Fluid flow evolution in the Alpine-related basement and sedimentary cover structures in the Southern Pyrenees: an integrated structural, petrographic and geochemical approach

Abstract

This PhD thesis examines the fracture-fluid interactions across different compressional structures affecting both Paleozoic basement and Mesozoic-Cenozoic cover lithologies in the Southern Pyrenees in order to investigate the relationships between fluid flow and deformation. The studied structures include the Estamariu thrust in the Pyrenean Axial Zone and the Sant Corneli-Bóixols anticline along the front of the Bóixols thrust sheet in the Southern-Central Pyrenees. The proposed methodology integrates field data with U-Pb dating and petrographic and geochemical (δ18O, δ13C, 87Sr/86Sr, clumped isotopes and elemental composition) analyses of vein cements, fault rocks and host rocks present in the study area. The Estamariu thrust resulted from a multistage late Paleozoic to Neogene tectonic evolution. Despite this thrust is known to be Variscan in origin, in the study area it places a Devonian pre-Variscan unit on top of a Stephano-Permian late to post-Variscan sequence, indicating that the structures within this thrust zone have to be post-Variscan. The contractional structures found at the contact between Devonian and Stephano-Permian units have been attributed to the Alpine reactivation of the Estamariu thrust. This Alpine-related deformation is consistent with the transposition of the Variscan regional foliation within the main thrust zone and with the formation of a subsidiary thrust zone in the footwall of the Estamariu thrust. Other structures found in the study area, such as extensional fractures, shear bands and normal faults, postdate the reverse structures and have been attributed to the Neogene extension. In the Sant Corneli-Bóixols anticline, the folded units involve Jurassic-mid Upper Cretaceous pre-compression and latest Cretaceous-Paleocene syn-orogenic sedimentary successions detached on Upper Triassic evaporites. Contrary to the Estamariu thrust, in the Sant Corneli- Bóixols anticline this PhD thesis provides the absolute age of deformation reporting 23 U- Pb dates measured in different sets of fracture-filling calcite cements. These U-Pb dates reveal Late Cretaceous to late Miocene age variations, which are coeval with growth strata deposition and Bóixols thrust sheet emplacement (dates from 71.2 ± 6.4 Ma to 56.9 ± 1.4 Ma), tectonic transport of the Bóixols thrust sheet above the southern Pyrenean basal thrust

(dates from 55.5 ± 1.2 Ma to 27.4 ± 0.9 Ma) and post-orogenic exhumation of the Sant Corneli-Bóixols anticline (dates younger than 20.8 ± 1.2 Ma). Throughout this long-lived deformation history, the geochemical data of the successive calcite veins allow to analyze the relationships between fluid flow and deformation in the two studied domains. In the Estamariu thrust, the high 87Sr/86Sr ratios of the different calcite cements indicate the interaction between the vein-forming fluids and radiogenic Paleozoic basement rocks. The geochemical evolution from the earliest to the latest calcite cements also evidence a progressive change in the fluid regime and composition during successive compressional and extensional tectonic events. The progressive increase in precipitation temperatures, from 50 ºC to around 210 ºC, and enrichment in δ18Ofluid, from -6.4 to +12 ‰SMOW, in cements attributed to the Alpine compression to cements from the Neogene extension, is probably linked to a higher extent of fluid-rock interaction with basement rocks. By contrast, during the latest stages of the Neogene extension, the geochemistry of the youngest calcite cements evidence the percolation of cold meteoric fluids that indicates a more significant change in the fluid regime, from upward to downward fluid migration. In the Sant Corneli-Bóixols anticline, the fluid origin and the extent of fluid-rock interaction varied in the several structural positions of the fold and according to the age and nature of their stratigraphy and the involved fracture networks. This evidences a compartmentalization of the fluid system. In the core of the anticline and in the lowest part of the synorogenic sequence from the footwall of the Bóixols thrust, the similar petrographic and geochemical features between successive calcite cements and host rocks indicate a closed fluid system, leading to high extent of fluid-rock interaction. This host-rock buffered fluid was likely derived from the surrounding Lower Cretaceous and Upper Cretaceous marine carbonates, respectively. Contrarily, along large faults, such as the Bóixols thrust, which affect the entire anticline and in the fold limbs, the fluid system was open. Large faults registered the upward migration of fluids in thermal and geochemical disequilibrium with surrounding host rocks, as attested by the light δ18O values of their associated vein cements, down to -14 ‰VPDB, and the high temperature of precipitation, up to 120 ºC. The fold limbs registered the infiltration of meteoric waters, corroborated by the low Sr contents and by the δ18O and δ13C

