Northern South Island Research Articles

Structural evolution and landscape development of a collapsed transpressive duplex on the Hope Fault, North Canterbury, New Zealand

This study examined the transpressional Conway segment of the Hope Fault in North Canterbury, the fastest moving fault of the Marlborough Fault Zone in northern South Island of New Zealand, in an attempt to reconstruct, via air photograph interpretation, detailed field mapping, and theoretical constraints, the styles of structural geometry of the late Quaternary deformation. We relate the evolving landscape to the development and modification of this fault in an active tectonic setting. The section of the fault zone studied is a 13 km long, 1.3 km wide, asymmetric transpressional reverse fault duplex, bounded to the southwest by the Lottery River and to the northeast by the Mason River, which tapers down to a single fault trace to the northeast. Between the bounding faults of the duplex are approximately 100 subsidiary fault scarps that initially formed an imbricate set of footwall propagating reverse/thrust faults. These faults became inactive and topographically unsupported when younger footwall propagating reverse faults migrated northeast along the main trace of the Hope Fault as the duplex also migrated to the northeast. The unsupported duplex structurally collapsed back into the dilating fault zone, causing reversal of slip on the imbricate reverse faults so that they became normal faults. As the duplex collapsed, the adjacent hanging wall was uplifted triggering landsliding, rapid incision by streams, formation of large alluvial fans, and minor normal fault gravity collapse structures. The footwall block outside the duplex became rapidly incised by streams and experienced widespread topographic slumping and associated structurally controlled, shallow‐level ridge renting. Stream dissection of flights of late Pleistocene aggradation/degradation surfaces also occurred at this time, leaving remnant flat‐topped hills between the dissected and slumped valleys. Our analysis of the interrelationships between the structural geology and the landscape over time is a new approach for the Hope Fault and reconfirms the necessity of integrating detailed structural geology with geomorphology in areas of active tectonism.

Stress directions in the shallow part of the Hikurangi subduction zone, New Zealand, from the inversion of earthquake first motions

The numerous earthquakes recorded during recent dense seismograph deployments in the northern South Island and southernmost North Island, and in the Raukumara Peninsula of New Zealand, provide an opportunity to investigate stress directions in the shallow part of the Hikurangi subduction zone. Here we invert for the stress tensor orientation for regions within both the subducted and overlying plates, using a new method that invokes the Coulomb failure criterion and considers all P-wave first motions together, regardless of whether or not they are sufficient to define single-event focal mechanisms. The inversion also provides information on which of the two planes with maximum Coulomb failure stress is the preferred fault plane. In the crust of the subducted plate, the least compressive stress (σ3 ) is closely aligned with the dip of the plate. The subducted plate is acting as an efficient stress guide, with slab pull from the deeper part of the plate being transmitted to shallow depths. Normal faulting on steeply dipping fault planes is favoured, suggesting that bending of the crust of the subducted plate is accomplished through bulk simple shear (akin to shuffling a vertical deck of cards). In the overlying plate in the northeastern South Island, the greatest and least compressive stresses (σ1 and σ3 ) are horizontal, thus favouring strike-slip faulting. The orientation of σ1 is close to that expected if the overlying plate is contracting in response to coupling at the underlying plate interface. However, sinistral motion on NW faults is favoured, whereas motion on the major NE–ENE surface faults is predominantly dextral. In the Raukumara Peninsula, thrust faulting on steeply dipping planes is favoured in the lower part of the overlying plate and near the plate interface. The stress regime at the plate interface does not favour interplate thrusting, and a weak interface is required for this to occur. It appears that strain in the overlying plate in both the northeastern South Island and Raukumara Peninsula is partitioned in time—conjugate faults may be active in taking up strain in the interseismic period between large events on the major faults.

