Heat flow and deep temperatures in the South-East basin of France: implications for local rheological contrasts
Résumé
Triassic salt at 5-10km depth may drive some of the recent tectonic features in south-eastern France. We estimate the likely temperature range of the salt using two different approaches. The first of these, based on the extrapolation of deep temperatures obtained in oil exploration wells, predicts temperatures at a depth of 8km to be in the range of 230-300 °C. However, this prediction could be biased by a lack of deep measurements and problems related to lateral heat transfer caused by thermal conductivity contrasts. The second approach can overcome these problems by modelling the actual heat transfer for appropriate basin geometry, including temperature-dependent thermal properties, and a mantle heat flow of 35 mW.m-2. This latter value enables us to reproduce available temperature measurements and surface heat flow data. Here we evaluate the stationary temperature field along two sections constrained by seismic profiles, one at a local scale across the Vistrenque graben and the other at a more regional scale across the South-East basin. Our findings suggest that the temperatures in the deepest parts of the evaporitic layer (11 km depth) can reach up to 398°C, but can be as low as 150 °C on the edge of the basin at the top of the salty layer. This temperature difference leads to important changes in salt viscosity. Results indicate that at a depth of 8 km, lateral viscosity contrasts within the evaporitic layer may reach 40. Such rheological contrasts might favour and amplify local subsidence, as seems to have been the case near the two Palaeogene half-grabens of Vistrenque and Valence, where deep hot zones are identified.