# Blossoming of the Pleistocene volcanism in the Ecuadorian Andes: a review based on new and recent geochronological data

Santiago Santamaría, Mathilde Bablon, Xavier Quidelleur, Pablo Samaniego, Jean-Luc Le Pennec, Silvana Hidalgo, Céline Liorzou

► To cite this version:

Santiago Santamaría, Mathilde Bablon, Xavier Quidelleur, Pablo Samaniego, Jean-Luc Le Pennec, et al.

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# Blossoming of the Pleistocene volcanism in the Ecuadorian Andes:
a review based on new and recent geochronological data

Santiago Santamaría a,*, Mathilde Bablon b, Xavier Quidelleur a, Pablo Samaniego c, Jean-Luc Le Pennec d, Silvana Hidalgo e, Céline Liorzou d

a GEOPS, Université Paris-Saclay, CNRS, Rue du Belvédère, 91405 Orsay, France

b Université Côte d'Azur, CNRS, IRD, Observatoire de la Côte d'Azur, Géoazur, Valbonne, France

c Laboratoire Magmas et Volcans, Université Clermont Auvergne, CNRS, IRD, OPGC, F-63000 Clermont-
Ferrand, France

d Geo-Ocean, Univ Brest, CNRS, Ifremer, UMR6538, IRD, Institut Universitaire Européen de la Mer, 29280
Plouzané, France

e Instituto Geofísico, Escuela Politécnica Nacional, Ap. 17-01-2759, Quito, Ecuador

* corresponding author (email: ssantamaria@yachaytech.edu.ec; sdsantiago@gmail.com). Now at: School of Earth Sciences, Energy and Environment, Universidad Yachay Tech, Urcuquí, Ecuador.

Keywords:

- Volcanic arc

- Ecuador

- K-Ar dating

- Geodynamics

- Carnegie Ridge

Key Points:

- We have compiled a dataset of 250 ages from 45 volcanoes in Ecuador

- Oldest volcanism mostly occurred in the Eastern Cordillera of the Quito segment

- Since about 1.4 Ma volcanic activity spread around the Quito segment

- Volcanic activity increased extensively throughout the arc since ~600 ka

- The Carnegie Ridge subduction influenced the Ecuadorian volcanism evolution

Word count:

- Abstract: 348 words

- Main body: 10497 words

Figures: 12; Tables: 2; Appendices: 5

Submitted to Bulletin of Volcanology; April 2024
ABSTRACT

The Ecuadorian arc is composed of an unusually high number of volcanoes, organized as along-arc alignments and across-arc clusters, in a relatively small area. Although several geochronological studies have been carried out in the last three decades, the eruptive history of the central zone of the arc remains poorly documented, preventing analysis of the initiation of volcanism of the whole arc. In this study, we present new K-Ar ages obtained from this central area, referred to as the Quito segment. These results were then incorporated into an updated comprehensive geochronological database of about 250 ages, allowing us to describe, at the arc scale, the spatial and temporal evolution of Quaternary volcanism in Ecuador. About eighty Quaternary volcanoes have been identified in the Ecuadorian Andes, 45 of which have been radioisotopically dated and/or identified as active or potentially active. The volcanic arc developed in three stages, characterized by an increase in the total number of active volcanoes. During the oldest Plio-Early Pleistocene stage, documented volcanic activity was mostly concentrated in the Eastern Cordillera of the Quito segment, with minor effusive eruptions in the southern Back-Arc. Since ~1.4 Ma, activity has spread to the surroundings of the Quito segment, and new edifices also appeared in the Western Cordillera and the Inter-Andean Valley. Towards the end of this intermediate stage (i.e., ~800 ka), volcanism occurred in isolated areas north and south of the Inter-Andean Valley. Finally, the late and current has been characterized by a remarkable increase in volcanic activity since ~600 ka. About 50 volcanoes were active during this stage. The spatial distribution of the Ecuadorian arc volcanism seems to be guided by deep mechanisms (i.e., slab geometry and age, amount and composition (fluids and melts) of slab input, mantle heterogeneities) and old crustal tectonic structures of the Western Cordillera, while neotectonics seems to influence the development of stratovolcanoes. In addition, we note that the spatial and temporal evolution of volcanism highlights the influence of the Carnegie Ridge and the young Nazca crust on the thermal regime of the subduction system, which in turn increases of volcanic activity in Ecuador.
# 1. INTRODUCTION

The study of the evolution of a volcanic arc is essential for understanding the processes that control the spatial distribution of volcanoes, the dynamics of their eruptions, and therefore, the associated risks to populations and communities. Thus, improving the fundamental knowledge of the relationships between volcanism and geodynamics becomes a necessary step to establish monitoring strategies for active volcanoes. Here, we present the analysis of the spatial and temporal evolution of Ecuadorian volcanoes, providing valuable insights into their relationship with the crustal structures and the slab configuration.

The Northern Andean Volcanic Zone results from the subduction of the oceanic Nazca plate beneath the northwestern margin of the continental South American plate (Fig. 1a; Hall and Wood, 1985). The volcanic activity in Ecuador developed over a large area, up to 270 km wide, and produced about eighty Quaternary volcanoes (Hall and Beate, 1991; Hall et al., 2008; Ramon et al., 2021). The recent eruptive activity of the Tungurahua, Pichincha, and El Reventador volcanoes during the 2000s highlighted the need to improve the knowledge of the evolution of Ecuadorian volcanoes and thus the associated volcanic hazard assessment. Therefore, several vulcanological, geochronological, geochemical, petrological, and geophysical studies have been carried out in the Ecuadorian Andes (e.g., Hall and Mothes, 2008a; Robin et al., 2010; Le Pennec et al., 2011), focusing mainly on volcanoes that were "active" during the Holocene or with morphological features suggesting "potential" Holocene activity (Hall and Beate, 1991). These studies revealed that at least twenty Ecuadorian volcanoes were active during the Holocene (e.g., Samaniego et al., 1998; Barba et al., 2008; Hall et al., 2008; Hidalgo et al., 2008; Robin et al., 2008; Bernard et al., 2014). Their activity resulted in the formation of thick volcaniclastic sequences that are now overlain by heavily populated areas, including the capital of Ecuador, Quito (Hall and Mothes, 2008b). Several authors have also suggested a possible link between the spatial distribution of volcanism and its geochemical variability in relation to the subduction of the Carnegie Ridge, crustal tectonics and the slab geometry (e.g., Litherland and Aspden, 1992; Barberi et al., 1988; Gutscher et al., 1999; Samaniego et al., 2002, 2005; Bourdon et al., 2003; Garrison et al., 2006; Chiaradia et al., 2009, 2021; Hidalgo et al., 2012; Ancellin et al., 2017).
Recently, several geochronological studies have shed light on the development of the Quaternary volcanism in Ecuador, with emphasis on the southern termination of the arc and its central and northern segments (e.g., Alvarado et al., 2014; Bablon et al., 2019, 2020a; Santamaría et al., 2023). In this work, in addition to compiling radioisotopic ages at the arc scale, we focus on the central zone of the arc, which has a relatively limited number of dated volcanic complexes. We present twelve new K-Ar ages obtained from ancient edifices from the Quito segment. This allows us to clarify the eruptive history of volcanoes from this densely populated area, and to provide new ages to expand the radioisotopic age dataset. We have also carefully selected and compiled the available geochronological data for the Pleistocene volcanism in Ecuador to present the first summary of the eruptive history of the whole volcanic arc based on K-Ar, $^{40}$Ar/$^{39}$Ar, and fission track ages.

## 2. GEOLOGICAL CONTEXT

### 2.1. Ecuadorian geological setting

Physiographically, Ecuador is composed of the Coastal Forearc and the Oriente Foreland basins separated by the Andes Mountains. The northern Ecuadorian Andes are divided into two parallel ranges, known as the Western and Eastern Cordilleras, with the Inter-Andean Valley between them (Fig. 1b). The basement lithologies and ages in these domains are widely variable (Egüez et al., 2017; Jaillard, 2022). For instance, the Coastal basin and the Western Cordillera are formed by Cretaceous MORB-like basalts accreted during the Late Cretaceous-Paleogene, overlain by Tertiary marine sediments and volcaniclastic deposits (Jaillard et al., 2004; Witt et al., 2019; Vallejo et al., 2019). Conversely, the Eastern Cordillera is composed of Paleozoic to Jurassic metamorphic rocks, mostly of continental affinity, and felsic to mafic igneous rocks (Litherland et al., 1994; Spikings et al., 2015; Villares et al., 2020). The Inter-Andean Valley is composed of Miocene-Pleistocene volcanic sediments that overlie a tectonic *mélange* of continental and oceanic rocks derived from both cordilleras (Aspden et al., 1995; Lavenu et al., 1995; Winkler et al., 2005). The basement of the Oriente basin corresponds to the Pre-Cambrian Guyanese craton, overlain by a thick Mesozoic to Cenozoic sedimentary sequence
(Ibanez-Mejia et al., 2011; Baby et al., 2013; Gutiérrez et al., 2019; Vallejo et al., 2021). The Miocene-Pliocene volcanic activity formed thick volcanic sequences (Lavenu et al., 1995; Vallejo et al., 2019, 2020; Chiaradia et al., 2021), presently exposed over both cordilleras, on which part of the Pleistocene-Holocene volcanic edifices developed. The thickness of the continental crust is estimated to be ~55-60 km for both the Western and Eastern Cordilleras (e.g., Koch et al., 2021).

Subduction of the oceanic Nazca plate led to the development of the Ecuadorian volcanic arc (Hall and Wood, 1985; Barberi et al., 1988). The Grijalva Fracture Zone (Fig. 1a), which resulted from the fragmentation of the ancient Farallon plate during the Early Miocene (~22-23 Ma), divides the Nazca plate into two distinct crusts, issued from the Galápagos spreading center to the north, and from the East Pacific rise to the south. Moreover, the eastward motion of the Nazca plate over the Galápagos hotspot led to the formation of the Carnegie Ridge, a 300 km-wide submarine mountain range built on the younger Nazca crust (Meschede and Barckhausen, 2000; Lonsdale, 2005; O'Connor et al., 2007). The complex structure of the Nazca plate and its corresponding thermal state affect the buoyancy of the subducted slab, which is then reflected in a change in dip angle from ~12° for the older crust (south of the GFZ) to >22° for the younger crust (north of the GFZ). This difference in subduction angle, combined with the convex shape of the continental margin, has induced a slab flexure beneath the southern part of the Ecuadorian Andes (Fig. 1a; Yepes et al., 2016; Portner et al., 2020; Araujo et al., 2021). The presence of the Carnegie Ridge has also been associated with notable variations in the nature of the metasomatic agent of the sub-arc mantle wedge. This agent has been postulated to be an aqueous fluid resulting from slab dehydration reactions, with variable inputs of a siliceous melt associated with partial melting of both the sedimentary and basaltic portions of the subducted slab (e.g., Barragan et al., 1998; Bourdon et al., 2003; Hidalgo et al., 2012; Ancellin et al., 2017; Narvaez et al., 2018). As previously proposed, partial melting of the slab could be associated with an increase in the thermal regime related to the presence of the Carnegie Ridge, changes in slab geometry, and/or differences in slab ages and rheology (e.g., Yepes et al., 2016; Ancellin et al., 2017; Narvaez et al., 2018). These issues could also influence the spatial configuration of the volcanic arc (Bablon et al., 2019, 2020a).
Furthermore, the oblique convergence of the Nazca Plate caused the motion of the northwestern edge of the South American plate known as the North Andean Sliver (Fig. 1a). The southeastern limit of its deformation zone corresponds to the Chingual-Cosanga-Pallatanga-Puná (CCPP) fault system. The CCPP fault system begins in the Gulf of Guayaquil, intersects the Ecuadorian Andes near the southern termination of the volcanic arc, and continues northward along the eastern edge of the Eastern Cordillera (Fig. 1; Witt and Bourgois, 2010; Alvarado et al., 2014; Nocquet et al., 2014; Jarrin et al., 2023).

