Eruptive chronology of the Acoculco caldera complex – A resurgent caldera in the eastern Trans-Mexican Volcanic Belt (México)

Journal of South American Earth Sciences xxx (xxxx) xxxx

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Eruptive chronology of the Acoculco caldera complex – A resurgent caldera in the eastern Trans-Mexican Volcanic Belt (México) Denis Ramón Avellána,∗, José Luis Macíasb, Paul W. Layerc, Giovanni Sosa-Ceballosb, Martha Gabriela Gómez-Vasconcelosd, Guillermo Cisneros-Máximob, Juan Manuel Sánchez-Núñeze, Joan Martíf, Felipe García-Tenoriob, Héctor López-Loerag, Antonio Polah, Jeff Benowitzc CONACYT – Instituto de Geofísica, Universidad Nacional Autónoma de México, Antigua Carretera a Pátzcuaro 8701, 58190 Morelia, Michoacán, Mexico Instituto de Geofísica, Universidad Nacional Autónoma de México, Antigua Carretera a Pátzcuaro 8701, 58190 Morelia, Michoacán, Mexico College of Natural Science, Mathematics and Geophysical Institute, University of Alaska Fairbanks, Fairbanks, AK 99775, USA d CONACYT – Instituto de Investigaciones en Ciencias de la Tierra, Universidad Michoacana de San Nicolás de Hidalgo, Francisco J. Mujica s/n, Felicitas del Río, 58040, Morelia, Michoacán, Mexico e Instituto Politécnico Nacional-CIIEMAD, Miguel Othón de Mendizábal s/n. Col. La Escalera, C.P. 07320, Del. Gustavo A. Madero, Ciudad de México, Mexico f Instituto de Ciencias de la Tierra Jaume Almera, CSIC, LLuis Sole Sabaris, s/n, 08028 Barcelona, Spain g División de Geociencias Aplicadas, Instituto Potosino de Investigación Científica y Tecnológica A.C., Camino a la Presa San José 2055, Lomas 4a Sección, C-P. 78216, San Luis Potosí S.L.P, Mexico h Escuela Nacional de Estudios Superiores, Universidad Nacional Autónoma de México, Antigua Carretera a Pátzcuaro 8701, 58190 Morelia, Michoacán, Mexico a

b c

A R T I C LE I N FO

A B S T R A C T

Keywords: Geology Geochronology Geothermal energy Acoculco caldera Puebla

The Acoculco caldera complex (ACC) is located in the eastern part of the Trans-Mexican Volcanic Belt in the northern part of the State of Puebla. The complex sits at the intersection of two regional fault systems with NESW and NW-SE orientations. The ACC was built atop Cretaceous limestones, the Zacatán basaltic plateau of unknown age, early Miocene domes (~12.7–10.98 Ma), and Pliocene lava domes (~3.9–3 Ma). Detailed field mapping and stratigraphy studies complemented by 40Ar/39Ar and 14C dating allowed the division of the ACC volcanic succession into 30 volcanic units. Based on the new results and previous studies, the ACC eruptive chronology was grouped in four eruptive phases: syn-caldera, early post-caldera, late post-caldera, and extracaldera. Inception of the ACC volcanism began around 2.7 Ma with the dispersion of an andesitic ignimbrite followed by the collapse of the magma chamber roof as attested by the presence of a lithic breccia in isolated parts of the caldera rim. The collapse produced a 18 × 16 km caldera depression which was partly filled by the ignimbrite (total volume of ~127 km3) followed by the establishment of an intracaldera lake of unknown total extension. Early post-caldera collapse activity (2.6–2.1 Ma) was restricted within the caldera producing 27 km3 of lava flows and domes dominantly of basaltic trachyandesite to basaltic composition. Late post-caldera collapse activity (2.0- < 0.016 Ma) migrated dominantly to the caldera rim and periphery emplacing 90 km3 of magma as rhyolitic domes, lava flows, scoria cones, and two younger ignimbrites. The 1.2 Ma Encimadas ignimbrite (26 km3) was vented through the eastern margin of the caldera and dispersed to the northeast, and the 0.6–0.8 Ma Tecoloquillo ignimbrite and dome (11 km3) erupted from the southwestern margin of the caldera. The most recent eruption of this phase was vented close to the southeastern caldera rim producing the Cuatzitzingo (< 16,710 ± 50 years BP) scoria cone. Extra-caldera activity (2.4–0.19 Ma) of the Apan–Tezontepec volcanic field produced scoria cones and lava flows of basaltic trachyandesite to basaltic andesite composition that are interbedded with the products of the caldera complex. Aeromagnetic data further constrains the edge of the caldera rim and is consistent with the presence of at least four intrusive bodies at depths of > 1 km hosted in the Cretaceous limestones. These bodies might represent a series of horizontal mafic intrusions located at different depths that provide the energy that maintains the Acoculco geothermal system active.

∗

Corresponding author. E-mail address: [email protected] (D.R. Avellán).

https://doi.org/10.1016/j.jsames.2019.102412 Received 6 August 2019; Received in revised form 2 November 2019; Accepted 9 November 2019 0895-9811/ © 2019 Elsevier Ltd. All rights reserved.

Please cite this article as: Denis Ramón Avellán, et al., Journal of South American Earth Sciences, https://doi.org/10.1016/j.jsames.2019.102412

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1. Introduction

a dry-hot rock reservoir by CFE (Lorenzo-Pulido et al., 2010) and is the site of an on-going European-Mexican effort to develop geothermal energy from non-conventional sources called GEMex Project (Calcagno et al., 2018). In this contribution, we analyzed previous studies combined with new fieldwork to present a simplified volcanological map and a new composite stratigraphic column of Acoculco. The results are assisted by 24 new 40Ar/39Ar ages and one 14C radiometric date of the volcanic units that combined with previous information set a refined evolution model of the caldera. This information is crucial to present the chronology of the caldera volcanic complex through time. To gain new insights on the caldera structure, we produced aeromagnetic models of the area, analyzed the morphostructural distribution of the volcanic units, and their correlation with subsurface units in geothermal wells. Based on this new information, we propose that the late Pleistocene shallow intrusions are still the heat source of geothermal activity beneath Acoculco.

Silicic collapse calderas are volcanic depressions resulting from the subsidence of the magma chamber roof caused by the rapid withdrawal of magma during an explosive eruption (Lipman, 2000; Martí et al., 2008; Geyer and Martí, 2014). The formation of a collapsed caldera is still an enigmatic geological phenomenon because of the structural complexity involved in such type of volcanic eruptions. Additionally, silicic caldera eruptions represent a high risk due to the large amount of magma and eruption rates involved (Costa and Martí, 2016). The resulting caldera depression may represent the site of important ore deposits and high enthalpy geothermal reservoirs. In many active volcanic areas, such as Iceland, New Zealand, Costa Rica, Japan, Indonesia, among several others, an important part of their government energy requirements is covered by the exploitation of geothermal resources associated with collapse caldera systems. This makes the study of caldera systems of great interest to modern societies. In México, geothermal energy has been used since 1959, when a program led by the Comisión Federal de Electricidad (CFE) started to explore and exploit some of these potential energy resources, most of them located in calderas of the Trans-Mexican Volcanic Belt (TMVB) (Hiriart et al., 2011). In the particular case of geothermal fields installed in calderas, it is very important to determine the mechanism that formed the caldera and the post-collapse internal structure (Bibby et al., 1995; Di Napoli et al., 2011; Molina et al., 2014; Afanasyev et al., 2015) to better understand fluid paths and the location of reservoirs. It is crucial to determine the exact structural limits of the caldera (e.g., Molina et al., 2014), the stratigraphy and age of the caldera-forming deposits and the distribution of thermal anomalies. In this respect, the Acoculco caldera located in the eastern TMVB (Fig. 1) represents an excellent case scenario to study the internal structure of the edifice, its eruptive chronology, and surface manifestations of geothermal activity concerning fractures and faults. Acoculco has been considered for years

2. Geological setting The Acoculco Caldera Complex (ACC) is located within the TMVB, a calc-alkaline volcanic arc produced by the subduction of the Cocos and Rivera plates beneath the North American plate at the Middle-American Trench (Pardo and Suárez, 1995) (Fig. 1). The ACC is located in the eastern sector of the TMVB (Pasquarè et al., 1991). Acoculco occurs ~140 km northeast of Popocatépetl volcano, which defines in this region the active front of the TMVB (Siebe et al., 1995; Macías et al., 2012) (Fig. 1). Acoculco is located over a 45-50 km-thick continental crust (Urrutia-Fucugauchi and Flores-Ruiz, 1996), and sits at 400 km from the trench, where the Cocos plate plunges into the mantle (PérezCampos et al., 2008). This area is under an NW-SE oriented extensional regime, as deduced from the alignment of volcanic vents, dike orientations, extension fractures, and kinematics of faults at the Apan-

Fig. 1. Regional tectonic configuration of Central México showing the active subduction of the Rivera and Cocos Plates beneath the North American Plate at the Middle-American Trench. The dotted black line depicts the boundary of the Trans-Mexican Volcanic Belt (TMVB) and the location of the Acoculco Caldera Complex and other volcanoes (LP, La Primavera; C, Fuego de Colima; T, Tancítaro; Am, Amealco; Hc, Huichapan; To, Nevado de Toluca; P, Popocatepetl; M, Malinche; O, Pico de Orizaba; and Hm, Humeros). The shaded relief map and bathymetric data was acquired and modified from Ryan et al. (2009). 2

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analysis with rose diagrams. Stereonet 9 software was used for plotting the fault planes and poles, from where the compression and tension areas were inferred into the synthetic right dihedral diagram from the two pairs of conjugate fault systems, according to their slip models (e.g., De Vicente et al., 1992). To estimate the volume of the volcanic units we used the geological map, the Digital Elevation Model (DEM), Spot-6 satellite images (1.5-m panchromatic and 6-m multispectral), and the shaded relief DEM (15-m resolution) in the ArcMap 9.3 software. The difference between the actual topography and the geomorphologic element of each unit was used to obtain a z value, and to create a 3D surface geology on the shaded relief DEM and the volume with the Surface Difference tool in ArcMap 9.3.

