Haast Schist Research Articles

Sediment provenance in the Murchison and Maruia basins, Aotearoa/New Zealand: a record of Neogene strike-slip displacement, convergence, and basement exhumation along the Australian–Pacific plate boundary

ABSTRACT The Murchison and Maruia basins are situated on the Australian Plate adjacent the Alpine Fault, an ideal place to study Cenozoic Australian–Pacific plate boundary evolution. Sandstone provenance was investigated using petrographic and detrital zircon U–Pb geochronologic methods and primarily varies geographically. Secondary up-section variation tracks spatiotemporal changes in basement exhumation. Eocene–middle Miocene western Murchison Basin sandstone is arkosic, derived locally from Devonian–mid-Cretaceous granitoids. Early Miocene–?Pliocene eastern Murchison and Maruia basin sandstone is lithic-/plagioclase-rich, reflecting a mixed provenance: proximal Carboniferous–mid-Cretaceous granitoids, and distal mid-Cretaceous Median Batholith and Permian–Triassic Eastern Province metasedimentary sources in the Pacific Plate. This confirms early Miocene Pacific Plate exhumation during initial Alpine Fault evolution began in northern Fiordland/western Otago but the contrast between eastern and western Murchison Basin indicates limited westward sediment dispersal. Higher proportions of Brook Street Terrane (BST) detrital zircon in middle Miocene eastern Murchison Basin sandstone suggests accelerated Pacific Plate exhumation at c. 15 Ma. By c. 10 Ma, the eastern Murchison Basin BST source had switched to the Australian Plate due to Waimea–Flaxmore fault reverse reactivation and dextral plate boundary strike-slip displacement, which by the late Miocene/Pliocene led to Rakaia Terrane-affinity Haast Schist replacing Caples Terrane as the main Maruia Basin sediment source.

Detrital zircon age studies of Haast Schist in western Otago and Marlborough, New Zealand: constraints on their protolith age, terrane ancestry and Au–W mineralisation

Detrital zircon U–Pb ages are reported from the Haast Schist in western Otago and Marlborough, South Island, New Zealand. The majority are from the Aspiring Lithological Association in Otago, which intervenes between Caples Terrane and Rakaia Terrane protoliths. These indicate provenances similar to those in established Jurassic and Late Triassic sandstones of the Waipapa Composite Terrane in the North Island. The populations are dominated (>60%) by Early Jurassic, Triassic and Permian zircons with only small proportions of pre-Permian zircons. In contrast, schists east of the Aspiring Lithological Association have substantial proportions (>20%) of early Paleozoic–Precambrian zircons and are thus assigned to the Triassic–Permian, Rakaia Terrane (Torlesse Composite Terrane), which is extensive across eastern Otago. However, three small areas of anomalous schist, with dominantly (80%) Carboniferous zircons, are considered the oldest level of the Rakaia Terrane protolith. Similar datasets from western Marlborough Schist confirm the correlation with western Otago Schist and are also assigned to the Waipapa Composite Terrane. The new Haast Schist datasets support a suspect terrane model in which both Rakaia and Waipapa Composite terrane protoliths have sediment provenances in eastern Australia and Au–W sources at their subsequent mineralisation sites. KEY POINTS Original protoliths are recognisable within the Haast Schist of New Zealand on the basis of distinctive detrital zircon age patterns. The Aspiring Lithological Association (Otago Schist) of western Otago, is correlated with a similar belt of schistose rocks (Marlborough Schist) in western Marlborough, and both are assigned to a Waipapa Composite Terrane protolith of Jurassic to Late Triassic age. The Jurassic-Late Triassic sandstones within Waipapa Composite Terrane are dominantly of granitoid provenance in eastern Australia. Detrital zircons of Carboniferous age in the Waipapa Composite Terrane support a possible New England Orogen provenance in northeastern NSW and southeastern Queensland, and also indicate possible primary sources for the gold and tungsten in the western Otago Schist and western Marlborough Schist. This result is in marked contrast to eastern Otago Schist where the protolith is Rakaia Terrane of Late Triassic age and whose provenance is in central-eastern Queensland.

