Miocene to Holocene Geological Evolution of the Lazufre Segment in the Andean Volcanic Arc GEOSPHERE; V

Home , Lastarria, Los Colorados (caldera)

Research Paper THEMED ISSUE: PLUTONS: Investigating the Relationship between Pluton Growth and Volcanism in the Central Andes

GEOSPHERE Miocene to Holocene geological evolution of the Lazufre segment in the Andean volcanic arc GEOSPHERE; v. 15, no. 1 José A. Naranjo, Francisco Hevia, Víctor Villa, and Cristián A. Ramírez https://doi.org/10.1130/GES01352.1 Departamento de Geología Regional, Servicio Nacional de Geología y Minería, Av. Santa María #0104, Providencia, Santiago 7520405, Chile

6 figures; 2 tables ABSTRACT during the period of March 2003–May 2005, and they proposed that the great CORRESPONDENCE: est deformation could have a magmatic origin between 7 and 15 km deep, jose.naranjo@sernageomin.cl The Lazufre bulging zone, in the area of the Pleistocene–Holocene Azufre, whereas a surface source related to hydrothermal fluids could have caused Cordón del Azufre, Bayo, and Lastarria volcanic complexes, has been a major only minor deformations detected during this same period. Later, for the pe CITATION: Naranjo, J.A., Hevia, F., Villa, V., and Ramírez, C.A., 2019, Miocene to Holocene geolog‑ focus of study over the past few decades. Since 1998, interferometric synthetic riod 2003–2006, Ruch et al. (2008, 2009) found that the deformation south of ical evolution of the Lazufre segment in the Andean aperture radar (InSar) analysis has shown structural deformation, likely a re- Lastarria volcanic complex affected an elongated area of 45 km by 55 km (1400 volcanic arc: Geosphere, v. 15, no. 1, p. 47–59, sult of an active magmatic and hydrothermal system. Our new mapping pro- km2) with a rise of up to ~3 cm/year. According to these authors, the defor https://doi.org/10.1130/GES01352.1. vides clues about the causes and possible consequences of this deformation, mation signal could be explained by the intumescence of a magmatic body based on the reinterpretation of important structures or regional lineaments. (located ~10 km deep) that is expanding with a lateral spreading rate of up to Science Editor: Raymond M. Russo Guest Associate Editor: Rodrigo del Potro The bulge is located upon the hanging wall of the east-vergent, Pedernales- 4 km per year. For the period 2003–2008, Anderssohn et al. (2009) detected a Arizaro NE-SW–trending Middle Miocene major thrust fault. The footwall of rise of 15–16 cm and considered that the magmatic body expansion occurred Received 29 April 2016 this fault was previously affected by a major explosive activity producing the at depths between 10 and 15 km. These authors estimated a constant lateral Revision received 4 October 2018 Los Colorados caldera at ca. 9.4–9.8 Ma, the source of the homonymous 115– spreading rate of the intrusion at ~5–10 km/year. In 2010, Budach et al. (2013) Accepted 8 November 2018 185 km3 ignimbrite. Conjugated at ~30° to the Pedernales-Arizaro thrust, the conducted a magnetotelluric study at Lastarria volcanic complex area, and Published online 21 December 2018 Imilac-Salina del Fraile oblique, slightly dextral strike-slip fault constitutes a they suggested that the rise of partial melts from the asthenospheric wedge, major structure in the area, which favored the opening of transtensive spaces, which would feed a potential magmatic reservoir beneath the volcanic center, parallel to the Los Colorados caldera–Lazufre bulge alignment. Notably, since was responsible for the regional strain. In this work, we have reinterpreted the Late Pliocene, volcanism has been concentrated in the Lazufre intumes- important structures or regional lineaments, and based on new mapping, we cence, including extrusion of ~120 km3 total lava volume. The lava accumula- provide clues on the causes and possible consequences of this deformation. tion rate estimated since Late Pliocene to the present at Lazufre bulging zone Constructional volcanoes (stratovolcanoes, volcanic complexes, and lavas) area is approximately one-third of the rate estimated for the generation of the are grouped primarily on the basis of subtle differences in erosion or preserva Los Colorados caldera. The migration of volcanic activity from this Miocene tion degrees, supported by lithological characterization. Integrated geochrono caldera area to the northwestern Lazufre bulging zone could be the conse- logical data and the use of satellite images allowed us to recognize several quence of local strain field variations that opened tectonic space and favored Upper Miocene to Pleistocene volcano remnants, pyroclastic cones, lavas, magmatic ascent and storage. and ignimbrites based on previous works as well as new mapping (Naranjo OLD G and Cornejo, 1992; Gardeweg et al., 1993, 1994; Kraemer et al., 1999; Richards INTRODUCTION and Villeneuve, 2002; Richards et al., 2006, 2013; Schnurr et al., 2007; Seg giaro et al., 2007; Naranjo, 2010; Naranjo et al., 2013a, 2013b, 2016, 2018). The The central Andes volcanic arc between 25° and 26°S comprises ~220 Up Pleistocene–Holocene Azufre, Cordón del Azufre, Bayo, and Lastarria volcanic OPEN ACCESS per Cenozoic volcanic centers with Holocene activity at the Lastarria, Cordón complexes are the youngest volcanoes in the area and were built over the del Azufre, and Bayo volcanic complexes (Naranjo et al., 2013a), which we refer Pliocene–Pleistocene Pirámide and Atalaya-Chuta complexes along a 40 km to as the Lazufre segment of the Andean arc. The Lastarria volcanic complex NNE-SSW–trending ridge (Fig. 2; Naranjo et al., 2013a). Additionally, the area has been a major study focus over the past few decades due to the identifica includes two Middle Miocene NE-SW as well as NW-SE main fault systems: tion of a regional deformation producing a bulge centered near the complex Pedernales-Arizaro and Imilac-Salina del Fraile faults (Fig. 1; Naranjo et al., (Fig. 1). Between 1998 and 2000, this area suffered an intumescence attributa 2018). Some authors have proposed a younger age (<10 Ma) for the main This paper is published under the terms of the ble to movements of magma and hydrothermal fluids at depth (Pritchard and transversal structures (lineaments) recognized in the area (e.g., Archibarca CC‑BY-NC license. Simons, 2002). According to Froger et al. (2007), this surface uplift persisted fault zone, Riller et al., 2001; Ramelow et al., 2006).