**2.2. Overview of the Ecuadorian volcanic arc**

Quaternary volcanism in Ecuador is characterized by a wide variety of well-preserved volcanic morphologies related to multiple eruptive styles and magmatic compositions. The resulting structures typically correspond to prominent composite cones with basal diameters of several kilometers, usually greater than 7 km (e.g., Cotopaxi volcano; Hall and Mothes, 2008a). Successive conduit migrations and/or changes in eruptive styles caused the superposition of edifices. Such events gave rise to large volcanic complexes or compound volcanoes, which are the most common structures observed in the Ecuadorian Andes (e.g., Pichincha volcanic complex; Robin et al., 2010). The construction processes of these "edifices" have also been interrupted by sector collapses, separating their eruptive histories into distinct cone-building stages (e.g., Tungurahua volcano; Hall et al., 1999; Bablon et al., 2018). Conversely, highly explosive eruptions have resulted in the formation of large calderas (e.g., Chalupas caldera; Beate, 1985; Bablon et al., 2020b). Also, less prominent structures such as lava domes (e.g., Pululahua dome complex; Andrade et al., 2021) and monogenetic cones with their corresponding lava flows (e.g., Tulabug cone; Bablon et al., 2019) have also been identified.

The Ecuadorian arc is usually described in terms of the geographic distribution of its numerous volcanoes along NNE-SSW alignments roughly parallel to the trench (e.g., Hall, 1977; Hall and Beate, 1991). These volcanoes built on oceanic basement (Western Cordillera and eastern Inter-Andean Valley) roughly constitute the Volcanic Front (yellow volcanic centers, Fig. 2), whereas those built on
continental basement (western Inter-Andean Valley and Eastern Cordillera) are called the Main Arc (orange and purple volcanic centers, Fig. 2). The Back-Arc volcanoes are located in the Sub-Andean zone (blue volcanic centers, Fig. 2). The magmas in each of these regions have distinctive geochemical and petrographic characteristics that lead to different eruptive styles and volcanic landforms (Hall et al., 2008; Ancellin et al., 2017). Notably, the along-arc distribution of these volcanoes is not uniform. For sake of clarity, we have divided the Ecuadorian arc into ~50 km-long segments perpendicular to the axis of the Volcanic Front (Fig. 2). Note that the boundaries of these segments roughly coincide with the Chota, Guayllabamba, Latacunga, and Riobamba valleys, where a smaller number of volcanoes are observed. The volcanoes located in the Tulcán, Ibarra, Machachi, and Ambato segments form across-arc “volcanic clusters” that include various composite stratovolcanoes and lava dome complexes. In these areas, the distance between volcanic summits ranges only from ~7 to ~15 km. These volcanic clusters cover areas of a few hundred square kilometers. We note that despite the short distance between volcanoes, their eruptive products show a wide geochemical variability resulting in multiple eruptive styles.

The volcanic products of the Ecuadorian arc have been classified as medium to high potassium calc-alkaline basaltic andesites to rhyolites (e.g., Hall et al., 2008; Hidalgo et al., 2012; Ancellin et al., 2017; Chiaradia et al., 2020), with exceptional occurrences of shoshonitic rocks in Back-Arc setting (e.g., Bourdon et al., 2003; Hoffer et al., 2008; Garrison et al., 2017). Overall, the concentrations of incompatible elements (e.g., K, Rb, Sr, Ba, La, Th) tend to increase with distance from the trench. Furthermore, a decrease in the concentrations of elements considered fluid-mobile (e.g., Ba, Pb, Cs, Li, B) relative to fluid-immobile elements (e.g., Be, Nb, Th, La) has been observed with increasing distance from the trench (e.g., Barragan et al., 1998; Bryant et al., 2006; Chiaradia et al., 2009; Hidalgo et al., 2012; Ancellin et al., 2017).

## 2.3. Volcanism in the Quito segment
The Quito segment includes the volcanoes located approximately between 0° and 0.4° S latitude (Fig. 2). Our study focuses on the volcanoes surrounding the Guayllabamba valley, where few ages are available (Fig. 3). Volcanoes in the Sub-Andean zone (e.g., Hoffer, 2008; Mothes and Hall, 2008a; Salgado et al., 2021) were excluded due to the limited access roads and rainforest cover, which preclude adequate sampling. A summary of the geological background for the volcanoes in the Quito segment is provided below.

**Pululahua** (3357 m asl.; lat. 0°05'N; long. 78°49'W) is a dacitic dome complex, mostly formed during the Holocene, located on the axis of the eastern edge of the Western Cordillera. Recent activity at Pululahua (2.3 - 2.6 cal ka BP; Papale and Rosi, 1993; Andrade et al., 2021) included several highly explosive events (VEI 4) that produced a caldera-like depression and culminated in the extrusion of several dacitic lava domes within the caldera (Andrade et al., 2021). Southward, the **Casitagua** volcano (3519 m asl.; lat. 0°33'S; long. 78°48'W) is formed by a sequence of andesitic lavas interbedded with pyroclastic deposits. These volcanic successions are overlain by the lacustrine sediments of the San Miguel Formation (Pacheco et al., 2014) and the Cangahua Formation, an Upper Pleistocene, mostly reworked, highly altered volcanic formation composed of tephra fallout and flow deposits (Clapperton, 1990; Hall and Mothes, 1997). This sequence is deformed by the Quito fault system. The summit area of Casitagua currently shows a 5 km-diameter horseshoe-shaped structure containing several dacitic lava domes. Multiple domes also occur to the east and southwest of Casitagua volcano. The Catequilla and Pacto satellite lava domes are interspersed among the Casitagua volcanic sequences. They have been dated at 833 ± 26 ka and 898 ± 15 ka using the K-Ar Cassignol-Gillot technique applied to groundmass and plagioclase phenocrysts, respectively (Pacheco, 2013; Alvarado et al., 2014). An imprecise whole-rock K-Ar age of 1400 ± 1400 ka was provided for the Lulunurcu satellite dome by Geothermica Italiana (1989).

Further south, the **Pichincha volcanic complex** (4776 m asl.; lat. 0°17'S; long. 78°60'W) is formed by three superimposed edifices (Robin et al., 2010). The Cinto edifice, dated by $^{40}$Ar/$^{39}$Ar at 1112 ± 24 ka, is the oldest edifice in the complex. It was later intruded by the Ungi lava dome, dated at 864 ± 48 ka. The Rucu Pichincha edifice is mainly composed of successions of andesitic lavas and
breccias formed between ~850 and ~262 ka, which partially cover the Cinto edifice. Further west, the Guagua Pichincha edifice, which is mostly dacitic in composition, rests on the western side of the Rucu Pichincha edifice. Its construction began as early as 52 ± 4 ka, and its most recent activity occurred between AD 1999 and 2001. Notably, several sector collapses have occurred during the eruptive history of Pichincha volcano (Robin et al., 2010). South of the El Cinto edifice lies La Carcacha volcano (3880 m asl.; Lat. 0°19'S; Long. 78°36'W), an old edifice (~1290 ka; Hidalgo, 2006) of the **Atacazo-Ninahuilca volcanic complex** (4455 m asl.; lat. 0°36'S; long. 78°62'W). The main cone-building stages of Atacazo occurred between ~220 and ~83 ka, followed by the extrusion of several satellite domes around ~71 ka and the Holocene high-explosive activity (VEI 4 or higher) of the Ninahuilca lava dome complex (Hidalgo, 2006; Santamaría et al., 2023). **Ilaló** (3188 m asl.; lat. 0°26'S; long. 78°42'W) is the only volcano in the Inter-Andean Valley within the study area. This highly eroded volcano is formed by sequences of andesitic lavas (Chiaradia et al., 2009) overlain by the Cangahua Formation. A whole-rock K-Ar age of 1620 ± 160 ka was obtained for a basaltic andesite lava (Barberi et al., 1988), although the location of the sampling site is not reported.

Several highly eroded volcanoes dominate the western edge of the Eastern Cordillera. To the north of the study area, **Pambamarca** volcano (4078 m asl.; lat. 0°08'S; long. 78°21'W) is formed by voluminous andesitic lava sequences overlain by a complex of dacitic and rhyolitic domes. Herradura is a *dome coulée* located in the summit area that, unlike most of Pambamarca's structures, shows scarce signs of erosion. Further south, **Cerro Puntas** (4558 m asl.; lat. 0°19'S; long. 78°21'W) is a volcano composed of sequences of monolithological breccias and lavas surrounded by incised glacial valleys. The southwestern flank of Puntas volcano partially covers the smaller **Coturco** volcano (3572 m asl.; lat. 0°21'S; long. 78°28'W), also called Cotohurcu or Leon Dormido (Bernard and Andrade, 2011). To the east, the **Chacana** caldera (4493 m asl.; Lat. 0°21'S; Long. 78°15'W) dominates the Eastern Cordillera, occupying an area 32 km long (N-S) and 20 km wide (E-W). The oldest volcanic products of Chacana are dated by $^{40}$Ar/$^{39}$Ar between 2150 ± 200 and 2710 ± 190 ka (Opdyke et al., 2006). The caldera-forming episode involved the emission of voluminous ignimbrite flows that spread radially along the caldera flanks, forming the Quiscatola-Yanaurcu and Tablones series. These sequences are
dated by obsidian fission-tracks between 1350 ± 90 and 1580 ± 70 ka, and at 810 ± 50 ka, respectively (Bellot-Gurlet et al., 2008). Two $^{40}$Ar/$^{39}$Ar ages are provided for the Quiscatola-Yanaurcu series at 1670 ± 190 and 1460 ± 50 ka (Opdyke et al., 2006). Alternatively, two glass K-Ar ages of 980 ± 13 and 810 ± 40 ka are consistent with the range of the Tablones series, although the sampling locations are not indicated (Barberi et al., 1988). The plateau created by these deposits surrounds the Pambamarca, Puntas and Coturco volcanoes. The subsequent Chacana activity phase corresponds to the uplift of the caldera floor caused by a shallow intracaldera intrusion (post-collapse resurgence stage), followed by a highly explosive episode and the emplacement of rhyolitic lava domes (Hall and Mothes, 2008c; Beate et al., 2010). The volcanic products of these episodes are dated between 450 and 160 ka (Opdyke et al., 2006; Bellot-Gurlet et al., 2008). The most recent major eruptions produced two 8 m-thick regional pumice-fallouts. The associated rhyolitic lava domes have been dated at 180 ± 20 and 170 ± 10 ka (Bigazzi et al., 2005). More recently, several andesitic and dacitic lava flows have been erupted, including the fissure eruptions of Pinantura (AD 1728) and Papallacta (AD 1773).

A sequence of voluminous andesitic lavas forms the northern flank of the Chacana caldera. These lava flows are associated with the **Izambi** volcano (4325 m asl.; Lat. 0°19'S; Long. 78°17'W), also called Gualimburo (Aguilera et al., 2007). Some authors consider this voluminous sequence to be part of the Chacana series, while the upper lava sequence (Pucará Chico peak) is assigned to the Izambi volcano (e.g., Hall and Mothes, 2008c; Bernard and Andrade, 2011). A basaltic andesitic lava was dated at 1300 ± 200 ka by whole-rock K-Ar (Barberi et al., 1988). Although the sampling location is not provided, the sample may correspond to the most accessible Gualimburo series. Alternatively, an obsidian block sampled near the Pucará Chico peak yielded a fission-track age of 370 ± 60 ka (Bigazzi et al., 2005).

# 3. METHODS

## 3.1. Sampling strategy
Sampling lavas for geochronology from the Quito Segment was carried out in 2020. Fresh lavas with low vesicle contents were sampled for K-Ar dating and whole-rock major and trace element analyses. Where possible, outcrops in both the summit and basal areas of the volcanic edifices were targeted (Fig. 3) in order to bracket the whole eruptive history of the studied volcanoes.