Cambrian arc evolution along the SE Gondwana active margin: A synthesis from Tasmania‐New Zealand‐Australia‐Antarctica correlations

Belts of Cambrian rocks with arc affinities in eastern Australia, Tasmania, New Zealand, and Antarctica are part of a single convergent margin, active over 30–40 Ma from the latest Early Cambrian to the Late Cambrian. Two of the most complete sequences are exposed in western Tasmania and the northern South Island of New Zealand (Takaka Terrane). Throughout the Middle Cambrian, magmatism in these two regions, in the Lachlan Fold Belt (SE Australia) and the Bowers Terrane (Antarctica) is represented by intra‐oceanic arc and back arc sequences. In the mid‐Middle Cambrian, collision of these arc segments with the proto‐Gondwana continent is recorded by obducted boninite‐bearing ophiolites in Tasmania and SE Australia. Postcollisional magmatism of latest Middle to early Late Cambrian age (e.g., Mount Read Volcanics of Tasmania and Stavely Volcanic Complex of Victoria) terminates convergent tectonics in SE Australia and Tasmania. In contrast, no postcollisional volcanism is known from the Bowers Terrane in Antarctica and from New Zealand. In Antarctica, Cambrian igneous activity in the Wilson Terrane and Transantarctic Mountains formed in an active continental margin setting and lasted through the Middle and Late Cambrian. This suggests that the Wilson Terrane is the lateral continuation on continental basement of the Bowers and Takaka Terrane arcs. The differences between the Australian ‐ Tasmanian and New Zealand ‐ Antarctic arc segments may result from a change in subduction polarity along the arc chain, suggested by structural features, sedimentation patterns, and isotope systematics. In the SE Australian and Tasmanian arc segments the proto‐Gondwana plate subducted beneath the Pacific plate whereas subduction in the Antarctic and New Zealand segments was of opposite polarity. Common to most Cambrian fragments in SE proto Gondwana is the tectonic overprint by the Ross‐Delamerian orogeny from the Middle Cambrian to Early Ordovician, thus paleogeographically linking all the fragments by the end of the Cambrian. The presence of Early Cambrian intra‐oceanic rocks at the SE proto‐Gondwana margin suggests separation of Laurentia from Gondwana prior to the Early Cambrian.

Seismic anisotropy from local earthquakes in the transition region from a subduction to a strike‐slip plate boundary, New Zealand

Shear wave splitting is used to investigate anisotropy in the crust and upper mantle in a subduction zone (lower half of the North Island of New Zealand), and its transition to oblique transform faulting (Marlborough area, northern South Island). In Mar !borough, delay times show almost no increase with depth, and it is most likely that the higher‐frequency phases used in this study respond mainly to lithospheric anisotropy. In the central Marlborough Fault System (MFS), fast polarizations are subparallel to the faults. Anisotropy is attributed to the presence of metamorphosed schist (eclogite), of 30±10 km thickness and located 50–80 km beneath the MFS. On the edges of the MFS, fast polarizations are parallel to the maximum compressive stress direction, consistent with crack‐induced anisotropy in the crust. The shear zone, which is as wide as the island in the mantle as inferred from SKS phases, seems to occur in a narrower zone in the crust. In the lower half of the North Island, fast polarizations from events at all depths are oriented parallel to the strike of the Hikurangi subduction zone as well as to the faults. Polarisations are similar to those of SKS phases, which mainly sample the mantle. This suggests that the lithosphere and the upper mantle asthenosphere deform in a coherent strike‐slip shear. We calculate 1.2±0.3% velocity anisotropy in the first 200 km of the mantle from increasing delay times with depth. In order to match the SKS delay times, this result requires the presence of anisotropic material down to 580±100‐km depth, or a change in anisotropy with depth, or frequency dependent splitting.

