Structural control on volcanoes and magma paths from local- to orogen-scale: The central Andes case

    Structural control on volcanoes and magma paths from local- to orogen-scale: The central Andes case A. Tibaldi, F.L. Bonali, C. Corazzato PII: DOI: Reference:

S0040-1951(17)30004-5 doi:10.1016/j.tecto.2017.01.005 TECTO 127377

To appear in:

Tectonophysics

Received date: Revised date: Accepted date:

10 August 2016 2 December 2016 7 January 2017

Please cite this article as: Tibaldi, A., Bonali, F.L., Corazzato, C., Structural control on volcanoes and magma paths from local- to orogen-scale: The central Andes case, Tectonophysics (2017), doi:10.1016/j.tecto.2017.01.005

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ACCEPTED MANUSCRIPT Structural control on volcanoes and magma paths from local- to orogen-scale: The central Andes case

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Tibaldi A.*, Bonali F.L.*, Corazzato C.^ * Department of Earth and Environmental Sciences, University of Milan Bicocca, Italy ^ Department of Science and High Technology, University of the Insubria, Como, Italy Abstract

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Assessing the parameters that control the location and geometry of magma paths is of paramount importance for the comprehension of volcanic plumbing systems and geo-hazards. We analyse the distribution of 1518 monogenic and polygenic volcanoes of Miocene-Quaternary age of the Central

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Volcanic Zone of the Andes (Chile-Bolivia-Argentina), and reconstruct the magma paths at 315 edifices by analysing the morphostructural characteristics of craters and cones. Then we compare

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these data with outcropping dykes, tectonic structures and state of stress. Most magma paths trend N-S, NW-SE, and NE-SW, in decreasing order of frequency. The N-S and NW-SE paths coexist in the northern and southern part of the study area, whereas N-S paths dominate east of the Salar de

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Atacama. Outcropping dykes show the same trends. The regional Holocene stress state is given by

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an E-W greatest horizontal principal stress. N-S and NNE-SSW reverse faults and folds affect deposits of 4.8, 3.2 and 1.3 Ma BP, especially in the central and southern study areas. A few NW-

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SE left-lateral strike-slip faults are present in the interior of the volcanic arc, part of which belong to the Calama-Olacapato-El Toro fault. The volcanic chain is also affected by several N-S- and NWSE-striking normal faults that offset Pliocene and Quaternary deposits. The results indicate different scenarios of magma-tectonic interaction, given by N-S normal and reverse faults and N-S fold

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hinges that guide volcano emplacement and magma paths. Magma paths are also guided by strikeslip and normal NW-SE faults, especially in the northern part of the study area. Zones with verticalized strata, with bedding striking NE-SW, also acted as preferential magma paths. These data suggest that at convergence zones with continental crust, shallow magma paths can be more sensitive to the presence and geometry of upper crustal weakness zones than to the regional state of stress.

Key words: Tectonic structures, volcano morphometry, Andes, magma pathway, state of stress.

1. Introduction This paper focuses on understanding which are the conditions that allow magma uprising in the uppermost crust in a compressional chain like the Andes. Magma storage and ascent are tightly linked to the structure and state of stress of the crust (Chaussard and Amelung, 2014; Tibaldi, 1

ACCEPTED MANUSCRIPT 2015), and a greatest principal stress (1) in horizontal position does not favour magma upwelling. In zones of plate convergence, the Andean-type tectonics has been considered a typical setting where subduction and related processes generate thickening of the lithospheric wedge above the

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Wadati-Benioff zone in a compressive regime (Kley et al., 1999; Cobbold and Rossello, 2003).

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Crustal thickening, in turn, produces broad tectonic uplift (Isacks, 1988; Kley and Monaldi, 1998; Beck and Zandt, 2002; McQuarrie, 2002) that can be decoupled from the main shortening events.

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For example, stable isotope data from the Altiplano of the Central Andes indicate that the average elevation was < 2 km before the Miocene, and then increased by about 2.5 km in the interval 10-6 Ma (Ghosh et al., 2006; Garzione et al., 2006, 2008). This suggests that in this area, the major uplift

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followed the main phases of shortening (Eichelberger et al., 2015). Volcanism was widespread in the central Andes during these Miocene uplift events, and also during Plio-Quaternary times, with

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voluminous rhyolitic ignimbrite eruptions, as the Altiplano-Puna Volcanic Complex (de Silva, 1989a; Kay and Coira, 2009; Ramos, 2009), and with the emplacement of several hundreds of volcanic cones (Trumbull et al., 2006).

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The reconstruction of the structure and geometry of the plumbing system of volcanoes is of

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paramount importance for understanding how magma reaches the surface (e.g. Bonali, 2013; Bonali et al., 2013,2015), and how the subvolcanic engine works (e.g. Tibaldi et al., 2011; Bistacchi et al.,

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2012; Tibaldi et al., 2013). Considering that tens of volcanoes of the central Andes are in a dormant stage, the comprehension of the structure of magma plumbing systems is also useful for the assessment of volcanic hazard. For example, the processes occurring at active volcanoes and leading to paroxysmal eruptive events can be addressed taking into consideration the structure of

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the shallower conduits (e.g. Chouet et al., 1997, 2008). Also the evaluation of the areas most prone to the opening of new vents and eruptive fissures is intimately linked to the comprehension of the structure of plumbing systems (e.g. Bonali et al., 2011; Tadini et al., 2014). Despite the importance of understanding the geometry and structure of shallow magma paths, this work had never been done in the central Andes. This is mostly due to the lacking of shallow geophysical data, to the logistical difficulty of the region with most volcanoes located in remote and hardly accessible areas, and to the scarcity of outcropping dykes that might give direct clues on the plumbing system at most eroded volcanoes. In this situation, the orientation of the shallow magma paths can be reconstructed by using a series of morphometric indicators measured on a single volcanic edifice (Tibaldi, 1995; Corazzato and Tibaldi, 2008; Kervyn et al., 2012; Germa et al., 2013; Hernando et al., 2014) or on aligned coeval volcanic centres (Adıyaman et al., 1998; Paulsen and Wilson, 2010). The first approach is based on the evidence that if the plumbing system of a volcano is characterized by parallel dykes, 2

ACCEPTED MANUSCRIPT this will result in a series of measurable morphostructural features linked to the dynamic of development of the crater zone and to the edifice growth (Nakamura, 1977; Tibaldi, 1995; Pasquarè and Tibaldi, 2003). The second approach is based on the consideration that a dyke intersecting the

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topographic surface will produce a series of aligned centres. These approaches can be particularly

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useful where other direct investigations cannot be carried out.

These situations can be complicated by parameters that are linked to the local setting, such as

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the dip of the substratum (Tibaldi, 1995), the presence of volcanoes grown on the flank of already existing edifices (Corazzato and Tibaldi, 2008), the presence of sector collapse scars, and a rapid reorganization of the stress state within the volcano (Tibaldi, 2015 and references therein). Also the

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regional tectonic setting is one of the most important factors in dictating the configuration of the magma plumbing system (e.g. Bonali et al., 2016). The tectonic influence on magma migration and

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volcanism has received a lot of attention recently, but several issues are still open and controversial. As an example, for decades volcanism and regional extensional tectonics have been thought to be tightly linked, as a horizontal least principal stress (σ3) and a vertical σ1 favour magma upwelling

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along vertical fractures perpendicular to σ3 (Anderson, 1951; Cas and Wright, 1987). For arc

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volcanism occurring at convergent margins, Nakamura et al. (1977) and Nakamura and Uyeda (1980) suggested that the overall tectonics within the arc should be strike-slip (with both horizontal

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σ3 and σ1) with magma ascending along vertical dykes parallel to the direction of σ1. By contrast, a pure contractional tectonic environment, with reverse or transpressional faulting, has been usually regarded as a highly unfavourable setting for volcanism (Cas and Wright, 1987; Glazner, 1991; Hamilton, 1995; Watanabe et al., 1999). Also the Plio-Quaternary tectonic evolution of the central

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Andes at the junction between Chile, Bolivia and Argentina, has been matter of debate, since normal, reverse and strike-slip faults have been found (Lahsen, 1989; Cladouhos et al., 1994; Riller and Oncken, 2003; González et al, 2009: Tibaldi et al., 2009; Montero Lopez et al., 2010; Zhou et al., 2013). In order to assess the influence of the regional and local structural and geomorphological settings on the development of volcanism in the central Andes, in this paper we present data collected over an area of 100,000 km2 between 20.5°S and 24.5°S, and 66.5°W and 68.7°W (Fig. 1). Data come from two different independent approaches: on one side we carefully evaluated the distribution in time and space of 1518 late Miocene-Quaternary volcanic centres, studied in detail their most reliable morphometric characteristics that might suggest the orientation of the shallow plumbing system, and mapped the outcropping dykes; on the other side, we mapped all the main faults that bear evidence of Miocene, Pliocene or Quaternary motions based on new field surveys, detailed satellite image interpretations and published maps, and completed this with the available 3

ACCEPTED MANUSCRIPT data on their kinematics and stress state. The comparison of these data sets allows us to improve the knowledge on the volcano-tectonic history of the region, and to assess the possible links among tectonic structures, magma paths and development of volcanism. Our case exemplifies the point that

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in a convergence zone with continental crust, magma paths can be more sensitive to the presence

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and geometry of upper crustal weakness zones than to the regional state of stress.