At Casitagua volcano, two samples were collected from massive andesitic lava flows located on its southern (20EQ51a) and western (20EQ52a) flanks. Three juvenile blocks (20EQ51b, 20EQ52b, 20EQ52c) were sampled from pyroclastic density current (PDC) deposits overlying the previously sampled lavas for geochemical comparison. A dacitic dome located within the horseshoe-like depression was also sampled (20EQ53). At Ilaló volcano, three massive lava flows were sampled at the base (19EQ01) and in the summit area (19EQ47, 19EQ48). The overlying thick deposits of the Cangahua Formation preclude the exposure of multiple outcrops on these volcanoes and thus prevented additional sampling.

Eight samples were collected from Pambamarca volcano. Four samples correspond to andesitic lavas located on the northern flank (20EQ56) and the summit area (20EQ59, 20EQ60, 20EQ61). Four samples were collected from the summit rhyolitic domes of Herradura (20EQ57, 20EQ58), Jambi Machay (20EQ62) and Quito Loma (20EQ63) (Fig. 3b). Only the northern flank of Puntas volcano was sampled due to poor access routes. However, four massive lavas were sampled at different elevations (20EQ64, 20EQ65, 20EQ67, and 20EQ68). Likewise, only two lavas could be sampled in the summit area of Coturco volcano (20EQ54, 20EQ55). Finally, we sampled a lava from the basal sequence on the southern side of the Antisanilla valley (20EQ85), located on the southern flank of the Chacana caldera.

## 3.2. K-Ar dating

Twelve samples were carefully selected for radioisotopic dating after thin-section examination (photographs available in Appendix A). We used the K-Ar dating method applying the unspiked Cassignol-Gillot technique (Gillot et al., 2006), which has been successfully applied previously to the Ecuadorian volcanic arc (Alvarado et al., 2014; Bablon et al., 2018, 2019, 2020b; Santamaría et al.,
2022, 2023), as well as to subduction and hotspot volcanism worldwide (e.g., Grosse et al., 2018, 2020; Hildenbrand et al., 2018; Dibacto et al., 2020). The Cassignol-Gillot technique is particularly suitable for dating volcanic rocks with low radiogenic argon ($^{40}$Ar*) contents diluted by atmospheric $^{40}$Ar, such as young Low-K lavas found in the Ecuadorian arc. Using a 180° sector multi-collector mass spectrometer, the electrical signals measured for the $^{40}$Ar and $^{36}$Ar extracted from a given sample are compared with those from an air aliquot measured under identical pressure conditions. The difference in the measured ratios allows determination of the $^{40}$Ar* produced by radioactive decay of $^{40}$K since closure of the sample. The $^{40}$Ar signal is calibrated by systematic measurements of the HD-B1 standard (24.18 ± 0.09 Ma; Schwarz and Trieloff, 2007) to convert the electrical signals into a certain number of atoms. Thus, the argon measurement process depends solely on the $^{40}$Ar* content of the sample, whose electrical signal is optimized by adjusting the initial sample mass. The measurement process was carried out at the GEOPS laboratory of the Université Paris-Saclay (France), where the detection limit of the mass spectrometer is close to 0.1% for $^{40}$Ar* (Quidelleur et al., 2001), allowing the dating of Holocene volcanic rocks with relatively low uncertainty (Gillot et al., 2006). The potassium (K) concentration is measured independently by flame absorption spectroscopy. MDO-G (Gillot et al., 1992) and BCR-2 (Raczek et al., 2001) standards are measured together with the analyzed samples for sake of comparison and correction purposes. Finally, the age is calculated from the $^{40}$K/K ratio in nature, the $^{40}$K to $^{40}$Ar decay constant (Steiger and Jäger, 1977), and the measured K and $^{40}$Ar* contents. The reproducibility of the measurements was verified within a 1-σ uncertainty range by replicating the whole process at least twice for each sample.

We prefer to apply this technique to the groundmass because it is the last phase to crystallize in equilibrium with the atmosphere after magma eruption. Phases crystallized before eruption, such as plagioclase phenocrysts, may carry inherited radiogenic argon biasing the calculated ages (e.g., Singer et al., 1998; Samper et al., 2008). Therefore, we consider K-Ar ages performed on crystals or whole-rock as older bounds of the real eruption date. To separate groundmass and crystal phases, samples were hand crushed and sieved to 63-80, 80-125 or 125-250 µm, depending on the average distance between phenocrysts occupied by groundmass. Subsequently, dilute acid (10% HNO$_3$) leaching, magnetic and
heavy-liquid (bromoform) separation procedures ensured that the recovered groundmass fraction remained homogeneous within a narrow density range and did not contain undetected weathered fractions. A full description of the sample preparation, analytical procedures, and age and uncertainty calculations can be found in Bablon et al. (2018).

**3.3. Whole-rock geochemical analyses**

Agate-crushed powders of all dated samples, together with 13 additional samples, were analyzed by Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES) following the analytical procedure described in Cotten et al. (1995) to determine their major and trace element contents. The relative uncertainties are < 1% for SiO₂, < 2% for other major elements, and < 5% for trace elements. The measuring process was carried out at the Laboratoire Geo-Ocean, Université de Bretagne Occidentale (France). The full dataset of 25 new whole-rock analyses is presented in Appendix B, where major element concentrations have been recalculated to a total of 100% on a water-free basis.

# 4. RESULTS

**4.1. K-Ar dating**

The samples selected for K-Ar dating show porphyritic to aphanitic textures with a mineral assemblage composed of plagioclase, orthopyroxene, clinopyroxene and amphibole crystals (Appendix A). Samples 20EQ62 and 20EQ63 from the summit domes of Pambamarca contain scarce biotite, while sample 20EQ63 contains unusually high amounts of orthopyroxene, amphibole, and biotite. The groundmass is generally composed of plagioclase, pyroxene, and scarce Fe-Ti oxide microlites hosted in a glassy matrix. Radioisotopic measurements were performed on groundmass only, except for samples 20EQ62, where plagioclase phenocrysts were used due to the intense alteration of the groundmass, and 20EQ54, where both groundmass and plagioclase phenocrysts were analyzed. K contents range from 0.964% to 1.411% for the groundmass and from 0.382% to 1.141% for the
plagioclases (Table 1). The minimum $^{40}$Ar* content is 6.7%, registered in sample 19EQ47 sample, with a mean of about 21%. Note that although the plagioclase fractions contain lower amounts of K, the $^{40}$Ar* content is significantly higher than the detection limit (Quidelleur et al., 2001), allowing accurate and reliable age determination using the Cassignol-Gillot technique.

Twelve new K-Ar ages are presented in Table 1 and Figure 3. Samples from Casitagua volcano yield ages of 785 ± 16 ka (20EQ51a) and 878 ± 13 ka (20EQ52a; Fig. 3a). A narrower age range of 1273 ± 20 ka (19EQ01) and 1112 ± 22 ka (19EQ47) is obtained for Ilaló volcano. Further east, two andesitic lavas from Pambamarca volcano are dated at 1261 ± 18 ka (20EQ60) and 1374 ± 21 ka (Fig. 3b; 20EQ56). Due to the highly weathered state of the groundmass phases observed in the summit lava dome samples from Pambamarca volcano, we analyzed plagioclase phenocrysts and obtained an age of 1292 ± 20 ka for the Jambi Machay dome (20EQ62; Fig. 3b). Three ages at 1084 ± 17 ka (20EQ67b), 1128 ± 16 ka (20EQ68), and 1132 ± 16 ka (20EQ66) were obtained for Puntas volcano (Fig. 3c). Due to the limited number of samples from Coturco volcano and the slight evidence of weathering, the measurements were performed on both groundmass and plagioclase phenocrysts extracted from the freshest sample 20EQ54. The calculated values were consistent within the 1-sigma uncertainty range, yielding a mean age of 1959 ± 28 ka (Fig. 3c). Finally, the lava from the old Chacana series sampled in the Antisanilla valley was dated at 2228 ± 34 ka (20EQ85; Table 1, Fig. 3a).

**4.2. Geochemical characterization**

Figures 4a and 4b show the Peccerillo and Taylor (1976) rock classification diagram for the samples collected in this study. Most of the samples are Low to Medium-K basaltic andesites and andesites with SiO₂ contents ranging from 53 to 63 wt.%, and K₂O contents ranging from 0.7 to 2.5 wt.%. Samples collected on the Casitagua volcano plot in the field displayed by the PDC deposits associated with this edifice (yellow symbols and fields, Fig. 4 a, b), sampled in the Quito basin, east of the Monjas river (Fig. 3a). Note that data from the Catequilla dome, located northeast of Casitagua (Fig. 3a), are also plotted in the same Casitagua PDC field (Pacheco, 2013). The andesitic lavas from
Ilaló volcano lie in the field described in previous studies (Chiaradia et al., 2009), except for sample 19EQ47, which has a lower silica content (red symbols and fields, Fig. 4b). Sample 20EQ85 form the old Chacana series is a basaltic andesite that plots outside the Chacana field (e.g., Chiaradia et al., 2009, 2014; Villares, 2010), although it appears to be consistent with the overall volcano trend (green symbols and fields, Fig. 4a, b). The samples from the Jambi Machay (20EQ62) and Quito Loma (20EQ63) summit domes of Pambamarca volcano are the only Medium-K rhyolites identified in this study (purple symbols, Fig. 4a). The samples from the Herradura dome (20EQ57 and 20EQ58) exhibit an unusual K₂O enrichment at low SiO₂ content, which classifies them as basaltic andesites belonging to the Shoshonitic series, which are quite common in Back-Arc volcanoes (e.g., Hoffer et al., 2008; Garrison et al., 2017).

Rare-Earth Elements (REE) normalized to chondrite and spider diagrams of trace elements normalized to the primitive mantle are shown in Figures 4c and 4d, respectively. In summary, these diagrams show fractionation between Light REE (LREE; e.g., La, Ce) and Heavy REE (HREE; e.g., Yb) with a slightly declining but homogeneous pattern, with no Eu anomaly observed for Casitagua, Ilaló, Puntas and Coturco volcanoes. A slight increase in REE concentration is observed between the Volcanic Front (Casitagua), Inter-Andean Valley (Ilaló) and the Main Arc (Puntas, Coturco), indicating a positive correlation between REE contents and distance from the trench (Fig. 4c-d), as previously documented at the arc scale (e.g., Ancellin et al., 2017). Sample 20EQ48 from Ilaló volcano distinguishes with significantly higher contents in most REE. The lavas of Pambamarca volcano exhibit a wide range of REE contents, with the andesitic samples, the summit rhyolitic domes, and the Herradura dome defining compositional fields consistent with those found in the Chacana caldera. In particular, the Herradura dome (20EQ57, 20EQ58) shows the highest REE enrichment of the whole dataset, creating a pattern remarkably similar to the Yuyos lava flow from the Chacana caldera (Fig. 3a; Chiaradia et al., 2014). Other lavas with similar petrographic and geochemical characteristics occur at Cayambe (Cono La Virgen; Samaniego et al., 2005) and Antisana (Cuyuja lavas; Hall et al., 2017) volcanoes. The primitive mantle-normalized trace element patterns reveal an overall enrichment in Large-Ion Lithophile Elements (LILE; e.g., Rb, Ba). A negative Nb anomaly is observed for the whole
dataset (Fig. 4d), typical of arc magmas. Only samples from the Jambi Machay (20EQ62) and Quito Loma (20EQ63) domes show significant negative P and Ti anomalies. These domes also have the highest depletions of Dy, Y and Yb (Fig. 4d).

