Gravity study of Los Humeros caldera complex, Mexico - structure and associated geothermal system
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Los Humeros caldera lies in the eastern portion of the Trans-Mexican Volcanic Belt. A geothermal system is associated with this silicic caldera. The volcano-sedimentary products of the caldera itself rest on a basement of calcareous rocks and granitic intrusives. A well-marked density contrast exists between these geologic units. Consequently, gravity measurements have proved to be a useful tool to estimate depths to the local basement. A detailed interpretation of the gravity anomaly of this caldera was conducted. Digital processing included vertical derivatives, downward, and upward continuations. In this way, it was possible to infer the main features of the subsurface structure of this caldera. The 3-D structural model so created was also constrained by geology and well data. It was tested by 3-D forward modelling. Two nested depressions are subsurficial expressions of two corresponding collapses. The western rim of the caldera complex is constituted by a structural high, representing a major weakness zone that controlled the location of several vents in the caldera complex. The geothermal system is located along this structure. A straightforward correlation is observed between the subsurficial structure (nested depressions, the minor central and major western structural highs respectively) and magnetic and resistivity anomalies (MT and d.c. resistivity studies). The enhanced permeability associated with the faulting along this major structural high accounts for the observed correlation with the resistivity data. Two relatively shallow heat sources are interpreted to exist at depths around 10 km. One is related to the actual geothermal production zone; the second, is located just to the southwest of the caldera complex. Because several other geological and geophysical parameters point to the possible existence of geothermal activity, this zone is considered a target for exploration, by means of wells, for geothermal fluids.
- Los Humeros