions from our work, and interferometric data522�(Velez et al. 2011). Therefore, the tectonic523�extension of the CVFS has modelled the large

Fig. 2.6 a and b detailed sketch of the structuressurveyed in two key areas. The black arrow in Fig. 2.6aindicates the trace of the main fault bordering the

northern side of the graben. For further explanation,please see Sect. 2.3.2. Location in Fig. 2.4b




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Fig. 2.7 Panoramic views and outcrops from Copahuevolcano and surrounding areas. Photo location inFig. 2.4a, b. a Copahue volcano western flank, with thevillage of Caviahue and the namesake lake in theforeground. b The summit crater of Copahue volcano,partially covered by a small ice cap. c Panoramic view ofthe Copahue Village Fault System striking ENE-WSW.The red dashed line outlines the fault plane, thatcorresponds to the fault indicated with black arrow in

Fig. 2.6a. d Close-up view of the previous fault, showingthe fault displacement: footwall to the left, hangingwall,partially covered by the snow, to the right. e Close-upview of Copahue Village fault plane: the pencil points outthe direction of the slickensides (for location, see the starin Fig. 2.4b). f NE-striking fault. The fault plane isintruded by dikes. g Close-up view of the dike strikingN50 °E. For further explanation, please see Sect. 2.3.2




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524 graben that also affects the Copahue village. This525 kinematics is in agreement with the estimated526 direction of the maximum horizontal stress in this527 area (N80 °E–N90 °E, Cembrano and Lara528 2009). In addition, the two tectonic trends529 (ENE-WSW and ESE-WNW, Figs. 2.4a and 2.5)530 may represent a conjugate fault system and are531 responsible for driving hot fluids to the surface532 (geothermal springs) in the area of Copahue533 village and Las Mellizas Lakes.

534535 2.4 Volcano Feeding System

536 During the Quaternary, the volcanic activity of537 Copahue has concentrated along the538 NE-trending, 90-km-long CCM (Fig. 2.1). Such539 lineament is the longest of the SVZ and it is540 interpreted as a crustal-scale transfer zone541 inherited from a Miocene rifted basin (Melnick542 et al. 2006). In particular, along this lineament543 are present the N60 °E-elongated Callaqui vol-544 cano to the SW, the Copahue stratovolcano, and545 the Caviahue-Agrio caldera to the NE (Fig. 2.2;546 Moreno and Lahsen 1986; Melnick et al. 2006).547 Local structures formed at the intersection of548 regional fault systems control the volcanic549 activity along the CAC alignment (Melnick et al.550 2006). The N60 °E-striking structural control of551 the CCM transfer zone is also suggested at local552 scale by Holocenic fault scarps affecting the553 volcano eastern slopes as well as by post-glacial554 vent alignments (Fig. 2.2; Melnick et al. 2006).555 The Holocene activity of Copahue is testified by556 aligned parasitic cones, fissure-related pyroclas-557 tic flows, along with activity from the summit558 craters, with basaltic-andesitic products (Melnick559 et al. 2006). In particular, the volcano is formed560 by the superimposition of several emission cen-561 tres along a main NE-striking fissure, and the562 volcano summit has nine craters aligned N60 °E,563 four of which post-glacial (Rojas Vera et al.564 2009) and the eastern-most being presently565 active (Velez et al. 2011). The NE-striking566 fracture system affects the most recent lava

567�flows in the NE flank of the volcano, together568�with the structures affecting the Chancho-Co569�(Rojas Vera et al. 2009). It has the same trend of570�the transtensive CVFS described in this chapter571�(cfr. 3.2). Based on the hypothesis that magmas572�rise along steeply-inclined intrusive sheets,573�propagating normal to the least principal hori-574�zontal stress (σHmin), and form dyke swarms575�(Dieterich 1988; Walter and Schmincke 2002)576�and aligned parasitic cones (Nakamura 1977;577�Tibaldi 1995; Corazzato and Tibaldi 2006; Bo-578�nali et al. 2011). Bonali (2013) proposed that579�magma at Copahue rises along vertical or sub-580�vertical planes striking about N60 °E, perpen-581�dicular to σHmin (in agreement with Melnick et al.582�2006). This is supported by the evidence that the583�Quaternary least principal stress axis (σ3) trends584�NW and it is mostly sub-horizontal, as suggested585�by the inversion of fault-slip data between 37 °S–586�46 °S in the SVZ (Lavenu and Cembrano 1999;587�Arancibia et al. 1999; Cembrano et al. 2000;588�Potent and Reuther 2001; Lara et al. 2006;589�Cembrano and Lara 2009). Furthermore Mamani590�et al. (2000), based on magnetotelluric sound-591�ings, suggest the possible presence of a magma592�chamber at a depth between 9 and 20 km.

