Fore-arc mantle wedge seismicity under northeast New Zealand

Relocated upper mantle seismicity, using a 3D velocity model, delineates three tightly clustered (about 12 km diameter) zones of seismicity that occur in the mantle wedge under the fore-arc region of the northern Hikurangi margin, North Island, New Zealand. These clusters extend from the subducting plate at about 55 km depth up to about 35 km, and coincide with strong seismic reflectivity recorded active source seismic reflection measurements, suggesting a fluids association. They have maximum magnitudes of about Mw 4.5. The clusters correspond closely with a change from low to high Vp and Qp, and with average Vp/Vs that is interpreted to be caused by the transition from serpentinised to normal peridotite. The location of this transition appears to correspond to the locus of dehydration of peridotite, suggesting that the seismicity is caused by dehydration embrittlement. The release of these fluids where the serpentinised mantle wedge reaches the locus of dehydration may provide a mechanism for transferring additional fluids or fluid fluxed partial melt into the adjacent back-arc basin and facilitate the generation of voluminous magmas that are erupted there. The occurrence of serpentinite in the fore-arc mantle under northeastern North Island may influence the shallow extent of interplate coupling under the region. The location and spacing of the clusters is consistent with thermally driven diapiric or fluid upwellings, but they also show a spatial relationship with proposed lithospheric fractures and associated volcanic seamounts on the subducting Hikurangi plateau. ► Three tightly clustered (about 12 km diameter) zones of seismicity delineated in the fore-arc mantle wedge. ► The clusters coincide with strong vertical seismic reflectivity suggesting a fluid association. ► The clusters correspond closely with the locus of dehydration of peridotite, suggesting that the seismicity is caused by dehydration embrittlement. ► Serpentinite in the fore-arc mantle may influence the depth extent of interplate coupling. ► Spacing of the clusters is consistent with thermally driven diapiric or fluid upwellings.