Milford Sound Research Articles

Agent-based modelling of evacuation scenarios for a landslide-generated tsunami in Milford Sound, New Zealand

Agent-based modelling is a useful tool for evacuation planning as it can increase understanding of the factors affecting potential evacuation outcomes. In New Zealand, Milford Sound has been shown to have a high risk from landslide-generated tsunami, with an estimated 1-in-1000-year wave runup of ∼17 m arriving on shore within 2–7 min. With an annual average of >1500 people visiting a day, there is potential for widespread loss of life. However, the number of people present varies substantially with time of day and season, yet how this affects the ability to evacuate remains unknown. This research developed an agent-based model to understand how many people can be safely evacuated in Milford Sound and explored how the number of people initially exposed affected the evacuation outcome alongside the effect of potential changes to evacuation messaging. Assuming a 17 m wave, the results suggest that currently no one can safely evacuate before the shortest wave arrival time regardless of the number of people present. Altering evacuation messaging results in minimal gains, with only ∼5 % of the exposed population reaching safety in time. This work demonstrates the importance of evacuation modelling for understanding risk in isolated tourism destinations where the population exposure can fluctuate dramatically across multiple timescales. Accounting for changing population exposure is essential to understand whether evacuation is a suitable risk treatment and can provide valuable information for determining safe levels of population exposure in locations with high hazard but limited evacuation options.

Expected direct costs of an oil spill in a UNESCO World Heritage area in New Zealand

AbstractFiordland National Park is a UNESCO site located in the remote, southwest of New Zealand's South Island, which attracts hundreds of thousands of visitors every year. One of its leading attractions is Milford Sound. An oil spill in this area could not only impact the industries that operate in Milford Sound but also impact all of the park. To improve the knowledge base about this area in regard to oil spills and policies regarding ship visitation, this study aimed to estimate the costs of clean‐up, impacts to fisheries, tourism, recreation and the wildlife/environment under three oil spill scenarios in Milford Sound. Minimum costs are estimated at NZD 140–154 million. These results demonstrate the need for proper planning and response measures, and appropriate domestic and international policies to help minimise the potential damages that may occur in the event of an oil spill in a remote and pristine area of New Zealand.

Recreational impacts of an oil spill in a world heritage area: a combined travel cost and contingent behaviour method approach

ABSTRACT Fiordland National Park is a UNESCO world heritage site that draws hundreds of thousands of visitors every year. An oil spill in this area could impact the visitation to not only Milford Sound, but to all of Fiordland National Park and the lower South Island. This study examined the impacts on the recreational value of Fiordland National Park under three oil spill scenarios in Milford Sound. Using a combined approach of travel cost and contingent behaviour methods revealed a consumer surplus of a visit to Fiordland National Park of NZD 534.76 and NZD 1,449.28 per person, with and without accommodation respectively. This leads to an aggregate consumer surplus estimate ranging from NZD 427.8–1,159.4 million for Fiordland National Park based on pre-COVID visitor numbers. The three oil spill scenarios represented small, medium, and large-scale oil spills, limiting various water-based activities at each step. This showed that for each increase in oil level, there was a decrease of NZD 164.10 and NZD 923.73 per person with and without accommodation respectively. This leads to an estimated minimum decrease of the welfare of visitors by 30.7–63.7 percent, or a decrease of NZD 131.3–734.0 million in the annual recreational value. Key Policy Highlights Aggregate annual recreational value estimates ranging from NZD 427.8 million – NZD 1.6 billion pre-COVID19. An oil spill occurring in Milford Sound demonstrated a minimum annual recreational value decrease of NZD 131.3 million over five years. These results demonstrate the need for appropriately funded prevention and preparedness policies to minimize possible damages.

Road impacts from the 2016 Kaikōura earthquake: an analogue for a future Alpine Fault earthquake?