Acoculco region (García-Palomo et al., 2002, 2018). The Acoculco caldera is bounded to the west by the Apan-Tezontepec volcanic field (ATVF) (~3–0.2 Ma) (García-Palomo et al., 2002; García-Tovar et al., 2015), to the north and to the east by Cretaceous limestones of the Sierra Madre Occidental (Avellán et al., 2018), and to the south by early Miocene volcanics of the TMVB (García-Palomo et al., 2002). Prior to the 80′s, the Acoculco geology was not extensively studied (e.g., Ledezma-Guerrero, 1987; Castro-García and Cordoba, 1994). The CFE produced the first regional map of the area (De la Cruz-Martinez and Castillo-Hernández, 1986) followed by other detailed studies such as rock dating (López-Hernández and Castillo-Hernández, 1997; LópezHernández and Martínez, 1996). Later, geophysical studies attempted to understand the internal structure of the caldera (Alatriste-Vilchis et al., 2005; Campos-Enriquez et al., 2003) and the hydrological conditions (Huizar-Álvarez et al., 1997) of the Apan region. The first volcanological study of the region was carried out by López-Hernández (2009), who concluded that Acoculco was an 18-km wide caldera, active from ~1.7 to ~0.2 Ma. These authors considered that Acoculco was nested within the larger 32-km wide Tulancingo Caldera, which was active between ~3.0 and ~2.7 Ma. Recently, Avellán et al. (2018) presented the first detailed geologic map and stratigraphy of the caldera supported by nine 40Ar/39Ar dates. These authors concluded that the caldera has a semi-circular shape (18–16 km) and that was active from 2.7 to 0.06 Ma, thus not corresponding with the previous suggested timing of the caldera formation (López-Hernández, 2009). This geology of Acoculco was used to present a simplified geologic map of the caldera to describe its geochemical evolution (Sosa-Ceballos et al., 2018) and a preliminary 3D model of the Acoculco subsurface structure (Calcagno et al., 2018).

4. Volcanic stratigraphy In this study we analyzed the regional (García-Palomo et al., 2002; López-Hernández, 2009) and local geologic maps of the ACC and its surroundings (Avellán et al., 2018; Sosa-Ceballos et al., 2018; Calcagno et al., 2018), and the available regional geochronological data. We compiled a new simplified geologic map (Fig. 2) supported by 24 new 40 Ar/39Ar and one 14C date (Tables 1 and 2). The map includes three pre-Acoculco units, two pre-caldera, 30 ACC, and nineteen extra-caldera units. The ACC units were subdivided in syn-caldera, early postcaldera, and late post-caldera, corresponding to different phases of the caldera formation defined by their distribution, stratigraphic position, age, mineral and chemical composition (Fig. 3). The chemical variations of the ACC rocks have been previously documented and discussed and will not be treated in this contribution (Sosa-Ceballos et al., 2018). Next, we succinctly described all units that span in age from ~2.7 Ma to < 0.016 Ma with their location given with respect to the caldera rim.

3. Methods Twenty-four whole-rock samples were crushed, sieved and washed in deionized water for 40Ar/39Ar isotopic dating (Table 1). For most samples, phenocrysts-free groundmass chips (whole rock) were separated for dating. For sample Ac11, both a plagioclase mineral separate and a whole rock sample were separated and dated, and for Ac69, two different rock types were analyzed. The samples were irradiated in position 5C at the McMaster University Nuclear Reactor in Hamilton, Canada, for 0.75 MWh. The standard mineral TCR-2 with an age of 28.619 Ma (Renne et al., 2010) was used to calculate the irradiation parameter, “J”. Samples Ac39 and Ac123, were irradiated with the standard mineral MMHb-1 with an age of 523.5. The standards were fused and the unknowns were step-heated, except for Ac90. The samples step-heated using a laser dating system consisting of 6W argon-ion laser at the Geophysical Institute, University of Alaska Fairbanks, following the technique described in Layer (2000) and Layer et al. (2009). For Sample Ac90, an obsidian, 7 small shards were fused, with 6 of the 7 yielding enough gas to calculate a fusion age. The samples were analyzed in a VG3600 mass spectrometer and the measured Ar isotopes were corrected for system blank, mass discrimination and Ca, K, and Cl interference reactions, according to procedures outlined in McDougall and Harrison (1999) and using the standards reported in Renne et al. (2010). System blanks were 2 × 10-16 mol 40Ar and 2 × 10-18 mol 36Ar, which are 5–50 times smaller than fraction volumes. Mass discrimination was monitored by running calibrated air shots. The structural analysis of the caldera system comprised a review of previous and new structural data, after the evaluation of digital elevation models, aerial photographs, topographic maps, fieldwork and the morphological evaluation of the landforms. The fault geometry was characterized by field measurements and geomorphic analyses of the faults using ArcMap 9.3 software on a 15 m-resolution digital elevation model from INEGI (Instituto Nacional de Estadística y Geografía). Grid references on maps are in the WGS 1984 UTM Zone 14N projection. Rozeta 2.0 software was used for plotting the fault data on stereographic projections (lower hemisphere) and to perform trend-frequency

4.1. Undifferentiated Cretaceous limestones (Ksl) The oldest rocks in the region are Cretaceous limestones (Ksl) of the Sierra Madre Oriental that are exposed to the northeastern, eastern and southeastern parts of the mapped area (Fig. 2). Good outcrops appear in the Tenexapa and Ajajalpan canyons, close to the towns of Chignahuapan and Zacatlán. At the Chignahuapan hot springs, these rocks occur as light-gray parallel stratified limestones with chert concretions, sometimes affected by vertical fractures filled with hydrothermal minerals (e.g., calcite). Although these rocks are not exposed inside the ACC, they were cut in the two geothermal CFE wells at depths of ~1,200 m in EAC-1 (López-Hernández, 2009), and 350–450 m in EAC-2 (Viggiano-Guerra et al., 2011). According to these authors, Ksl was intruded by a light-gray phaneritic granite found at the bottom of EAC1 drill hole at depths from 1800 to 2000 m. The intrusion produced a methamorphic aureole of skarns (Viggiano-Guerra et al., 2011). A sample analyzed from this well was an aplite made of alkali feldspar, plagioclase, quartz, amphibole, chlorite, and Fe-Ti oxides. An isochron 40 Ar/39Ar age of this sample yielded an age of 183 ± 36 ka (Table 1, isochron age; Fig. 4A), which may correspond to either younger intrusions beneath Acoculco or a reset age of an older regional plutonic body (López-Hernández et al., 2009; Calcagno et al., 2018).

4.2. Zacatlán basaltic plateau (Za) Za is a dark-gray, up to 200 m thick basaltic lava plateau that discordantly overlies the Cretaceous limestones near the Chignahuapan and Zacatlán towns (Fig. 2). The lava flow is aphanitic with columnar jointing and spheroidal weathering. The age of this unit is unknown, however, its stratigraphic position indicates that Za is younger than the Cretaceous limestones. 3

4

Ac94A Ac15

Ac90

Ac103

Ac80

Ac69A

Ac44

Ac69B

Ac113

Ac76

Ac98B

Ac107

Ac94B

Ac92

Ac72

Ac100

Ac82

Ac37

Ac89

Ac104

Ac11

Ac11

Early postcaldera Early postcaldera Early postcaldera TMVB TMVB

Early postcaldera Syn-caldera

Late postcaldera Late postcaldera Late postcaldera Late postcaldera Late postcaldera Late postcaldera Late postcaldera Late postcaldera Late postcaldera Late postcaldera Late postcaldera Late postcaldera Late postcaldera Late postcaldera Late postcaldera Late postcaldera Syn-caldera