593594�2.5 Elastic Interactions Between595�Recent Faulting and Copahue

596�This section explores how active tectonics is597�capable of influencing the normal volcanic598�activity at Copahue volcano. Active tectonics599�along the SVZ is represented by faulting that600�produces earthquakes both along the subduction601�zone and along the intra-arc active faults. Such602�phenomena are capable of influencing the normal603�volcanic activity by inducing stress changes604�(static, quasi static and dynamic), as suggested in605�the literature (Hill et al. 2002; Marzocchi et al.606�2002; Manga and Brodsky 2006). Several607�authors pointed out that the delay between the608�earthquake and the following volcanic events can609�be from seconds to years, because of the




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610 complexity of volcanic systems (Linde and Sacks611 1998; Nostro et al. 1998; McLeod and Tait 1999;612 Walter and Amelung 2007; Eggert and Walter613 2009), despite the present lack of a thorough614 constraint. Regarding the SVZ, Watt et al. (2009)615 noticed that the overall eruption rate in the616 Southern Volcanic Zone (SVZ) increased after617 the two Chilean earthquakes of August 1906 (Mw

618 8.2) and May 1960 (Mw 9.5), and this increase619 occurred one and 3 years after the earthquakes.620 The static stress change is the variation in the621 stress field from just before an earthquake to622 shortly after the seismic waves have decayed623 (Hill et al. 2002). Such change is invoked as an624 explanation for eruption-leading processes625 occurring in regions close to the fault rupture626 (e.g., Walter and Amelung 2007; Bonali 2013;627 Bonali et al. 2013). Quasi-static stress change628 occurs over a period of years to decades, and it is629 associated with slow viscous relaxation of the630 lower crust and upper mantle beneath the epi-631 centre of a large earthquake (Freed and Lin 2002;632 Marzocchi 2002a, b; Marzocchi et al. 2002).633 Dynamic stress, associated with the passage of634 seismic waves, is often regarded as a possible635 eruption trigger at greater distances (Linde and636 Sacks 1998; Manga and Brodsky 2006; Delle637 Donne et al. 2010).638 A new method, regarding the static stress639 changes imparted by the earthquakes on the640 reconstructed volcano magma pathway (e.g.641 Bonali et al. 2013; Bonali 2013), was followed.

642643 2.6 Conceptual Model644 and Modelling Strategy

645 In order to study a possible relationship between646 earthquakes and volcanic activity in the Copahue647 area, the normal stress change, imparted on the648 volcano magma pathway (e.g. Nostro et al. 1998;649 Walter 2007; Bonali et al. 2012) by Mw ≥ 8650 earthquakes occurred in front of the SVZ, was651 calculated (Bonali et al. 2013). The numerical652 models using the Coulomb 3.3 software (Lin and653 Stein 2004; Toda et al. 2005) was performed.

