From Early Contraction to Post-Folding Fluid Evolution in the Frontal Part of the Bóixols Thrust Sheet (Southern Pyrenees) as Revealed by the Texture and Geochemistry of Calcite Cements

Abstract

Structural, petrological and geochemical (δ 13 C, δ 18 O, clumped isotopes, 87 Sr/ 86 Sr and ICP-MS) analyses of fracture-related calcite cements and host rocks are used to establish a fluid-flow evolution model for the frontal part of the Bóixols thrust sheet (Southern Pyrenees). Five fracture events associated with the growth of the thrust-related Bóixols anticline and Coll de Nargó syncline during the Alpine orogeny are distinguished. These fractures were cemented with four generations of calcite cements, revealing that such structures allowed the migration of different marine and meteoric fluids through time. During the early contraction stage, Lower Cretaceous seawater circulated and precipitated calcite cement Cc1, whereas during the main folding stage, the system opened to meteoric waters, which mixed with the connate seawater and precipitated calcite cement Cc2. Afterwards, during the post-folding stages, connate evaporated marine fluids circulated through newly formed NW-SE and NE-SW conjugate fractures and later through strike-slip faults and precipitated calcite cements Cc3 and Cc4. The overall paragenetic sequence reveals the progressive dewatering of Cretaceous marine host sediments during progressive burial, deformation and fold tightening and the input of meteoric waters only during the main folding stage. This study illustrates the changes of fracture systems and the associated fluid-flow regimes during the evolution of fault-associated folds during orogenic growth.During the geodynamic evolution of fold and thrust belts and related foreland basins, fluid migration controls diagenetic processes and the propagation of fractures and faults. In turn, the fracture geometry and architecture conditions their role as either conduits or seals for fluids, thus controlling fluid distribution [13,14]. In this geodynamic setting, the fluid sources change with time as foreland basins usually evolve from marine to continental conditions [15,16]. Moreover, thrusts may act also as paths for deep-sourced fluids while fold-associated fractures for low-temperature meteoric fluids [17,18].Structural analysis of fractures, together with petrographical and geochemical studies of vein calcite cements and their host rocks, allow to assess the interplay between rock deformation and diagenetic reactions, the degree of fluid-rock interaction, the fluid-flow regime during deformation and the relative timing of fluid circulation [18][19][20]. Likewise, the type and origin of fluids can be unraveled [21][22][23], and thus decipher if the fluids that formed the different veins flowed locally in a closed paleohydrogeological regime [24][25][26] or in a relatively open system with possible interaction between fluids from different sources [16,27]. Studies integrating the evolution of fracture systems and related cements are needed to constrain the fluid-flow history of an area during orogenic growth and, therefore, understand the nature and origin of fluids that circulate through time, the diagenetic process evolution, changes in...

Extensional reactivation of the Penninic frontal thrust 3 Myr ago as evidenced by U–Pb dating on calcite in fault zone cataclasite

“…2. Domains represented by dashed red, black and blue lines are from the literature (Nardini et al, 2019;Rossi and Rolland, 2014, and references therein). The green domain corresponds to veins associated with brittle-ductile structures.…”

Extensional reactivation of the Penninic Frontal Thrust 3 Ma ago as evidenced by U-Pb dating on calcite in fault zone cataclasite

“…The western Late Eocene Flysch (Champsaur sandstone) gave the lowest δ 13 C ratios of -0.3 ‰ and a δ 18 O ratio of 21.9 ‰. Analysed brittle-ductile veins either related to the compressional or to the onset of the extensional tectonic phases stand relatively close to their host rocks and to the meteoric water field defined by Nardini et al (2019) (Fig. 6).…”

“…lines are from the literature (Nardini et al, 2019;Rossi and Rolland, 2014 and references therein). The coloured green domain corresponds to veins associated to brittle-ductile structures which show similar isotopic compositions as their host rocks.…”

“…Based on several studies in the frontal parts of Alpine orogens (Smeraglia et al, 2020;Nardini et al, 2019), these isotope signatures are thought to be representative of meteoric water system from the most superficial domains. Three important parameters are involved to control this surface-derived fluid regime: (i) the presence of a shallow impermeable clay layer and (ii) evidences of pressuresolution (iii) and finally the lack of large-scale structures (section 4.1. and Fig.…”

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

“…This fault-controlled fluid flow could have been continuous through time or could be related to intermittent pulses. Fault control on upflowing of hot fluids along fault systems is a common process in different geological settings and has been reported in the Great Basin, USA (Faulds et al, 2010;Nelson et al, 2009), in western Turkey (Faulds et al, 2010), in the southern Canadian Cordillera (Grasby and Hutcheon, 2001), and in southern Tuscany, Italy (Liotta et al, 2010).…”