Ac42

Aco901

ACC stage

Sample

Pald Pdhd

Srl

Vtal

Mtal

Aai

Atad

Aai

Alrd

Tal

Cbd

Amrc

Crcd

Trcd

Ard

Eri

Arcd

Msf

Mrcd

Arcd

Tr1

Tr1

aplitic dike

Plc

Unit

2216042 2190111

2208919

2205459

2198103

2200645

2195570

2200645

2196999

2201609

2202522

2213875

2216042

2213879

2199502

2204497

2190307

2199284

2208434

2194852

2192344

2192344

2203029

2199245

North

Location

585241 579819

574826

593663

586869

584104

592761

584104

584295

587830

592368

590387

585241

583057

578889

596510

595629

575914

572356

591775

586149

586149

589693

593432

East

– 10.98 ± 0.07 Ma 3,620 ± 22 10.74 ± 0.06 Ma

Whole rock Whole rock

–

2,553 ± 110

–

2,323 ± 48

2,199 ± 24

2,185 ± 65

2,179 ± 26

2,041 ± 38

1,870 ± 36

1,708 ± 54

1,600 ± 35

1,438 ± 24

1,394 ± 8

1,360 ± 15

1,283 ± 88

1,278 ± 14

1,145 ± 14

1,084 ± 22

1,066 ± 42

998 ± 36

2,288 ± 53

2,113 ± 28

2,279 ± 86

2,183 ± 19

1,991 ± 33

1,864 ± 38

1,717 ± 40

1,562 ± 42

1,413 ± 27

1,339 ± 8

1,329 ± 12

1,190 ± 54

1,240 ± 12

1,139 ± 10

2,850 ± 197

1,363 ± 54

921 ± 38

756 ± 10

762 ± 9

No plateau Fractions = 4 56.4% of MSWD = 2.16

39

Ar rel.

Fractions = 5 99.2% of 39Ar rel. MSWD = 0.69 Fractions = 8%99.4 of 39Ar rel. MSWD = 0.71 Fractions = 6 92.4% of 39Ar rel. MSWD = 0.49 Fractions = 5 78.7% of 39Ar rel. MSWD = 2.23 Fractions = 4 87.1% of 39Ar rel. MSWD = 2.45 Fractions = 8 99.8% of 39Ar rel. MSWD = 2.29 Fractions = 5 94.4% of 39Ar rel. MSWD = 1.83 Fractions = 4 78.9% of 39Ar rel. MSWD = 2.08 Fractions = 4 76.0% of 39Ar rel. MSWD = 1.83 Fractions = 5 71.6% of 39Ar rel. MSWD = 1.22 Fractions = 6 95.9% of 39Ar rel. MSWD = 0.20 Fractions = 6 97.2% of 39Ar rel. MSWD = 0.57 Fractions = 4 90.5% of 39Ar rel. MSWD = 2.64 Fractions = 7 99.6% of 39Ar rel. MSWD = 0.71 Fractions = 5 96.2% of 39Ar rel. MSWD = 1.59 Fractions = 4 82.3% of 39Ar rel. MSWD = 2.33 Fractions = 3 87.7% of 39Ar rel. MSWD = 0.71 Fractions = 4 87.2% of 39Ar rel. MSWD = 0.97 Fractions = 7 99.1% of 39Ar rel. MSWD = 0.35 611 ± 72

614 ± 82

–

Ar rel.

–

39

Fractions = 7 79.0% of MSWD = 2.23 No plateau

Plateau information

71 ± 17

Plateau Age (ka)

28 ± 12

Integrated Age (ka)

Whole rock

Whole rock

Pumice

Whole rock

Pumice

Whole rock

Whole rock

Whole rock

Whole rock

Whole rock

Whole rock

Whole rock

Whole rock

Obsidian

Pumice

Whole rock

Whole rock

Pumice

Plagioclase

Whole rock

Whole rock

Material

–

-

2,321 ± 47

2,316 ± 70

2,113 ± 100

2,207 ± 31

2,018 ± 56

1,854 ± 32

1,738 ± 26

1,608 ± 32

1,462 ± 59

1,410 ± 12

1,392 ± 27

1,309 ± 526

–

1,130 ± 9

1,136 ± 49

1,036 ± 43

1,171 ± 81

755 ± 13

633 ± 72

183 ± 36

121 ± 32

Isochron Age (ka)

Fractions = 4 Init. 40Ar/36Ar = 295.9 Fractions = 6 Init. 40Ar/36Ar = 285.3 Fractions = 5 Init. 40Ar/36Ar = 289.3 Fractions = 6 Init. 40Ar/36Ar = 273.2 Fractions = 7 Init. 40Ar/36Ar = 294.1 Fractions = 8 Init. 40Ar/36Ar = 294.9 Fractions = 7 Init. 40Ar/36Ar = 285.1 Fractions = 5 Init. 40Ar/36Ar = 299.3 Fractions = 8 Init. 40Ar/36Ar = 288.9 Fractions = 8 Init. 40Ar/36Ar = 308.2 Fractions = 4 Init. 40Ar/36Ar = 255.6 Fractions = 7 Init. 40Ar/36Ar = 295.9 Single shot fusions on 6 Probability = 0.33 No isochron No isochron

Fractions = 7 Init. 40Ar/36Ar = 291.2 Fractions = 10 of 14, 2 Init. 40Ar/36Ar = 303.4 Fractions = 6 Init. 40Ar/36Ar = 297.2 Fractions = 8 Init. 40Ar/36Ar = 299.4 Fractions = 7 Init. 40Ar/36Ar = 287.5 Fractions = 8 Init. 40Ar/36Ar = 304.4 Fractions = 4 Init. 40Ar/36Ar = 290.3 Fractions = 8 Init. 40Ar/36Ar = 300.5 No isochron

Isochron information

± 4.8 MSWD = 0.42 chips, MSWD = 1.15

± 26.6 MSWD = 0.14

± 9.4 MSWD = 0.54

± 12.8 MSWD = 4.76

± 8.9 MSWD = 1.98

± 80.2 MSWD = 0.78

± 2.8 MSWD = 1.55

± 2.3 MSWD = 0.63

± 58.6 MSWD = 0.21

± 4.6 MSWD = 0.97

± 9.2 MSWD = 3.04

± 17.6 MSWD = 2.92

± 5.0 MSWD = 1.69

± 6.2 MSWD = 2.38

± 2.3 MSWD = 2.29

± 3.5 MSWD = 0.28

± 9.0 MSWD = 0.77

± 12.1 MSWD = 0.64

± 3.2 MSWD = 1.83 runs ± 0.8 MSWD = 0.46

Table 1 List of dated samples from rock units of the Acoculco Caldera Complex. See text for map units. All ages (in ka) are reported at the 1-sigma level. For most samples, the 40Ar/39Ar plateau age is the interpreted age (shown in bold) of the unit. Sample Aco901 showed evidence of excess argon, so the isochron age is the interpreted age. The age of sample Ac90 is a weighted mean age of six single-step fusions, and for sample Ac94a, no plateau or isochron could be calculated. MSWD: Mean Square Weighted Deviation. See text for analytical methods and standards used. List of rock units of the Acoculco Caldera Complex dated in this study and their 40Ar/39Ar ages.

D.R. Avellán, et al.

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Fig. 2. Simplified geological map of the Acoculco Caldera Complex after Avellán et al. (2018), Sosa-Ceballos et al. (2018), and Calcagno et al. (2018). The map contains the distribution of 30 stratigraphic units and a composite stratigraphic column supported by 24 new 40Ar/39Ar ages and two 14C dates (Fig. 3). This map shows towns, main roads, the location of stratigraphic logs, samples with age determinations, faults, the zone of intense hydrothermal alteration, and the two CFE exploratory wells. The DEM was obtained from the INEGI topographic data with a horizontal resolution of 15-m. The geologic profile at the bottom correlates the surface units of this map with the CFE stratigraphic logs of the EAC-1 and EAC-2 wells with the location of shallow intrusions.

4.3. Peñuela dacitic dome complex (~13 to 10 Ma) (Mv)

method at 10.98 ± 0.07 Ma (Table 1, plateau age; Fig. 4B).

These rocks are exposed southern of the ACC and are correlative to the first beginnings of regional volcanism of the eastern part of the TMVB (Fig. 2). The oldest pre-caldera rocks are the Peñuela dacitic (Pdd unit) and Quexnol andesitic dome complexes exposed to the SW and SE parts of the ACC, respectively (Fig. 3). The Peñuela dome was dated with the K-Ar method at 12.7 ± 0.6 Ma (García-Palomo et al., 2002). Another sample of this rock (Pdd) was dated here with the 40Ar/39Ar

4.4. Pre-caldera units (~3.9 – 3.0 Ma) (Pc) The Puente (Pald) and Terrerillos (Tdld) lava domes are exposed to the N-NW and SW of the geologic map, respectively (Figs. 2 and 3). These rocks are light-gray andesitic to dacitic lava domes with greenishgray enclaves. Pald yielded in this work an integrated age of 3,620 ± 22 ka (Table 1; Fig. 4C). García-Tovar et al. (2015) reported a

Table 2 Radiocarbon date of paleosol sample performed during study. The location is in UTM coordinates. Analyses performed at the International Chemical Analysis Inc. Florida, USA. Unit

Clc

Outcrop

Ac1

Description (sample)

Paleosol below Clc

Location North

East

2197465

594866

Lab. Code

Pretreatment

Conventional age

Calibrated Age

18OS/0373

AO

16,710 ± 50 BP

Cal 18410 - 18020 BC

5

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D.R. Avellán, et al.