# 5. DISCUSSION

## 5.1. Comparison between new and previous ages from the Quito segment

Overall, our new K-Ar ages are in agreement with previously published studies. The Catequilla and Pacpo domes, located northeast and southeast of Casitagua volcano (Fig. 3a), were previously dated between 833 ± 26 and 898 ± 15 ka using the same unspiked K-Ar method (Pacheco, 2013; Alvarado et al., 2014). We dated two andesitic lava flows sampled on the flanks of Casitagua volcano and obtained close ages of 785 ± 16 and 878 ± 13 ka (20EQ51a and 20EQ52a, respectively; Fig. 3a). These new data suggest that the eruptive activity of Casitagua was contemporaneous with the emplacement of the Catequilla and Pacpo satellite domes. Furthermore, this period of eruptive activity is stratigraphically consistent with the K-Ar age of 1152 ± 30 ka obtained from the underlying basaltic andesite lavas assigned to the Pisque Formation (Fig. 3a; Alvarado et al., 2014).

In the Inter-Andean Valley, groundmass ages acquired from the Ilaló volcano suggest an activity between 1112 ± 22 and 1273 ± 20 ka (19EQ47 and 19EQ01, respectively; Table 1). This range is younger than the rather poorly constrained whole-rock K-Ar age of 1620 ± 160 ka previously reported (EC 76; Barberi et al., 1988). Although the exact sampling location of EC 76 was not provided, it appears to correspond to the lava 19EQ01, located in an easily accessible outcrop on the northwestern flank of Ilaló (Fig. 3a; P. Mothes, pers. comm.). We interpret the age difference to be due to (1) the presence of phenocrysts within the whole-rock fraction that may carry inherited argon; and/or (2) the inclusion of weathered grains for which potassium may have partially leached out (e.g., Samper et al., 2008; Schaen et al., 2020). Note that such a bias toward older ages has already been noted for other lavas in the Ecuadorian arc dated on whole-rock fractions (e.g., Bablon et al., 2018, 2019) and elsewhere (e.g., Quidelleur et al., 1999, 2021; Samper et al., 2008).
The ages obtained in this study were not biased by differences in the ratio of microcrystals to glass in the groundmass. Samples from the Ilaló volcano, for instance, contain different amounts of glass in their groundmass. Sample 19EQ01 has a groundmass rich in microcrystals (80%), while sample 19EQ47 has lower amounts of crystals (60%). However, the ages of both samples (1273 ± 20 ka and 1112 ± 22 ka, respectively) are consistent with their relative stratigraphic position. Similar statements can be made for other volcanoes described here or in other studies (e.g., Bablon et al., 2018, 2020a; Santamaría et al., 2022; Santamaría et al., 2023; Almeida et al., 2023). Note that vitreous samples, including obsidians and pumices, have also been successfully dated in agreement with stratigraphy and with ages provided by other dating techniques (e.g., Bablon et al., 2020b; Santamaría et al., 2023).

**5.2. Geochronological data compilation of the Ecuadorian volcanic arc**

In order to constrain the onset of eruptive activity of the current Ecuadorian arc, we have compiled available geochronological data for Ecuadorian volcanoes (Fig. 5). Note that ¹⁴C ages are not considered here because they have been largely used for the detailed study of the recent eruptive history of volcanism in Ecuador, due to their limitation to about 50 ka towards their older bound. Nevertheless, a compilation of these data is offered in Santamaría (2017). Low quality data, such as whole-rock K-Ar ages, have been discarded (see section 5.1). Similarly, ⁴⁰Ar/³⁹Ar ages without minimum quality criteria, such as plateau ages calculated with at least three consecutive steps corresponding to at least 50% of the total ³⁹Ar released, and having an associated isochron with an initial ⁴⁰Ar/³⁶Ar ratio indistinguishable from the atmospheric value (e.g., Fleck et al., 1977), have also been removed. Finally, following Ramon et al. (2021), we considered volcanic systems to be “potentially active” or “active”, if there was evidence of Holocene or historical activity, respectively (Table 2).

We followed the approach of considering eruptive subsystems that show spatial and/or temporal overlap (e.g., Rucu Pichincha and Guagua Pichincha volcanoes) as a single volcano (e.g., Pichincha volcanic complex; Robin et al., 2010). A thorough bibliographic review led to the identification of 78 individual volcanic systems in the Ecuadorian arc (Appendix C), 45 of which have published
radioisotopic ages in the Ecuadorian arc (Table 2). This database includes nearly 250 ages (Appendix C), with 145 recent K-Ar ages obtained on groundmass (Fig. 5; Alvarado et al., 2014; Telenchana et al., 2017; Bablon et al., 2018, 2019, 2020a, 2020b; Andrade et al., 2021; Santamaría et al., 2022, 2023; this study; Almeida et al., 2023), 89 $^{40}$Ar/$^{39}$Ar ages measured on groundmass or separated crystalline phase (e.g., Monzier et al., 1999; Hammersley, 2003; Samaniego et al., 2005; Hidalgo, 2006; Opdyke et al., 2006; Hoffer, 2008; Hoffer et al., 2008; Hall et al., 2017; Robin et al., 2010; Le Pennec et al., 2011; Bellver-Baca et al., 2020), as well as 15 fission track ages (e.g., Bigazzi et al., 1992, 2005; Bellot-Gurlet et al., 2008).

## 5.3. Pleistocene eruptive history of the Ecuadorian arc

Figure 6 shows the timing of activity based on the radioisotopic ages database presented above (Appendix C) and summarized in Table 2, as well as stratigraphic and morphological data. Remarkably, the volcanic activity was not uniformly distributed in space and time. Based on the number of active volcanoes, three time-intervals of volcanic activity can be identified from Figure 6, with an early stage older than 1.4 Ma, an intermediate stage between 1.4 and 0.6 Ma, and a late stage for activity younger than 0.6 Ma. Accordingly, a summary of the Pleistocene eruptive history of the Ecuadorian arc is presented below.

### 5.3.1. Early stage: > 1.4 Ma

The few oldest available ages provide evidence for volcanic activity occurring from at least 2.5 and up to 1.4 Ma in the Quito and Ambato-Latacunga segments (dark yellow fields, Fig. 7). Note that the occurrence of coeval activity in other segments cannot be excluded due to sampling bias towards the most recent units and the limited number of published studies focusing on Early Pleistocene volcanism. In addition, the gradual erosion of the oldest deposits and their coverage by younger products prevents us from defining a precise age for the onset of the arc construction, although geochronological data seem to point to ~2.5 Ma.
## Quito segment

$^{40}$Ar/$^{39}$Ar plateau ages reported for the **Chacana** caldera (Western Cordillera) suggest that active volcanism occurred in the center of the present-day Chacana caldera at ~2.6 ± 0.2 Ma (Opdyke et al., 2006). We have obtained here a consistent, albeit much more precise, K-Ar age of 2228 ± 34 ka (20EQ85; Table 1) for a basal lava flow that forms the southern flank of the caldera. To the east, the construction of **Coturco** volcano appears to be contemporaneous with this early stage of Chacana, as suggested by our K-Ar age of 1959 ± 28 ka (20EQ54; Table 1) acquired from a summit lava flow. No additional age from this period has been reported for the Western Cordillera or the Sub-Andean zone in the Quito segment.

## Coeval activity in the Ambato and Latacunga segments (Back-Arc)

The low-volume, mostly effusive volcanism that occurred in the Sub-Andean zone formed several cone-shaped landforms of a few tens of meters height (Cacalurco and Chuvaurcu cones), as well as thin lava sequences south of the town of Puyo (Hoffer et al., 2008; Ball, 2015). A $^{40}$Ar/$^{39}$Ar age of 2770 ± 20 ka was obtained from the northern cone of **Cacalurcu** volcano (Fig. 7; Hoffer et al., 2008). Note that intense erosion and scarce exposures prevented accurate mapping in this zone. To the north of Puyo, a lava sequence identified as “**Mera Lavas**” was dated by $^{40}$Ar/$^{39}$Ar at 1980 ± 50 ka (Fig. 7; Hoffer et al., 2008). The source of these lavas remains unknown.

## Old ages reported from the Ibarra segment

Four $^{40}$Ar/$^{39}$Ar ages ranging from 3610 ± 60 to 61 ± 20 ka (Béguelin et al., 2015) have been reported for the Yanaurcu de Piñán volcanic system from the Ibarra segment. Such a very long duration of activity is puzzling, as it has not been observed for any other composite volcano of similar size from the whole Ecuadorian arc. Based on their isotopic ratios (Fig. 8 in Béguelin et al., 2015), it can be
suggested that the Andesitic Old Yanaurcu formation (~3.6 Ma) has a different origin than the younger
(<172 ± 20 ka) products of the Yanaurcu de Piñán volcano. These old products could be related to the
Late Miocene Pugarán formation, which covers part of the Western Cordillera and is dated at 5.0 ± 2.9
Ma (Boland et al., 2000), or to the Miocene-Pliocene Angochagua formation of the Eastern Cordillera,
which is dated between 6.31 ± 0.10 and 3.65 ± 0.07 Ma (Barberi et al., 1988; Boland et al., 2000).

**5.3.2. Intermediate stage: 1.4 to 0.6 Ma**

During the intermediate construction stage, several new edifices were constructed in the Quito
segment and in the southern area of the Ibarra segment (dark to light green fields, Fig. 7). At the end of
this stage, new volcanic activity occurred in the Tulcán segment to the north and in the Machachi and
Ambato segments to the south, where only a few new structures appeared.

***Quito segment***

In the Eastern Cordillera, the activity of the **Chacana** caldera (dark green field, 8) is associated
with massive lava sequences overlain by voluminous ignimbrite deposits and obsidian flows (Hall and
Mothes, 2008c; Beate and Urquizo, 2015). The sequence outcropping on the western caldera flank has
been dated between 1350 ± 90 and 1580 ± 70 ka (Bigazzi et al., 2005; Opdyke et al., 2006). To the
north, the overlying El Tablón obsidian flow, dated at 810 ± 50 ka (Bigazzi et al., 2005), provides the
younger bound for the Chacana caldera formation. Notably, the eastern flank of the oldest Coturco
volcano (Fig. 8) seems to be covered by these sequences, further supporting the new K-Ar age of 1959
± 28 ka obtained here for this volcano (Table 1).

Further north, the **Pambamarca** and **Puntas** volcanoes (Fig. 8) were built on the northwestern
and western flanks of the Chacana caldera. The eruptive activity of these andesitic volcanoes occurred
while the Chacana caldera was already active, as indicated by our K-Ar ages ranging from 1261 ± 18
to 1374 ± 21 ka for Pambamarca volcano, and from 1132 ± 16 to 1084 ± 17 ka for Puntas volcano
(Table 1). The Pambamarca activity ended with the extrusion of the summit rhyolitic dome complex at 1292 ± 20 ka (20EQ62). We propose that the main stage of the Chacana caldera may be more recent or contemporaneous with Pambamarca and Puntas volcanoes, considering that the Chacana ignimbrite series partially covered these edifices (Hall and Mothes, 2008c). The age of the post-caldera activity at Chacana is defined by the Plaza de Armas lava sequence, located south of the caldera (Figs. 3, 8) and dated at ~726 ka (Pilicita, 2013), which is roughly coeval with the El Tablón obsidian flow west of the caldera (Ta, Fig. 8), dated at 810 ± 50 ka (Bigazzi et al., 2005).

Volcanic activity in the Western Cordillera seems to have first appeared during this stage. In the southern Quito segment, **La Carcacha** volcano (Atacazo-Ninahuilca volcanic complex) was active at ~1300 ka (LC, Fig. 8; Hidalgo, 2006). It was followed to the north by **El Cinto** volcano (Pichincha volcanic complex), dated at 1112 ± 24 ka (Fig. 8; Robin et al., 2010). El Cinto edifice was later intruded by the Ungi dome at 910 ± 7 ka (Robin et al., 2010). Further north, the activity of **Casitagua** volcano built an andesitic main edifice surrounded by several domes between 898 ± 15 and 785 ± 16 ka (Fig. 8; Alvarado et al., 2014; this study). Then, the construction of the **Lower Rucu Pichincha** edifice took place between the Casitagua and El Cinto volcanoes. The lava sequences of this volcano have been dated between ~850 and ~590 ka (Robin et al., 2010; Alvarado et al., 2014) and are thus contemporaneous with the Casitagua volcano.

