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.

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The increasing number of field investigations and various controlled benchtop and largescale experiments have permitted the evaluation of a large number of processes involved in the formation of maar-diatreme volcanoes, the second most common type of smallvolume subaerial volcanoes on Earth. A maar-diatreme volcano is recognized by a volcanic crater that is cut into country rocks and surrounded by a low-height ejecta rim composed of pyroclastic deposits of few meters to up to 200 m thick above the syn-eruptive surface level. The craters vary from 0.1 km to up to 5 km wide and vary in depth from a few dozen meters to up to 300 m deep. Their irregular morphology reflects the simple or complex volcanic and cratering processes involved in their formation. The simplicity or complexity of the crater or the entire maar itself is usually observed in the stratigraphy of the surrounding ejecta rings. The latter are composed of sequences of successive alternating and contrastingly bedded phreatomagmatic-derived dilute pyroclastic density currents (PDC) and fallout depositions, with occasional interbedded Strombolian-derived spatter materials or scoria fall units, exemplifying the changes in the eruptive styles during the formation of the volcano. The entire stratigraphic sequence might be preserved as a single eruptive package (small or very thick) in which there is no stratigraphic gap or significant discordance indicative of a potential break during the eruption. A maar with a single eruptive deposit is quantified as monogenetic maar, meaning that it was formed by a single eruptive vent from which only a small and ephemeral magma erupted over a short period of time. The stratigraphy may also display several packages of deposits separated either by contrasting discordance surfaces or paleosoils, which reflect multiple phases or episodes of eruptions within the same maar. Such maars are characterized as complex polycyclic maars if the length of time between the eruptive events is relatively short (days to years). For greater length of time (thousands to millions of years), the complex maar will be quantified as polygenetic. These common depositional breaks interpreted as signs of temporal interruption of the eruptions for various timescales also indicate deep magma system processes; hence magmas of different types might erupt during the formation of both simple and complex maars. The feeding dikes can interact with groundwater and form closely distributed small craters. The latter can coalesce to form a final crater with various shapes depending on the distance between them. This observation indicates the significant role of the magmatic plumbing system on the formation and growth of complex and polygenetic maar-diatreme volcanoes.

Abstract. Structural studies in active caldera systems are widely used in geothermal exploration to reconstruct volcanological conceptual models. Active calderas are difficult settings to perform such studies mostly because of the highly dynamic environment, dominated by fast accumulation of primary and secondary volcanic deposits, the variable and transient rheology of the shallow volcanic pile, and the continuous feedbacks between faulting, secondary porosity creation, and geothermal fluid circulation, alteration and cementation that tend to obliterate the tectonic deformation structures. In addition, deformation structures can be also caused by near- and far-field stress regimes, which include magmatic intrusions at various depths, the evolving topography and regional tectonics. A lack of consideration of all these factors may severely underpin the reliability of structural studies. By rebutting and providing a detailed discussion of all the points raised by the comment of Norini and Groppelli (2020) to the Urbani et al. (2020) paper, we take the opportunity to specify the scientific rationale of our structural fieldwork and strengthen its relevance for geothermal exploration and exploitation in active caldera geothermal systems in general and, particularly, for the Holocene history of deformation and geothermal circulation in the Los Humeros caldera. At the same time, we identify several major flaws in the approach and results presented in Norini and Groppelli (2020), such as (1) the lack of an appropriate ranking of the deformation structures considering an inventory method for structural analysis; (2) the misinterpretation and misquoting of Urbani et al. (2020) and other relevant scientific literature; and (3) irrelevant and contradictory statements within their comment.

Summary This study focuses in the analysis of the internal structure of the upper 3 km of Los Humeros caldera and the relation of electrical and hydrothermal anomalies. For this purpose, we measured, processed, and interpreted 78 broadband magnetotelluric (MT) soundings. We performed a 3D inversion of the data set (ModEM) using all MT soundings, although only half of the available frequencies per sounding due to limited computed power. We also carried out the 2D inversions (NLCG) of the invariant determinant along two orthogonal profiles (EW and NS) crossing the caldera structure; their comparison yields similar resistivity and structural models results. The resistivity modeling is complemented with the results of a joint 3D inversion of an accurate gravity database of 720 stations, and total field aeromagnetic data (SGM) from the caldera crater. The combined results provide novel details about the structure of the shallow geothermal reservoir of the resurgence caldera complex hosting the active hydrothermal system. Density and resistivity models show the existence of a composed crater basin structure separated by an EW high density structure; the northern basin is associated to the LH crater, whereas the southern basin associates to the emergent LP caldera basin. The magnetization model indicates that there is a common source for the magnetic volcanic products observed at the caldera surface, and that the Los Potreros (LP) fault is the more magnetized fault of the geothermal system. The propylic zoning under the geothermal field, which according to the MT model results has resistivities above ∼100 ohm-m, was extrapolated using this and additional criteria to obtain the distribution of other hypothetical propylitic zones of hydrothermal potential.