Geothermal gradient and heat flow in the state of Rio de Janeiro

Abstract

Results of geothermal studies carried out at 72 localities have been used in evaluation of temperature gradient and heat flow values of the upper crust in the state of Rio de Janeiro. The investigations included temperature logs in boreholes and wells, calculation of geothermal gradients, measurements of thermal conductivity and determination of heat flow density. In addition, estimates of temperature gradients and heat flow were also made for areas of thermo-mineral springs, based on the so-called geochemical methods. Analysis of these data sets, after incorporation of appropriate corrections (for the perturbing effects of drilling operations, topography and climate changes) has allowed for the first time a better understanding of the regional distribution of thermal gradients and heat flow within the study area. The results obtained indicate that geothermal gradient values are in the ranges of 14 to 26oC/km in Precambrian metamorphic terrain and 19 to 33oC/km in areas of Phanerozoic sedimentary basins. Most of the rock formations are characterized by thermal conductivity values varying from 2.2 to 3.6 Wm-1K-1. Consequently regionally averaged mean heat flow values are found to fall in the interval of 40 to 70 mW/m2. Computer generated contour maps reveal that geothermal gradients and heat flow are systematically high in the western compared to the eastern parts of the state of Rio de Janeiro. There are indications that this geothermal anomaly is probably associated with the belt of Tertiary alkaline intrusives, between Itatiaia and Cabo Frio. Residual heat of large scale magma intrusions in the later part of the Tertiary period may be one of the possible mechanisms responsible for this thermal anomaly.