Abstract
Fission-track dating and track length analysis were carried out on 11 samples collected from surface outcrops (n = 5) and the Kawaue boring core (n = 6) in the region of the Atera Fault. This fault is a large, active sinistral fault with a dip-slip component of displacement. Mean zircon fission-track ages are ∼76 Ma and ∼69 Ma at sites located southwest and northeast of the fault, respectively. Mean apatite fission-track ages outside and within the fault zone are ∼42 Ma and ∼22 Ma, respectively. Mean track lengths in U-4 zircon, collected from within the fault zone, indicate that only this sample is partially annealed, with an apparent bimodal track length distribution. Forward modeling and considerations of annealing kinetics suggest that this sample was slowly cooled from ∼300 °C since ∼40–60 Ma. We have calculated the excess erosion of the hanging wall northeast of the fault, and true dip displacement of the fault, assuming that these processes control the age differences across the fault. These calculations lead us to three main conclusions: (1) Dip-slip displacement of the Atera fault since 70 Ma is ∼1 km. This calculated displacement is similar to that estimated from basement rocks and/or topography across the fault. (2) The present-day fracture zone along the fault experienced heating post-20 Ma, but prior to the Quaternary. (3) Some minor magmatic intrusions or another heating event, followed by slow cooling, has affected sample U-4 at ∼40–60 Ma. Our first conclusion is consistent with previous studies that concluded the present-day Atera Fault activity commenced in the early Quaternary. Our second conclusion indicates that this fracture zone has existed from ∼20 Ma through to the Quaternary.
