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The Los Humeros volcanic complex, a 21 × 15 km diameter caldera edifice nesting volcanic domes and a complex faulting structure, is located at the eastern edge of the Trans-mexican volcanic belt (TMVB). It is a young edifice (<0.5 Ma) that hosts one of the five main geothermal fields of Mexico with still an important energy production (∼65 kW installed capacity). Being one of the more studied producing fields, the geothermal system of the caldera is largely unknown at depths greater than ∼2.4 km, which is the approximate penetration range of the available geothermal wells. Here we present the results of a geophysical survey in Los Humeros caldera and surroundings with the aim to provide further insight on the physical characteristics of the geothermal system at depths greater than 2.4 km. The survey comprised 70 broadband magnetotelluric (MT) soundings distributed within and in the periphery of the caldera edifice of which we present here three EW profiles. We also accomplished a mesh of 718 accurately leveled gravity stations. These data sets were complemented with 13 TDEM soundings for static shift control, as well as with the aero-magnetic digital chart of the area (#E14-3, SGM, 2004). The MT data analysis yielded an average electric azimuth of N23W for the central profile where the field production is concentrated, which is quite consistent with the mapped NW-SE geological structure. However, at individual frequency ranges the strike follows the local faulting structures, most of which are apparently controlled by the deeper crater structure. At the production zone, the conductivity model reveals the existence of an eastward dipping resistive body, which follows the isotherms registered at the wells. The inclined conductivity interface above it seems to play an important role in controlling the heat and fluid flow towards shallow depths. Petrographic studies of well samples provide evidence of mineralogical assemblages that suggest magnetite-metasomatose hydrothermal alteration. The production zone coincides with maximum gravity and magnetic gradients, at the western edge of the well-defined circular crater anomaly. At this point the central MT section (profile 1) shows the shallower depth (2–3 km) to the relatively high resistivity (∼400–500 ohm-m) and magnetized intrusive-like body. The interpreted geophysical and surface geological data backed by well data support a reservoir and plume model structure consisting of a resistive propylitic core that feds the geothermal field through fractures and deep- seated faults. Surface conductors associated with stratified mineralization produced from lixiviation of geothermal fluids are well differentiated from deeper conductors. The better preserved northern sector of the Humeros caldera yields an anomalous deep conductor at depths of 6–7 km below sea level (profile 5) as well as at the central production sector (profile 1) whereas the anomalous conductivity zone at the southern profile (profile 6) could be as shallow as 5 km below sea level. These anomalous conductivity zones are expected to be associated to the primary energy source of the Los Humeros geothermal system in the form of partial fusion or hypercritical trapped fluids within the upper crust, conditions that have been concluded to prevail along cordilleras of the American continent (e.g. Hyndman, 2017). [ABSTRACT FROM AUTHOR]

The objective of this study is the analysis of the south of Chicontepec Paleocanyon (PC) and the relation of electromagnetic anomalies. For this purpose, we measured, processed, and interpreted 16 broadband magnetotelluric (MT) soundings, disposed of in an orthogonal profile that covers the inside and the boundaries of the Paleocanyon. We performed a 3-D inversion of the data set (ModEM) using the range of periods between 10−3–102 s. In addition, we compared the result of 3-D inversion and the interpolation of 1-D inversion in their invariant rotational along the profile, where it is watched that they are similar in the resistivity anomalies and geological structures. The interpretation of the 3D resistivity inversion model shows the heterogeneous geometry and boundaries of the Paleocanyon and the contact between sediments and the top of the basement. Furthermore, based on the resistivity contrast, the information from wells-drilled, and seismic sections, it is possible to delimitate some anomalies that perhaps have the probability of hydrocarbon existence. • Comparison of sections 3D MT inversion and seismic in the Chicontepec Paleocanyon. • Resistive contrasts were correlated to the geological structures and the basement. • 3D Magnetotellurics is a powerful technique for oil and gas exploration • MT surveying can help seismic information to detect oil/gas reservoirs [ABSTRACT FROM AUTHOR]