654�Calculations are made in an elastic half-space655�with uniform isotropic elastic properties follow-656�ing Okada’s (1992) formulae that allow to cal-657�culate the static normal stress change resolved on658�a volcano magma pathway, independently from659�the rake angle of the receiver structure and from660�the friction coefficient used in the model. The661�reconstructed magma pathway of Copahue vol-662�cano was assumed as a N60 °E-striking vertical663�plane, as explained in Sect. 2.4, and different664�attitudes were also tested by changing the dip665�angles (50°, 70° and 90°). Its reconstructed666�geometry also mimics the Quaternary geometry667�of active faults reported in this work, striking668�ENE-WSW. The upper crust was modelled as an669�elastic isotropic half-space characterized by a670�Young’s modulus E = 80 GPa and a Poisson’s671�ratio ν = 0.25 based on Mithen (1982), King et al.672�(1994), Lin and Stein (2004) and Toda et al.673�(2005); a lower value of the Young’s modulus674�would only have the effect of reducing the675�magnitude of static stress changes. Regarding the676�finite fault model used to simulate earthquake677�effects, for the 1960 earthquake the fault solu-678�tions proposed by Barrientos and Ward (1990)679�based on tsunami wave form inversion was used,680�whereas for the 2010 earthquake the fault solu-681�tions proposed by Lin et al. (2013), based on682�InSAR, GPS and teleseismic data, was used. The683�input stress field for both earthquakes is reported684�in Tab 1, based on T, N and P axis of related685�focal mechanisms. Furthermore, it has been686�evaluated the possible contribution of687�earthquake-induced static stress changes due to688�crustal earthquakes occurred between the 1907689�and the 2010 Chile earthquakes available in a690�digital database provided by USGS (http://691�earthquake.usgs.gov). Fault geometries and692�kinematics are based on focal mechanism solu-693�tions (http://earthquake.usgs.gov; http://www.694�gmtproject.it/) when available, and geometries695�and kinematics of active faults reported in the696�literature (Melnick et al. 2006; Cembrano and697�Lara 2009). Fault dimensions were based on698�empirical relations (Wells and Coppersmith699�1994; Toda et al. 2011). The earthquake-induced700�normal stress changes on Copahue feeding




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701 system, possibly imparted by earthquakes with702 Mw ≥ 4 and a depth between 0 and 70 km (http://703 earthquake.usgs.gov), was also resovled.

704705 2.7 Interaction Between706 the Subduction Zone707 and the Volcano

708 It is commonly suggested that large subduction709 earthquakes are capable of inducing static stress710 perturbations on the fault hanging wall block,711 also promoting volcanic eruptions (Barrientos712 1994; Lin and Stein 2004; Walter and Amelung713 2007; Bonali 2013; Bonali et al. 2013). Static714 stress change is the difference in the stress field715 from just before an earthquake to shortly after the716 seismic waves have decayed (Hill et al. 2002),717 and it is proposed as a mechanism capable of718 leading eruptions in regions close to the fault719 rupture (e.g., Walter and Amelung 2007; Bonali720 et al. 2013). Altought a first theory suggested that721 magma could be squeezed upward by increased722 compressional stress in the crust surrounding a723 magma chamber close to its critical state (e.g.724 Bautista et al. 1996), more recent findings agreed725 that earthquake-induced static (permanent) nor-726 mal stress reduction on magma pathway727 (unclamping) could promote dyke intrusion and728 following eruptions (e.g. Hill et al. 2002; Walter729 2007). Furthermore, Walter and Amelung (2007)730 suggested that earthquake-induced volumetric731 expansion can increase the magma-gas pressure732 and encourage eruptions in a time frame of years.733 Finally, Bonali et al. (2013) advanced the734 hypothesis that unrest at volcanoes with deep735 magma chambers is encouraged by magma736 pathway unclamping.737 Results of numerical modelling suggest that738 both the 1960 and 2010 subduction earthquakes739 were capable of inducing unclamping at vertical740 or subvertical magma pathway of Copahue vol-741 cano (Fig. 2.8). Regarding the 1960742 earthquake-induced static stress changes743 (Fig. 2.8a, b), unclamping is higher for a vertical744 or SE-dipping magma pathway and it increases745 with depth, while it decreases with depth for a

