Fluid Flow during Paleogene Compression in the Linking Zone Fold and Thrust Belt (Northeast Spain)

Abstract

The Linking Zone (northeast Spain) is a fold and thrust belt that, together with the Catalan Coastal Range and the Iberian Chain, constitutes the southern flank of the Ebro Basin. Compressive deformation occurred during the late Eocene–early Miocene, affecting a Hercynian basement, a Mesozoic cover, and syntectonic Tertiary conglomerates. The integration of the structural study of the fractures with the petrologic and geochemical study of the cements filling the fractures reveals two episodes of fluid circulation during compressive deformation. The first episode occurred during the early stages of the fold and thrust belt development and includes two different fluids responsible for precipitation of two types of calcite cement. The first type of calcite cement is characterized by 18O ranging from 9 to 3.8 Peedee belemnite (PDB), 13C ranging from 5.6 to 2.6 PDB, a 87Sr/86Sr ratio of 0.70768, between 440 and 3565 ppm of Mg, between 275 and 720 ppm of Fe, between 180 and 410 ppm of Mn, and Sr content always below 275 ppm (below the detection limit). The fluids precipitating these cements were meteoric fluids that were derived directly from the surface and circulating through the undeformed and highly porous Tertiary syntectonic conglomerates in an open paleohydrogeological system. The second type of calcite cement is characterized by 18O ranging from 10.1 to 5.4 PDB, 13C ranging from 7.5 to 3.5 PDB, a 87Sr/86Sr ratio ranging between 0.70759 and 0.70778, between 350 and 5150 ppm of Mg, Fe content as much as 11,490 ppm, Mn content below 180 ppm (below the detection limit), and Sr content as much as 795 ppm. The fluid that was precipitating these cements was originally a meteoric fluid that evolved to a formation water composition probably because of a high interaction with Mesozoic rocks. The Triassic shales and evaporites of the detachment levels served as barriers, forcing the fluids to move laterally above them. The second episode of fluid circulation occurred during the last stages of fold and thrust belt development, after the major uplift of the interior belt and formation of the relief. Two different fluids are recognized during this episode as being responsible for precipitation of two types of calcite cements. The first type of calcite cement is characterized by 18O ranging from 3.4 to 3.3 PDB, 13C ranging from 4.0 to 3.8 PDB, 87Sr/86Sr ratio of 0.70845, between 475 and 4200 ppm of Mg, Fe always below 275 ppm (below the detection limit), between 180 and 655 ppm of Mn, and between 280 and 850 ppm of Sr. The second type of calcite cement is characterized by 18O ranging from 12.7 to 8.1 PDB, 13C ranging from 6.4 to 3.9 PDB, a 87Sr/86Sr ratio ranging between 0.70792 and 0.70795, between 260 and 5240 ppm of Mg, Fe content as much as 1155 ppm, Mn content as much as 925 ppm, and Sr content as much as 925 ppm. These two groups of cements that precipitated from meteoric waters evolved to a formation water composition probably because of interaction with Mesozoic and Paleozoic rocks, being the low 18O of the second probably because of high temperatures of precipitation. A late episode of meteoric fluid circulation, occurring during the Tertiary extension, is recorded by the calcite cements filling normal faults, which are characterized by 18O ranging from 9.2 to 7.6 PDB, 13C ranging from 2.6 to +1.1 PDB, 87Sr/86Sr ratio of 0.70788, between 870 and 2695 ppm of Mg, between 470 and 1420 ppm of Mn, and Fe and Sr always below the detection limit, which is 275 ppm for both. The study of the fluid-flow evolution during the Linking Zone fold and thrust belt development shows that the main factors controlling the fluid-flow dynamics are the existence of a high relief, whereas the host rocks and the type of structure are not important factors.