Central South Island Research Articles

Mapping the Moho beneath the Southern Alps continent‐continent collision, New Zealand, using wide‐angle reflections

A wide‐angle stack using all onshore‐offshore, and onshore seismic data from the SIGHT'96 experiment provides a direct three‐dimensional (3‐D) image of the Moho below the continental collision zone through the South Island of New Zealand. A bright reflector sequence (up to 4 s thick), the base of which coincides with the PmP modeled Moho, extends throughout most of the lower crust and bends downward asymmetrically beneath the Southern Alps. The crustal root reaches a maximum depth at ∼15 s (45 km), beneath the regional (−80 mGal) Bouguer gravity low and is flanked east and west by shallower Moho where the average crustal thickness is ∼20 km. The 3‐D structure of the crustal root in the central South island is moderately well resolved by the SIGHT'96 experiment.

Continental crust under compression: A seismic refraction study of South Island Geophysical Transect I, South Island, New Zealand

The 1996 South Island Geophysical Transect (SIGHT) active source seismic survey was designed to show the style of lithospheric thickening due to late Cenozoic oblique convergence across the Australian‐Pacific plate boundary in New Zealand. As part of this study, two seismic refraction lines were shot across central South Island and offshore extensions of the continental crust in the Tasman Sea and Pacific Ocean. We present the data and a 603 km long seismic velocity profile of the crust and uppermost mantle along one of these seismic transects. A tomographic inversion of 62,563 travel times from crustal and upper mantle refractions and wide‐angle reflections resulted in a model with a two‐layer crust. Upper crustal velocities were between 5.9 and 6.3 km/s, and lower crustal velocities were between 6.5 and 7.0 km/s. Continental compression has locally reduced the seismic velocities in the Pacific plate crust by 0.2–0.3 km/s, a possible effect of high strain and fluids in the crust. The thickening of the crust from 28 km at the east coast of South Island to 37 km beneath the Southern Alps can account for about 25% of the 80–110 km shortening of Pacific plate crust, while the rest must be accounted for by rapid erosion of Mesozoic sedimentary rocks on the west side of the orogen. In our model the lower crust forms a continuous 2–6 km thick layer beneath central South Island. The asymmetric topography of the Southern Alps is reflected in the crustal root which has a steeper flank at the west coast. This observation is consistent with westward underthrusting of Pacific lower lithosphere beneath South Island that has been suggested in earlier studies.

The Alpine Fault biogeographic hypothesis revisited.

The New Zealand Alpine Fault is a major tectonic feature located on the mutual boundary of the Australian and Pacific Plates which is hypothesized to have undergone some 470 km of right-lateral displacement. The Nelson and Westland provinces have moved north-east relative to the rest of the South Island. Many plant and animal taxa show a conspicuous distribution gap in the central South Island, and traditionally this has been explained by glacial extirpation of the central populations. However, the gap is usually filled by a closely related taxon. Furthermore, the taxa involved occupy a wide ecological range, and include intertidal algae, shorefishes, lowland trees, and alpine herbs and insects that thrive around glaciers. The Alpine Fault biogeographic hypothesis [Heads, M., 1998a. Biogeographic disjunction along the Alpine Fault, New Zealand. Biol. J. Linn. Soc. 63, 161-176; Heads, M., 1998b.Coprosma decurva (Rubiaceae), a new species from New Zealand. NZ J. Bot. 36, 65-69.], based on track and phylogenetic analyses, proposes that the disjunction has instead been caused by movement on the fault pulling apart plant and animal populations. Wallis and Trewick (2001) [Wallis, G.P., Trewick, S.A., 2001. Finding fault with vicariance: a critique of Heads (1998). Syst. Biol. 50, 602-609.] provided a critique of this idea and pointed out what they felt were nine problems with it. These problems are answered here and shown to result from Wallis and Trewick's misunderstanding of aspects of geology and biology involved in the hypothesis. In particular, they have confused the age of inception of the fault with the age of movement along it, and, by neglecting the related central taxa in the "gap", have assumed the biogeographic hypothesis to be an uninformative two-area statement, whereas in fact it is an informative three- or four-area statement.