Cretaceous molybdenite in metasomatic epidosite associated with the Pounamu ophiolite, New Zealand

ABSTRACTEpidosites occur in association with pelagic and quartzofeldspathic metasediments overlying the Pounamu suprasubduction zone ophiolite. They are dominated by epidote with minor diopside, tremolite, titanite, zircon and quartz. Detrital zircon age spectra are similar to those of associated quartzofeldspathic schist, with minimum ages of Cretaceous (c. 106 Ma). Epidosite was derived by metasomatic replacement of this metasedimentary protolith by Ca-rich fluids, likely derived from the underlying ophiolite. One epidosite contains laminae and veinlets of molybdenite, which yields a Re/Os age of c. 89.7 ± 0.6 Ma. Metasomatic formation of epidosite and molybdenite mineralisation occurred during the interval between c. 106 and c. 70 Ma when the Pounamu terrane component of the Alpine Schist underwent accretion, subduction, hydrothermal alteration, mineralisation and metamorphic recrystallization. Molybdenite occurrence is unique in the context of Haast Schist metallogenesis, with Mo most likely derived by leaching and redistribution from the sedimentary cover of the Pounamu ophiolite.

Chatham Schist

ABSTRACTNew structural, petrographic, whole rock chemical and mineral chemical data from the Chatham Schist, the easternmost part of the Haast Schist, are presented. In combination with existing geochronological data, these bring knowledge of the basic geological framework of the Chatham Schist to the same standard as the rest of the Haast Schist. Chatham Schist is dominated by greyschist whose protoliths are correlated with the Permian-Triassic Rakaia Terrane of the Torlesse Composite Terrane. Graded bedding can be recognised in a few places in the northern Chatham Schist, but no fossils have been found. Metamorphic grade on Chatham Island is pumpellyite-actinolite facies and textural grade varies from TZ2A-3. Transitional greenschist-amphibolite facies, garnet-biotite zone Chatham Schist has been sampled in a xenolith from a Pliocene volcano and in a Southeast Chatham Terrace dredge. The age of Chatham Schist prograde metamorphism is Middle to Late Jurassic. At least two phases of penetrative deformation, and a third, brittle faulting phase, are recognised. Chatham Schist was rapidly exhumed in the late Early Cretaceous, possibly as a result of collision of the Hikurangi Plateau against the Gondwana supercontinent. Detrital garnets and spinels now found on Chatham Islands beaches were not sourced from Chatham Schist.

Two-sided accretion and polyphase metamorphism in the Haast Schist belt, New Zealand: Constraints from detrital zircon geochronology

Two-sided accretion and polyphase metamorphism in the Haast Schist belt, New Zealand: Constraints from detrital zircon geochronology

High-level stratigraphic scheme for New Zealand rocks

We formally introduce 14 new high-level stratigraphic names to augment existing names and to hierarchically organise all of New Zealand's onland and offshore Cambrian–Holocene rocks and unconsolidated deposits. The two highest-level units are Austral Superprovince (new) and Zealandia Megasequence (new). These encompass all stratigraphic units of the country's Cambrian–Early Cretaceous basement rocks and Late Cretaceous–Holocene cover rocks and sediments, respectively. Most high-level constituents of the Austral Superprovince are in current and common usage: Eastern and Western Provinces consist of 12 tectonostratigraphic terranes, 10 igneous suites, 5 batholiths and Haast Schist. Ferrar, Tarpaulin and Jaquiery suites (new) have been added to existing plutonic suites to describe all known compositional variation in the Tuhua Intrusives. Zealandia Megasequence consists of five predominantly sedimentary, partly unconformity-bounded units and one igneous unit. Momotu and Haerenga supergroups (new) comprise lowermost rift to passive margin (terrestrial to marine transgressive) rock units. Waka Supergroup (new) includes rocks related to maximum marine flooding linked to passive margin culmination in the east and onset of new tectonic subsidence in the west. Māui and Pākihi supergroups (new) comprise marine to terrestrial regressive rock and sediment units deposited during Neogene plate convergence. Rūaumoko Volcanic Region (new) is introduced to include all igneous rocks of the Zealandia Megasequence and contains the geochemically differentiated Whakaari, Horomaka and Te Raupua supersuites (new). Our new scheme, Litho2014, provides a complete, high-level stratigraphic classification for the continental crust of the New Zealand region.

