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

Abstract

S U M M A R YRecent 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 which coincides with the upper plane of the dipping seismic zone and extends to ca. 65 km depth with the subducted Hikurangi Plateau, which is about 17 km thick prior to subduction. The mantle wedge is generally imaged as a low Vp, high Vp/Vs feature. However, there are significant changes evident in the wedge along the strike of the subduction zone. The region where Vp is lowest (7.4 km s −1 ) and Vp/Vs is highest (1.87) occurs at 65 km depth, immediately west of the Taupo caldera. This region is best interpreted as a significant volume of partial melt, produced by the reaction of fluid released by dehydration of the subducted plate with the convecting mantle wedge. The region with lowest Vp, while paralleling the underlying dipping seismic zone, is located about 30 km from the upper surface of the zone. Material 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.

Subduction of the oceanic Hikurangi Plateau and its impact on the Kermadec arc

“…A similar B10-km thick crust-mantle transition layer with lower seismic velocities (that is, Vp ¼ 7.0-7.7 m s À 1 ) has also been identified beneath the Izu-Bonin-Mariana and Tonga arcs, interpreted to be composed of a mixture of crustal and mantle materials 45,46 . Likewise, beneath the central Taupo Volcanic Zone, a zone of reduced seismic velocities of ZVp ¼ 7.4 m s À 1 (Vp/Vs ¼ B1.87) extends from the Moho to the slab-mantle wedge interface suggesting diapirically rising hydrated and low density material 47 . Lower seismic velocities of Vp ¼ B7.7, (and density ¼ B3.0 g cm À 3 ; Vp/Vs ¼ B1.85), which are similar to the velocities observed at the crust-mantle boundary beneath arcs have been measured in exhumed subduction mélange rocks with compositions intermediate between chlorite schists and jadeite (at T ¼ 600°C and P ¼ 2 GPa).…”

Teleseismic constraints on the geological environment of deep episodic slow earthquakes in subduction zone forearcs: A review

“…10b). Reyners et al (2006) Taken together, slow earthquake observations, thermal modeling and seismic velocity models indicate that temperature may be the main factor controlling the depth and lateral extent of the seismogenic zone at the Hikurangi margin . In the northeast, the higher slab temperatures and fluid generation (with inferred high pore-fluid pressure) lower the depth to the brittle-ductile transition, as well as the depth of unstable to stable sliding, compared to the southwest.…”

“…There is an abundance of high-resolution three-dimensional regional velocity models from P-and S-wave travel-time tomography studies (Eberhart-Phillips and Reyners, 2012;Reyners et al, 2006); in comparison, teleseismic scattering studies of the Hikurangi margin are sparse. Henrys et al (2013) showed P-wave velocity structure of the forearc system from the active source data beneath the southern North Island.…”

Tectonics of the Taranaki region, New Zealand: Earthquake focal mechanisms and stress axes

“…1) is thought to overlie the juxtaposition of a thin continental crust (25 km) to the north against a normal thickness crust (36 km) to the south (Stern et al 1987), which is marked by the Taranaki-Ruapehu Line (TRL). From the orientation of the regional gravity anomaly, Stern et al (1987) considered the TRL to be aligned east-west, although Reyners et al (2006), using 3D velocity tomography results, think it is aligned northwest-southeast, parallel to the dip of the subducting Pacific plate beneath the North Island. Although the area is seismically active (Sherburn & White 2005), there are no known active faults, possibly because the earthquakes are concentrated deeper than 25 km below the surface (Sherburn & White 2005).…”