0# Llullaillaco vn.

Llullaillaco Arizaro

Fig. 2

Pajonales Río Grande 0# Lastarria v.c. I m

i l a

- S a Figure 1. Lazufre interferogram (taken 0# l Cordón del Azufre v.c. i n from Anderssohn et al., 2009) on digi- a tal elevation model image showing the d e extended intumescence attributable to l F magma movements since at least 1998. r a Late Pleistocene to Holocene volcanoes t ile l (Lazufre-Bayo volcanic complexes) are lo- u fa f a u cated within the bulged area. Main faults l r o t of the segment, Los Colorados caldera a z (LCC) and ignimbrite (LCI) with ages (see r i - A Table 1), are also indicated. Location of s Figure 2 is also specified (red dashed box). l e a n 9.8±0.5 Locations for Figure 3 sections (A, B and C) r De la Isla 9.797±0.035 8.161±0.056 e (& 9.6±0.2 are identified. d 9.419±0.042 A 9.849±0.036 e t Antofalla P 8.644±0.045 l 9.620±0.032 u Aguilar a f

Las Parinas a l l

a f

o t

Grande n

8.31±0.12 (& B 9.01±0.03 Argentina ± Chile 8.65±0.03 0102030 8.466±0.015 (& C 9.09±0.04 Km

Inferred fault ((Thrust fault LCC rim Salar Lineament Dextral oblique-slip Explosion crater LCI

GEOSPHERE | Volume 15 | Number 1 Naranjo et al. | Geological evolution of the Lazufre segment 49 Research Paper

Using geochronological data, geological knowledge of the area of interfero mass (Table 1). Approximately one kilogram of sample was crushed and metric synthetic aperture radar (InSar)–detected deformation has remarkably sieved to obtain grain sizes of 100–250 µm. Minerals were separated using increased and allowed us to define in detail the geological context and the an isodynamic Frantz magnetic separator and heavy liquids, followed by temporal and structural evolution of the area (Naranjo et al., 2013a, 2013b). handpicking under binocular microscope. Samples were mounted in high- Thus, we are able to map important structures or regional lineaments previ purity Al discs (with space for 21 samples), together with sanidine crystals ously defined and interpreted by other authors (i.e., Riller et al., 2001; Reijs and of Fish Canyon standard sample (28.02 ± 0.1 Ma, Renne et al., 1998). Sam McClay, 2003; Ramelow et al., 2006). The purpose of this paper is to provide ples were irradiated in a RECH-1 nuclear reactor at Comisión Chilena de the geological and structural evolutionary context of the Lazufre deformation Energía Nuclear, La Reina, Santiago (CCHEN) nuclear reactor facility, during zone and to explain its occurrence. a period of 22 h, in a Herald-type pool, at 5 Mw power energy. Samples in side the reactor were mounted in a stable and rotating position, surrounded by Cd shell. Rotating system was implemented to obtain the homogeneous METHODOLOGY J factor for the different samples across the disk. After irradiation, sam ples were introduced in a copper disc, with potassium bromide cover, and Fieldwork was conducted through seven field seasons between 2011 and mounted in the sample chamber of the sample zone of the spectrometer, 2015 and involved regional-scale sampling of fresh volcanic rocks and geo under ultra-high vacuum conditions. Samples were heated by successive

logic mapping (at 1:100,000 scale) supported by multiple satellite image in power steps induced by a CO2 laser, with maximum energy of 30 W. Follow terpretation and SRTM digital elevation models (DEMs) visualized as 3D tools ing each set of three analytical steps, blank measurements were made to in Google Earth Pro. Most localities corresponded to lava fronts around vol correct successive measurements. Noble gases extracted from the sample canic complexes. The ignimbrite stratigraphy is based on descriptions of 15 were separated using a cool finger trap and one getter (ST101), operated naturally exposed cut-bank sections of ~150 visited sites. Correlations between at 2.2 Å. The purified sample was introduced into the MAP 215-50 mass ignimbrite deposits were made based on stratigraphic order and macroscopic spectrometer. Isotope content for masses 36Ar, 37Ar, 38Ar, 39Ar, and 40Ar were physical characteristics, such as color, grain size, texture, lithology, and min measured using a high-resolution electron multiplier, together with base eralogy (Fig. 3). line measurements for each heating step. Volcano volumes were calculated using DEMs. The total volume of the vol Obtained apparent ages for each heating step take into consideration cor canic edifice was considered on a flat 3D base delimited by the edges of each rections for interference of isotopes generated during irradiation from K, Ca, mapped volcano and volcanic complex. We used the methodology proposed and Cl. The criteria to define a “plateau age” correspond to three or more suc by Grosse et al. (2009, 2012) to obtain the morphometric parameters of vol cessive steps overlapping on error at 2σ level and containing >50% of released canoes in the area. The delimitation of the base of the volcanoes was plot 39Ar (Fleck et al., 1977). The decay constants for 40K used for calculations are ted using satellite images and geologic maps at a scale of 1:100,000 (Naranjo those proposed by Steiger and Jäger (1977). The atmospheric ratio for 40Ar/36Ar et al., 2013a, 2013b, 2016), to take into account the geology of each volcanic is assumed at 295.5. complex and the extension of their products. Morphometric parameters such as altitude, absolute height, and basal area were estimated using the Global Mapper 11.02 program. Each volcano complex limit was drawn in a first DEM1. SUMMARY OF THE VOLCANIC GEOLOGY OF THE Then a 3D image of the modeled base of the building was created, allowing LAZUFRE SEGMENT (25°S–25°40′S) us to obtain more realistic parameters than those calculated with a horizontal base. The line that delimits each volcano complex is represented by points A complete overview of the Oligocene–Holocene explosive volcanism with information of coordinates and height, which allowed generating a new that includes large deposits of ignimbrite that are interbedded with depos DEM2. By combining 1 and 2, a third DEM3 was generated, which, along with its of constructional volcanoes is presented in Naranjo et al. (2018) for the its height, was finally integrated to obtain the volume. 24.5° and 27° S Andean volcanic arc segment. These authors indicate that The geochronology framework is based on 75 radiometric analy there is no evidence of systematic migration of the arc in the past 25 m.y.; ses and includes 24 K-Ar ages obtained by Naranjo and Cornejo (1992), the width of the arc has been variable, and the overlap of younger vol 13 by Kraemer et al. (1999), four in Schnurr et al. (2007), 13 40Ar/39Ar ages canoes that grow on older volcanic structures is a characteristic feature of by Naranjo (2010), 17 by Naranjo et al. (2013a), two by Richards and Ville the area. neuve (2002), and 14 by Richards et al. (2013) (see Naranjo et al., 2018, The volcanic arc in the Lazufre segment (25°S–25°40′S), in particular, has supplemental material). In addition, 40Ar/39Ar geochronological dating for been built on mainly Paleozoic volcanic and metasedimentary basement four ignimbrites and two lava flows was carried out at Isotope Geology Lab rocks. In this segment, the presence of an explosive structure identified as oratory from SERNAGEOMIN on mineral separates of biotite and ground Los Colorados caldera constitutes an outstanding feature, as well as the

