Observations of very high P -velocities in the subducted slab, New Zealand, and their relation with the slab geometry

236

315

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.

Section: Discussion Of Resultssupporting

confidence: 87%

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

Reyners¹,

Eberhart‐Phillips²,

Stuart³

et al. 2006

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315

“…The mantle of the subducting plate has Vp of 8.0–8.7 km s −1 , similar to Chadwick's (1997) and Robinson's (1986) models. The high Vp has been observed in other studies and anisotropy has been considered an explanation (Galea 1992). While there is likely some anisotropy in the subducting plate, our local earthquake inversion method uses numerous ray paths with a wide range of azimuths and the 3‐D velocities will be generally representative of average velocities rather than fast anisotropic velocities.…”

Section: Discussionsupporting

confidence: 60%

Crustal heterogeneity and subduction processes: 3-DVp, Vp/VsandQin the southern North Island, New Zealand

Eberhart‐Phillips¹,

Reyners

Chadwick

et al. 2005

S U M M A R YAlong the southern Hikurangi subduction zone in the southern North Island, New Zealand, the subduction zone changes from volcanic with moderately weak interseismic coupling, to non-volcanic with strong coupling in Wellington. To investigate how material heterogeneity relates to subduction processes, we compute 3-D seismic properties using 342 earthquakes and 2 shots recorded by a 165-station seismic array. The time base for some telemetered temporary stations was not stable, which required revising the inversion code to allow an additional origin time related to the telemetered stations. Progressive inversions from 2-D to fine 3-D were carried out for Vp, Vp/Vs and Qp. Combined interpretation of these three seismic properties highlights features of the subduction zone. The subducted slab is imaged as a high-velocity, high-Q feature. In the overlying crust, from southeast to northwest, there is low velocity in the accretionary wedge, high velocity in the axial ranges and low velocity in the Wanganui basin. High Vp/Vs and low Qp characterize the crust east of the North Island dextral fault belt. High Vp/Vs is imaged above the subducted slab to 40 km depth in the northern half of the study area. Low Qp is also prominent above the slab, particularly trenchwards of the volcanic region. In marked contrast to the high Vp/Vs, the low Qp above the slab extends as a continuous feature up to the low Qp in the shallow crust underlying the volcanic region. The low Qp and high Vp/Vs are correlated with the region of weaker coupling and are consistent with significant subducted sediments with associated fluid release. Vp/Vs is particularly sensitive to overpressured fluid in the compressional forearc, whereas Qp also images the hydrostatically pressured fluid transport in the more permeable extensional volcanic region. The rheology of the lower crust of the overlying plate is a factor in large-scale subduction coupling as evidenced by the terrane boundaries and Haast schist in the section near Wellington. The changing rheology of the lower crust of the overlying plate in the southern North Island also provides a possible explanation for the southward cessation of volcanism.

“…Some earthquakes to the northeast of the TVZ were rejected because the observed and expected backazimuths of the initial P ‐wave arrival were notably different, probably due to the effect of refraction along the Kermadec and Hikurangi subduction zones (e.g. Ansell & Gubbins 1986; Galea 1992). The term ‘backazimuth’ used throughout this study refers to the direction from the station towards the earthquake epicentre, measured positive clockwise from north.…”

Section: Receiver Function Data and Methodsmentioning

confidence: 99%

Shear wave velocity variation across the Taupo Volcanic Zone, New Zealand, from receiver function inversion

Bannister¹,

Bryan²,

Bibby³

2004

S U M M A R YThe Taupo Volcanic Zone (TVZ), New Zealand is a region characterized by very high magma eruption rates and extremely high heat flow, which is manifest in high-temperature geothermal waters. The shear wave velocity structure across the region is inferred using non-linear inversion of receiver functions, which were derived from teleseismic earthquake data. Results from the non-linear inversion, and from forward synthetic modelling, indicate low S velocities at ∼6-16 km depth near the Rotorua and Reporoa calderas. We infer these low-velocity layers to represent the presence of high-level bodies of partial melt associated with the volcanism. Receiver functions at other stations are complicated by reverberations associated with nearsurface sedimentary layers. The receiver function data also indicate that the Moho lies between 25 and 30 km, deeper than the 15 ± 2 km depth previously inferred for the crust-mantle boundary beneath the TVZ.