Fig. 3. Detailed stratigraphic column of the units and eruptive stages of the Acoculco Caldera Complex composed of 30 units marked in the geological map of Fig. 2. To the right appear the plateau age of units, the plateau age sorted by age, and their estimated total volume.

contact between Aai and the underlying Pdd and Pald units was not observed, however, accidental fragments of these units occur in the ignimbrite. At site 24, occurs a massive lithic breccia (mlBr) made of heterolithologic lavas and scarce pumice (Fig. 5C and D). This breccia (5–7 m thick) contains angular accidental lavas set in a coarse to fine ash matrix. Some lava fragments are aphanitic (light-gray and ochre), and other porphyritic (dark-gray, greenish, and ochre) with clinopyroxene and plagioclase phenocrysts. The mlBr matrix contains reddish aphanitic lithics and isolated crystals of plagioclase, pyroxene and amphibole, disseminated pumice and silty minerals. Laterally, this mlBr grades to a massive fine ash layer enriched in pumice and crystals of plagioclase and amphibole. In three sites (24, 65, and 114) mlBr is interbedded between flow units of Aai (Fig. 5D). The Aai unit was transected in the EAC-1 exploratory well at depths

K-Ar age of 3.0 ± 0.4 Ma for Tdld, which is consistent with its stratigraphic position. 4.5. Syn-caldera unit (Acoculco andesitic ignimbrite, ~2.7 Ma) (Sc) The Acoculco andesitic ignimbrite (Aai) is a yellow to white massive deposit. It consists of rounded pumice and angular to sub-angular accidental lava fragments (gray, pink and banded) supported by a matrix of coarse to fine ash (Fig. 5A). Tube-banded pumice (greenish-gray to white) with alkali feldspar and amphibole phenocrysts is also present. The Aai crops out in small gullies and in some places of high topographic relief where it is generally covered by younger deposits (e.g., Hbl, Srl, Atal, Fig. 2). In the southwestern and northern parts of the caldera, Aai underlies ≤40 m thick lacustrine deposits (Fig. 5B). The 6

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Fig. 4. 40Ar/39Ar age spectra and inverse isochron diagrams of dated rocks of the Acoculco Caldera Complex (see Table 1). Plateau and Isochron ages, where calculated, are reported at the 1-sigma level. For Ac90, the 6 single-step fusion results are shows as a single probability density histogram (see Table 1).

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Fig. 5. Photos of selected outcrops of the Acoculco andesitic ignimbrite (Aai) with its different lithological facies. A) Aspect of the welded massive structure of the ignimbrite with pumice fragments embedded in a lightyellowish ash matrix. Hand lens for scale. B) Lacustrine deposits that partly covered the Aai with parallel plane stratification. They are made of interbedded massive fine sandy beds with laminar-stratified clayed beds. Notice that the succession is gently tilted. C) Co-ignimbrite lag breccia (mlBr) of the Acoculco ignimbrite transected by E-W structures and NW-SE faults. Notice that the lava lithics are supported by a light-brown to yellowish ashy matrix with pumice. D) Co-ignimbrite lag breccia (mlBr) at the bottom of the outcrop in contact with the Acoculco ignimbrite. The color change marks a difference in the texture and lithological components of the deposits. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

inside the caldera rim in the south, southwest, and northern parts. It consists of a tilted alternation of white clayed laminae, and dark-gray cm-thick, volcaniclastic beds. These beds are made of rounded lava fragments set in a fine-grained matrix barren of fossils. At sites 69 and 119, ls overlies the Acoculco ignimbrite (Aai) (Figs. 2 and 3).

between 210 and 560 m, where it covers the Pald unit (Pre-caldera units) and is overlain by the Pedernal rhyolitic lava unit (Pdl, which is a late post-caldera unit) (Fig. 2). Well EAC-1 shows nearly 800 m of volcanic materials resting atop a skarn, which was interpreted as the metamorphosed calcareous basement (López-Hernández et al., 2009; Viggiano-Guerra et al., 2011). These authors interpreted the volcanic column from top to bottom as: Acoculco ignimbrite (0–130 m), Cruz Colorada dacite (130–210 m), Alcholoya ignimbrite (210–580 m), and las Minas rhyodacite (580–790 m). However, based on the new geologic map and revised stratigraphy of Acoculco, we consider that the successions would correspond to our Pdl, Aai and Pald units, respectively (Fig. 3). The Pedernal rhyolitic lava (Pdl) crops out around the CFE drill holes as a highly altered vesicular rock with feldspars, plagioclase, quartz and mafic unrecognizable minerals and recrystallized lithic clasts. We consider that these highly altered lavas were previously mistaken as pumice blocks and erroneously described as the Acoculco ignimbrite (López-Hernández, 2009). Also, we propose that their Alcholoya ignimbrite (~2.7 Ma; López-Hernández, 2009) described between 210 and 580 m depth would correspond to our Acoculco ignimbrite Aai (Fig. 5A). Unfortunately, Aai was not recognized in the succession of well EAC-2. This well only shows 340 m of volcanic infill (Pdl and Pald units) resting over the skarn including 200 m of the porphyritic unit (Viggiano-Guerra et al., 2011) or our Pdl unit. As mentioned before, EAC-1 and EAC-2 wells are 500 m apart, but their stratigraphy suggests that they are separated by a fault that is not recognizable at surface. By averaging these thicknesses in the EAC-1 exploratory well, gullies and outcrops described at the surface (~470 m), we estimate a minimum Aai volume of 127 km3. A pumice fragment separated from Aai was dated with the 40Ar/39Ar method on plagioclase at 2,732 ± 185 ka (Avellán et al., 2018). In this work, we obtained two new whole-rock 40Ar/39Ar dates of Aai pumice samples that yielded younger ages of 2,041 ± 38 and 2,185 ± 65 ka (Table 1, plateau age; Fig. 4D-E). However, Aai underlies several earlypost caldera lava flows as Aguila (Atal, 2.44 Ma), and Manzanito (Mtal, 2.2 Ma), and the Sayula dome (Srl, ~2.55 Ma) (see Figs. 2 and 3). Based on these stratigraphic relationships, we concluded that the age of Aai must be older than 2.55 Ma for which the 40Ar/39Ar on plagioclase result (2,732 ± 185 ka) is the best age approximation of the caldera collapse. A sequence of ≥40 m thick lacustrine sediments (ls) is exposed

4.6. Early post-caldera units (~2.6-~2.1 Ma) (Epc) Four of these units are exposed inside the caldera depression as basaltic to basaltic trachyandesite lava flows (Hbl, Atal, Vtal and Mtal) that partially cover Aai (Figs. 2 and 3). They are highly eroded and have asymmetric morphologies similar to flatirons with their apex towards the center of the caldera developing a sub-radial exorheic drainage. These units typically appear as light-gray to dark-gray, blocky lava flows with porphyritic to aphanitic textures. These lavas frequently present greenish, yellowish and reddish hydrothermal alteration zones, and host xenoliths of sub-rounded limestones, sandstones, and fine grain granite. Two units were dated with the 40Ar/39Ar method at 2,323 ± 48 ka (Vtal) and 2,199 ± 24 ka (Mtal) (Table 1, plateau age; Fig. 4F-G). Avellán et al. (2018) obtained a 40Ar-39Ar whole-rock age of 2,441 ± 234 ka for Atal. The age of Hbl is unknown; nevertheless, it underlies the Srl dome dated at ~2.55 Ma (see below). The other two early-post-caldera units (Srl and Atad) are exposed to the external northwestern and southeastern caldera rim, respectively. The Srl unit (2,553 ± 110 ka; Table 1; Fig. 4H) consists of gray to black and brown, banded obsidian lava flows with holohyaline texture and partially devitrified to light-gray spherulites and lithophysae. The Atad unit (2,179 ± 26 ka; Table 1; Fig. 4I) is a blocky greenish to light-gray, porphyritic lava flow. For all the early post-caldera units we estimated a volume of 27 km3. 4.7. Late post-caldera units (~2.0 - ~0.016 Ma) (Lpc) These units are represented by 11 domes (Alrd, Lrd, Amrc, Prld, Crcd, Trcd, Crd, Ard, Crl, Arcd and Mrcd), 5 lava flows (Coal, Tal, Cual, Pdl and Prld), 2 ignimbrites (Eri, Tr1), and 4 scoria cones (Plc, Tlc, Clc1 and Clc2). The Alrd, Lrd, Amrc, Crcd, Trcd, Crd, Ard, Crl Arcd and Mrcd dome and lava flow units occur on the caldera border and periphery (Fig. 2). These structures have predominant rhyolitic compositions with coulée and asymmetrical morphology delimited by very steep levées. The 8

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Fig. 6. 40Ar/39Ar age spectra and inverse isochron diagrams of dated rocks of the Acoculco Caldera Complex (see Table 1). Plateau and Isochron ages, where calculated, are reported at the 1-sigma level.

