Dating shallow thrusts with zircon (U-Th)/he thermochronometry-the shear heating connection

New zircon (U-Th)/He (ZHe) ages from a shallow (<6-7 km) thrust fault zone and surrounding wall rocks in the Helminthoid Flysch of the Ligurian Alps were measured to test the applicability of the thermochronometer for dating brittle or brittle-ductile faults. The ages are integrated with X-ray diffraction analysis of clay minerals and fluid inclusion microthermometry on vein-filling minerals to constrain the temperature conditions of the damage zone and the wall rocks during thrusting. The wall rocks yield pre-depositional inherited ZHe ages (125.3 ± 15 to 312.3 ± 37 Ma) while ages from the fault core are reset (28.8 ± 3.4 to 33.8 ± 4.0 Ma). This is consistent with independent geological and thermochronometric evidence for early Oligocene motion of the thrust. This implies that the fault zone exceeded 200 °C during faulting, and confirms the illite crystallinity and fluid inclusion constraints, which indicate temperatures of 220-300 °C in the fault zone, while in those the wall rocks were <180-200 °C. Thermal modeling of the fault zone suggests that the shear heating associated with the fault motion is an efficient mechanism for generating temperature increases of 50-70 °C during a displacement of 10-25 km in 2-10 m.y. Our results underscore the validity of the ZHe technique for dating brittle or brittle-ductile faults characterized by relatively high strain rate.