Mantle Tectonics Beneath New Zealand Inferred from SKS Splitting and Petrophysics

Geophys. J. Int. (2005) 163, 760–774 doi: 10.1111/j.1365-246X.2005.02725.x

Mantle tectonics beneath New Zealand inferred from SKS splitting and petrophysics

Mathieu Duclos,1,2 Martha K. Savage,1 Andr´ea Tommasi2 and Ken R. Gledhill3 1Institute of Geophysics, School of Earth Sciences, PO Box 600, Victoria University of Wellington, Wellington 6001, New Zealand. E-mail: [email protected] 2Laboratoire de Tectonophysique, ISTEEM, CNRS, Universit´e Montpellier 2, France 3Institute of Geological and Nuclear Sciences, Lower Hutt, New Zealand

Accepted 2005 June 27. Received 2005 February 21; in original form 2004 October 31

SUMMARY We constrain the extent of lithospheric and asthenospheric deformation beneath New Zealand by coupling measurements of shear wave splitting of teleseismic waves and petrophysical analysis of mantle xenoliths. SKS splitting for the central South Island and the eastern part of the North Island confirms earlier observations. The fast S-wave polarization directions are roughly NE/SW, which is parallel to the Alpine Fault in the South Island and to the strike of the Hikurangi subduction zone in the North Island. This suggests that flow parallel to the plate boundary extends up to 160 km away from the boundary. Departures from this pattern are restricted to the southernmost station and the two northernmost stations and indicate changes in the mantle flow or a more complex anisotropic pattern, such as a dipping axis of symmetry or heterogeneous anisotropy. Analysis of mantle xenoliths from the Raglan (North Island), Dunedin (South Island) and Chatham Island regions allows us to constrain the lithospheric contribution to the observed seismic anisotropy. The intrinsic S-wave anisotropy is 1.5 per cent higher for the South Island samples (∼5.0 per cent versus ∼3.5 per cent for the North Island), indicating a strong lithospheric deformation that could explain the major part of the observed splitting. In the North Island, the intrinsic lithospheric anisotropy is too low to explain the SKS splitting times. This indicates that asthenospheric deformation probably plays a major role there. For the Chatham Island, the anisotropy pattern remains unresolved as the xenolith suggests strong lithospheric anisotropy but no present splitting is measured. Key words: mantle tectonics, New Zealand, petrophysics, seismic anisotropy, shear wave splitting. GJI Tectonics and geodynamics

thenospheric deformations. New Zealand is the perfect place for INTRODUCTION the study of the mantle flow beneath an oblique convergent plate When a shear wave enters an anisotropic medium, it is split in boundary given the good knowledge of the relative movements of two perpendicular directions corresponding to the fast and slow the plates creating the boundary and the large number of geophysi- polarization directions of the anisotropic body for its particular cal studies conducted there recently. Moreover, lithospheric mantle path (Crampin 1985). Using a three-component seismometer, one xenoliths can be retrieved in the North and South Islands as well can record these two split phases and evaluate their time differ- as the Chatham Islands. The petrophysical study of these xenoliths ence and the direction of polarization of the fast component of the allows constraints on the lithospheric contribution to the observed S wave (termed ‘fast direction’ hereafter). Flow in the shallow man- splitting. tle (at depths <200–250 km) leads to alignment of the [100]-axis of olivine parallel or subparallel to the shearing direction (Nico- GEODYNAMIC CONTEXT las & Christensen 1987) and the fast direction measured with tele- OF THE STUDY seismic shear wave splitting indicates the flow direction under the recording seismic station. Due to the lack of vertical resolution of The two main islands of New Zealand are situated on a segment this technique, two end-member hypotheses can be considered; one of the obliquely convergent boundary between the Australian and links the anisotropy to the deformation of the lithosphere, the other Pacific plates (Fig. 1). The relative movement of these plates varies links the anisotropy to an asthenospheric flow. The most probable from 47 mm yr−1 along the northern end of the Hikurangi margin hypothesis is that the SKS wave records both lithospheric and as- down to 38 mm yr−1 along the Alpine Fault (DeMets et al. 1994).

760 C 2005 RAS Mantle tectonics beneath New Zealand 761

Figure 1. Major structures and geodynamic regions of New Zealand: the Hikurangi subduction zone and the associated Central volcanic region (CVR), the Marlborough Fault System (MFS), the Alpine Fault and the Puysegur subduction zone. The stars represent the localities from which our mantle xenolithswere obtained.

From North to South, we can divide New Zealand into four major gur subduction is not well constrained. The South Island of New geodynamic regions (Fig. 1). The North Island is characterized by Zealand is thus a transition between two subductions of opposite the active west-dipping Hikurangi subduction zone and the Central dip, the Hikurangi and Puysegur subduction zones, through a set of volcanic region that is undergoing active volcanism through backarc dextral transpressive faults: the MFS and the Alpine Fault. Here, we extension (Adams & Hatherton 1973). In the South Island, the major focus on three of the major regions, the two subduction zones and tectonic feature is the Alpine Fault. This dextral transform boundary the Alpine Fault regions. has been active for ∼45 Ma (Cooper & Norris 1994; Walcott 1998). About 6.4 Ma ago, a change in direction of the plate motions led to a PETROPHYSICS DATA stronger compressional motion, transforming the Alpine Fault into a transpressive system with a vertical component of movement three In order to constrain the lithospheric contribution to the observed times smaller than the horizontal one (Walcott 1998). This com- splitting, we conducted a joint petrological and petrophysical anal- pression might have started even earlier, possibly 20 Ma ago but ysis of eight mantle xenoliths brought up to the surface by vari- with less vertical movement, as inferred from a recent palaeogeog- ous phases of intraplate basaltic volcanism. Three spinel-lherzolites raphy reconstruction taking into account a microplate southwest of (9225, 9231 and 5548) were brought to the surface by intraplate New Zealand, the Macquarie Plate (Cande & Stock 2004). Between basaltic volcanism active from 2.8 Ma to 1.7 Ma in the Raglan area these two systems, the Marlborough Fault System (MFS) and its in the North Island (Kear 1994). Three spinel-lherzolites and one extension in the lower North Island, the Wellington region, are a spinel-harzburgite come from the Dunedin area in the South Island transition zone composed of a set of dextral transpressive faults. In (D104A, D105C, D106C, D120B). In that area, the volcanic ac- the southernmost part of the South Island, the Fiordland region is tivity, consisting mainly of mantle derived alkali basalts (Bishop an anomalously elevated zone (Oliver & Coggon 1979), linked to &Turnbull 1996), began at 21 Ma, ending around 9 Ma (Coombs the steeply northeast-dipping Puysegur subduction zone (Eberhart- et al. 1986; Coombs 1987). The last sample, the spinel-lherzolite Phillips & Reyners 2001; Smith 1971). The transition between the NZ56CH comes from a 60 Ma old volcanic ﬂow from the Chatham Alpine Fault, which disappears in the Tasman Sea, and the Puyse- Islands (Hoke et al. 2000).

