Eastern Trans-Mexican Volcanic Belt Research Articles

Environmental and anthropogenic factors mediating the functional connectivity of the mesquite lizard along the eastern Trans-Mexican Volcanic Belt.

Functional connectivity, the extent to which a landscape facilitates or impedes the dispersal of individuals across the landscape, is a key factor for the survival of species. Anthropogenic activities, such as urbanization, agriculture and roads, negatively impact functional connectivity of most species, particularly low-vagility species like lizards. Here, we examine how a landscape modified by anthropogenic activities affects the functional connectivity, at both broad and fine scales, of a widely distributed generalist lizard Sceloporus grammicus in the eastern Trans-Mexican Volcanic Belt, Mexico. We estimated for the first time the species' genetic structure, gene flow and functional connectivity in agricultural and forest zones using genomic data, a comprehensive landscape characterization and novel methods including gravity models. Our results showed not only marked genetic differentiation across the study region but also that functional connectivity is maintained for tens of kilometres despite S. grammicus low vagility. Specifically, we found that substrate and air temperature facilitated connectivity over broad and fine scales, respectively, while agricultural cover, relative humidity and slope were important for connectivity and gene flow. Contrastingly, forest cover and roads favoured (broad-scale) and limited (fine-scale) connectivity, likely associated with movement facilitated by small forest patches and with thermoregulation. Altogether, these results support that S. grammicus alternates its thermoregulatory behaviour depending on the distance travelled and the habitat environmental conditions, and that it can disperse through relatively modified landscapes, mainly using agricultural zones. The information obtained is crucial to understanding the response of lizards to current anthropogenic pressures and their potential to adapt.

A window on the lithospheric mantle beneath the eastern Trans-Mexican volcanic belt: Insights from pargasite-bearing mantle xenocrysts from the Holocene Ocotenco maar volcano

In this short communication, we report and discuss for the first time the occurrence of pargasite-bearing pyroxenite mantle macro-xenocrysts in a basaltic-andesite juvenile material erupted from the Holocene Ocotenco maar at the Serdán-Oriental Basin Volcanic field, situated in the eastern sector of the Trans-Mexican Volcanic Belt. Through an integrated approach based on microtextural observations, mineral chemistry, geothermobarometric modelling, and comparisons with the existing literature, we determined the xenocrysts equilibrated at ca. 0.8 GPa and 1100 °C, compatible with a pargasite-bearing, upper lithospheric mantle setting. The different textures (resorbed vs. unzoned) of xenocrysts, not only provide evidence of a mantle melting event, but also enabled the discrimination between disrupted mantle fragments and peritectic solid phase generated during the pargasite breakdown. Moreover, peculiar characteristics of the xenocrysts such as the dark pargasite rims and the melt embayment in clinopyroxene, together with the lack of phenocrysts from magma fractionation, the swallow-tail texture of groundmass plagioclase, and a vesicle-rich textures, cumulatively suggest a short-term (days to months?) eruptive scenario with a fast melt extraction from the source region to a rapid ascent through the crust and a sudden explosive eruption by the intense interaction of the ascending melt with a source of water close to the surface.

Geochemistry and 40Ar/39Ar dating of the Sierra de Santa Marta in the tectonically controversial Los Tuxtlas Volcanic Complex (Veracruz, Mexico)

