Crustal Structure Of The Northern Hikurangi Margin, New Zealand: Variable Accretion And Overthrusting Plate Strength Influenced By Rough Subduction

Exploring the structure of convergent margins is key to understanding megathrust slip behavior and tsunami generation. We present new wide-angle and marine multichannel seismic data that constrain the crustal structure and accretion dynamics of the northern Hikurangi margin. The top of the basement of the Hikurangi Plateau is overlain by a rough, 2-3 km thick layer of volcanic cover with P-wave velocities (V-P) between 3 and 5 km/s. This volcanic cover contributes significantly to seismic reflectivity beneath the shallow subduction plate boundary. The frontal prism structure varies along-strike from similar to 25 km wide with imbricate thrust faults where accretion of trench sediments is undisrupted, to narrower (similar to 14 km) with slumps and branching, irregular thrust fault geometries, which may reflect lower sediment supply or past seamount collisions. A large thrust fault network in the inner prism with a seismically fast hanging wall indicates a mechanical boundary between a seismically faster deforming backstop and the seismically slower frontal prism. Near the coastline, V-P increases between 2.8 and 4 km/s at 2-8 km depth and is 0.5-1.7 km/s slower than the southern Hikurangi margin. Low seismic wavespeeds and low vertical velocity gradients in the inner prism support the hypothesis that a weak overthrusting plate contributes to historic tsunami-earthquakes and long duration seismic ground motion.