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

Abstract

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 (Chaînons Béarnais), significant fluid flow occurred along a major décollement 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 décollement 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.

Triassic evaporites: a vast reservoir of brines mobilized successively during rifting and thrusting in the Pyrenees

“…10D and E). Such a process was inferred in the case of the late silicification at Urdach: mixing trends between saline and dilute end-members are typical of the early silicification, and during later stages, quartz cement formed from hypersaline brine suggesting that the unconformity was isolated due to the deposition of the Cenomanian flysh, limiting or stopping the venue of seawater (Nteme Mukonzo et al, 2021).…”

“…Several studies consider these water-rock interactions as representative of syn-rift processes and are dated in some instances (albitisation: Poujol et al, 2010Mgmetasomatism: Boulvais et al, 2006;Boutin et al, 2016; breccias with silicifications and carbonate cements: Debroas et al, (2010), Asti et al, (2019), Incerpi et al, 2020, Nteme Mukonzo et al, 2021; see later the compilation of available ages). Table 1 provides details about localities (host-rock, inferred ages of the formation when available).…”

“…In Chainons Bearnais (Salardon et al, 2016, Nteme Mukonzo et al, 2021, salinities of FI in cements synchronous of the rift climax, range from 10 to 37 wt. % eq.…”

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

“…Angrand et al, 2020). Since the concepts of crustal hyper-extension and sub-continental mantle exhumation were applied in the NPZ (Jammes et al, 2009;Lagabrielle et al, 2010;Lagabrielle and Bodinier, 2008), the role of salt tectonics during rifting and inversion has been emphasized as a major element of the Pyrenean rift evolution based on numerical experiments (e.g., Grool et al, 2019;Duretz et al, 2019;Jourdon et al, 2020) and field studies (Jammes et al, 2010;Lagabrielle et al, 2020). Despite this important effort, the temporal and spatial relationships between the basement-cover decoupling in the Triassic evaporites and mantle exhumation during rifting…”

“…To date, the formation of breccias in or near the base of early Cretaceous extensional basins in the Pyrenees have been attributed to :(i) debris flows reworking footwall rocks of high-angle normal fault scarps (Debroas, 1990(Debroas, , 1987Souquet et al, 1985) or low-angle detachment systems (Jammes et al, 2009;Masini et al, 2014), (ii) dissolution-collapse brecciation processes during pre-and syn-rift salt mobilization (Canérot et al, 2005(Canérot et al, , 2004Canérot and Lenoble, 1993;Labaume and Teixell, 2020), (iii) hydraulic fracturing and dolomitization caused by rift-related fluid activities (Incerpi et al, 2020b;Salardon et al, 2017), (iv) fluidassisted (brines) tectonic brecciations along extensional decollement localized in the Triassic evaporites and the pre-rift limestones (Clerc et al, 2016(Clerc et al, , 2015Lagabrielle et al, 2020;Mukonzo et al, 2021) or found in association with ultramafic rocks (Asti et al, 2019;Clerc et al, 2012;de Saint Blanquat et al, 2016;Lagabrielle et al, 2016Lagabrielle et al, , 2019a in-situ topbasement cataclastic breccias across low-angle detachment fault (Masini et al , 2014;Cadenas et al, 2021). These studies are based on observations made at different locations along the strike of the Pyrenean rift and therefore are valuable only at a given rift segment.…”