Fluid Dynamics in a Thrust Fault Inferred from Petrology and Geochemistry of Calcite Veins: An Example from the Southern Pyrenees

Abstract

Petrographic and geochemical analyses (δ¹⁸O, δ¹³C, ⁸⁷Sr/⁸⁶Sr, clumped isotopes, and elemental composition) coupled with field structural data of synkinematic calcite veins, fault rocks, and host rocks are used to reconstruct the episodic evolution of an outstanding exposed thrust zone in the Southern Pyrenees and to evaluate the fault behavior as a conduit or barrier to fluid migration. The selected thrust displaces the steeply dipping southern limb of the Sant Corneli-Bóixols anticline, juxtaposing a Cenomanian-Turonian carbonate unit against a Coniacian carbonate sequence. Successive deformation events are recorded by distinct fracture systems and related calcite veins, highlighting (i) an episodic evolution of the thrust zone, resulting from an upward migration of the fault tip (process zone development) before growth of the fault (thrust slip plane propagation), and (ii) compartmentalization of the thrust fault zone, leading to different structural and fluid flow histories in the footwall and hanging wall. Fractures within the footwall comprise three systematically oriented fracture sets (F1, F2, and F3), each sealed by a separate generation calcite cement, and a randomly oriented fracture system (mosaic to chaotic breccia), cemented by the same cements as fracture sets F1 and F2. The formation of fractures F1 and F2 and the mosaic to chaotic breccia is consistent with dilatant fracturing within the process zone (around the fault tip) during initial fault growth, whereas the formation of the latest fracture system points to hybrid shear-dilational failure during propagation of the fault. The continuous formation of different fracture systems and related calcite cementation phases evidences that the structural permeability in the footwall was transient and that the fluid pathways and regime evolved due to successive events of fracture opening and calcite cementation. Clumped isotopes evidence a progressive increase in precipitation temperatures from around 50°C to 117°C approximately, interpreted as burial increase linked to thrust sheet emplacement. During this period, the source of fluid changed from meteoric fluids to evolved meteoric fluids due to the water-rock interaction at increasing depths and temperatures. Contrary to the footwall, within the hanging wall, only randomly oriented fractures are recognized and the resulting crackle proto-breccia is sealed by a later and different calcite cement, which is also observed in the main fault plane and in the fault core. This cement precipitated from formation fluids, at around 95°C, that circulated along the fault core and in the hanging wall block, again supporting the interpretation of compartmentalization of the thrust structure. The integration of these data reveals that the studied thrust fault acted as a transverse barrier, dividing the thrust zone into two separate fluid compartments, and a longitudinal drain for migration of fluids. This study also highlights the similarity in deformation processes and mechanisms linked to the evolution of fault zones in compressional and extensional regimes involving carbonate rocks.

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1. Introduction

The study of outcrop analogues in fractured carbonate reservoirs is important to better understand the characteristics and evolution of synkinematic fracture systems and their control on fluid migration during crustal deformation [1–4]. In areas undergoing compressional regimes, the largest fluid fluxes, mass transfer, and heat transport commonly occur along the main thrust faults and related fracture networks because of the loading induced by thrust sheet emplacement [5–7]. By contrast, fluid flow rates in adjacent rock-matrix and poorly connected synkinematic fractures are commonly very low and fluid composition are often rock-buffered [6, 8]. In some cases, the development of thrust systems may also inhibit vertical fluid transport inducing fluid overpressure [9–11] leading to hydraulic fracturing [12–14]. Whether a fault zone will constitute either a conduit or barrier to fluid migration depends, among other factors, on the architecture of the fault zone and the permeability associated with the developed structures [15, 16]. Since the fault zone consists of a fault core, which is usually formed of low-permeability fault rocks, and a damage zone, which mainly includes extensional fractures and faults, overall permeability of the fault is conditioned by the amount, the spatial distribution, and the internal composition of these two fault zone elements [15, 17]. Besides, such structural permeability is dynamic and may vary spatially and temporally across the fault zone due to successive episodes of fracture opening and cementation [18, 19].