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2. Geological-structural background

The central Andes, between 20.5°S and 24.5°S, are composed of four structurally and geologically different regions (Fig. 1B): i) to the east, the Eastern Cordillera that corresponds to the

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front of imbricated thrust slices, mostly located in Bolivia (Main Andean Thrust, MAT); ii) towards the west, the Altiplano - Puna Plateau (PP), which extends from south-western Bolivia to north-

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western Argentina; iii) in the middle, the chain of volcanoes along the Chile-Bolivia and ChileArgentina borders, whose tectonics is poorly understood, and that mostly coincides with the Central Volcanic Zone (CVZ); iv) to the west, the Western Cordillera of northern Chile (WCC),

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characterized by N-S to NNW-SSE folds and reverse faults. To the north and to the south the study

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area is bordered, respectively, by the large Uyuni and Arizaro salares. A Palaeozoic to Miocene sequence of metamorphic and sedimentary rocks constitutes the

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eastern region of imbricated thrust slices (Marsh et al., 1992). The slices are arranged as a sequence of foreland-dipping duplexes following an about E-W direction of shortening (Herail et al., 1996; Kley, 1999). Folds are characterized by dominant N-S-trending hinge lines. The volcanic belt is characterized by eruptive centres mainly aligned along a NW-SE corridor in the northern sector,

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and N-S in the southern sector. The western coastal region is made of Paleozoic meta-sedimentary rocks and Mesozoic plutons and volcanic rocks (Scheuber et al., 1994; Riquelme et al., 2003). Here, the most important structure is the strike-slip Atacama fault system (Scheuber et al., 1995; Taylor et al., 1998; Riquelme et al., 2003). In the studied area, the oldest rocks are represented by intrusive plutons of Paleozoic age. The oldest sedimentary deposits are represented by marine rocks of Jurassic-Late Cretaceous age. Early Miocene sedimentary rocks crop out in a scattered way, being mostly covered by a sequence of Early Miocene lava flows and Late Miocene lava flows and ignimbrites. In particular, several largely-dissected stratovolcanoes of Late Miocene age have been recognized in the northern part of the study area (Tibaldi et al., 2008). Pliocene-Quaternary volcanic deposits, resulting from a dominantly effusive activity, belong to more recent stratovolcanoes. Their products are interlayered with Pliocene and Pleistocene ignimbrites that have been erupted in main phases of caldera

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ACCEPTED MANUSCRIPT collapse, the largest structures being the Pastos Grandes (PG), La Pacana (LP), Panizos (CP), Vilama (VC) and Cerro Guacha (CG) calderas (Fig. 1B, SGM, 1997a). Contractional deformation dominated for most of the Cenozoic tectonic evolution of the

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central Andes (De Celles and Horton, 2003; Deeken et al., 2006; Strecker et al., 2007). The

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convergence between the Nazca and the South-American plates caused shortening perpendicular to the orogen and crustal thickening, which led to a large-scale uplift and to the formation of the

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Andean Altiplano - Puna Plateau (PP) (Isacks, 1988; Allmendinger et al., 1997; Elger et al., 2005). Compressional deformation began in Late Cretaceous in the Domeyko system and Coastal Cordillera (Mpodozis et al., 1995; Bascuñan et al., 2015), and reverse faulting and folding fully

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developed in the Eocene-Oligocene (Jordan et al., 1997; Kraemer et al., 1999; Coutand et al., 2001; Carrapa et al., 2005; Mpodozis et al., 2005), with maximum shortening during the Neogene

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Quechua phase of the Andean orogeny (Jordan and Alonso, 1987; Allmendinger et al., 1997). The orogenesis was interrupted during late Oligocene-early Miocene times when a period of extensional relaxation took place (Pananont et al., 2004; Winocur et al., 2015). Based on some authors (Gubbels

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et al., 1993; Cladouhos et al., 1994), the shortening phase ceased at 9-10 Ma in the southern

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Altiplano and northern PP, whereas for Gonzalez et al. (2009) it has been active up to the Quaternary in the volcanic chain immediately west of the PP. Marrett et al. (1994) showed that the

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compressional phase ended at 2-4 Ma in the southern PP, and Baby et al. (1995), Gubbels et al. (1993), Moretti et al. (1996) and Echavarria et al. (2003) demonstrated that in late MiocenePliocene times, contractional deformation shifted eastwards in the thin-skinned Subandean foldand-thrust belt.

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Extension as well was documented in the central Andes, mainly in the PP and close surroundings, with two main orientations: one parallel to the orogen and one perpendicular (review in Daxberger and Riller, 2015). The N-S, orogen-parallel extension was first advanced by Allmendinger (1986), Allmendinger et al. (1989) and Marrett et al. (1994). Its onset was dated back to the Late Miocene-Early Pliocene along the southern margin of the PP by Montero-Lopez et al. (2010). Zhou et al. (2013) found < 1 Ma normal faults in the Pasto Ventura region of the southern PP, providing a NE-SW to NNE-SSW extension direction that is oblique to the local orogen trend. This obliquity has been explained by Alvarez et al. (2015) by the inception of the Copiapo ridge over the northern part of the Chilean shallow subduction zone. An extension oblique to the orogen and younger than 3.5 to 7 Ma was found by Schoenbohm and Strecker (2009) at the southern margin of the PP. A late Pliocene-Quaternary NE-SW extension was also found by Tibaldi et al. (2009) at some sites in the area here studied. Various directions of Neogene-Quaternary extension were found by Daxberger and Riller (2015) in the PP, comprising an E–W to NW–SE trend of 5

ACCEPTED MANUSCRIPT extension that they interpreted as due to the formation of footwall synclines and hanging-wall anticlines, and an orogen-parallel trend. Finally, Cladouhos et al. (1994) cited the presence of < 9 Ma minor normal faults in the northern part of the PP.

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The volcano distribution was studied by Trumbull et al. (2006), who found out that the onset

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of deformation preceded that of magmatism, and that the distribution of volcanic centres and domains of shortening deformation vary independently from each other. Although they suggested

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that, on a local scale, the distribution of volcanoes is influenced by shallow-crustal structures, such a study was beyond their scope.

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3. Methods 3.1. Age of volcanic edifices

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The studied 1518 volcanic centres have been divided into four groups that correspond to edifices of different ages. First, we collected all the published data on the absolute age of those volcanoes that had been radiometrically-dated (Ramírez and Huete, 1980; Gardeweg and Ramírez,

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1987; De Silva, 1989b; Marsh et al., 1992; SGM, 1997a, b; Global Volcanism Program, 2013).

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These volcanoes have then been grouped by their age: i) Miocene; ii) Pliocene; iii) Pleistocene; iv) Holocene. We must be aware that only relatively few volcanic deposits have been radiometrically-

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dated respect to the very large number of edifices, and thus the attribution of each volcano to a given age group represented a main issue. For that, we integrated different approaches: first of all, Miocene, Pliocene and Quaternary volcanoes were distinguished based on the stratigraphic relations between their deposits and a series of wide ignimbrite plateaux, which correspond to five major

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dacitic ignimbrite eruptions of the San Bartolo Group: the Artola Member (9.4 Ma), the Sifon Member (8.3-8.2 Ma), the Pelon Member (7.0-5.5 Ma), the Puripicar Member-Atana Member (4.2 Ma), plus the ignimbrite deposits belonging to the Chaxas Formation (4.3-1.1 Ma), the Talabre Formation (2.2 Ma) and the Purico Formation (1.3 Ma) (review in De Silva, 1989b). Stratigraphic relations were also used in the case of lavas poured out by different volcanoes. Where the stratigraphic relations were not evident, we coupled a morphological approach consisting in assigning a possible relative age by comparing lava texture, degree of erosion of volcano slopes, and amount of hydrothermalization at a given volcano respect to a close and similar radiometrically- or stratigraphically-dated cone. These observations result from the integration of published geological maps with new checks, made locally in the field wherever possible, and more systematically on Google Earth. Respect to the previous work of Trumbull et al. (2006), our extensive use of Google Earth, in connection with all the available bibliography, allowed to identify, analyse and classify not only a larger number of vents, but also in a more systematic and 6

ACCEPTED MANUSCRIPT detailed way. Trumbull et al. (2006) in fact, recognized about 1450 volcanic centres in the latitude range 14-28°S, whereas we identified and studied 1518 centres just in the latitude range of 20.524.5°S. Centres of Pleistocene age are those cones that were radiometrically-dated or that show

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clear morphological evidence of much less erosion respect to edifices of the other groups, and that

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are not known to have had eruptions in the Holocene (last 10 ka) or historical times, also based on the Smithsonian Catalogue (Global Volcanism Program, 2013). Holocene centres are those listed in

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the Smithsonian Catalogue or that show no evidence of glacial erosion although reaching very high elevations. As an elevation threshold, we used the Local Late Glacial Maximum (LLGM) detailed snowlines of Klein et al. (1999). These authors show that the LLGM snowlines located between

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4000 and 4400 m above the sea level (a.s.l.) in the northern part of our study area, and between 4600-4800 m a.s.l. in the southern study area. For example, Figure 2A shows a deeply eroded

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Pliocene volcanic dome that can be compared with the intact dome of Figure 2B. The latter reaches an altitude of 5272 m (northern part of study area) and since it does not show any evidence of erosion and glacial abrasion, a Holocene age was assigned. Figure 2C shows an eroded and altered

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stratovolcano of Miocene age, which can be compared with the much less eroded stratovolcano of

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Pliocene age (Fig. 2D).

Some uncertainty remains on the age attribution due to the huge amount of remote and

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inaccessible volcanoes in the study area. Whereas there are undisputable differences between volcanoes of Miocene age respect to those of Quaternary age, especially Holocene ones, and thus we are quite confident on their relative age attribution, we are aware that some volcanoes dated only by indirect methods may have an uncertain age. Anyway, at the present state of knowledge of the

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area, our results on age differentiation represent the best possible effort.

3.2. Volcano types

Volcanic centres have been first subdivided into polygenic and monogenic types. For Polygenic type, the criteria base on the presence of multiple deposits highlighting a long-lasting eruptive activity, the cone dimension, and the presence of a summit crater zone (Fig. 3). These edifices include both stratovolcanoes made of lava flows and pyroclastic successions, and huge lava cones. The threshold cone dimension is 1 km in diameter size, based on the distribution of diameters of the Smithsonian Institution Global Volcanism Program data base, but special attention was given to those edifices that have basal diameter in the range 1-3 km in order to distinguish huge monogenic cones from small polygenic volcanoes. Each of these was field checked, wherever possible, or carefully evaluated by published data and Google Earth analyses. All the monogenic edifices corresponding to domes, pyroclastic cones and lava flow vents, resting on the flank of a polygenic 7

ACCEPTED MANUSCRIPT volcano, have been grouped as Vent on polygenic flank. This has been done to fulfil one of the criteria (Nakamura, 1977) to assess magma paths on polygenic volcanoes as will be described in the following section. The category Domes on the substrate has been attributed to those edifices that

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had not grown on the flank of polygenic volcanoes, and based on the typical shape of volcanic

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domes given by steep flanks and a horizontal to slightly convex summit part, made of more viscous lavas that produced short and very thick flows. The category Monogenic on the substrate

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corresponds to the most mafic monogenic edifices, which had not grown on the flank of a polygenic volcano, and including pyroclastic cones, maars and vents of low-viscosity lava flows. The recognition of lava flow vents is important because in the study area there are some lava fields that,

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as a whole, constitute a prominent cone-like morphology; anyway, the lava flows have been poured out from several, sometimes scattered, vents, and do not coincide with a real polygenic edifice,

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because no, long lasting, summit crater zone can be distinguished.