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Figure 2. Olivine and enstatite measured lattice preferred orientations. Leftmost three plots are lower hemisphere equal area projection for olivine LPO, contours at 0.5 multiple of a uniform distribution interval. Rightmost three plots are lower hemisphere projection for enstatite LPO, dotted due to low number of measurements. A horizontal line marks an observed foliation plane where it is present. Other samples’ LPOs are rotated to align the [100] axes with the X direction. The number of crystals measured is shown in the upper left corner.

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Table 1. Modal composition, main mineralogical characteristics and average modelled temperature of the xenoliths. Mg# = 100 × (Mg/(Mg+Fe)) with Mg and Fe the atomic percentage of magnesium and iron respectively. Cr# = 100 × (Cr/(Cr+Al)) with Cr and Al the atomic percentage of chromium and aluminium, respectively. Sample Modal composition (%) Main mineralogical characteristics Avg temperature Olivine OrthoPx ClinoPx Spinel Mg# Olivine Mg# OrthoPx Mg# Spinel Cr# Spinel Al Content (%) (◦C) 5548 75 8.5 6.5 3 89.8 89.9 73.6 14.7 5.13 1005 9225 54 23 13.5 4 89.9 90.1 72.7 15.7 5.62 1030 9231 63 19 8 2 90.8 91.1 66.2 38.5 4.37 1160 D104A 56 20 18 3 89.7 90.8 79.5 10 4.91 950 D105C 65 26 6 2 91.5 92.5 79.1 27.1 3 935 D106C 81 12 3 1 90.7 92.3 58.9 61 2.2 1050 D120B 64 18 13 2 90.2 90.8 78.5 12.7 5.4 1045 NZ56CH 67 24 5 1 92.1 92.1 67 55.4 1.6 670

Analysis of the microstructure by optical and electron microscopy samples), from secondary activation of low-temperature [001] slip, and measurement of strain-induced crystal preferred orientations by or from dynamic recrystallization (Tommasi et al. 2000). indexation of electron backscattered diffraction (ESBD) patterns are We calculated the average seismic properties for the North and used to unravel the deformation history. In order to characterize the South Islands by adding the olivine and pyroxene LPOs of all the lithospheric mantle composition at each site, modal and mineral samples from each area (Fig. 3). The North Island average sam- compositions (Table 1) are determined through analysis of electron ple presents a 3.45 ± 0.4 per cent maximum S-wave polarization backscattered images of thin sections and microprobe analysis, re- anisotropy for a wave travelling in the foliation plane normal to the spectively. The mineralogy and chemistry of the xenoliths is detailed flow direction; the direction of polarization of the fast S wave is par- in Appendix A. allel to the [100] axis of the olivine crystals, so it is parallel to the flow EBSD patterns are measured on polished thin sections using a direction marked by the lineation. Isotropic directions are observed scanning electron microscope (SEM) (Newbury & Yakowitz 1975). for waves traveling at 45◦ to the lineation and to the normal to the For each sample, we measured the crystallographic fabric of all foliation plane. For P waves, the maximum velocity (8.48 km s−1) major minerals: olivine, orthopyroxene and clinopyroxene. Mea- is aligned with the [100] axis of the olivine minerals and the slowest surements were operator controlled and performed on 2 mm spaced direction (8.02 km s−1)isnormal to the foliation plane. A 5.2 ± 0.3 parallel profiles along the long axis of the thin section on a grain per cent anisotropy is calculated for the P waves. by grain basis. Each time the observed pattern changed, a new mea- For the South Island average, the calculated S-wave anisotropy surement was performed. is maximum (4.9 ± 0.5 per cent) for a wave traveling in the fo- Once the modal composition of the sample and the preferred ori- liation plane normal to the flow direction. Similarly to the North entation of the major phases have been determined, we derive, based Island average, isotropic S-wave propagation directions are ob- on the single-crystal elastic constant tensors, the seismic properties served at 45◦ to the foliation plane. The maximum P-wave velocity of the aggregate (Mainprice 1990; Mainprice & Silver 1993). This (8.53 km s−1)isaligned with the [100] axis of the olivine minerals program assumes a Voigt–Reuss–Hill average of the elastic stiff- and the minimum P-wave velocity (7.99 km s−1) perpendicular to nesses of all constituent crystals in order to determine the rock’s the foliation plane, yielding 6.6 ± 1.1 per cent anisotropy for the elastic properties. Based on these data, we infer the velocity dis- P-waves. The sample from the Chatham Islands (Figs 2 and 3) tribution of the P waves, the anisotropy of the S waves and the gives a 5 per cent maximum S-wave anisotropy for a wave trav- polarization direction of the fast S wave for a particular incidence elling in the foliation plane normal to the flow direction and it is angle for each particular sample. isotropic for waves travelling at 45◦ to the lineation and to the nor- The measured LPOs for olivine and enstatite are shown in Fig. 2. mal to the foliation plane. The fast propagation direction for P waves All peridotites present clear olivine high-temperature deformation (8.64 km s−1)isaligned with the [100] axis of the olivine minerals fabrics. In those samples which display well-developed foliation for the sample with a perpendicular slow direction (8.06 km s−1) and lineation (D104A & D105C), the [100] axes are oriented in the in the foliation plane. Maximum anisotropy for P waves is lineation direction while [010] axes are normal to the foliation, sug- 6.9 per cent. gesting dominant activation of the high-temperature (010)[100] slip system. Dynamic recrystallization by subgrain rotation, with [001] SKS WAVE SPLITTING DATA as the rotation axis in agreement with dominant (010)[100] slip (Nicolas & Poirier 1976) is one possibility to explain the stronger We use previously analysed SKS splitting data from the SAPSE concentration of [001] axes relative to [100] and [010] axes in spinel deployment (Southern Alps Passive Seismic Experiment) (Klosko lherzolite D104A. In the other samples, the joint analysis of the et al. 1999), from the POMS2 array in the southern North Island olivine and orthopyroxene lattice preferred orientations also suggest (Marson-Pidgeon et al. 1999) and from the 1995 Panda deployment the activation of the high-temperature (010)[100] and (100)[001] (Audoine et al. 2004) along with new data collected by a set of systems for olivine and orthopyroxene, respectively. Except for sam- broadband stations deployed in New Zealand in 2001 by the Insti- ple D120B, olivine and enstatite LPO are much stronger in the South tute of Geological and Nuclear Sciences (IGNS) (Cowan & GeoNet Island peridotites than in North Island ones. The North Island sam- Project Team 2002). This deployment is part of the GeoNet pro- ples and D120B display olivine LPO characterized by a stronger con- gram and some of the new stations were deployed in areas with- centration of [010] and a girdle distribution of [100] in the foliation out any previous seismic anisotropy measurements. We also report plane. Such an LPO pattern may result from a compressional compo- measurements at new permanent stations located on previous tem- nent in the deformation field (unlikely in the case of the North Island porary sites. The method used in order to measure the anisotropy