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parts of the caldera rim, respectively. The Paila unit directly overlies the Atad and Eri post-caldera units and Tulimán unit lies discordantly on top of Srl post-caldera unit. The Cuatzitzinguito (Clc1) and Cuatzitzingo (Clc2) scoria cones lies on the southern flank of the Paila unit (Fig. 3). All these scoria cones are composed of massive poorly-sorted fallout beds with dense blocks and bread-crust scoria and spatter bombs, as well as, black to dark-gray blocky lava flows associated with effusive activity. The Tulimán scoria cone was dated with the 40Ar/39Ar method at 63 ± 9 ka (Avellán et al., 2018). A 40Ar/39Ar age of 71 ± 17 ka was obtained here for the Paila scoria cone (Table 1; Fig. 6L). A paleosol underneath a fallout tephra of the Cuatzitzinguito scoria cone yielded a 14C age of 16,710 ± 50 BP (Table 2). We did not obtain an age for the Cuatzitzingo scoria cone, however, its stratigraphic position indicates that it is likely younger than 16 ka. We estimated a total volume for all the late-post caldera units including Eri and Trl of 90 km3.

rocks of these units typically appear as light-gray to pinkish-gray, banded to massive obsidian lavas with mottled structure given by abundant spherulites and lithophysae (Fig. 3). López-Hernández (2009) and García-Tovar et al. (2015) reported K-Ar ages on hornblende and whole-rock of the Lrd (1,700 ± 400 ka), Crd (1,300 ± 600 ka), and Crl (1,274 ± 27 ka). In this work, seven new units were dated with the 40 Ar/39Ar method (Table 1) yielding ages of 1,870 ± 36 ka (Alrd) (Fig. 4J), 1,438 ± 24 ka (Amrc) (Fig. 4K), 1,394 ± 8 ka (Crcd) (Fig. 4L), 1,360 ± 15 ka (Trcd) (Fig. 6A), 1,283 ± 88 ka (Ard) (Fig. 6B), 998 ± 36 to 1,145 ± 14 ka (Arcd) (Fig. 6C and D) and 1,066 ± 42 ka (Mrcd) (Fig. 6E), that are in agreement with their stratigraphic position. On the other hand, on the western edge the undifferentiated Maguey unit formed of a sequence of pyroclastic surges and fall with a 40Ar/39Ar age of 1,084 ± 22 ka (Table 1; Fig. 6F). The Coal, Tal, Cual, Pdl and Prld units are situated inside the caldera partially overlying the syn-caldera and early post-caldera units (Fig. 2). They appear as a stack of bedded lava flows with andesitic (Coal, Tal, and Cual) and rhyolitic (Pdl and Prld) compositions. The andesitic lava flows are porphyritic, black to dark-gray in color, with reddish to yellowish intense hydrothermal alteration (Fig. 3). The Coal unit was dated by Avellán et al. (2018) with the 40Ar/39Ar method at 2,027 ± 40 ka. A dike 40Ar/39Ar age that cuts this unit yielded 1,600 ± 35 ka (Table 1; Fig. 6G). In this study we dated the Tal unit with the same method at 1,708 ± 54 ka (Table 1; Fig. 6H). López-Hernández (2009) obtained a 40Ar/39Ar age of 1,600 ± 200 ka for the Cual unit. The rhyolitic lava flows (Pdl and Plrd units) are pinkish-gray to pinkish-white porphyritic rocks. These units are white and highly hydrothermally altered lavas that crop out in the vicinity of Pedernal and Acoculco towns. The lava flows are corroded by hydrothermal fluids along highly vesicular breccia structures, where phenocrysts and matrix have been replaced by alteration minerals. López-Hernández (2009) reported two K-Ar ages for these lavas of 1,600 ± 100 ka (Pdl), and 1,400 ± 200 ka (Prld). The Encimadas unit (Eri) is a rhyolitic ignimbrite widely exposed in the east-northeast external parts of the ACC. It has a moderately dissected plain that mantles the Zacatlán basaltic plateau and partially covers some of the post-caldera units. Eri is a welded ignimbrite with several flow units that appear as massive, light-gray to white, beds. Each bed consists of matrix-supported fine ash particles with feldspar and quartz phenocrysts. It has an approximated volume of 26 km3. López-Hernández (2009) reported a 40Ar/39Ar age on sanidine of Eri at 1,300 ± 200 ka. A similar 40Ar/39Ar age of 1,278 ± 14 ka was obtained in this study (Table 1, plateau age; Fig. 6I). The Tecoloquillo ignimbrite (Tr1) is a rhyolitic ignimbrite widely exposed to the south-southwestern part of the caldera (Fig. 2). South of the caldera, Tr1 partially covers some units belonging to the pre-caldera, syn-caldera and post-caldera. The Tr1 unit consists of two main beds; the lowermost part is massive, monolithologic, brittle and matrixsupported with highly friable pumice fragments embedded in a medium to fine ash matrix. Both pumice and matrix contain bipyramidal quartz and alkali feldspar phenocrysts. The upper part is massive with pinkgray, corroded lava blocks, supported by a crumbly medium ash matrix. The upper part of Tr2 is a rhyolitic dome made of angular light-pink to gray lava blocks. The rock is moderately vesicular, fibrous and porphyritic with quartz, alkali feldspar and amphibole phenocrysts. One pumice sample collected from the basal part of the Tr1 unit yielded a 40 Ar/39Ar age on plagioclase of 611 ± 72 ka (Table 1; Fig. 6J). Another pumice sample that yielded older age of 762 ± 9 ka (Table 1; Fig. 6K). López-Hernández (2009) reported a 40Ar/39Ar age on sanidine of 0.8 ± 0.1 Ma for this unit. The last four late-post-caldera units are cinder cones (Paila, Tulimán, Cuatzitzinguito and Cuatzitzingo) and associated lava flows of basaltic andesite composition. These scoria cones occur above or close to the topographic caldera rim (Fig. 2). The Paila (Plc) and Tulimán (Tlc) cinder cones are exposed on the southeastern and northwestern

4.8. Extra-caldera units (~2.4-0.19 Ma) (ATVF) Nineteen scoria cones and four small-shield volcanoes of the ATVF occur around the caldera complex (Fig. 2). Four of these scoria cones, known as Amanalco (Asc; 2,408 ± 58 ka; Avellán et al., 2018), Huixtepec (Hsc), Tecolote (Tsc) and Apapasco (Asc) are located ca. 7 km to the southeast of the caldera border. Five scoria cones called Buenavista (Bsc), Comal (Csc), Calandria (Csc), Toronjil (Tsc), and Tezontle (Tsc) and the Coatzetzengo small-shield-volcano are located at ca. 4 km to the northwest. Three scoria cones named Moxhuite (Msc; 239 ± 34 ka; Avellán et al., 2018), Matlahuacala (Msc), and Cazares (Csc) lie discordantly on top of the Encimadas ignimbrite at ca. 6 km to the east. Two small-shield volcanoes, Camelia (Clc; 2,033 ± 84 ka; Avellán et al., 2018) and Tetelas (Tlc, 1,060 ± 84 ka; Avellán et al., 2018) are located at 10 km to the south of the caldera border. Three scoria cones are aligned in a NW-SE direction, these are Tecajete (Tsc) (1,235 ± 62 ka), Blanco (Bsc; 1,274 ± 62 ka; Avellán et al., 2018) and Hermosa (Hsc). Another, four scoria cones, Coliuca (Clc; 188 ± 6 ka K-Ar age; García-Tovar et al., 2015), Colorado (Csc), El Conejo (Csc) and Tezoyo (Tsc), and the Coyote small-shield volcano are situated at ca. 7 km to the southwest of the caldera border. 5. Characterization of fault systems The ACC is in a highly tectonized region affected by NW-SE, NE-SW and E-W structures (Fig. 7A). The NW-SE and NE-SW structures belong to two regional fault systems. The NW-striking (NNW to NW) fault system contains the oldest regional structures. Some of these faults follow the trend of older structures, such as fold axes and thrust faults from the Laramide orogeny in the Sierra Madre Oriental (Suter et al., 1984; Rocha et al., 2006; Lermo et al., 2009), exposed just north of the study area. The NE-oriented Cenozoic extension (Henry and ArandaGómez, 1992) was active in this region from middle to late Miocene creating NW-striking normal faults and NE-striking strike-slip faults synchronous with regional volcanism (Andreani et al., 2008). In the ACC, the NW-SE fault system is represented by several major normal and oblique (right-lateral component) faults and numerous minor fault strands with lengths between 2 and 5 km, with an average azimuth of 130° dipping mainly to the NE at average angles of 50°, creating horst and graben-like structures (García-Palomo et al., 2002, Fig. 7A). The Manzanito structure, also known as the Tulancingo-Tlaxco fault system (López-Hernández, 2009), is the most representative fault pertaining to this system in the ACC (Avellán et al., 2018). It has an en échelon array of normal faults, which extends for ca. 30 km (Fig. 6B) and displaces the 1.7 Ma Lobera rhyolitic dome by at least 145 m, and the 1.07 Ma Minilla rhyolitic dome by 120 m (this study). Also, at around 1.6 Ma, the ~160°-striking and ~2.5 km long Colorada basaltic dikes were emplaced at the southeastern part of the caldera (Fig. 2; Coal; Avellán et al., 2018). But there is no evidence of recent (Holocene) activity in 10