ABSTRACTThe 2016 MW 7.8 Kaikōura earthquake involved complex rupture of multiple faults for > 170 km, generating strong ground shaking throughout the upper South Island leading to widespread landsliding. As a result of surface fault rupture and landslides, State Highway (SH) 1 and SH70 were blocked, isolating Kaikōura and the surrounding communities, and necessitating evacuations by air and sea. In all these respects, the Kaikōura earthquake can be considered an analogue for a future Alpine Fault earthquake, providing lessons for the necessary emergency response. Landslide blockages primarily occurred where surrounding slopes averaged > 17° and where peak ground acceleration was > 0.43 g, peak ground velocity was > 41 cm/s, or the modified Mercalli intensity was > 7.9. Using a potential future Alpine Fault scenario earthquake, this study identifies locations on other key state highway routes that have similar predictive variables that may, therefore, become blocked in a future earthquake. This suggests that SH6 between Hokitika and Haast, SH73 near Arthur’s Pass, and SH94 south of Milford Sound are all likely to be affected. This will necessitate the evacuation of large numbers of spatially distributed tourists as well as the resupply of isolated local populations. The possibility of bad weather along with a lack of sea ports south of Hokitika will likely make such activities challenging. Contingency planning based on experiences from the Kaikōura earthquake is therefore necessary and likely to prove invaluable following an Alpine Fault earthquake.

Radiogenic and stable Sr isotope ratios (87Sr/86Sr, δ88/86Sr) as tracers of riverine cation sources and biogeochemical cycling in the Milford Sound region of Fiordland, New Zealand

This study reports radiogenic Sr isotope ratios (87Sr/86Sr), stable Sr isotope ratios (δ88/86Sr), and major ion concentrations for river, rock, sediment, soil, and plant samples collected from the Cleddau and Hollyford catchments in the Milford Sound region of Fiordland, New Zealand. The catchments primarily drain gabbro, but some tributaries access limestone and volcanogenic sediments. The goal of the study was to understand controls on riverine δ88/86Sr values in a landscape with multiple factors that may influence chemical weathering, including dense vegetation, high rainfall, and abundant, freshly-eroded Holocene fluvio-glacial and landslide debris.Rivers draining gabbro have higher δ88/86Sr values than bedrock, by as much as ∼0.14‰, and the δ88/86Sr values strongly correlate with molar Ca/Sr ratios (R2=0.69). Leaching of rocks and sediment reveals no evidence for the preferential dissolution of minerals having high δ88/86Sr values and Ca/Sr ratios. In-stream Sr isotope fractionation seems unlikely because comparison against 87Sr/86Sr and Ca/Sr ratios demonstrates that riverine δ88/86Sr values conservatively trace water-mass mixing. The riverine data are best explained by the input of soil water, which is distinct from potential bedrock end-members (i.e., silicates and carbonates) based on δ88/86Sr but indistinguishable in terms of Ca/Sr and 87Sr/86Sr. While strontium isotope fractionation during secondary mineral formation and pedogenesis is possible, clay mineral formation is minor and most soils are poorly developed. Instead, soil water δ88/86Sr values more likely reflect plant uptake. Plant samples yielded a wide range of δ88/86Sr values, but on average, they are lower than those for bedrock, consistent with the expectation that plants preferentially incorporate lighter Sr isotopes. Mass-balance constraints, together with 87Sr/86Sr ratios, indicate that soil water δ88/86Sr values are ∼0.30‰ higher than bedrock δ88/86Sr values, and mixing calculations show that the plant-fractionated soil water pool contributes ∼27% of the riverine Sr. For tributaries accessing limestone and volcanogenic sediments, Ca/Sr and 87Sr/86Sr ratios appear consistent with two-component mixing between silicate and carbonate weathering, but δ88/86Sr values reveal a third contribution from soil water inputs, similar to gabbro catchments.The results of this study suggest that Sr isotopes behave conservatively during water mass mixing and stream transport but non-conservatively in soil, where plant uptake can elevate soil water δ88/86Sr values relative to bedrock. Plant uptake, or related biogeochemical processes, such as ion-exchange on organic matter surfaces, also appear to modify soil water Ca/Sr ratios. Many studies use 87Sr/86Sr and Ca/Sr ratios to apportion riverine solutes between silicate and carbonate weathering, but Ca/Sr ratios may be non-conservative in densely vegetated areas. The stable Sr isotope tracer shows promise for resolving riverine cation sources, as well as effects from biological cycling.