A new ctenostome bryozoan ectosymbiotic with terminal‐moult paddle crabs (Portunidae) in New Zealand

Triticella capsularis sp. nov. (Bryozoa, Ctenostomata) is newly described as an ectosymbiont of paddle crabs (Ovalipes catharus) in central New Zealand (Bay of Plenty to northern South Island). The bryozoan produces the longest zooids known in the genus Triticella, with colonies forming a “fur” up to almost 10 mm thick on large crabs, mostly males. The densest area of colonisation is the ventral anterior half of the crab. The bryozoan lives only on O. catharus and probably benefits from its “messy” and voracious feeding habits, opportunities for gene exchange during crab swarming behaviour, and dispersal. There is no synchrony between reproduction of the bryozoan and moulting cycles of the crab, as the bryozoan achieves reproductive colony size only on old terminal‐moult crabs. Although visually striking when dense, the bryozoan growth is only superficial and affects neither the behaviour of the crab nor the quality of its meat.

Reinstatement of Sophora longicarinata (Fabaceae—Sophoreae) from northern South Island, New Zealand, and typification of S. microphylla

Sophora longicarinata is reinstated at species rank, having recently been placed as a variety of S. microphylla. Sophora longicarinata is recognised by a suite of characters, including its lack of a juvenile growth phase, its shrubby, slender growth habit, sometimes producing several main stems from below ground level, and often suckering, its small, dark green, and numerous leaflets, and often twisted pedicels and flowers that are inverted in bud. Sophora longicarinata is restricted to marble and other limestone areas in northern Nelson and western Marlborough, is sparsely distributed, has few plants at each locality, and is considered to be a local endemic. A lectotype for S. microphylla is selected from the Banks and Solander herbarium at the Natural History Museum, London.

Strain partitioning in the transition area between oblique subduction and continental collision, Hikurangi margin, New Zealand

Spatial differences in late Quaternary structural style and deformation rates indicate a complex pattern of strain partitioning within the transition area from oblique subduction beneath southern North Island to oblique continental collision in northern South Island, New Zealand. The late Quaternary structure of the offshore southern Hikurangi margin is determined here using seismic reflection profiles, MR1 side‐scan sonar swath images, EM12Dual multibeam swath bathymetry and backscatter images, and sediment core and dredge samples. Where oblique (50°) subduction of the oceanic Hikurangi Plateau is occurring beneath southern Wairarapa region, the steep, dissected, upper margin consists of a wedge of rocks that are being deformed by strike‐slip and reverse faults. Beneath the lower margin an accretionary wedge of Quaternary age has developed rapidly by frontal accretion of a thick succession of turbidites on the Pacific Plate. Estimates of late Quaternary geological strain rates have been made on two margin‐perpendicular transects across southern North Island and the offshore margin to budget the predicted NUVEL‐1A plate motion. More than 33–55 % of the total plate motion is expressed in the offshore part of the upper plate above the subduction detachment in the central Hikurangi margin between 40°S and 41°S. This offshore deformation could potentially include margin‐parallel fault slip of up to 14–15 mm/yr. Across the upper plate, strain is partitioned into zones of folding and thrust faulting and possibly more than one zone of strike‐slip faulting, but the partitioning may be incomplete with oblique‐slip thrusting an important component of the deformation. The wide distribution of deformation within the upper plate partially reflects strong coupling between the subducting oceanic plateau and the leading edge of the overriding plate. This coupling increases toward the southwest, where the continental part of the subduction zone beneath Marlborough may now be inactive or very strongly coupled. Toward the southwest the transpressive deformation style of the upper Wairarapa margin continues into the upper Marlborough margin and onshore into the strike‐slip Marlborough Fault System of northern South Island. In contrast, the accretionary wedge narrows to the southwest and terminates in southeastern Cook Strait, approximately above the transition from subducted oceanic crust to subducted continental crust of the Pacific Plate. Farther southwest beneath the lower Marlborough margin, low‐strain folding and thrust faulting is occurring, but the style of deformation is not typical of the accretionary wedge. This shortening results either from a small amount of subduction slip beneath Marlborough and/or from contractional strain partitioned to the southeast of the continental transpression zone. Whereas the initial onset of continental collision occurs in Marlborough, the surface expression of deformation of the Pacific Plate occurs south of 42°30′S. Normal faulting of the northwestern margin of the continental Chatham Rise possibly reflects flexural extension of the Pacific Plate crust at the southern end of the subduction zone, whereas folding and thrust faulting of the north Canterbury coastal region results from NW‐SE contraction of the delaminated upper crust entering the Southern Alps collision zone.

