MODERATE CARBONATE CRYSTALLIZATION TEMPERATURES ON THE LOUTRAKI FAULT, GREECE: WHERE IS THE EVIDENCE FOR SHEAR HEATING?

Outcropping at the southeast end of the Corinth rift, the most rapidly extending region in Europe, the Loutraki Fault is a S-block-down normal fault that separates the Perachora Peninsula horst from the Lechaion Gulf-Saronic Gulf grabens that probably last slipped during the Pleistocene. Carbonate clumped isotope measurements were made on select carbonate microstructures from the Loutraki Fault to determine if any of these fault phases preserve a signature of high-temperature crystallization, e.g., those developed during "shear heating." We find that comminuted clasts of Pindos Limestone country rock preserve the highest temperatures of c. 95 to 120°C, whereas adjacent ultracataclasite bands (UB) record temperatures in the range 60 to 80°C. The two coolest temperatures of c. 62°C were measured in UB within 5 mm of the outcropping principal slip surface (PSS). A linear correlation of UB crystallization temperature vs distance into the footwall from the outcropping PSS is c. 5°C/cm over a distance of c. 10 cm. Calculated equilibrium d¹⁸O water compositions for the UB nearest the PSS are in the range 2 to 4 permil but, for country rock clasts and UB deeper in the footwall, are >8 permil. Given that temperature increases with distance into the footwall, we find a zone of cooling on and near the active PSS and find no evidence for spikes in crystallization temperature that would be expected from shear heating. Calculated water compositions are in part consistent with marine, rather than meteoric, water, and the cooling trend adjacent to the PSS does not appear to be the result of meteoric influx. The c. 25°C difference in temperatures recorded in Pindos country rock clasts may indicate either partial re-equilibration with surrounding fault gouge or sampling of country rock from different levels of crust from as deep as c. 3 km and separated from each other by as much as 1 km vertically.