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Fig. 7. A) Regional structural map modified from García-Palomo et al. (2018). Dark gray areas correspond to horsts and light gray areas correspond to grabens. Dike, scoria cones and slickensides symbols show extension direction. B) Structural map of the Acoculco Caldera Complex showing the main structural features, field sites (outcrops) and the caldera rim (this study). C) The fault planes were plotted on stereographic projections in the lower hemisphere. The trend-frequency polar diagram (rose diagram) was obtained with Rozeta 2.0, and the fault planes and poles were plotted with Stereonet 9. D) A slip model showing an orthorhombic symmetry of NW- and NE- oriented faults moving under the same strain field. Black zones correspond to 100% compatible compression, and white zones 100% tension areas. 11

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parallel to the NE-trending fault system), but also as part of the ring fault system that controlled the caldera collapse and exerted the main control on the location of post-caldera vents (Fig. 2). The southwestern caldera rim coincides with the NW-SE Manzanito fault, while the northern caldera rim is marked by the Atotonilco scarp (Avellán et al., 2018). The sub-linear to sub-circular collapsed structures form a 18 × 16 km rhombohedral-shape, dipping towards the central part of the ACC. Left-lateral movement in the NE-SW faults during the middle Miocene (García-Palomo et al., 2000, 2018) and/or the right-lateral movement of the NW-SE faults likely was responsible for the fracturing and creation of the space necessary to accommodate the magma reservoir beneath the caldera (pull-apart basins in transtensional regimes; e.g., Bursik, 2009; Saxby et al., 2016). Activity of the NE-SW and NWSE regional systems continued after the caldera formation and modified the trace of the caldera border, causing its displacement at several points (e.g., northeastern tip of the topographic caldera rim, outcrops 27 and 28; Fig. 7B). Tectonic movements affected the interior of the caldera until very recent time, causing displacements of intra-caldera blocks and disturbing the position of intra-caldera volcanic vents and products. For example, E-W faulting in the central part of the caldera (outcrops 24, 32 and 54; Fig. 7B). Caldera reactivation during the emission of the 26 km3 Encimadas (~1.28 Ma) and 11 km3 of the Tecoloquillo ignimbrites (~0.6–0.76 Ma) was possibly originated near or along the E and SW borders of the caldera, respectively. Therefore, it is probable that the caldera used these faults systems to nucleate the rest of the ring fault that controlled the collapse event, as it has occurred in other well documented calderas (see Aguirre-Díaz et al., 2008; Martí et al., 2013; Molina et al., 2014). Despite the strong, selective erosion that has affected parts of the area, the northern topographic caldera rim has not retreated significantly from its original position. This may imply that caldera subsidence continued for a long time ( ± 500 ka, between Encimadas and Tecoloquillo eruptive events), dissecting younger rocks emplaced close or through the ring faults. Conversely, in the eastern side of the ACC, neither the morphological nor the structural border are visible at surface, however, it is depicted in the aeromagnetic map (Fig. 8A). At the surface, the eastern border is not visible because it has been buried by younger deposits and it is also likely an uneven collapse of the caldera. The E-W intra-caldera fault system is limited to the caldera interior and therefore is could represent local deformation related to the resurgence of the caldera. The NE-SW and NW-SE fault systems coexist in the ACC (Fig. 7B). This can be explained by two different constructive phases. The first one took place in the Miocene, ruled by ~ NE-oriented extension, forming ~ NW-SE dip-slip faults and ~NE-SW sinistral strike-slip faults (Henry and Aranda-Gómez, 1992). The second phase took place in the Pliocene-Pleistocene, and it is controlled by ~ NW-oriented tectonic extension, forming ~ NW-SE dextral strike-slip faults and ~NE-SW dipslip faults (oblique and normal faults with a minor left-lateral component; García-Palomo et al., 2018). This is consistent with the left-stepping en échelon geometry of the NW-SE Manzanito fault originated by right-lateral slip. This latter phase of NW-oriented extension is still active (García-Palomo et al., 2018). Therefore, both fault systems occur in the same region and are still active under the same stress regime. This is possible because the NW-SE structures are acting as transfer faults of the NE-SW normal faults. In order to demonstrate that both fault systems are moving under the same strain field, a synthetic right dihedra diagram was created for the two pairs of conjugate fault sets, according to the mean measured planes (Fig. 7D). This slip model implies an orthorhombic symmetry of faults, where superposed areas divide the figure into the different compressional and tensional areas in the ACC.

the local NW-striking faults. The NE-striking (NNE to NE) Apan-Tlaloc Fault System (GarcíaPalomo et al., 2018), Tenochtitlan-Apan Fault System (García-Palomo et al., 2002) or Tenochtitlan Shear Zone (De Cserna et al., 1988), has been active since the Miocene and is still active (García-Palomo et al., 2002, 2018, Fig. 7). This is the most important fault system in the region, consisting of normal faults with a left-lateral component with an average azimuth of 040° dipping both to the NW and SE with an average dip angle of 75° that present two generations of striae (~30 and 80°). This system has created regional horst and graben-like structures, obeying a NW-oriented extensional regime that affects eastern México (this study and García-Palomo et al., 2018, Fig. 7A). In general, this fault system shows a good geomorphic expression represented by several major faults (e.g. Apan-Tlaloc Fault and Chignahuapan Fault) and numerous minor fault strands and fractures with 1–4 km individual lengths. Field exposures show prominent scarps; the ~1.3 Ma Canoas rhyolitic dome is displaced by the Atotonilco fault (northern caldera rim) by 150 m, and the ~2.2 Ma Ajolotla trachyandesitic dome is displaced by the Chignahuapan fault by 200 m (this study). The most recent volcanic structures of the ACC (Plc with 71 ka, and Tlc with 63 ka units) are cut by these faults (this study; Figs. 2 and 7B). The NE- and NW-striking normal fault systems intersect each other (García-Palomo et al., 2002; Lermo et al., 2009) creating an orthogonal arrangement of grabens, half-grabens and horsts. The NE-SW RosarioAcoculco Horst (García-Palomo et al., 2002) is delimited to the west by the 235°-trending and NW-dipping Apan-Tlaloc Fault (Mooser and Ramírez, 1987; Huizar-Álvarez et al., 1997) and to the east by the 055°trending and SE-dipping Chignahuapan Fault (Avellán et al., 2018). The ACC is a volcano-tectonic depression inside the Rosario-Acoculco Horst, delimited by parallel faults but with opposite dipping. As for the E-W structures, they are represented by at least 10 E-W striking (~N085°) and SE-dipping normal fault strands. They can only be observed inside the ACC, mainly affecting post-caldera volcanism in the central part of the ACC (this study; Figs. 2 and 7). 6. Aeromagnetic data We produced an aeromagnetic map by reprocessing the airborne data of the Mexican Geological Survey obtained in 2000 (Fig. 8A). This map shows different aeromagnetic anomalies inside or outside of the ACC. The outer domains may be caused by regional and topographic anomalies. Instead, inner domains are possibly associated to four positive local anomalies (subdomains) and conforming a semi-circular shape of the Acoculco caldera. The first subdomain is located in the central part of the ACC; it shows an NE-oriented elongated shape (9.7 km long and 4.8 km wide) with magnetic intensities between −81.8 and 57.8 nT. The second anomaly is located at the southeastern boundary of the ACC, ENE-oriented, 6.6 km long and 4.1 km wide with magnetic intensities between −135.1 and 25.7 nT. The third anomaly is located in the south-central portion of the ACC, NE-oriented, 8.9 km long and 3.5 km wide with magnetic intensities between −64.5 and 36.5 nT. The fourth anomaly appears in the western part of the ACC, NW-oriented, 5.4-km long and 3-km wide with magnetic intensities between 84.5, and −2 nT. The map shows a contrast between the caldera (higher magnetic values) and the surrounding areas (lower magnetic values). 7. Discussion 7.1. Tectonic implications The regional tectonic setting has controlled the formation and evolution of the ACC. The ACC formed on top of the Rosario-Acoculco horst, which is bounded by the Apan and Tlaxco-Chignahuapan grabens (Fig. 7). Furthermore, some pre-existing tectonic faults were used as pathways for dike intrusions and volcanism (aligned scoria cones

7.2. Aeromagnetic interpretation and geothermal implications Previous studies of the area used the aeromagnetic and gravimetric 12

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Fig. 8. A) Image of the Magnetic Field Reduced to the Pole (MFRP) overlaid on a DEM of the ACC region. The black dots represent the CFE wells. B) A 3D inversion model of the magnetic susceptibility produced with the Geosoft's VOXI software by using the magnetic vector inversion method (Ellis et al., 2012). In this image four anomalies related to intrusive bodies occur at depths between 1,000 and > 2,500 m below the surface.