The Pounamu terrane, a new Cretaceous exotic terrane within the Alpine Schist, New Zealand; tectonically emplaced, deformed and metamorphosed during collision of the LIP Hikurangi Plateau with Zealandia

The petrologically renowned Haast Schist, previously acknowledged as having formed by Jurassic collision and amalgamation of the exotic Rakaia and indigenous Caples terranes during late Mesozoic orogenesis, contains a component of the Alpine Schist whose protolith was deposited in the mid-Cretaceous. An air-fall tuff deposited on the ophiolitic Pounamu Ultramafic Belt (PUB) has an eruption age of 108Ma. The PUB is in tectonic contact to the east with Rakaia terrane-derived Alpine Schist of Triassic age and is overlain by turbiditic metasediments that have Cretaceous (109–140Ma) detrital zircon populations. The dominant zircon component of these sediments is Permo-Triassic, compatible with autocannibalistic reworking from the Torlesse Composite terrane. Reconnaissance analysis of the Alpine Schist indicates that Cretaceous protoliths extend at east 200km along strike, while possible correlatives elsewhere in New Zealand suggest that the original basin may have been thousands of kilometres in extent. Models for the emplacement of these exotic Cretaceous sediments, the proposed Pounamu terrane, are explored. Our favoured interpretation is that the Cretaceous Alpine Schist protoliths were deposited in an ensimatic sedimentary basin north of earlier Mesozoic subduction zones. Multi-phase deformation and metamorphism occurred during collision of the LIP Hikurangi Plateau with Zealandia, resulting in the emplacement of an ophiolite (PUB)-soled allochthonous thrust sheet of the Pounamu terrane onto the previously metamorphosed Rakaia–Otago Schist–Caples massif. Based on new and regional evidence, this collision and the climactic metamorphism of the Pounamu terrane occurred at ~72Ma.

Late Jurassic detrital zircons from the Haast Schist and their implications for New Zealand terrane assembly and metamorphism

The youngest detrital zircon age groups from three samples within the Haast Schist in northwest Otago are Late Jurassic (154, 155, 160 Ma), as determined by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis of U–Pb isotopes in individual zircons. It is inferred that this is the maximum age of sedimentation for these samples, which is within the range of ages for Haast Schist metamorphism (145–180 Ma). This maximum sedimentation age is at least 50 Ma younger than the previously inferred depositional ages for the Caples and Rakaia terranes which are the protoliths of Haast Schist. The zircon age populations within the samples are also different from those found within the Rakaia and Caples terranes, implying different sedimentary sources and possibly a different terrane. The detrital zircon populations are comparable to those found within the Waipapa Terrane in the North Island.

Cretaceous sedimentation and metamorphism of the western Alpine Schist protoliths associated with the Pounamu Ultramafic Belt, Westland, New Zealand

The Pounamu Ultramafic Belt (PUB), part of the Alpine Schists, comprises a metamorphosed ophiolitic sequence overlain by a thin pelagic unit, a massive quartzofeldspathic biotite rock and quartzofeldspathic turbidites derived from a Torlesse/Otago Schist source. Constituent zircon populations from the biotite rock are euhedral/prismatic and are unabraded. One sample is near unimodal, with a U–Pb SHRIMP age of 106.6±1.4 Ma (2σm), while the other has two peaks at 108.0±1.6 Ma and 71.9±1.8 Ma. The older age is interpreted to represent the age of igneous activity and sedimentation, while the younger peak is interpreted to represent the age of subsequent metamorphism. The whole-rock geochemistry of the biotite rock, its field distribution and zircon morphology are consistent with an origin by eruption of a dacitic air-fall tuff. The base of the Pounamu sequence divides predominantly psammitic Triassic metasediments to the east from intensely deformed, but regionally west-facing, Cretaceous metasediments to the west. These Cretaceous metasediments have undergone multiphase deformation and metamorphism by 72 Ma, a timing that contrasts markedly with previous interpretations for Jurassic amalgamation of the Haast Schists.

