Structure and late Cenozoic tectonics of the southern Two Thumb range, mid Canterbury, New Zealand

Abstract

Two crosscutting fault sets dominate the southern Two Thumb Range—the Forest Creek Faults, and the Fox Peak Faults. The Fox Peak Faults extend from the Fairlie Basin northward across Butler Saddle and into Forest Creek. To the south, they consist of two north‐striking, west‐dipping, reverse fault zones along the eastern flank of the Two Thumb Range. The eastern edge of the Fairlie Basin consists of an anticlinal fold. This becomes an east‐dipping reverse fault in the north of the basin. North of Butler Saddle, there are four Fox Peak Faults, which extend to Forest Creek where they are cut by or end against the Forest Creek Faults. The Forest Creek Faults consist of a set of paired reverse faults, striking northeast and dipping to the northwest and the southeast. They can be traced from at least the Rangitata River to Lake Tekapo and may extend even further. Late Quaternary vertical slip rate estimates from the southern Two Thumb Range are in the order of 1–1.5 mm/yr. Geodetically derived contemporary velocities and strain rates show a minimum in γ1 over this region. We suggest that the southern Two Thumb Range was actively uplifting during the early middle Quaternary at rates higher than those measured. The rates measured today reflect a slowing in uplift rate during the late Quaternary as the loci of deformation shifted southwest to the range front.