Intermediate depth earthquakes beneath Nelson, New Zealand, and the southwestern termination of the subducted Pacific plate

There has been debate on how convergence of mantle lithosphere is accommodated during continental collision in the central South Island of New Zealand–whether by intracontinental subduction or continuous deformation of mantle lithosphere. Intermediate depth earthquakes are sparse beneath the central South Island, and abundant seismicity 130–230 km deep within the Hikurangi subduction zone to the northeast ends abruptly beneath the Nelson region of the northern South Island. This termination of seismicity has previously been interpreted as marking the southwestern edge of the subducted slab. Here we investigate the stress and strain regime at this termination of activity, and find that it is very similar to that within the subducted plate both updip and along strike towards the northeast. Our results are consistent with subduction continuing further southwest, albeit largely aseismically, as suggested by shallow earthquakes in the northeastern South Island.

Strain modelling, seismic anisotropy and coupling at strike-slip boundaries: applications in New Zealand and the San Andreas fault

Abstract Despite similar surface transform faulting behaviour, observed shear-wave splitting patterns in the California and New Zealand plate boundary regions are markedly different. To better understand the origin of the seismic anisotropy in these regions we model mantle flow and strain for a variety of strike-slip plate boundary scenarios. Simple relations between the flow or strain and elastic anisotropy are assumed to determine the integrated splitting in shear particle motion along teleseismic paths. Strain-controlled models fit the observations in New Zealand and California better than simplified flow-controlled models. Fast shear polarizations are progressively rotated toward the shear plane over time, and even a constant-viscosity model provides a good fit to the fast directions in New Zealand and southern California. The constant viscosity implies strong coupling between the surface and the deeper mantle. To fit the lack of decrease in delay times with distance from the fault, the relationship between delay time and strain must saturate at small strains. If this is the case, then strain in southern New Zealand and southern California may be small, equivalent to that achieved along an infinite fault by about 3–10 Ma of their present motion. Stratified viscosity allows more rapid rotation of fast directions toward fault-parallel than occurs in isoviscous models, and can explain the nearly fault-parallel fast directions in the central South Island. Different aspects of the northern California results are fitted with different models, but a rapid change in viscosity with depth is needed to produce the full effects of the behaviour previously modelled as two layers of anisotropy suggesting vertical decoupling.

Diet of mammalian predators in braided river beds in the central South Island, New Zealand

In New Zealand, five of the six endemic bird species that breed primarily in South Island braided river beds are classed as threatened. A major cause of decline for these species is predation by introduced mammals, and predator-trapping programs are undertaken in the braided rivers of the Mackenzie Basin to protect them. Trapping programs carried out between September 1997 and April 2001 provided the opportunity to investigate predator diet from the gut contents of 375 cats (Felis catus), 371 ferrets (Mustela furo) and 86 stoats (Mustela erminea). As a percentage frequency of occurrence of the main prey items, cat diet consisted of lagomorphs (present in 70% of guts), birds (in 47%), lizards (30%) and invertebrates (36%). Ferret diet consisted of lagomorphs (69%) and birds (28%). Stoat diet consisted of lagomorphs (50%), birds (51%), lizards (21%) and invertebrates (23%). The frequency of occurrence of birds in all three predators was higher in the spring/summer of 1997 – immediately after rabbit haemorrhagic disease (RHD) was introduced – than in any other previous diet study on these braided rivers. This suggests that RHD did lead to increased predation pressure on birds, at least in the short term.

A new species of Galaxias (Teleostei: Galaxiidae) from the Mackenzie Basin, New Zealand

Galaxias macronasus n. sp. is described from three small upper tributaries of the Waitaki River System, Mackenzie Basin, central eastern South Island. It is distinctive in having a prominently rounded snout profile, with eyes well below profile, only 12–14 caudal fin rays, and 4–6, usually 5, pelvic fin rays. Morphologically, the new species most closely resembles G. paucispondylus and G. divergens which, with G. prognathus and G. cobitinis, are labelled the “pencil galaxias” owing to their elongate, slender, pencil shape. G. macronasus resembles these species in features of osteology of the cranium, suspensorium, and pectoral girdle. Gene sequencing strongly supports distinct taxonomic status of the new form, but provides a different perspective on its phylogenetic relationships. This conflict remains unresolved.