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Vp AVs Fast S wave Polarization

North Island

Max.Velocity = 8.48 Max.Anisotropy = 3.45 % Anisotropy = 5.2 %

South Island

Max.Velocity = 8.53 Max.Anisotropy = 4.93 % Anisotropy = 6.6 %

Chatham Island

Max.Velocity = 8.64 Max.Anisotropy = 5.28 % Anisotropy = 6.9 %

Figure 3. Calculated North Island, South Island and Chatham Island average xenolith’sseismic properties (P-wave velocity, S-wave anisotropy and fast S-wave polarization plane). Lower hemisphere equal-area projections. P-wave velocity and S-wave anisotropy plots have 0.1 km s−1 and 0.5 per cent contour intervals, respectively. parameters, direction of polarization of the fast S wave (‘fast direc- No reliable measurements were obtained for the FWVZ (Far West tion’) and SKS splitting time between the fast and slow S wave, on T-Bar, Ruapehu) station. Finally, the PWZ (Pawanui, Hawke’s Bay) teleseismic SKS wavesisdescribed in Appendix B (Silver & Chan station, sitting in the south of Hawke’s Bay, yields an average 15 ± 1991). 8◦ fast direction and an average splitting time of 2 ± 0.4 s, therefore, In the North Island, the directions of polarization of the fast S wave showing a small rotation from the trench azimuth. These results are (Table 2 and Fig. 4) are roughly parallel to the strike of the Hikurangi consistent across the backazimuth range available (Table 2). subduction zone and consistent with the previously observed SKS In the South Island, the results obtained (Table 2 and Fig. 4) are splitting in the southernmost part of the island (Marson-Pidgeon in agreement with the previous study by Klosko et al. (1999). Fast et al. 1999) and in the central volcanic region (Audoine et al. 2004). directions are generally sub-parallel to the strike of the Alpine Fault However, the KNZ (Kokohu Rd, Hawke’s Bay) and TOZ (Tahuroa for the stations in the central part of the island. MQZ (Mc Queen’s Rd, Waikato) stations, on the east coast of the Raukumara peninsula Valley, Canterbury) and DSZ (Denniston, West Coast), on opposite and on the west side of the Central volcanic region, yield different sides of the island, display parallel fast directions and comparable results for waves coming from different back azimuths. Most direc- splitting times (1.5 s). The station RPZ (Rata Peak, Canterbury) tions of polarization of the fast S wave are parallel or sub-parallel also shows Alpine Fault parallel fast directions. However, at the to the Hikurangi trench but rays coming from the west (270 ± 10◦ southernmost station, WHZ (Whether Hill, Southland), the fast di- back azimuth) yield NW/SE to N/Sfast directions. Different split- rection rotates counter clockwise to an angle of 30◦ to the Alpine ting parameters for different back azimuth were also recorded at Fault direction. This station yields 17 ± 7◦ fast direction, parallel a number of other stations in the central North Island during the to measurements already performed in the South of New Zealand CNIPSE (Central North Island Passive Seismic Experiment) de- by Klosko et al. (1999). The SKS splitting time here is large, up to ployment and in two portable stations in the South Island (Hofmann 3.4 s with an average value of 2.7 ± 0.4 s. 2002; Reyners & Stuart 2002). Backazimuthal dependence of the splitting parameters has also been observed at the permanent IRIS DISCUSSION station in Wellington (Marson-Pidgeon & Savage 2004a) and on a temporary deployment in southern North Island (Marson-Pidgeon We make the classical assumption that the anisotropy measured is &Savage 2004b). linked to only one anisotropic layer with a horizontal symmetry

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Table 2. Summary of the new SKS measurements. ‘Null X’ means that the possible directions of anisotropy that would yield a null measurement are X and X + 90◦. STATION EVENT DATE (yyyyddd-hh:mm) (◦) δt (s) Back azimuth (◦) DSZ 2001174-20:33 null 41◦ 118.4 DSZ 2001188-09:38 null 42◦ 120.4 DSZ 2001257-04:45 null 39◦ 36.1 DSZ 2002236-18:40 48 ± 4 1.64 ± 0.18 70.0

KNZ 2001013-17:33 3 ± 11.5 1.76 ± 0.72 232.3 KNZ 2001174-20:33 null −59◦ 114.2 KNZ 2001285-05:02 null −50◦ 28.4 KNZ 2001332-14:32 null 73◦ 77.4 KNZ 2002108-16:08 48 ± 4.5 2.05 ± 0.34 71.6 KNZ 2002145-05:36 44 ± 5 1.95 ± 0.32 12.3

MQZ 2001174-20:33 48 ± 6 1.8 ± 0.55 118.0 MQZ 2001188-09:38 null 31◦ 119.9 MQZ 2001322-21:59 null −50◦ 303.5 MQZ 2002179-17:19 42 ± 2 1.69 ± 0.14 330.9

PWZ 2001145-00:40 23 ± 8 1.56 ± 0.42 340.0 PWZ 2001174-20:33 null 31◦ 114.8 PWZ 2001188-09:38 null 14◦ 116.8 PWZ 2002090-06:52 null 26◦ 311.0 PWZ 2002167-02:46 16 ± 6 2.12 ± 0.44 89.1

RPZ 2001174-20:33 null 4248◦ 119.2 RPZ 2001188-09:38 42 ± 8 1.8 ± 0.41 121.1

TOZ 2001048-20:11 null 37◦ 27.8 TOZ 2001188-09:38 null −75◦ 117.0 TOZ 2001285-05:02 null 37◦ 29.3 TOZ 2002148-16:45 −17 ± 4 1.4 ± 0.26 312.0

URZ 2001188-09:38 null 46 116.5

WHZ 2001174-20:33 null 24◦ 121.7 WHZ 2001188-09:38 21 ± 2.5 3 ± 0.56 123.6 WHZ 2002032-21:55 null 15◦ 331.5 WHZ 2002087-04:56 20 ± 5 2.9 ± 0.54 129.1 WHZ 2002169-13:56 null 36◦ 132.4 WHZ 2002212-00:16 null 30◦ 98.0

