Reprocessed and newly acquired seismic data provide new constraints on lithospheric flexure profiles beneath the Hawaiian Islands. We use these new observations and three-dimensional numerical models of lithospheric deformation combining elasticity, brittle failure, low-temperature plasticity (LTP) and high-temperature creep deformation mechanisms to constrain the thermal structure and rheology of the oceanic lithosphere lithosphere. When simulating normal oceanic lithospheric conditions with experimentally-derived LTP flow laws, the lithosphere flexes with too little amplitude and over too large a wavelength compared to observations. This result supports prior studies which call on the need to (1) adjust the LTP flow laws or, alternatively, to (2) account for magma-assisted flexural weakening of the lithosphere. Here, models that explore reductions in the activation energy of LTP are able to explain the observations of flexure with a smaller reduction than previously suggested. Models that explore elevated temperatures attributed to hotspot magmatism localized beneath the island edifices also produce close fits to the observed flexural profiles. Although the two factors cannot be distinguished based on fits to the flexure profiles, magma-assisted flexural weakening is supported by recent studies of geothermobarometry of pyroxenite xenoliths from O‘ahu, seismic structure and patterns of seismicity beneath the Hawaiian chain. If magma-assisted flexure is a common phenomenon at other ocean islands and seamounts, it could explain global trends in effective elastic plate thickness at those settings as well as at subduction zones and fracture zones.

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.

Seafloor spreading at slow rates can be accommodated on large-offset oceanic detachment faults (ODFs), that exhume lower crustal and mantle rocks in footwall domes termed oceanic core complexes (OCCs). Footwall rock experiences large rotation during exhumation, yet important aspects of the kinematics - particularly the relative roles of rigid block rotation and flexure - are not clearly understood. Using a high-resolution numerical model, we explore the exhumation kinematics in the footwall beneath an emergent ODF/OCC. A key feature of the models is that footwall motion is dominated by solid rotation, accommodated by the concave-down ODF. This is attributed to a system behaviour in which the accumulation of distributed plastic strain is minimized. A consequence of these kinematics is that curvature measured along the ODF is representative of a neutral stress configuration, rather than a ‘bent’ one. Instead, it is in the subsequent process of ‘apparent unbending’ that significant flexural stresses are developed in the model footwall. The brittle strain associated with apparent unbending is produced dominantly in extension, beneath the OCC, consistent with earthquake clustering observed in the Trans-Atlantic Geotraverse at the Mid-Atlantic Ridge.