values of the vein cements, from -8 to -6 ‰VPDB and down to -10 ‰VPDB, respectively, which are typical values of meteoric carbonates. In the fault zone of the Bóixols thrust, successive fracture systems and related calcite cements highlight an episodic evolution of the thrust zone. The presence of early extensional fractures and a chaotic breccia is consistent with the formation of dilatant fracturing within a 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 Bóixols thrust. Similarly, the different structural and fluid flow histories in the footwall and hanging wall of the Bóixols thrust indicate a compartmentalization of the thrust zone. Clumped isotopes applied to vein cements from the footwall evidence a progressive increase in precipitation temperatures from 50 ºC to 117 ºC, approximately, and an enrichment in δ18Ofluid, from -1.8 to +5.5 ‰SMOW. This has been interpreted as a change in the fluid source from meteoric fluids to evolved meteoric fluids due to water-rock interaction at increasing depths and temperatures. Contrary to the footwall, clumped isotopes applied to vein cement from the hanging wall, which is the same cement found in the fault core, revealed temperatures around 95 ºC and δ18Ofluid up to +1.9 ‰SMOW. This has been interpreted as the migration of formation waters from the fault core and towards the hanging wall. Therefore, the Bóixols thrust likely acted as a transverse barrier, dividing the thrust zone into two separate fluid compartments, and a longitudinal drain for migration of fluids. Altogether, the maximum temperatures and δ18Ofluid, up to 120 ºC and +5.5 ‰VSMOW, obtained in the Sant Corneli-Bóixols anticline, implying 3-4 km depth and presence of formation waters, respectively, together with the 87Sr/86Sr ratios of the vein cements, within the range of values documented in the sedimentary cover, discard the transfer of fluids between the basement and the sedimentary cover in the Southern-Central Pyrenees. This indicates that the paleohydrological system in the Sant Corneli-Bóixols anticline was restricted to the Bóixols thrust sheet above the Upper Triassic detachment level. This evaporitic detachment likely acted as a lower fluid barrier, preventing the input of fluids from deeper parts of the belt, as interpreted in other areas of the Pyrenees and in other detached thrust systems as the Central Appalachians and Sierra Madre Oriental.

Finally, comparing the results obtained in both studied domains with those reported in previous contributions, this study assesses the fluid flow and deformation relationships at regional scale. On the one hand, this thesis highlights a common fluid flow behavior along strike in the central-eastern part of the southern Pyrenees, where major faults acted as transfer zones for migration of fluid in thermal and geochemical disequilibrium with adjacent host rocks, whereas background fracturing recorded the presence of fluids that highly interacted with surrounding host rocks. Additionally, our data indicates that regardless of the fluid origin and the tectonic context, the fluids that have interacted with basement rocks have a higher 87Sr/86Sr ratio (> 0.710) than those that have circulated through the sedimentary cover (< 0.710). Lastly, extensional deformation structures in the eastern Pyrenees have acted as conduits for hydrothermal fluid migration in Neogene times similarly as in the northern part of the Catalan Coastal Ranges. These fluids likely interacted with basement rocks before ascending through fault zones and related fractures.