Reducing predation of orb‐web spiders by controlling common wasps (Vespula vulgaris) in a New Zealand beech forest

1. Introduced common wasps (Vespula vulgaris) reach high densities in the beech forests (Nothofagus spp.) of the northern South Island, New Zealand, and may be having a severe impact on populations of native invertebrates. An experimental approach was used to test whether reducing the abundance of common wasps increases the probability of native invertebrates surviving. Garden orb‐web spiders (Eriophora pustulosa) were used because they were easy to collect and could be trained to build webs on a standard frame. Thirty orb‐web spiders were placed out on web‐frames in each of four study sites in beech forest during periods of low, medium and high wasp abundance, and their rate of removal was measured over a 4‐h period. Wasp numbers were reduced by poisoning in two study sites. Using wasp‐abundance data from weekly Malaise trap samples in the poisoned and non‐poisoned sites, the probability of spider survival over the entire wasp season was modelled and the level of wasp control necessary to protect natural populations of orb‐web spiders was estimated.2. Wasp abundance and the probability of spider survival were negatively correlated, and smaller spiders were likely to survive longer than larger spiders. At the peak of the wasp season, significantly more spiders survived in the poisoned areas than in the non‐poisoned areas.3. The probability of a spider surviving to the end of the wasp season was virtually nil in the non‐poisoned sites (5.08 × 10–18), but was also very unlikely in the poisoned areas (3.47 × 10–5).4. The survival model predicts that wasp abundance would need to be reduced by 80–89.5% over the entire wasp season to protect populations of orb‐web spiders.5. Extrapolation from the model predicts that the invertebrate taxa most vulnerable to wasp predation may have already been removed from the beech forest ecosystem during the 40 years of wasp occupation.

Rates and mechanics of rapid frontal accretion along the very obliquely convergent southern Hikurangi margin, New Zealand

Analysis of seismic reflection profiles, swath bathymetry, side‐scan sonar imagery, and sediment samples reveal the three‐dimensional structure, morphology, and stratigraphic evolution of the central to southern Hikurangi margin accretionary wedge, which is developing in response to thick trench fill sediment and oblique convergence between the Australian and Pacific plates. A seismic stratigraphy of the trench fill turbidites and frontal part of the wedge is constrained by seismic correlations to an already established stratigraphic succession nearby, by coccolith and foraminifera biostratigraphy of three core and dredge samples, and by estimates of stratigraphic thicknesses and rates of accumulation of compacted sediment. Structural and stratigraphic analyses of the frontal part of the wedge yield quantitative data on the timing of inception of thrust faults and folds, on the growth and mechanics of frontal accretion under variable convergence obliquity, and on the amounts and rates of horizontal shortening. The data place constraints on the partitioning of geological strain across the entire southern Hikurangi margin. The principal deformation front at the toe of the wedge is discontinuous and represented by right‐stepping thrust faulted and folded ridges up to 1 km high, which develop initially from discontinuous protothrusts. In the central part of the margin near 41°S, where the convergence obliquity is 50°, orthogonal convergence rate is slow (27 mm/yr), and about 75% of the total 4 km of sediment on the Pacific Plate is accreted frontally, the seismically resolvable structures within 30 km of the deformation front accommodate about 6 km of horizontal shortening. At least 80% of this shortening has occurred within the last 0.4±0.1 m.y. at an average rate of 12±3 mm/yr. This rate indicates that the frontal 30 km of the wedge accounts for about 33–55% of the predicted orthogonal contraction across the entire plate boundary zone. Despite plate convergence obliquity of 50°, rapid frontal accretion has occurred during the late Quaternary with the principal deformation front migrating seaward up to 50 km within the last 0.5 m.y. (i.e., at a rate of 100 km/m.y.). The structural response to this accretion rate has been a reduction in wedge taper and, consequently, internal deformation behind the present deformation front. Near the southwestern termination of the wedge, where there is an along‐the‐margin transition to continental transpressional tectonics, the convergence obliquity increases to >56°, and the orthogonal convergence rate decreases to 22 mm/yr, the wedge narrows to 13 km and is characterized simply by two frontal backthrusts and landward‐verging folds. These structures have accommodated not more than 0.5 km of horizontal shortening at a rate of < 1 mm/yr, which represents < 5% of the predicted orthogonal shortening across the entire plate boundary in southern North Island. The landward‐vergent structural domain may represent a transition zone from rapid frontal accretion associated with low basal friction and high pore pressure ratio in the central part of the margin, to the northern South Island region where the upper and lower plates are locked or at least very strongly coupled.