Although numerous studies based primarily on structural and numerical data have provided conceptual and analytical models on the architecture, mechanical properties, and fluid flow along fault zones [7, 15, 20], there exist only a few studies coupling field data and geochemistry of synkinematic minerals filling fractures that characterize the fluid migration through a thrust zone [21–25] and its spatial behavior as a conduit or barrier system [26, 27]. An outstanding exposed thrust in the Southern Pyrenees was chosen as a case study to evaluate qualitatively the fault-related permeability and its control on the fluid flow within and around the fault zone. Here, we combine structural, petrological, and geochemical data of calcite veins and host rocks present in the studied thrust zone. Therefore, the main objectives of this paper are (i) to determine the origin, composition, and temperature of the vein-forming fluids and the timing of fluid migration in relation to the fracturing events and (ii) to discern the fluid pathways, the extent of fluid-rock interaction, and the transfer of fluids across a fault zone during thrusting. The field and lab results are then compared with other studies reporting fluid flow within fault zones in other geological settings to generalize our conclusions to fault zones in carbonate settings.

2. Geological Setting

The Pyrenees constitute an asymmetrical and doubly verging orogenic belt that resulted from the Alpine convergence between the Iberian and European plates from Late Cretaceous to Oligocene, causing the inversion of previous Mesozoic rift basins and their incorporation into the thrust system [28–32]. The Pyrenean structure consists of a central antiformal stack of basement-involved thrust sheets from the axial zone [30], flanked by two oppositely vergent fold-and-thrust belts and their related Cenozoic Aquitaine and Ebro foreland basins [30, 33] (Figure 1(a)).

[figures omitted; refer to PDF]

The South Pyrenean fold-and-thrust belt comprises a piggy-back imbrication of south verging and east-west striking thrust sheets involving Mesozoic and Tertiary cover rocks detached above Late Triassic evaporites [34] and transported southwards over the Ebro foreland basin. From north to south, they are the Bóixols thrust sheet, emplaced during the Late Cretaceous to Paleocene, the Montsec thrust sheet, originated during the Paleocene to late Ypresian, and the Serres Marginals thrust sheet, formed during the Lutetian-Oligocene [35, 36] (Figure 1(a)). In the frontal part of the Bóixols thrust sheet, a south verging and east-west trending fault-related anticline developed in relation to the propagation of the Bóixols thrust (the Sant Corneli-Bóixols anticline; Figures 1(b) and 1(c)). It is characterized by a gently dipping northern backlimb and a vertical to overturned southern forelimb [37].

Rocks cropping out along the Bóixols anticline comprise a large stratigraphic record ranging in age from Jurassic to Oligocene and deposited during the main tectonic events that affected the study area [38] (Figures 1(b) and 2). The stratigraphic sequence includes prerift Jurassic limestones and dolostones, synrift Lower Cretaceous limestones, marls and clays, preorogenic (postrift) Late Cenomanian to Santonian limestones, sandy limestones and marls and synorogenic Late Santonian to Oligocene marls and clays, sandstones, sandy limestones, and conglomerates ([38] and references therein).

[figure omitted; refer to PDF]

In the central part of this anticline, the southern tilted forelimb is offset by minor low-dipping south-directed thrusts developed after the main folding phase, as evidenced by the displacement of already folded beds [38–40]. Therefore, they have been interpreted as postfolding accommodation structures or as hanging wall splays of the main Bóixols thrust [39, 40, 42]. One of these minor thrust faults is well exposed 2 km southeast of the Bóixols village (15 km of the L511 road), in the southern-central Pyrenees (Figure 3), exhibiting a complex calcite-filled fracture network developed in the fault core and damage zone of both the footwall and hanging wall of this reverse fault. This outcrop represents an exceptional field analogue to observe in a 300 m long outcrop the variation in deformation mechanisms developed across a fault zone and its contribution as seal or conduit for fluids. In this area, the studied thrust juxtaposes the Upper Cretaceous Santa Fe Formation against the Collada Gassó Formation (Figures 2 and 3). The Collada Gassó Formation [43] is formed of Coniacian grainstones with the variable presence of quartz, marly limestones, and limestones with abundant fossil content and has a general thickness of 150 to 250 m [38]. The Santa Fe Formation [44] is basically constituted of Cenomanian-Turonian limestones with a general thickness of 20 m [38].

[figure omitted; refer to PDF]

[figures omitted; refer to PDF]

The transition from the fault core to the hanging wall damage zone is marked by discrete S-C sigmoidal shears without the development of foliated cataclasite, or by a discrete, discontinuous slip plane mineralized with calcite (Figure 5(c)).

