Paleogeothermal Gradients across an Inverted Hyperextended Rift System (Mauléon Fossil Rift,Western Pyrenees)

Examples of fossil and present-day passive margins resulting from mantle exhumation at the ocean–continent transition appear to have developed under conditions of high mantle heat flow. The pattern of geothermal gradients along these hyperextended margins at the time of rifting is of interest for exploration of geothermal and petroleum resources, but is difficult to access. 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. Data from a set of 155 samples from densely spaced outcrops and boreholes, analyzed using Raman spectroscopy of carbonaceous material, shed light on the distribution of geothermal gradients across the inverted hyperextended Mauléon rift basin during Albian and Cenomanian time, its period of active extension. The estimated paleogeothermal gradient is strongly related to the structural position along the Albian-Cenomanian rift, increasing along a proximal-distal margin transect from ~34°C/km in the European proximal margin to ~37–47°C/km in the two necking zones and 57–60°C/km in the hyperextended domain. This pattern of the paleogeothermal gradient induced a complex competition between brittle and ductile deformation during crustal extension. A numerical modeling approach reproducing the thermal evolution of the North Pyrenees since 120 Ma suggests that mantle heat flow values may have peaked up to 100 mW.m-2 during the rifting event. We demonstrate that the style of reactivation during subsequent convergence influences the thermal structure of the inverted rift system.