3.3. Morphometric parameters of volcanoes

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The morphometric characteristics of monogenic and polygenic volcanoes provide a great deal

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of geometric information that can help in distinguishing buried magma-feeding fractures. The method has been successfully applied to pyroclastic cones (e.g.: Settle 1979; Pasquarè et al., 1988;

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Tibaldi et al., 1989; Tibaldi, 1995; Corazzato and Tibaldi, 2006; Paulsen and Wilson, 2010), to volcanic domes (e.g.: Pasquarè and Tibaldi, 2003), and to polygenic edifices (e.g.: Nakamura, 1977; Nakamura et al., 1977). The use of morphometric data as an indirect method for locating shallow magma-feeding fractures can be particularly useful in areas where an extensive cover of volcanic

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and epiclastic deposits prevents from a direct identification of volcanic structures, as in the central Andes case. Among the nine parameters advanced by Tibaldi (1995) we have selected and measured those most useful in inferring the geometry of the magma-feeding fractures in the study area. The morphometric values were measured after a careful inspection of the substrate inclination around each cone, since the cone morphology can be influenced by the dip of an inclined substrate topography (> 9° for pyroclastic cones, Tibaldi, 1995). The growth of a new cone on the flank or nearby a pre-existing cone has also been taken into account, since also this configuration can influence the final morphology of the younger volcano. Volcanoes that show evidence of sector or flank collapse scars were discarded for morphometric measurements, because lateral failure can deeply condition the successive cone growth (Tibaldi et al., 2008, 2010a; Tibaldi, 2015). We selected 315 volcanoes, of which the following morphometric parameters were measured, taking into consideration that the measured features parallel the shallow magma-feeding fracture

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ACCEPTED MANUSCRIPT (for a detailed explanation see Tibaldi, 1995, Pasquarè and Tibaldi, 2003 and Corazzato and Tibaldi, 2006): 1) Azimuth orientation of crater elongation, only for well-preserved landforms (Fig. 4A);

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2) Azimuth orientation of cone-base elongation, only if the base limit of the cone is not

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obscured by more recent deposits (Fig. 4B);

the summit zone of a stratovolcano (Fig. 4C);

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3) Azimuth of the alignment of the centre points of craters of the same or similar age lying on

4) Azimuth of the alignment of the centre points of craters of the same age lying on the substrate (Fig. 4D);

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5) Azimuth of the apical graben (or crease) structures on domes (Fig. 4B); 6) Azimuth of the line connecting the two depressed points along a well-preserved pyroclastic

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crater rim (Fig. 4A).

3.4. Age and kinematics of tectonic structures and stresses In order to date the structures that affect the rock successions and to recognize different

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tectonic phases, we used the stratigraphic relationships and the ages of the deposits. The relative age

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of the different tectonic phases has been assigned by analysing the crosscutting relationships of the structures both with the lithostratigraphic units, among the fault sets, and among faults and folds.

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Wherever possible, we measured fault strike, dip direction and angle. The sense of displacement has been determined in the field wherever possible, by combining both recrystallized fibres, Riedel microfractures on slip planes (sensu Petit, 1987) and offset of stratigraphic markers, layering or other structures. In a few cases, especially in very remote areas, it

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was possible to recognize the offset on oblique views on Google Earth. Finally, the database has been integrated with information from published papers and geological maps. In volcanic lithotypes, as those in the study area, it is necessary to pay particular attention to collect reliable fault slip indicators: the criteria proposed by Tibaldi (1996) have been applied to distinguish slickensides produced by actual tectonic slip from textures induced by lava layers moving at different speeds within a flow. To investigate the state of stress, we used the few available data on striated fault planes collected in the study area by Tibaldi et al. (2009), who determined the stress tensors through classical inversion methods, using the Sperner et al. (1993) approach. For the Holocene stress state, no striated fault planes of that age have been found within the study area. We thus used the data of the World Stress Map and, in order to obtain a picture of the regional stress state, we considered also the surrounding areas.

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ACCEPTED MANUSCRIPT 4. Results 4.1. Distribution of volcanic centres in space and time In the study area there are 647 polygenic edifices (comprising stratovolcanoes and lava cones),

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84 domes and 346 more basic monogenic centres (Fig. 3). At each eroded volcano we assigned one

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vent located at the uppermost point of the edifice, and wherever possible we counted also all the other recognizable vents (441) located on the volcano flanks. The maximum width of the volcanic

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arc, measured perpendicularly to the arc as the spacing between the two most distant vents, is 100 km in the northernmost part of the study area, 110-130 north of the Salar de Atacama basin, 75-97 km along this basin, and 180-210 km in the southernmost part of the study area. If we consider the

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largest concentration of vents, thus excluding the external scattered vents, the arc width is 90-100 km in the north, decreasing gradually to 70 km north of the Atacama basin, then 35-45 km along it,

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and finally around 65 km in the southernmost part of the study area. It is evident from the maps of volcanic centres subdivided by age (Fig. 5) that the oldest volcanoes (Miocene age) are equally distributed along the entire width of the volcanic arc. Pliocene

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volcanoes also occupy the entire width of the Miocene arc in the northern part of the study area, but

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then their distribution decreases south of 23°S to a width of 20-30 km. Pleistocene volcanoes are much less scattered in an E-W sense, giving rise to a more linear distribution in a N-S sense. This

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N-S orientation is not continuous along the entire volcanic arc but, more in detail, it is possible to recognize a series of parallel segments arranged with a left-stepping geometry. Four main N-S segments can be recognized from the northern part of the study area down to the latitude of the southernmost part of the Atacama basin. Further south, Pleistocene volcanoes become more

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scattered, with an arc width of 80-85 km. It is worth mentioning that with this age there is also the Tuzgle Quaternary volcano (Fig. 1B) and the monogenic cones nearby, located east of the main arc and of the studied area as well. Finally, Holocene domes and pyroclastic cones, as well as the stratovolcanoes with a Holocene activity, are located in the same areas of Pleistocene volcanoes. In particular, the youngest Holocene and Pleistocene volcanoes are located in the western part of the previous Miocene and Pliocene arc.

4.2. Space distribution of morphometric parameters and magma paths Figure 6A shows all trends obtained from the measured morphometric parameters in the studied area. The rose diagram shows two maxima in correspondence of N-S (peak at N00-10°) and NW-SE (peak at N130-140°), followed by the NE-SW (peak at N30-40°) direction, and very few scattered data in other directions. The same trends have been measured using different morphometric indicators, as can be appreciated for example in Figure 4B where a dome has both 10

ACCEPTED MANUSCRIPT the maximum base axis and the crease structure trending NW-SE. The fact that the azimuth distribution is very similar for all morphometric parameters indicates that the shape of the analysed volcanoes is controlled by some consistent mechanism, and confirms that the used morphometric

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parameters can give reliable results in the interpretation of the shallow magma-feeding paths.

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In Figure 7A we plotted at each volcano a segment that represents the most probable orientation of the shallow dykes that fed magma to the crater, obtained by morphometric

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measurements. In the northernmost part of the study area (north of 21°30’S), the dominant trends of magma paths are the N-S and NW-SE. More to the south (between 21°30’S and 22°20’S), cones show a dominant NW-SE trend, which is reflected both in the morphometry of the single cones and

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in the alignment of cones of the same, or similar, age. It is important to note that in the study area there are the Pastos Grande and La Pacana calderas that also show a clear NW-SE elongation (Fig.

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1B). For example, in Figures 8A-B it is possible to observe a series of volcanoes located on a horizontal substrate that have all the parameters suggesting a possible NW-SE feeding system: they are aligned, each composed of an elongated single cone, and show elliptical craters with the same

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orientation of the maximum axes. This type of interpretation is supported by a series of sites where

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it is possible to see at the same time the NW-SE elongation of the cone base and of the crater and of the fracture in the substrate: in Figure 9A there is a pyroclastic cone, resting directly on the

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ignimbrite plateau, which has a NW-SE elongated base. The substrate is affected by a swarm of NW-SE fractures located just in correspondence of this cone and passing exactly below the crater, possibly representing the features linked to the uprising dyke. In Figure 9B there is an elliptical dome (known as Pabellon dome) that has the maximum axis of the base striking NW-SE, exactly

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with the same orientation of the crease structure lying on its upper surface that represents the weakness zone crossing the dome produced by the uprising dyke (see Pasquarè and Tibaldi, 2003, for a more-in-depth explanation). These two morphostructural features are perfectly parallel to the underlying fracture zone that links the normal fault that crops out NW and SE of the dome. At the latitude of the northern limit of the Salar de Atacama basin (south of 22°20’S), the distribution of magma paths changes: the three directions N-S, NW-SE and NE-SW are present, in decreasing order of frequency (Fig. 7A). It is important to stress that the pattern here is complex and errors might occur, unless taking into consideration the morphometric parameters also of single cones. As an example, Figure 10 portrays a series of polygenic volcanoes aligned NE-SW. After carefully checking their stratigraphic relationships, it has been possible to reconstruct their relative age of emplacement, resulting in a gradual shift of the vents from NE towards SW. Examining each cone in detail, the morphometric indicators suggest that the cones had built through NW-SE-striking magma-feeding paths. This can be fully appreciated in the box of Figure 10A, where the recentmost 11

ACCEPTED MANUSCRIPT cone shows a summit zone with a series of craters of comparable age that are aligned NW-SE, as well as each crater is elongated in the same direction, and the line connecting the depressed points of the crater rim also trends NW-SE. Such orientation indicates a NE-SW trend of the local least

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principal stress (3). Although all the other four cones of the area of Figure 10 are more eroded, and

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thus do not show a well preserved summit crater zone, it is worth noting that all of them had suffered lateral failure towards the southwest. Also such constant orientation of the direction of

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collapse is consistent with NW-SE-striking dykes feeding magma to the summit zone. The N-S trend of magma paths is dominant at the latitude of the central Atacama basin, although some cones still present morphometric parameters that suggest NW-SE magma feeding

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paths. As in the northern part of the study area, also in this sector of the volcanic chain there are complex interactions between alignments of volcanoes and real orientation of the dominant magma

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paths. As already introduced in the previous section, there are several alignments of polygenic stratovolcanoes or smaller lava volcanoes, and/or domes that have N-S orientation. By a more detailed examination, geological reconnaissance at crucial sites allowed to decipher the real

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dominant morphometric features, which locally are evidently N-S, but in some case are different.

