Fluid-rock interactions along detachment faults during continental rifting and mantle exhumation: the case of the Urdach lherzolite body (North-Pyrenees)

The North Pyrenean Zone (NPZ) corresponds to the paleo-passive margin of the North Iberia plate at the foot of which subcontinental mantle exhumed during Albian times. Rare bodies of exhumed mantle rocks associated with strongly sheared lenses of continental crust are scattered among the NPZ metasediments. In the Urdach massif (Chainons Bearnais), significant fluid flow occurred along a major decollement at the basement-Trias interface. Fluids with a broad range of salinity (10 to 38 wt.% NaCl equiv.), indicative of mixing between brines and more diluted waters, produced strong silicification of breccias. Brines circulated at around 240-280°C under lithostatic pressures at about 6 ± 1 km depth. Fluids became increasingly saline towards the final stages. Then, the syn-deposition of Cenomano-Turonian flysch layers progressively isolated the lower aquifers close to the decollement where Triassic brines were predominant. The release and the migration of significant volumes of brines during the stretching and squeezing of the Triassic evaporites played a critical role in the mineralogical and rheological transformations that occurred during the Pyrenean Cretaceous rifting event.