A pattern-oriented model for assessing effects of weather and freshwater discharge on black coral (Antipathes fiordensis) distribution in a fjord

Doubtful Sound and other New Zealand fjords are unique in that they provide suitable habitat for a black coral species (Antipathes fiordensis) at depths as shallow as 4m. Research suggests the upper depth limit at which black corals can survive in Doubtful Sound is controlled by gradients in salinity (as opposed to light levels) within buoyant low-salinity layers (LSL). The thickness of the LSL in Doubtful Sound varies along the fjord and fluctuates as a function of rain, wind and freshwater tailrace discharge from the Manapouri Hydroelectric Power Station. A pattern-oriented survivorship model was developed to predict mortality and upper-depth distribution of black corals in Doubtful Sound in response to changes in the LSL. Model parameters were estimated using observed patterns of black coral distribution and time-series salinity measurements over a range of depths. Predicted upper depth limits in Doubtful Sound and a reference fjord (Milford Sound) were validated against field observations of black coral depth distributions on the adjacent rock walls. The model accurately forecasted depths of corals in mid to outer regions of Doubtful Sound, but underestimated upper depth limits in the inner region of both fjords. The discrepancies in inner fjord locations suggest that other factors, such as sedimentation, are also limiting establishment of long-lived corals in some locations. Data from hydrodynamic simulations of the LSL in Doubtful Sound were fed into the survivorship model to investigate black coral distributions under varying rain, wind and tailrace discharge scenarios. Results indicate that weather, rather than tailrace discharge, drives long-term patterns in the vertical distribution of black corals in Doubtful Sound.

Interspecific and Geographical Variation in Trace Metal Concentrations of New Zealand Scallops

ABSTRACT The concentrations of selected trace metals were measured in commercially harvested New Zealand scallop species: Chlamys delicatula (queen scallop) from the Otago Coastal Shelf and Pecten novaezelandiae (dredge scallop) collected from 6 sites around the South Island, New Zealand (Titirangi Bay, Collingwood, Milford Sound, Breaksea Sound, Paterson Inlet, and the Chatham Islands). Bivalves were dissected into individual parts, the tissues lyophilized by freeze-drying, and then digested by heating with concentrated HNO3 and H2O2. The resulting solutions were analyzed by inductively coupled atomic emission spectrophotometry to determine the Cd, Cu, Zn, and Fe concentrations. For P. novaezelandiae, the trace metals in whole body tissues were site-specific with mean values ranging from 18–59 µg/g dry weight (dwt) for most sites and high Cd in scallops collected from the Chatham Islands (mean, 332 µg/g dwt). Cu concentrations were low, 6–11 µg/g; average concentrations of Zn ranged from 57–127 µg/g, and...

The distribution and behaviour of Patiriella mortenseni and P. regularis in the extreme hyposaline conditions of the Southern New Zealand Fiords