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As an example of a real N-S trend, in Figure 11 it is possible to observe the summit part of one of these N- S aligned volcanic edifices. In this case, a series of clues suggest that the single edifice has

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been subject to magma uprising along a N-S-striking weakness zone, based on the presence of: i) about N-S-striking fractures, ii) coeval N-S-aligned craters, iii) elongated craters with N-S maximum axis, iv) eruptive vents, dome spikes and tumuli aligned along a N-S-trending zone, v) a summit pyroclastic cone with a N-S-trending maximum base axis, and vi) N-S orientation of the

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line connecting the crater rim depressed points. In Figure 12 instead, there is an example of eruptive centres that are located close to a zone of N-S aligned volcanoes but show a different magma path; a series of consistent morphometric parameters indicate here a NNE-SSW possible shallow magmafeeding dyke, based on: i) a NNE-elongated older dome, above which ii) there is a younger NNEelongated dome, iii) a series of eruptive points along a NNE-trending strip, and iv) the alignment of the domes with another small eruptive centre located to the NNE. This example is located at the latitude of the southern part of the Atacama basin, where there are three main orientations of morphometric indicators down to the southern limit of the study area.

4.3. Time distribution of magma paths Figure 13 shows the distribution of magma paths with time. The rose diagrams show the presence of constant dominant directions in the N-S and NW-SE ranges throughout all the investigated intervals. WNW-ESE and ENE-WSW trends are present only in the Miocene, and a 12

ACCEPTED MANUSCRIPT NNW-SSE trend appears only in the Holocene. The lack of magma paths in the E-W range is noteworthy. More in detail, NW-SE magma paths dominate in all periods in the northern and southern parts of the study area, whereas N-S paths are mostly present in the central part, east of the

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Salar de Atacama. Finally, a few NE-SW paths are present in the Miocene, Pliocene and Pleistocene

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times, and are concentrated in the northern part of the study area.

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4.4. Dykes

A total of 24 dykes were observed at eroded volcanoes (Fig. 6C), whereas other 644 dykes were measured in the substrate located close to the volcanic chain (Figs. 6D-E). All the substrate

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outcrops in the study area were meticulously surveyed in the field wherever possible, or by satellite images. Dykes were recognised and mapped in the north-western (Fig. 7B), north-eastern (Fig. 7C),

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south-western (Fig. 7D) and southern (Fig. 7E) part of the study area (Fig. 7). The host rocks are represented by Palaeozoic to Tertiary deposits, and no absolute ages are available for these dykes. We are aware that part of the dykes surveyed in the Palaeozoic rocks might correspond to very old

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intrusions, but their crosscutting relationships, the presence of faults that offset some dyke sets

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while hosting younger dykes, are representative of a long history of magmatic intrusions. Although the youngest part of this history may be of Tertiary times, due to age uncertainties it will be used

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just as a comparison with the magma paths obtained by our morphometric study. The orientations of dykes, instead, surveyed into Oligocene-Miocene deposits can be directly merged with our magma path data-base, having a Neogene age based on the crosscutting relationships with the host rocks. The orientations of all measured outcropping dykes are represented in the rose diagram of

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Figure 6B, which shows three dominant strikes: NW-SE, N-S and NE-SW in decreasing order of frequency. These orientations are present in all the surveyed rocks, although in older rocks of Palaeozoic and Cretaceous-Lower Tertiary age, the E-W-striking dykes are abundant or even dominant, respectively (Figs. 6D-E). The dykes intruded into the younger Oligocene-Miocene deposits show the two dominant N-S and NE-SW orientations (Fig. 6C), followed by the NW-SE subordinate trend. Considering the change in dyke orientation with space, in the north-western part of the study area (Fig. 7B) the dominant orientation of dykes is NE-SW, followed by the ESE-WNW and ENEWSW orientations. The ENE-WSW dykes are in most cases inclined, suggesting they had suffered tilting and are older. The NE-SW-striking dykes, instead, are mostly vertical and some of them also intruded sequences of deposits belonging to volcanoes of probable Miocene age. It is necessary to note that dykes locally present a radial arrangement in plan view, which can be attributed to a local stress field induced by the presence of central volcanoes (Tibaldi, 2015, and references therein). In 13

ACCEPTED MANUSCRIPT the north-eastern part of the study area, the most frequent dykes strike NE-SW, followed by N-S (Fig. 7C). In the south-western part of the study area, immediately south of the Atacama Basin, the dominant orientation of dykes is from WNW-ESE- to NW-SE, followed by the N-S and WNW-

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ESE orientations, in decreasing order of frequency (Fig. 7D). The NW-striking dykes show a

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dominant dip towards NE, suggesting they had suffered tilting. Also several NE-striking dykes are not vertical. The dykes striking around N-S are instead vertical, indicating a younger age. A deeply

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eroded volcano, of probable Miocene age, resting in the middle of the area where dykes crop out, is intruded by a NNW-SSE-striking dyke that passes to a N-S orientation, confirming that dykes with these orientations should be the youngest in this area. In the southern part of the study area, N-S is

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the dominant orientation of the dykes, followed by other more scattered trends (Fig. 7E). Here, some dykes striking NNW-SSE to N-S and other striking NNE-SSW are tilted and thus are the

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oldest. Other dykes, especially striking N-S and NNW-SSE, are vertical and younger. The dykes affecting two eroded volcanoes of Miocene age strike from SW-NE to WSW-ENE. In some places, it was possible to reconstruct the offset relationships between dykes of

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different generations. Figures 14A-B, located in the southernmost part of the study area (see Fig. 7),

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show a series of older NNE- to NE-striking dykes offset by NW-SE-striking left-lateral strike-slip faults. Successively, NW-striking dykes emplaced along these faults. Figure 14C shows a more

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recent N-S-striking dyke belonging to a swarm present in the north-eastern part of the study area. The example in Figures 14D-E, located in the southern part of the study area, shows a series of older NNE-striking dykes offset by NW-SE-striking left-lateral strike-slip faults, along which younger dykes have emplaced. The NW-SE dykes, in turn, are cut by a 10-m-large N-S-striking

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dyke that here results the youngest intrusion.

4.5. Tectonic structures

Starting from the north, along the southern border of the Salar de Uyuni, there is a series of main folds with N-S to NNE-SSW hinge lines (Tibaldi et al., 2009) (Fig. 15). The deposits affected by the westernmost fold are dated 25 to 17 Ma (Elger et al., 2005). South of the Salar de Uyuni, extensive lava flow fields, volcanic edifices and ignimbrite plateaux are present, and there are locally outcrops of altered volcanic rocks of Upper Miocene age. Both the folded strata and the Miocene volcanic deposits are affected by reverse faults and strike-slip faults (Tibaldi et al., 2009). The reverse faults dip to the SE or to the NW and show pure dip-slip motion. The stress tensor, computed with the striated reverse faults, is given by a NW-SE-trending horizontal 1 and a NESW-trending horizontal intermediate principal stress (2). The strike-slip faults strike WNW-ESE and ENE-WSW, with left-lateral and right-lateral strike-slip motions respectively. The stress tensor 14

ACCEPTED MANUSCRIPT is given by a horizontal 1 ranging from WNW-ESE to NW-SE, and a horizontal 3. All the described sets of structures are offset by a series of high-angle normal faults and joints with N-S to NW-SE strike. The computed stress tensor for the normal faults gives an E-W-trending horizontal

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3 and a vertical 1.

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Lavas and ignimbrite deposits dated 7.5-5.7 Ma BP, 5.9-5.4 Ma BP, and 5.4 ± 0.3 Ma BP (Baker and Francis, 1978; SGM, 1997b; Ramírez and Huete, 1980) are affected by NW- to NNW-

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striking and NE-striking normal faults and extensional joints. The NW-striking faults mostly dip at high angle to the southwest. The resulting stress tensor computation indicates a dominant NE-SWtrending horizontal 3 and a NW-SE-trending horizontal 2, although there are sites with a NE-SW

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2 and a NW-SE 3 (Tibaldi et al., 2009; Fig. 15).

The longest faults of the northern area (those with a length > 3 km) strike NW-SE and N-S in a

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decreasing order of frequency (Fig. 15). A series of normal faults with these orientations offset the deposits of volcanoes with an estimated Pliocene-Pleistocene age. The downthrown block is always to the west (e.g. Figs. 8C-D). In the north-eastern part of the study area, a NNE-SSW-striking fault

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offsets a volcano of late Miocene age with a downthrown western block. To the west, an ignimbrite

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plateau of Pliocene age is offset by a series of N-S-striking normal faults with downthrow to the west. In the centre of the volcanic arc, a series of lava flows of estimated Pliocene age are offset by

faults (Figs. 16A-B).

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both NW-striking normal faults, downthrown to the SW, and NW-striking left-lateral strike-slip

More to the south (i.e. northwest of the Pastos Grandes Caldera), a swarm of NW-striking

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normal faults affects volcanoes of both Pliocene and Pleistocene age, and ignimbrite deposits of Pliocene age. All the faults dip to the SW. This is particularly evident in the area immediately southeast of the Ollagüe volcano, where a main NW-striking normal fault offsets the ignimbrite plateau that disappears towards the SW along a 25- to 40-m-high scarp (Figs. 15 and 16C-D). A series of en-échelon normal faults, striking between NW-SE and NNW-SSE, are located on the prosecution of this scarp and affect lava flows of Pleistocene age. All of them, apart one, show a downthrow to the southwest. On the ideal prosecution of this fault swarm there is the Pastos Grandes Caldera (Baker, 1981; de Silva, 1989a; de Silva and Francis, 1991). This volcano-tectonic structure has a marked asymmetry, with the northern side defined by a 9-km-large swarm of normal faults dipping to the SW. All these faults are rectilinear in plan view, apart from the southwesternmost one that bends assuming a NNW-SSE strike at its SE termination (Fig. 15). The centre of the caldera hosts a resurgent dome, which is characteristically elongated NW-SE, and affected by a series of normal faults with the same orientation and with converging dips. South-west of the

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ACCEPTED MANUSCRIPT resurgent dome, a series of parallel NNW-SSE and WNW-ESE faults are present. They bear evidence of normal motions and dip towards the NE or SW. South-west of the Pastos Grandes Caldera, a major system of NW-striking normal faults is

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present from the Azufre volcano to north of the Inacaliri volcano (Figs. 15 and 17). Two major

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faults (10 to 18 km long) show converging dips and 150-m-high scarps forming a symmetric graben, whose floor is affected by other minor faults (2 to 4 km long) with an oblique arrangement

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respect to the main faults. All these faults offset a series of NW-SE-aligned stratovolcanoes of late Miocene-Pliocene age, whereas the main fault bounding the graben to the NE is sealed by a lava dome (Pabellon) dated at 80-130 ka BP (Renzulli et al., 2006). As the Pabellon dome stands exactly

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on the main fault trace, its emplacement was guided by the fault. A minor fault, also striking NWSE, offsets both the graben floor and the dome with a normal motion.