Both the footwall and hanging wall damage zones comprise multiple sets of systematically and randomly oriented calcite-cemented fractures (i.e., veins) developed at both sides of the fault core (Figures 5 and 6). Although in both cases, there is a progressive increase in deformation and vein intensity from the protolith towards the fault core (Figure 5(a)), the geometry of veins differs from one to another damage zone.

In the footwall damage zone, there are two areas showing different vein characteristics (Figure 5(a)). The first area is around 30 m wide from the protolith and is characterized by three systematically oriented vein systems (F1 to F3) with clear crosscutting relationships between them (Figures 5(a)–5(c)). F1 consists of NNW-SSE extensional veins dipping between 50 and 70° to the SW. They are up to 1–2 m long and less than 2 cm thick (Figures 5(a), 5(b), and 6(a)). F2 is characterized by E-W extensional veins steeply dipping to the S-SW. These veins are several meters long and up to 5 cm thick (Figures 5(a), 5(b), and 6(b)). F3 consists of N-S and NE-SW en échelon conjugate sets of tension gashes (i.e., hybrid extensional-shear calcite veins) gently dipping towards the NW (Figure 6(c)). These veins are centimetric-sized and have sigmoidal shapes. The second area observed in the footwall covers around 15-20 m between the first area and the fault core and is dominated by randomly oriented calcite veins defining a dilational mosaic to chaotic breccia [52] (Figure 6(d)).

By contrast, in the hanging wall, the damage zone is narrower, only concentrated in the 15-20 meters near the fault core (Figure 6(e)), and it is defined by randomly oriented calcite veins defining an incipient crackle breccia (proto-breccia) [52] (Figure 6(f)). Fracture density is lower than within the footwall damage zone.

4.3. Calcite Cements of Veins and Breccias

The damage zones in the footwall and hanging wall not only differ in the type and distribution of fracture systems, as previously described, but also they also show different fracture-filling calcite cements, characterized by distinct petrological and geochemical features.

In the footwall, three calcite cement generations (Cc1 to Cc3) have been observed filling the fracture systems F1 to F3 and two of those calcite cements (Cc1 and Cc2) are also present in the mosaic to chaotic breccia (randomly oriented fractures).

Calcite cement Cc1 exhibits a milky to slightly brownish color in hand sample and consists of up to 1 mm-sized anhedral crystals showing mechanical twinning and featuring a blocky to elongated blocky texture, in which crystals grow syntaxially. This cement shows a dark to light orange cathodoluminescence (Figure 8(a)). Calcite Cc1 precipitated in fractures F1 and in the mosaic to chaotic breccia of the footwall.

[figure omitted; refer to PDF]

Cc2 displays a white to translucent aspect in hand sample. It is formed of anhedral to subhedral calcite crystals, ranging in size from approximately 0.2 to 5 mm, with a blocky to elongated blocky texture. The elongated blocky Cc2 cement grows syntaxially from the fracture walls (Figures 8(a) and 8(b)). Crystals present abundant twin planes and show a nonluminescent to bright orange concentric zonation under cathodoluminescence in which the zoning pattern generally coincides with the crystal growth (Figure 8(b)). Calcite Cc2 precipitated in fractures F2 as well as in fractures F1 and in the mosaic to chaotic breccia of the footwall after Cc1.

Cc3 exhibits a milky appearance in hand sample and is constituted of up to 0.5 mm anhedral blocky, locally elongated crystals. It has mechanical twinning and presents a dull orange luminescence (Figure 8(c)). Cc3 precipitated in fractures F3.

On the other hand, in the hanging wall and in the fault core, only one calcite cement (Cc4) is observed. Cc4 precipitated in the main fault plane and in randomly oriented fractures. This cement has a milky aspect in hand sample and is characterized by up to 1 mm anhedral crystals with a nonluminescent to dark brown luminescence (Figure 8(d)).