Intermediate-Depth Earthquakes in a Region of Continental Convergence: South Island, New Zealand

It is rare to find earthquakes with depths greater than 30 km in continent–continent collision zones because the mantle lithosphere is usually too hot to enable brittle failure. However, a handful of small, intermediate-depth earthquakes (30–97 km) have been recorded in the continental collision region in central South Island, New Zealand. The earthquakes are not associated with subduction but all lie within or on the margins of thickened crust or uppermost mantle seismic high-velocity anomalies. The largest of the earthquakes has M_L 4.0 corresponding to a rupture radius of between 100 and 800 m, providing bounds on the upper limit to the rupture length over which brittle failure is taking place in the deep brittle–plastic transition zone. The earthquake sources may be controlled by large shear strain gradients associated with viscous deformation processes in addition to depressed geotherms.

Carbon storage along a stand development sequence in a New Zealand Nothofagus forest

Indigenous forests cover about 6.4×10 6 ha or 24% of the land area of New Zealand and thus form a major carbon (C) pool, but there are few estimates of C storage for these forests, especially at different stages of stand development. C storage and distribution in stemwood, coarse woody debris, forest (floor litter (L) + fermentation /humus(FH)-layers) and upper (0–0.1 m) mineral soil were measured along a stand development sequence in montane zone mountain beech ( Nothofagus solandri var. cliffortioides) forest in the Craigieburn Range, central South Island. C storage in all pools peaked in 125-year-old pole stands (219 Mg/ha) and declined as the stands matured (>150 years old, 192 Mg/ha), and were replaced by seedling stands (10 years old, 152 Mg/ha) following windthrow, to reach a low point (114 Mg/ha) in 25-year-old sapling stands. Coarse woody debris provided an important pool for C storage following catastrophic forest disturbance, and varied reciprocally with stemwood along the development sequence. Above-ground components showed a much greater range (88 Mg/ha) with stand development than forest floor and mineral soil components (16 Mg/ha). Stemwood and coarse woody debris components varied by 136 and 76 Mg/ha, respectively, while litter, the FH-layer and upper mineral soil C varied by 4, 17, and 12 Mg/ha, respectively. The sum of forest floor and mineral soil C ranged between 53 Mg/ha (sapling stage) and 69 Mg/ha (pole stage), and did not differ significantly with age. The litter and FH-layers, and mineral soil were re-sampled after a period of 8 years in the sapling and pole stands. C storage in the litter layers did not change significantly over this period. C storage in the FH-layer declined significantly by 9.7 Mg/ha in the pole stage, but showed little change in the sapling stage. It is hypothesised that C loss resulting from ongoing decomposition in the FH-layer was balanced by continuing inputs of coarse woody debris in the sapling stage, but not in the pole stage, where the coarse woody debris resource was relatively small. There was no change in mineral soil C storage over the 8-year period. It is concluded that, in the absence of disturbance, C storage in the forest floor may vary substantially over periods of <10 years, in contrast to the mineral soil where it appears relatively stable over decadal time intervals.

Relation between surface velocity field and shear wave splitting in the South Island of New Zealand

The relative motion between the Pacific and Australian plates across the central South Island of New Zealand, as measured by repeated GPS measurements, is distributed over a region more than 80 km wide. The surface distribution of velocity is fit almost equally well by a model in which deformation of the lower lithosphere is concentrated into very narrow slip zones below one or two major faults and by a model in which the lower lithosphere deforms in a broad shear zone. Observations of the splitting of SKS waves permit distinction between the two models on the basis of the finite strain they predict. Deformation in the deep lithosphere that was limited to narrow shear zones would not produce measurable splitting; hence any splitting observed should not correlate with surface tectonics. However, the distribution of fast propagation directions revealed by observations of shear wave splitting do reflect surface tectonics. In particular, the orientations of fast propagation directions are consistent with the finite strain calculated by assuming that the distribution of velocity at depth within the lithosphere resembles the distribution of velocity measured at the surface by GPS. The observed distribution of splitting can be reproduced with an RMS misfit of 15°, provided that the velocity field is integrated over at least the time interval since significant convergence across the Southern Alps began at about 6.5 Ma.