In the Guayllabamba valley (Inter-Andean Valley), the activity of the **Ilaló** volcano occurred between 1273 ± 20 and 1112 ± 22 ka, forming an edifice composed of andesitic lava sequences (Fig. 8). Then, the emission of voluminous basaltic andesitic lavas at the northern border of the Guayllabamba valley was dated at 1152 ± 30 ka (Pisque Formation, L3, Fig. 8; Alvarado et al., 2014) and 1038 ± 87 ka (pre-Mojanda lavas, L2, Fig. 8; Bablon et al., 2020a). Finally, the activity of the **Pan de Azúcar** volcano in the Sub-Andean zone (Fig. 8) occurred during this stage at 1150 ± 10 ka (Hoffer, 2008).

*Extension to the Ibarra and Machachi segments*
During this stage, volcanic activity was also present in the Ibarra and Machachi segments (Fig. 8). **Viejo Cayambe** (Fig. 8) is an ancient edifice formed to the northeast of the Chacana and Pambamarca volcanoes in the Eastern Cordillera (Ibarra segment). This edifice, dated between 1108 ± 11 and 1050 ± 5 ka, was formed by andesitic to dacitic lava sequences that culminated in a caldera-forming eruption (Samaniego et al., 2005). Further south, the activity of **Santa Cruz** volcano (Fig. 7) occurred in the Inter-Andean Valley ~40 km south of La Carcacha volcano (Machachi segment). This edifice was formed by andesitic lava series dated at 702 ± 11 ka and several dacitic domes in the summit area (Santamaría et al., 2022). Furthermore, several obsidian pebbles have been identified in the Cosanga river, in the Sub-Andean zone between the Quito and Machachi segments. These fragments are associated with an unknown source located in the Western Cordillera, probably the **Cosanga** or **Bermejo** centers (Mothes and Hall, 2008a). Two fission-track ages suggest that some of these obsidians were emitted during eruptions that occurred at 670 ± 6 and 290 ± 20 ka (Bellot-Gurlet et al., 2008).

### Coeval activity in other segments

Within the Tulcán segment, the lower **Cerro Negro** edifice (Western Cordillera), located on the Colombian border (Fig. 7), was active at 883 ± 19 ka (Telenchana et al., 2017; Bablon, 2018). Further south, in the Latacunga segment, two K-Ar groundmass ages show that the **Sagoatoa** volcano (Fig. 7) was active from at least 826 ± 12 ka to 799 ± 12 ka (Bablon et al., 2019). We note that two unreliable ages at 1850 ± 190 and 1850 ± 240 ka (Lavenu et al., 1995; Opdyke et al., 2006) were obtained from a sequence of basaltic andesite lavas north of **Chinibano** volcano (Fig 7). However, based on erosional features and comparison with other eroded volcanoes, we suggest that this volcano may have been constructed during the intermediate stage of the Ecuadorian arc.

### 5.3.3. Late stage: from 600 ka

During the late construction stage of the Ecuadorian volcanic arc, a striking increase in the number of active volcanoes can be observed in almost all segments (dark and light blue fields, Fig. 9),
with at least 50 volcanoes active during this period. Although nearly twenty volcanoes remain poorly studied (most of them from the Tulcán and Latacunga segments), Figure 9 suggests that most of the volcanic activity in Ecuador extended over time from the Quito segment both northward and southward. Considering the available data, it is highly plausible that the eruptive history of most of these poorly studied volcanoes occurred during this stage, i.e., the last ~600 ka. As the eruptive histories of the Ibarra, Machachi, Latacunga, Ambato, and Riobamba segments have been presented in detail elsewhere (Bablon et al., 2019, 2020a; Santamaría et al., 2023), we only detail the eruptive history of the Quito segment here.

## Quito segment

Volcanic activity in the Quito segment was widespread in both Cordilleras and in the Sub-Andean zone. In the Western Cordillera, the eruptive history of the Rucu Pichincha edifice extended to ~150 ka, including the sector collapse of its western flank (Robin et al., 2010). The westward migration of the vent caused the formation of the Guagua Pichincha edifice as early as ~60 ka, including two sector collapses and the extrusion of several dacitic domes (Robin et al., 2010). The last eruption of Guagua Pichincha took place in 1999-2002 AD. Both edifices form the **Pichincha volcanic complex**.

To the north, the highly explosive activity of the **Pululahua dome complex** occurred during the Holocene (Andrade et al., 2021). In the Eastern Cordillera, several obsidian and lava flows were emitted during the resurgent phase of the **Chacana caldera**. The Mullumica and Callejones flows, dated between 200 and 180 ka, are associated with major Plinian eruptions whose pumice fallout deposits, known as the Pifo layers, are key stratigraphic markers of the Guayllabamba basin (Bigazzi et al., 2005; Hall and Mothes, 2008c). To the north of the Chacana caldera, the **Izambi volcanic series** was formed at 370 ± 60 ka (Bigazzi et al., 2005), while the underlying lava sequences probably belong to Chacana (e.g., Aguilera et al., 2007; Hall and Mothes, 2008c). Unlike the adjacent segments, volcanic activity in the Inter-Andean Valley appears to be absent during this stage. However, thick volcaniclastic sequences from both Cordilleras were deposited in the Inter-Andean Valley, resulting in the Guayllabamba and Chiche formations overlying the lacustrine deposits of the San Miguel formation (Villagómez, 2003;
Pacheco et al., 2014). Further west, several rhyolitic eruptions occurred on the western margin of the Cordillera Occidental, to the east of the Chacana caldera (e.g., Cosanga and Bermejo centers; Mothes and Hall, 2008a). Although the eruptive histories of these volcanoes remain poorly documented, radiocarbon determinations and stratigraphic relationships suggest that their activity continued into the Holocene (e.g., El Dorado and Pumayacu centers; Hall and Mothes, 2010). Further west, unpublished $^{40}$Ar/$^{39}$Ar ages suggest that the **Sumaco** volcano (Back-Arc) was active between 255 ± 32 and 200 ± 8 ka (Appendix C; M. Fornari pers. com.), with its most recent activity occurring between the 16th and 20th centuries (Salgado et al., 2021).

## Holocene volcanism

Among the 78 Quaternary volcanoes identified in our study (Appendix C), 25 were active during the Holocene (Fig. 9). They include Antisana (Hall et al., 2017), Atacazo-Ninahuilca (Hidalgo et al., 2008), Calpi cones (Bablon et al., 2019), Cayambe (Samaniego et al., 1998), Chacana (Hall and Mothes, 2008c), Chachimbiro (Bernard et al., 2014), Chimborazo (Barba et al., 2008), Cotacachi-Cuicocha (Almeida et al., 2023), Cotopaxi (e.g., Hall and Mothes, 2008a), Cubilche (Navarrete et al., 2020), Huañuna (Hall et al., 2017), Iliniza (Santamaría et al., 2022), Imbabura (Le Pennec et al., 2011), Pichincha (Robin et al., 2008), Pululahua (Andrade et al., 2021), Quilotoa (e.g., Di Muro et al., 2008; Mothes and Hall, 2008b), Reventador (e.g., Hall et al., 2004), Sangay (Monzier et al., 1999), Soche (Beate, 1994; Hall and Mothes, 2008b), Sumaco (Salgado et al., 2021), Tungurahua (e.g., Hall et al., 1999; Le Pennec et al., 2013, 2016), and volcanoes on the western side of the Eastern Cordillera El Dorado, Huevos de Chivo, and Pumayacu (Hall and Mothes, 2010). Although there is no clear evidence of significant explosive activity during the Holocene at the Chiles-Cerro Negro volcano (Santamaría et al., 2017), the unrest episode recorded in 2014 AD (Ebmeier et al., 2016) and current fumarolic activity suggest that this volcano is still potentially active. Finally, in the absence of more detailed studies, we do not exclude that other volcanoes may have erupted during the Holocene. For instance, the extrusion of the present-day poorly eroded domes of the Mojanda-Fuya Fuya volcanic complex (e.g., Panecillo
dome; Robin et al., 2009) and Pambamarca volcano (i.e. Herradura dome; this study) could have
occurred during the Holocene.

**5.4. Relationship between volcanism and tectonics**

The ancient NE-SW crustal structures of the Andean Range (black lines, Fig. 10a; Litherland and Aspden, 1992) appear to partially control the alignments of the Ecuadorian arc, especially for frontal volcanoes. Comparable relationships between crustal tectonic structures and the arrangement of volcanoes have also been documented in other Andean volcanic zones (e.g., Acocella et al., 2007; Cembrano and Lara, 2009; González et al., 2009; Salas et al., 2017; Tibaldi et al., 2017) and even worldwide (e.g., Takada, 1994; Tibaldi and Bonali, 2017; Bonali et al., 2018). Indeed, recent evidence suggests that magma storage, differentiation, and mobilization in the Earth's crust are significantly influenced by tectonic controls. Variations in lithology and melt fraction within mush reservoirs create rigidity contrasts that influence the depth of sill intrusion. (e.g., Jackson et al., 2018; van Wyk de Vries and van Wyk de Vries, 2018). Thus, several authors highlighted the arrangement of eruptive centers and/or dykes as a response to regional tectonic stress (e.g., Delaney et al., 1986; Cembrano and Lara, 2009; Sychev et al., 2019; Díaz et al., 2020). Moreover, the distribution of volcanoes and unrest periods also appear to be influenced by local stress regimes and the mechanical properties of the bedrock, which ultimately affect the magma pressure in the conduit (e.g., Gudmundsson, 2006; Gudmundsson and Philipp, 2006; Ebmeier et al., 2016). Consequently, the spatial distribution of volcanoes can result from both deep (i.e., slab dehydration reactions, mantle heterogeneities) and shallow (i.e., tectonics, basement structure) controls. Following the approach of Cembrano and Lara (2009), we discuss below the volcano-tectonic relationship for present-day active and inactive (inherited) crustal structures of the Ecuadorian arc.

***Volcanoes over inherited basement structures***
In the Western Cordillera, edifices of the Volcanic Front overlie the ancient margin-parallel Pujilí and Toachi thrust-belts sutures, which could serve as passive pathways for magma ascent. These NNE-SSW trending fault systems (black lines, Fig. 10a) separate the Cretaceous oceanic plateau basalts and ophiolites units (e.g., San Juan ($K_{Sj}$) and Pallatanga ($K_{Pa}$) Formations) to the East, from the Paleogene to Eocene volcanic and volcanoclastic units (e.g., Macuchi Formation ($PcE_M$)) to the West (Hughes and Bermúdez, 1997; Hughes and Pilatasig, 2002; Vallejo et al., 2019). The Chachimbiro, Cotacachi, Pichincha, Iliniza, Sagoatoa, and Chimborazo volcanoes from the Western Cordillera are located over the Pujilí-Pallatanga suture zone, whereas Almas Santas and Quilotoa, the westernmost volcanoes of the arc, lie above the Chimbo-Toachi suture zone (Fig. 10a; Hughes and Pilatasig, 2002; Vallejo et al., 2019). The relationship between the basement structures and distribution of volcanism is less obvious in the Eastern Cordillera. However, we note that the eruptive centers of Pambamarca, Puntas, Sincholagua and Cotopaxi on one side, and Cayambe, Chacana, Antisana, and Chalupas on the other side show N-S alignments (Fig. 10a). Although the reverse fault structures of the western margin of the Eastern Cordillera are hidden by a thick Pliocene volcanic cover (e.g. Pisayambo ($PL_{Py}$) and Angamarca ($PL_{An}$) Formations), their existence is inferred from the scarce exposures further north in the Chota Valley and in the Pisayambo zone to the south (Litherland et al., 1994; Winkler et al., 2005).