One of the most debated volcanic areas along the eastern coast of Mexico is Los Tuxtlas Volcanic Complex located in the south of the Veracruz state. This volcanic province stands out as an isolated massif between the Trans-Mexican Volcanic Belt to the NW and the Chiapanecan Volcanic Belt to the SE. Los Tuxtlas complex is surrounded by two large alluvial plains formed by the Papaloapan and Coatzacoalcos rivers, and include the active volcano San Martín Tuxtla, the last eruption of which occurred in 1793. The ages, distribution, morphology and geochemistry of the volcanic vents and lavas, particularly the co-existence of the alkaline and subalkaline suites, offer an excellent opportunity to understand, in space and time, the tectono-magmatic history of Los Tuxtlas in comparison with other similar volcanic complexes in the eastern Trans-Mexican Volcanic Belt. Most of the volcanic centers of Los Tuxtlas show NW-SE alignments. This vent arrangement is associated with a left-lateral fault system known as the Veracruz fault, the northwestern expression of which are the submarine volcanoes that form the Anegada High. This fault might be a reactivation of the ancient Tamaulipas-Oaxaca fault related to the opening of the Gulf of Mexico. Based on geomorphological, geochemical, petrographical, and 40Ar/39Ar data, we propose three different stages of volcanic activity for Los Tuxtlas: The older Montepío-El Vigía (7–1.4 Ma), the intermediate Santa Marta (1–0.5 Ma), and the younger San Martín Tuxtla (50 ka to present). The main focus region of this paper is the stratovolcanoes of the Sierra de Santa Marta, formed by at least four major NW-SE aligned volcanic edifices, which from NW to SE are Encanto (950 m.a.s.l; identified and named in this work for the first time), Yohualtajapan (1450 m.a.s.l), Santa Marta (1685 m.a.s.l), and San Martín Pajapan (1250 m.a.s.l). The geological map, the 40Ar/39Ar dates, and the geochemistry data indicate a NW-SE migration of the volcanic activity of the four stratovolcanoes that have created the sierra de Santa Marta. Petrography and chemistry of rocks suggest that the subalkaline rocks of the three magmatic stages were fed by subduction activity of the Cocos Plate. The alkaline suite is likely associated with asthenosphere upwelling and slab edge melting, facilitated by a slab tear of the south Cocos plate evolved since the Early Miocene.

New constraints on the seismotectonics and crustal structure of the Eastern Trans-Mexican Volcanic Belt based on local seismicity, focal mechanisms and a preliminary state of stress evaluation

After the occurrence of unusual upper crustal seismic activity within the eastern sector of the Trans-Mexican Volcanic Belt (TMVB) in 2019, an area of generally low seismicity, an effort was made to interpret the origin. Concerns leading to the careful analysis presented here, were manifold: Shallow clustered seismicity can indicate active volcanism or related processes, as well as hydrothermalism. Additionally, the area was struck by a major earthquake (Mw 6.4) on January 4, 1920, which had devastating effects on the area's complex geological features and caused hundreds of deaths and significant material damage. As a result, several events of the 2019 sequence of unusual seismic activity have been relocated to precisely know the location. The 34 earthquakes of the 2019 seismic sequence occur in a relatively small area of less than 1600 km2. The seismic sequence can be further subdivided into 2 clusters, separated by the Pico de Orizaba – Cofre de Perote volcanic chain, which itself separates two tectonostratigraphic terranes, is a topographic divide and represents the limit to a considerable topographic drop. We also used the seismic sequence to constrain further the state of local stress and henceforth observe the tectonic regime of the area. For this purpose, focal mechanisms of the events with Mc > 4 (this sequence and previous data) were determined and the inversion of the stress tensor of this group was carried out. Similarly, the state of stress for the central zone of the TMVB was recalculated to establish its possible variations towards the eastern part of the TMVB. We show that the state of stress of the Eastern/Easternmost TMVB presents a transtensional regime, similar to the central TMVB, which has been suggested is mainly caused and controlled by the geometry of the subduction of the Cocos plate beneath the North American plate. A review of Rayleigh wave dispersion curves, calculated through interferometry of ambient seismic noise, a change is observed that suggests some variation in the structure of the eastern and easternmost areas. This contribution is relevant because little is known about the seismotectonics of the TMVB's eastern sector, in part because of the low cortical seismicity reported along the TMVB, the reduced coverage of seismic stations in this region and the distance from the trench Middle American Trench (MAT), where most of the important tectonic processes in Mexico take place. Recently, with the incorporation of a regional seismic network, the detection of small-magnitude events in the study area has significantly been improved.