Plate coupling in the northern South Island and southernmost North Island, New Zealand, as illuminated by earthquake focal mechanisms

Subduction of the Pacific plate in the northern South Island and southernmost North Island of New Zealand is transitional, insofar as the crustal thickness of the Pacific plate increases significantly along strike in the northern South Island. Focal mechanisms of 145 events shallower than 100 km in this region have been determined using both first motion polarity data and amplitudes of seismogram envelopes. The stress regime in the subducted plate appears to be dominated by slab pull. T axes in both the upper and lower planes of the dipping seismic zone generally parallel the local dip of the zone, and the average azimuth of these T axes is rotated some 25° clockwise out of the direction of dip of the subducted plate. This can be related to the asymmetrical shape of the subducted slab. In contrast, the stress regime in the overlying plate appears to be dominated by subhorizontal compression. Low‐angle thrust events near the plate interface in Cook Strait and the southernmost North Island concentrate in two areas which may mark the updip and downdip edges of a locked region identified from Global Positioning System (GPS) observations. An absence of low‐angle thrust events near the plate interface in the northern South Island and the tendency of P axes of events in the subducted plate to become more horizontal suggest that plate coupling there is stronger than in the southernmost North Island. Differential coupling at the plate interface provides a viable mechanism for producing the large tectonic rotations seen in the northern South Island.

Continental subduction and three‐dimensional crustal structure: The northern South Island, New Zealand

The three‐dimensional Vp and Vp/Vs structure of a region where subduction transitions to oblique transform faulting has been determined using arrival times from 579 local earthquakes recorded during a temporary deployment, and 3146 earthquakes have been relocated. Between 40 km and 100 km depth, the subducted plate is imaged as a relatively low‐velocity feature in the uppermost mantle, reflecting the continental nature of the subducted crust in this region. An increase in amplitude of this low‐velocity feature from northeast to southwest can be related to an increase in the thickness of the crust of the subducted plate in this direction. Velocity variations within the subducted and overlying plates show some spatial correlation. This suggests an interaction between the plates which extends well beyond the plate interface and is consistent with other geophysical and geological evidence that the plate interface beneath Marlborough is currently not accommodating much active subduction. In the overlying plate, the Awatere fault is a major structural feature, associated with a low‐velocity zone extending to 23 km depth. There is a marked change in structure near this fault, with seismic velocities being lower to the southeast. A relatively high level of seismicity occurs in this region of lower seismic velocities, suggesting a relationship between the two. A possible explanation for this is elevated pore pressures caused by fluids derived from dehydration of the continental subducted crust. The low‐velocity region in the overlying plate coincides with the region of most intense active deformation, suggesting it is relatively weak.