A Immediately south of C Quebrada Los Colorados caldera Barrancas Blancas Ash LapilliBombs 91 m Massive lapilli-tu

Ash Lapilli Bombs 80 m Aguas Blancas Volcano lavas Pumice-rich massive lapilli-tu 8.644 ± 0.045 Ma, Laguna Verde Ignimbrite 8.161 ± 0.056 Ma ca. 3.8 Ma Lithic and amme rich welded lapilli-tu 80 m

B Quebrada Massive lithic breccia 9.419 ± 0.042 Ma Tres Puntas Laguna Verde Ignimbrite Ash LapilliBombs ca. 3.8 Ma COL-2 47 m Fiamme rich welded lapilli-tu

8.97 ± 0.11 Ma Fiamme rich highly welded lapilli-tu 60 m COL-2 9.797 ± 0.035 Ma 40 m 8.71 ± 0.03 Ma COL-2 Lava-like vitrophyre 60 m

9.849 ± 0.036 Ma Intrusive

8.31 ± 0.12 Ma Blocky lava ow 8.13 ± 0.09 Ma, 8.466 ± 0.015 Ma 8.65 ± 0.03 Ma Tiny normal jointing 40 m COL-1 20 m COL-1 Platy jointing 40 m 8.55 ± 0.03 Ma 8.66 ± 0.03 Ma 9.620 ± 0.032 Ma Normal-graded pumices COL-1 9.56 ± 0.03 Ma Sharp contact Gradational contact

COL-0 Erosion contact San Andrés 9.09 ± 0.04 Ma 20 m Ignimbrite

20 m

Figure 3. Key sections exhibiting the main Los Colorados ignimbrite (LCI) facies and stratigraphic relationships. The location of each COL-0 section (A to C) is shown in digital elevation model images of Figure 1. Some stratigraphic San Andrés Ignimbrite León Muerto and lithologic features are shown in corre- ca. 10 Ma granodiorite sponding photographs on Figure 4. Ages listed ca. 254 Ma in Table 1 are also indicated. Younger ages of biotites in columns B and C are due to Ar Barrancas Blancas Ignimbrite ca. 10 Ma degassing produced by the younger Laguna Verde ignimbrite above LCI.

TABLE 1. SUMMARY OF NEW Ar/Ar AGES AND PUBLISHED AGES OF LOS COLORADOS IGNIMBRITE AND RELATED LAVAS Age spectrum Isochron analysis UTM WGS84 19S Integrated Ar/Ar Plateau Ar/Ar 40Ar/36Ar Isochron age Analyzed Sample EN Lithology Published age age (Ma ± 2σ) age (Ma ± 2σ)N*** MSWD intercept (Ma ± 2σ)N*** MSWD material Reference 100513-1A 547055 7099707 Vitrophyric ignimbrite8.13 ± 0.09* –––– ––––Hornblende Naranjo et al. (2016) 280214-6C 558222 7153231 Andesite–8.15 ± 0.13 8.161 ± 0.056 8 of 8 0.09 295.2 ± 3.0 8.16 ± 0.16 8 of 8 0.11 Groundmass This work 140513-1A 550428 7117524 Ignimbrite8.31 ± 0.12* –––– –––– BiotiteNaranjo et al. (2016) 190214-2 548300 7100403 Vitrophyric ignimbrite8.466 ± 0.015* –––– –––– BiotiteNaranjo et al. (2016) 140513-9 554165 7105539 Ignimbrite8.55 ± 0.03* –––– –––– BiotiteNaranjo et al. (2016) 280214-5 559114 7149639 Basaltic andesite–8.628 ± 0.087 8.644 ± 0.045 8 of 8 1.3295.4 ± 2.8 8.641 ± 0.096 8 of 8 1.4GroundmassThis work 140513-8 554060 7105467 Vitrophyric ignimbrite8.65 ± 0.03* –––– –––– BiotiteNaranjo et al. (2016) 100513-5C 548382 7098475 Ignimbrite8.66 ± 0.03* –––– –––– BiotiteNaranjo et al. (2016) 140513-1B 550428 7117524 Ignimbrite8.71 ± 0.03* –––– –––– BiotiteNaranjo et al. (2016) 140513-5 553624 7110362 Ignimbrite8.97 ± 0.11* –––– –––– BiotiteNaranjo et al. (2016) 100513-5B 548382 7098475 Ignimbrite 9.09 ± 0.04* –––– –––– BiotiteNaranjo et al. (2016) 280214-6B 558222 7153231 Ignimbrite–9.404 ± 0.087 9.419 ± 0.042 8 of 8 1.35 295.7 ± 3.8 9.413 ± 0.070 8 of 8 1.5 BiotiteThis work 150513-9 550667 7117002 Ignimbrite 9.56 ± 0.03 –––– –––– BiotiteNaranjo et al. (2016) ID-65** 594351 7153873 Ignimbrite (pumice) 9.6 ± 0.2 –––– ––––Plagioclase Kraemer et al. (1999) 280214-4A 558618 7149313 Pumice – 9.371 ± 0.081 9.620 ± 0.032 6 of 8 1.2294.0 ± 7.7 9.64 ± 0.11 6 of 8 1.4 BiotiteThis work 280214-6A 558222 7153231 Ignimbrite–9.788 ± 0.097 9.797 ± 0.035 8 of 8 0.89295.4 ± 1.5 9.792 ± 0.049 8 of 8 0.98 BiotiteThis work ID-47** 592898 7155703 Ignimbrite (pumice) 9.8 ± 0.5 –––– –––– Biotite Kraemer et al. (1999) 280214-7 572544 7152641 Vitrophyric ignimbrite– 9.53 ± 0.10 9.849 ± 0.036 6 of 8 0.3296.6 ± 5.9 9.829 ± 0.095 8 of 8 0.31 BiotiteThis work Note: MSWD—mean square of weighted deviate. Preferred ages in bold. *Possibly reset age. **K-Ar ages N***—number of plateau/isochron steps used in regression