These major fault systems separate the metamorphic groups formed during the Triassic anatexis and posterior accretion of the Jurassic para-autochthonous terrains (Litherland et al., 1994; Villagómez et al., 2011; Spikings et al., 2015). For instance, the group composed of Pambamarca, Puntas, Sincholagua and Cotopaxi volcanoes was constructed over the Peltetec suture zone (Litherland et al., 1994; Vallejo et al., 2019). Further west, the Mojanda, Ilaló, Pasochoa, and Rumiñahui alignment might also indicate the presence of inherited basement structures beneath the Inter-Andean Valley. Nonetheless, exposures of the basement of the Inter-Andean valley are scarce, resulting in an incomplete understanding of the suture architecture created by the accretion of the Western Cordillera terrains (Litherland et al., 1994; Hughes and Pilatasig, 2002). Farther east, the Back-Arc volcanoes are built near basement-involved thrust belt faults related to the Napo Uplift, which was formed by the eastward thrusting of the Western Cordillera (Bès de Berc et al., 2005; Gutiérrez et al., 2019). Although several of these structures are still active, their extent and behavior remain poorly documented.
*Volcanoes over present-day compressional or trans-pressional tectonics*

Although several active fault systems (fs) have been identified in the Ecuadorian Andes (red lines, Fig. 10b; e.g., Alvarado et al., 2016), their relationship to volcanic landforms is not straightforward (Andrade, 2009). For instance, few volcanoes are found near or over the CCPP-fs, the major tectonic structure in Ecuador that connects several trans-pressive and reverse subsystems from the Gulf of Guayaquil (Fig. 1a) to the eastern border of the Eastern Cordillera. The CCPP-fs intersects the Inter-Andean Valley in the Ambato segment and runs through the Calpi cones and the Igualata, Huisla, and Mulmul volcanoes (e.g., Bablon et al., 2019; Baize et al., 2020), but does not coincide with the whole Ambato volcanic cluster. Figure 10b shows that many volcanoes from the arc do not overlie major fault-systems, although their evolution may be influenced by fault kinematics (Tibaldi et al., 2007; Alvarado et al., 2016). For instance, the Billecocha subsystem (B-fs) may have influenced the orientation of the Cotacachi sector collapses as well as their subsequent reconstruction patterns (Jomard et al., 2021; Almeida et al., 2023). Further east, the Alambi subsystem (A-fs) played a key role in the preferential growth patterns of the Imbabura volcano (Andrade et al., 2019). The central part of the arc lies in the interaction zone between the Quito and Latacunga reverse fault systems (QL-fs), which are expressed to the north and south of the segment as parallel strands of folds overlying major west-dipping, blind, *en echelon* thrust faults (Fiorini and Tibaldi, 2012; Alvarado et al., 2014, 2016). Notably, several segments of the Quito-fs are curved around volcanic edifices suggesting that the onset of volcanic activity preceded tectonic activity (Alvarado et al., 2014).

**5.5. Relationship between volcanism, the Nazca slab and induced crustal stress**

Numerical reconstructions can be used to estimate the present-day bulk volume of volcanic edifices and infer the volume of material accumulated at the end of their construction phases (e.g., Grosse et al., 2013; Germa et al., 2015; Ricci et al., 2015; Bablon et al., 2018, 2020a; Santamaría et al., 2022; Santamaría et al., 2023). A summary of the available volumetric data for Ecuadorian volcanoes,
along with their activity periods, is presented in Appendix D. These data show a first-order correlation
between the area (S) covered by each volcanic edifice and the volume (V) of accumulated material, with
the relationship V = 0.42 S (Figure 11). This equation was then used to derive a rough estimate of the
cone-building volume for the nine dated stratovolcanoes for which no volumetric data are reported in
the literature (i.e., Casitagua, Chachimbiro, Coturco, Ilaló, Pambamarca, Pan de Azúcar, Puntas,
Reventador, Yanahurcu de Piñán). Since the eruptive centers with the largest basal areas are located
near 0° latitude (Bablon et al., 2020a), the highest volumes of magma were erupted by volcanoes in the
Quito and Ibarra segments. Figure 12 shows an exponential increase of the cumulative volume (V) for
the whole arc through time (t) according to the relationship V = 2974e⁻⁰.⁰⁰³t (R²=0.977), with a total
volume of about 2800 km³ for the last 2 Myr. The volume of erupted material increased sharply during
the late evolutionary stage, from ~500 to ~2760 km³. The erupted volume (DRE) for the Chalupas,
Cuicocha, and Quilotoa calderas were included in this estimate (Crosweller et al., 2012). It must be
acknowledged that these data are rough estimates that require further investigations. Accordingly, future
studies must take into account several factors, including tephra dispersion, tephra versus lava
production, volcanic deposit density variations, numerical reconstruction techniques, and uncertainty
in numerical values (e.g., Yamamoto et al., 2018; Bablon et al., 2020a; O'Hara and Karlstrom, 2023;
Santamaria et al., 2023). Moreover, the lack of geochronological and stratigraphic data for several
volcanoes may affect the accuracy of these estimates. For instance, the Chacana caldera lacks adequate
estimates of the volume of material erupted during its long-lived explosive activity, which has been
extensively eroded and/or widely dispersed. Whatsoever, Figure 12 provides an interesting overview of
the evolution of the Ecuadorian arc, with a strong increase in magmatic productivity coeval with the
northward and southward extension of the arc observed during the Late stage since 600 ka.

The spatial distribution of volcanic clusters toward the center of the Ecuadorian arc, together
with long-lived volcanism exclusively located within the Quito and Ibarra segments, and with the widest
range of incompatible element contents for products of edifices located around 0.5°S (Figure E1; e.g.,
Ancellin et al., 2017), suggests the highest magma generation rates. The Nazca slab beneath Ecuador
has a contorted surface, where the projection of the Grijalva FZ coincides with an inferred slab flexure
at depth (Fig. 1a; Yepes et al., 2016). Bablon et al. (2019) proposed that the apparent southward extension of volcanism from the Ambato segment, above the Grijalva FZ, may have been influenced by changes in the slab geometry, and therefore changes in pressure and temperature conditions, during the last 1 Myr. Additional mechanisms are needed to explain the spreading of volcanism observed since the early stage of the Ecuadorian arc, must be invoked. Since at least ~2.5 Ma, intense volcanism seems to be mainly present in the Quito segment, which lies above the projection of the Carnegie Ridge (Fig. 1a). Recent geochemical signatures highlight that the sub-arc mantle wedge is heterogeneously metasomatized by aqueous fluids and siliceous melts from slab dehydration and partial melting (Ancellin et al., 2017; Narvaez et al., 2018). This evidence may indicate a higher thermal gradient of the sub-arc slab consistent with the position of the Carnegie Ridge. Thus, the thermal regime of the subducting young Nazca crust and the Carnegie Ridge likely favored magma generation since the Early Pleistocene.

The increase in the emission rate of volcanic products, and thus the spread of volcanism described above, may also be related to variations in the crustal stress field, in addition to changes in the magmatic production rate associated with variations in slab geometry and rheology. Indeed, the timing of the increase in the volcanic activity seems to coincide with other regional tectonic events, such as: (1) the cessation of the Late Pliocene WNW-ESE compressional phase registered in the Chota basin (Fig. 2; northern Ecuadorian Andes) prior to the Late Pleistocene, as indicated by the deformed dykes and sills (AFT age: 3.7 ± 1.7 Ma) unconformably overlain by the undeformed volcaniclastic rocks (ZFT age: 0.5 ± 0.2 Ma; Barragán et al., 1996; Winkler et al., 2005); (2) the formation of the Quito-Latacunga microblock during the Pleistocene as a consequence of the latest stage of the North Andean Sliver deformation zone from the Western Cordillera to the east of the Eastern Cordillera, through the creation (or reactivation) and abandonment of tectonic structures over time to accommodate convergent directions (Alvarado et al., 2016); and (3) the Late Pleistocene increase in subsidence and deposition rates reported in the Gulf of Guayaquil (Fig. 1a), recorded by the up to 4 km thick sediment stack, dated by foraminiferal chronobiozones and marine transgression-regressive structures, overlying the thinner Pliocene series (Witt et al., 2006; Witt and Bourgois, 2010; Loayza et al., 2013; Peuzin et al., 2023).
These surface observations of upper crustal dynamics during the Pleistocene may reflect changes at depth, probably related to increased plate coupling at the subduction interface and the presence of the subducting Carnegie Ridge and South American plate (e.g., Margirier et al., 2023).

# 6. CONCLUSIONS

After three decades of vulcanological research on the Ecuadorian Andean volcanism, the numerous published geochronological data have been compiled, helping to improve knowledge of the eruptive history of Quaternary volcanoes and to define the spatial and temporal evolution of volcanism in the whole arc. The ~250 radioisotope ages included in this geochronological data compilation show that the Ecuadorian arc was constructed in three main stages, with an overall increase in the number of active volcanoes. Based on our dataset, we propose that (1) volcanism began at least 2.5 Ma ago in the central part of the Ecuadorian arc (Quito segment) with the formation of the Coturco volcano and the onset of activity of the Chacana caldera in the Eastern Cordillera. Minor volcanic activity seems to have occurred in the southern Back-Arc (Ambato segment) with the emission of remote scoria cones and lava flows (Hoffer et al., 2008). (2) Volcanic activity extended around the Guayllabamba valley at ~1.4 Ma, as evidenced by products from Cayambe (Samaniego et al., 2005), Pambamarca, Puntas, Casitagua (this study), Pichincha (El Cinto and Rucu Pichincha; Robin et al., 2010), and La Carcacha (Atacazo volcanic complex; Hidalgo, 2006), followed by the construction of the Ilaló volcano and the emission of the Pre-Mojanda (Bablon et al., 2020a) and Pisque lavas (Alvarado et al., 2014) in the Inter-Andean Valley. Toward the end of the intermediate stage (i.e., ~900-600 ka), single edifices appeared in the northern (Cerro Negro; Telenchana et al., 2017; Bablon, 2018), south-central (Santa Cruz; Santamaría et al., 2022), and southern (Sagoatoa; Bablon et al., 2019) zones of the Inter-Andean Valley. (3) A remarkable increase in volcanic activity has occurred since ~600 ka, with at least 50 volcanoes active during this interval. Rather than being homogeneously distributed along the arc, several of these volcanoes were located in areas of a few tens of square kilometers, forming E-W elongated volcanic clusters (e.g., Bablon et al., 2019, 2020a; Santamaría et al., 2023). At least 25 of these volcanoes were active during the Holocene.
The high concentration of volcanoes in the central part of the arc since the beginning of Quaternary volcanism spatially coincides with the inland projection of the Carnegie Ridge. The thermal regime of the subducting young Nazca crust and the Carnegie Ridge thus likely favored magma generation from the Early Pleistocene to the present. Furthermore, the progressive increase of the plate coupling at the subduction interface together with the oblique convergence of the Nazca Plate contributed to the uplift of the Andes and the deformation of the South American plate, with the development of regional crustal fault systems. Therefore, the northward and southward migrations of the arc volcanoes reported here can be explained by the interplay of several factors, including deep control (i.e., slab geometry and age, slab-derived dehydration and melting reactions, mantle heterogeneities), and tectonic reactivation of old structures inherited from the continental crust.
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FIGURES