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East of the Pastos Grandes Caldera, the 5.3 Ma old ignimbrite plateau is offset by a series of normal faults with different orientation, nominally NE-SW, NNW-SSE, N-S and WNW-ESE, in order of decreasing total length (Fig. 15).

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The central area, between the Pastos Grandes caldera and La Pacana Caldera, is characterized

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by the dominance of NW-SE-striking faults (Fig. 15). In correspondence of the Tocorpuri volcano, a series of NW- to NNW-striking faults with fresh scarps are present, and they offset relatively

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young (Pliocene-Quaternary) deposits. All of them apart one dip to the SW, scarps are in the order of metres to tens of metres high, and motions are of dominant normal type. In the western part of the central area, the ignimbrite plateau is affected by three main faults striking (from east to west) NE-SW, N-S and NW-SE. The latter is 15 km long and shows normal

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motions with downthrow of the south-western block. An ignimbrite plateau south-west of Tocorpuri is offset by NE- and NW-striking faults (Fig. 15). The core of the volcanic arc, west of the Guacha Caldera, is affected by a swarm of NW-SE to NNW-SSE faults showing normal motions with dominant downthrow to the SW. The Guacha Caldera shows inner structures suggesting the presence of a central resurgent dome. As at the Pastos Grandes Caldera, also in this case the faults associated to the resurgent dome strike NW-SE and secondarily E-W. South of the Licancabur and Juriques volcanoes there is a swarm of NNE-SSW- to N-Sstriking faults (Figs. 15 and 18). The southern slope of the Juriques volcano, up to the crater rim, is affected by a series of NNE-SSW-striking normal faults with converging dips that offset the recentmost deposits of Holocene age (Fig. 18B). The north-eastern slope of the volcano and the crater rim are instead offset by a NW-SE-striking fault. These data indicate that the Juriques volcano had emplaced at the intersection between these structures. The NNE-SSW faults extend 16

ACCEPTED MANUSCRIPT southward into the ignimbrite plateau composed of the 1.35 Ma old Purico Formation (De Silva, 1989b). These faults intersect another fault swarm trending N-S and pointing to the Licancabur volcano. The summit of this volcano is affected by two parallel N-S faults. The western part of the

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Purico ignimbrite plateau, at a distance of 11 km from the eastern border of the Salar de Atacama, is

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offset by a N-S-striking rectilinear fault zone with a very fresh morphology. The fault scarps are vertical and face to the west. The ancient river gullies are vertically offset, indicating dip-slip

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motions. The fault zone, with a total length of 8.6 km, is composed of six fault segments with some overlap zone in a right-stepping arrangement.

A series of NW-striking faults are located between this N-S fault swarm and the La Pacana

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Caldera (Fig. 15). The kinematics of these faults is not clear, and some of them have an en-échelon arrangement within a N-S corridor located on top of the most prominent morphological scarp of all

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the area. This scarp goes from the Chile-Bolivia border southwards, across the Lascar Volcano and running east of the Salar de Arizaro (Fig. 15). It is an asymmetrical ridge with the scarp facing to the East, and was named “Miscanti Ridge” by Gonzalez et al. (2009). These authors interpreted the

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ridge as a frontal antiform that deforms also ignimbrite and lava deposits of Pliocene-Pleistocene

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age. The scarp runs for more than 200 km and is locally obscured by volcanoes that had grown directly on top of it. The scarp height ranges from a few tens of meters in the north, to 850 m near

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the south-western part of the Salar de Arizaro. Here a complex flexural fold crops out in the substrate with a vergence towards the east. A similar structure has also been found by Gonzalez et al. (2009) at several sites more to the north. Compressional structures given by reverse faults that affect the ignimbrite deposits of the 1.35-Ma-old Purico Formation were found near Talabre village,

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whereas other reverse faults offset the 3.2-3.1 Ma old Tucúcaro-Patao ignimbrite, south of the Atacama basin (Fig. 15).

The Atacama basin is a complex depression with structures of different kinematics and origin. To the east it is limited by a gentle west-dipping ignimbrite plateau. The northern part of the plateau is affected by the already mentioned N-S normal fault swarm, and the southern part by the reverse faults, mostly with an east vergence. The western side of the basin is affected by N-S to NNE-SSW reverse faults and folds of the Cordillera de la Sal. Part of these structures is tectonics in origin and part is linked to salt tectonics and gravitational instability due to salt withdrawal (Pananont et al., 2004; Jordan et al., 2010). The La Pacana caldera is characterized by well-defined south-western and south-eastern boundaries, and by the presence of a 33-km-long resurgent dome in the middle. Similarly to the other described calderas, also this resurgent dome is NW-SE elongated, although it shows a NE-SW culmination in its south-eastern part. East of La Pacana Caldera there is a series of N-S folds and 17

ACCEPTED MANUSCRIPT reverse faults. Based on Acocella et al. (2007) some of these faults have dominant strike-slip kinematics of possible Miocene-Quaternary times, although it has not been possible to constrain the age.

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South of the La Pacana caldera there are scattered, NNW-SSE-striking normal faults to the

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west, and N-S normal faults to the east. East of this caldera, there is a series of NNE-SSW-striking faults that are not vertical and that might represent reverse faults based on the local dip of the fault

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plane towards the west and the eastward-facing scarp. These faults offset an ignimbrite layer dated at 4.8 Ma (Gardeweg and Ramirez, 1987). Southeast of the caldera, the N-S normal faults, belonging to an about 10-km-long swarm, have converging dips and define a graben. The faults

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offset ignimbrites of the Atana Formation and a series of andesitic lava flows resting above another ignimbrite layer dated at 2.4 Ma by Gardeweg and Ramirez (1987). This graben should hence be

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Quaternary in age. Further south-east there is the Quaternary segment of the Calama-Olacapato-El Toro left-lateral strike-slip fault (Bonali et al., 2012; Lanza et al., 2013; Fig. 15). Finally, the southernmost part of the study area is characterized by scattered normal faults,

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mainly striking N-S and NNE-SSW. They mostly offset volcanic deposits belonging to deeply

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4.6. Holocene state of stress

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eroded volcanoes of possible late Miocene age, since they rest below the Pliocene ignimbrites.

Figure 20 plots the indicators of Holocene stress orientation from the World Stress Map Project (Heidbach et al., 2008) in the region surrounding the studied area. The WSM does not have any data in our study area, and the data outside are mostly represented by earthquake focal mechanisms (67

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events), followed by stress obtained from fault attitude and primary sense of offset (5 data), and from boreholes (2 data). For the focal mechanisms, we plotted only those obtained from upper crustal seismic events (depth < 20 km). The earthquake data from the WSM indicate a dominant N80-90° orientation of the horizontal greatest principal stress (Hmax) all around the studied area, with some scattering ( 10°) around this orientation. The focal mechanisms indicate that the azimuth of Hmax is always represented by the azimuth of the P-axis, equivalent to 1, whereas the T-axis (2) is vertical, corresponding to reverse faulting. Along the Calama-Olacapato-El Toro fault, seismic events are transcurrent with horizontal 1 and 3. The fault data indicate a N90-100° peak of the Hmax with some scattering of  10°. Boreholes give different inputs but are represented by only two data that can be biased by local settings. Within the studied area, the stress tensors obtained only by the recentmost faults show that during Quaternary times, normal faults developed following a dominant NE-SW 3 in the northern part of the study area, corresponding to a NW-SE Hmax (Tibaldi et al., 2009). It is important to 18

ACCEPTED MANUSCRIPT remind that during Quaternary times also contractional deformations developed, especially in the southern part of the study area, although data on striated fault inversion are not available. Anyway, the dominant N-S strike of reverse faults and fold hinge lines corresponds to an E-W contractional

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direction, consistent again with an E-W Hmax.

5. Discussion

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5.1. Distribution of volcanism

Volcanoes are not homogeneously distributed throughout the study area: the largest concentration of vents is present in the northern and southernmost part of the area, where the

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volcanic arc is 100 km and 65 km wide (excluding the external scattered vents). Instead, east of the Atacama basin, there are fewer vents concentrated in a 35-km-wide arc. Variations in the intensity

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and distribution of volcanism along the arc had already been recognized in the past (Coira et al., 1982, 1993; Allmendinger et al., 1997; Kay et al., 1999; Trumbull et al., 2006), although in the present paper we reached a higher detail on volcano distribution and age respect to previous

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analyses.

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By examining the map of the age of volcanic centres (Fig. 5), Miocene volcanoes are equally distributed along the entire width of the volcanic arc, with the exception of the area east of the

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Atacama basin where the arc is thinner and there are less volcanoes. The Pliocene volcanoes describe a less wide volcanic belt (about 70 km) than the Miocene arc, but with a quite homogenous width from north to south. Pleistocene volcanoes are much less scattered in an E-W sense; they tend to give rise to a more linear distribution in a N-S sense, with the exception of the northern part

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where there are fewer centres. South of the Atacama basin, the Pleistocene volcanoes are concentrated in a NE-SW corridor. Finally, the Holocene volcanoes are located along the same areas where there are the Pleistocene volcanoes. The presence of fewer volcanoes during the Miocene east of the Atacama basin might be apparent if we consider that the extensive Pleistocene and Holocene activity has buried the older volcanoes here. Anyway, during the Miocene there were fewer volcanoes at the latitude of the Atacama basin also east of the Pleistocene-Holocene arc (where the young cover is missing), suggesting that the lower frequency of volcanic centres east of Atacama is real. This arrangement may be explained as a combination of effects acting at shallow and deep level. At the shallow level, previous papers suggested that a major change in the Miocene-Pliocene tectonics occurred at 22°S (Lamb et al., 1997): undeformed Early Miocene ignimbrites north of this latitude show that significant contractional deformation in northern Chile ceased in the Early Miocene, followed by a relative uplift of the Western Cordillera (Baker and Francis, 1978). South 19

ACCEPTED MANUSCRIPT of 22°S instead, the same ignimbrites were involved in a thrust belt (Jolley et al., 1990; Gonzalez et al., 2009). Our structural field data confirm the presence of Pliocene-Quaternary contractional tectonics especially south of 23°S with reverse faults and folds, despite some normal faults are also

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present. The wide Miocene arc north of 22°S may have resulted from crustal stress relaxation

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following the main preceding shortening phase. From 22°S to 23°S, we can see an increase in the number of volcanic centres during the Pliocene (Fig. 5B) respect to the Miocene, reflecting a

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decrease in shortening. These data indicate that magmatism followed intense phases of contractional deformation, especially north of 22°S, consistently with a similar conclusion reached for the northern part of CVZ by Sébrier and Soler (1991) and McQuarrie et al. (2005), and for the

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central part of CVZ by Trumbull et al. (2006). South of 22°S, magmatism was coeval with shortening during middle-late Miocene, and also during Plio-Quaternary times south of 23°S,

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consistently with the data of Elger et al. (2005) showing time-parallelism between the general trend of shortening rates and volcanism in the southern Altiplano plateau. From the point of view of the possible causes at deep level of the geometry of the volcanic arc,

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the eastward shift of volcanoes of all ages at the latitudes of the northern and southern tips of the

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Salar de Atacama basin had already been interpreted based on the study of spatial distribution of vp, vp/vs ratios and Qp values (attenuation-1) from seismic tomography (Schurr and Rietbrock, 2004).

