Seismic Depth Images and Velocity Models at the Southern Hikurangi Subduction Margin: Insights into Emergent Deformation and Dewatering

We apply 2D tomographic inversions and pre-stack depth migrations to multi-channel seismic data near the southern end of the Hikurangi subduction margin, east of New Zealand. This is the region where subduction transitions into dextral strike-slip motion in a zone of highly-oblique convergence, close to the epicentre of the 2016 Mw 7.8 Kaikōura earthquake. We use these data and methods to explore the southward extent of active subduction between the Pacific and Australian plates. The data lend themselves to investigations into emergent deformation (near the deformation front) and dewatering processes within the wedge.

The seismic velocities clearly resolve a broad and pronounced low velocity zone beneath the deforming wedge, a feature that has also been observed further north on this part of the margin. The subduction interface develops at the top of the low velocity zone. Amplitude-versus-offset (AVO) character, and a lack of anomalously high-amplitude reflections, indicate that the low velocities are not likely to be caused by widespread free gas. Rather, they represent lower bulk sediment velocities characteristic of this incoming sequence.

In the overlying wedge, gentle folding and proto thrusts develop well seaward of the bathymetric expression of the deformation front. Free gas accumulates beneath the seafloor in areas where splay faults and proto thrusts intersect the base of gas hydrate stability. Disturbances of the base of gas hydrate stability, albeit subtle, show evidence for localised dewatering (the movement of warm fluids along faults).

By providing new insight into the nature of the subduction interface, the development of compressional deformation and wedge dewatering, our results help to unravel the processes occurring in this complex tectonic transition at the southern Hikurangi margin.