<figure><img src="image_2.png" /></figure>

**Figure 1.** **a)** Geodynamic setting of the Ecuadorian margin. White arrow indicate the direction of the Nazca plate motion relative to South America (DeMets et al., 2010). Topography and bathymetry from the GEBCO 2020 program. Slab depth and Carnegie Ridge inferred extension from Yepes et al. (2016) and Gutscher et al. (1999), respectively. **CCPP:** Chingual-Cosanga-Pallatanga-Puná fault system (Alvarado et al., 2016). **NAS:** North Andean Sliver; **SAP:** South American Plate. **b)** Major geomorphological provinces of Ecuador (modified from Aspden et al., 1992).
<figure><img src="image_3.png" /><figcaption>**Figure 2.** Segmentation of the Ecuadorian volcanic arc. Each section is named after the main city it contains, which are highlighted with black bold letters. Volcanoes' shapes taken from Bernard and Andrade (2011), and are colored according to their distance from the trench. Examples of volcanic landforms: (1) Pululahua lava dome complex, (2) Pichincha volcanic complex, (3) Cotopaxi volcano, (4) Chalupas caldera, (5) Tungurahua volcano, (6) Tulabug cone. Main valleys represented as blue letters.</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>**Figure 3.** Hill-shaded digital surface model (topography from the Sigtierras program) of the Quito Segment showing the sampling locations and geochronological results. Numbers correspond to the last two digits of sample names (19EQxx or 20EQxx according to the recollection year). K-Ar dated samples are represented with solid symbols. K-Ar ages from Alvarado et al. (2014) are shown with black diamonds. Volcanoes shown as black bold letters. Volcanic landforms shown as red letters. Rivers and valleys represented as blue letters. Coordinates are in Universal Transverse Mercator (UTM) zone 17.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Figure 4.** **a-b)** K₂O vs SiO₂ diagram (Peccerillo and Taylor, 1976) for eruptive products of the Quito segment volcanoes. HK: high-K, MK: medium-K, and LK: low-K calc-alkaline series. Colored areas represent the composition fields created by bibliographic data for Casitagua, Chacana, and Ilaló volcanoes (Pacheco, 2013; Chiaradia et al., 2009, 2014). Shaded areas represent the composition fields of the volcanoes from Western Cordillera, Eastern Cordillera, and Back-Arc based on the Georoc database (http://georoc.mpch-mainz.gwdg.de/georoc/). **c)** Rare Earth Elements normalized to chondrites, and **d)** Incompatible trace elements normalized to the primitive mantle diagrams (Sun and McDonough, 1989) for the Quito segment. The sampled volcanoes were organized according to their position in the Western Cordillera (yellow lines), Inter-Andean Valley (red lines), and Eastern Cordillera (blue and turquoise lines). Chacana (green lines) and Pambamarca (purple lines) samples showed in separated panels. Shaded area refers to bibliographic data from Chacana (Bryant et al., 2006; Chiaradia et al., 2009, 2014).</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Figure 5.** (Caption provided above)</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Figure 5.** Summary of the location of published K-Ar, <sup>40</sup>Ar/<sup>39</sup>Ar and obsidian Fission Track (FT) ages obtained on products of the current Ecuadorian arc, and of the new K-Ar ages presented in this study (black dots, blue squares, and green dots, respectively). Sampled volcanoes are colored in function to their distance to the trench. Details of the sampled unit, dated material, and references are available in Appendix C.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>**Figure 6.** Stages of eruptive history of the Ecuadorian arc. Individual cone-building stages represented according to the position of each volcano relative to the Volcanic Front axis. The cone-building stages of a lifespan lest than ~100 kyr are represented as solid dots. The oldest stages of the volcanoes in the Tulcán and Latacunga segments remain poorly constrained.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>**Figure 7.** Sketch of the Early (dark yellow) and Intermediate (green) stages of eruptive history of the Ecuadorian arc. The structures crated during the Intermediate stage colored by age: 1.4-1.2 Ma (dark green), 1.2-1.0 Ma (green), and 1.0-0.6 Ma (light green). The locations of the Pre-Chacana (L1), Pre-Mojanda (L2), Pisque Fm (L3), and Mera eruptive centers are undefined, thus they are represented as oversized dots. Landforms referred in the text shown as red letters as in Figure 3. An: Antisanilla; EC: El Cinto; LC: La Carcacha; QY: Quiscatola-Yanaurcu series; RP: Rucu Pichincha.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Figure 8.** Sketch of the Quito segment at the end of the Intermediate stage. Volcanic landforms colored as in Figure 6. **Early stage events:** emission of the pre-Chacana lavas (L1, An, QY) and construction of the Coturco volcano. **Intermediate stage events:** formation of the early Chacana series followed by the construction of the Pambamarca, Puntas, and Viejo Cayambe edifices. Further east, construction of Ilaló, El Cinto (EC), and La Carcacha (LC), and emission of the Pre-Mojanda (L2) and Pisque (L3) lavas. Activity of Pan de Azúcar volcano occurred in the Sub-Andean zone. The last edifices to be constructed were Casitagua and Rucu Pichincha (RP). The deposits created by the formation of the Chacana Caldera (Gu, Ta, Pl) partially covered the Pambamarca, Puntas, and Coturco volcanoes.</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>**Figure 9.** Sketch of the Late (blue) stage of the eruptive history of the Ecuadorian arc. Structures created during this stage colored by age: 600-300 ka (dark blue) and <300 ka (light blue). Locations of small eruptive centers (<5 km in diameter) represented as oversized dots. Volcanoes with Holocene activity highlighted with yellow triangles. See Appendix C for a detailed view and volcano names.</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>**Figure 10.** a) Simplified geological map of the central and northern Ecuadorian Andes showing the basement-inherited structures beneath the volcanic arc (Modified from Hughes and Bermúdez, 1997; Litherland et al., 1994; Egüez et al., 2017; Vallejo et al., 2019). Geological units mentioned in the text represented as black letters. PLAn: Angochagua; PcEM: Macuchi; KPa: Pallatanga; PLPy: Pizayambo; KSj: San Juan. b) Tectonic map of the same area describing the active faults as red lines. A-fs: Ambi; B-fs: Billecocha; C-fs: Cosanga; L-fs: Latacunga; M: Machachi; Q-fs: Quito; and S-fs: Salado fault systems as red letters.</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>**Figure 11.** Correlation chart between construction bulk-volume (V; km³) and area covered by the edifice (S; km²) for the volcanoes of the Ecuadorian volcanic arc.</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>**Figure 12.** Cumulative construction bulk-volume (km³) over time. Error bars on the x-axis reflect the estimated duration of the construction stages of each volcanic edifice. Error bars in the volume calculation (y-axis) are too small to be visible compared to the cumulative bulk-volume. The data used are given in Appendix D.</figcaption></figure>
<table><caption>**Table 1.** New K-Ar ages obtained for the volcanoes of the Quito segment. Column headings indicate sample name, outcrop nature and relative location, sample coordinates projected using the Universal Transverse Mercator (UTM) coordinate system (Zone 17), potassium (K) content in percent, radiogenic argon content ($^{40}$Ar*) in percent and in $10^{11}$ atoms per gram, age obtained for each measurement, and weighted mean age in ka given with a $1-\sigma$ uncertainty. All measurements were performed on groundmass, except for three analyses performed on plagioclase phenocrysts (**).</caption><thead><tr><th>Sample</th><th>Location and Unit</th><th>Longitude (m)</th><th>Latitude (m)</th><th>K (%)</th><th><sup>40</sup>Ar* (%)</th><th><sup>40</sup>Ar* (10<sup>11</sup> at/g)</th><th>Age ± 1σ (ka)</th><th>Mean age (ka)</th></tr></thead><tbody><tr><td colspan="9"><strong>Casitagua volcano</strong></td></tr><tr><td>20EQ52a</td><td>Lava flow, E flank</td><td>782567</td><td>9995974</td><td>1.329</td><td>20.3<br>19.7</td><td>12.1227<br>12.2556</td><td>873 ± 13<br>883 ± 13</td><td>878 ± 13</td></tr><tr><td>20EQ51a</td><td>Lava flow, SE flank</td><td>780212</td><td>9992204</td><td>1.411</td><td>7.5<br>6.9</td><td>11.6027<br>11.5223</td><td>787 ± 15<br>782 ± 16</td><td>785 ± 16</td></tr><tr><td colspan="9"><strong>Ilaló volcano</strong></td></tr><tr><td>19EQ01</td><td>Lava flow, E flank</td><td>784978</td><td>9973735</td><td>0.964</td><td>14.5<br>13.9</td><td>12.7775<br>12.8238</td><td>1273 ± 20<br>1274 ± 20</td><td>1273 ± 20</td></tr><tr><td>19EQ47</td><td>Lava flow, N flank</td><td>787784</td><td>9972350</td><td>1.133</td><td>7.1<br>7.1<br>6.7</td><td>12.9692<br>13.2883<br>13.2122</td><td>1096 ± 22<br>1123 ± 22<br>1116 ± 23</td><td>1112 ± 22</td></tr><tr><td colspan="9"><strong>Pambamarca volcano</strong></td></tr><tr><td>20EQ56</td><td>Lava flow, NW flank</td><td>805673</td><td>9997815</td><td>1.241</td><td>19.5<br>21.5</td><td>17.804<br>17.8125</td><td>1373 ± 21<br>1374 ± 20</td><td>1374 ± 21</td></tr><tr><td>20EQ62**</td><td>Jambi Machay dome, summit area</td><td>810553</td><td>9989550</td><td>0.382</td><td>12.4<br>18.2</td><td>5.1262<br>5.1773</td><td>1285 ± 21<br>1297 ± 20</td><td>1292 ± 20</td></tr><tr><td>20EQ60</td><td>Lava flow, summit area</td><td>809077</td><td>9991663</td><td>1.586</td><td>37.2<br>39.2</td><td>20.7881<br>21.0203</td><td>1254 ± 18<br>1268 ± 18</td><td>1261 ± 18</td></tr><tr><td colspan="9"><strong>Puntas volcano</strong></td></tr><tr><td>20EQ66</td><td>Lava flow, E flank</td><td>809562</td><td>9981566</td><td>1.449</td><td>31.7<br>33.2</td><td>17.0642<br>17.2136</td><td>1127 ± 16<br>1137 ± 16</td><td>1132 ± 16</td></tr><tr><td>20EQ68</td><td>Lava flow, E flank</td><td>806008</td><td>9982702</td><td>1.362</td><td>30.9<br>33.2</td><td>16.0579<br>16.0570</td><td>1128 ± 16<br>1128 ± 16</td><td>1128 ± 16</td></tr><tr><td>20EQ67b</td><td>Lava flow, E flank</td><td>807261</td><td>9982195</td><td>1.034</td><td>14.3<br>12.8</td><td>11.6670<br>11.7687</td><td>1080 ± 17<br>1089 ± 18</td><td>1084 ± 17</td></tr><tr><td colspan="9"><strong>Coturco volcano</strong></td></tr><tr><td>20EQ54</td><td>Lava flow, summit</td><td>802112</td><td>9976733</td><td>1.610</td><td>47.7</td><td>33.1602</td><td>1971 ± 28</td><td>1959 ± 28</td></tr><tr><td>20EQ54**</td><td>area</td><td></td><td></td><td>1.141</td><td>43.4</td><td>23.2191</td><td>1948 ± 28</td><td></td></tr><tr><td colspan="9"><strong>Chacana Caldera</strong></td></tr><tr><td>20EQ85</td><td>Lava flow, Antisanilla valley</td><td>795498</td><td>9951500</td><td>1.319</td><td>19.6<br>16.4</td><td>30.7537<br>30.6488</td><td>2232 ± 34<br>2224 ± 34</td><td>2228 ± 34</td></tr></tbody></table>