axis. In reality, multiple layers are likely (Audoine et al. 2004; ples used came from the two major geodynamic contexts studied Marson-Pidgeon & Savage 2004a) but the poor azimuthal coverage with shear wave splitting: the mantle wedge above a subduction hinders the determination of multiple anisotropic layer solutions or zone (North Island samples, Raglan) and at 200 km distance from a of dipping layers (Silver & Savage 1994). We further make the sim- large-scale strike-slip fault (South Island samples, Dunedin). They, plifying assumption that the anisotropy measured on the Raglan area therefore, provide information on the level of anisotropy we can ex- xenoliths is representative of the North Island lithospheric mantle, pect in the lithospheric mantle in both situations. While SKS split- that the one measured on the Dunedin area xenoliths is representa- ting integrates all anisotropic contributions, deep or shallow, under tive of the South Island lithospheric mantle and that the Chatham a station, Pn waves sample the anisotropy of a region just under the Island sample represents the area at which the station is located. This Moho where our xenoliths come from. assumption is not perfect since the tectonic regime probably varies Using these lithospheric mantle anisotropy values and our knowl- laterally. However, there are no other known xenolith’slocalities and edge of the present day lithospheric structure (Stern et al. 2000) and at least, it is a better assumption than using an average xenolith from deformation direction (Walcott 1998) along with data from previ- elsewhere in the world, as is done in most other studies. Also, we ous SKS and teleseismic S studies (Klosko et al. 1999; Marson- interpret the data in conjunction with previous SKS wave and local Pidgeon et al. 1999; Audoine et al. 2004) and Pn studies (Smith S-wave splitting results and Pn measurements. & Ekstrom 1999; Scherwath et al. 2002), we can infer an expected lithospheric fast direction and splitting time for the North Island and the Central South Island (Fig. 5). For the North Island, we infer IPetrophysics data and the source of the observed small lithospheric splitting (less than 0.8 s) assuming a lithospheric seismic anisotropy thickness of 70 km and 3.5 per cent intrinsic anisotropy with a The main purpose of the petrophysics study was to constrain the NW/SE extension parallel fast direction. For the South Island how- lithospheric contribution to the observed SKS splitting. The sam- ever, the lithospheric fast direction should be parallel to the Alpine

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170 175

Australian Plate -35 -40

Chatham Islands 5.0 % Auckland Pacific TOZ 3.5 % Plate URZ Macquarie Island FWVZ 160 180 KNZ

-40 PWZ

DSZ SNZO

MQZ Alpine Fault RPZ Banks Peninsula

2 secs -45 Fiordland Previous studies

This study

Nulls 5.0 % WHZ No measurements Analysed xenoliths

4 % Pn Anisotropy

Figure 4. Measurements of anisotropy in New Zealand on a map of active faults (thin lines). The bars show SKS splitting measurements obtained in this study (dark grey) along with results from previous studies (light grey; see text for references). The length of the bar is proportional to the observed SKS splitting time and its azimuth is the fast direction. The stars show stations where no reliable measurements were obtained. ‘Null’ measurements are represented by a cross at the station, showing the two possible fast direction that would yield no splitting. Pn measurements (Scherwath et al. 2002; Smith & Ekstrom 1999) are also shown. Xenoliths’ intrinsic S-wave anisotropy values are plotted on their origin area. The symbol for the Chatham Island corresponds to null measurements for all backazimuths.

Fault (∼N40◦–45◦) with large lithospheric splitting (1.5 s) given the linked to the Alpine Fault dextral shearing. This mode of deforma- thickness (∼150 km) of the lithosphere and its intrinsic anisotropy tion will tend to create a vertical foliation and a horizontal lineation (5 per cent). (Nicolas & Poirier 1976), thus a strong anisotropic medium for ver- Knowing the anisotropy we can expect from the lithospheric tically travelling waves. For the South Island, it is therefore likely mantle of New Zealand (Fig. 5), we can interpret the SKS and Pn that the maximum 5.0 per cent S-wave anisotropy we measured on anisotropy measurements (Fig. 4) and propose an anisotropy model the samples is representative of the lithospheric mantle anisotropy for the three areas of New Zealand we study. sampled by the SKS waves. Petrophysical studies conducted on Red Hill, Red Mountain and the Dun Mountain (South Island) permian ophiolites, which repre- II Continental transfrom: central South Island sent mantle material from underneath the South Island (Christensen In the South Island, the seismic anisotropy inferred from the 1984a,b), show a 4–5 per cent anisotropy at the scale of the massif, xenoliths is higher than in the North Island. The individual comparable to the results we obtained for the South Island mantle samples display a higher range of maximum shear wave polariza- xenoliths. The South Island lithospheric mantle seems thus more tion anisotropies, from 3.8 to 5.8 per cent. The average sample has anisotropic than the North Island’s one. a maximum shear wave polarization anisotropy of 5.0 per cent. In For the South Island samples, the P-wave anisotropy measured the case of the South Island, the major lithospheric deformation is is 6.9 per cent. The Pn anisotropy is in good agreement with Smith

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170 175 presented in this paper along with other studies (Klosko et al. 1999; Audoine et al. 2000; Koelher 2003) point toaN35◦–45◦ Efast direction for the entire width of the South Island. Second, local S waves -35 suggest a narrower zone of anisotropy in the Marlborough region above the subducting slab (Audoine et al. 2000). Finally, xenoliths suggest the lithospheric mantle intrinsic anisotropy, though higher than in the North Island, is unlikely to be over 5 per cent for the eastern part of the South Island. Average splitting time measured with shear wave splitting is close to 1.4 s for the whole central South Island. If we assume a 5 per cent 0.8 sec lithospheric mantle anisotropy based on the xenoliths’ seismic properties, a lithosphere thickness of 120 km is necessary. Stern et al. -40 (2000), using teleseismic P-wave delays, showed that a lithospheric ‘blob’ under the Southern Alps was likely, with lithospheric thicknesses over 150 km. Molnar et al. (1999) and Klosko et al. (1999) both linked the observed fast directions and SKS splitting times to a widespread deformation of the lithospheric mantle under a trans- 1.5 sec pressive tectonic regime. While the deformation is localized in the crust on the Alpine Fault, they inferred that, in the lithospheric mantle and/or the lower crust, the deformation must widen to at least the width of the South Island. However, the variation observed with Pn measurements and local S phases is clearly not matched by this