4.4. Oxygen and Carbon Isotopes

The δ¹⁸O and δ¹³C isotopic composition of the calcite cements Cc1 to Cc4, adjacent host rocks, and fault rocks is summarized in Table 1 and presented in Figure 9. The calcite cement Cc0 in the interparticle porosity of the grainstone from the Collada Gassó Formation (footwall) has δ¹⁸O values between -7.2 and -6.7‰VPDB and δ¹³C values between -0.5 and +0.7‰VPDB. The micrite matrix of the wackestone from the Santa Fe Formation (hanging wall) shows δ¹⁸O values between -6.2 and -5.8‰VPDB and δ¹³C values between +2.1 and +2.2‰VPDB. The carbonate micritic matrix from the cataclasite yields δ¹⁸O values between -7.1 and -6‰VPDB and δ¹³C values between +2.3 and +3‰VPDB.

Table 1

δ¹⁸O, δ¹³C, and ⁸⁷Sr/⁸⁶Sr isotopic signature of the calcite cements, fault rocks, and host rocks. The ${\Delta }_{47}$, δ₁₈O_fluid, and precipitation temperatures obtained from clumped isotopes are also included. Fw means footwall and Hw means hanging wall.

Sample	Fracture	Cement	δ18O ‰VPDB	δ13C ‰VPDB	87Sr/86Sr	${\Delta }_{47}$	δ18Ofluid ‰VSMOW	$T$ (°C)
Bx1B	F2	Cc1	-8.2	-1.7	0.707707	0.619/0.642	-1.8 to -0.1	42 to 51
Bx 3	F1	Cc1	-7.3	-1.8
Bx36.I	Breccia (Fw)	Cc1	-7.6	-2
Bx36.IV	Breccia (Fw)	Cc1	-7.4	-1.6
Bx36.V	Breccia (Fw)	Cc1	-6.5	-1.5
Bx3.III	F1	Cc2	-8.2	-3.7
Bx1A.I	F2	Cc2	-7	-4.6	0.707699	0.562/0.589	+3.7 to +5.9	64 to 78
Bx1B.II	F2	Cc2	-6.3	-4.6
Bx3.II	F1	Cc2	-7	-3.3
Bx36.II	Breccia (Fw)	Cc2	-7.8	-5.6	0.707695
Bx36.III	Breccia (Fw)	Cc2	-7.2	-6.3
Bx35	F2	Cc2	-5.4	-4	0.707765
Bx1A.II	F3	Cc3	-13	-1.9
Bx2A	F3	Cc3	-12.1	-2.9	0.707698	0.498/0.515	+3.2 to +5.5	105 to 117
Bx2B	F3	Cc3	-11.9	-2.8
Bx4	F3	Cc3	-11.7	-3.2
Bx5	Thrust plane	Cc4	-12.9	+0.4	0.707771	0.53/0.535	+0.7 to +1.9	93 to 96
Bx6	Breccia (Hw)	Cc4	-12.3	+2.4
Bx7.I	Breccia (Hw)	Cc4	-13.1	+0.4	0.707715
Bx7.II	Breccia (Hw)	Cc4	-11.9	+0.9
Bx 8	Breccia (Hw)	Cc4	-12.3	-0.6
Bx66.C	Slip plane	Cc4	-12	+1.6
Bx1.HR	Host rock (Fw)	Cc0	-7.2	-0.5
Bx2A.HR	Host rock (Fw)	Cc0	-6.7	+0.7	0.707606
Bx3.HR	Host rock (Fw)	Cc0	-7	+0.1
Bx7	Host rock (Hw)		-5.8	+2.2
Bx8	Host rock (Hw)		-6.2	+2.1	0.707718
Bx6.I	Fault rock (matrix)		-6.9	+2.9
Bx6.II	Fault rock (matrix)		-7.1	+3
Bx66	Fault rock (matrix)		-6.6	+2.6
Bx67	Fault rock (matrix)		-6	+2.3

[figures omitted; refer to PDF]

The calcite cements Cc1 to Cc4 exhibit a wider range of values (Figure 9). Cc1 has δ¹⁸O values ranging between -8.2 and -6.5‰VPDB and δ¹³C values between -2 and -1.5‰VPDB. Cc2 is characterized by δ¹⁸O values between -8.2 and -5.4‰VPDB and δ¹³C values between -6.3 and -3.3‰VPDB. Cc3 shows δ¹⁸O values between -13 and -11.7‰VPDB and δ¹³C values between -3.2 and -1.9‰VPDB and Cc4 has δ¹⁸O values between -13.1 and -11.9‰VPDB and δ¹³C values between -0.6 and +2.4‰VPDB. Calcite cements precipitated in the footwall (Cc1, Cc2, and Cc3) exhibit negative δ¹³C isotopic values, whilst the calcite cement precipitated within the fault plane and hanging wall (Cc4) displays heavier δ¹³C values (Figure 9(a)). In addition, the δ¹⁸O values also exhibit two different trends within the four calcite cements: Cc1 and Cc2 show δ¹⁸O values similar to those of Cc0 and host rocks (Figures 9(a) and 9(b)), whereas Cc3 and Cc4 have lighter δ¹⁸O values with respect to the host carbonate values (Figures 9(a) and 9(b)).