<table><caption>**Table 2.** Volcanic systems with radioisotopic ages. Segment refers to Figure 6. Oldest and youngest ages are given in ka. Volcanoes with a Holocene activity are considered as potentially active (Pot. Active); those with an historic activity or in eruption are considered as active (Bernard and Andrade, 2011); n.d.: not determined or unknown. Dating tech. (dating technique) and minerals used: wr (whole-rock), gms (groundmass), plg (plagioclase), obs (obsidian), FT (fission-tracks). References, 1: Bablon (2018); 2: Béguelin et al. (2015); 3: Bellver Baca et al. (2020); 4: Almeida et al. (2023); 5: Le Pennec et al. (2011); 6: Bablon et al. (2020a); 7: Samaniego et al. (2005); 8: Andrade et al. (2021); 9: Alvarado et al. (2014); 10: Robin et al. (2010); 11: Bigazzi et al. (1992); 12: Bigazzi et al. (2005); 13: Bellot-Gurlet et al. (2008); 14: Hoffer, (2008); 15: Hall et al. (2017); 16: Hidalgo (2006); 17: Santamaria et al. (2023); 18: Santamaria et al. (2022); 19: Hammersley (2003); 20: Opdyke et al. (2006); 21: Bablon et al. (2019); 22: Bablon et al. (2018); 23: Samaniego et al. (2022); 24: Hoffer et al. (2008); 25: Monzier et al. (1999).</caption><thead><tr><th>Segment</th><th>Edifice - Abbreviation</th><th>Lat. (°N)</th><th>Long. (°E)</th><th>Oldest age (ka)</th><th>Youngest age (ka)</th><th>Dating tech.</th><th>Reference</th></tr></thead><tbody><tr><td>Tulcan</td><td>Chiles - Cerro Negro - CCN</td><td>0.817</td><td>-77.936</td><td>883 ± 19</td><td>Pot. Active</td><td>K-Ar gms</td><td>1</td></tr><tr><td>Ibarra</td><td>Yanaurcu de Piñán - YNP</td><td>0.482</td><td>-78.33</td><td>3610 ± 60</td><td>61 ± 20</td><td><sup>40</sup>Ar/<sup>39</sup>Ar gms</td><td>2</td></tr><tr><td>Ibarra</td><td>Chachimbiro - CCB</td><td>0.467</td><td>-78.312</td><td>406 ± 20</td><td>Pot. Active</td><td><sup>40</sup>Ar/<sup>39</sup>Ar gms</td><td>3</td></tr><tr><td>Ibarra</td><td>Cotacachi - Cuicocha - CTC</td><td>0.361</td><td>-78.349</td><td>173 ± 4</td><td>Pot. Active</td><td>K-Ar gms</td><td>4</td></tr><tr><td>Ibarra</td><td>Imbabura - IMB</td><td>0.254</td><td>-78.181</td><td>47 ± 6</td><td>Pot. Active</td><td><sup>40</sup>Ar/<sup>39</sup>Ar wr; K-Ar gms</td><td>5; 6</td></tr><tr><td>Ibarra</td><td>Cubilche - CUB</td><td>0.230</td><td>-78.132</td><td>45 ± 5</td><td>n.d.</td><td>K-Ar gms</td><td>6</td></tr><tr><td>Ibarra</td><td>Cushnirumi - CHS</td><td>0.191</td><td>-78.328</td><td>411 ± 8</td><td>383 ± 6</td><td>K-Ar gms</td><td>6</td></tr><tr><td>Ibarra</td><td>Cusín - CUS</td><td>0.159</td><td>-78.148</td><td>517 ± 8</td><td>495 ± 12</td><td>K-Ar gms</td><td>6</td></tr><tr><td>Ibarra</td><td>Mojanda - Fuya Fuya - MFF</td><td>0.133</td><td>-78.293</td><td>1038 ± 87</td><td>28 ± 5</td><td>K-Ar gms</td><td>6</td></tr><tr><td>Ibarra</td><td>Cayambe - CAY</td><td>0.025</td><td>-77.988</td><td>409 ± 4</td><td>Active</td><td><sup>40</sup>Ar/<sup>39</sup>Ar wr</td><td>7</td></tr><tr><td>Quito</td><td>Pululahua - PUL</td><td>0.035</td><td>-78.466</td><td>18 ± 3</td><td>Pot. Active</td><td>K-Ar gms</td><td>8</td></tr><tr><td>Quito</td><td>Casitagua - CAS</td><td>-0.034</td><td>-78.479</td><td>1152 ± 30</td><td>785 ± 16</td><td>K-Ar gms</td><td>This study; 9</td></tr><tr><td>Quito</td><td>Pambamarca - PAM</td><td>-0.08</td><td>-78.209</td><td>1374 ± 21</td><td>1261 ± 18</td><td>K-Ar gms</td><td>This study</td></tr><tr><td>Quito</td><td>Pichincha - PCH</td><td>-0.176</td><td>78.6</td><td>1100 ± 10</td><td>Active</td><td><sup>40</sup>Ar/<sup>39</sup>Ar wr</td><td>10</td></tr><tr><td>Quito</td><td>Puntas - PUN</td><td>-0.188</td><td>-78.205</td><td>1132 ± 16</td><td>1084 ± 17</td><td>K-Ar gms</td><td>This study</td></tr><tr><td>Quito</td><td>Izambi - IZA</td><td>-0.193</td><td>-78.172</td><td>370 ± 60</td><td>n.d.</td><td>FT obs</td><td>11; 12</td></tr><tr><td>Quito</td><td>Coturco - STU</td><td>-0.211</td><td>-78.286</td><td>1959 ± 28</td><td>n.d.</td><td>K-Ar gms</td><td>This study</td></tr></tbody></table>
<table><tbody><tr><td>Quito</td><td>Chacana - CHN</td><td>-0.214</td><td>-78.185</td><td>2710 ± 190</td><td>Active</td><td>FT obs.;<br/>$^{40}$Ar/$^{39}$Ar<br/>gms</td><td>11; 12; 13</td></tr><tr><td>Quito</td><td>Ilaló - ILA</td><td>-0.263</td><td>-78.419</td><td>1273 ± 20</td><td>1112 ± 22</td><td>K-Ar gms</td><td>This study</td></tr><tr><td>Quito</td><td>Pan de Azúcar - PDA</td><td>-0.432</td><td>-77.719</td><td>1150 ± 10</td><td>n.d.</td><td>$^{40}$Ar/$^{39}$Ar wr</td><td>14</td></tr><tr><td>Quito</td><td>Antisana - ANT</td><td>-0.485</td><td>-78.143</td><td>378 ± 38</td><td>Pot. Active</td><td>$^{40}$Ar/$^{39}$Ar<br/>gms</td><td>15</td></tr><tr><td>Machachi</td><td>Atacazo - ATA</td><td>-0.357</td><td>-78.619</td><td>1290 ± 10</td><td>Pot. Active</td><td>$^{40}$Ar/$^{39}$Ar<br/>gms</td><td>16</td></tr><tr><td>Machachi</td><td>Pasochoa - PAS</td><td>-0.467</td><td>-78.481</td><td>472 ± 8</td><td>423 ± 10</td><td>K-Ar gms</td><td>17</td></tr><tr><td>Machachi</td><td>Corazón - COR</td><td>-0.531</td><td>-78.66</td><td>178 ± 32</td><td>67 ± 4</td><td>K-Ar gms</td><td>17</td></tr><tr><td>Machachi</td><td>Sincholagua - SIN</td><td>-0.538</td><td>-78.372</td><td>312 ± 6</td><td>n.d.</td><td>K-Ar gms</td><td>17</td></tr><tr><td>Machachi</td><td>Rumiñahui - RUM</td><td>-0.581</td><td>-78.507</td><td>202 ± 8</td><td>n.d.</td><td>K-Ar plg</td><td>17</td></tr><tr><td>Machachi</td><td>Almas Santas - ALS</td><td>-0.59</td><td>-78.854</td><td>374 ± 7</td><td>364 ± 7</td><td>K-Ar gms</td><td>17</td></tr><tr><td>Machachi</td><td>Cosanga - COS</td><td>-0.603</td><td>-77.991</td><td>670 ± 60</td><td>290 ± 20</td><td>FT obs</td><td>11ou 12</td></tr><tr><td>Machachi</td><td>Santa Cruz - SCR</td><td>-0.652</td><td>-78.633</td><td>702 ± 11</td><td>60 ± 3</td><td>K-Ar gms</td><td>18</td></tr><tr><td>Machachi</td><td>Iliniza - ILI</td><td>-0.663</td><td>-78.716</td><td>353 ± 6</td><td>Pot. Active</td><td>K-Ar gms</td><td>18</td></tr><tr><td>Machachi</td><td>Cotopaxi - COT</td><td>-0.681</td><td>-78.438</td><td>537 ± 11</td><td>Active</td><td>K-Ar obs</td><td>17</td></tr><tr><td>Machachi</td><td>Chalupas - CLP</td><td>-0.781</td><td>-78.329</td><td>459 ± 9</td><td>169 ± 1</td><td>$^{40}$Ar/$^{39}$Ar<br/>plg</td><td>19</td></tr><tr><td>Latacunga</td><td>Chinibano - CNB</td><td>-0.959</td><td>-78.476</td><td>1670 ± 190</td><td>n.d.</td><td>$^{40}$Ar/$^{39}$Ar<br/>gms</td><td>20</td></tr><tr><td>Latatcunga</td><td>Sagoatoa - SAG</td><td>-1.155</td><td>-78.669</td><td>826 ± 12</td><td>799 ± 12</td><td>K-Ar gms</td><td>21</td></tr><tr><td>Ambato</td><td>Vizcaya - VIZ</td><td>-1.369</td><td>-78.414</td><td>253 ± 5</td><td>n.d.</td><td>K-Ar gms</td><td>22</td></tr><tr><td>Ambato</td><td>Puñalica - PLC</td><td>-1.399</td><td>-78.678</td><td>18 ± 3</td><td>n.d.</td><td>K-Ar gms</td><td>21</td></tr><tr><td>Ambato</td><td>Huisla - HUI</td><td>-1.4</td><td>-78.572</td><td>612 ± 10</td><td>492 ± 9</td><td>K-Ar gms</td><td>21</td></tr><tr><td>Ambato</td><td>Carihuairazo - CRH</td><td>-1.408</td><td>-78.751</td><td>512 ± 9</td><td>157 ± 5</td><td>K-Ar gms</td><td>23; 21</td></tr><tr><td>Ambato</td><td>Mera lavas - MER</td><td>-1.408</td><td>-77.927</td><td>2670 ± 60</td><td>1920 ± 100</td><td>$^{40}$Ar/$^{39}$Ar wr</td><td>24</td></tr><tr><td>Ambato</td><td>Mulmul - MUL</td><td>-1.438</td><td>-78.543</td><td>174 ± 3</td><td>145 ± 4</td><td>K-Ar gms</td><td>21</td></tr><tr><td>Ambato</td><td>Puyo cones - PUY</td><td>-1.46</td><td>-77.914</td><td>190 ± 70</td><td>n.d.</td><td>$^{40}$Ar/$^{39}$Ar wr</td><td>24</td></tr><tr><td>Ambato</td><td>Chimborazo - CHB</td><td>-1.467</td><td>-78.817</td><td>87 ± 8</td><td>Pot. Active</td><td>$^{40}$Ar/$^{39}$Ar<br/>gms - K/Ar<br/>gms</td><td>23; 21</td></tr><tr><td>Ambato</td><td>Tungurahua - TUN</td><td>-1.47</td><td>-78.444</td><td>293 ± 10</td><td>Active</td><td>K-Ar gms</td><td>22</td></tr><tr><td>Ambato</td><td>Igualata - IGL</td><td>-1.493</td><td>-78.641</td><td>376 ± 10</td><td>107 ± 11</td><td>K-Ar gms</td><td>21</td></tr><tr><td>Riobamba</td><td>Calpi cones - CAL</td><td>-1.638</td><td>-78.731</td><td>62 ± 4</td><td>Pot. Active</td><td>K-Ar gms</td><td>21</td></tr></tbody></table>
<table><tr><td>Riobamba</td><td>Licto cones</td><td>-1.7837</td><td>-78.615</td><td>183 ± 9</td><td>n.d.</td><td>K-Ar gms</td><td>21</td></tr><tr><td>Riobamba</td><td>Sangay - SAN</td><td>-2.005</td><td>-78.342</td><td>380 ± 70</td><td>Active</td><td><sup>40</sup>Ar/<sup>39</sup>Ar gms</td><td>25</td></tr></table>