-45 model. Together Pn, local S and SKS data imply that the strain in the mantle is more consistent across the South Island at depth than in the shallower part of the lithospheric mantle. A recent comparative study on a number of large scale continental strike-slip faults showed that most of them present properties that could be explained by a large-scale ‘wrench-fault type’ fabric (Vauchez & Tommasi 2003), that is, a vertical foliation plane with Figure 5. Predicted lithospheric anisotropy direction and lithospheric split- a subhorizontal lineation direction. This type of fabric will create ting times from xenoliths’ data and tectonic information for the North Island a strong anisotropic medium, particularly for vertically travelling and the central South Island. waves like SKS, and can be widespread in the lithospheric mantle, even if the deformation is localized in the uppermost crust. Compar- & Ekstrom (1999) ∼8 per cent Pn anisotropy in the Marlborough ing the Alpine Fault to other studies on major continental strike-slip region with fast directions parallel to the Alpine fault trend (Fig. 4). faults, like the Red River Fault (Pham et al. 1995) or the Great Glen Scherwath et al. (2002) inferred a minimum estimate of 11 ± 3 per Fault (Helffrich 1995; Barruol et al. 1997), we propose that, be- cent P-wave anisotropy with the fastest direction parallel to the fault. cause of the Alpine Fault dextral shearing, a zone, narrow at Moho From this data they evaluated a 7 ± 3 per cent S-wave anisotropy depth but at least as wide as the entire South Island deep in the value, for the vicinity of the Alpine Fault on the Australian Plate lithospheric mantle, presents a ‘wrench fault type’ fabric. This fab- side (Fig. 4). ric could extend down to the asthenosphere, widening even further In a vertical foliation plane parallel to the Alpine Fault with with depth. The location of the deeper extent of the Alpine Fault an horizontal direction, the Pn measurements parallel to the is important to infer the location of likely maximum deformation. Alpine Fault should be close to the maximum P-wave velocity A number of recent studies using analysis of near surface deforma- measured in our xenoliths while Pn measurements perpendicular to tion pattern (Little et al. 2002) point to a 45◦ east-dipping Alpine the Fault should be close to the minimum P-wave velocity measured. Fault in the crust, which means the lower part of the crustal fault, Our maximum and minimum P-wave velocities of respectively at the Moho, is likely located around the centre of the South Island. 8.53 km s−1 and 7.99 km s−1 compare with Haines (1979) 8.3 km s−1 The Pn anisotropy measurements using SIGHT data on both sides P-wave velocity and Scherwath et al. (2002) maximum and mini- of the South Island are roughly at equal distance from the fault lo- mum P-wave velocities of 8.6 km s−1 and 7.7 km s−1, respectively, cation at the Moho and present a big difference. Our interpretation if the azimuth at which they were measured is taken into account. of this deformation inhomogeneity is that the Australian side of the The somewhat stronger anisotropy of Scherwath et al. (2002) could fault is deformed up to the Moho while the Paciﬁc side is not. simply be caused by a location closer to the Alpine Fault. Baldock The dynamic model we propose for the Central South Island (2004) studied SIGHT data (Stern 2002) offshore the eastern side upper mantle tectonics is derived from models by Molnar et al. of the South Island. There, the anisotropy measured with Pn waves (1999) and Vauchez & Tommasi (2003) and from the low Pn between Alpine Fault parallel and Alpine Fault perpendicular di- anisotropy on the eastern (Paciﬁc) side of the plate boundary. It rections is lower, 6 ± 2 per cent for the southernmost intersection is divided in three phases. An initial phase (Fig. 6a) started at 45 Ma (SIGHT Lines 2 and 3E) and virtually no anisotropy (0 ± 2 per cent) where all deformation occurred via strike-slip motion on a verti- for the northernmost intersection (SIGHT Lines 1 and 3E). cal proto Alpine Fault or transform fault. The deformation is lo- For the central part of the South Island, where the crustal defor- calized on the fault in the crust but distributes in the mantle to mation is mostly accommodated by the Alpine Fault and the other form a wide region of deformation. This period corresponds to the major strike-slip faults of the Marlborough region, we have three ma- age of extraction of the South Island xenoliths. The second phase jor sources of information. First, the SKS splitting measurements, (Fig. 6b) started with the change in motion of the boundary and the

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Phase 1 -45 to -7 Ma: strike slip motion -36o Dunedin volcanoes

A 1

Xenoliths 5% anisotropy

-38o Phase 2 -7 to -2 Ma: collision and possibly subduction

1 2 B

Fossil ? Active

174o 176o 178o

Figure 7. Modiﬁed from Audoine et al. (2004). Summary of the SKS and lo- Phase 3: -2 to present : collision and lower crust subduction cal S splitting measurements for the Central volcanic region. One can notice "Alpine Fault" the trench-parallel fast direction of the SKS splitting, almost perpendicular C to the extension parallel splitting from the local S phases.

3 1 2 Anisotropy varies with the orientation of the foliation and lin- Scherwath et al. high anisotropy Baldock et al. low anisotropy eation, which varies with the type of deformation considered and so varies with the geodynamic environment. However, independently of the deformation regime and the orientation of the foliation plane and lineation direction, the intrinsic anisotropy values measured in the xenoliths are not sufficient to explain the SKS splitting time mea- Figure 6. Cartoon showing the proposed dynamic model for the central sured through pure lithospheric deformation. P-wave anisotropy on South Island. The dark shaded areas show the wrench fault deformation the North Island samples averaged 5 per cent with maximum and zone. The light grey areas represent zones where wrench fault deformation −1 was (and might still be) active. See text for details on the dynamic model. minimum P-wave velocities being 8.48 and 8.05 km s , respectively. This is about twice the 2.7 per cent anisotropy value obtained by Smith & Ekstrom (1999) for the Central volcanic region. How- appearance of a compression component at 7 Ma, possibly even ever, averaging over many samples at non-optimal orientation, as as early as 20 Ma (Cande & Stock 2004), and corresponds to the occurs in in situ studies, is likely to give a lower value than the one earliest stage of continental collision. The deformation in the deep measured on the xenoliths, so the two values may be consistent. lithospheric mantle migrates westward, possibly associated with an Multiple sets of information about the North Island upper man- early stage of subduction. The final phase (Fig. 6c) represents the tle exist. First, SKS splitting measurements indicate that a trench most recent evolution of the South Island. The collision is now ◦ ◦ ◦ parallel fast direction, N30 –40 ,iswidespread from the East Coast well marked. The Alpine Fault dip is close to 45 (Little et al. (station PWZ) to the Waikato area (station TOZ). The splitting mea- 2002) and its lowest part has migrated more eastward. The defor- surements in stations TOZ and KNZ also show a backazimuth depen- mation area has widened further, but only reaches the Moho on the dency. Pn anisotropy and shear wave splitting analysis using local Australian side of the plate boundary, hence the difference in the phases suggest vertical and lateral variations in anisotropy (Audoine Pn anisotropy with high anisotropy on the Australian side (Scher- et al. 2004) (Fig. 7). The absence of good quality measurements in wath et al. 2002) and low or no anisotropy on the Pacific side. The the Taupo volcanic zone (Station FWVZ) might be linked to the high widespread deformation over the whole width of the South Island attenuation observed in this area (Satake & Hashida 1989; Salmon creates the constant deformation field observed in the SKS split- pers. comm.) or to complicated lateral variations in anisotropy. Fi- ting measurements. There is no clear indication for the lower limit nally, petrophysics data on the North Island xenoliths indicate that of the strike-slip deformation in the mantle, but if we admit a 5 per the lithospheric mantle intrinsic S-wave anisotropy is unlikely to be cent anisotropy of the lithospheric mantle, as derived from the xeno- over 3.5 per cent. liths, 150 km are needed in order to explain our average splitting A 3.5 per cent anisotropy implies that, in order to explain the times. average splitting time in the North Island of 1.2 s by lithospheric anisotropy alone, the lithosphere should be 150 km thick. Litho- sphere thickness has not been measured under the North Island but, III Subduction zones: Hikurangi and Fiordland in an extensional context, thicknesses up to that value are highly un- In the North Island, the individual samples from the Raglan area likely. For example, lithospheric thicknesses in the Basin and Range gave low shear wave polarization anisotropy; maximum values vary are less than 50 km (Beghoul et al. 1993) and have been evaluated as from 3.3 to 3.8 per cent and the average sample displayed a 3.5 per 80 km in the East African Rift (Artemieva & Mooney 2001). And for cent maximum. the stations close to the trench, as the subduction of the Hikurangi