4.5. Clumped Isotopes

Temperatures in °C and δ¹⁸O_fluid in ‰VSMOW of the vein-forming fluids are calculated from the measured ${\Delta }_{47}$ using the formula of [54, 55], respectively (Table 1 and Figure 10). The ${\Delta }_{47}$ value for Cc1 ranges between 0.619 and 0.642, which translates into temperatures of 42–51°C and δ¹⁸O_fluid of -1.8 to -0.1‰VSMOW. For Cc2, ${\Delta }_{47}$ is between 0.562 and 0.589, implying temperatures between 64 and 78°C and δ¹⁸O_fluid between +3.7 and +5.9‰VSMOW. For Cc3, ${\Delta }_{47}$ varies between 0.498 and 0.515, which translates to temperatures between 105 and 117°C and δ¹⁸O_fluid between +3.2 and +5.5‰VSMOW. Finally, ${\Delta }_{47}$ values of Cc4, between 0.53 and 0.535, imply temperatures of 93 to 96°C and δ¹⁸O_fluid between +0.7 and +1.9‰VSMOW. In the footwall, the calculated temperatures progressively increase from Cc1 to Cc3 and the δ¹⁸O_fluid is lighter in Cc1, whilst Cc2 and Cc3 have a similar value.

[figure omitted; refer to PDF]

Calcite cement Cc2 is found in fractures F1 and F2 and within the mosaic to chaotic breccia from the footwall. This cement has similar ⁸⁷Sr/⁸⁶Sr ratios and δ¹⁸O values to Cc1 but precipitated at higher temperatures (between 64 and 78°C) and from heavier δ¹⁸O_fluid (between +3.7 and +5.9‰VSMOW). These higher temperatures and the δ¹⁸O_fluid imply hot formation waters. However, the low δ¹³C values (Figure 9), the diluted Mg, Sr, and REY concentrations (Figure 12), and the calculated Mg/Ca and Sr/Ca molar ratios of Cc2 reveal the involvement of meteoric fluids with increasing influence of organic-derived carbon [85–89] (Figure 12(b)). The CL zonation of Cc2, alternating between non- and bright luminescent zones, the presence of positive and negative Ce anomalies (Figures 12(b) and 12(c)), and the oscillations in the Fe and Mn contents (Figure 12(a)) indicate oxidizing-reducing fluctuation, typical of the meteoric environment [6, 90–93]. These facts indicate that Cc2 precipitated from formation fluids that likely evolved from heated meteoric waters during the development of the second fracturing event (F2). The evolution of these fluids probably involved water-rock interactions during progressive increase in burial depths and temperatures linked to thrust sheet emplacement [94–96].

Calcite cement Cc3, occluding fractures F3, shows an ⁸⁷Sr/⁸⁶Sr ratio similar to cements Cc1 and Cc2 (Figure 11), δ¹³C within the same range of values of Cc2 (Figure 9(a)), and a lighter δ¹⁸O than the previous cement generations and the adjacent host carbonates (Figures 9(a) and 9(b)). Such depletion is related to hotter precipitation conditions (between 105 and 117°C). On the other hand, the δ¹⁸O_fluid (between +3.2 and +5.2‰VSMOW), the high Sr, Mn, and Fe contents (Figure 12(a)), and the calculated Mg/Ca and Sr/Ca molar ratios for the vein-forming fluids are within the range of formation waters [57, 87, 95]. The homogeneous bright-orange luminescence points to complete precipitation of Cc3 during a single precipitation event [97], and the presence of a negative Ce anomaly indicates oxic precipitation conditions [84] (Figure 12(b)).

