HEMATITE (U-TH)/HE THERMOCHRONOMETRY CONSTRAINTS ON INTRAPLATE STRIKE-SLIP TECTONICS, THE KUH-E-FAGHAN FAULT, CENTRAL IRAN

The Kuh-e-Faghan strike-slip fault system (KFF), defining the northern edge of the Lut Block in Central Iran, developed through a pulsed history of fault propagation and fault-related exhumation due to the Arabia-Eurasia continental collision. Here we integrate structural and fault-rock textural analysis with new and previously published apatite fission-track (AFT) and apatite (U-Th)/He (He) data, chlorite thermometry, and hematite He dating of hematite-coated brittle fault surfaces to refine patterns of late Miocene-Pliocene burial and exhumation associated with the propagation of the KFF. Twenty-nine hematite He dates from three striated, metallic luster hematite-coated slip surfaces from KFF fault cores and damage zone yield individual dates from ~12-2 Ma. Petrographic analysis and chlorite thermometry of a polyphase fossil fluid circulation system in the KFF fault core document mineralization and fluid circulation transitioned from a closed system characterized by pressure solution and calcite mineralization to an open system characterized by hot (T≈239±10°C) fluid flow and hematite mineralization. Hematite microtextural and grain-size analysis reveals primary and secondary synkinematic hematite fabrics, no evidence of hematite comminution, and similar HeHe closure temperatures (~60-85 °C) in each sample. Integration of these data with thermal history modeling of new AFT and previously published apatite He results shows fault activity is characterized by an early stage of fault nucleation, fluid circulation, hematite mineralization, and eastward horizontal propagation of the fault system from ~10 to 7 Ma. Hematite He, AFT, and apatite He data also track a second phase of fault system activity involving fault-related exhumation initiating at ~7 Ma and continuing until present time. Our new data constrain the onset of the well-recognized Late Miocene-Pliocene regional tectonic reorganization of the Iranian Plateau. Our results illustrate the utility of fault rock hematite He dating, when combined with chlorite thermometry and conventional low-temperature thermochronometry, to constrain the nature and evolution of fault system fluids, deformation mechanisms, and timing of fault zone processes.