Ancient thermal anomaly of an active fault system: Zircon fission‐track evidence from Nojima GSJ 750 m borehole samples

Abstract

To understand better heat generation and transfer along faults, fission track (FT) thermochronologic analysis was performed on zircon separates from Cretaceous granitic rocks around the Nojima fault, activated during the 1995 Kobe earthquake. Samples were collected from the Geological Survey of Japan (GSJ) 750 m borehole and nearby outcrop. Zircon FTs showed reduced mean lengths and ages for samples less than ∼10 m away from the fault at depth. This is interpreted as representing an ancient thermal anomaly by heat transfer or dispersion via fluids at the deep crustal interior because the degree of FT annealing is correlated to that of deformation/alteration of borehole rocks. From extrapolation of data in the mean FT age‐length plot as well as from model age calculation, it is suggested that last cooling took place at ∼30–40 Ma.