Timing of Cretaceous extension and Miocene compression in northeast South Island, New Zealand: Constraints from Rb‐Sr and fission‐track dating of an igneous pluton

The Tapuaenuku Plutonic Complex is a large alkaline intrusion that was emplaced into Upper Jurassic‐Lower Cretaceous Torlesse greywacke/argillite near the axis of the Inland Kaikoura Range (northern South Island, New Zealand). Concordant Rb‐Sr, K‐Ar, and titanite/zircon fission‐track cooling ages of circa 105 to 93 Ma record a phase of igneous activity that was related to a period of mid‐Cretaceous extension in South Island, New Zealand. The updoming and uplift which accompanied the extension and igneous activity are also reflected by the uplift of the marine Split Rock Formation above sea level where it was eroded. On this was deposited a terrestrial sequence followed by shallow marine deposits which may represent the onset of a long period of transgressive marine sedimentation, the result of slow thermal subsidence after magmatism and rifting, from Late Cretaceous to Oligocene. This burial of the pluton is identified not only from the sedimentary relationships of the local stratigraphy but also by the apatite fission‐track ages which record later exhumation. During this time, New Zealand remained passively part of the Pacific Plate. In the early Miocene at circa 22 Ma the intrusion cooled through the partial annealing zone of apatite (approximately 120°–60° C). Exhumation was the result of early Miocene thrusting and shortening in Marlborough and inception of the present day transpressive plate boundary between the Indo‐Australian and Pacific Plates through New Zealand. Further evidence for this period of uplift and accompanying denudation is seen through the large quantities of coarse‐grained clastic sediments which were deposited in Marlborough at this time. The uplift rate required to raise the top of pluton to its present altitude (2885 m), based on a constant rate from 22 Ma until the present day, is approximately 0.2 mm yr−1. However, estimates of present‐day uplift rates of 10 mm yr−1 from summit heights and raised marine benches suggest that two periods of late Cenozoic uplift must have occurred with a long period of quiescence between, a conclusion supported by a major mid‐Miocene unconformity below Quaternary sediments in Marlborough.

Germination and seedling growth of an endangered native broom,Chordospartium muritaiA.W.Purdie (Fabaceae), found in Marlborough, South Island, New Zealand

Abstract In the wild, adult Chordospartium muritai shrubs and trees up to 6 m tall are confined to a single population in the northern South Island. There are few juveniles and seedlings have only recently been recorded. Seed viability, germination requirements, fate of seeds in the soil seed bank, and seedling growth were investigated. Seeds required scarification, after which a high proportion germinated over a wide range of temperatures, but particularly between 20°C and 24°C. The few seeds that survived a year's burial were still viable, suggesting the effects of soil scarification. Seedling growth was very slow and 65% of seedlings that survived the first 2 years reached a height of only 12 cm. Site factors including past grazing, competition from herbs, and lack of suitable substrates, are considered largely responsible for the failure of C. muritai regeneration. The management implications of the results are discussed.

The kinematics of northern South Island, New Zealand, determined from geologic strain rates

Relative motions within the distributed plate boundary zone of northern South Island, New Zealand, are determined through an inversion of geologic strain rate estimates. The Quaternary fault slip rate estimates define the shear strain rates, and rock uplift rates provide information on the horizontal divergence rates. An erosion rate to rock uplift rate ratio along with a crustal compensation factor is estimated in order to convert rock uplift rates to horizontal divergence rates. Because of the uncertainty in erosion rates, horizontal divergence rates are given a large standard error of . The three horizontal strain rate components obtained from these data completely define the horizontal velocity gradient tensor. Strain rate distributions are matched with spline polynomial functions, which can be constrained to behave rigidly within specified regions, such as the Pacific or Australian plates. Inversion of the strain rate distribution, assuming uniform erosion rates across the northern South Island, yields a velocity field that has small differences in both magnitude (10% larger) and direction with the NUVEL‐1A plate motion model between Pacific and Australian plates. A revised strain rate data set, obtained from a variable erosion model in which erosion rates are a linear function of the log of the average annual rainfall magnitudes, yields a velocity field with expected directions that are indistinguishable from the NUVEL‐1A plate motion model between Pacific and Australian plates, but velocity magnitudes are still 10–15% higher than the plate motion model. Therefore the average values of slip rate on strike‐slip faults in Marlborough, required by the NUVEL‐1A plate motion model, are typically close to the low end of the published range of slip values for those structures. The major strike‐slip structures within the Marlborough region are accommodating 80–100% of the total plate motion between Australia and Pacific plates on northern South Island; as much as 20% of the relative motion could be accommodated by shear generated by folding or thrust faulting with a single orientation between the major structures. The magnitude of plate motion contributed by the major faults indicates that the distributed geodetic shear strains measured across northern South Island, and discussed by Bibby [1981] and Walcott [1984], are not a form of irrecoverable strain but rather a feature of the strain field that will eventually be released primarily as concentrated slip on the strike‐slip structures within the region. Strain modeling indicates that the earthquake moment release over the last 150 years within the northwest Nelson province, which is west of the Marlborough region, is at least half an order of magnitude higher than the long‐term rate of moment release. Overall, the northwest Nelson region is playing only a minor role in taking up the long‐term plate motion.