Aquesta tesi estudia les interaccions entre fracturació i fluids a diferents estructures (plecs i zones de falles) que afecten els materials del sòcol Paleozoic y de la cobertora Mesozoica- Cenozoica a la Zona Sud-Pirinenca. L’objectiu de la tesi és reconèixer les relacions entre migració de fluids i deformació utilitzant com a anàlegs l’encavalcament d’Estamariu a la Zona Axial i l’anticlinal de Sant Corneli-Bóixols a la part frontal del mantell de Bóixols, Unitat Sud-Pirinenca Central. A cada estructura s’integren dades de camp amb datacions U- Pb i diversos anàlisis petrogràfics i geoquímics (δ18O, δ13C, 87Sr/86Sr, cumpled isòtops i composició elemental) de ciments de calcita, roques de falla i roques encaixants. L'encavalcament d'Estamariu és el resultat d'una evolució tectònica que comprèn des del Paleozoic superior al Neogen, mentre que a l’anticlinal de Sant Corneli-Bóixols, les edats U- Pb revelen deformació des del Cretaci superior fins al Miocè superior. Durant aquesta història de deformació, les dades geoquímiques de les successives venes de calcita permeten analitzar les relacions entre la migració de fluids i la deformació a les dues zones d’estudi. A l’encavalcament d’Estamariu, l'evolució geoquímica dels ciments de calcita evidencia un canvi progressiu en el règim i la composició dels fluids durant successius episodis tectònics de compressió i extensió, de migració ascendent a percolació de fluids. A l’anticlinal de Sant Corneli-Bóixols, l’origen dels fluids i el grau d’interacció fluid-roca van variar en les diverses posicions estructurals del plec i segons l’edat i la naturalesa de la seva estratigrafia i les fractures relacionades. La qual cosa evidencia una compartimentació del sistema de fluids. Al nucli de l’anticlinal i a la part més baixa de la seqüència sinorogènica del bloc inferior del l’encavalcament de Bóixols, les característiques petrològiques i geoquímiques similars entre successius ciments de calcita i les seves roques encaixants indiquen un sistema tancat de fluids i un grau alt d’interacció fluid-roca. Contràriament, al llarg de grans falles, com l’encavalcament de Bóixols, que afecten tot l’anticlinal i als flancs del plec, el sistema de fluids estava obert, registrant la migració ascendent de fluids en desequilibri tèrmic i geoquímic amb les roques encaixants i la infiltració d’aigües meteòriques, respectivament. Tot plegat, aquests resultats indiquen que al anticlinal de Sant Corneli-Bóixols el sistema paleohidrològic estava restringit al mantell de Bóixols per sobre del nivell de desenganxament del Triàsic superior. Aquest nivell evaporític probablement va actuar com un segell evitant l’entrada de fluids de parts més profundes de la serralada, tal com s’interpreta en altres zones del Pirineu i en altres sistemes d’encavalcaments com els Apalatxes centrals i la Sierra Madre Oriental. Finalment, comparant els resultats obtinguts en els dos dominis estudiats amb els resultats reportats en altres contribucions prèvies, aquesta tesi analitza la relació entre migració de fluids i deformació a escala regional. D’una banda, aquesta tesi posa de manifest un comportament comú del flux de fluids al llarg de la part central-oriental de la Zona Sud-

Pirinenca, on les falles principals van actuar com a zones de transferència per a la migració de fluids en desequilibri tèrmic i geoquímic amb roques encaixants, mentre que les fractures de escala menor van registrar la presència de fluids equilibrats amb els seus encaixants. A més, les nostres dades també indiquen que, independentment de l’origen del fluid i del context tectònic, els fluids que han interaccionat amb les roques del sòcol tenen una relació de 87Sr/86Sr (> 0,710) superior als fluids que han circulat per la cobertora sedimentària (<0,710). D’altra banda, les estructures de deformació extensiva, tant al Pirineu oriental com a la part nord-est de la Cadena Costera Catalana, han actuat com a conductes per a la migració de fluids hidrotermals al Neogen i en l’actualitat. Aquests fluids probablement van interactuar amb les roques del sòcol abans d’ascendir a través de zones de falla i les fractures relacionades.