Review of geology and geomorphology of the Xalapa Monogenetic Volcanic Field, eastern Trans-Mexican Volcanic Belt

Monogenetic volcanic fields have associated volcanic hazards that are not always well constrained due to the lack of complete and well established geological and geomorphological studies, field boundaries, volcano inventories, and of detailed reconstruction of individual volcano eruptive histories. This work presents a detailed review and further insights into geology and geomorphology of the Xalapa Monogenetic Volcanic Field (XMVF), achieved by completing a comprehensive revision of all previous works on the XMVF, along with new data produced from extensive fieldwork, field and digital mapping, radiocarbon dating, and morphometric and density analyses. The main results are: (i) a renewed inventory of 72 XMVF monogenetic volcanoes - a total number larger than previously recorded (59); (ii) identification of six new and not previously identified monogenetic volcanoes: La Natividad, La Pedrera, El Puente, Rancho Sicomoro, and Puentecillas scoria cones, and the Tenampa maar; (iii) recognition of four possible new volcanoes integrating the Macuiltépetl cluster, recorded in historical documents; (iv) corrections and new volcanoes identified in the volcano clusters of Pextlan, La Joya, Ocotepec, and Cruz Negra; (v) geological mapping of all lava flows associated with the 72 monogenetic volcanoes; (vi) comprehensive morphometric analyses of all 72 reported volcanoes; (vii) three new radiocarbon ages corresponding to 780 ± 60 BP and 1340 ± 60 BP lava flows of El Volcancillo scoria cone, and to 1300 ± 60 BP and 1115 ± 80 BP lavas of La Pedrera volcano; (viii) new evidence of the late Pleistocene to Holocene age of the Acatlán scoria cone; (ix) new outline of the boundaries of the XMVF based on fieldwork and remote sensing; and (x) volcanoes and lava flows density and distribution maps. From these results, we conclude that, in case of renewed monogenetic activity in the XMVF, the most likely deterministic hazard model would comprise effusion of lavas of variable length (3–40 km), of up to extremely long tube-fed pahoehoe lava flows; explosive activity ranging from typical to violent Strombolian style eruptions producing scoria cones, extensive fallouts, and associated pyroclastic density currents; and phreatomagmatic style activity producing maars, tuff rings and/or tuff cones, and sequences of wet pyroclastic density currents. Hazard distributions in these scenarios would be mainly controlled by topography, local tectonics, groundwater level and basement rocks lithology, and would mostly affect the north and south flanks of Cofre de Perote volcano, as well as central XMVF sectors where the metropolitan area of the city of Xalapa and towns of Naolinco and Acatlán locate. From this review, we conclude that without any further studies of geology and chemistry of every single volcano, the western boundaries of the XMVF will remain unclear, and the associated volcanic risk will remain underestimated. This work provides a fundamental basis from which further and more detailed work could be attained.

Hydrogeochemistry and geothermometry of thermal springs in the eastern Trans-Mexican Volcanic Belt

This study was aimed at investigating the hydrogeochemical and isotopic characteristics of low-temperature hydrothermal springs in six study sites located in the eastern portion of the Trans-Mexican Volcanic Belt, in order to evaluate the source of the thermal water, rock-water interactions, and reservoir temperature. In addition, based on this, possible similarities and differences among these thermal springs were explored. Samples of thermal waters (12 samples) and non-thermal waters (6 samples) were collected during dry and rainy seasons. Stable isotopes (18O, 2H) as well as major, minor, and trace ions were analyzed for, and saturation indices and reservoir temperatures were estimated. Thermal water temperatures ranged from 33 to 55°C and the pH was slightly acid (and slightly basic in only one case). The electrical conductivity ranged widely, from 0.2 to 27 mS/cm, although most samples had > 8 mS/cm. Using a Piper diagram, three hydrochemical facies were identified: Na-Cl, Na-Ca-SO4-HCO3, and Na-HCO3. The notable minor and trace elements and ions were B, Li, As, and F− . δ18O and δ2H values plot very close to the meteoric water line, although samples from three of the study sites showed a possible isotope exchange between water and minerals at high temperatures. By estimating reservoir temperatures using solute geothermometers and saturation indices, it was possible to determine similarity among the values obtained (90 to 190°C). These data indicate that the hydrochemistry of the thermal waters reflects a water source that is related to geothermal brines in some cases, and with the mixing of a thermal fluid and a recently infiltrated water in other cases, as well as water-rock interactions (mostly with silicates) and carbonate precipitation.