Echinoderms are a stenohaline phylum, however some species can tolerate varying degrees of exposure to hyposaline conditions. These exposures typically are limited in duration to a few hours to no more than a day or two, and hyposalinity levels range from ~ 8‰ to < 30‰. The unique climatological and hydrographic conditions in Fiordland of southwest New Zealand produce a near-freshwater layer (Low Salinity Layer = LSL) on top of marine (34‰) water. The relative depth of the LSL varies both tidally and with rainfall. This halo-boundary presents a particularly stressful osmotic challenge to organisms found in shallow water. A series of transects in Doubtful Sound and the more northern Milford Sound quantified differences in density and bathymetric distribution of two species of seastars . The surveys revealed that Patiriella mortenseni occurs primarily below the LSL whereas Patiriella regularis is found well within the range of the LSL. Video observations of P. regularis over a rising tide were correlated with salinity measurements and showed a strong correlation between increased salinity and general vertical displacement upwards of the seastars observed moving. Activity coefficients (AC) were used to estimate the degree of neuro-muscular coordination and stress levels of these seastars in two hyposalinity experiments. Reciprocal transplants showed that P. regularis performed equally well in deeper, marine-salinity waters, as in shallow hyposalinity conditions, whereas P. mortenseni showed significantly impaired performance in the hyposaline conditions. These performance estimates were refined in a second experiment by quantifying AC of both species in a seawater dilution series. To rule out the possibility of heretofore unknown osmoregulatory capacity, we exposed both species to 0‰, 5‰, 15‰, 25‰, and 34‰ and recorded the osmolarity of the body fluid after 24 h. P. mortenseni died at salinities below 25‰. In contrast, P. regularis survived and the body fluid was nearly isotonic with the surrounding water. Two final experiments established the hyposalinity-tolerance limits of P. regularis. In a righting-time (RT) experiment we exposed P. regularis to hyposaline water of 5‰ for: 12, 24, 36, 48, 60, 84, and 133 h and recorded righting times immediately after re-immersing them in sea water. All individuals survived and the mean RT ranged from 18 min to 27 h (control group = 1.6 min). Finally, a lethal-exposure experiment to 0‰ (deionized water) confirmed that P. regularis can withstand far greater levels of extreme hyposalinity than has been reported for any other echinoderm.

Milford Sound, New Zealand

Milford Sound, New Zealand

Transport of low‐aH2O dehydration products to melt sites via reaction‐zone networks, Milford Sound, New Zealand

AbstractThe gabbroic/dioritic Pembroke Hornblende Granulite (PHG) of Milford Sound displays a geometrically simple mesoscopic network of sub‐planar garnet reaction zones (GRZ) in which the meta‐igneous hornblende granulite has been depleted of Na, Si, and H2O, and c. 25 vol.% almandine‐rich garnet has formed. Some studies postulate the initial presence of melt along the centres of all GRZ, explaining the frequent absence of feldspathic veins by selective melt loss. A more parsimonious model is necessitated by structural evidence and, together with chemical data, suggests a relationship between mid‐range metasomatic transport and anatexis. The Pembroke outcrops show a process of incipient melting of gabbro/diorite in an environment of relatively low aH2O in lithologies that have limited free quartz. A non‐equilibrium steady state is proposed, in which a sodic dehydration fluid moves some distance via the GRZ network towards areas of partial melting. Only in these areas are Na and Si reconstituted as albite, with more garnet as byproduct, having avoided the need for melt percolation. The combined structural and chemical evidence directs a focus on mass transport in low‐aH2O gabbroic environments. In subsequent events of shearing and complete transposition, both sets of garnet – the atypical GRZ residue and partial melt melanosomes – were inherited by the Milford Gneiss ‘facies’ of the PHG.

Residency pattern of bottlenose dolphins Tursiops spp. in Milford Sound, New Zealand, is related to boat traffic

A population of bottlenose dolphins inhabits 7 of the 14 fjords that compose Fiordland, New Zealand. One of these fjords, Milford Sound, supports a large tourism industry that results in intense boat traffic. Bottlenose dolphins regularly visited Milford Sound and tour boats interacted with them during these visits. I studied the factors affecting the frequency of visits to Milford Sound by relating the residency pattern of dolphins in this fjord to oceanographic parameters and variations in boat traffic between December 1999 and February 2002. Boat traffic was the only variable that could explain the frequency of dolphin visits to Milford Sound. Dolphins spent less time in Milford Sound during seasons of intense boat traffic. Moreover, when dolphins visited this fjord, they spent more time at the entrance of the fjord when boat traffic was intense, out of the reach of tour boats. It seems that dolphins avoid Milford Sound when traffic is heavy. This avoidance could have long-term implications for the demography of the population.

