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.
Highlights
Large-scale faults at orogenic fronts are privileged places to study the relationships between long-term deformation processes with short-term episodic fluid circulations
As determined in the Lannemezan-1 well, the northern part of Baronnies Trough located on the hanging wall of the North Pyrenean Frontal Thrust (NPFT) is filled by Albo-Cenomanian syn-rift successions which sit on the Upper Triassic evaporites (Fig. 3)
Further studies are certainly needed to validate this hypothesis, but so far it remains the most likely explanation. This is the first study in the Pyrenees that combined fluid inclusions study to thermochronological studies in a structural framework
Summary
Large-scale faults at orogenic fronts are privileged places to study the relationships between long-term deformation processes with short-term episodic fluid circulations. Foreland thrusts are known to usually act as conduits for fluids (Travé et al, 2000; Cruset et al, 2018; Smeraglia et al, 2020) driving circulation of different types of fluids dependent on their tectonic history and sources (e.g., Travé et al, 2007; Lacroix et al, 2014). Nature and timing of the fluid circulations in the central part of the South Pyrenean Zone are well documented for Cenozoic synorogenic deformation phases (e.g., Rye and Bradbury, 1988; Travé et al, 2000; Lacroix et al, 2014; Beaudoin et al, 2015; Crognier et al, 2018; Cruset et al, 2018). One of the main syn-orogenic faults carrying fluids in the north of Pyrenees is the North Pyrenean Frontal Thrust (NPFT; Connan and Lacrampe-Couloume, 1993) which separate the Aquitaine Foreland Basin from the North Pyrenean Zone (NPZ; Fig. 1)
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