Geology of Las Minas: an example of an exhumed geothermal system (Eastern Trans-Mexican Volcanic Belt)

ABSTRACT The Las Minas area corresponds to an exhumed geothermal system considered a proxy for the deep part of the nearby Los Humeros active geothermal system. The stratigraphic succession is made up of: Palaeozoic-Miocene granitoids, a thick Jurassic- Cretaceous carbonate succession, Neogene lava flows and volcano-sedimentary deposits. Linked to a Miocene magma intrusion, marble and skarn rock-volumes developed by contact metamorphism and geothermal fluid flow. Faults are arranged in SW- and NNW-striking systems. These controlled the morphological evolution and favored Neogene-Quaternary dyke emplacement. Faulting gave rise to a tectonic depression where lacustrine sediments and pyroclastics deposited. Skarn rocks are mainly located at fault intersections and along pre-existing discontinuities, suggesting the role of bedding and/or fractures in channeling deep fluids. Results give inputs for exploration at depth of Los Humeros geothermal system.

Anatomy of the magmatic plumbing system of Los Humeros Caldera (Mexico): implications for geothermal systems

Abstract. Understanding the anatomy of magma plumbing systems of active volcanoes is essential not only for unraveling magma dynamics and eruptive behaviors but also to define the geometry, depth, and temperature of the heat sources for geothermal exploration. The Pleistocene–Holocene Los Humeros volcanic complex is part of the eastern Trans-Mexican Volcanic Belt (central Mexico), and it constitutes one of the most important exploited geothermal fields in Mexico with ca. 90 MW of produced electricity. With the aim to decipher the anatomy (geometry and structure) of the magmatic plumbing system feeding the geothermal field at Los Humeros, we carried out a field-based petrological and thermobarometric study of the exposed Holocene lavas. Textural analysis, whole-rock major-element data, and mineral chemistry are integrated with a suite of mineral-liquid thermobarometric models. Our results support a scenario characterized by a heterogeneous multilayered system, comprising a deep (depth of ca. 30 km) basaltic reservoir feeding progressively shallower and smaller discrete magma stagnation layers and batches, up to shallow-crust conditions (depth of ca. 3 km). The evolution of melts in the feeding system is mainly controlled by differentiation processes through fractional crystallization (plagioclase + clinopyroxene + olivine + spinel). We demonstrate the inadequacy of the existing conceptual models, where a single voluminous melt-controlled magma chamber (or “Standard Model”) at shallow depths was proposed for the magmatic plumbing system at Los Humeros. We instead propose a magmatic plumbing system made of multiple, more or less interconnected, magma transport and storage layers within the crust, feeding small (ephemeral) magma chambers at shallow-crustal conditions. This revised scenario provides a new configuration of the heat source feeding the geothermal reservoir at Los Humeros, and it should be taken into account to drive future exploration and exploitation strategies.

Stratigraphy and evolution of the Holocene Aljojuca Maar volcano (Serdán-Oriental basin, Eastern Trans-Mexican Volcanic Belt), and implications for hazard assessment