The role of tourist icons for sustainable tourism

This paper provides a discussion of tourist icons as pull factors of a destination and problems associated with icons. According to interviews held with major wholesalers in Australia and inbound tour operators in New Zealand, tourist icons vary in importance to different segments of the market, with coach tourists placing more importance on icons than free independent travellers. Moreover, the need to base overseas marketing on icons depends on the maturity of the targeted market. A tourist survey in New Zealand revealed that Milford Sound and Rotorua were indeed the most commonly known attractions, but that in addition to these two key icons, tourists were attracted to natural features in general, such as mountains and glaciers. This paper argues that an overemphasis on tourist icons potentially leads to unsustainable tourism development, for the following reasons: tourists travel large distances to visit the spatially very dispersed tourist icons, and this results in considerable amounts of greenhouse gas emissions, even if they visit New Zealand for a short period of time; the high visitation levels at popular sites result in infrastructural bottlenecks, reaching carrying capacity, which results in potentially significant environmental and social impacts; and the uneven geographical distribution of tourism spending is a missed opportunity for much-needed regional development. The current ‘100% Pure’ campaign by Tourism New Zealand potentially contributes to a more holistic promotion of New Zealand as an attractive ‘natural landscape’ destination.

AERIAL SURVEYS FOR COASTAL DOLPHINS: ABUNDANCE OF HECTOR'S DOLPHINS OFF THE SOUTH ISLAND WEST COAST, NEW ZEALAND

AbstractLine‐transect surveys of abundance depend critically on the estimation of detection probability, which includes corrections for availability and visibility. In this aerial line‐transect survey for Hector's dolphin (Cephalorhynchus hectori) we recorded dive times from a helicopter to estimate the proportion of time that dolphins could be seen at or near the water surface and were therefore “available” to be counted. The proportion of available sightings that were counted on transect lines was estimated by comparing sightings by two independent teams of two observers in the fixed‐wing plane. The survey covered the area between Farewell Spit and Milford Sound, off the west coast of the South Island of New Zealand. Survey effort was stratified according to existing distribution data. A total of 142 separate sightings was made on 1,355 km of trackline. Average availability for fifty Hector's dolphin groups observed from the helicopter (161 dive/surface cycles) was 46.3% (CV = 4.2%). Data from the two independent observer teams indicated that 96.2% (CV = 2.3%) of the dolphin groups that were at the surface and on the trackline were seen. The abundance estimate for the South Island west coast, corrected for visibility and availability, was 5,388 Hector's dolphins (CV = 20.6%). The total population estimate for South Island Hector's dolphins is 7,270 (CV = 16.2%).

Trace element partitioning during high‐P partial melting and melt‐rock interaction; an example from northern Fiordland, New Zealand

AbstractPods of granulite facies dioritic gneiss in the Pembroke Valley, Milford Sound, New Zealand, preserve peritectic garnet surrounded by trondhjemitic leucosome and vein networks, that are evidence of high‐P partial melting. Garnet‐bearing trondhjemitic veins extend into host gabbroic gneiss, where they are spatially linked with the recrystallization of comparatively low‐P two‐pyroxene‐hornblende granulite to fine‐grained high‐P garnet granulite assemblages in garnet reaction zones. New data acquired using a Laser Ablation Inductively Coupled Plasma Mass Spectrometer (LA‐ICPMS) for minerals in various textural settings indicate differences in the partitioning of trace elements in the transition of the two rock types to garnet granulite, mostly due to the presence or absence of clinozoisite. Garnet in the garnet reaction zone (gabbroic gneiss) has a distinct trace element pattern, inherited from reactant gabbroic gneiss hornblende. Peritectic garnet in the dioritic gneiss and garnet in trondhjemitic veins from the Pembroke Granulite have trace element patterns inherited from the melt‐producing reaction in the dioritic gneiss. The distinct trace element patterns of garnet link the trondhjemitic veins geochemically to sites of partial melting in the dioritic gneiss.