In conclusion, the progressive depletion in δ¹⁸O values from Cc1 to Cc3, as well as the continuous increase in precipitation temperatures and enrichment in δ¹⁸O_fluid and in the elemental composition, evidences a progressive change in the fluid regime during ongoing deformation. This change may result from a continuous shift in the fluid composition, due to the evolution of the meteoric waters, or from a progressive change in the fluid origin, from percolation of diluted meteoric waters to the upflow of hotter formation fluids. The evolution of the fluid system is also attested by the disequilibrium between Cc3 and its adjacent host rocks (Figure 9(b)) and may be indicative of a continuous opening of the fluid system, which is in turn attributed to the increase in fracture density and superposition of different fracturing events [4, 98–102]. Previous contributions reported the relationships between folding, fracturing, and fluid migration in the eastern part of the Sant Corneli-Bóixols anticline (around the Coll de Nargó area, Figure 1(b)) [37]. During the latest stages of deformation in the frontal part of the anticline, these authors also reported a change in the fluid origin and composition, that is, from percolation of meteoric waters, circulating at approximately 40°C, to migration of hotter formation fluids, at temperatures around 90°C [103]. Consequently, the comparison between both studies suggests a common fluid behavior during the latest-folding to postfolding stages of deformation in the whole anticline.

5.3.2. Fluid System in the Hanging Wall

Calcite cement Cc4 precipitated within the main fault plane, within the fault core, and within the randomly oriented fractures (crackle proto-breccia) located in the hanging wall. The similarity between δ¹³C values in Cc4 and its host carbonates, together with the high Mg contents and the REY pattern showing a well-defined negative Ce anomaly and a positive Y anomaly, similar to that of the host rocks, indicates buffering of the precipitating fluid by the hanging wall carbonates (Figures 9 and 12). This buffered composition of the fluids is also recorded in the δ¹⁸O_fluid obtained from clumped isotopes and yielding values between +0.7 and +1.9‰VSMOW. However, the depletion in δ¹⁸O values in Cc4 in relation to the adjacent rocks suggests that the system was opened to advection of the vein-forming fluid (Figures 9(a) and 9(b)). The Mg/Ca and Sr/Ca molar ratios calculated for the fluid that precipitated Cc4 reveal the presence of formation waters, which precipitated at temperatures between 93 and 96°C (according to clumped isotope thermometry). The ⁸⁷Sr/⁸⁶Sr ratio of Cc4, similar to that of the hanging wall host rock (0.707718) in the crackle proto-breccia (0.707715), but more radiogenic in the fault plane (0.707771) (Figure 11), reveals a channelized ascending fluid migrating through the thrust plane towards the hanging wall, where it progressively increased the fluid-rock interaction. Cretaceous evaporated seawater is the most probable source for these formation fluids, if we take into account that (i) Cretaceous carbonates are broadly present around and underlying the studied thrust, (ii) the ⁸⁷Sr/⁸⁶Sr ratios of Cc4 are consistent with Cretaceous seawater and the δ¹⁸O_fluid is slightly more enriched with respect to marine values (Figure 10), and (iii) during the Late Cretaceous, there was a transition from marine to lagoonal and thus more restricted and evaporative conditions [104]. Upward migration of Cretaceous connate seawater was also reported during the postfolding stages of evolution in the eastern part of the Sant Corneli-Bóixols anticline [37].

5.4. Thrust Zone Evolution

The successive deformation stages, associated with different fracture systems, record an episodic evolution of the studied thrust zone. We interpret this evolution to result from an upward propagation of the fault tip leading to distributed deformation within the process zone before propagation of the fault [71, 89, 105, 106].

During initial fault growth, deformation at the fault tip, i.e., within the process zone, was dominated by concentration of dilatant (extensional) fracturing, giving rise to fractures F1 and the mosaic to chaotic breccia from the footwall (Figure 13(a)). The fluid associated with this initial deformation stage responsible for precipitation of calcite cement Cc1 was a host-rock buffered meteoric fluid. Meteoric fluids probably infiltrated the system on high structural reliefs that were likely elevated during growth of the Sant Corneli-Bóixols anticline (Figure 13(a)). These fluids warmed up to ~50°C at depth by a normal geothermal gradient and then migrated through diffuse deformation developed in the process zone before the growth and propagation of the thrust slip plane (Figure 13(a)) [71, 89, 105]. As deformation continued, new dilatant fractures F2 developed, and F1 fractures reopened and randomly oriented fractures from the breccia continued forming. This synchronicity is evidenced by the presence of the same calcite cement (Cc2) in the aforementioned fractures. The second fluid flow event, associated with the second deformation stage, was characterized by the progressive evolution of meteoric fluids due to water-rock interactions at increasing depths and temperatures (Figure 13(b)). During these two initial stages of deformation, the remote stress fields varied locally in the process zone, allowing for the formation of steeply dipping fractures (F1 and F2) (Figures 13(a) and 13(b)).