Cuenca de la Independencia (CI) is a semi-arid basin in Central Mexico, subdivided into four administrative hydrogeological regions, and comprised of two hydrogeological units: a granular aquifer of lacustrine origin overlying a fractured acidic igneous aquifer. Although many geophysical soundings were carried out within the basin, no geophysical data exceeding 500 m of depth has ever been reported; the actual conceptual models were generated only from shallower geophysical studies, geological and hydrogeochemical mapping. We performed a geophysical survey in CI to determine the geometry of the hydrogeological units through Audio-Magnetotellurics (AMT), Time Domain Electromagnetics (TDEM), and Vertical Electrical Soundings (VES). Since the acquired data set from the different applied geophysical methods are multidimensional and band-limited, some well-known empirical relations between the applied methods were used to improve the subsurface imaging, turning data into broadband-integrated and space-time scaled into the MT frequency domain. We present a methodology on how to handle this integrated information through a 2-D inversion approach. The results reveal that, even though there are two hydrogeological units, they could not be vertically distinguished, thus is not possible to refute the previous idea of direct vertical communication between the two aquifers. Moreover, a clear lateral transition in resistivity models was not found, so the administrative division of CI would only obey political-administrative purposes. • Cuenca de la Independencia aquifer is an important ground water source in Mexico. • Space- and time-scaling can increase resolution of 2-D resistivity models. • MT-methodologies can be applied to other geophysical methods under right conditions. • Resistivity distribution suggest that CI is a single hydrogeological system. • 2-D resistivity models show both aquifers vertically semi-connected. [ABSTRACT FROM AUTHOR]

The Los Humeros Volcanic Complex (LHVC) is a large silicic caldera complex in the Trans-Mexican Volcanic Belt (TMVB), hosting a geothermal field currently in exploitation by the Comisión Federal de Electricidad (CFE) of Mexico, with an installed capacity of ca. 95 MW of electric power. Understanding the structural architecture of LHVC is important to get insights into the interplay between the volcano-tectonic setting and the characteristics of the geothermal resources in the area. The analysis of volcanotectonic interplay in LHVC benefits from the availability of subsurface data obtained during the exploration of the geothermal reservoir that allows the achievement of a 3D structural view of the volcano system. The LHVC thus represents an important natural laboratory for the development of general models of volcano-tectonic interaction in calderas. In this study, we discuss a structural model of LHVC based on morphostructural and field analysis, integrated with well logs, focal mechanism solutions and magnetotelluric imaging. The structural analysis suggests that inherited regional tectonic structures recognized in the basement played an important role in the evolution of the magma feeding system, caldera collapses and post-caldera deformations. These inherited weak planes have been reactivated by resurgence faults and post-caldera magma-driven hydrofractures under a local radial stress field generated by the shallow LHVC magmatic/hydrothermal system. The local stress field induced caldera resurgence and volcanotectonic faulting. The results of this study are important to better constrain the structural architecture of large caldera complexes. Also, our study is useful to understand the structure of the Los Humeros geothermal field and support the exploration of deeper Super-Hot Geothermal Systems (SHGSs) and engineering of Enhanced Geothermal Systems (EGSs) for electric power production in the LHVC and other active resurgent calderas. • Geomorphological, field, subsurface and geophysical information are integrated into a new 3D model of LHVC; • Magmatic radial stress field, overwhelming the regional stress field, induces caldera resurgence; • Volcanotectonic deformations and hydrofractures reactivated inherited tectonic weak planes; • Resurgence faults and volcanotectonic stress field control the location of structures producing geothermal fluids. [ABSTRACT FROM AUTHOR]

