MANTLE WEDGE SEISMICITY UNDER THE WESTERN MARGIN OF THE RAUKUMARA PENINSULA, NORTHEAST NORTH ISLAND, NEW ZEALAND

Crustal seismic reflection data across the eastern Bay of Plenty and Raukumara Peninsula margin image an intriguing localised zone of strong reflectivity at a depth of about 35 - 40 km (12 - 16 s twt), that coincides with a local increase in seismicity. The zone lies between the Hikurangi subduction zone along eastern North Island and the extensional back-arc basin within central North Island (Taupo Volcanic Zone - TVZ). The zone is within the mantle wedge as Moho is clearly imaged on the seismic reflection data and the top of the subducting slab is clearly defined by seismicity. The distribution of seismicity within the region shows the occurrence of two similar local seismicity hots spots in the mantle wedge, about 60 km to the northeast and to the southwest of the original zone, sub-parallel to the plate boundary strike. The hotspots lie 40 km east of the eastern margin of the TVZ in the south and about 25 km east in the north. The southern hotspot lies approximately across strike from the change from andesite to rhyolite volcanism in the TVZ. In 3D the zone of seismicity forms a narrow column, about 10 - 15 km in diameter, in the mantle wedge from the subducted plate at 50 km to about 30 km depth. The strong reflectivity suggests a partial melt or fluid origin, but the subducted plate is too shallow here for normal back-arc melt generation. A possible cause of the hotspots is fluid, probably partial melt, resulting from dewatering of the subducted Hikurangi Plateau, but the cause of the limited extent along the plate margin to the south is unknown.