Extensional deformation along the Footwall Fault below the Hyde-Macraes Shear Zone, Otago Schist, New Zealand

Abstract

We describe ductile-to-brittle structures across the Footwall Fault directly below the Hyde-Macraes Shear Zone in the Otago Schist. These indicate that pervasive deformation along the Footwall Fault was due to northeast-southwest crustal extension. Structures below the Footwall Fault are ductile top-to-the-northeast extensional shear bands that formed in a penetrative lower greenschist facies foliation. With progressive deformation, shear bands formed at higher angles to the foliation, culminating in the shear bands being cut by lowand high-angle normal faults. In the hanging wall, extensional structures are much less pervasive and comprise a few high-angle normal faults. This sharp contrast in structural evolution of footwall and hanging wall resembles the tectonic evolution of major extensional shear zones worldwide. We propose that mid Cretaceous normal movement on the lowangle Footwall Fault accompanied the formation of the steeper Waihemo Fault further north. On a regional scale, much of the boundary between the Otago Schist core and its northern flank is cut by mid Cretaceous normal faults. Zircon fission-track ages from the footwall range from 79 ± 13.7 to 46.1 ± 6.2 Ma (2σ uncertainties) and young in the direction of hanging wall transport. One sample from the hanging wall yielded a zircon fission-track age of 76 ± 12.9 Ma. The fission-track ages are hard to interpret as zircon across the fault zone was affected by partial or full annealing. In part, the ages confirm mid Cretaceous movement but may also be interpreted to suggest additional reactivation of the Footwall Fault at slow inferred slip rates. ARTICLE HISTORY

K-Ar fault-gouge dating in the Lower Buller gorge constrains the formation of the Paparoa Trough, West Coast, New Zealand

“…Similar Late Cretaceous apatite fission-track ages occur in several other places in the South Island (Kamp 1997;Ring and Bernet 2010;Lang et al 2018). Ring et al (2018a) reported zircon fission-track ages of 79-46 Ma from the footwall of the Hyde-Macraes shear zone in the Otago Schist indicating NE-SW extensional reactivation. Bishop (1992) compiled nearly 400 King (2000) and the Paparoa Trough.…”

Middle to Late Miocene Age for the End of Amphibolite‐Facies Mylonitization of the Alpine Schist, New Zealand: Implications for Onset of Transpression Across the Alpine Fault

“…Hence, the notion of a Late Cretaceous through Paleogene wider “Alpine Fault thermotectonic corridor” might be an artifact of Neogene transpression. In addition to the Late Cretaceous/Paleogene ages for high‐T metamorphism, there are also numerous Cretaceous/Paleogene low‐T thermochronologic ages across the South Island (Ring, Mortimer, et al, ; Tippett & Kamp, ) and Stewart Island (Reiners et al, ; Ring et al, ) (Table S6), which have hitherto hardly been considered in compilations for Late Cretaceous/Paleogene ages (Briggs et al, ; Cooper & Palin, ; Mortimer, ). However, when the lithosphere is deforming, there needs to be deformation compatibility, and ages for lower crustal high‐grade metamorphism are linked in some way with low‐T thermochronologic ages in the upper crust.…”

“…Similar Late Cretaceous apatite fission track ages occur in several other places in the South Island (Batt et al, ; Kamp, ; Lang et al, ). Ring, Mortimer, et al () reported zircon fission track ages of 79 to 46 Ma from the footwall of a normal fault in the Otago Schist. These ages show a systematic and robust age trend indicating NE‐SW extension, which probably reactivated a mid‐Cretaceous normal fault.…”