Tectonic shortening via sediment compaction in the subduction trench: Southern Hikurangi margin, New Zealand

Horizontal compressive stress in subduction zones is accommodated by both brittle and ductile deformation, from the deep megathrust to the seafloor. Compaction of sediments is an important, but poorly understood, component of the overall strain budget. Combining seismic depth imaging, P-wave velocities, sediment physical properties, and age constraints, we characterize horizontal shortening in the southern Hikurangi trench (New Zealand) and assess its contribution to the accommodation of plate motion. A distinct change in the lateral seismic velocity gradient of trench sediments, ∼20 km seaward of the main frontal thrust, marks the onset of lateral compaction and is accompanied by an onset of tectonically driven stratigraphic thickening. Compaction of the trench wedge has accommodated ∼3 km of shortening, far exceeding the <450 m of shortening from macroscopic fault heave and folding at the main frontal thrust. Further, we estimate that trench compaction accounts for >20% of the time-averaged, long-term convergent strain rate, thereby significantly reducing the amount of strain that would otherwise be accommodated by slip on the shallow megathrust and overlying upper plate faults. Detailed analyses of trench and outer wedge compaction are critically important for understanding shallow megathrust strain partitioning, which has implications for the megathrust’s slip behavior and tsunamigenic potential.