Terrestrial heat flow in the Paraná Basin, southern Brazil

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Abstract
We present 56 new heat flow values from the intracratonic Paraná Basin in southern Brazil. This large basin is filled with up to 5 km of Paleozoic and Mesozoic sedimentary rocks. In the Late Jurassic‐Early Cretaceous a great igneous event capped most of the basin surface with flood basalts up to 1700 m thick. Geothermal gradients computed from 79 deep exploration boreholes range from 20 K km−1 to 30 K km−1 with the lower gradients generally located in the central part of the basin. Thermal conductivities were determined on 247 core samples. The harmonic mean thermal conductivity of the section encountered by the boreholes 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 thickening of the basalt cap toward the basin center. Surface heat flow values for the 56 sites range from 40 mW m−2 to 75 mW m−2, with larger and more variable values (50–70 mW m−2) occurring along the eastern margin of the basin in the region without basalt cover. The heat flow 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 discount advective effects as an explanation of the heat flow pattern because if a topographically driven flow system existed, it would diminish heat flow in the elevated recharge area along the basin margin and augment heat flow in the discharge area along the basin axis, opposite to what is observed. Wholly conductive models show that larger‐scale thermal conductivity contrasts produced by the flood basalts do not refract significant heat into the surrounding higher‐conductivity sedimentary section on the periphery of the basalts. Other model calculations show 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 thermal regime within the basin and that the observed heat flow pattern is not likely to result from intrabasinal causes. The observed pattern likely reflects the larger‐scale thermal structure of the lithosphere of this region, developed at the time the flood basalts were generated and extruded.