746�NW-dipping magma pathway, and it results in747�clamping at a depth between 5–9 km for the 50°-748�NW-dipping pathway (Fig. 2.8b). Regarding the749�2010 earthquake-induced static stress changes,750�the magma pathway (Fig. 2.8c) is always affected751�by a normal stress reduction (unclamping) within752�a depth range of 1–9 km below the volcano base753�(Fig. 2.8d). Magnitude of unclamping slightly754�decreases with depth, and ranges from −0.258 to755�−0.147 MPa (Bonali 2013), and decreases with756�decreasing dip angle of the magma pathway757�(Fig. 2.8d). Although positive feedbacks due to758�dynamic and post-seismic stress changes could759�not be excluded, large subduction earthquakes760�seems to be capable of favouring volcanic761�activity at Copahue due to earthquake-induced762�static (permanent) normal stress reduction on its763�magma pathway (e.g. Hill et al. 2002; Walter764�2007) at a distance of 257–353 km and up to765�3 years following subduction earthquakes (Bo-766�nali 2013; Bonali et al. 2013). Such hypothesis is767�supported by some suggestion proposed in768�Marzocchi (2002a, b) where coseismic stress769�changes are candidate to promote eruptions in the770�0–5 years after earthquakes as far as 100–300 km771�from the epicentre (i.e. 38 h after the 1960 Val-772�divia 9.5 Mw earthquake the Cordon Caulle773�erupted; Moreno and Petit-Breuilh 1999, Collini774�et al. 2013). Regarding the time delay, Marzocchi775�(2002a, b) has proposed that: (i) the inertia of the776�volcanic system in reacting to the static stress777�changes, (ii) a non perfect elasticity of the crust778�and/or (iii) stress corrosion effect (e.g., Main and779�Meredith 1991), are processes candidate to780�explain a time delay of 0–5 years. Other mech-781�anisms that could have promoted unrest at Co-782�pahue, following such earthquakes, regard783�postseismic-induced stress/strain (Marzocchi784�2002a, b; Marzocchi et al. 2002) and dynamic785�stress changes (Manga and Brodsky 2006). As786�proposed by Marzocchi et al. (2002) and Freed787�and Lin (2002), quasi-static stress change, which788�is a post-seismically-induced effect, could pro-789�mote eruptions over a period of years to decades.790�Marzocchi (2002a, b) suggested a time window791�of about 30–35 years after earthquakes, com-792�patibly to the relaxation time of a viscous793�asthenosphere (Piersanti et al. 1995, 1997; Pollitz




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794 et al. 1998; Kenner and Segall 2000). Regarding795 the dynamic stress changes, such effects may796 excite and promote ascent of gas bubbles, and797 consequently magma ascent (Manga and Brod-798 sky 2006), the details of the feedback mecha-799 nisms remaining unclear and are nowadays

800�discussed in the literature. It is proposed that801�dynamic effects is capable of promoting bubble802�growth, including adjective overpressure (Linde803�et al. 1994), rectified diffusion (Brodsky et al.804�1998; Ichihara and Brodsky 2006) and shear805�strain (Sumita and Manga 2008). Copahue

Fig. 2.8 a Contour map of the 1960 Mw 9.5earthquake-induced normal stress change resolved onthe N60 °E-striking Copahue magma pathway at a depthof 4.5 km below the volcano base. Red colours representa normal static stress reduction on the receiver plane, bluecolours represent an increase. The black triangle locatesCopahue volcano. The finite fault model used is fromBarrientos and Ward (1990), and b shows the normalstress change for vertical as well as inclined receiver

surfaces (attitude expressed as dip direction/dip angle), ina depth range of 1–9 km below the volcano. c Contourmap of the Mw 8.8 2010 earthquake-induced normalstress change resolved on the N60 °E-striking Copahuemagma pathway at a depth of 4.5 km below the volcanobase. The finite fault model used here is from Lin et al.(2013), and d shows the normal stress change for verticalas well as inclined receiver surfaces in a depth range of1–9 km below the volcano




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806 experienced 9 eruptions since 1900 (Global807 Volcanism Program Digital Information Series),808 five of which in the last 21 years (Fig. 2.9). In809 detail, four eruptions occurred in the time-frame810 1992–2000, the recentmost eruption (2012)811 occurred instead after the 2010 Chile earthquake,812 under unclamping conditions of the magma813 pathway. The Copahue magma pathway was814 affected by normal stress reduction also after the815 1960 Valdivia earthquake, and the volcano816 erupted certainly in 1961.

817818 2.8 Effect of the Surrounding819 Structures on the Volcano

820 Our analysis of the possible contribution of821 earthquake-induced static stress changes due to822 crustal earthquakes with Mw 4–6.6 (http://823 earthquake.usgs.gov) reveals that only one out

824�of 2727 earthquakes (Fig. 2.10a) was capable of825�inducing static normal stress changes on the826�Copahue magma pathway (Fig. 2.10b). Such827�earthquake occurred on 31/12/2006 with Mw of828�5.6 at a distance of about 11 km from the vol-829�cano. The hypocenter was 20.6 km deep (http://830�earthquake.usgs.gov) and the kinematics was831�dextral strike-slip (Fig. 2.10a). Results of the832�numerical modelling carried out considering833�different dip angles of the N60 °E-striking834�magma pathway suggest that such surface suf-835�fered a normal stress reduction when vertical or836�subvertical, SE-dipping. A subvertical837�NW-dipping pathway is instead sometimes838�affected by normal stress increase with depth.839�When considering a less inclined NW-dipping840�pathway, no relevant normal stress changes are841�observed (Fig. 2.10b). Whatever the case, the842�stress change imparted by the 2010 earthquake is843�two orders of magnitude greater than the contri-844�bution of the 2006 seismic event.