Regional metamorphism of the Early Palaeozoic Greenland Group, South Westland, New Zealand

Early Palaeozoic basement rocks in South Westland consist of regionally metamorphosed Greenland Group of probable Ordovician depositional age. Correlative and formerly contiguous Gondwana margin rocks are also found in Antarctica and eastern Australia. Major protoliths are variably foliated sandstone and mudstone, with rare scattered calcareous lenses. Field, petrographic and electron microprobe data reveal a progressive, regional, southeast-wards increase in metamorphic grade in a 2600 km2 area between Martins Bay and Fox Glacier. We define new chlorite, biotite–albite, biotite–calcic plagioclase and sillimanite–microcline zones. Minerals such as cordierite and andalusite, along with semi-schistose textures, indicate a low P/T (pressure/temperature) Buchan style of regional metamorphism spanning the lower greenschist to upper amphibolite facies. This contrasts with medium to high P/T metamorphism in the Haast Schist east of the Alpine Fault. In South Westland, peak metamorphism is loosely constrained as Devonian to Carboniferous (330–375 Ma) in age; the present-day isograd pattern was mostly established by the Late Cretaceous and has not been strongly influenced by Alpine Fault deformation.

3-D imaging of Marlborough, New Zealand, subducted plate and strike-slip fault systems

SUMMARY We present a 3-D seismic velocity model of the northern South Island, New Zealand, using several combined passive and active-source data sets. This area is the transition from Hikurangi subduction in the North Island to oblique continental collision along the Alpine fault and the Southern Alps, and geodetically observed deformation is consistent with plate motion being primarily taken up on a series of rotating strike-slip faults. We carry out simultaneous inversion for Vp and Vp/Vs and hypocentres using earthquake data recorded by the permanent seismic network and three temporary seismic arrays, also using active-source marine airgun data recorded at onshore stations. We image the subducted Hikurangi slab as a continuous unbroken feature extending from the active subduction zone to the north. Beneath the Marlborough region, the 3-D hypocentres define the subducted slab as a smooth curved feature to over 250-km depth, whereas below the junction of the Awatere and Alpine faults, the seismicity abruptly ends at 100-km depth. The subducting Pacific plate has a thick crust, including greywacke upper crust, which slows subduction. The interaction of the subducted plate with the translating overlying plate forms a 50-km-long zone of apparent crustal thickening with Vp < 7.0 km s−1 observed to 40-km depth. The Australian Plate has a high-velocity block to 65-km depth, aligned with the Separation Point pluton and trending oblique to the subducted slab. This zone of thicker Australian lithosphere may also hinder subduction. The Marlborough faults trend about 25° to the strike of the underlying subducted slab, so that the relation of faults to the slab varies along the fault strikes, and the overlying plate ranges in thickness from 20 to 50 km along the faults. Thus, the fault interaction with the subducted slab varies, from translating material directly above the subducting plate, to bottoming into a thick ductile region. The lower crust has high Vp/Vs under the Marlborough faults (except for a region of Haast schist), which may indicate excess fluid released by the subducted plate.

Plio-Pleistocene paleoclimate in the Southwest Pacific as reflected in clay mineralogy and particle size at ODP Site 1119, SE New Zealand