C 2005 RAS, GJI, 163, 760–774 Mantle tectonics beneath New Zealand 769

Plateau operates, it is unlikely there is any thick sub-Australian Plate lithosperic mantle under them. S-wave anisotropy in the Australian lithosphere is, therefore, not enough to explain the observed splitting. Thus, we assume that most of the splitting is acquired in the asthenospheric mantle beneath the North Island and/or in or under the subducting slab. Trench parallel fast directions are recorded in many places both above and under subducting slabs (e.g. review by Fischer et al. 1998; Savage 1999). Two main causes are considered if the fast direction is assumed parallel to the flow. The first links trench parallel fast directions to a roll-back effect from the slab. In subduction zones where the slab pull is high, the subducting plate will tend to sink and push the mantle underneath sideways (Russo et al. 1996). The other possibility is that the slab simply acts as a rigid barrier to the mantle flow (Marson-Pidgeon et al. 1999). In New Zealand, the Hikurangi plateau, a cretaceous large igneous Figure 8. Cartoon showing the proposed model for the North Island. CVR province (Davy & Wood 1994; Wood & Davy 1994; Mortimer & stands for Central volcanic region. See text for details. Parkinson 1996) with a 12- to 15-km-thick crust, is being subducted along the Hikurangi trench. We cannot rule out the effect that the Hikurangi slab might have on our splitting measurements. There is Holtzman et al. (2003) analysed the effect of the presence of still a disagreement over the origin and formation of this plateau, but melt on deformation. They found that, for certain types of flow, most agree that it was once attached to the Manihiki plateau further through melt segregation and deformation partitioning, the direction North and was separated from it by a rifting process ultimately of polarization of the fast S wave could become perpendicular to leading to the now dead Osbourne ridge (Billen & Stock 2000). the flow direction. The dehydration from the slab produces fluids, The fast direction of oceanic lithosphere is generally perpendicular leading to partial melting of the mantle wedge (e.g. Stern 2002). If to the spreading ridge it originated from (Hess 1964). Therefore, the majority of the splitting is acquired in the mantle wedge, it is in the case of the Hikurangi plateau, since the original ridge is therefore possible that the trench parallel fast directions measured perpendicular to the Kermadec trench, the fast direction frozen in indicate in fact a trench perpendicular direction of flow, therefore the lithospheric mantle of the Hikurangi plateau is likely to display parallel to the extension direction. a trench-parallel azimuth, adding to the total splitting measured. Finally, while much of the splitting is acquired below the litho- Also, from a local earthquakes study (Matcham et al. 2000), trench sphere of the North Island, a lithospheric component of deformation parallel fast direction with a 4.4 per cent anisotropy was determined is likely, possibly linked to the Alpine Fault dextral movement and for the slab under the Wellington region. marked at the surface by the trace of the junction magnetic anomaly Another possibility for trench parallel fast direction resides in the (Hunt 1978). Audoine et al. (2004) note a more complicated pattern asthenospheric mantle wedge between the subducting and the over- of anisotropy measured from local earthquakes. Some of their vari- riding plates. Although flow processes in the mantle wedge are still ation is caused by high frequencies measuring crustal anisotropy. poorly understood (e.g. Stern 2002, for a review), some recent petro- Yet, their observations of splitting with fast direction perpendicu- logical data on a fossil mantle wedge section in Alaska (Mehl et al. lar to the trench under the western Central volcanic region suggest 2003) have shown clear indications for trench parallel flow within deeper mantle flow in the trench perpendicular fast direction in the the mantle wedge. Also, splitting studies on local S waves coming extension region (Fig. 7). Our results at station TOZ are consistent from the slab through the mantle wedge (Gledhill 1993; Gledhill & with this model. Stuart 1996; Audoine et al. 2004) have shown that trench parallel The model we, therefore, propose for the North Island upper fast polarization direction of S waves occurs in the mantle wedge mantle tectonics, following and confirming the work by Audoine beneath the central North Island. Classical interpretation would then et al. (2004), is a small lithospheric deformation of the Australian suggest trench-parallel flow in the mantle wedge. However, recent Plate, a mantle wedge trench parallel fast direction, indicating trench studies (Jung & Karato 2001; Holtzman et al. 2003) have concluded parallel or trench perpendicular flow under special conditions (high that, in presence of water or partial melt, the fast polarization di- water content and/or presence of melt), trench-parallel fast direction rection could be different from the flow direction. Jung & Karato in the oceanic slab and a trench-parallel asthenospheric flow beneath (2001) showed that a high water fugacity and high stress may change the subducting slab probably linked to the slab acting as a barrier to the olivine dominant glide system from (010)[100] to (010)[001], flow (Fig. 8). The large average SKS splitting time could therefore creating what the author called a B fabric, where the [100] axes of be linked to a succession of anisotropic layers with the same fast olivine are oriented perpendicular to the flow direction. Particularly direction. in the case of the mantle wedge, where fluids from the dehydration The measurements obtained in the Fiordland area are similar to of the slab are likely to be present, this mode of deformation has the ones in the North Island. They point to a N15◦–20◦ fast direc- to be considered. The B fabric observed by Jung & Karato (2001) tion and an average SKS splitting time of 2.5 ± 0.6 s. This large has since been evidenced in naturally deformed peridotites equili- splitting time can not be explained with a simple shearing fabric brated under high-pressure conditions in subduction environments in the lithosphere alone. Indeed, assuming an anisotropy of 5 per (Mizukami et al. 2004). However, none of the samples we analysed cent the lithosphere thickness will be over 200 km. Even if some of presented a B fabric, and numerical modelling of the development the splitting in that area is acquired through lithospheric deforma- of LPO in olivine aggregates predicts that trench-parallel flow is tion linked to strike-slip motion in Fiordland, the major part of the more likely than water in order to explain the observed anisotropic splitting must be acquired in the asthenosphere. The change in fast pattern in subduction zones (Kaminski 2002). direction, from N45 in Central South Island to N15 in Southern New