The western slope of Cerro Domuyo in northern Patagonia is characterized by thermal springs with boiling waters, Quaternary silicic domes, and pyroclastic deposits that suggest the existence of a geothermal reservoir. According to geochemical studies, the reservoir may have a temperature of 220 °C and one of the largest advective heat fluxes reported for a continental volcanic center. In this paper, we propose a more refined conceptual model for the Domuyo geothermal area, based on a geological survey supported by U Pb, U Th, and Ar Ar geochronology and by magnetotelluric and gravity surveys. Our study indicates that the Domuyo Volcanic Complex (DVC) is a Middle Pleistocene dome complex overlying middle Miocene to Pliocene volcanic sequences, which in turn cover: 1) the Jurassic-Early Cretaceous Neuquén marine sedimentary succession, 2) silicic ignimbrites dated at ~186.7 Ma, and 3) the Paleozoic metamorphic basement intruded by ~288 Ma granite bodies. The volcanic cycle in the DVC is distinctly bimodal, characterized by the emplacement of massive silicic domes and less voluminous olivine basalts on its southern slope. A major collapse of the central dome at ~600 ka produced a voluminous (19.4 km3 and 133 km2) block-and-ash flow, and associated pyroclastic flows, that filled a valley to the southwest at distances up to ~30 km from Cerro Domuyo summit. This was followed by a period of intense effusive activity that formed the Cerro Guitarra, Cerro Las Pampas, Cerro Domo, and Cerro Covunco silicic domes. The last two domes are the youngest and largest edifices, dated at 0.50 Ma (Ar Ar age) and 0.25 Ma (U Th age). Pre-Cenozoic successions were affected by N-S reverse and thrust faults that were later displaced by an ENE-WSW-trending transtensional belt. The basement rocks at the northern termination of the Cordillera del Viento anticlinorium were also displaced towards the east-northeast by this belt, which is observed NNW of Cerro Domuyo. The DVC was emplaced within this zone of crustal weakness. The integration of geologic observations with magnetotelluric and gravity data, allowed us to develop an updated conceptual model of the geothermal system. The geothermal reservoir is inferred at a depth of less than 2 km within pre-Pliocene fractured rocks, bounded by ~WSW-ENE trending faults and sealed by the pyroclastic deposits and rhyolitic lavas of the DVC. The location of most thermal springs is not directly controlled by faults. Instead, flows emerge at the contact between the fractured and faulted basement and the caprock. • The Domuyo Volcanic Complex (DVC) is a silicic dome complex formed in the Pleistocene. • The central dome collapsed at ~600 ka producing a voluminous (19.4 km3) pyroclastic deposit. • The youngest domes were emplaced at ~500 ka and 250 ka. • The DVC was emplaced within a ENE-WSW trending transtensional zone of crustal weakness. • The geothermal reservoir is hosted in pre-Pliocene rocks sealed by the DVC pyroclastics and lavas. [ABSTRACT FROM AUTHOR]

• The Juchipila graben is an unconventional geothermal system located in central Mexico. • Thermal water has anomalous concentrations of F, B, Li, and As. • δ2H and δ18O indicate a common meteoric source but different evaporation processes. • Mantle helium is transported into the system through deep-rooted faults. This work presents hydrochemical results for groundwater and dissolved gas samples collected from a thermal and cold aquifer in the Juchipila Basin, in southern Sierra Madre Occidental, central Mexico. Thermal springs in the Juchipila Basin reach temperatures of 60 °C, these manifestations are not related to recent or active volcanism as are all the known geothermal fields in Mexico. The thermal waters (>32 °C) are Na-HCO 3 and Na-SO 4 type, with an anomalous concentration of F, B, Li, and As. Their chemistry likely results from water-rock interaction processes. The cold waters (<32 °C) have a Ca-HCO 3 composition typical of recent infiltration and shallow flow, but they have an anomalous concentration of NO 3. The δ2H and δ18O indicate a common meteoric source for the warm and cold water plotting along an evaporation line. The waters have higher CO 2 and He concentrations than the air-saturated water. The helium composition is mainly atmospheric and terrigenous with a mantle helium contribution of up to 14%. This suggests that faults affecting the region are deeply rooted, permitting mantle helium uprise. Geothermometry gives mean reservoir temperatures of 58–102 °C. Based on these results, we propose a model of hydrothermal circulation in the Juchipila Basin, in which rainwater infiltrates deeply through the graben edges fault system, dissolves ions and crustal helium, incorporates mantle helium, while heated by the geothermal gradient, and eventually surges and mixes with the cold, shallow aquifer along faults cutting the whole succession within the graben. [ABSTRACT FROM AUTHOR]