remarkable concentration of Pliocene–Middle Holocene volcanism (<3.5 Ma) caldera wall is partially buried with scalloped landslide scars and their corre in the area of Lazufre intumescence. Two main NE-SW– and NS-oriented sponding deposits (Fig. 2). thrust systems dominate the structural architecture of this arc segment A remarkable 14-km-diameter negative aeromagnetic anomaly had been (Fig. 1; Naranjo et al., 2018). recognized in the LCC (see figure 3 in Chernicoff et al., 2002 and figure 2 in Richards et al., 2013). This anomaly includes the area where Aguas Calientes, Río Grande, and Abra Grande volcanoes (<7 Ma) grew, indicating the latency Los Colorados Caldera and Ignimbrite of the magmatic system (Fig. 2). In addition, the subsided caldera floor is over lain by a fill of thick ignimbrites, landslide breccias, and caldera-lake sediments Immediately adjacent to the SE of the present Lazufre intumescence, we (Salar de Aguas Calientes). A 35–70-m-thick outflow ignimbrite sheet contains recognized a 33-km-diameter Los Colorados caldera (LCC), partially outlined proximal lithic breccias and comprises stratified multilayered deposits with by Ruch and Walter (2010), the source of the Los Colorados ignimbrite (LCI) significant variations in facies lithologies that are traceable up to 80 km to the (Figs. 1 and 2). The almost circular caldera collapse destroyed Middle Miocene south from LCC (Fig.1). This ignimbrite invariably overlies the 9.2–9.8 Ma San volcanoes with reported ages older than 10 Ma and hosts volcanoes of ca. Andrés ignimbrite to the south, and is itself covered by 8.16–8.64 Ma lavas, 7 Ma (Richards et al., 2013). Relatively young lavas of the Pleistocene–Holo and massively by the 3.8 Ma Laguna Verde ignimbrite at the southernmost cene Lastarria–Cordón del Azufre and Bayo volcanic complexes (<3.5 Ma) outcrops (Naranjo et al., 2018) (Table 2; Figs. 3B and 3C). Seggiaro et al. (2007) flowed into the caldera, obliterating its NW margin, while well-exposed SW mapped these facies lithologies as separated Los Colorados, Los Patos, and and southern topographic boundaries (caldera rim) reach up to 800-m-high Vitrofídica ignimbrites. scarps. This conspicuous caldera wall exhibits Early Miocene ignimbrite The LCI is a dark-pink to dark-gray pumice-rich ignimbrite with two main sheets interbedded within Atacama Gravels piedmont deposits. The SE cal facies layers (COL-1 and COL-2; Table 2 and Fig. 3). The lower facies corre dera rim is roofed by the Late Miocene Alto Quebrada Honda volcano, which sponds to a massive lapilli-tuff deposit with characteristic dark-pink color, with subsequently suffered a sector collapse originating a debris avalanche deposit beige, brown, and gray, fibrous and tubular pumices, along with a few purple, (including substrate pre-caldera blocks) into the caldera floor (Fig. 4A). The black, and reddish, porphyritic and scoriaceous lithics, supported in a shard-

GEOSPHERE | Volume 15 | Number 1 Naranjo et al. | Geological evolution of the Lazufre segment Downloaded from https://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/15/1/47/4619088/47.pdf 52 by INGEMMET user on 07 June 2019 on 07 June 2019 by INGEMMET user Downloaded from https://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/15/1/47/4619088/47.pdf Research Paper covered by the Laguna Verde ignimbrite, 65 km SSW of the LCC rim. Red arrow indicates vehicle for scale. lower fiamme-rich layer, and the upper part corresponds to a fiamme-rich massive lapilli tuff (yellow bar for scale). (F) View to the south of southernmost outcrops of LCI, W, from location A in Figure 1, exhibiting the upper levels of LCI, covered by lava 0.056 Ma Aguas Blancas 8.161 ± flow, SSW of LCC rim. 9 km Platy jointing characterizes the facies are shown: a lower reddish layer grades upwards to (C) a glassy brown vitrophyre and (D) an upper gray vitrophyre with tiny normal joints. (E) Panoramic view to the avalanche deposits emplaced on the caldera floor are shown (see Fig. 2 for details). (B) Panoramic view to the SE, taken 12 km SSE of the LCC (yellow bar for scale). Three LCI part of the LCC rim. In the foreground, piedmont deposits above the Early Miocene ignimbrites are exposed on the caldera wall; in the center of the image, volcanoes and debris Figure 4. Photographs illustrating different aspects of Los Colorados caldera (LCC) and Los Colorados ignimbrite (LCI). (A) Panoramic view to the north taken from the southern COL- COL- Laguna D C F A 2 2 Ve rd e Ignimbri LC I te 20 Lithics-fiamm Plat m Fiamme-ric yj Lava-lik ointed e- ev rich massive hh itroph highly ighl yr yw lapilli e welded elde 25 tu dl ff B m apilli lapilli E tu tu ff ff

