Evidence of phyllosilicate alteration processes and clay mineral neoformation promoted by hydrothermal fluids in the Padul Fault area (Betic Cordillera, SE Spain)

Geochemical changes and authigenic clays were detected in the fault rocks of the Padul Fault. An enrichment from the protolith to the ultracataclasites in Si, Al, Fe, Ca, Ti and also Zn but an impoverishment in Mg was detected. Although the protolith (dolostones) and fault rocks are mostly composed of carbonates, the fault rocks are also characterized by the presence of fine-grained clays in the matrix (mainly chlorite and Mg-rich biotite, but also smectite and punctually talc). Neither chlorite nor talc were detected in the protolith. The application of chlorite thermometry gives temperatures around 140–220 °C for chlorites lamellae located on the fault plane. These chlorites are compositionally homogeneous, whereas the chlorites detected in the cataclasites are more variable, although both of them are Mg-rich and have almost no Fe. As well, chlorites from the cataclasites usually include Zn in their composition and, as observed at nanoscale, they are genetically related to biotites, which come from the protolith. Talc grains (< 1 μm) are always found between dolomite and calcite. These observations point to clay mineral reactions in the fault rocks as the chlorite and talc precipitation promoted by circulation of fault-controlled hydrothermal fluids close to 200 °C. Other clays such as smectite are the result of the final step of the hydrothermal activity in the fault. The identification of authigenic clay minerals, which cause weakening mechanisms, is consistent with the geodetic characterization of the Padul Fault, which plays an important role in aseismic deformation.