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Fig. 9. Eruptive stages during the formation of the Acoculco Caldera Complex described as pre-caldera (A), syn-caldera (B), early post-caldera (C) late post-caldera (D), and still late post-caldera and extra-caldera (E). After the caldera collapse two episodes of caldera reactivation generated the Encimadas (Eri), and Tecoloquillo (Tr1) rhyolitic ignimbrites. Outside of the caldera rim, the products of the Acoculco caldera are interbedded with deposits of the Apan-Tezontepec volcanic field (extra caldera volcanism).

caldera as reconstructed from the distribution of the Acoculco ignimbrite, the co-ignimbritic breccia, the Atotonilco scarp, and the Manzanito fault (Avellán et al., 2018; this work) (Fig. 7). The aeromagnetic anomalies suggest a rough elliptical limit of the caldera that may be bounded to the east by the approximated venting location of the Encimadas ignimbrite and to the southeast by the basaltic andesite Paila scoria cone. The positive aeromagnetic anomaly located at the NE portion of the ACC may be produced by the Zacatlán basaltic plateau

data of the Acoculco-Atotonilco region obtained by Petroleos Mexicanos (PEMEX) in 1980 (García-Estrada, 2000; López-Hernández, 2009). These authors interpreted a depression bounded by faults coincident with gravimetric data and low magnetic values with much higher values at the interior defining the border of the Acoculco caldera. This coincides with our interpretation of the airborne data displayed in the aeromagnetic map of Fig. 8A. This map shows an approximated match between the edge of the 18 × 16 km Acoculco 14

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Field (Avellán et al., 2018; Sosa-Ceballos et al., 2018).

that is overlaid by the Encimadas ignimbrite and both cover the Cretaceous limestones (Fig. 2). The positive anomalies located to the SW and NW of the ACC may be associated with intrusive bodies that gave place to monogenetic volcanoes of the ATVF. The interior of the caldera is not homogeneous but includes higher and lower magnetic values that might reveal the presence of small-scale horsts and grabens combined with the presence of shallow intrusive bodies (i.e., laccoliths, sills) of basaltic and basaltic and intermediate composition intruded inside the Cretaceous limestones at depths (> 1 km; Fig. 8B). These positive anomalies inside the caldera could have been magma bodies that fed the ca. 2.6 Ma Huistongo, 2.2 Ma Manzanito, 0.71 Ma Paila and < 16 ka Cuatzitzingo eruptions (Figs. 2, 3 and 7). The anomalies described in Fig. 8A have rough NE-SW orientations separating a local horst on top of which the CFE exploratory wells were drilled, cutting skarns at ~790 m deep (EAC-1), and argilitic limestones at 350 m deep (EAC-2) (Viggiano-Guerra et al., 2011). These positive magnetic anomalies might represent intrusive bodies injected during the post-caldera phase (Huistongo lavas 2.6 Ma), which may have an important role to keep the geothermal system active. Sosa-Ceballos et al. (2018) concluded that after the caldera collapse some 2.7 Ma ago, the stress field on the ACC magma chamber and its immediate surroundings was modified. Such change hindered magma migration through the collapsed reservoir and promoted lateral tension zones where dikes and sills converge and eventually form new magma chambers. Due to the active tectonics in the Acoculco area, a new plumbing system developed after the caldera collapse and gradually favored the ascent of deep seated, independent peralkaline and calcalkaline mafic magmas. In addition, Sosa-Ceballos et al. (2018) found that magma mixing-heating is the main magmatic process that modified the ACC rock suite. Hence, the aeromagnetic anomalies might represent the horizontal intrusion zones where mafic magma accumulates as a consequence of the upper crustal deformation produced by the collapse. These intrusion zones serve as heating elements of magma reservoirs (that eventually might evolve into small magma chambers) and yield the energy that sustain the Acoculco geothermal system. When magma did not migrate horizontally, it flowed upwards, sometimes getting tapped on its way to the surface evolving to silicic compositions es got trapped as the aplitic dike dated at 183 ± 36 ka found in EAC-1 (Table 1). These young felsic intrusions have reactivated the system providing heat for the hydrothermal activity and Holocene hydrothermal explosions (Canet et al., 2015a and b). López-Hernández (2009) also suggested the possible presence of an intrusive body, whose top is at > 1000 m depth and that was not reached by EAC-1 well. Although, we believe these intrusions also contribute to the geothermal system, their volume and abundance are not yet known and probably can be estimated by geophysical methods to constraint their energy input. The distribution of magnetic anomalies inside the caldera also reveals the presence of tectonic lineaments with NE-SW and NW-SE orientations, likely associated to horst and grabens blocks and coinciding with those intra-caldera and caldera ring faults observed and measured in the field, suggesting that such faults were used as vent zones for postcaldera volcanism (Figs. 7 and 8). These faults may have also favored the ascent of gas with isotopic compositions (N2/He, 3He/4He, 13C, 15N) of both MORB- and arc-type signatures (Peiffer et al., 2014), and high values of CO2 and 3He/4He ratio (R/Rair = 8.5), that suggest an active magmatic source at depth under the ACC (Polak et al., 1982).

7.3.1. Caldera formation (~2.7 Ma) Prior to the formation of the caldera, the Puente and Terrerillos lava domes had been extruded in the area (Fig. 9A). These domes of andesitic and dacitic composition and calc-alkaline affinity were emplaced during the Pliocene between 3.9 and 3 Ma. After a few hundred thousand years (0.3 Ma), a large amount of andesitic magma stagnated at depth preparing for a major eruption. This calc-alkaline magma overpressured the encasing rock initiating the emission of an andesitic pyroclastic density current (Acoculco ignimbrite). The continuous emission of the ignimbrite eventually diminished the magmatic pressure and weakened the roof of the magma chamber triggering its collapse and forming a 18 × 16 km asymmetric caldera (Fig. 9B). The collapse followed older structures as the NW-SE Manzanito fault in the western and southwestern parts and generated a semi-circular caldera ring (Atotonilco scarp) at the north (Fig. 7B). The ignimbrite crops out in the western-southwestern and northern parts of the caldera interior because in other locations it was covered by younger deposits. The use of older fractures as magma feeders facilitated a high discharge rate at the beginning of the eruption, as discussed by Costa and Martí (2016). This developed into massive proportions, thus precluding the formation of a vertical eruption column and the deposition of fallout deposits, which have not been recognized neither outside the caldera nor in the intra-caldera wells. The occurrence of a co-ignimbrite breccia found in relative small, isolated areas at the north, west and southwestern parts of the caldera rim points to emissions sites of the ignimbrite. The fact the ring fault scarps and the topographic caldera rim are clearly visible at the western half of the caldera, but are hidden at the eastern side, suggest that caldera-collapse could have been more intense in the western sector than in the eastern sector. We cannot fully inform on how the caldera-forming eruption progressed, as most if its corresponding deposits have been eroded outside the caldera or are now totally hidden by post-caldera rocks at the interior of the caldera depression. However, the presence of a large number of normal faults with different orientations affecting intra-caldera rocks suggests that caldera-collapse could have occurred in a piecemeal-trapdoor fashion through several stages of volcanism. 7.3.2. Early-post caldera phase (2.6–2.2 Ma) After the caldera formation, there followed a relatively short (~0.1 Ma) quiescence period during which an intra-caldera shallow lake developed with lacustrine sedimentation (Fig. 5B). The early postcaldera volcanism is represented by eruptions that occurred mainly in the interior of the caldera with a minimum total volume of 27 km3 (Fig. 9C). This volcanism vented along the ring faults and intra-caldera normal faults that formed or acted during caldera collapse to then be reactivated by the extensional tectonics that have affected the area until present (García-Palomo et al., 2018). These modifications of the local stress field allowed the ascent of peralkaline basaltic and basaltic trachyandesite magmas that were generated by partial melting of a metasomatized mantle, genetically unrelated to the calc-alkaline magmas that dominated the pre-caldera and caldera activities (Sosa-Ceballos et al., 2018). Both suites of magma mixed and formed the early postcollapse dome complexes on the ring of the caldera. 7.3.3. Late-post caldera phase (~2.0 - ~0.016 Ma) After a quiescent period of around 0.2 Ma the caldera entered a new phase of volcanism along the caldera rim dominated by the emission of domes and lava flows, two ignimbrites, and four scoria cones (Fig. 9D). An extended period of intermittent volcanism occurred between 2 and 1.3 Ma with the occurrence of at least 13 effusive eruptions along the caldera rim. Between 2 and 1.6 Ma, eruptions emitted bimodal andesitic (Coal, Tal, Cual units) and rhyolitic (Alrd, Lrd units) products to then erupted rhyolitic effusive magmas until 1.28 Ma (Ard unit) at the southwestern part of the caldera rim. At about 1.27 Ma, a rhyolitic

7.3. Volcanic evolution Based on our synthetic map, refined volcanic stratigraphy, structural analysis, and interpretation of the aeromagnetic anomalies, the ACC was emplaced through the Cretaceous limestones, the Peñuela dacitic domes (~13–11 Ma), and the Zacatlán basaltic plateau (Figs. 2 and 3). Rocks of the ACC were divided into 30 units formed in different stages of the caldera evolution from pre, syn, early-post, late-post caldera and extra-caldera volcanism of the Apan-Tezontepec Volcanic 15

