Fracturing and fluid migration during Palaeogene compression and Neogene extension in the Catalan Coastal Ranges, Spain

The Catalan Coastal Ranges (NE part of the Eastern Iberian Margin) correspo nd to a system of grabens formed at the north-western margin of the Valenci a Trough. Extensional activity in the Catalan Coastal Ranges occurred at le ast from early to late Miocene and reactivated earlier transpressive faults related to the Palaeogene compression. In the central part of the Catalan Coastal Ranges, tectonic micro and macrostructures (faults, joints, styloli tes) are well developed in the Mesozoic (mainly Cretaceous) limestones as w ell as in the Miocene graben fill deposits. In these rocks, seven generatio ns of fractures, which formed during different tectonic phases, have been d istinguished. Type 1-4 fractures affect only the Cretaceous limestones, typ e 5 and 6 fractures the Cretaceous and Miocene deposits, and type 7 fractur es only the Miocene sediments. The fractures are filled with calcites, and locally with an internal sediment or a dolomite cement. The Cretaceous host -rock has an average delta(18)O value of -4.3 parts per thousand PDB (Peede e Belemnite), an average delta(13)C value of +0.6 parts per thousand PDB, a Sr-87/Sr-86 ratio of 0.70741, up to 5630 p.p.m. of Mg2+, up to 2615 p.p.m. of Sr2+, and up to 1560 p.p.m. of Fe2+. Type 2 fractures are related to Pa laeogene compression. The calcite cement filling this type of fracture has an average delta(18)O value of -8.2 parts per thousand PDB, an average delt a(13)C value of -0.6 parts per thousand PDB, a 87Sr/86Sr ratio of 0.70714, up to 4560 p.p.m. of Mg2+, up to 3275 p.p.m. of Sr2+, and up to 3540 p.p.m. of Fe2+. These results indicate a fluid characterized by a high rock-fluid interaction approaching a closed system equilibrium. Type 5 fractures are related to the syn-rift stage. The calcite cement fill ing this type of fracture has an average delta(18)O value of -6.9 parts per thousand PDB, an average delta(13)C value of -4.3 parts per thousand PDB, a Sr-87/Sr-86 ratio of 0.70787, up to 5375 p.p.m. of Mg2+, up to 1750 p.p.m . of Sr2+ and up to 2855 p.p.m. of Fe2+. These results indicate a fluid cha racterized by a low rock-fluid interaction and an open hydrogeological syst em. The cements filling the compressional fractures are characterized by un dulose extinction, subgrain formation and deformed mechanical twin planes i ndicating formation under stressed conditions. In contrast, cements filling extensional fractures are characterized by translucent crystals with unifo rm extinction, indicating free growth not subjected to stress.