Ray Wood

Gravity and magnetic modelling of the Hikurangi Plateau

Abstract Gravity modelling of the Hikurangi Plateau, east of North Island, New Zealand, indicates the crust thickens from about 10 km in the north to about 15 km adjacent to the Chatham Rise. It appears to thicken by a lesser amount to the west, towards the Hikurangi Trough. The Chatham Rise crustal thickness is 23–26 km, thinning to the south towards the Bounty Trough and east of the Chatham Islands. The gravity models indicate the eastern margin of the Hikurangi Plateau intersects the Chatham Rise at about 176°W. The area north of the Chatham Rise and east of 176°W could be either part of the Hikurangi Plateau crustal block, or oceanic crust thickened by sedimentary deposits. Postulated segmentation of the down-going slab beneath the North Island is not reflected in variations in crustal structure east of the trench. Magnetic modelling confirms the interpretation from seismic reflection data of widespread volcanism much of which may date from plateau formation.

The Hikurangi Plateau

Abstract Recently collected geophysical data reveal the basement and sedimentary structure of the Hikurangi Plateau presently being subducted beneath the North Island of New Zealand. We have subdivided the plateau into 6 areas on the basis of basement structure. Two basins separated by a basement high trend subparallel to the Chatham Rise, probably filled with Mesozoic sediments. Another is parallel with the present plate margin along the east coast of the North Island and is filled with Late Cenozoic sediments. The northern part of the plateau is characterised by shallow volcanic basement. The southeast part of the region is complex but has several kilometers of sediments deformed by volcanic intrusions. At least two episodes of volcanism have occurred. The older, probably Mesozoic episode, produced numerous large, isolated edifices, some exposed as seamounts and others buried by sediments. Activity resumed in the Neogene, resulting in the emplacement of smaller knolls and widespread intrusions. Variation in crustal thickness north of the Chatham Rise supports the hypothesis of a convergent margin along the north flank of the Chatham Rise in the Mesozoic. The structure of the Hikurangi Plateau may explain the origin of the postulated physiographic variations and segmentation of the downgoing slab along the active Kermadec-Hikurangi subduction margin.

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.

Paleogene seafloor spreading in the southeast Tasman Sea

Magnetic anomalies 11–18 have been identified in the southeast Tasman Sea, the area of ocean crust southwest of Fiordland, New Zealand, west of the Puysegur Trench (the present plate boundary) and southeast of the older (80–58 Ma) ocean crust of the Tasman Sea basin. Structures associated with two changes in spreading direction are preserved in the area. Spreading between the Australian and Pacific plates began in the Eocene, about 40 Ma in this area, orthogonal to Cretaceous-Paleocene spreading between them in the Tasman Sea. The boundary between the two ages of ocean crust is abrupt and associated with what we have interpreted as marginal uplift blocks and rift basins of the Resolution Ridge system. Rifting appears to have propagated northeast along a fracture zone. The northern blocks of the Resolution Ridge system may be isolated fragments of continental crust of the Campbell Plateau. Between anomalies 18 and 11 (40–30 Ma) the spreading rate was about 1.5 cm/yr. Swath mapping and satellite altimetry data show that a second change in relative plate motion direction began shortly after anomaly 11, about 30 Ma, and led to a change from tension to transcurrent motion along the plate boundary. The timing of the changes in spreading direction agrees with tectonic events interpreted from onshore data.

Morphostructure of an incipient subduction zone along a transform plate boundary: Puysegur Ridge and Trench

Multibeam bathymetric and geophysical data reveal a major strike-slip fault that extends along the summit of the Puysegur Ridge east of the Puysegur Trench. The northward structural development of this ridge-trench system illustrates the evolution of an incipient subduction zone along a transform plate boundary that has been subjected to increasing transverse shortening during the past 10 m.y. At the southern end of the trench, where subduction has not yet started, the Puysegur Ridge has a narrow (<50 km) steep-sided cross section, and the axial strike-slip fault separates a shallow (125–625 m), flat-topped eastern crest from a deeper (400–1600 m) western crest; these characteristics indicate differential uplift during the initial stage of shortening. On the lower plate an incipient, 5.2-km-deep trench developed in conjunction with normal and reverse faults, suggesting strong interplate coupling across the trench. Northward, the ridge broadens linearly to 80 km wide, its western flank has locally collapsed, and the ridge summit has subsided, possibly by 1.5 km, suggesting that the interplate coupling decreases and that a Benioff zone is being formed. Concomitant to the northward ridge evolution, the trench deepens to 6.2 km and normal fault throws increase along its outer wall, indicating greater flexure of the downgoing plate.