Sea Level Variability on the West Coast of New Zealand

Abstract Variations in coastal sea level in the weather band (periods 5–20 days) on the west coat of New Zealand can largely be explained in terms of wind-forced coastal-trapped waves. Long-wave theory was applied to the North Island shelf with some success in hindcasting sea level at New Plymouth. On the southern South Island shelf, the sea level signal was found to propagate like a mode-one coastal-trapped wave, but the complex topographic variations made application of simple long-wave theory impossible. The sea level signal in the south of the region was highly correlated with alongshore wind stress over the shelf region to the north, suggesting that alongshore wind stress is the major forcing mechanism. In contrast, the generation of coastal-trapped waves by a coastal flux through Taranaki Bight/Cook Strait appears to be the dominant mechanism over the northern and central South Island shelf, from the present and previous studies.

Sustainability of Beilschmiedia tawa-Dominated Forest in New Zealand: Population Predictions Based on Transition Matrix Model Analysis

Beilschmiedia tawa (Lauraceae) is a common canopy tree which is often dominant in lowland forests in the North Island and northern South Island of New Zealand. The sustainability of B. tawa-dominated forests was investigated at Pureora Forest Park, west of Lake Taupo, central North Island, where a range of sites with different extents of disturbance by logging was studied. Demographic studies-estimates of seedfall, recruitment, growth, and mortality rates-yielded data for life history tables. Based on these, Leslie matrix models were used to determine the rate of increase of five populations. Of these, three logged populations were apparently declining, whereas unlogged forest showed moderate population increase. These population studies suggested that B. tawa is a K-selected species capable of regeneration only within forest. The smaller size-classes are shade tolerant and stems accumulate in the stripling size-class. High light conditions are needed for growth from this class to the sapling class.

Structure of the Hanmer strike-slip basin, Hope fault, New Zealand

Hanmer basin (10 x 20 km), located in northern South Island, New Zealand, is evolving where two major segments of the dextral strike-slip Hope fault are projected to converge across a 6- to 7-km-wide releasing step-over. The structural geometry and development of Hanmer basin does not conform to traditional pull-apart basin models. The respective fault segments do not overlap but are indirectly linked along the southwest margin of the basin by an oblique normal fault. The Hope River segment terminates in an array of oblique normal faults along the northwestern basin range front, and east-west-striking normal faults on the west Hanmer Plain. Faulted Holocene alluvial-fan surfaces indicate west Hanmer basin is actively subsiding evolving under north-south extension. The Conway segment along the southeastern margin of the basin terminates in a complex series of active fault traces, small pop-up ridges, and graben depressions. Early basin-fill sediments of Pleistocene age are being folded, elevated, and dissected as the eastern part of Hanmer basin is progressively inverted and destroyed by north-south contraction. The north margin of the basin is defined by a series of topographic steps caused by normal faulting outside of the area of the releasing step-over. These normal faults we interpret to reflect large-scale upper crustal collapse of the hanging-wall side of the Hope fault. New seismic reflection data and geologic mapping reveal a persistent longitudinal and lateral asymmetry to basin development. Four seismic stratigraphic sequences identified in the eastern sector of the basin thicken and are tilted southward, with insequence lateral onlaps occurring to the north and east, and also onto basement near the fault-controlled basin margins. The basin depocenter currently contains >1000 m of sediment adjacent to the south margin and is disrupted by faulting only at depth. In the western part of the basin, the sediment fill is thinner ( Today the rate of basin deepening under transtension at the western end is matched by its progressive inversion and destruction under transpression in the eastern sector, with the oldest basin fill now being recycled. We propose a hybrid model for Hanmer strike-slip basin, one in which geometric elements of a fault-wedge basin (downward and upward tipped, spindle-shaped ends) are combined with those of a pull-apart basin (step-over region between the major fault segments). We also conclude that changes in fault geometry (releasing and restraining bends and step-overs) at a variety of scales and over short distances control the development of the extensile and contractile parts of the basin and three-dimensional basin asymmetry. Strain partitioning is complex and cannot be related simply to local reorientation of the regional stress field.