Finally, the thrust slip plane propagated through the process zone. Frictional processes occurred along the thrust surface as well as the progressive weakening of the fault zone, which led to the formation of the foliated cataclasite, characterized by S-C shear fabric, pressure solution seams, calcite mineralization, and carbonate fine-grained matrix. This micritic matrix is petrographically similar to that of the hanging wall protolith (Figures 4(c) and 7(b) and 7(c)) and displays comparable isotopic composition (Figure 9). These observations indicate that the cataclasite matrix derived from the hanging wall carbonates. During this period of thrust sheet emplacement, fractures F3 developed. We interpret that the orientation of these fractures represents the far-field stress regime unaltered by faulting (Figure 13(c)). The calcite cement Cc3, precipitated in fractures F3, reflects the presence of formation fluids and the progressive increase in precipitation temperatures, up to ~117°C, which could indicate progressive burial during fault growth from calcite cement Cc1 to Cc3 [107].

Later, the thrust acted as a preferential pathway for fluids that only infiltrated in the hanging wall block (Figure 13(c)) as indicated by the presence of the same calcite cement (Cc4) along the thrust slip plane, in the fault core and within the crackle proto-breccia from the hanging wall. Thus, this observation corroborates the compartmentalization of the fluid system. As proposed in other settings where faults compartmentalize fluid flow between the footwall and hanging wall, the presence of veins without a preferred structural orientation (i.e., randomly oriented veins) is indicative of overpressured fluid migrating during deformation. Pressured fluids were likely expelled from Cretaceous carbonates due to sediment compaction during thrust sheet emplacement [77] (Figure 13(c)).

The interpreted evolution of the studied fault zone is similar to models already proposed in other extensional settings during upward propagation of normal faults deforming carbonate rocks [71, 80, 89, 105, 108, 109]. Therefore, the comparison between these studies allows us to provide insights into deformation processes and mechanisms related to propagation of faults in different geological settings. Interestingly, all these contributions reported an initial stage of fault nucleation related to the development of a process zone (in the fault tip) before individualization of the fault plane. This period was characterized by the formation of hydraulic extension fracturing and brecciation and the presence of a fluid in chemical equilibrium with the host rocks. Finally, the progressive deformation resulted in the propagation of the fault (slip plane) through the process zone and the circulation of external fluids along the main slip surfaces (i.e., longitudinal fluid migration). These studies also reported an evolution of the fracture connectivity, a continuous opening of the fluid regime, and a progressive change in the fluid composition and origin during the evolution of the fault zone [80, 89, 105].

5.5. Structural Permeability: Thrust Sealing Capability and Fluid Pressuring

The petrology and geochemistry of the studied calcite cements indicate that the thrust separates two compartments with different deformation and fluid flow patterns. It is therefore likely that the thrust acted as a transversal barrier for fluids migrating between the footwall and hanging wall. The barrier role of the thrust is attributed (i) to the poor permeability of the foliated cataclasite and its micritic carbonate matrix with concentration of clay minerals associated with pressure-solution surfaces and (ii) to the nonporous character of the hanging wall carbonates (i.e., wackestones). In the latter case, although it has not been measured, the permeability is likely lower in the hanging wall wackestones with respect to the footwall grainstones. Therefore, the thrust acted as a transversal barrier and a longitudinal drain from fluids [15].