GEOSPHERE | Volume 15 | Number 1 Naranjo et al. | Geological evolution of the Lazufre segment 53 Research Paper

TABLE 2. DESCRIPTIVE CHARACTERISTICS OF LOS COLORADOS IGNIMBRITE A. Immediately south of Los Colorados caldera B. Quebrada Tres Puntas C. Quebrada Barrancas Blancas Light-gray welded pumice-rich lapilli tuff. Contains gray, yellow, Gray welded pumice-rich lapilli tuff with deformed pumices Gray lithic- and fiamme-rich lapilli tuff, lithics, and fiammes <3 cm, and pinkish fiammes up to 2 cm, scarce lithics <1.5 cm and spherulitic fiammes up to 3 cm, 3% crystals of with 5% crystals of plagioclase and biotite, immersed in a white COL-2 and 3% crystals of plagioclase and biotite, and even less plagioclase and biotite and scarce lithics <1 cm, in a to light-gray ash matrix with coarse glass shards. The upper part orthopyroxene. The matrix is white to light gray made of deformed shard-rich ash matrix. The upper part has platy of the deposit has platy jointing. shard-rich ashes. The lower 10 to 12 m has platy jointing. jointing and slightly higher degree of welding. Slightly orange-pink pumice-rich massive lapilli tuff with brown, Reddish-pink pumice-rich massive lapilli tuff with brown-reddish pink, white, and greenish vesicular fibrous pumices up to fibrous tubular pumices (up to 20 cm across) that grade Reddish-pink welded fiamme-rich lapilli tuff. Brown and gray 15 cm (but mostly <5 cm), 5% crystals of plagioclase, biotite, upward to fiammes. Scarce lapilli-sized lithics and 3% of fibrous pumices up to 4 cm, 3% crystals of plagioclase less clinopyroxene, and orthopyroxene, and very scarce lithics crystals of plagioclase and biotite, and some clinopyroxene. COL-1 and biotite, with less quartz, amphibole, and scarce <1 cm, immersed in a pink to light-orange shard-rich ash matrix. The matrix is made of shard-rich reddish-pink ashes. The lithics <2 cm. The matrix is composed of pink ashes with The welding degree increases to the top, forming a reddish pink base corresponds to a gray massive lapilli tuff similar to the eutaxitic deformed shards. fiamme-rich lapilli tuff and then a dark-brown to black vitrophyre matrix of the upper deposit, and the upper part corresponds with tiny normal jointing in its upper part. Finally, welding degree to a fiamme-rich tiny normal jointing vitrophyre. decreases, forming a red fiamme-rich lapilli tuff. Pink unconsolidated pumice-rich massive lithic breccia. Light-pink massive lapilli tuff with normal-graded pumices; yellow, Massive unconsolidated lithic breccia, with subround black to Pink pumices <5 cm and white <2 cm, lithics <3 cm. light-brown, pink, white, and white with pink border fibrous COL-0 reddish up to 30-cm-diameter volcanic clasts immersed in a 3% crystals of plagioclase and biotite, less amphibole, pumices up to 30 cm, scarce lithics <3 cm and 5% crystals of pink- to beige-colored fine ash matrix. orthopyroxene, and clinopyroxene. Pink shard-rich ash plagioclase, biotite, and less orthopyroxene and quartz, in a matrix. light-pink shard-rich ash matrix.

rich, ash-sized matrix with biotite and quartz crystals. At proximal facies, it (2005), for example, biotites can be partially but uniformly outgassed, pos includes a basal, massive, lithic lag-breccia layer. The degree of welding grad sibly because of enhanced diffusion parallel to the cleavage, obtaining ages ually increases upwards. Thus, the upper facies varies from a black fibrous, younger than real eruption age. glassy, welded layer with decimeter fiamme to a dark-gray lava-like vitrophyric level (Fig. 4B). Locally, subaqueous facies with tiny normal jointing filled a basin that was formed between reverse faults to the SE of the caldera, gener Pre-Caldera Rocks ating a 5.5-km-diameter and 400-m-deep vapor explosion pit. These facies are distributed immediately to the SE of the caldera margin and may evidence the Pre-volcanic arc oldest basement rocks are a succession of Ordovician– presence of a lake at that age (Figs. 1 and 4B–4D). The welding of the higher Permian metasedimentary rocks, Permian dacitic breccias and lavas (La Tabla level decreases again, turning into gray porphyric-lithic–rich pink and yellow Formation), and Permian monzogranites and granodiorites (Seggiaro et al., granular pumices (Fig. 4E). 2007; Naranjo et al., 2013b). Late Eocene to Oligocene sandstones with inter According to the deposit exposures, we consider minimum average thick bedded conglomerates formed the Paleogene backarc intermontane deposits ness of 50 m with an original distribution area of ~2300 km2. Assuming all known as Vizcachera Formation (Seggiaro et al., 2007). These deposits are un mapped, nearly continuous outcrops, a total 115 km3 volume was calculated conformably mantled by Oligo-Miocene piedmont deposits as a consequence (Figs. 1 and 3). However, since the western distribution of LCI is obscured by of the Andean uplift (Atacama Gravels, Mortimer, 1973; Naranjo and Paskoff, younger volcanoes, this volume is underestimated. On the other hand, the cal 1985; Nalpas et al., 2008). Two Early Miocene ash-flow tuff units (Río Frío and dera topographic depth average based on 11 measurements is ~400 m. Given Pajonales ignimbrites) sourced at Aguilar caldera, 30 km to the southwest, are an area of 464 km2 for the collapsed caldera floor, we can estimate a 185 km3 also interbedded within the Atacama Gravels (Naranjo et al., 2013b; Naranjo volume for the space left by the caldera subsidence. The latter would corre et al., 2018). spond to a more realistic estimation. Remnants of constructional volcanoes and lava flows dated at 11.76 ± The eastern outcrops of LCI have ages between 9.4 and 9.8 Ma (Table 1 and 0.2 Ma (8 km north of LCC, Ar/Ar, sample AR-337, Richards et al., 2013), Fig. 3A), which are in agreement with K-Ar ages reported by Kraemer et al. 9.95 ± 0.39 Ma (within LCC floor, Ar/Ar, sample AR-421, Richards et al., 2013), and (1999; samples ID-47 and ID-65, Table 1). However, slightly younger ages be 10.5 ± 0.7 Ma (19.5 km southwest of LCC, K-Ar, GN-493, Naranjo and Cornejo, tween 8.1 and 9.1 Ma have been obtained for the western outcrops of this 1992) are distributed in the area surrounding LCC (Fig. 2). The overlap of nu ignimbrite (Naranjo et al., 2016; Figs. 3B, 3C, and 4F). The latter ages could merous younger volcanoes obscures the continuity of their extension to the have been reset as LCI was mantled by the 3.8 Ma Laguna Verde ignimbrite, west during the 14–10 Ma period (Naranjo et al., 2018). Nevertheless, they are producing argon loss through the massive heating event. According to Dickin particularly exposed along the LCC scarp.