Regional endemism in New Zealand grasses

Regional endemism is evident in 60 grass taxa among the 182 species and infraspecific taxa that comprise the New Zealand endemic grass flora. The pattern of regional endemism matches that described earlier for dicotyledons with high frequencies in Nelson‐Marlborough of northern South Island, and in southern South Island in Otago, Southland, and Fiordland. Low frequencies in southern North Island and central South Island are also consistent with patterns earlier described. In northern North Island grass endemism is low, consistent with the long history of forests in that region. Habitat preference is evident for most regional endemics with rupestral and edaphic constraints dominating. Taxonomic rank is the first imperative in establishing status as a regional endemic. Disjunctions fragment species into genetically isolated populations with their own evolutionary potentialities. These populations, unadorned by taxonomic rank, are treated as simple species disjunctions unlike those supported by an infraspecific taxonomic foundation that qualify for regional endemism. This distinction seems biologically inconsistent. Regional grass endemics conform in chromosome number, reproductive biology, and habitat selection to other congeneric species except for rare departures. A comparison of the frequencies in regional distributions of endemic Gramineae and endemic herbaceous Compositae showed a high level of agreement, unlike that with endemic Cyperaceae. Among the tribes of New Zealand Gramineae, Agrostideae have a lower ratio of regional endemics:total endemics than in tribes Poeae and Danthonieae, primarily because of the lack of coherent morphological‐geographic patterns in species of wide amplitude.

Fluid generation and pathways beneath an active compressional orogen, the New Zealand Southern Alps, inferred from magnetotelluric data

Forty‐one wideband magnetotelluric (MT) soundings were collected in a 150‐km‐long transect across the Southern Alps of the central South Island of New Zealand, an active compressional orogen. Decomposed MT impedance tensors, vertical magnetic field relations, and reconnaissance soundings at two locations off line imply an approximately two‐dimensional geometry here with average regional geoelectric strike of ∼N40°E, similar to surface geologic trends. Two independent, two‐dimensional inversion algorithms were applied to the MT data, and both imply a concave‐upward (U‐shaped), middle to lower crustal conductive zone beneath the west central portion of the island. The average conductivity of this zone in the strike direction appears to be much higher than that required across strike and may represent anisotropy or along‐strike conductive strands narrower than the transverse magnetic (cross‐strike) mode MT data can resolve. The deep crustal conductor under the Southern Alps is interpreted to represent mainly a volume of fluids arising from prograde metamorphism within a thickening crust. Fluid interconnection and electrical conduction are promoted by shear deformation. The conductor rises northwestward toward the trace of the Alpine Fault but attains a near‐vertical configuration at a depth of ∼10 km and reaches close to the surface 5–10 km inland of the fault trace itself. The transition to vertical orientation at this depth is interpreted to occur as fluids ascend across the brittle‐ductile transition in uplifting schist and approach the surface through induced hydrofractures. The high‐grade schist becomes resistive after depletion of fluids and continues to extrude toward the Alpine Fault. Shallow extensions of the deep high conductivity are coincident with modern, hydrothermal veining and gold mineralization interpreted to be of deep crustal provenance. To the southeast, high conductivity also reaches the surface coincident with a major back thrust fault zone of the doubly vergent Southern Alps orogen, which also exhibits evidence for expulsion of high‐temperature fluids. The higher conductivity inferred along strike (possible anisotropy) could reflect more efficient fluid interconnection in this higher‐strain direction, as well as possible contributions by sheared, fluid‐deposited graphite. Conductivity of the uppermost mantle of the South Island is low, consistent with advection of cold mantle lithosphere into the underlying asthenosphere as suggested by P wave delay studies.