Structure and kinematics of oblique continental convergence in northern Fiordland, New Zealand

Southeast of the Alpine Fault in Fiordland, New Zealand, Late Tertiary faults envelope exposures of Early Cretaceous high- P (12–16 kbar) granulite facies orthogneisses. These exposures allowed us to examine how pre-Cenozoic structures influenced variations in the degree and style of kinematic partitioning within a zone of oblique continental convergence. Fault-slip data and kinematic modelling show that oblique-slip and reverse displacements preferentially were partitioned east of the Alpine Fault onto curved NE-striking surfaces and moderately dipping (30–52°), N- and NW-striking surfaces, respectively. Approximately 3.0–5.5 km of exhumation and 6.5–7.0 km of oblique-dextral displacement occurred on faults that display palm tree-style geometries and subhorizontal detachments in profile. This heterogeneous style contrasts with the near horizontal slip on subvertical surfaces that define the Alpine Fault north of Milford Sound. The principal axes of instantaneous strain we determined using faults are compatible with principal stress axes derived from earthquakes. Our data indicate that mechanical anisotropies created by inherited structures controlled the locations and highly variable geometries of faulting since 7–9 Ma. The effects of these pre-existing structures and a focusing of contractional deformation along the northeast margin of Fiordland resulted in an unusually high degree of strike-slip partitioning across the southernmost onshore segment of the Alpine Fault.

Transformation of two‐pyroxene hornblende granulite to garnet granulite involving simultaneous melting and fracturing of the lower crust, Fiordland, New Zealand

AbstractGranulite facies gabbroic and dioritic gneisses in the Pembroke Valley, Milford Sound, New Zealand, are cut by vertical and planar garnet reaction zones in rectilinear patterns. In gabbroic gneiss, narrow dykes of anorthositic leucosome are surrounded by fine‐grained garnet granulite that replaced the host two‐pyroxene hornblende granulite at conditions of 750 °C and 14 kbar. Major and trace element whole‐rock geochemical data indicate that recrystallization was mostly isochemical. The anorthositic veins cut contacts between gabbroic gneiss and dioritic gneiss, but change in morphology at the contacts, from the anorthositic vein surrounded by a garnet granulite reaction zone in the gabbroic gneiss, to zones with a septum of coarse‐grained garnet surrounded by anorthositic leucosome in the dioritic gneiss. The dioritic gneiss also contains isolated garnet grains enclosed by leucosome, and short planar trains of garnet grains linked by leucosome. Partial melting of the dioritic gneiss, mostly controlled by hornblende breakdown at water‐undersaturated conditions, is inferred to have generated the leucosomes. The form of the leucosomes is consistent with melt segregation and transport aided by fracture propagation; limited retrogression suggests considerable melt escape. Dyking and melt escape from the dioritic gneiss are inferred to have propagated fractures into the gabbroic gneiss. The migrating melt scavenged water from the surrounding gabbroic gneiss and induced the limited replacement by garnet granulite.

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.

Late Quaternary slip rates and slip partitioning on the Alpine Fault, New Zealand

Published geological data on Late Quaternary offsets on the Alpine Fault, New Zealand, have been assembled into a common format and analysed with respect to uncertainties. Uncertainties arise mainly from measurement of offset features, relating apparent offsets to actual fault slip, and dating the offset features. Despite the considerable uncertainties, the data form a coherent set consistent with a relatively constant rate of strike-slip of 27±5 mm/year between Milford Sound and Hokitika. This rate represents 70–75% of the fault-parallel interplate motion. North of the confluence with the Hope Fault, the rate drops substantially. Dip-slip rates, on the other hand, show considerable variation along strike, rising to a maximum of more than 10 mm/year in the central section and decreasing to zero at the southern end. Partitioning of c. 25% of the interplate slip on to structures east of the Alpine Fault occurs in the central section, consistent with predictions from critical wedge models. The partitioning of all the fault-normal component of displacement on to other structures in the south may be related, in part, to a doubling in width of the deforming wedge to the east. Most probably, however, the fault-normal displacement is mainly accommodated by underthrusting of the Australian plate offshore, due to a change in the nature of the crust from continental to oceanic.