Aljojuca maar is located in the eastern Trans-Mexican Volcanic Belt, within the Serdán-Oriental Basin, which is characterized by Quaternary bimodal monogenetic volcanism. Aljojuca maar has an irregular shape, with an eastern embayment structure that forms an E-W alignment with at least three older scoria cones to the east. The crater walls expose a sequence of pre-maar volcaniclastic deposits, with a lava flow linked to the older scoria cones and a paleosol at its top; the latter indicates that a significant hiatus occurred between the growth of the scoria cones and the eruption of the Aljojuca maar, whose deposits overlie the soil. Strong interactions of the ascending magma with groundwater led to the emplacement of a ~30-m-thick maar sequence. Our detailed stratigraphic study shows that the eruption can be grouped into two main stages, comprising eight stratigraphic units deposited during five eruptive phases. The first stage reflects the development of the main crater (a maar sensu stricto) and is recorded by six stratigraphic units (from A to F). The eruption began with a strombolian pulse, followed by phreatomagmatic activity (unit A), forming dilute pyroclastic density currents. In the second phase, phreatomagmatic explosions prevailed with different degrees of fragmentation, from juvenile and lithic-rich lapilli fallout (unit B), to less diluted pyroclastic density currents (unit C), and a spatially restricted strombolian pulse. The third phase includes a main-vent enlargement event represented by a chaotic and heterolithic breccia (unit D). In the fourth phase, dilute pyroclastic density current deposits (unit E) dominate, with minor chaotic breccias (unit F). The second stage (fifth phase) includes the occurrence of a possibly fissural E-W structure, which formed a scallop in the eastern flank of the maar (eastern embayment structure), breaking the previously formed main crater's eastern flank and producing a 4-m-chaotic breccia (unit G) followed by the occurrence of a dilute pyroclastic density current (unit H), whose energy enabled the current to climb to the closest scoria cone. The migration of the explosive loci toward the eastern flank of the maar reflects a tectonic control by the dominant regional east-trending structural pattern. Several radiocarbon ages support that the eruption of Aljojuca took place between 2.7 ka (age of charcoal fragments under the ejecta ring) and ~2.2 ka (age of the oldest lake sediments in the maar crater). This has important implications for the hazard assessment of volcanism in this area, which had not been considered previously as a potentially active system.

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

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 NE-SW 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 extra-caldera. 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.

Late Pleistocene to Holocene activity of Alchichica maar volcano, eastern Trans-Mexican Volcanic Belt

Detailed volcano-stratigraphy and 40Ar/39Ar dating are used for the first time to reconstruct the eruptive evolution of the Alchichica maar and to contribute to the understanding of maar volcanism within the eastern Trans-Mexican Volcanic Belt (TMVB). The maar excavated in an area where previous eruptions included Hawaiian, Strombolian, and phreatomagmatic types. Alchichica is characterized by an irregular shape tephra ring with a bulk volume of ∼0.088 km3 based on high-resolution digital terrain model calculations. The maar deposits comprises two stratigraphic units (lower and upper) with contrasting lithofacies characteristics suggesting different depositional conditions and variations in eruption behavior. These units correspond to: (1) vent-clearing stage with explosions producing a succession of massive tuff-breccia intercalated with massive lapilli-tuffs; (2) pulsating phreatomagmatic eruptions producing a series of accretionary lapilli-rich tuffs that alternate with dominantly well-stratified tuffs and lapilli-tuffs deposited from pyroclastic density currents. The geochronological data provide first constraints about the young age (Late Pleistocene-Holocene) of the Alchichica maar. The presence of the maar and some of the other pre-maar eruptions suggest long-lasting dispersed volcanism in a well-defined, but small area, where rejuvenated monogenetic vents cut into earlier ones, having significant implications for hazard assessments in the region.

Wide-Area InSAR Survey of Surface Deformation in Urban Areas and Geothermal Fields in the Eastern Trans-Mexican Volcanic Belt, Mexico

This paper provides the first wide-area Interferometric Synthetic Aperture Radar (InSAR) survey of the whole eastern Trans-Mexican Volcanic Belt (42,200 km2). The aims are to identify ground deformation hotspots within major urbanized areas and rural valleys, establish baselines in geothermal exploration sites, and analyze deformation at geothermal exploitation sites and its relationship with energy production. The whole 2003–2010 ENVISAT C-band SAR archive available over the region was processed with the Small BAseline Subset (SBAS) InSAR method to retrieve over 840,000 coherent targets and estimate their ground displacement rates and time series. Land subsidence hotspots due to aquifer drawdown are found within the city of Puebla (up to −53 mm/year vertical rates, groundwater pumping for industrial use), Tlaxcala and Apizaco (−17 mm/year, industrial and public), the valley of Tecamachalco (−22 mm/year, agricultural), Tulancingo (−55 mm/year, public, industrial and agricultural), and in the eastern Mexico City metropolitan area (−44 mm/year, agricultural). The baseline for the Acoculco caldera complex shows widespread ground stability. Conversely, localized subsidence patterns of −5 to −10 mm/year exist around Las Derrumbadas and Cerro Pinto in the Serdán-Oriental basin, due to intense groundwater pumping for agriculture. A well-defined land subsidence area with −11 mm/year maximum rates is found at Los Humeros volcanic complex within Los Potreros collapse, correlating well with energy production infrastructure location and historical steam production rates. Field surveys carried out in Acoculco and Los Humeros in 2018 provide supporting evidence for the identification of hydrothermal manifestations, and understanding of the landscape and surface deformation patterns within the geothermal fields.