Clay mineralogical and particle size data from Ocean Drilling Program (ODP) Site 1119 reflect processes associated with the deposition of the Canterbury Drifts and the evolution of the New Zealand south-eastern shelf, and are strongly correlated with Pliocene–Pleistocene climatic and oceanographic fluctuations. Elevated smectite contents at the base of Site 1119 (latest Early Pliocene ∼ 3.9–3.5 Ma) also occur regionally in the Southern Ocean and point towards an alternative sediment source other than nearby New Zealand terranes. The abrupt reduction in smectite content at ∼ 3.5 Ma and replacement by dominant chlorite–illite assemblages corresponds to the onset of Late Pliocene cooling climate conditions and more intense onland physical weathering. High illite contents in Late Pliocene–Early Pleistocene (∼ 3.0–1.7 Ma) sediments suggest a more southern terrane (e.g. Haast Schist) source influence and widened shelf conditions. An upward increase in clay content reflects an increased supply of glacier-derived detrital sediment, concurrent with global climate deterioration during the Late Pliocene–Pleistocene. Grainsize fining of the background mean sortable silt (mss) signal indicates reduced Southland Current flow, consistent with its forced seaward migration across Site 1119 over time due to continual eastward shelf progradation. Clay mineralogy and particle size data at ODP Site 1119 exhibit a relationship with both global climatic conditions and with Southwest Pacific paleoceanographic and paleoclimatic regimes.

River capture and Main Divide migration in the Haast River catchment assessed from the fluvial distribution of sodalite‐bearing dike rocks and fenites

A sodalite tinguaite dredged from the bed of the Clutha River, at the Clyde Dam site in Central otago, shares mineralogical, geochemical, and hand specimen characteristics with a sodalite tinguaite collected from near the Twirligig, Burke River, south Westland. Tinguaites have a restricted geographic distribution, occurring as intrusions into the host Haast Schists in the Haast‐Burke River area of the oligocene‐Miocene Alpine Dike Swarm. A second boulder from Clyde, with the mineralogy albite‐magnesioriebeckite‐aegirine is interpreted as a fenite, a rock type that also crops out extensively in the Haast‐Burke River area. Fenites occur at the margins of evolved dike rocks and are produced by metasomatic alteration of the country rock. The occurrences of the tinguaite and fenite at Clyde require that the position of the Main Divide originally lay west of the Burke River, enabling rocks exposed in that catchment to have been transported into the Clutha drainage system. The 25 km eastward migration of the Main Divide required to explain the tinguaite and fenite distribution was achieved through the Haast River capturing successively the Clarke, landsborough, Burke, and Wills Rivers in response to erosion associated with deformation on the Pacific‐Australian plate boundary.

Three‐dimensional attenuation structure of central and southern South Island, New Zealand, from local earthquakes

The three‐dimensional attenuation properties of the South Island, New Zealand, are obtained from local earthquake t* (spectral decay) data. We extend the spectral fitting method used to obtain t* by enabling limited frequency dependence for Q. Observed Qp heterogeneity relates to both active geological processes and to constituent tectonic blocks, with high Qp for the Haast schist. The most pronounced low Qp features in the upper crust correspond to zones of recent seismicity, rather than the most significant faults. The central Alpine fault, a dipping, oblique‐slip fault, forms the western boundary to a low Qp volume in the ductile crust. Particularly low Qp is imaged at the base of the Alpine fault at 30‐km depth and may be related to metamorphic fluid release in the crustal root, a region of predicted high strain. Conversely, the southern Alpine fault, which is a vertical strike‐slip fault, forms the western boundary to the high Qp schist region. In the southern South Island, the plate boundary becomes a subduction zone, exhibiting unusual Qp character. Moderately high Qp is associated with the subducted Australian plate. There is also a high Qp, high Vp feature in the Pacific mantle to 160‐km depth that appears to cause the subducting slab to bend to near vertical. This feature may represent mantle shortening since the Miocene. Regional attenuation rates for Fiordland earthquakes, calculated from the 3‐D Qp model, show large variations over the South Island; the low Qp in the crustal root has a significant effect on passing raypaths.

Establishing the P–T path for Alpine Schist, Southern Alps near Hokitika, New Zealand

AbstractPressure–temperature pseudosections for ‘greyschist’ (metamorphosed greywacke and argillite) from the Alpine Schist (Haast Schist group) near Hokitika (Southern Alps, New Zealand) are used to gain new insights into its metamorphic history. The rocks were metamorphosed at relatively low‐grade conditions associated with the first appearance and initial growth of garnet in the stability field of albite. The measured and predicted garnet compositional zoning data are used to construct an overall P–T path by combining P–T path results from nearby rocks that have a range of MnO contents. The P–T path obtained is steep from ∼380 °C/2.5 kbar up to ∼490 °C/8.5 kbar, then recurves sharply with garnet growth continuing during early decompression to ∼500 °C/6.5 kbar. Most garnet growth in the study area took place in the stability field of albite, with oligoclase appearing only during decompression, when the peristerite gap was entered. On appearance of oligoclase, there is a marked decrease in the CaO content of garnet. The preservation of mineral assemblages from near‐peak temperature conditions can be understood in terms of the P–T path subsequently becoming tangential to water content contours, during cooling with further decompression.

