The North Pyrenean Frontal Thrust: structure, timing and late fluid circulation inferred from seismic and thermal-geochemical analyses of well data

Abstract

During orogenesis, large-scale thrusts as orogenic fronts can act as conduits and/or barriers for fluid flow. Unravelling the timing and modes of tectonic activation of large-scale faults is crucial to understanding the relationship between fluid flow and deformation. The North Pyrenean Frontal Thrust (NPFT) corresponds to a major basement-involved thrust responsible for the northward overthrust of the pre-orogenic sediments on top of the Aquitaine Foreland Basin. This study questions the timing of activation of this thrust, its geometry, the nature of the last fluids, which circulated there, and its role on the circulation of fluids. The structural study confronted to published thermochronology data led to determine the timing of the two tectonic activations during the NPFT compression phase and to relate them to the fluid circulations. We constrain the first activation at Campanian times and link it to the leak of the deep gas reservoir present in depth, as the NPFT acted as a conduit. Then the NPFT acted as a barrier, probably due to the breccia consolidation during the Paleocene quiescence period. Finally, the Eocene-Oligocene reactivation led to fluid circulation of high salinity fluids from the Triassic evaporites leaching. This latter event is associated with a fracturing event and the late generation of calcite veins studied here. This is the first study in the Pyrenees directly applied to the NPFT which uses the association between fluid inclusions study, seismic and thermochronological data. It highlights that the NPFT may be an important structure responsible of the leakage of deep hydrocarbons reservoirs. It also shows the importance of the determination of the activation steps of large-scale faults to decipher the origin of fluid circulations in space and time.

Evolution of a low convergence collisional orogen: a review of Pyrenean orogenesis

“…2020: Ortiz et al, 2020). The Aquitaine basin overlies a saltrich Aptian-Cenomanian rift system that widens westward (Tugend et al, 2014a: Angrand et al, 2018: Gómez-Romeu et al, 2019: García-Senz et al, 2020: Andrieu et al, 2021: Barré et al, 2021. Pre-orogenic Jurassic and Cretaceous diapiric growth sourced from Triassic evaporites has long been recognized across this region (James and Canérot, 1999: Canérot et al, 2005: Biteau et al, 2006: Serrano et al, 2006.…”

“…These faults variably record both early inversion (Late Cretaceous) and Eocene-Oligocene thrusting (Fig. 7; Rocher et al, 2000;Rougier et al, 2016;Grool et al, 2018;Fillon et al, 2020;Labaume and Teixell, 2020;Barré et al, 2021).…”

“…4) demonstrate that the thermal perturbation inherited from Aptian-Cenomanian rifting maintained temperatures during early orogenesis above the sensitivity limit of low-temperature thermochronometers (40-300 °C; e.g., Carrapa, 2010;Peyton and Carrapa, 2013), thus delaying cooling until Eocene collision. Late Cretaceous inversion structures are nevertheless reported along the western North Pyrenean Thrust Front (e.g., Jourdan et al, 1998;Barré et al, 2021). Thermal modelling of new low temperature thermochronology data from an adjacent deep borehole in the proximal foreland basin (Fillon et al, 2020) shows a clear signal of early orogenic heating due to synorogenic burial.…”

Evolution of rift-related cover-basement decoupling revealed by brecciation processes in the eastern Pyrenees

“…Fluids that form brines in the pre-to synrift cover are often characterized by elevated temperature and salinity. They are interpreted to result from the dissolution of Triassic evaporites (Cathelineau et al, 2021;Motte et al, 2021;Barré et al, 2021).…”