Post-Caldera Constructional Volcanoes in the Lazufre Segment regional east-vergent thrust faults are recognized along the Lazufre segment (Fig. 1). A NNW-SSE lineament is also recognized. The east-vergent Peder After formation of the LCC, numerous stratovolcanoes, volcanic com nales-Arizaro is a thrust fault (Naranjo et al., 2013a) that extends for more than plexes, and lavas were placed across the volcanic arc, including those within 260 km from the pre-Cordillera cutting across the volcanic arc to the NE. The and on the rim of LCC, such as Aguas Calientes, Río Grande, and Abra Grande fault scarp locally exceeds 900 m at Salar Aguilar obliterating the morpho volcanoes. Those that grew inside the LCC reached a total volume of ~75 km3, logical NW scarp of the Lower Miocene Caldera Aguilar, which last collapsed and were erupted ca. 7 Ma (Figs. 2, 4A, and 5). 16.5 m.y. ago. The explosion of the 22-km-diameter Juan de la Vega Caldera, Notably, the two subsequent volcanic stages were concentrated within the in turn, cut the fault scarp at 14 Ma in that sector (see figures 2A and 5 in current Lazufre bulging area (Fig. 2). The first stage occurred during the Late Naranjo et al., 2018). Thus, the last activity of Pedernales-Arizaro thrust fault is Pliocene to Pleistocene period and totaled 85 km3, and the latest started at constrained to ca. 14 Ma. On the other hand, between La Isla and Río Grande the Late Pleistocene and have persisted until the Christian Era (i.e., Caletones salars, its escarpment was covered by Upper Miocene to Pliocene volcanoes, Cori-Escorial, Lastarria, Azufre, Cordón del Azufre, and Bayo volcanic com emplaced along 90 km on the hanging wall. The Lazufre intumescence com plexes, Naranjo, 2010; Naranjo et al., 2013a), with a total volume of ~33 km3 prises the northern part of these volcanoes (Fig. 1). (Figs. 2 and 5). Parallel to the Pedernales-Arizaro thrust fault, 75–90 km to the southeast, the 170-km-long, NE-oriented Antofalla thrust fault is the eastern border of the volcanic arc in this segment (Fig. 1). According to Kraemer et al. (1999), Tectonism this fault was active since the Late Oligocene. Volcanoes dated at 11 Ma were built above this structure, sealing its last activity (Kraemer et al., 1999; Richards A dominantly compressive tectonic regime has been recognized in the et al., 2006). southern Puna from the Late Oligocene to Holocene (Allmendinger et al., 1997; Between these structures, we recognized shorter, up to 70-km-long Kraemer et al., 1999; Naranjo et al., 2018). Various relevant NE-SW–oriented, east-verging thrusts, with escarpments up to 700 m, 15–25 km east of Salar de

Figure 5. Volumes of volcanic products and ignimbrite accumulated from the Late Miocene in the Lazufre–Los Colorados Caldera segment in the Andean volcanic arc. Although it is not possible to know the volume of volcanoes before the collapse of the caldera, it is noteworthy that volumes involved for each stage since then are similar (see text for details). LCI— Los Colorados ignimbrite.