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

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

New Zealand's Geological Foundations

New Zealand is a fragment of Gondwana that, before Late Cretaceous sea floor spreading, was contiguous with Australia and Antarctica. Only about 10% of the area of continental crust in the wider New Zealand region (Zealandia) is emergent above sea level as the North and South Islands. No Precambrian cratonic core is exposed in onland New Zealand. The Cambrian to Early Cretaceous basement can be described in terms of nine major volcano-sedimentary terranes, three composite regional batholiths, and three regional metamorphic-tectonic belts that overprint the terranes and batholiths. The terranes (from west to east) are: Buller, Takaka, Brook Street, Murihiku, Maitai, Caples, Bay of Islands (part of former Waipapa), Rakaia (older Torlesse) and Pahau (younger Torlesse). The western terranes are intruded by three composite batholith (>100 km 2) sized belts of plutons: Karamea-Paparoa, Hohonu and Median, as well as by numerous smaller plutons. Median Batholith (including the Median Tectonic Zone) is a recently-recognised Cordilleran batholith that represents the site of subduction-related magmatism from ca. 375–110 Ma. Parts of the terranes and batholiths are variably metamorphosed and deformed: Devonian and Cretaceous amphibolite-granulite facies gneisses are present in Buller, Takaka, Median and Karamea-Paparoa units; Jurassic-Cretaceous subgreenschist-amphibolite facies Haast Schist overprints the Caples, Bay of Islands and Rakaia Terranes; Cretaceous subgreenschist facies Esk Head and Whakatane Mélanges bound the Pahau Terrane. In the South Island, small areas (<5 km 2 total) of Devonian, Permian, Triassic and Jurassic Gondwana sequences have been identified. In the North Island a widespread Late Jurassic overlap sequence, Waipa Supergroup (part of former Waipapa Terrane), has recently been proposed.

Phylogeny, biogeography and adaptive radiation of Pachycladon (Brassicaceae) in the mountains of South Island, New Zealand

AbstractAim To report analyses and propose hypotheses of adaptive radiation that explain distributional patterns of the alpine genus Pachycladon Hook.f. – a morphologically diverse genus from New Zealand closely related to Arabidopsis thaliana.Location South Island, New Zealand.Methods Morphological and nrDNA ITS sequence phylogenies were generated for Pachycladon. An analysis is presented of species distributional patterns and attributes.Results Phylogenetic analyses of morphological characters and nrDNA ITS sequence data were found to be congruent in supporting three New Zealand clades for Pachycladon. Monophyletic groups identified within the genus are geographically distinct and are associated with different geological parent materials. Distribution maps, latitude and altitude range, and data on geological parent material are presented for the nine named and one unnamed species of Pachycladon from New Zealand.Main conclusions (a) Panbiogeographic hypotheses accounting for the origin and present‐day distribution of Pachycladon in New Zealand are not supported. (b) Species diversity and distributions of Pachycladon are explained by a Late Tertiary–Quaternary adaptive radiation associated with increasing specialization to geological substrates. Pachycladon cheesemanii Heenan &amp; A.D.Mitch. is morphologically similar to the closest overseas relatives. It is a geological generalist and has wide latitudinal and altitudinal ranges, and we suggest it resembles the ancestral form of the genus in New Zealand. Pachycladon novae‐zelandiae (Hook.f.) Hook.f. and P. wallii (Carse) Heenan &amp; A.D.Mitch. are a southern South Island group that predominantly occurs on Haast Schist, are polycarpic, have lobed leaves, and lateral inflorescences. Pachycladon enysii (Cheeseman) Heenan &amp; A.D.Mitch., P. fastigiata (Hook.f.) Heenan &amp; A.D.Mitch., and P. stellata (Allan) Heenan &amp; A.D.Mitch. are restricted to greywacke in the eastern South Island, and are facultatively monocarpic, have serrate leaves, and stout terminal inflorescenes. (c) Present distributions of Pachycladon species may relate to Pleistocene climate change. Pachycladon enysii reaches the highest altitude of New Zealand species of Pachycladon and is most common in the Southern Alps in Canterbury. We propose that this species survived on nunataks at the height of the last glaciation. In contrast, P. fastigiata grows at a lower altitude and is absent from the high mountains of the Southern Alps. We suggest it was extirpated from this area during the last glaciation.