Active Crustal Deformation in the Trans‐Mexican Volcanic Belt as Evidenced by Historical Earthquakes During the Last 450 Years

AbstractThe Trans‐Mexican Volcanic Belt (TMVB) is oblique to the subduction zone. Background seismicity is very low. Nevertheless, two large instrumental earthquakes were recorded: the 1912 Acambay (Mw 6.9) and the 1920 Jalapa (Mw 6.4) earthquakes. Macroseismic data were systematically searched for crustal earthquakes on the TMVB contained in historical accounts. The macroseismic reports were inverted for the best fitting magnitude and epicenter. The western end of the TMVB shows the highest level of seismic activity. An earthquake is reported in 1568 (Mw 7.2 ± 0.2) and a second large event took place in 1875 (Mw 7.0 ± 0.2). Two other earthquakes are reported in the western TMVB in 1611 and 1749 (Mw 6.4). The 1858 earthquake (Mw 7.6 ± 0.3), in the central TMVB occurred on faults located to the west of the 1912 Acambay event. The eastern TMVB is less active. An earthquake in 1575 (Mw 5.7) occurred east of the city of Puebla. The 1887 (Mw 6.2 ± 0.2) and the 1920 Jalapa earthquakes are the larger events in this region. The seismicity observed from the instrumental and historical seismicity catalogs show that active tectonic deformation occurs throughout the TMVB and that it is not confined to specific regions where instrumental earthquakes have been located and active faults mapped. The number of large earthquakes in the TMVB, suggest unique large‐scale deformation of this complex volcanic belt. The presence of large earthquakes in this tectonic province, where 40% of the population of Mexico live, underlines the importance of reevaluating hazard in this densely populated region.

Evidence of a shallow magma reservoir below Los Humeros volcanic complex: Insights from the geochemistry of silicate melt inclusions

Los Humeros volcanic complex (Eastern Trans Mexican Volcanic belt) is one of the three most important geothermal fields in Mexico and is currently producing electricity (ca. 90 MW). It is therefore pertinent to constrain the magma chamber characteristics, such as its depth, and the magmatic evolution of the complex in order to better define the superficial geothermal activity. In this paper we present the first geochemical data (major and trace elements) for 15 silicate melt inclusions and their five host rocks from Los Humeros volcanic complex.We constrained the evolution of the volcanic complex by coupling X-ray spectrometry, ICP-MS, microprobe, X-ray microtomography and NanoSIMS analyses together with thermodynamical modeling on both whole rocks and silicate melt inclusions.A ponding reservoir located within an interval betwwen 5 and 13 km at depth is evidenced by silicate melt inclusions entrapment pressures. This ponding depth's interval is in agreement with previous studies by thermal modeling, resistivity and geobaromatry done in the same area. The geothermal activity of Los Humeros volcanic area is expected to be the result of the cooling of the magma in the shallow chamber. A large geochemical variability, in less than 1 Ma, is demonstrated by trace and volatile element compositions evolution through time. Subduction contamination is expected into the mantle source with the highest contamination event at 69 Ka, during the second caldera event of Los Humeros volcanic complex.