Fig. 2.9 The graph reports the eruptions occurred atCopahue since 1900 AD (Global Volcanism ProgramDigital Information Series) and significant earthquakes

(Mw > 8) along the SVZ. Marks represent the first day ofeach reported year. VEI-Volcanic Explosivity Index




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845846 2.9 Discussion

847 The role of structures in a volcanic area is848 unquestionable because they control the magma849 storage and drive the magma rising, as well as850 the location of vents and the evolution of a vol-851 cano (e.g. Bellotti et al. 2006; Norini et al. 2013).852 In this way, by examining the structural data853 published up to now on Copahue volcano (see854 Sect. 2.3.1), it is evident that a comprehensive855 model is lacking and a large discussion is856 ongoing. In this section, all these available data857 were summarised, showing that there are some858 divergences among them and the related models.859 In addition, the new field data presented here (see860 Sect. 2.3.2), which cover only some portions of861 the volcanic area and consider mainly the recent862 and present volcano-tectonic activity, were863 considered.864 The long geological history of this sector of865 the Andes results in a really complex pattern of866 structures, with the superimposition of different867 tectonic phases. In detail, the complex structural

868�setting displayed by Melnick et al. (2006)869�(Fig. 2.3) shows the presence of several genera-870�tions of faults because of the variable position in871�time and somewhere in space of σhmax in the872�entire area. South of Copahue volcano there is a873�set of structures parallel to each others, striking874�NNE-SSW and with conflicting kinematics, as875�strike-slip faults parallel to normal faults and876�folds (anticlines and synclines). In addition,877�perpendicular to this direction a thrust system is878�emplaced. These features cannot be easily rec-879�onciled and could be explained to be the result of880�two or more generations of structures and/or of881�the heritage of older phases. Also in the Anfite-882�atro area, close to the Copahue village, there is883�another evident incompatibility between a set of884�reverse faults associated with fold anticlines and885�the normal faults parallel to them, and the geo-886�thermal field alignment (Melnick et al. 2006,887�Fig. 2.3).888�In view of the main structures affecting Co-889�pahue volcano there is a general agreement890�among the authors that the ENE-WSW-striking891�lineament is the most important, because of

Fig. 2.10 a Stars represent the crustal earthquakes withMw ≥ 4 (http://earthquake.usgs.gov) occurred in thetime-frame between the 1907 and the 2010 Chileearthquake. The focal mechanism of 31/12/2006 (Mw

5.6) is shown (http://earthquake.usgs.gov; http://www.gmtproject.it/), the red line representing fault strike andkinematics (Melnick et al. 2006; Cembrano and Lara

2009). b 2006 Mw 5.6 earthquake-induced normal stresschange resolved on the N60 °E-striking reconstructedCopahue magma pathway. Vertical as well as inclinedreceiver surfaces are taken into account (attitudeexpressed as dip direction/dip angle), in a depth rangeof 1–9 km below the volcano




