Seismic imaging of the New Zealand subduction zone

This thesis provides a comprehensive seismic investigation of the Hikurangi subduction zone in New Zealand, aiming to improve our understanding of its lithospheric and upper mantle structure from the shallow crust to the mantle transition zone (MTZ). By employing both P-to-S and S-to-P receiver function methods on data from permanent and temporary seismic networks, this work images key seismic discontinuities to explore their geodynamic implications. We use S-to-P receiver functions to constrain the Moho and the lithosphere-asthenosphere boundary (LAB) across the North Island. The Moho of the overriding Australian Plate is imaged at a depth of 15–30±3 km, while the subducting Pacific Plate's Moho is found at 31±3 km to 48±5 km. The Australian Plate LAB is identified at 62–80±8 km depth, with evidence suggesting ponded asthenospheric melt sharpens this discontinuity. Beneath the Pacific Plate, a shallow LAB is inferred at 90–105±10 km, consistent with an inferred LAB melt channel in previous work. Crucially, the location of inferred melt beneath the Pacific Plate corresponds spatially with the transition to where the plates become locked, suggesting that the buoyancy of this melt may contribute to enhanced plate locking at shallower depths. Beneath the Taupō Volcanic Zone (TVZ) we use P-to-S receiver functions to characterise its complex magma plumbing system at Taupō, with S-to-P receiver functions supporting interpretations. We image a positive phase beneath the Taupō caldera, with a lower boundary at 10−16±2 km depth, interpreted to be the base of a silicic mush zone, corresponding to a maximum estimated magma mush volume of ∼836 km3 based on synthetic modelling. A widespread negative discontinuity between 13±2 km and 26±3 km depth is interpreted as the top of a lower-crustal partial melt layer. The Moho is observed to deepen from 28±3 km in the northwest to 39±4 km in the southeast, a variation likely driven by a combination of magmatic addition causing thickening in the southeast and extensional rifting causing thinning in the northwest. Finally, the topography of the 410 km and 660 km discontinuities bounding the MTZ is constrained across the entire New Zealand region. P-to-S receiver functions reveal a significantly thickened MTZ (by 6−42±5 km) near the subducting Pacific slab, consistent with a cool thermal anomaly (−46K to −323K). In contrast, significant MTZ thinning (by 28–64±14 km) is observed southeast of the South Island behind the slab, supporting the presence of a hot mantle upwelling with a thermal anomaly of +215K to +492K. Evidence for slab-induced hydration is found northwest of the active slab, while two distinct bands of MTZ thickening further northwest correlate spatially with ancient subduction trenches (30–50 Ma), indicating that stagnant slab remnants have left a lasting imprint on the MTZ structure. Together, these findings provide an integrated view of how ongoing and past subduction processes shape New Zealand's dynamic tectonic environment.