Geochemistry, mineralogy, and metamorphic history of kyanite‐orthoamphibole‐bearing Alpine Fault mylonite, South Westland, New Zealand

Mylonite outcropping in the Alpine Fault Zone at Makawhio River, South Westland, shows mineralogy unlike rocks found elsewhere in the Haast Schist. The Makawhio mylonites comprise kyanite‐orthoamphibole hornblendite, garnet amphibolite, chlorite‐hornblende schist, kyanite‐plagioclase schist, and talc‐tremolite rocks, are characterised by ultrabasic to intermediate chemistry, show extreme enrichment in Cr and Ni, and collectively include kyanite, margarite, fuchsite, scapolite, and the first reported occurrence of gedrite and anthophyllite from the Eastern Province. These unusual mylonites form a pod‐like body enveloped by chlorite‐hornblende schist within Torlesse‐derived metasediments. The geochemistry and mineralogy of the Makawhio rocks suggests an origin as a veined ophiolitic rock metasomatised before or during upper amphibolite facies Alpine Schist metamorphism and mylonitisation, resulting in a localised occurrence of rocks atypical of the Haast Schist overall. These kyanite‐bearing, metasomatised mylonites are juxtaposed with quartzofeldspathic and calc‐silicate mylonites, and with amphibolites derived from ocean‐floor basalt, within the Alpine Fault Zone. The Makawhio River kyanite‐bearing mylonites and associated rocks probably represent a tectonised sliver of the Pounamu Ultramafic Belt ophiolite, which is exposed to the north and south as part of the imbricated basement to the Torlesse Terrane along the western margin of the Southern Alps. The range of lithologies and unusual nature of the mylonites at Makawhio River may also represent an extension of the Torlesse Caples Terrane boundary incorporated into the plate boundary zone during oblique convergence and dextral shear on the Alpine Fault.

The effect of crustal anisotropy on reflector depth and velocity determination from wide-angle seismic data: a synthetic example based on South Island, New Zealand

When deriving velocity models by forward modelling or inverting travel time arrivals from seismic refraction data, a heterogeneous but isotropic earth is usually assumed. In regions where the earth is not isotropic at the scale at which it is being sampled, the assumption of isotropy can lead to significant errors in the velocities determined for the crust and the depths calculated to reflecting boundaries. Laboratory velocity measurements on rocks collected from the Haast Schist terrane of South Island, New Zealand, show significant (up to 20%) compressional (P) wave velocity anisotropy. Field data collected parallel and perpendicular to the foliation of the Haast Schist exhibit as much as 11% P-wave velocity anisotropy. We demonstrate, using finite-difference full-wavefield modelling, the types of errors and problems that might be encountered if isotropic methods are used to create velocity models from data collected in anisotropic regions. These reflector depth errors could be as much as 10–15% for a 10-km thick layer with significant (20%) P-wave velocity anisotropy. The implications for South Island, New Zealand, where the problem is compounded by extreme orientations of highly anisotropic rocks (foliation which varies from horizontal to near vertical), are considered. Finally, we discuss how the presence of a significant subsurface anisotropic body might manifest itself in wide-angle reflection/refraction and passive seismic datasets, and suggest ways in which such datasets may be used to determine the presence and extent of such anisotropic bodies.