Additionally, considering the footwall damage zone as a high fractured compartment, its structural permeability is qualitatively evaluated. In this compartment, systematically oriented fractures (F1 to F3) were cemented by three generations of calcite cement (Cc1 to Cc3) and the randomly oriented fractures were cemented by Cc1 and Cc2. This evidences that the structural permeability was transient and that successive episodes of fracturing added new pathways to fluids that were rapidly occluded by calcite precipitation and sealing [18, 70]. According to theoretical models, the presence of existing fractures appropriately oriented for reactivation prevents the formation of new fracturing events [10]. Therefore, a new fracturing episode developed when the previous fracture system regained cohesive strength by calcite precipitation [10]. The fact that the calcite cements Cc1 to Cc3 vary geochemically through time demonstrate that the fluid pathways, path lengths (associated with different fracture sizes), and the extent of fluid-rock interaction changed during the thrust nucleation (process zone development) and growth (thrust slip plane propagation), which has been corroborated by the different fluid compositions and temperatures obtained from clumped isotopes. In the same location, a previous study [42] documented the distribution of fractures developed in the footwall and discussed the permeability properties of the footwall damage zone in terms of porosity creation and occluding processes based on structural analysis and crosscutting relationships between veins. These authors also concluded that the footwall damage zone had to be considered as a low-quality reservoir analogue because any porosity developed during fracturing was rapidly sealed by cementation processes [42].

The comparison between the above described scenario and the model proposed in the Oman Mountains [26] highlights that fluid behavior during thrusting greatly varies in different geological settings. In particular, the analysis of calcite cements precipitated across the fault zone in both studies reveals that the thrust faults acted as transversal barriers to fluid migration but were preferential paths for channelized fluids along the thrust planes (longitudinal drains). In the studied outcrop, fluids in the footwall did not reach the hanging wall because of the impermeable fault core rock (foliated cataclasite) and a different fluid migrated upwards along the thrust plane and towards the hanging wall. By contrast, in the outcrop of Oman [26], fluids migrated along the thrust fault and towards the footwall, the only block in which syntectonic fractures developed. Our contribution and that of [26] highlight the importance of a good understanding of the timing of fracturing and the associated fluids in order to characterize the hydraulic behavior of a fault zone.

6. Conclusions

The studied thrust displaces a subvertical Upper Cretaceous carbonate succession located in the southern limb of the Sant Corneli-Bóixols anticline, in the Southern Pyrenees. Structural data together with petrological and geochemical analyses of synkinematic calcite veins developed within the fault zone constrain the evolution of the structural permeability and the thrust sealing capability and provide insights into the paleofluid system at the time of thrusting:

(i)

The heterogeneous distribution of deformation mechanisms across the thrust zone resulted in strong host rock dissolution within the fault core and calcite precipitation in veins within the damage zone

(ii)

The evolution of the thrust zone resulted from the upward propagation of the fault tip, producing distributed deformation in the process zone and subsequent propagation of the fault (slip plane), similar to the models already proposed for normal faults deforming carbonate rocks. During this evolution, the different deformation and fluid flow histories in the footwall and hanging wall indicate that the fault core acted as a transversal barrier due to its low permeability associated with the foliated cataclasite arrangement and matrix cementation. In the footwall, three systematically oriented fracture systems (F1 to F3) and randomly oriented fractures developed during progressive deformation. The structural permeability in the footwall damage zone was transient, and successive episodes of fracturing added new pathways to fluids but were rapidly occluded by calcite precipitation. By contrast, in the hanging wall, only randomly oriented fractures are present

(iii)

Three calcite cements (Cc1 to Cc3) precipitated within the fracture systems developed in the footwall. The petrology and geochemistry of Cc1 to Cc3 reveal a progressive change in the fluid origin and/or composition from percolation of meteoric fluids at temperatures between 42 and 51°C to upward migration of hotter formation waters at temperatures between 105 and 117°C. On the other hand, only one calcite cement (Cc4) precipitated in the hanging wall and in the fault core from hot formation waters at 95°C. These formation fluids were probably expelled from Cretaceous carbonates and migrated along the thrust plane towards the hanging wall during deformation. The comparison between the present study and previous data reported in the eastern part of the Sant Corneli-Bóixols anticline reveals the same fluid regimen and evolution of the fluid system during the postfolding stages of deformation in the whole anticline

Acknowledgments

This research was carried out within the framework of the Dirección General de Investigación Científica y Técnica Spanish Project (PGC2018-093903-B-C22) (Ministerio de Ciencia, Innovación y Universidades/Agencia Estatal de Investigación/Fondo Europeo de Desarrollo Regional, Unión Europea) and the Grup Consolidat de Recerca “Geologia Sedimentària” (2017-SGR-824). The PhD research of DM-L is supported by the FPI2016 (BES-2016-077214) Spanish program from MINECO.

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