Mitochondrial DNA phylogenetics of the Galaxias vulgaris complex from South Island, New Zealand: rapid radiation of a species flock

Mitochondrial control region sequence variation was examined in the Galaxias vulgaris complex, a group of freshwater‐limited galaxiid fishes endemic to South Island, New Zealand. Phylogenetic analyses were used to test the monophyly of seven non‐migratory (G. vulgaris complex) and one migratory (G. brevipinnis) species. Newly‐described taxa, some diagnosed on the basis of subtle differences in morphology, are associated with strongly monophyletic mtDNA lineages. Although the current taxonomy is supported generally, sequence data suggest that Southland G. anomalus should be attributed to G. gollumoides, whereas Southland and Stewart Island G. depressiceps represent a new southern lineage. Molecular clock calibrations suggest that migratory and non‐migratory forms, separated by a maximum of 6·4% sequence divergence, diverged no earlier than the mid‐late Pliocene. Biogeographical implications of high diversity (eight lineages) in the south (Otago‐Southland) and low diversity (one lineage) in central South Island (Canterbury) are discussed. The unresolved relationships among species may reflect a rapid evolutionary radiation, essentially a star phylogeny.

A focused look at the Alpine fault, New Zealand: Seismicity, focal mechanisms, and stress observations

The Alpine fault is the Pacific‐Australian plate boundary in the South Island of New Zealand. This study analyzes 195 earthquakes recorded during the 6 month duration of the Southern Alps Passive Seismic Experiment (SAPSE) in 1995/1996 and two ML 5.0 earthquakes and aftershocks in 1997, which occurred close to the central part of the Alpine fault. Precise earthquake locations are derived by simultaneous inversion for hypocenter parameters, a one‐dimensional velocity model, and station corrections. Together with focal mechanisms calculated using a first motion and amplitude ratio method, these results provide a picture of the seismotectonics in the central South Island over a 6 month period. Moment tensor inversions of three earthquakes provide an independent means of comparison to the focal mechanisms derived using the amplitude/first motion method. To validate our observations over time, we compare the SAPSE seismicity with the seismicity recorded by the New Zealand National Seismic Network (NZNSN) and a local network at Lake Pukaki east of the Southern Alps (6 months versus 8 years). Our study indicates that the Alpine fault releases elastic strain seismically from the surface down to 10–12 km depth between Milford Sound in the south and the Hope fault in the north. The seismicity rate of the Alpine fault is low but comparable to locked sections of the San Andreas fault, with large earthquakes expected. Seismicity decreases north of Bruce Bay at the Alpine fault and within a triangular region along the Alpine fault located between the Hope and Porters Pass fault zones. We interpret this as the result of deformation distributed on the Alpine fault and the Hope and Porters Pass fault zones. The base of the seismogenic zone is fairly uniform at 12 km ± 2 km over large parts of the South Island. The high Alps region has a shallower base of the seismogenic zone, indicating localized elevated temperatures east of the Alpine fault. Most of the study region deforms under a uniform stress field with a maximum principal horizontal shortening direction of 110°–120°, similar to geodetic observations and plate motions. This confirms that the region is not undergoing strain partitioning. The earthquake data show that the deformation away from the Alpine fault is distributed on mainly NNE trending thrust faults and strike‐slip transfer faults with a maximum seismogenic depth of 12 km.