A new look at the New Caledonia Basin, an extension of the Taranaki Basin, offshore North Island, New Zealand

Abstract The New Caledonia Basin is a major bathymetric trough extending north-westwards from Taranaki and continuing past New Caledonia. This paper is concerned with the 250 km of the feature closest to New Zealand and adjacent to the Taranaki Shelf. The New Caledonia Basin appears to have formed as the surface expression of a failed rift system. Gravity models, constrained by seismic reflection and refraction data, indicate thinned crust below the basin. The date of inception of rifting is not known, but it must be pre-Cenozoic and may be pre-Cretaceous. New seismic reflection data reveal tensional block faulting within the basin and on the flanks of the Challenger Plateau and the West Norfolk Ridge. Offset gravity anomalies and apparent compressional faulting observed on seismic data provide evidence for transverse, possibly transform, faulting accompanying and facilitating oblique extension. More than 1.5 s (TWT) of sediments were deposited during rifting. This is equivalent to a thickness of up to 3 km. They are likely to consist of Cretaceous clastics and volcaniclastics deposited in fluvial and marginal marine environments. Terrigenous clays and mudstones probably cap the rift sequence. Post-rift sedimentation commenced with thick authigenic limestone, reflecting regional thermal subsidence and remoteness from a clastic source. There is little record of tectonic activity until at least the Late Eocene, and probably not until the Early Miocene when volcanic activity began. Seismic sections show domal uplift at the base of the Challenger Plateau and the formation of several large submarine volcanoes. This activity may have been related to renewed rifting in the Bellona Trough. An average thickness of approximately 0.5 km of Plio-Pleistocene sediments onlap the margins of the basin. Channels and fans are evident on the seismic data and account for the distribution of a significant portion of the sediment eroded from the rising Southern Alps. The fragmentation of this part of the Gondwana margin resembles the evolution of the North-east Atlantic. In particular, the structure of the Rockall Trough closely resembles that of the New Caledonia Basin. Hydrocarbon source rocks may be present in the rift sequence, which has been buried to a depth great enough for thermal maturation. Structural and stratigraphic traps may exist at a number of levels.

Zealandia: Earth’s Hidden Continent

A 4.9 Mkm2 region of the southwest Pacific Ocean is made up of continental crust. The region has elevated bathymetry relative to surrounding oceanic crust, diverse and silica-rich rocks, and relatively thick and low-velocity crustal structure. Its isolation from Australia and large area support its definition as a continent—Zealandia. Zealandia was formerly part of Gondwana. Today it is 94% submerged, mainly as a result of widespread Late Cretaceous crustal thinning preceding supercontinent breakup and consequent isostatic balance. The identification of Zealandia as a geological continent, rather than a collection of continental islands, fragments, and slices, more correctly represents the geology of this part of Earth. Zealandia provides a fresh context Nick Mortimer, GNS Science, Private Bag 1930, Dunedin 9054, New Zealand; Hamish J. Campbell, GNS Science, P.O. Box 30368, Lower Hutt 5040, New Zealand; Andy J. Tulloch, GNS Science, Private Bag 1930, Dunedin 9054, New Zealand; Peter R. King, Vaughan M. Stagpoole, Ray A. Wood, Mark S. Rattenbury, GNS Science, P.O. Box 30368, Lower Hutt 5040, New Zealand; Rupert Sutherland, SGEES, Victoria University of Wellington, P.O. Box 600, Wellington 6140, New Zealand; Chris J. Adams, GNS Science, Private Bag 1930, Dunedin 9054, New Zealand; Julien Collot, Service Géologique de Nouvelle Calédonie, B.P. 465, Nouméa 98845, New Caledonia; and Maria Seton, School of Geosciences, University of Sydney, NSW 2006, Australia in which to investigate processes of continental rifting, thinning, and breakup.

