Estrutura Termal da Litosfera nos segmentos cratônicos de Salvador e São Francisco

Abstract

Regional variations in geothermal gradients and heat flow across the Sao Francisco craton and the neighboring metamorphic fold belts are examined on the basis of geothermal data from the highland areas of eastern Brazil. The database employed include results of geothermal measurements in 63 localities. The results indicate that the northeastern segment of cratonic area and the adjacent metamorphic fold belts are characterized by mean geothermal gradients in the range of 12 to 18oC/km. The estimated heat flow values are found to fall in the range of 35 to 55mW/m2, with the cratonic area having low values relative to those in the fold belts. On the other hand, the southwestern segment of this craton and the Sao Francisco sedimentary basin are characterized by higher gradient values, in the range of 20 to 24oC/km, and the corresponding heat flow values are in excess of 60mW/m2. Crustal thermal models have been developed to examine the implications of the observed intra-cratonic variations in heat flow. These models take into consideration variation of thermal conductivity with temperature as well as change of radiogenic heat generation with depth. Vertical distributions of seismic velocities were used in obtaining estimates of radiogenic heat production in crustal layers. Crustal temperatures are calculated based on a procedure that makes simultaneous use of the Kirchoff and Generalized Integral Transforms, providing thereby analytical solutions in 2D and 3D geometry. The results point to temperature variations of up to 250oC at the Moho depth, between the northern and southern segments of the craton. There are indications that such differences are responsible for the contrasting styles of deformation patterns in the adjacent metamorphic fold belts.