Hydration of the crust and upper mantle of the Hikurangi Plateau as it subducts at the southern Hikurangi margin.

• Hikurangi Plateau crust at the southern Hikurangi margin is ≈12 km thick. • A fast-upper mantle resides at a depth of ≈50 km beneath Hikurangi trough. • Crustal and upper mantle P-wave-speeds reduced by ≈10% beneath the Hikurangi trough. • Hikurangi Plateau is hydrated to depths of ≈50 km beneath the Hikurangi trough. Controlled-source seismic studies at most subduction zones show that bending of the subducting plate results in reduced seismic wave-speeds in the crust and upper mantle near the trench. Similar studies also have found unusually high P-wave-speeds (V p) in the upper mantle under oceanic plateaus. Onshore-offshore seismic profiling at the southern Hikurangi margin, where the ≈120 Ma old oceanic Hikurangi Plateau is subducting, indicates that a fast (V p ≈8.7±0.2 km/s) upper mantle layer lies beneath a ≈25 km thick mantle layer with more regular wave-speeds (V p ≈8.0±0.2 km/s) under the Hikurangi trough. This is consistent with previous findings of upper mantle V p ≈8.7-9.0 km/s in the margin-parallel direction under the North Island (≈100 km northwest of the deformation front) at depths ≈8-10 km below the Moho. Our profiles are margin-perpendicular, thus we show that the upper mantle lid of the subducting Pacific Plate is characterized by unusually high P-wave-speeds along all azimuths. We find an area of lowered V p in the ≈12±1 km thick Hikurangi Plateau crust beneath the trough. This drop in V p is ≈10%, and a similar drop in V p is deduced to depths of 25±2 km into the upper mantle. We interpret that the increase in thickness of the regular mantle beneath the trough results from the formation of a low-velocity zone in the faster upper mantle layer; this zone formed from serpentinisation by hydration through bending-induced normal faults and/or due to crack porosity introduced by thermal cracking, further enhanced by bending-related faulting. Thus the "regular mantle" (V p ≈8 km/s) is not in fact regular, but rather the high-speed mantle has mechanically bent, fractured, and altered. The absolute depth of fast mantle V p under the Hikurangi trough is around 50 km. The onset of the lower band of seismicity of the double seismic zone and high upper mantle V p under the North Island is observed at similar depths. This is consistent with the hypothesis that the lower band of earthquakes in a double seismic zone is due to antigorite dehydration processes, a hydrous mineral formed in the low velocity zone in the upper mantle beneath the trough. Our study on the Hikurangi margin is different, as the subducting plate here contains a ≈120 Ma old oceanic plateau with a ≈12 km thick crust, but the results are similar to other subduction margins where regular oceanic crust is subducting. [ABSTRACT FROM AUTHOR]