Las Parinas. These structures apparently controlled the distribution of a basin Lazufre bulging zone and the Late Miocene collapsed LCC form a NW-SE– that possibly housed a shallow-water lake, where the subaqueous facies of the aligned area of 65-km-long by 40-km-wide, which is arranged at a conjugate Los Colorados ignimbrite originated and an ~5-km-diameter phreatic explo 30° angle with respect to the ca. 14–13 Ma east-vergent Pedernales-Arizaro sion crater was formed (Figs. 1 and 4B–4D). thrust fault (Fig. 6). The LCC is located in the footwall block, and Lazufre is The 190-km-long, Imilac–Salina del Fraile fault extends from Salar Punta located in the hanging-wall block, >500 m topographically above the LCC rim. Negra to Salar Antofalla along a NNW-SSE strike. This fault is covered by We propose a cause-and-effect relationship between a maximum compression volcanoes dated at ca. 8 Ma (Naranjo and Cornejo, 1992), and its extension NW-SE (large arrows in Fig. 6), which would have controlled the ancient activ to the north controls the volcano alignment since that time, including Río ity of the Pedernales-Arizaro thrust fault, with the generation of extensional Grande (2–1 Ma), Caletones Cori-Escorial (0.5–0.1 Ma), Llullaillaco (<1.5 Ma), domains oblique to the direction of maximum compression (small arrows in and Negrillar (1.2 Ma) volcanoes (Fig. 6) (Naranjo and Cornejo, 1992; Garde Fig. 6), which would have favored the ascent and accumulation of large vol weg et al., 1994; Richards and Villeneuve, 2001, 2002; Naranjo et al., 2013a). At umes of magma, originating the LCC reservoir and Lazufre intumescence. Ac the southern Imilac–Salina del Fraile fault, we recognized the Salina del Fraile cording to Ruch and Walter (2010), local extensions could explain the NE-SW pull-apart basin, previously identified as a NNE sinistral strike-slip fault (Reijs elongation of the Lazufre bulging zone. We suggest the structural and temporal and McClay, 2003). Nevertheless, our observations allow us to redefine this evidence in the Lazufre segment implies very strong tectonic control on the basin as a consequence of a NW-SE dextral transtensive fault as shown by emplacement timing of both LCC and the Late Pliocene to Holocene magmatic the oblique dextral 2.5 km displacements of the western coast of the Antofalla systems, thus promoting their maturity. Salar (Figs. 1 and 6). This is in agreement with structural observations given by The volcanic volumes emitted through time are shown in Figure 5 by the Seggiaro et al. (2007, p. 39). In fact, these authors argued that a dextral running volume estimates and accumulation rates along the LCC-Lazufre bulging zone fault with a north-south course was transferred southward in an anticline fold lineament. The LCC formed at ca. 9.4–9.8 Ma, and, judging by the ages of the with an axis of sigmoidal NNW-SSE–oriented geometry. volcanoes in its surroundings, it can be estimated that its reservoir of at least 185 km3 could be formed within a 2 m.y. time interval at a magmatic accu mulation average rate of ~92,500 m3/yr. During the ca. 7–3.5 Ma interval, this DISCUSSION rate decreased to ~21,400 m3/yr (Fig. 5). Subsequently, from the Late Pliocene, the activity of the volcanic arc moved to the NW, to the Lazufre ~1500 km2 The results do not allow us to identify a correspondence between the dis bulging area, at a lava emission rate of ~34,000 m3/yr for the 3.5–1 m.y. period tribution of the volcanoes and the NW Archibarca and Culampaja lineaments (Figs. 2 and 5). Finally, during the Late Pleistocene–Holocene time interval, nor their proposed kinematics (i.e., Riller et al., 2001; Chernicoff et al., 2002; lava emission rates for these active volcanoes have remained at an average Ramelow et al., 2006; Richards et al., 2013). In contrast, based on geographic similar to the previous period with ~33,000 m3/yr. Thus, it seems evident that relationships, it is likely that the main structure recognized in the area, the NE the magmatic accumulation rate that preceded the formation of the LCC was Pedernales-Arizaro thrust fault, favored the formation of LCC and the volcanic nearly three times the estimated rates for the Late Pliocene to Holocene period migration to the Lazufre bulging zone through an inherited NW structure, con at Lazufre. For comparison, Ruch et al. (2009) estimated volume changes of jugated at ~30° to the main fault and parallel to the Imilac–Salina del Fraile fault ~13,000 m3/yr during the period 2003–2008 A.D., but attributable to a possible (Fig. 6). Based on available evidence, we consider that the migration of the vol source to be located within the Lastarria volcano edifice. However, Remy et al. canism from the LCC area to the Lazufre bulging zone may be a consequence (2014) estimated a volume change rate at Lazufre for the ~1996–2010 period, of variations in the shallow crust strain field along the kinematically inactive, whatever the source type, was ~12.5 × 106–14.8 × 106 m3/yr, volumes that are but inherited structures mentioned above, which efficiently favored the gener ~150–640 times greater than the rates estimated in Figure 5. In addition, they ation of magmatic plumbing systems, through conjugated extensions due to considered these volume estimations compatible with the peak intrusion rates tensional strain fields (see figure 7 in Naranjo et al., 2018). proposed by de Silva and Gosnold (2007) for the building of upper-crustal plu Although the identification of LCC has been previously delineated (e.g., tons associated with the Neogene volcanic activity of Altiplano-Puna volcanic Figure 1 in Ruch and Walter, 2010), we provide a geological-geomorphological complex, 250 km north of Lazufre. We think that, unless a caldera explosion description of this remarkable volcanic structure, the ignimbrite that originated would be imminent in the Lazufre area, such a ground surface displacement from its explosion, and the time context of its activity. In addition to new Ar/Ar would be mainly interpreted as strain partitioning tectonic deformation at ages, we reinterpret some of those provided by Richards et al. (2013), which brittle crust levels, rather than instantaneous intrusion rates (de Saint Blan allows us to give them an adequate geological framework. For example, three quat et al., 2006). Strain partitioning has been argued to be the result of varia ages of Early Miocene ignimbrites were sampled in the LCC wall landslide de tions in the stress field of shallow crustal regions, promoting the formation posits, which include Paleozoic substrate blocks, Early Miocene ignimbrites, of extensional fracture domains and magma ascent (Naranjo et al., 2018, and and ancient lavas (Figs. 2 and 4A). references therein).

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GEOSPHERE | Volume 15 | Number 1 Naranjo et al. | Geological evolution of the Lazufre segment 57 Research Paper