Genetic diversity in New Zealand Galaxias vulgaris sensu lato (Teleostei: Osmeriformes: Galaxiidae): a test of a biogeographic hypothesis

AimTo test whether a geologically recent region with low endemism shows low genetic diversity in a freshwater fish species complex, in contrast to an adjacent area with high diversity. The low endemism is generally postulated to be the combined effect of Pleistocene glaciations and the unstable nature of the recent gravel outwash from young mountains to the west. This rock aggradation has formed a plain, crossed by several braided rivers. The faunas of these rivers have the potential to interchange through flooding or course changes.LocationSouthern and central South Island, New Zealand (NZ). We define the boundary of these two biogeographical regions as the Waitaki River, the most southerly braided river. To the north lies the Canterbury Plain, formed from coalesced alluvial fans; to the south is the Otago peneplain and associated mountain ranges.MethodsPrevious work has revealed considerable genetic structuring in the stream‐resident fish Galaxias vulgaris Stokell sensu lato, including a species complex in the southern region of South Island, NZ. Using isozyme electrophoresis, we have now examined genetic variation among a total of 58 samples of non‐migratory river galaxias from 24 river systems in South Island: 16 in the central region and eight to the south.ResultsWhereas samples from the central region do exhibit significant genetic structuring (Wright’s FST=0.331), they are more genetically homogeneous (mean Nei’s D= 0.032) than populations to the south of the Waitaki River (FST=0.826, mean D=0.374).Main conclusionsThe results concur with our expectations of differences between the central and southern regions. We suggest that the divergence in the south reflects geological stability, whereas relatively shallow divergences within the central region reflects the more recent formation of the Canterbury Plain and its unstable braided rivers. The genetic structuring of this group has been promoted by the absence of the marine juvenile phase that is found in five other members of the genus native to NZ. This diversity of the southern region mirrors its diverse flora and invertebrate fauna, and has conservation implications that parallel those resulting from our improved knowledge of the NZ herpetofauna through the application of genetic analysis.

Scree weta phylogeography: Surviving glaciation and implications for Pleistocene biogeography in New Zealand

The Pleistocene glaciation is thought to have had a profound impact on the distribution of endemic biota. The intraspecific phylogeography of the alpine‐adapted scree weta, Deinacrida connectens Ander, was surveyed throughout its range in the South Island, New Zealand using mitochondrial cy‐tochrome oxidase I DNA sequence data. Seven distinct genetic lineages were evident from mtDNA haplotypes, with each occupying mountain ranges in discrete geographic regions. Genetic distances among lineages were up to 8.2%, whereas within‐lineage distances reached only 2.8%. The inferred age of lineages and the striking phylogeographic structure exhibited by D. connectens indicates that it radiated in response to Pliocene mountain building. Maintenance of this structure is likely to relate to the combined effects of mountain‐top isolation during Pleistocene interglacials and ice barriers to dispersal during glacials. Two lineages are endemic to the central South Island, an area regarded as species poor due to glacial‐extirpation of much of the biota. It appears that D. connectens survived across much of the South Island in a mosaic of ecological, rather than one or few, regional refugia. The Pleistocene biogeography of New Zealand in general is discussed in the light of this hypothesis.

Seismic and geodetic constraints on plate boundary deformation across the northern Macquarie Ridge and southern South Island of New Zealand

SUMMARY In the southern New Zealand region the boundary between the Australian and Pacific plates changes from oblique continental convergence in the central South Island of New Zealand to interoceanic transcurrent motion along the Macquarie Ridge. Source parameters of recent (post-1964) upper crustal events, Mw≥5.3, in the region 43–53°S, 155–175°E are determined using P- and SH-waveform analysis in order to investigate the nature of strain accommodated seismically throughout the region. Repeated occupation of three geodetic networks across the southern South Island provides estimates of contemporary crustal strain. Seismicity over the last 35 years shows uneven spatial and temporal distribution, with clusters of events occurring in regions along the plate boundary that exhibit structural variation. Focal mechanisms of individual earthquakes vary widely but are consistent with the accommodation of long-term estimates of the relative plate motion. Across the southern South Island, geodetically measured principal contraction directions are consistent with P-axis orientations. The mode of accommodation of the relative plate motion varies throughout the region, influenced by the change in crustal composition on either side of the plate boundary, the underlying structure as a relic of previous relative motions, and the variation of obliquity of the relative motion with respect to these structures.

