Influence of basement rocks on fluid evolution during multiphase deformation: the example of the Estamariu thrust in the Pyrenean Axial Zone

Abstract

Abstract. Calcite veins precipitated in the Estamariu thrust during two tectonic events are studied in order to (i) decipher the temporal and spatial relationships between deformation and fluid migration in a long-lived thrust and (ii) determine the influence of basement rocks on the fluid chemistry during deformation. Structural and petrological observations constrain the relative timing of fluid migration and vein formation, whilst geochemical analyses (δ13C, δ18O, 87Sr∕86Sr, clumped isotope thermometry, and elemental composition) applied to the related calcite cements and host rocks indicate the fluid origin, pathways, and extent of fluid–rock interaction. The first tectonic event, recorded by calcite cements Cc1a and Cc2, is attributed to the Alpine reactivation of the Estamariu thrust. Analytical data indicate that these cements precipitated from heated meteoric fluids (temperatures in the range of 50 to 100 ∘C) that had interacted with basement rocks (87Sr∕86Sr > 0.71) before upflowing through the thrust zone. The second tectonic event, attributed to the Neogene extension, is characterized by the reactivation of the Estamariu thrust and the formation of normal faults and shear fractures sealed by calcite cements Cc3, Cc4, and Cc5. Analytical data indicate that cements Cc3 and Cc4 precipitated from hydrothermal fluids (temperatures between 130 and 210 ∘C and between 100 and 170 ∘C, respectively) that had interacted with basement rocks (87Sr∕86Sr > 0.71) and been expelled through fault zones during deformation. In contrast, cement Cc5 probably precipitated from meteoric waters that likely percolated from the surface through small shear fractures. The comparison between our results and already published data in other structures from the southern Pyrenees suggests that regardless of the origin of the fluids and the tectonic context, basement rocks have a significant influence on the fluid chemistry, particularly on the 87Sr∕86Sr ratio. Accordingly, the cements precipitated from fluids that have interacted with basement rocks have significantly higher 87Sr∕86Sr ratios (> 0.710) with respect to those precipitated from fluids that have interacted with the sedimentary cover (< 0.710), which involves younger and less radiogenic rocks.

Origin of 87Sr enrichment in calcite cements in Jurassic limestones (Eastern Paris Basin, France)

“…(2) Waters derived from deeper Triassic aquifers or basement fluids (Muñoz-López et al, 2020) injected into the shallower Jurassic carbonate aquifers, as evidenced in the central Paris Basin Matray, 1993a, 1993b;Worden and Matray, 1995);…”

Origin and distribution of calcite cements in a folded fluvial succession: The Puig‐reig anticline (south‐eastern Pyrenees)

“…This would suggest fluidrock interaction between the cement-forming fluids and the radiogenic silicic clasts of the host rocks sourced from the Palaeozoic basement in the Axial Zone. Muñoz-L opez et al (2020a) studied the influence of basement rocks on fluid chemistry in other outcrops elsewhere in the Pyrenees, and concluded that calcite cements precipitated from fluids that strongly interacted with basement rocks could have resulted in a significantly higher 87 Sr/ 86 Sr ratio (>0.710) in nearby areas. The 87 Sr/ 86 Sr ratios of Cc3 in our study area are lower than this value, probably because carbonate clasts dominate over silicic clasts in the zone where Cc3 precipitated.…”

“…Many studies mainly focus on unravelling the relationships between fluid flow, deformation and fluid–rock interaction in fold‐and‐thrust belts, whereas their adjacent foreland basins are significantly less well‐documented. For example, in the South Pyrenees, an ideal natural laboratory due to its well‐preserved and well‐exposed outcrops, most studies focused on the cover thrust sheets (Travé et al ., 1997; Lacroix et al ., 2011, 2014, 2018; Beaudoin et al ., 2015; Crognier et al ., 2018; Cruset et al ., 2018, 2020a,b; Nardini et al ., 2019; Muñoz‐López et al ., 2020a,b), whereas only a few studies involved the Ebro Foreland Basin (Travé et al ., 2000, 2007; Cruset et al ., 2016). On the other hand, compressional settings have high potential for carbon capture and storage (CCS) (Sun et al ., 2020).…”