Imaging subduction from the trench to 300 km depth beneath the central North Island, New Zealand, withVpandVp/Vs

Abstract

SUMMARY Recent dense deployments of portable digital seismographs have provided excellent control on earthquakes beneath the central North Island of New Zealand. Here we use a subset of the best-recorded earthquakes in an inversion for the 3-D Vp and Vp/Vs structure. The data set includes 39 123 P observations and 18 331 S observations from 1239 earthquakes and nine explosions. The subducted plate is imaged as a high Vp, low Vp/Vs feature. Vp within the mantle of the subducted slab is almost always >8.5 km s−1, which requires the ca. 120 Myr slab to be unusually cold. The low Vp/Vs within the subducted plate closely parallels the lower plane of the dipping seismic zone. It most likely indicates fluid resulting from dehydration of serpentine in the slab mantle, and the earthquakes themselves are likely to be promoted by dehydration embrittlement. We identify a region with Vp 8.0 km s−1 directly above the dipping seismic zone can be interpreted as sinking, entrained with the motion of the subducted slab and forming a viscous blanket that insulates the slab from the high-temperature mantle wedge. Material in the overlying low Vp region can be interpreted as rising within a return flow within the wedge. The volcanic front appears to be controlled by where this dipping low Vp region meets the base of the crust. The thickness of the backarc crust also shows significant variation along strike. In the central Taupo Volcanic Zone (TVZ) the crust is ca. 35 km thick, while southwest of Mt Ruapehu the crust thickens by ca. 10 km. There is no significant low Vp zone in the mantle wedge in this southwestern region, suggesting that this thicker crust has choked off mantle return flow. The seismic tomography results, when combined with constraints on mantle flow from previous shear-wave splitting results, provide a plausible model for both the distribution of volcanism in the central North Island, and the exceptional magmatic productivity of the central TVZ.