Spatio-temporal variation of fluid flow behavior along a fold: The Boixols-Sant Corneli anticline (Southern Pyrenees) from U-Pb dating and structural, petrographic and geochemical constraints

This study integrates field structural data, petrographic and geochemical (delta 18O, delta 13C, Delta 47, 87Sr/86Sr, and elemental composition) analyses and U-Pb dating of calcite veins cutting the B ' oixols-Sant Corneli anticline (Southern Pyrenees) in order to date and to investigate the spatio-temporal relationships between fluid flow and fold evolution. This E-W trending anticline grew from Late Cretaceous to Paleocene at the front of the B ' oixols thrust sheet deforming pre-growth and growth sedimentary sequences. U-Pb dating reveals Late Cretaceous to late Miocene deformation ages, which agree with the age of growth strata deposition and the sequence of deformation interpreted from field and microstructural data. Dates coeval (71.2 +/- 6.4 to 56.9 +/- 1.4 Ma) and postdating (55.5 +/- 1.2 to 27.4 +/- 0.9 Ma) Upper Cretaceous to Paleocene growth strata are interpreted to record: (i) the growth of the B ' oixols-Sant Corneli anticline during the B ' oixols thrust emplacement, and (ii) the tightening of the anticline during the southern tectonic transport of the South-Central Pyrenean Unit. Other ages (20.8 +/- 1.2 to 9.0 +/- 4.6 Ma) postdate the folding event and have been associated with the collapse of the B ' oixols-Sant Corneli anticline. The geochemistry of calcite veins indicates that the fluid flow behavior varied across the B ' oixols-Sant Corneli anticline through its growth, showing a compartmentalized fluid system. In the hinge of the anticline and in the upper Santonian to middle Campanian syn-orogenic sequence along the footwall of the B ' oixols thrust, the similar petrographic and geochemical features between all calcite cements and host rocks point towards a locally-derived or well-equilibrated fluid system. Contrarily, along large faults such as the B ' oixols thrust, and in the anticline limbs, the geochemistry of vein cements indicates a different scenario. Cements in large faults yielded the lightest delta 18O values, from -8 to -14 %0VPDB, and variable enrichment in delta 13C, 87Sr/86Sr, elemental composition and delta 18Ofluid. This is interpreted as the migration of fluids, through fault zones, that evolved from distinct fluid origins. Cements in the fold limbs exhibit delta 18O and delta 13C between -8 and -6 %0 VPDB and between -10 and + 2 %0VPDB, respectively, the lowest Sr contents and the lowest precipitation temperatures, suggesting that the anticline limbs recorded the infiltration and evolution of meteoric waters. The paleohydrological system in the B ' oixols-Sant Corneli anticline was restricted to the B ' oixols thrust sheet. The Upper Triassic evaporitic basal detachment likely acted as a lower fluid barrier, preventing the input of fluids from deeper parts of the Pyrenean crustal thrust system. This study provides a well-constrained absolute timing of fracturing and fluid flow during basin inversion and folding evolution and highlights the suitability of U-Pb geochronology to refine the age of fractures and veins and their sequential evolution in orogenic belts.

Automated summary

This study integrates field structural data, petrographic and geochemical analyses and U-Pb dating of calcite veins to investigate the relationships between fluid flow and fold evolution. The results show that the deformation of the anticline is consistent with the age of growth strata deposition. The geochemistry of calcite veins indicates a compartmentalized fluid system and different fluid origins along large faults and in the anticline limbs.