Deep (>20 km) crustal seismicity is typically associated with cold Precambrian lithosphere. However, previous studies have reported that the depth above which 90% of seismicity occurs (D90) in the Southland region of New Zealand is 30-35 km, even though it is situated in relatively young Mesozoic crust at the eastern edge of the transpressive Pacific-Australian plate boundary. To examine this unusual lower crustal seismicity further, we deployed 19 short period seismometers between October 2022-2023 through the Southland Otago Seismic Array (SOSA). Supplemented by the permanent GeoNet network and an ongoing array to the west in Fiordland, SOSA provided a station spacing of ~30 km across this 150 km wide region. In total, SOSA recorded 85 events in and around Southland. These events are not uniformly distributed, with an abrupt decrease in seismicity east of Fiordland. We also identified six spatial-temporal clusters of elevated seismicity across eastern Fiordland and Southland. The distribution of SOSA event depths confirms the presence of lower crustal seismicity in Southland (D90 = 32 km). This D90 estimate is significantly deeper than suggested by event depths to the north in Otago (D90 = 14 km). Using 1D lithospheric strength profiles, we suggest that the embrittlement of Southland’s mid-lower crust reflects the emplacement of mafic terranes in this region during Mesozoic subduction at the Gondwana margin. Our study therefore highlights that a mafic lower crust can lead to deep seismicity within a region undergoing transpression.

Simulating present-day solid Earth deformation and volatile cycling requires integrating diverse geophysical datasets and advanced numerical techniques to model complex multiphysics processes at high resolutions. Subduction zone modeling is particularly challenging due to the large geographic extent, localized deformation zones, and the strong feedbacks between reactive fluid transport and solid deformation. Here, we develop new workflows for simulating 3-dimensional thermal-mechanical subduction and patterns of volatile dehydration at convergent margins, adaptable to include reactive fluid transport. We apply these workflows to the Hikurangi margin, where recent geophysical investigations have offered unprecedented insight into the structure and processes coupling fluid transport and solid deformation across broad spatiotemporal scales. Geophysical data sets constraining the downgoing and overriding plate structure are collated with the Geodynamic World Builder, which provides the initial conditions for forward simulations using the open-source geodynamic modeling software ASPECT. We systematically examine how plate interface rheology and hydration of the downgoing plate and upper mantle influence Pacific–Australian convergence and seismic anisotropy. Models prescribing a dry rheology and a plate boundary viscosity of 5e20 Pa s best reproduce observed plate velocities. Conversely, models considering hydrated olivine fabrics best reproduce observations of seismic anisotropy. Predicted patterns of slab dehydration and mantle melting correlate well with observations of seismic attenuation and arc volcanism. These results suggest that hydration-related rheological heterogeneity and related fluid weakening may strongly influence slab dynamics. Future investigations integrating coupled fluid transport and global mantle flow will provide insight into the feedbacks between subduction dynamics, fluid pathways, and arc volcanism.