Paleogeothermal Gradients Across an Inverted Hyperextended Rift System: Example of the Mauleon Fossil Rift (Western Pyrenees)

The fossil rift in the North Pyrenean Zone, which underwent high temperature-low pressure metamorphism and alkaline magmatism during Early Cretaceous hyperextension, was studied to explore the geothermal regime at the time of rifting. In this work, we combined Raman lab analysis and thermal numerical modelling to shed light on the distribution of geothermal gradients across the inverted hyperextended Mauleon rift basin during Albian and Cenomanian time, its period of active extension. Data were acquired from a set of 155 samples from densely spaced outcrops and boreholes, analyzed using Raman spectroscopy of carbonaceous material. The estimated paleogeothermal gradient is strongly related to the structural position along the Albian-Cenomanian rift, increasing along a proximal-distal margin transect from similar to 34 degrees C/km in the European proximal margin to similar to 37-47 degrees C/km in the two necking zones and 57-60 degrees C/km in the hyperextended domain. This pattern of the paleogeothermal gradient induced a complex interaction between brittle and ductile deformation during crustal extension. A numerical model reproducing the thermal evolution of the North Pyrenees since 120 Ma suggests that mantle heat flow values may have reached 100 mW/m(2) during the rifting event. This model reveals that above the thermal pulse, the temperature gradient varied within a small range of 55 to 62 degrees C/km, as inferred from RSCM peak temperatures. We demonstrate that the style of reactivation during subsequent convergence influenced the thermal structure of the inverted rift system.