NW-SE Pliocene-Quaternary extension in the Apan-Acoculco region, eastern Trans-Mexican Volcanic Belt

The Apan-Acoculco area is located in the eastern portion of the Mexico basin and the Trans-Mexican Volcanic Belt. The area is transected by right-stepping variably dipping NE-SW normal faults. The Apan-Tlaloc Fault System is a major discontinuity that divides the region into two contrasting areas with different structural and volcanic styles. a) The western area is characterized by a horst-graben geometry with widespread Quaternary monogenetic volcanism and scattered outcrops of Miocene and Pliocene rocks. b) The eastern area is dominated by tilted horsts with a domino-like geometry with widespread Miocene and Pliocene rocks, scattered Quaternary monogenetic volcanoes and the Acoculco Caldera. Gravity data suggest that this structural geometry continues into the Mesozoic limestones. Normal faulting was active since the Pliocene with three stages of extension. One of them, an intense dilatational event began during late Pliocene and continues nowadays, contemporaneously with the emplacement of the Apan-Tezontepec Volcanic Field and the Acoculco caldera. Statistical analysis of cone elongation, cone instability, and the kinematic analysis of faults attest for a NW50°SE±7° extensional regime in the Apan-Acoculco area. The activity in some portions of the Apan-Tlaloc Fault System continues today as indicated by earthquake swarms recorded in 1992 and 1996, that disrupted late Holocene paleosols, and Holocene volcanism.

Geologic Map of Los Humeros volcanic complex and geothermal field, eastern Trans-Mexican Volcanic Belt

We present an updated version of the geologic map of Los Humeros Volcanic Complex (LHVC) and geothermal field, based on acomplete revised characterization of the rock units and contacts, structural features, stratigraphy and recent radiometric dating withmodern methods (U/Th, 40Ar/39Ar, Carrasco-Núñez et al., 2017, in press), together with the use of high resolution Digital TerrainModel at 1m resolution and Google Earth optical imagery. Improvements of this version include refined stratigraphic sequence,revised classification of each lithostratigraphic unit, updated structural features and geochronologic data; all together providingconstraints to support a new evolutionary volcanic history for LHVC. Main changes with previous works involve the recently dis-covery of much younger ages for the main caldera-forming eruption. Much younger ages were also obtained for other importantexplosive phases. These findings reveal the existence of a much younger long-lived magmatic system with Holocene activity and ahigh geothermal potential that requires a further assessment for exploration and volcanic hazard purposes.

Subsurface stratigraphy and its correlation with the surficial geology at Los Humeros geothermal field, eastern Trans-Mexican Volcanic Belt

Los Humeros is one of the main Pleistocene silicic volcanic complexes and the third geothermal field in exploitation in Mexico. It is located in the eastern sector of the Trans-Mexican Volcanic Belt (TMVB) and has a long and complex geologic evolution involving the formation of at least two large nested calderas associated with very large ignimbrite-forming eruptions during the last 0.46Ma (Ferriz and Mahood, 1984). Due to its young activity, it has an active geothermal field (Yáñez and García, 1980) producing about 65MW. Several wells for both exploration and exploitation of geothermal fluids have been drilled in the caldera interior, showing the subsurface stratigraphy and structure of the volcanic center. A revised lithostratigraphic characterization of Los Humeros subsurface together with petrographic, geochemical and 40Ar/39Ar-based geochronology data support correlation of rock formations at depth with surface geology. This study shows the very heterogeneous nature of the subsurface stratigraphy of Los Humeros volcanic complex, helping to refine its internal structure and evolution. This provides a more realistic configuration of the heterogeneities of the geothermal reservoir and the vertical and lateral distribution of permeable conditions controlling the configuration of the geothermal reservoir. These results can be important to guide the prospection of potential areas for the possible expansion of the geothermal field.