CONCLUSIONS Chernicoff, C., Richards, J., and Zappettini, E., 2002, Crustal lineament control on magmatism and mineralization in northwestern Argentina: Geological, geophysical, and remote sens ing evidence: Ore Geology Reviews, v. 21, p. 127–155, https://doi .org/10.1016/S0169-1368 The global mapping of the area around the InSar Lazufre intumescence (02)00087-2. has remarkably improved during the past few years, allowing us to define in de Saint Blanquat, M., Habert, G., Horsman, E., Morgan, S.S., Tikoff, B., Launeau, P., and Gleizes, G., 2006, Mechanisms and duration of nontectonically assisted magma emplacement in the detail the geological context and temporal and structural evolution of the area. upper crust: The Black Mesa pluton, Henry Mountains, Utah: Tectonophysics, v. 428, p. 1–31, The bulging, interpreted as the consequence of the presence of a 10-km-depth https://doi.org/10.1016/j.tecto.2006.07.014. magmatic body, is located upon the hanging wall of Pedernales-Arizaro thrust, de Silva, S.L., and Gosnold, W.D., 2007, Episodic construction of batholiths: Insights from the a major NE-SW–trending Middle Miocene fault. On the footwall block of this spatiotemporal development of an ignimbrite flare-up: Journal of Volcanology and Geo thermal Research, v. 167, no. 1–4, p. 320–335, https://doi.org/10.1016/j.jvolgeores.2007.07.015. fault and immediately SE of InSar-identified Lazufre intumescence, the explo Dickin, A.P., 2005, Radiogenic Isotope Geology: Cambridge, UK, Cambridge University Press, sion of the 30-km-diameter Los Colorados caldera produced the eponymous 510 p., https://doi.org/10.1017/CBO9781139165150. 40 39 ca. 9.4–9.8 Ma and 115–185 km3 ignimbrite. Fleck, R.J., Sutter, J.F., and Elliot, D.H., 1977, Interpretation of discordant Ar/ Ar age-spectra of Mesozoic tholeiites from Antarctica: Geochimica et Cosmochimica Acta, v. 41, no. 1, p. 15– Since the Late Miocene–Pliocene, migration of volcanic activity from the 32, https://doi.org/10.1016/0016-7037(77)90184-3. Los Colorados caldera shifted toward the bulged Lazufre area. This migration Froger, J.L., Remy, D., Bonvalot, S., and Legrand, D., 2007, Two scales of inflation at Lastarria- could have been the result of variations in the shallow-crust strain field along Cordon del Azufre volcanic complex, central Andes, revealed from ASAR-ENVISAT interfero metric data: Earth and Planetary Science Letters, v. 255, no. 1–2, p. 148–163, https://doi .org a NW-oriented old inherited structure, conjugated to the NE-SW Pedernales - /10.1016/j.epsl.2006.12.012. Arizaro thrust fault. The upper brittle crust heterogeneity is superficially rep Gardeweg, M., Ramirez, C.F., and Davidson, J., 1993, Mapa geológico del área del Salar de Punta resented by geological units of different competence and structures. Thus, we Negra y del volcán Llullaillaco: Región de Antofagasta: Servicio Nacional de Geología y suggest that the study of the relationship between tectonics, magmatism, and Minería, Documentos de Trabajo No. 5, scale 1:100,000, 1 sheet. Gardeweg, M., Pino, H., Ramírez, C.F., and Davidson, J., 1994, Mapa Geológico del Área de Imilac volcanism must be addressed on a local scale. y Sierra de Almeida 1:100,000: Región de Antofagasta: Servicio Nacional de Geología y Although the lava accumulation rate estimated from the Late Pliocene to Minería, Documentos de Trabajo no. 7, scale 1:100,000, 1 sheet. the present is much lower than the rate estimated for the generation of the Grosse, P., van Wyk de Vries, B., Petrinovic, I.A., Euillades, P.A., and Alvarado, G., 2009, Mor phometry and evolution of arc volcanoes: Geology, v. 37, p. 651–654, https://doi .org/10.1130 LCC, it is crucial to keep monitoring in the short to middle term to detect an /G25734A.1. eventual variation in the deformation rate through different methods. Also, in Grosse, P., van Wyk de Vries, B., Euillades, P.A., Kervyn, M., and Petrinovic, I.A., 2012, Systematic order to improve estimates of magmatic accumulation rates, it would be valua morphometric characterization of volcanic edifices using digital elevation models: Geomor ble to perform magmatic residence studies of the LCI and subsequent lavas. phology, v. 136, p. 114–131, https://doi.org/10.1016/j.geomorph.2011.06.001. Kraemer, B., Adelmann, D., Alten, M., Schnurr, W., Erpenstein, K., Kiefer, E., van den Bogaard, P., and Görler, K., 1999, Incorporation of the Paleogene foreland into the Neogene Puna pla teau: The Salar de Antofalla area, NW Argentina: Journal of South American Earth Sciences, ACKNOWLEDGMENTS v. 12, p. 157–182, https://doi.org/10.1016/S0895-9811(99)00012-7. Mortimer, C., 1973, The Cenozoic history of the southern Atacama Desert, Chile: Journal of the This work is a contribution to the Plan Nacional de Geología (PNG) of Servicio Nacional de Geological Society of London, v. 129, no. 5, p. 505–526, https://doi.org/10.1144/gsjgs.129.5 Geología y Minería (National Geology and Mining Survey) (abbreviated SERNAGEOMIN), Chile, .0505. and the cooperation agreement for geological and thematic maps elaboration with Servicio Nalpas, T., Dabard, M.P., Ruffet, G., Vernon, A., Mpodozis, C., Loi, A., and Herail, G., 2008, Sedi Geológico y Minero (abbreviated SEGEMAR), Argentinean Geological Survey. We thank Marcos mentation and preservation of the Miocene Atacama Gravels in the Pedernales-Chañaral Lienlaf for his valuable drawing support. We are greatly appreciative of Gonzalo Núñez, José Luis Area, Northern Chile: Climatic or tectonic control?: Tectonophysics, v. 459, p. 161–173, Díaz, Hugo Neira, Roberto Flores, Eduardo Martínez, and René Urbina for invaluable assistance in the field. The authors are grateful for the valuable and important suggestions made by refer https://doi.org/10.1016/j.tecto.2007.10.013. ees Jorge Clavero, Morgan Salisbury, and Rodrigo Iriarte. Finally, we greatly appreciate Charles Naranjo, J.A., 2010, Geología del Complejo Volcánico Lastarria, Región de Antofagasta: Servicio Stern for the final revision of the manuscript and Rodrigo del Potro for editorial handling. This is Nacional de Geología y Minería, Carta Geológica de Chile, Serie Geología Básica, No. 123, a contribution to the PLUTONS project (http://plutons .science .oregonstate .edu), financed by the scale 1:25,000, 1 sheet, 33 p. text. National Science Foundation. 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