New Zealand's Darran Complex and Mackay Intrusives; Rb-Sr whole-rock isochrons in the Median Tectonic Zone

The Cretaceous opening of the Tasman Sea, followed by uplift east of the Alpine Fault, has given access to the roots of a Mesozoic convergent Gondwana margin in western New Zealand. This long term margin contains the complex Median Tectonic Zone (MTZ) of recent authors and is pefectly exposed between Milford Sound and the Darran Range. In the western part of this profile, in the Milford Gneiss Complex, earlier MTZ history is obscured by Cretaceous metamorphism and structural transposition. However, the Pembroke Granulite is well preserved, and the metamorphic curtain gradually lifts toward the east. Rb-Sr whole-rock isochron dates and initial 87 Sr/ 86 Sr (I o )values are presented. The Darran Complex yields a whole rock isochron of 133 + or - 17 Ma, concordant with published zircon U-Pb and hornblende K-Ar results. The Mackay Intrusives (Mistake Diorite and Hut Suite) yield a whole rock isochron of 224 + or - 4 Ma (MSWD = 1.21) in agreement with published U-Pb dates of Mistake Diorite. Rafts of Christina Gneiss in Mackay Intrusives were subject to partial melting at the time of the Mackay Intrusive activity. However, the Darran Complex also appears spectacularly intruded by the Christina-Crosscut irruptives of Mackay granitoids, which plot precisely on the 224 Ma Mackay Rb-Sr whole rock isochron. Obvious ways of resolving this paradox are (1) that the Darran Complex has undergone thermal resetting after its intrusion by Mackay magma, and (2) the Christina-Crosscut irruptives are late, back-intrusive Darran differentiates containing assimilated Mackay material. These solutions fail on the grounds of heat transfer (Mackay isochron was not re-set) or of heat balance (surmised irruptives excessively contaminated for the available heat). A way past the constraints is the passive tectonic emplacement of the Mt. Crosscut irruptive as a dike-like roof pendant. The Mt. Christina irruptive is dated less reliably, is tectonically less modified, and may represent a late Darran liquid. A much older age and lower I o of the Darran correlative Rotoroa Complex may be inherited from a common Rotoroa/Darran protolith. I o increases systematically from Rotoroa to Western Darran (Milford/Pembroke), to Darran rocks, and finally to Western Fiordland Orthogneiss (WFO), which is chemically distinct from the Pembroke Granulite and Darran Complex.

Cretaceous high‐P granulites at Milford Sound, New Zealand: metamorphic history and emplacement in a convergent margin setting

Granulite facies orthogneiss of the Arthur River Complex (ARC) at Milford Sound, western Fiordland records a complex Early Cretaceous magmatic and orogenic history for the Pacific Gondwana margin that culminated in the emplacement and burial of a dioritic batholith, the Western Fiordland Orthogneiss (WFO). Enstatite‐bearing mafic to intermediate protoliths of the ARC and WFO intruded the middle to upper crust. The early deformation history of the ARC is preserved in the Pembroke Granulite, where two‐pyroxene S1 assemblages that reflect P&lt;8 kbar and T &gt;750 °C were only patchily recrystallized during later deformation. S1 is cut by garnet‐bearing, leucogabbroic to dioritic veins, which are cut by distinctive D2 fractures involving anorthositic veins and garnet–diopside–plagioclase‐bearing reaction zones. These zones are widespread in the ARC and WFO and record conditions of P≈14 kbar and T &gt;750 °C. Garnet–clinopyroxene‐bearing corona reaction textures that mantle enstatite in both the ARC and WFO reflect Early Cretaceous burial by approximately 25 km of continental crust. Most of the ARC is formed from the Milford and Harrison Gneisses, which contain steeply dipping S4 assemblages that envelop the Pembroke Granulite and involve garnet, hornblende, diopside, clinozoisite, rutile and plagioclase, with or without kyanite. The P–T history of rocks in western Fiordland reflects pronounced Early Cretaceous convergence‐related tectonism and burial, possibly related to the collision and accretion of island arc material onto the Pacific Gondwana margin.