Carboniferous granite basement dredged from a site on the southwest margin of the Challenger Plateau, Tasman Sea

Abstract Discordant zircon fractions from a granite sample dredged off a basement horst on the western margin of the Challenger Plateau yield a 335 ± 7 Ma lower intercept date interpreted as the crystallisation age of the granite. This age, and the modal composition of the granite, is similar to that of the Karamea Suite of Westland and Nelson, New Zealand, and some Tasmanian granites. The concordia upper intercept date of 1747 ± 300 Ma implies the presence of Proterozoic continental crustal material in the source region of the granite. The Challenger granite is distinct from older S‐type granites of southeastern Australia and I‐type granites of northern Victoria Land and Marie Byrd Land, Antarctica. Subsequent to emplacement, the granite was brecciated and hydrothermally altered. A K‐Ar age of 95 Ma on hydrothermal sericite indicates that this event overlapped with a major crustal extension event recorded in Westland and Nelson, and predates the oldest known sea floor in the Tasman Basin by at least 11 Ma.

Structure and seismic stratigraphy of the western Challenger Plateau

Abstract The Challenger Basin is a newly discovered sedimentary basin beneath the western margin of the Challenger Plateau. Seismic and gravity data near the site of two Deep Sea Drilling Project holes (DSDP 284 and 593) show more than 2 km of sediment in the basin, probably formed in the early stages of Late Cretaceous rifting in the Tasman Sea. Thinning of about 5 km occurred before separation took place to the southwest and stranded this portion of the rift system. Regional subsidence ensued in the early Cenozoic, followed by a major phase of submarine basaltic volcanism about 38 Ma ago. This volcanism may have been related to the onset of spreading southwest of New Zealand. Subsequent sedimentation has been virtually entirely pelagic. The Oligocene unconformity commonly observed in the New Zealand region was not found in the wells, presumably because of either oceanographic changes induced by tectonic events, or redeposition from adjacent highs. Changes in ocean current patterns and biogenic productiv...

Cretaceous‐Tertiary tectonic history of the Fiordland margin, New Zealand

Abstract Integration of regional seismic reflection data with swath bathymetry and imagery data and onshore geology has revealed the complex sequence of tectonic events that have affected the development of the offshore Fiordland margin. There are no core or dredge samples to constrain the timing of these events, but comparison with recent work on the Tertiary evolution of the Australia‐Pacific plate margin provided a temporal framework for the subsurface interpretation. The integrated dataset allows the prediction of the age and likely lithology of the seismic sequences, the timing of tectonic events, and details of how structures along the Fiordland plate margin developed in response to changes in relative plate motion direction. We conclude that Cretaceous rifting in the Tasman Sea formed half‐grabens along Caswell High and beneath the Fiordland Basin. Up to several kilometres of Cretaceous to Early Eocene sediments, probably terrestrial and marginal marine sediments derived from the Campbell Plateau, are preserved in these grabens. A second phase of rifting in the Eocene, associated with the formation of the Southeast Tasman Sea, resulted in the separation of the Caswell High and Fiordland Basin block from the Campbell Plateau and the start of its journey northwest, northeast, and finally east to its present position adjacent to Fiordland. The rift sediments are overlain by a relatively uniform sequence, probably Late Eocene to Mid Miocene deep‐water carbonates. Strike‐slip faults developed along the continental shelf and slope in the Miocene, and increasing margin‐normal shortening in the Pliocene‐Pleistocene, led to the development of thrust faults along the margins of sediment lobes in the Fiordland and Milford Basins. The youngest sediments are up to several kilometres thick, probably Late Miocene‐Quaternary elastics derived from Westland and Fiordland.