TERRESTRIAL HEAT-FLOW IN THE PARANA BASIN, SOUTHERN BRAZIL

We present 56 new heat flow values from the intracratonic Parana Basin in southern Brazil. This large basin is filled with up to 5 km of Pal eozoic and Mesozoic sedimentary rocks. In the Late Jurassic-Early Cret aceous a great igneous event capped most of the basin surface with flo od basalts up to 1700 m thick. Geothermal gradients computed from 79 d eep exploration boreholes range from 20 K km(-1) to 30 K km(-1) with t he lower gradients generally located in the central part of the basin. Thermal conductivities were determined on 247 core samples. The harmo nic mean thermal conductivity of the section encountered by the boreho les decreases from 3.0 W m(-1) K-1 at the eastern basin margin to 2.0 W m(-1) K-1 in the basin center; this variation is related to the thic kening of the basalt cap toward the basin center. Surface heat flow va lues for the 56 sites range from 40 mW m(-2) to 75 mW m(-2), with larg er and more variable values (50-70 mW m(-2)) occurring along the easte rn margin of the basin in the region without basalt cover. The heat fl ow in the central part of the basin (40-50 mW m(-2)) is less than that on the basin margin by about 15 mW m(-2) and is more uniform. We disc ount advective effects as an explanation of the heat flow pattern beca use if a topographically driven now system existed, it would diminish heat flow in the elevated recharge area along the basin margin and aug ment heat flow in the discharge area along the basin axis, opposite to what is observed. Wholly conductive models show that Larger-scale the rmal conductivity contrasts produced by the flood basalts do not refra ct significant heat into the surrounding higher-conductivity sedimenta ry section on the periphery of the basalts. Other model calculations s how that the heat flow at the surface reflects the heat input from the basement with only minor, if any, redistribution within the basin. We conclude that the thermal data indicate a dominantly conductive therm al regime within the basin and that the observed heat flow pattern is not likely to result from intrabasinal causes. The observed pattern li kely reflects the larger-scale thermal structure of the lithosphere of this region, developed at the time the flood basalts were generated a nd extruded.