Regions of anomalous geothermal fields in the State of Tocantins, Central Brazil
We report recent progress in determination of geothermal gradients and heat flow in the State of Tocantins, Central Brazil. This region lying between the Amazonas and Sao Francisco cratons has been affected by metamorphic folding events (Brasilia and Araguaia) during Proterozoic times. This area is also characterized by moderate micro-seismic seismic activity. Results of recent investigations have revealed the presence of several areas where geothermal gradients and heat flow have values higher than normal, which is considered atypical of stable tectonic settings. In southern parts of the State of Tocantins heat flow values are higher than 80 mW/m2. Extrapolations based on near surface heat flow data point to crustal temperatures in excess of 200°C at depths less than 5 km. However, there are no evidences of magmatic intrusions at shallow intra-crustal depths. In the absence of other geologic source mechanisms and tectonic events the process responsible for high heat flow has been postulated to be enhanced heat transport by carbonic gas flow in the upper crust. This possible alternative is supported by observations of carbonic gas flow at sites of thermal springs within the study area and also in geothermal areas in the neighboring state of Goiás. Model simulations of deep crustal geotherms indicate that temperatures may approach levels of partial fusion at the crust mantle boundary.