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After the formation of the caldera, volcanic activity stopped for a while, permitting the formation of an intra-caldera lake as suggested by sediments covering part of the caldera-forming ignimbrite. Renewed volcanic activity emplaced several domes and lava flows along the caldera borders and intra-caldera faults that modified the caldera depression. Post-caldera volcanism also affected the areas around the caldera and was controlled by the main regional fault systems that also influenced the formation of the caldera. The ACC was formed at the intersection of NE-SW and NW-SE fault systems. These fault systems have controlled the formation and evolution of the ACC, the regional subsidence, and venting of syn- and post-caldera volcanism inside and outside the caldera. The presence of positive aeromagnetic anomalies associated to intrusive bodies (sills and dikes) at the interior of the Acoculco caldera makes it a high potential target for geothermal exploration.

explosion occurred at the eastern part of the caldera dispersing to the east and northeast the Encimadas ignimbrite (26 km3). We assume that magmatic overpressure in a shallow magma chamber triggered the rhyolitic eruption that represents the second largest event of the caldera. Immediately after this major eruption occurred, four rhyolitic eruptions took place in the S-SW parts of the caldera between 1.27 and 1.06 Ma. These eruptions particularly emitted the voluminous Ailitla coulée dome to the south (Arcd; 12.4 km3), the Cabezas lava to the southwest (Crl), a minor explosive event that dispersed small-volume pyroclastic density currents (Msf), and the Minilla coulée come to the west (Mrcd). These effusive eruptions preceded the occurrence of another explosive rhyolitic event that occurred some 0.2 Ma later. This eruption was vented at the southwestern edge of the caldera rim (0.6–0.76 Ma), at the intersection with the NW-SE Manzanito fault. This eruption dispersed the Tecoloquillo rhyolitic ignimbrite (11 km3) to the southwest of the caldera rim, ending with the extrusion of a rhyolitic summit dome. The Tecoloquillo ignimbrite represents the third largest explosive event of the ACC after which activity associated with the caldera apparently declined for a period of circa 0.5 Ma. However, renewed activity of the caldera started 0.071 Ma with the emplacement of the La Paila (Plc) at the southeast, the Tulimán scoria cone and lava flow (0.063 Ma) at the northwest, and the Cuatzitzinguito (16 ka) and Cuatzitzingo ca. 0.01 Ma scoria cones at the southeast. These magmas erupted basaltic andesite to basaltic trachyandesite products (SosaCeballos et al., 2018). During this late post-caldera period, the peralkaline magma suite gradually dominated over the calc-alkaline suite (Sosa-Ceballos et al., 2018) (Fig. 9C-D). This process resulted in an undistinguishable unique source for the early post-collapse magmas, whereas trace elements geochemistry suggests a relatively homogeneous source for the late post-collapse rhyolites (Sosa-Ceballos et al., 2018). The total volume of magma erupted during late post-caldera phase was ~90 km3. We believe that during this phase of the caldera evolution, the resurgence of the caldera floor began, as suggested by uplifted and tilted lacustrine sediments (250°/30SE) exposed inside the caldera (Fig. 2) covering the Acoculco ignimbrite. The morphology of the caldera floor suggests that the resurgence was more important in the southwestern portion, which is in accordance with shallow intrusions of magma revealed by aeromagnetic data that could cause the caldera resurgence. According to Sosa-Ceballos et al. (2018), these shallow magma intrusions were originated by post-caldera deformation that promoted the formation of sills and dikes above the collapsed reservoir and might be the heat source of the Acoculco geothermal system. Finally, the area covered by the ACC products is ca. 856 km2 with a total minimum volume ca. 143 km3. By assuming that the present extension of deposits is minimum, we calculated that activity was dominated by nearly 80% of effusive volcanism with three major bursts of explosive volcanism that represent 20% of the ACC. As summarized above, Acoculco has been a site of persistent volcanism since 2.7 Ma that include the presence of a young intrusion (183 ± 36 ka) at the bottom of the EAC-1 well. This intense magmatism, as well as Holocene hydrothermal explosions (Canet et al., 2015a, 2015b) and geothermal manifestations (Peiffer et al., 2014), indicate that the complex is still active and could represent a site to develop a geothermal field.

Acknowledgements This study was partially funded by the Centro Mexicano de Innovación en Energía Geotérmica (CeMIE-Geo) project P15 and GEMex 4.4. to J.L. Macías. J. Marti is grateful for the MECD, Spain (PRX16/00056) grant. We thank F. Mendiola, G. Reyes-Agustín and S. Cardona for their technical support during the laboratory analyses and D.E. Torres-Gaytan for his support during the aeromagnetic map generation. We appreciate the discussions and input provided by C. Arango and C. Canet. We appreciate the constructive comments by J. Aranda and two anonymous reviewers. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jsames.2019.102412. References Afanasyev, A., Costa, A., Chiodini, G., 2015. Investigation of hydrothermal activity at Campi Flegrei caldera using 3D numerical simulations: extension to high temperature processes. J. Volcanol. Geotherm. Res. 299, 68–77. Aguirre-Díaz, G.J., Labarthe-Hernández, G., Tristán-González, M., Nieto-Obregón, J., Gutiérrez-Palomares, I., 2008. The ignimbrite flare-up and graben caldera of the Sierra Madre Occidental, México. In: Gottsmann, J., Martí, J. (Eds.), Caldera Volcanism: Analysis, Modeling and Response. Elsevier, Amsterdam, pp. 143–174. Alatriste-Vilchis, D., Campos-Enriquez, O., Huizar-Alvarez, R., Marines-Campos, R., 2005. La estructura sub-superficial de la subcuenca de Tecocomulco. La Laguna de Tecocomulco Geo-Ecologia de un desastre. Universidad Nacional Autónoma de México, Instituto de Geología, Publicación especial 3, 33–48. Andreani, L., Le Pichon, X., Rangin, C., Martínez-Reyes, J., 2008. The southern Mexico block: main boundaries and new estimation for its Quaternary motion. Bulletin de la Société géologique de France 179 (2), 209–223. https://doi.org/10.2113/gssgfbull. 179.2.209. Avellán, D.R., Macías, J.L., Layer, P.W., Cisneros, G., Sánchez-Núñez, J.M., GómezVasconcelos, M.G., Pola, A., Sosa-Ceballos, G., García-Tenorio, F., Reyes-Agustín, G., Osorio-Ocampo, S., García-Sánchez, L., Mendiola, I.F., Marti, J., López-Loera, H., Benowitz, J., 2018. Geology of the late Pliocene – Pleistocene Acoculco caldera complex, eastern trans-Mexican volcanic belt (México). J. Maps 15 (2), 8–18. Bibby, H.M., Caldwell, T.G., Davey, F.J., Webb, T.H., 1995. Geophysical evidence on the structure of the Taupo Volcanic Zone and its hydrothermal circulation. J. Volcanol. Geotherm. Res. 68 (1–3), 29–58. Bursik, M., 2009. A general model for tectonic control of magmatism: examples from long valley caldera (USA) and El Chichón (México). Geofis. Int. 48 (1), 171–183. Calcagno, P., Evanno, G., Trumpy, E., Gutiérrez-Negrín, L.C., Macías, J.L., CarrascoNúñez, G., Liotta, D., the GEMex T3.1 team, 2018. Preliminary 3-D geological models of los humeros and Acoculco geothermal fields (Mexico) – H2020 GEMex project. Adv. Geosci. 1, 1–13. Campos-Enríquez, J.O., Alatriste-Vilchis, D.R., Huizar-Álvarez, R., Marines-Campos, R., Alatorre-Zamora, M.A., 2003. Subsurface structure of the Tecocomulco sub-basin (northeastern México basin), and its relationship to regional tectonics. Geofis. Int. 42 (1), 3–24. Canet, C., Hernández-Cruz, B., Jiménez-Franco, A., Pi, T., Peláez, B., Villanueva-Estrada, R.E., Salinas, S., 2015a. Combining ammonium mapping and short-wave infrared (SWIR) reflectance spectroscopy to constrain a model of hydrothermal alteration for the Acoculco geothermal zone, Eastern México. Geothermics 53, 154–165. Canet, C., Trillaud, F., Prol-Ledesma, R.M., González-Hernández, G., Peláez, B., Hernández-Cruz, B., Sánchez-Córdova, M.M., 2015b. Thermal history of the Acoculco geothermal system, eastern México: insights from numerical modeling and

8. Conclusions The Acoculco collapse caldera was originated at about ~2.7 Ma ago in response to the eruption of 127 km3 of the andesitic ignimbrite of the Acoculco unit. The caldera collapse episode occurred in part along preexisting regional NE-SW trending faults (southwestern, western and northwestern sectors), but also along newly formed ring faults (northern sector and probably the eastern sector, but this is not visible). The collapse likely occurred in a piecemeal-trapdoor fashion, in which intra-caldera blocks bounded by pre-existing or newly formed normal faults gravitationally collapsed in a partly emptied magma chamber. 16

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