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These authors showed a volume of rheologically strong rocks beneath the Atacama basin, surrounded by weak regions. The strong block hinders fluids and melts from propagating to the west, resulting in the eastward shift of the arc. For Riller et al. (2006), based on the analysis of isostatic residual gravity, there are buried, upper-crustal discontinuities bounding the north-eastern

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and south-eastern margins of the Salar de Atacama. These faults may have localized the Cenozoic volcanic activity, which would explain the eastward shift of the volcanic arc. It is worth noting that there is a gradual decrease in the arc width from the Miocene to the Quaternary, with the youngest volcanoes of Pleistocene and Holocene age located in correspondence of the western front of the previous arcs (Fig. 5). Variations in the location and width of a volcanic arc respond to changes in slab dip (e.g. Tatsumi and Eggins, 1995; Ramos et al., 2002) or in converge rate (Lara and Folguera, 2006), although it has been stressed that the complex distribution of the volcanic centres of the CVZ in time and space may be the effect of multiple causes (Trumbull et al., 2006). These authors suggest that, in order to directly correlate surface volcanism with slab dip, it is necessary a near-vertical ascent of magmas through the mantle wedge and the crust. We agree that the configuration of the Miocene to Quaternary arcs was conditioned also by other factors, such as crustal heterogeneity, deformation, and lithospheric thickening, which may have perturbed the vertical ascent of magmas (Trumbull et al., 2006). Anyway, the general 20

ACCEPTED MANUSCRIPT orientation of the arc during the Miocene-Quaternary time periods did not change significantly, whereas the arc thinning and concentration at the western front can be interpreted as the main effect of slab steepening, and/or a decrease in convergence rate, the latter consistent with data from

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Somoza (1998) and Kendrick et al. (2003).

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With regard to the types of volcanic centres, it is possible to observe that monogenic volcanoes are present everywhere but with a different distribution: north of 23°S they are scattered along a 70-

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90-km-wide region. South of 23°S, monogenic cones are concentrated in rows trending especially N-S and NE-SW, in a 20-50-km-wide region. The predominance of contractional tectonics south of 23°S may explain the presence here of concentrated rows of monogenic cones: this volcano type is

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normally characterized by ephemeral, short-lived conduits. In this area, more subject to shortening, monogenic centres aligned along a few major weakness zones. In the northern area instead, the

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presence of the strike-slip faults, locally remobilised as normal faults, created the conditions for the scattering of monogenic centres. 5.2. Orientations of magma paths

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In the northern part of the study area (north of lat. 23°) and south-east of the Atacama basin,

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the NW-SE magma paths are dominant, but there are also several volcanoes with N-S paths. This coexistence of N-S and NW-SE magma paths can be observed in all the studied time windows apart

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from the Holocene, suggesting that it has been a long stable setting. During Holocene times, this coexistence might not be detectable due to the lower amount of volcanic centres respect to the other wider investigated time windows. More precisely, the N-S magma paths correspond to the N0-10° range, which is exactly parallel to the subduction trench along the studied area. This range can

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represent the effect of the deep distribution of magmatic diapirs that form and rise following the depth contours of the subduction zone. The N0-10° range might thus correspond to a first-order feature linked to the general orientation of the magma production zone. The NW-SE magma paths are parallel to the several faults we found in the study area. These faults show normal or strike-slip motions (Figs. 21A-B). The latter are consistent with the already known transcurrent faults that cut obliquely across the volcanic chain. One of these is the CalamaOlacapato-El Toro fault that, based on Bonali et al. (2012) and Lanza et al. (2013), shows Quaternary left-lateral strike-slip motions along its eastern segment across the Puna Plateau. Across the volcanic chain, instead, these authors suggest that the fault is older and acts simply as a weakness zone to facilitate magma upwelling. The presence of several faults with the same NW-SE orientation and Quaternary normal motions suggest that these slip planes might represent the reactivation of older strike-slip faults. Since transcurrent faults are usually vertical, they can act as deep weakness zones intercepting magma batches (Tibaldi, 1992). Magma upwelling can be further 21

ACCEPTED MANUSCRIPT facilitated, because the transcurrent faults are here reactivated under a changed state of stress with a vertical 1 (normal faulting). We thus conclude that the NW-SE magma paths and the parallel alignments of several volcanoes in the northern and southern parts of the study area reflect the

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control of regional faults. This control is exerted passively due to the presence of the deep weakness

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zones under a favourable local extensional state of stress during the Quaternary. During the previous Miocene-Pliocene periods, strike-slip motions occurred; the persistence of NW-SE magma

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paths further supports the idea that magma can rise along strike-slip fault planes (Fig. 21A) (Tibaldi et al., 2010b; Tibaldi, 2015; Spacapan et al., 2016). A more general control of the NW-SE faults on volcanism, and especially on the caldera development in the Puna Altiplano, was suggested also by

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Riller et al. (2001, 2006), Caffe et al. (2002), Chernicoff et al. (2002), Petrinovic et al. (2005), and Ramelow et al. (2006). In regards to calderas, we found that their escarpments mostly strike NW-SE

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and secondarily NNE-SSW. Several intracaldera volcanoes also show the same trends. Moreover, all the resurgent intracaldera horsts are elongated NW-SE. These data suggest that regional tectonic faults guided the geometry of calderas during both deflation and inflation phases, and that magma

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feeding paths should have the same dominant orientations.

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The presence of these dominant magma paths, obtained by the analysis of the morphometric characteristics of the volcanoes, is validated by the study of the outcropping dykes. The direct

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comparison of the total magma paths and surveyed dykes clearly indicates that the ranges and peaks of azimuth frequencies are similar (Fig. 6A-B). Moreover, from the point of view of the evolution of dyke orientations with time, several field examples indicate that the E-W and NNE-SSW dykes are the oldest, followed by the NW-SE intrusions and finally by the N-S dykes. The two youngest

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dyke trends are identical to the most diffuse NW-SE and N-S magma paths. It is also noteworthy that field data show older dykes offset by NW-striking left-lateral strike-slip faults that, in turn, are intruded by NW-SE dykes. The latter is a clear further indication of the direct control exerted by the planes of transcurrent faults on magma intrusions. East of the Atacama basin, magma paths show a prevailing N-S trend throughout all the investigated periods. From a structural point of view, this area is affected by dominant N-S normal faults in the northern part, and N-S reverse faults and folds in the southern part (Figs. 21C-D), which may act as weakness zones along which magma rises. As a consequence, in the shallow crust here the magma paths are influenced by the N-S-striking tectonic structures. This is a complementary possibility respect to the previously introduced concept that magma batches with an original N-S orientation may originate from deep levels giving rise to N-S magma paths also at the shallower level.

22

ACCEPTED MANUSCRIPT 5.3. Local control on volcanic centres vs. regional tectonic stress field One of the most astonishing results of the present work is the almost absence of magma paths parallel to the plate convergence direction. Classical models predict that intrusions may radiate from

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the central conduit of a volcano following the force exerted by magma overpressure along the main

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conduit. This corresponds to hydraulic fracturing with propagation of radial dykes (review in Acocella and Neri, 2009). Following Nakamura’s (1977) model, at some distance from the central

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conduit dykes tend to intrude parallel to the regional Hmax because tectonic regional forces become more influent (larger) than local magma forces. Above subduction zones, the regional Hmax usually coincides with 1 and with the direction of plate convergence (Nakamura and Uyeda, 1980). If the

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regime is transcurrent, with horizontal 1 and , magma can rise up along tension fractures corresponding to the bisector of the acute dihedral angle composed by the conjugate strike-slip

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faults (T fractures of Riedel’s, 1929, shear model). These fractures are thus parallel to  and normal to . This model was positively applied, for example, to the Aleutian-Alaska volcanic arcsubduction zone (Nakamura et al., 1977).

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In the study area, the direction of convergence between the South American plate and the

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Pacific (Nazca) plate is N76-79° (Norabuena et al., 1998). Between 22°S and 25°S along the northern Chile subduction zone, from 20 to 50 km in depth the stress axis σ1 is oriented in the

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convergence direction (Delouis et al., 1996). Around our study area, the Holocene state of stress, obtained from data of the World Stress Map Project, is characterized by the constant E-W orientation of Hmax related to crustal earthquakes - focal depth ranges 8-39 km. Within the study

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area, the northernmost part has been characterized by a NW-SE Hmax during Quaternary times (Tibaldi et al., 2009). If we thus apply Nakamura’s (1977) model to the study area, we should expect to find magma paths mostly trending E-W, and very subordinately trending NW-SE in the northernmost part of the study area. The E-W trend is instead missing, both based on magma paths and dyke orientations. Only the dykes surveyed in the oldest, Palaeozoic rocks, and only in the NW and SW parts of the study area, show also E-W trends but, given their ages, cannot be directly compared to the Holocene state of stress. We interpret the lacking of magma paths parallel to the E-W regional Hmax as the effect of local pre-existing weakness zones that exert a passive larger control on magma upwelling with respect to the mechanical work necessary to produce new fractures. This explanation is valid if we can assume that a dense network of mechanical discontinuities had been produced by tectonic stresses before magma upwelling, and in fact this model can be validated taking into account that the tectonic history of the central Andean chain goes back to pre-Miocene times (Almendinger et al., 1997). During the Cretaceous-Paleogene period, deformation occurred accompanied by the 23

ACCEPTED MANUSCRIPT development of different volcanic arcs that gradually migrated eastwards (Huene and Scholl, 1991). Deformation involved both the pre-Cordillera system and the Eastern Cordillera (Reutter et al., 1996; Lamb et al., 1997). At 25 Ma ago, the western Cordillera had already reached about the 50%

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of its present altitude (Gregory-Wodzicki, 2000). The various tectonic phases that had developed

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from Cretaceous to Paleogene times, produced a series of reverse faults and folds with a dominant N-S orientation in the study area, constituting part of the network of weakness zones that would

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have been successively used as magma paths. For which concerns the pre-existence also of the NWSE-striking mechanical discontinuities, we have to consider the flare up of volcanic activity between 12 and 2 Ma ago that was interpreted as produced by a series of causes, comprising: i) an

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increase in the plate convergence rate (de Silva, 1989a), ii) an increase in the dip of the subducting slab (Kay et al., 1994), and iii) a change in the tectonic deformation style, which is from dominant

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reverse to transcurrent along main NW-SE-striking faults (Riller et al., 2001). We are keen on to considering the latter explanation as the most plausible for the development of the other main set (NW-SE) of discontinuities used for magma rising.

