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

Abstract

Structural 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.

Interplay of Cretaceous transpressional deformation and continental arc magmatism in a long-lived crustal boundary, central Fiordland, New Zealand

“…Faults also record increased uplift (Sutherland et al, 2009) owing to the interaction between the subducted portions of the Hikurangi Plateau and Pacific plate beneath Fiordland (Klepeis et al, 2019a). In the study area, these features are part of the Misty fault segment (Klepeis et al, 2019a;Klepeis et al, 2019b), a reactivated northern extension of the Spey-Mica Burn fault system (Fig. 2).…”

“…L 3 plunges gently to the W-SW (see Fig. 2B of Klepeis et al, 2019b). S 2 foliation deflections, sigmoidal pods with intact S 2 /L 2 structures, C′-type shear band cleavages, sigmoidal feldspar porphyroclasts with tails of chlorite or biotite, and inclined quartz subgrains all suggest top-to-the-W/SW displacement.…”

“…The Cozette Pluton in this part of the map area displays the same gneissic foliation with mm-thick quartz ribbons and quartz + biotite lineation as is present at Camp 2. These exposures are found in the footwall of a major Cenozoic reverse fault (Misty fault; see Klepeis et al, 2019b). However, the portions of this fabric mapped east of the Misty fault in the Cozette Pluton in Figure 3 represent a minimum extent.…”

“…Continental arcs tend to be long-lived features that are subject to changes in plate motions and growth by the accretion of outboard arcs and terranes-characteristics that create crustal anisotropies and influence the rates and locations of magma addition. Field-based and modeling studies investigating structural reactivation show that the process can range in scale from small individual fault surfaces to crustal-scale structures formed by terrane accretion, deformation, and magmatism (e.g., Bitencourt and Nardi, 2000;Scott et al, 2011;Miller and Becker, 2014;Klepeis et al, 2019aKlepeis et al, , 2019bNabavi et al, 2020). Inherited structures have both the ability to localize deformation and to act as conduits for the transport of melt and fluids (e.g., Naganjaneyulu and Santosh, 2011;Lu et al, 2012;Klepeis et al, 2019a).…”

“…Recent work shows that inherited (Carboniferous) crustal-scale boundaries are preserved within the arc (Andico et al, 2017;Klepeis et al, 2019a) and have concentrated both Early Cretaceous magmatism and deformation. Importantly, structures and intrusions originating from different paleodepths are exposed at the surface along these boundaries due to the tilted nature of the section and extensive Cenozoic reverse faulting (e.g., Klepeis et al, 2019aKlepeis et al, , 2019b, making the area ideal for tracking the migration of deformation within the crustal column and for establishing temporal relationships between deformation and magmatism.…”