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.