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We also consider that the NW-SE transcurrent faults were locally reactivated as normal faults,

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further facilitating their use as magma paths. The presence of normal faults within this active orogen has already been explained by the local reorientation of stress tensor due to crustal

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thickening and topographic load, which lead to the collapse of some sectors of the orogen (Tibaldi et al., 2009; Bonali et al., 2012; Lanza et al., 2013; Giambiagi et al., 2016). Our results point out that magma can rise along planes oblique to the principal stress axes (Fig. 21A), or even along planes that are perpendicular to the coeval 1, as testified to by the presence of

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several Holocene NNW-SSE- to N-S-trending magma paths (Figs. 21C-D). This is consistent with the observation of Currie and Ferguson (1970) that pre-existing fracture zones might directly “capture” upwelling magma irrespective of the orientation of 1 and 3. Our results are also consistent with the volcano-tectonic setting of the Southern Volcanic Zone where Cembrano and Lara (2009) found magma paths severely misoriented with respect to the coeval stress state and that followed old reverse faults. To understand this behaviour, it is necessary to remind that dyking can occur through intrusion into a newly formed fracture if magmatic pressure (pm) exceeds: the lithostatic pressure (pl), plus the horizontal compressive stress in the host rocks perpendicular to the dyke, plus the host rock tensile strength (Gudmundsson, 1995, 2006, 2012). If instead the host rock is characterized by the presence of mechanical discontinuities inherited from previous deformation phases, these planes have no cohesion (or very poor cohesion in the case of sealing effects) and magma can propagate along them if magmatic overpressure (po = pm − pl) exceeds the compressive stress acting perpendicular 24

ACCEPTED MANUSCRIPT to that plane. Although magma usually tends to intrude along planes containing σ1 and σ2, the presence of faults in the volcano substratum may facilitate the intrusion of dykes along them by reducing the magma pressure necessary to exceed the rock strength. A similar situation has been

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found also south of the study area (in the Southern Volcanic Zone), in correspondence of active

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volcanoes, by Bonali et al. (2015).

The control exerted by inherited structures must be extended also to stratification and folds.

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The bedding of sedimentary rocks represents mechanical discontinuities that can be used as magma paths, as it is the case for the volcanoes located in the north-eastern part of the study area, where some volcanic centres of Pliocene and Pleistocene age show NE-SW magma paths. These

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volcanoes are located above a succession of sedimentary rocks with sub-vertical to vertical strata attitude. The strike of the bedding planes is exactly parallel to the magma paths and may thus have

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acted as weakness zones.

Also folds can guide magma paths, as exemplified in the study area by the presence of several volcanoes resting directly above folds and showing magma paths parallel to the hinge lines. This

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situation has been very recently recognized also in other places; for example, the Cerro Negro

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intrusive complex was emplaced in the Chos Malal fold-and-thrust belt in the Neuquén Andes, Argentina (Gürer et al., 2016). Absolute dating and field observations showed that the emplacement

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of dykes and sills was coeval with the tectonic development of the fold-and-thrust belt. Dykes were emplaced perpendicular to the main shortening direction following anticlinal hinges. This suggests that dykes may emplace following ramp faults at a deeper level (e.g. Tibaldi, 2008), and the stresses related to the fold outer-arc stretching at a shallower level (Gürer et al., 2016). A similar control

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exerted by folds has been observed also for felsic magmas at the Holland’s dome granite injection complex, Damara belt (Namibia) (Kruger and Kisters, 2016): also in this case magma rose along vertical sheets parallel to the fold hinge, even if other subsidiary conjugate sets of sheets had developed.

5.4. Mutual feedback between magma and tectonism In the study area there is a series of sites where it has been possible to recognize deformations linked to magma pressure, gravity and tectonics. The volcano-tectonic characteristics of the interior of the giant La Pacana, Pastos Grande, and Guacha calderas are remarkably similar to each other and with the Azufre-Inacaliri volcanic complex (Fig. 17). All these areas are characterized by the presence of major (> 10 km long) NW-striking normal faults with converging dips defining graben systems (Fig. 15). All these grabens are located on top of NW-SE alignments of polygenic volcanoes and some monogenic edifices, and rest at altitudes > 4500-5000 m asl. The origin of 25

ACCEPTED MANUSCRIPT these structures in the interior of calderas can be attributed to the expansion of a shallow magma chamber giving rise to the formation of a resurgent horst. These grabens can thus be interpreted as the effect of differential uplift due to magma forces, from the causative point of view. From the

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point of view of the geometrical influence, since all these grabens have a strike identical to that of

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the regional NW-SE faults, we have already suggested, in the previous chapter, a regional tectonic control on their orientation. The Azufre-Inacaliri volcanic complex, instead, is not related to any

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caldera, but here the graben is limited only to the uppermost part of the volcano row. Although these normal fault scarps are tens of metres high, the faults abruptly end in correspondence of the outer volcano slopes and do not extend into the substrate. We interpret this setting as the evidence

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of a gravity force responsible for the formation of the graben, possibly in connection with volcano spreading. Spreading is justified by the huge mass of this volcanic row and by the concurrent possible presence of clay-salty deposits below it (a salar deposit crops out at the northern base of

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this volcanic complex), similar to other field cases elsewhere (e.g. Borgia et al., 1990) and to laboratory experimental data (Walter et al., 2006). The orientation of the graben may have been

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influenced by the regional faults or by the elongation of the volcanic row. Once these grabens form,

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triggered by caldera resurgence or gravity, they act as weakness zones that further guide the emplacement of new volcanic edifices, in a sort of feedback effect. This is the case, for example, of

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the Pabellon dome that grew exactly above one of the major bounding faults of the Azufre-Inacaliri graben (Fig. 17), or of other centres grown in correspondence with the caldera resurgent horsts. Another striking example of mutual feedback effect between tectonism and magmatism can be found at a larger scale. East of the Salar de Atacama, several volcanoes rest on top of folds. Some of

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these folds are strongly asymmetrical, as the Miscanti Ridge, suggesting they are linked with blind reverse faults. As explained earlier, the fold hinge and fault may have controlled magma migration resulting in the N-S magma paths. Similarly, the swarm of N-S tectonic structures guided here the development of the N-S corridor of volcanism as a whole. Such a concentrated development of volcanoes and their magma conduits should have lubricated the faults, facilitating their further development, consistently with similar interpretations based on laboratory and field data elsewhere (Brown et al., 1999; Bruhn et al., 2005; Schilling et al., 2006). This might explain why the Miscanti Ridge and its southern prolongation shows such a high scarp (up to 1000 m high) and long trace (six segments from 15 to 70 km long).

6. Conclusions

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ACCEPTED MANUSCRIPT We analysed the distribution of 1518 monogenic and polygenic volcanoes of MioceneQuaternary age in the Central Volcanic Zone of the Andes, and reconstructed the magma paths at 315 volcanic centres by analysing the morphostructural characteristics of craters and cones.

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From the Miocene to the Quaternary the arc width decreased, with the Holocene volcanoes

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mainly located along the western part of the previous volcanic belt (with the exception of the Tuzgle volcano). The volcanic belt always shifted eastwards at the latitude of the Salar de Atacama,

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where the arc was also thinner during the Miocene and Pliocene.

Most magma paths trend N-S, NW-SE, and NE-SW, in decreasing order of frequency. The NS and NW-SE paths coexist in the northern and southern part of the study area for every period. N-S

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paths dominated east of the Salar de Atacama during Pliocene, Pleistocene and Holocene. Orientation of dykes mapped in deeply eroded areas show the presence of the same trends, and field

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data suggest that the N-S dykes are the most persistent during time. The regional Holocene stress state is given by an E-W greatest horizontal principal stress. During at least Pliocene and Pleistocene times, the main direction of shortening should also have

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been about E-W, consistent with the development of N-S and NNE-SSW reverse faults and folds,

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and NW-SE left-lateral strike-slip faults. The volcanic chain is also affected by several N-S- and NW-SE-striking normal faults that offset both Pliocene and Quaternary deposits.

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Our results indicate that N-S normal and reverse faults and N-S folds guide volcano emplacement and magma paths. Fold hinges especially controlled magma paths and location of volcanoes east of the Salar de Atacama. NW-SE faults also guided magma paths in the northern part of the study area. Zones of verticalized strata, with bedding striking NE-SW, also acted as

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preferential magma paths in the north-eastern study area. These data suggest that at convergence zones, shallow magma paths can be more sensitive to the presence and geometry of upper crustal weakness zones than to the regional state of stress.

Acknowledgements We acknowledge the useful suggestions of Andres Folguera and Adelina Geyer on a previous version of the manuscript. This study has been done in the framework of the International Lithosphere Program - Task Force II “Structural and rheological constraints on magma migration, accumulation and eruption through the lithosphere”.

