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Data gathered from two geothermal exploration wells in the Acoculco caldera, within the Trans-Mexican Volcanic Belt, indicated that temperature is high enough for economic utilization, but permeability is insufficient. Hence, heat exploitation at this location may only be possible by Enhanced Geothermal System (EGS) technologies. To evaluate the potential for EGS development, a wide range of exploration work has been carried out in the framework of the international research project GEMex. In this manuscript, we present best estimates of the in-situ stress field conditions at the site - an important, yet highly uncertain, parameter for stimulation planning. The study is based on geological data, drilling parameters, geophysical logging, laboratory measurements on collected rock samples, and statistical analyses. The in-situ stress regime of the Acoculco geothermal area can be described as transtensional with a maximum horizontal stress striking in the NE-SW direction, a pore pressure gradient of 8.73 MPa·km-1, a minimum horizontal stress gradient of 22.8 ± 3.3 MPa·km-1, a vertical stress gradient of 24.3 ± 1.5 MPa·km-1, and a maximum horizontal stress gradient of 42.9 ± 28.5 MPa·km-1. Based on the predicted stress tensor, we estimate the maximum pressure required to enhance the rock permeability and discuss the potential EGS development options for the Acoculco geothermal area.

Travertines are continental carbonates that precipitate from spring waters, mainly due to CO2 degassing. They are widely quarried as building stone, which has led to numerous (often abandoned) travertine quarry outcrops. These and other outcrops, however, merely represent the surface expressions of the complex architecture of travertine bodies and their deep–rooted feeding systems. Integration of field observations and geochemical analyses with crucial subsurface geophysical data is required in order to fully understand travertine systems and their relationship to the local geological framework. Such a multi-methodological approach is illustrated here for the case of the abandoned Cukor quarry. Radio-magnetotelluric (RMT) and Electrical Resistivity Tomography (ERT) surveys provide unique insights into local tectonics, geobody architecture and the topography preceding travertine deposition. Altogether, the lens–shaped, isolated domal geobody with spring-proximal lithofacies and subvertical bedding, in addition to the contrasting geochemistry and age with surrounding travertine, enabled the unequivocal reconstruction of the dismantled Cukor fissure ridge. This study illustrates the high potential of this multi–methodological approach on poorly exposed, strongly weathered, quarried and tectonically deformed travertine bodies, and provides information on the complex feeding system beneath travertine deposits. Their reconstruction can be interpreted in light of the neo–tectonic expression of regional uplift, helps to complete the regional tectonic history and provides insights into older landscapes that lack preservation or would no longer be immediately recognized at the surface.

Linking the architecture of structural conduits with the hydrothermal fluids migrating from the reservoir up to the surface is a key-factor in geothermal research. A contribution to this achievement derives from the study of spring-related travertine deposits, but although travertine depositional systems occur widely, their feeding conduits are only rarely exposed. The integrated study carried out in the geothermal Gölemezli area, nearby the well-known Pamukkale area (Denizli Basin, western Anatolia, Turkey), focused on onyx-like calcite veins (banded travertine) and bedded travertine well exposed in a natural cross-section allowing the reconstruction of the shallower part of a geothermal system. The onyx-like veins represent the thickest vein network (> 150 m) so far known. New field mapping and structural/kinematic analyses allowed to document a partially dismantled travertine complex (bedded travertine) formed by proximal fissure ridges and distal terraced/pools depositional systems. The banded calcite veins, WNW-trending and up to 12 m thick, developed within a > 200 m thick damaged rock volume produced by parallel fault zones. Th/U dating indicates a long lasting (middle-late Pleistocene) fluids circulation in a palaeo-geothermal system that, due to its location and chemical characteristics, can be considered the analogue of the nearby, still active, Pamukkale system. The isotopic characteristics of the calcite veins together with data from fluid inclusions analyses, allow the reconstruction of some properties (i.e. temperature, salinity and isotopic composition) and processes (i.e. temperature variation and intensity of degassing) that characterized the parent fluids and the relation between degassing intensity and specific microfabric of calcite crystals (elongated/microsparite-micrite bands), controlled by changes/fluctuations of the physico-chemical fluid characteristics. © 2017 Elsevier B.V.