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Fiordland CONCLUSION WHZ The Australian-Pacific plate boundary in New Zealand presents a EFF South Island (Pacific Plate) paradox. Despite two different geodynamic situations, subduction Australian Plate under the North Island and Fiordland, strike-slip faulting in the centre, the fast directions measured using SKS splitting are parallel Wrench fault deformed area? to the major structures. Four possible sources of trench parallel anisotropy have been identified for the subduction zones in this study: the asthenospheric mantle both below and above the slab and Trench parallel flow the lithospheric mantle of the slab and of the overriding plate. Al- ? Trench parallel flow ? ? though we cannot quantify the respective contribution of each layer, the joint analysis of the petrophysics and of the shear wave splitting Figure 9. Cartoon showing the proposed model for southernmost New data allows us to conclude that in the North Island and in south- Zealand and Fiordland. EFF stands for East Fiordland Fault. Same as Fig. 6 ernmost New Zealand, the source of the anisotropy is likely to be for the dark shaded areas. See text for detailed information on this model. mainly asthenospheric. For the central South Island however, where strike-slip deformation is the main deformation process, the two possible sources of Alpine Fault parallel anisotropy are in the upper Zealand, has been attributed to a diminishing effect of the Alpine mantle, lithosphere and/or asthenosphere. Considering the xeno- Fault (Little et al. 2002; Savage et al. 2004). Yet the big SKS split- liths’ anisotropy and the modelled lithospheric thickness under the ting time measured cannot be explained by a simple diminution of South Island, we suggest that the Alpine Fault parallel anisotropy the Alpine Fault effect. Another pattern of deformation is therefore may result essentially from strike-slip (or transpressional) deforma- likely. The fast direction corresponds to the strike of the Puysegur tion in the lithospheric mantle. The model we propose takes into subduction zone. It is also parallel to the east Fiordland strike-slip account the tectonic history of the South Island and the variation fault. in dynamics with time of this plate boundary. The measured 5 per The station with the highest SKS splitting times measured, WHZ, cent anisotropy in the xenoliths implies that the depth extent of the sits almost exactly above the eastern edge of the Puysegur slab, wrench fault deformed area of the mantle must be 120 km to ex- imaged by (Eberhart-Phillips & Reyners 2001). Also, this station plain our average splitting times. Finally, in the case of the Chatham is close to the surface trace of the east Fiordland fault. Assuming a islands, this study fails to explain the observed absence of splitting trench-parallel asthenospheric flow around the slab, we can infer that given the observed xenolith’s anisotropy. the splitting measured here is high because of the combination of two deformation patterns displaying the same fast direction (Fig. 9). Finally, the backazimuth dependence observed in some North Is- land stations could have multiple origins. We cannot test for them ACKNOWLEDGMENTS because of the poor azimuthal coverage. However, we note that dif- The authors wish to acknowledge the support of the New Zealand ferent results appear for rays travelling in an east–west direction. The GeoNet programme for providing the seismic data, and from possible origins for such dependence include the dipping slab’slitho- Leonore Hoke (GNS and Victoria University) and Philippa Black spheric mantle, lateral inhomogeneities in the lithosphere and/or the (Auckland University) for lending us some of their thin sections asthenosphere and, possibly, a deeper signal linked to anisotropy in and rock samples. Financial support was provided by the Royal So- the lower mantle, as evidenced North of New Zealand (Wookey et al. ciety of New Zealand through the Marsden Fund and the French 2002), or in the D” layer (Kendall & Silver 1996, 1998; Hofmann ‘Ministère de l’éducation nationale, de l’enseignement supérieur et 2002). A lateral inhomogeneity effect could be tested using Favier de la recherche’ through their international collaboration scheme. et al. (2004) sensitivity kernels method. We thank Edouard Kaminski and an anonymous reviewer for their helpful comments. We acknowledge the useful discussions and contributions of our colleagues David Mainprice, Tim Stern, Sonja Hoffman, Etienne Audoine, Alain Vauchez and Leonore Hoke. The IV Chatham islands seismic processing was performed using the Seismic Analysing The S-wave splitting measurements at the Chatham Islands station Code (SAC) ((Tapley et al. 1990); http://www.llnl.gov/sac/). Some indicate that the mantle under the island is isotropic to vertically of the figures were produced using GMT (Wessel & Smith 2001). travelling waves. The measurement we performed on the xenolith indicate that the uppermost mantle is 5 per cent anisotropic. This situation has already been observed at a number of other places, REFERENCES such as Tahiti in French Polynesia and Reunion Island (Barruol & Hoffman 1999). There, anisotropy measurements on xenoliths in- Adams, R.D. & Hatherton, T., 1973. 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Russo, R.M., Silver, P.G., Franke, M., Ambeh, W.B. & James, D.E., 1996. might be an indication of equilibration at high temperature (Brey & Shear-wave splitting in Northeast Venezuela, Trinidad and the eastern Kohler 1990) or of some enrichment of the mantle by deep source Caribbean, Phys. Earth planet. Int., 95, 251–275. material. The observed high Al contents are typical of continental Satake, K. & Hashida, T., 1989. Three-dimensional attenuation structure mantle peridotites, confirming the continental character of the New beneath the North Island, New Zealand., Tectonophysics, 159, 181– Zealand lithosphere. The Chatham Island lherzolite NZ56CH, in 194. contrast, is characterized by more magnesian olivines (Mg# = 92.1) Savage, M.K., 1999. Seismic anisotropy and mantle deformation: what and no Al-enrichment in the pyroxenes, so the continental signature have we learned from shear wave splitting?, Rev. geophysics, 37(1), 65– is weaker, perhaps reflecting its position at the edge of the continental 106. shelf. Savage, M.K., Fischer, K.M. & Hall, C.E., 2004. Strain modelling, seismic All peridotites exhibit coarse-grained porphyroclastic mi- anisotropy and coupling at strike-slip boundaries: Applications in New Zealand and the San Andreas Fault, in Vertical Coupling and Decoupling crostructures characteristic of deformation by dislocation creep un- > ◦ in the Lithosphere, pp. 9–40, eds. Grocott, J., Tikoff, B., McCaffrey, K.J.W. der high-temperature, low-stress conditions (T 1100 C). Olivine &Taylor, G., Geological Society of London, London. porphyroclasts are usually up to 5 mm long and display curvilinear Scherwath, M., Melhuish, A., Stern, T. & Molnar, P., 2002. Pn anisotropy grain boundaries and well-developed and widely-spaced (100) sub- and distributed upper mantle deformation associated with a continental grain boundaries (Fig. A1). Their shape-preferred orientation marks transform fault, Geophys. Res. Lett., 29(8), 16-1–16-4. the foliation and lineation in spinel-lherzolites D104A and D105C. Silver, P.G. & Chan, W.W., 1991. Shear wave splitting and subcontinental Interpenetrating olivine-olivine grain boundaries indicate active mantle deformation, J. geophys. Res., 96, 16 429–16 454. grain boundary migration that, in some unusual samples (e.g. Silver, P.G.& Savage, M.K., 1994. The interpretation of shear-wave splitting parameters in the presence of two anisotropic layers, Geophys. J. Int., 119(3), 949–963. Smith, G.P. & Ekstrom, G., 1999. A global study of Pn anisotropy beneath continents, J. geophys. Res., 104, 963–980. Smith, W.D., 1971. Earthquakes at shallow and intermediate depths in Fiord- land, New Zealand, J. geophys. Res., 76, 4901–4907. Stern, R.J., 2002. Subduction zones, Rev. Geophysics, 40, 3-1–3-38. Stern, T.A., Molnar, P., Okaya, D. & Eberhart-Phillips, D., 2000. Teleseismic P-wave delays and modes of shortening the mantle beneath the South Island, New Zealand, J. geophys. Res., 105(B9), 21 615–21 631. Stern, T.A., Okaya, D.A. & Scherwath, M., 2002. Structure and strength of a continental transform from onshore-offshore seismic profiling of South Island, New Zealand, Earth, Planets, Space, 54, 1011–1019. Tapley, W.C.,Tull,J.E., Miner, L. & Goldstein, P., 1990. SAC2000 Command reference manual Version 10.5d, University of California. Tommasi, A., Mainprice, D., Canova, G. & Chastel, Y., 2000. Viscoplas- tic self-consistent and equilibrium-based modeling of olivine lattice preferred orientations: implications for the upper mantle seismic anisotropy, J. geophys. Res., 105, 7893–7908. Vauchez, A. & Tommasi, A., 2003. Wrench faults down to the astheno- Figure A1. High-temperature, coarse-grained porphyroclastic microstruc- sphere: Geological and geophysical evidence and thermo-mechanical ture in lherzolite D104A. The foliation and lineation (X) are marked by elon- effects, in Intraplate Strike-Slip Deformation Belts, pp. 15–34, eds. gation of olivine (Ol) porphyroclasts with well-developed and widely-spaced Storti, F., Holdsworth, R.E. & Salvini, F., Geological Society of London, (100) subgrain boundaries as well as grain boundary migration features. London. Enstatite (En) and spinels (Sp) are also labelled.