Thermal history of the Acoculco geothermal system, eastern Mexico: Insights from numerical modeling and radiocarbon dating

Acoculco is a geothermal prospective area hosted by a volcanic caldera complex in the eastern Trans-Mexican Volcanic Belt. Surface manifestations are scarce and consist of gas discharges (CO2-rich) and acid-sulfate springs of low temperature, whereas hydrothermal explosive activity is profusely manifested by meter-scale craters and mounds of hydrothermal debris and breccias. Silicic alteration extends for several square kilometers around the zone with gas manifestations and explosive features, affecting surficial volcanic rocks, primarily tuffs and breccias. In the subsurface, an argillic alteration zone (ammonium illite) extends down to a depth of ∼600m, and underneath it a propylitic zone (epidote–calcite–chlorite) occurs down to ∼1000m.Thermal logs from an exploratory borehole (EAC-1, drilled in 1995 down to 1810m) showed a conductive heat transfer regime under high geothermal gradient (∼140°C/1000m). In contrast, the thermal profile established from temperatures of homogenization of fluid inclusions—measured on core samples from the same drill hole—suggests that convection occurred in the past through the upper ~1400m of the geothermal system. A drop in permeability due to the precipitation of alteration minerals would have triggered the cessation of the convective heat transfer regime to give place to a conductive one. With the purpose of determining when the transition of heat transfer regime occurred, we developed a 1D model that simulates the time-depth distribution of temperature. According to our numerical simulations, this transition happened ca. 7000years ago; this date is very recent compared to the lifespan of the geothermal system. In addition, radiocarbon chronology indicates that the hydrothermal explosive activity postdates the end of the convective heat transfer regime, having dated at least three explosive events, at 4867–5295, 1049–1417 and 543–709y cal. BP. Therefore, hydrothermal explosions arise from the self-sealing of the Acoculco geothermal system, involving a natural hazard that could affect future geothermal-power infrastructure.

Kaolin from Acoculco (Puebla, Mexico) as raw material: Mineralogical and thermal characterization

AbstractThe present study determined the mineralogy and thermal properties of kaolin from Acoculco (Puebla), at the eastern Trans-Mexican Volcanic Belt and compared it with the nearby deposits of Agua Blanca (Hidalgo) and Huayacocotla (Veracruz). The mineralogy of the kaolins was determined by X-ray diffraction, infrared spectroscopy and scanning electron microscopy. Thermal behaviour was studied by differential thermal analysis, dilatometry and hot-stage microscopy. The Acoculco deposit is composed mainly of kaolinite and SiO2minerals. In the case of Agua Blanca and Huayacocotla, alunite is abundant in places and minor anatase is also present locally. The Acoculco kaolins are Fe-poor and relatively rich in some potentially toxic elements (Zr, Sb, Pb). They undergo a relatively small amount of shrinkage (∼3–4 vol.%), during firing at 20–1300°C and cooling down to 20°C, except when &gt;10 wt.% alunite is present. These kaolins are a suitable raw material for the ceramics industry. Other applications (pharmaceuticals, cosmetics) would require an enrichment process to eliminate impurities such as Fe oxides.

Rhyolitic phreatomagmatism explored: Tepexitl tuff ring (Eastern Mexican Volcanic Belt)

Rhyolitic Tepexitl tuff ring in the Serdán-Oriental Basin of the eastern Trans-Mexican Volcanic Belt is a young, well-preserved circular crater — a simple morphology that belies a more complex eruptive history. Field observations, ash studies (granulometry, componentry, and morphology) and volatile data (LOI and XRF) allow the broad division of the Tepexitl deposits into two sequences: a lower sequence characterized by fine-grained deposits with abundant bomb and lapilli sags, and an upper sequence of coarse-grained deposits that show little to no deformation. The early eruptions at Tepexitl are interpreted to have been dominated by discrete, highly efficient, phreatomagmatic blasts, which caused a progressive deepening of the eruptive center (lower sequence), followed by a transition to dominantly magmatic behavior in the upper sequence. Dome growth occurred at the end of the eruption, but subsequent retrogressive explosions triggered by external water destroyed all trace of the original dome morphology. Molten fuel-coolant interaction (MFCI) is commonly thought to cause the repetitive water–magma interaction in phreatomagmatic eruptions, but a process by which magma and viscous felsic magmas can mingle prior to explosive interaction has not been described. A viable mechanism for rhyolitic MFCI, based upon field work at Tepexitl and laboratory experiments, requires that fluidized sediments intrude marginal fractures in the rhyolite magma, creating enough interfacial surface area to initiate phreatomagmatic explosions from within the interior of a rising plug or dome.