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Figure catpions

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Figure 1. (A) Location of the study area in the CVZ and other active volcanic segments of the Andes: Northern Volcanic Zone (NVZ), Central Volcanic Zone (CVZ), Southern Volcanic Zone (SVZ), Austral Volcanic Zone (AVZ) (modified from de Silva, 1989). (B) Shaded view of the studied area with the main structures affecting this sector of the Andes. MAT — Main Andean thrust (Herail et al., 1996), WCC Western Cordillera of northern Chile, PP Altiplano – Puna Plateau, Us Uyuni salar, As Arizaro salar, LC Lipez-Coranzuli fault system, AG Archibarca-Galan fault system, COT Calama-Olacapato-El Toro fault system, CFN Culampaja- Farallon Negro fault system (Riller et al., 2001; Matteini et al., 2002a; Petrinovic et al., 2005; Mazzuoli et al., 2008), AFS Atacama Fault system (Rutland, 1974; Riquelme et al., 2003). Modified after Tibaldi et al (2009) and Bonali et al. (2012). Main calderas in the study area: CG Cerro Guacha, CP Cerro Panizos, LP La Pacana, PG Pastos Grandes, VC Vilama. Red triangle locates Tuzgle volcano. Blue rectangle locates the studied area and Fig. 3. Figure 2. Examples of volcanoes of clearly different age. (A) Eroded dome of Pliocene age (4.8  0.2 Ma, Chamaca dome, 23°29’24.99”S – 67°28’59.95”W); (B) Holocene dome (Guayaques, 22°54’04.21”S – 67°36’28.68”W) that reaches 5272 m of altitude and does not show any sign of erosion and glacier abrasion; (C) eroded and altered stratovolcanoes of Miocene age (10.2  1.7 Ma, 23°29’31.34”S – 67°22’01.47”W); (D) more preserved stratovolcano of Pliocene age (3.5  0.7 Ma, Cerro Hualitas, 23°01’14.32”S – 67°25’09.16”W). (Images from Google Earth, dating from Gardeweg and Ramirez, 1987). Figure 3. Location of all the 1518 studied volcanic centres, distinguishing: i) polygenic volcanoes; ii) vents located on the flanks of polygenic volcanoes (domes, pyroclastic cones, lava flow vents);

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ACCEPTED MANUSCRIPT iii) domes grown on the substrate; iv) other more mafic monogenic centres (pyroclastic cones, maars and low-viscosity lava flow vents) grown on the substrate.

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Figure 4. Examples of volcanic centres showing clear morphometric features; the dashed lines, or the converging arrows, represent the measured azimuth for the inferred magma path. (A) Elongation of a well-preserved crater located on top of a stratovolcano and the same alignment of the two crater-rim depressed points highlighted by the orange line (22°01’01.11”S 67°46’09.31”W); (B) elongated dome with a continuous outcropping base with maximum axis trending NW-SE, and parallel apical graben (crease) structure showed by the red arrows (23°5'26.58"S - 67°42'14.60"W); (C) alignment of craters of similar age located on the summit of a stratovolcano (Lascar, 23°21'57.00"S - 67°44'5.18"W); (D) alignment of eruption sites of the same age lying directly on the substrate (21°59'38.25"S - 67°50'0.54"W). (Images from Google Earth).

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Figure 5. Distribution of volcanic centres by type and time interval: (A) Miocene, (B) Pliocene, (C) Pleistocene, and (D) Holocene. In brackets the number of vents for each time interval. The dashed line shows the boundary of the total area where the Miocene-Quaternary volcanic arc developed.

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Figure 6. (A) Rose diagram showing strike of all inferred magma paths. Rose diagrams of strike of measured outcropping dykes (locations in Fig. 7): (B) all study areas, and intruding rocks of (C) Oligocene age, (D) Upper Cretaceous-Lower Tertiary age, (E) Cambrian-Ordovician-Silurian age; In brackets, the number of dykes for each set.

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Figure 7. (A) Orientation of all 315 shallow magma feeding paths reconstructed in the present work. Boxes locate the areas where dykes (black colour) have been surveyed: (B) north-eastern area, (C) north-western area, (C) south-western area and (D) southern area. In subareas, dykes are in red colour. Rose diagrams show dyke strike (number of dykes in brackets).

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Figures 8. (A) Satellite image and (B) superimposed structural sketch map of a series of volcanoes located on a horizontal substrate whose parameters all suggest a possible NW-SE feeding system: they are NW-aligned, have NW-elongated single cone, and have NW-trending maximum axis of the elliptical craters (22°1'19.17"S - 67°46'11.38"W ). (C) Satellite image and (D) interpretation of NW-striking normal faults offsetting a volcano of Pliocene age (fault scarps in grey). The downthrown block is to the southwest. Location: box in Fig. 8A. Figure 9. (A) Pyroclastic cone with a NW-SE elongated base and the substrate affected by a swarm of NW-SE fractures located just in correspondence of the cone and passing exactly below the crater (20°51'58.58"S - 68°7'24.93"W). (B) Elliptical dome with the maximum axis of the base and the crease structure on its upper surface that strike NW-SE, consistent with the same orientation of the hidden fracture that connects the fault cropping out NW and SE of the dome (21°50'26.13"S 68°9'10.04"W). Figure 10. (A) Satellite image (modified from Google Earth) and (B) structural sketch map of a series of polygenic volcanoes aligned NE-SW (20°42'46.80"S - 68°32'38.49"W): the morphometric indicators suggest that each cone has been built through NW-SE-striking magma-feeding paths. In the inset of Figure 10A, the recentmost cone (Irruputuncu) shows a summit zone with a series of craters of comparable age that are aligned NW-SE, and each crater is elongated in the same direction. The white arrows indicate the NE-SW trend of the local least principal stress 3. Figure 11. (A) Satellite image (modified from Google Earth) and (B) structural sketch map of the summit part of one of the N-S aligned volcanic edifices (22°37'42.00"S - 67°52'49.95"W). A series 39

ACCEPTED MANUSCRIPT of clues suggest that the single edifice has been subject to magma uprising along a N-S-striking weakness zone: i) about N-S-striking fractures, ii) coeval N-S-aligned craters, iii) elongated craters with N-S maximum axis, iv) eruptive vents, dome spikes and tumulus aligned along a N-S-trending zone, and v) a summit pyroclastic cone with a N-S-trending maximum base axis.

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Figure 12. (A) Satellite image (modified from Google Earth) and (B) structural sketch map of a series of vents located close to a zone of N-S aligned volcanoes but showing a different NNE-SSW magma path. The morphometric parameters are: i) a NNE-elongated older dome, above which ii) there is a younger NNE-elongated dome, iii) a series of eruptive points within a NNE-trending strip, and iv) the dome is aligned with another small eruptive site located to the NNE. Small black arrows indicate the lava flow direction. The red arrows indicate the inferred direction of local least principal stress.

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Figure 13. Distribution of magma path orientations with time: (A) Miocene, (B) Pliocene, (C) Pleistocene, and (D) Holocene. Rose diagrams show frequency of azimuth distribution of magma paths for each time interval. Number of magma paths for each set is reported in brackets. The dashed line shows the boundary of the total area where the Miocene-Quaternary volcanic arc developed.

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Figure 14. Examples of dykes in the study area. (A) Elaborated image from Google Earth and (B) interpretation showing a series of older NNE- to NE-striking dykes offset by NW-SE dykes; the NWstriking dykes emplaced along left-lateral strike-slip faults. Note that some linear features are roads. (C) A more recent N-S-striking dyke. (D) Elaborated image from Google Earth and (E) interpretation showing a series of older NNE-striking dykes offset by NW-SE dykes that emplaced along left-lateral strike-slip faults. The latter, in turn, are cut by a 10-m-large N-S-striking dyke.

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Figure 15. Structural map of the study area with main Neogene-Quaternary faults and folds based on new data from this work, González et al (2009), Tibaldi et al. (2009), Sernageomin (2003), Bonali et al. (2012), Marsh et al. (1995), Salfity and Monaldi (1998), Servicio Geológico Nacional (1996). Rose diagrams show fault strikes classified by kinematics, as well as fold hinge lines. LP La Pacana caldera, CP Cerro Panizos caldera, PG Pastos Grandes caldera, GC Guacha caldera, VC Vilama caldera, SA Salar de Atacama, SAR Salar de Arizaro, SU Salar de Uyuni, TV Tocorpuri volcano, LV Licancabur volcano, JV Juriques volcano, MR Miscanti Ridge, COT CalamaOlacapato-El Toro fault, T Talabre village. Figure 16. (A-B) Example of lava flows of estimated Pliocene age offset by left-lateral strike-slip, NW-striking faults (21°4'45.12"S - 68°15'37.16"W) (C-D) Example of normal faults detected near Ollagüe volcano, Chile-Bolivia border (21°24’32.61”S - 68°06’38.28”W). Figure 17. (A) Elaborated satellite image (from Google Earth) and (B) geological-structural interpretation of the Inacaliri graben, Chile. Note the presence of volcanic centres of PliocenePleistocene (deposits in green) and Holocene age (deposits in yellow) that are aligned along the faults (e.g. the Pabellon lava dome, 21°50'20"S - 68°09'15"W). Azufre volcano is located at 21°47'00" - 68°14'19". Figure 18. (A) Area located east of the Atacama desert, affected by N-S- to NNE-SSW-striking normal faults running from the Licancabur and Juriques volcanoes southwards (22°56’21.49”S – 67°52’30.78”W). Both the crater zone (B) and the southern slope (C) of the Juriques volcano are offset by a graben-like structure striking NNE-SSW. Triangles point to the fault scarps. (D) Recent

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Figure 19. (A) Satellite image with oblique view (modified from Google Earth) and (B) structural sketch map of an area affected by recent normal faults with converging dips, defining a graben-like structure. (C) Example, by oblique view of a modified Google Earth image, of a vertical fault with downthrown of the tectonic block located to the right side (i.e. to the East). The surface trace of the fault scarp is very fresh. (D) Field close view of a N-S-striking normal fault showing a dip-slip offset of an old river bed (23°47’31.04”S – 67°20’31.66”W). (E) Field distant view of the same N-S fault portrayed in Figure 19D.

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Figure 20. Azimuth of maximum horizontal stress from the World Stress Map Project (Heidbach et al., 2008) in the region surrounding the studied area (blue rectangle). Focal depth ranges from 8 to 39 km.

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Figure. 21. Sketch (not to scale) of magma path orientation in the study area based on outcropping dykes and volcano morphometric parameters. In A and B magma paths are oblique to the trench strike and to the plate convergence direction. In A magma path is also oblique to the principal stress axes. In C and D magma path strike is perpendicular to the greatest principal stress 1 ad to convergence. In C magma paths may use the reverse fault planes, while in D they use the hinge zone of folds.

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ACCEPTED MANUSCRIPT Highlighst We compared tectonic structures, volcanoes and magma paths in central Andes

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Different scenarios of magma-tectonic interaction explain complex magma path

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Magma tracked strike-slip faults or vertical strata oblique to principal stress axes

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Magma emplaced also at reverse faults and fold hinge normal to regional compression

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Shallow magma paths are more sensitive to preexisting structures than regional stress

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