The enigma of the Arthur's Pass, New Zealand, earthquake: 1. Reconciling a variety of data for an unusual earthquake sequence

The 1994 Arthur's Pass earthquake (MW6.7) is the largest in a recent sequence of earthquakes in the central South Island, New Zealand. No surface rupture was observed, the aftershock distribution was complex, and routine methods of obtaining the faulting orientation of this earthquake proved contradictory. We use a range of data and techniques to obtain our preferred solution, which has a centroid depth of 5 km, M0=1.3 × 1019 N m, and a strike, dip, and rake of 221°, 47°, 112°, respectively. Discrepancies between this solution and the Harvard centroid moment tensor, together with the Global Positioning System (GPS) observations and unusual aftershock distribution, suggest that the rupture may not have occurred on a planar fault. A second, strike slip, subevent on a more northerly striking plane is suggested by these data but neither the body wave modeling nor regional broadband recordings show any complexity or late subevents. We relocate the aftershocks using both one‐dimensional and three‐dimensional velocity inversions. The depth range of the aftershocks (1–10 km) agrees well with the preferred mainshock centroid depth. The aftershocks near the hypocenter suggest a structure dipping toward the NW, which we interpret to be the mainshock fault plane. This structure and the Harper fault, ∼15 km to the south, appear to have acted as boundaries to the extensive aftershock zone trending NNW‐SSE. Most of the ML ≥ 5 aftershocks, including the two largest (ML6.1 and ML5.7), clustered near the Harper fault and have strike slip mechanisms consistent with motion on this fault and its conjugates. Forward modeling of the GPS data suggests that a reverse slip mainshock, combined with strike slip aftershock faulting in the south, is able to match the observed displacements. The occurrence of this earthquake sequence implies that the level of seismic hazard in the central South Island is greater than previous estimates.

Upper mantle anisotropy in the New Zealand Region

Shear‐wave splitting parameters of fast polarization direction (Φ) and delay time (δt) are determined using data from the Southern Alps Passive Seismic Experiment (SAPSE), on the South Island of New Zealand and in the surrounding region. Our results clearly show that Φ are subparallel to trends of the Alpine and Marlborough Faults, and to the Pacific‐Australian plate boundary. The δt values range from 0.6–2.2 s with an average value of 1.6 s; the largest values are from the central South Island. The main source of the observed shear‐wave splitting is an anisotropic region between 40–400 km. The width of the zone is approximately 200 km. We attribute the coincidence of surface structural trends with the measured Φ, and the large δt values, to significant shear deformation in a 200 km thick zone along the plate boundary extending from the surface to deep within the upper mantle.

A Retrospective Analysis of the Establishment and Dispersal of the Introduced Australian Parasitoids Xanthopimpla rhopaloceros (Krieger) (Hymenoptera: Ichneumonidae) and Trigonospila brevifacies (Hardy) (Diptera: Tachinidae) within New Zealand

Two Australian parasitoids, Xanthopimpla rhopaloceros (Krieger) and Trigonospila brevifacies (Hardy), were introduced to New Zealand to control the light-brown apple moth (Epiphyas postvittana Walker). Dispersal by the parasitoids has since occurred naturally and with the aid of releases in fruit-growing areas. The present geographical range of the parasitoids includes all the North Island and some offshore islands to latitude 41 20 S. X. rhopaloceros is also present to latitude 41 48 S in the South Island. Comparisons of these distributions with those in Australia indicate that climatic conditions may have played a major role in the areas of establishment of both species in New Zealand. The mean winter temperature may be a limiting factor in the dispersal of T. brevifacies and X. rhopaloceros in New Zealand. Other factors that have probably aided the successful dispersal of the parasitoids include the wide distribution of host Tortricidae and the occurrence of tortricid host plants. The areas of New Zealand that appear suitable for further colonization by T. brevifacies include northern areas of the South Island, and both parasitoids could disperse further into suitable climate areas of the east and west coasts of the central South Island. The rate of dispersal for X. rhopaloceros was estimated at 13-24 km/year, and for T. brevifacies at 8-15 km/year.