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892 active vents and summit craters alignment. This893 feature represents the local expression of the894 regional CMC structure (Figs. 2.1, 2.2, 2.3 and895 2.4), a transfer fault system that links the LOFZ896 to the ACFZ (Folguera and Ramos 2009; Velez897 et al. 2011) along with three volcanoes are898 located (Callaqui, Copahue and Mandolegue).899 This fault system extends in the area close to the900 Copahue Village and Anfiteatro where vertical901 displacements, mainly reversal, associated with a902 right-lateral component have been identified.903 Some of these structures are named Chancho-Co904 and Copahue Fault (Fig. 2.3) (e.g. Folguera et al.905 2004; Melnick et al. 2006; Rojas Vera et al. 2009906 with some differences among their models). In907 our neotectonic survey (Figs. 2.4, 2.6 and 2.7)908 any evidence of recent reverse faults was iden-909 tified; instead, based on our field data, they were910 considered as normal faults with a dextral911 strike-slip component. The subvertical geometry912 of the faults (and their graben-like arrangement),913 the displacements, the presence of open fractures914 (up to 20 cm wide) and the few slickenlines915 suggest a normal-oblique kinematics for these916 structures. This normal fault system was named917 as Copahue Village Fault System (CVFS) was918 finally proposed. The CVFS kinematics is in919 agreement with the deflation observed by Velez920 et al. (2011), confirming that these are the re-921 centmost structures in the area, and thus they are922 more recent than the Chanco-Co structure. As923 suggested by Melnick et al. (2006), no evidence924 of folds on the topographic surface was identi-925 fied; the lava flows display a tabular geometry,926 and local folding can be explained with the927 rotation due to the normal movements (drag)928 along the fault plane. The coeval presence of fold929 anticlines and parallel reverse faults can be930 related to the same stress field, but in this case the931 σhmax has to be rotated (about 90°) with respect932 to the regional one (N80 °E–N90 °E; Cembrano933 and Lara 2009). On the contrary, considering the934 new structural data, the resulting σhmax is in935 agreement with the regional one and the main936 geothermal sources are located in extensional and937 transtensional settings associated to a general938 deflation observed by InSAR data (Velez et al.939 2011). Another evidence of extensional regime

940�for the CVFS is suggested by the geometry of the941�plumbing system, striking almost parallel to this942�fault system. A previous shortening phase943�marked by folds and thrusts cannot be excluded,944�as recognised in the field and described in many945�outcrops by, among the others, Folguera et al.946�(2004), Melnick et al. (2006) and Rojas Vera947�et al. (2009). Conversely, the normal pattern948�recognised in our recent survey close to the949�Anfiteatro needs to be confirmed by more950�extensive surveys, also to recognise possibile951�diverse recent tectonic phases. Taking into952�account all the available data and models pro-953�posed for the area close to the Copahue village,954�an in-depth study appears necessary to better955�understand the structural setting and its evolu-956�tion, allowing to recognise two or more possible957�tectonic phases. In addition, further studies958�should address the determination of the dis-959�placement rate of the active structures, because of960�their importance for volcanic and seismic hazard961�and for geothermal exploitation.962�Results of numerical models show that active963�tectonics is capable of perturbing the normal964�activity at Copahue due to static stress transfer.965�In the studied cases both 1960 and 2010 sub-966�duction earthquakes induced a normal stress967�reduction on volcano magma pathway, favouring968�dyke intrusion (e.g., Hill et al. 2002; Walter969�2007; Walter and Amelung 2007). Minor events,970�occurred along intra-arc faults, were also capable971�of producing the same effect, but with minor972�intensity.

973974�2.10 Final Remarks

975�This review presents the state-of-art about tec-976�tonic data in the CAC and surroundings area as977�well as includes some new neotectonic data978�around Copahue volcano. The main points are979�summarised as follow:

980�1. Different structural models have been pro-981�posed right now, but a comprehensive model982�is lacking and further in-depth studies are983�necessary.




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984 2. A main ENE-WSW striking structure affects985 Copahue volcano and represents the local986 expression of the CCM transfer zone. This987 structure is inferred to control the magma988 pathway and the volcano evolution. There is989 no agreement on the normal or reverse kine-990 matics related to the present activity of the991 CVFS. The possible reverse movement of this992 fault system needs a rotation of the regional993 σhmax, while the normal one is in agreement994 with it.995 3. A better knowledge of the CVFS activity, the996 potential presence of two or more phases and997 the understanding of its Holocenic strain rate998 are fundamental goals for further and in-depth999 structural studies. In fact, a more detailed and

1000 complete tectonic setting of the CAC should1001 represent a fundamental result also for the1002 general geological-structural model of this1003 area, providing structural constraints for the1004 magma storage, pathway and rising, and1005 representing an essential issue also for vol-1006 canic hazard assessment, as well as for1007 enancing the exploitation of the geothermal1008 system.1009 4. Active tectonics is capable of perturbing the1010 normal activity at Copahue volcano due to1011 earthquake-induced static stress changes. As1012 result of numerical modelling, both 1960 and1013 2010 large subduction earthquakes induced1014 unclamping on Copahue feeding system,1015 favouring following eruptions. Very low1016 unclamping was also induced by one crustal1017 earthquake occurred after the pre-2012 erup-1018 tion and before the 2010 earthquake.

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