Heat flow, thermal structure and thermal evolution of the Parana Basin, southern Brazil.

Abstract

I report 56 new heat flow measurements for the Parana Basin of southern Brazil, a large Phanerozoic basin covered with flood basalts in the Mesozoic. The heat flow increases from 40 mW m$\sp{-2}$ in the central region of the basin (the region of thickest basalt cover) to 75 mW m$\sp{-2}$ at the eastern basin margin. This pattern of heat flow can be explained by crustal and mantle structures associated with the generation and extrusion of flood basalts which divert deep mantle heat away from the central region into the thinner lithosphere underlying the basin margin and surrounding folded belts. A least squares inversion of the temperature and stratigraphic data yields best fitting geothermal gradients for representative formations and a best fit subsurface temperature field for the Parana Basin. This temperature field is in agreement with results of modeling of a purely conductive regime. Advective heat transport models suggest that the effects of basin scale subsurface fluid flow are likely to be small and if present would cause heat flow variation opposite to that observed. Numerical models of temperature histories in the sediments and of surface heat flow through time suggest that a basin-initiating thermal event would have been of little consequence to any but the earliest sediments. The history of the basin has been predominantly one of subsidence, interrupted by regional uplift. The last depositional event was the extrusion of flood basalts at 135-130 Ma over most of the basin surface, with associated intrusions in the sediments. The accompanying burial increased the temperatures within the sediments by up to 40$\sp\circ$C. 'Illite crystallinity' data appear to reflect the combined effects of increasing temperature and time with burial. The I/S ratio of mixed-layer illite smectite measured on core samples taken in proximity to sill intrusions is compatible with the present-day temperatures controlled by the geothermal gradient and suggests that short-lived high temperatures associated with the igneous activity did not affect significantly the I/S ratio. A comparison of the thermal history of two sites of different subsidence/uplift histories confirms that the thermal maturation of the sediments is dominated by the long term subsidence history and is little affected by igneous activity.