Serpentinization and Magmatic Distribution in a Hyperextended Rift Suture: Implication for Natural Hydrogen Exploration (Mauléon Basin, Pyrenees)

The Mauléon basin is a world-class example of hyperextended rift suture. The basin possesses key attributes of an optimal hydrogen target, namely mantle, at shallow depth with tectonic structures rooted into it. Natural H2 seepages have been recognized at the surface in the foothills. Yet distribution and quantification of serpentinization within the mantel piece representing the potential H2 source has not been addressed while this aspect is crucial to consider further exploration. We discuss these aspects using joint gravimetric and magnetic 2D forward modeling along two orthogonal transects. 2D forward modeling shows that serpentinization gradually increases from bottom (20 km depth) to top reaching a maximum amount of nearly 76% (8 km depth). The N-S transect evidence that serpentinization fronts are northward inclined, suggesting a N-S serpentinization gradient responsible for the long wavelength gravity and magnetic anomalies. This orientation matches that of detachment within the former hyperextended domain, which exhumed the mantle during the Cretaceous. The W-E transect shows that serpentinization also increase toward the east reaching its maximum amount against the Barlanès lithospheric structure. The latter also coincides with the main short wavelength magnetic anomaly recognized in the basin. Forward geophysical modeling reveals that this anomaly could be linked to the presence, at shallow depth, of an alkaline magmatic body or a shallower piece of highly serpentinized subcontinental mantle both attesting for the paroxysm of the Cretaceous rifting phase. Finally, we propose a conceptual model of the H2 life cycle in the Mauléon basin and discuss the implications for H2 exploration.