Marine reserve site selection along the Abel Tasman National Park coast, New Zealand: Consideration of subtidal rocky communities

AbstractAt present, marine reserves do not represent the full range of community types throughout New Zealand.To assist with the placement of a marine reserve along the Abel Tasman National Park coast (northern South Island), dominant subtidal laminarian and fucoid algae, echinoids and herbivorous molluscs were quantitatively investigated. Results from 100 quadrats collected from 19 random transects at six selected sites showed that algae and grazer assemblages varied between granite and limestone substrata.Granite had a high percentage cover of crustose coralline algae (mean 82%–90%), a sublittoral fringe of brown macroalgae and no Ecklonia radiata or red foliose algae. Limestone sites were distinguished by a relatively low percentage cover of coralline algae (mean 13%) and high cover of foliose red algae and E. radiata (2%–36% cover and 0.2–13.9 stipes m−2, respectively).On limestone, molluscs Turbo smaragdus and Cookia sulcata, and the echinoid Evechinus chloroticus were larger than those on granite. On limestone sites with little macroalgae, herbivore size was intermediate. Grazers were more abundant on granite than limestone (mean 34.6 m−2, and 10.8 m−2 respectively).Differences in herbivore composition were recorded between granite substrata, while both algal and herbivore composition varied between limestone sites.We suggest that a variety of environmental factors including substratum influence algal and herbivore assemblages along the Abel Tasman coast.It is recommended that selection of a marine reserve site or sites along the coast of Abel Tasman National Park recognizes differences in community structure both between and within limestone and granite substrata.

Exceptionally deep earthquakes in the northern South Island, New Zealand

Abstract The intermediate depth seismicity of the northern part of the South Island has been found in the past to extend to a maximum depth of about 230 km. On 1990 Jun 22, an event occurred which was routinely located at a depth of 348 km in this region. Subsequently, a data search revealed another event in 1988 of similar depth and location, less well determined but acceptable. Both events have a magnitude ML = 3.4–3.5. In the future, consideration of the deep structure in this region should take into account the possible extension of seismicity to greater depths.

The 1968 May 23 Inangahua, New Zealand, earthquake: An integrated geological, geodetic, and seismological source model

Abstract The 1968 Inangahua, New Zealand, earthquake occurred in the West Coast Basin and Range Province, northwest of the main plate boundary zone in northern South Island. At MS 7.4, it is not the largest known earthquake in the province, but it has been the subject of thorough seismological, geological, and geodetic documentation. Re interpretation of past observations and more recent data, in the light of new structural and tectonic theories, has produced a new source model for the earthquake. The data suggest that at least 4 m of reverse slip occurred on a fault plane dipping 45° to the northwest beneath the northern part of the Grey‐Inangahua Depression, an area previously inferred to be on the footwall of major reverse faults bounding the ranges on either side of the depression. The seismogenic fault may have propagated north and south across older geological structures in recent times. Faulting within basement is occurring on pre‐existing faults and is accommodating some of the compressional compo...