C 2005 RAS, GJI, 163, 760–774 Mantle tectonics beneath New Zealand 773

D106C), gives rise to cm-scale olivine grains (abnormal grain depth of 70–75 km. A classic continental geotherm (Fowler 1990) growth). Olivine neoblasts are usually smaller than 1 mm and do not links temperatures of 900◦C with depths over 70 km. This together display any clear substructure. Enstatite, diopside and spinel form with core–rim gradients suggest heating of the lithosphere beneath aggregates, which in some samples (D104A, D105C) are elongated New Zealand. parallel to the foliation. Large orthopyroxene crystals often display corroded shapes with olivine fillings. This corrosion is not linked APPENDIX B: SPLITTING METHOD to the basalt because the xenoliths display sharp clear-cut edges. This suggests that orthopyroxene corrosion is due to intramantel- The selection of the events is based on two main parameters: focal lic magma–rock reactions previous to extrusion (Kelemen et al. distance and magnitude. Only events occurring in a distance range 1992). from 80◦ to 130◦ from a station are considered. This allows us to The presence of spinel in all samples implies that they equilibrated avoid most of the phase interaction between SKS and SKKS waves at depths between 35 and 75 km (0.8–1.8 GPa, (O’Neill 1981)). and teleseismic S phases or secondary KS phases (like PKS, sSKS, Equilibrium temperatures are determined with clinopyroxene- etc.). The minimum magnitude considered is 5.8, which is small orthopyroxene geothermometers (Wells1977; Brey & Kohler 1990). given the level of noise on oceanic islands’ broadband data. Ninety- The North Island spinel-lherzolites show high equilibrium temper- eight events were considered for measurements, with the largest atures (Table 1). Average temperatures obtained using core compo- being an Mw 8.2 event from the Chile subduction zone (2001 June sitions range from 1000◦Cto1050◦C. Moreover, 9231 and 9225 23). Final selection is based on a qualitative examination of the show evidence for a heating event, with rim analyses indicat- signal to noise ratio and resulted in 21 events being considered ing equilibrium temperatures higher than 1150◦C. High equilib- for measurement. We use different band pass filters, ranging from rium temperatures (∼1050◦C) are also observed in South Island 0.04–0.1 Hz to 0.05–0.8 Hz. Despite heavy filtering, the signal to samples D120B and D106C, while samples D104A and D105C noise ratio remains low most of the time, limiting the number of present lower equilibrium temperatures, around 950◦C. No sys- measurements. tematic core–rim variation is observed in these samples. Finally, To evaluate S-wave splitting, we use the algorithm developed by the Chatham sample displays a low equilibrium temperature, under Silver & Chan (1991). This program assumes that, at a good signal 700◦C. Equilibrium temperatures of 1000–1150◦C obtained for the to noise ratio, the fast and slow S waves are identical apart from North and South Island xenoliths are high for a maximum origin their amplitude and the delay between them. The program rotates

SKKS SKKS sSKS sSKS pSKS pSKS SKS b SKS radial a radial

transverse transverse corrected radial

Z comp corrected transverse

event: 01013 Angle: 3˚ +/- 11 delay: 1.76 s +/- 0.72

c d

Figure A2. Example of ‘good’ quality splitting measurement using the Silver & Chan (1991) algorithm. (a) shows the original recording from the KNZ seismometer and the measurement window (grey area). (b) represents the uncorrected and corrected radial and transverse components. (c) shows the horizontal particle motion and the corrected waveforms. (d) shows the SKS splitting time and fast direction for the measurement, with a 95 per cent conﬁdence area (grey area). See text for details on the method.

C 2005 RAS, GJI, 163, 760–774 774 M. Duclos et al.

SKKS SKKS sSKS sSKS pSKS pSKS a SKS b SKS radial radial

transverse transverse corrected radial Z comp corrected transverse

event: 01174 Angle: -59˚ +/- 22.5 delay: 2.72 s +/- 3.38

c d

Figure A3. Example of a ‘null’ ranked measurement from the KNZ seismometer. The method and the plots are identical to Fig. A2. the seismograms into the back azimuth of the event, therefore, ob- particle motion and a small confidence area. In an isotropic case, taining the radial and transverse components, and performs a grid the result will show no energy on the transverse component before search of the splitting parameters (splitting time and fast direction) correction, a best fit fast direction perpendicular or parallel to the required in order to best remove the energy on the transverse com- initial polarization and an undetermined SKS splitting time. That ponent. The observed horizontal particle motion, elliptic in case of kind of result is called a ‘null’ (Fig. A3). If the initial polarization anisotropy, will appear linear after correction, if the chosen splitting of a wave is parallel or perpendicular to the fast or slow direction of parameters are good. The quality of the measurement is evaluated the anisotropic medium, the result will be a null despite the presence through different graphs (Fig. A2). One first compares the wave- of anisotropy. Thus, a null measurement indicates only that there is forms of the corrected radial and transverse components. Then, the no apparent anisotropy for that particular back azimuth. Multiple horizontal particle motion plots are compared. Finally, we look at null results are therefore needed, from different back azimuths, in the quality of the fit. After correction, a high quality measurement order to say that the mantle under a station is isotropic to vertically will have little or no energy on the transverse component, a linear travelling waves.

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