The Age and Origin of Miocene‐Pliocene Fault Reactivations in the Upper Plate of an Incipient Subduction Zone, Puysegur Margin, New Zealand

Abstract

AbstractStructural observations and 40Ar/39Ar geochronology on pseudotachylyte, mylonite, and other fault zone materials from Fiordland, New Zealand, reveal a multistage history of fault reactivation and uplift above an incipient ocean‐continent subduction zone. The integrated data allow us to distinguish true fault reactivations from cases where different styles of brittle and ductile deformation happen together. Five stages of faulting record the initiation and evolution of subduction at the Puysegur Trench. Stage 1 normal faults (40–25 Ma) formed during continental rifting prior to subduction. These faults were reactivated as dextral strike‐slip shear zones when subduction began at ~25 Ma. The dextral shear zones formed part of a transpressional regime (Stage 2, 25–10 Ma) that included minor reverse motion and modest uplift above the leading edge of the subducting slab as it propagated below Fiordland. At 8–7 Ma (Stage 3), trench‐parallel faults accommodated the first and only episode of pure reverse motion. Reconstructions confirm that these faults formed when the slab reached mantle depths and collided with previously subducted crust. At 5–4 Ma (Stage 4), trench‐parallel faults accommodated oblique‐reverse motion when an oceanic ridge collided obliquely with the Puysegur Trench. Stages 3 and 4 both accelerated rock uplift and topographic growth in short pulses. A return to strike‐slip motion occurred after ~4 Ma (Stage 5) when deformation localized onto the Alpine Fault. These results highlight how the rock record of faulting links displacements occurring at Earth's surface to events occurring both at the trench and deep within the lithosphere during subduction.