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Mortimer N.,Institute of Geological & Nuclear Sciences
Journal of Structural Geology | Year: 2014

Most of the South Island of New Zealand lies within an Eocene-Recent continental shear zone related to Pacific-Australia plate motion. Macroscopic finite strain in this shear zone has, in the past, been tracked through the deformation of the Dun Mountain Ophiolite Belt. This paper identifies additional sub-vertical basement strain markers including: Buller-Takaka Terrane boundary, Darran Suite and Jurassic volcanic belt within the Median Batholith, Taieri-Wakatipu-Goulter Synform axial trace, Esk Head Melange and bedding form surfaces within the Buller, Takaka and Torlesse terranes. An analysis of the oroclinal bend over the entire Zealandia continent shows that it is a composite feature involving pre- as well as post-Eocene bending of basement structures. Satisfactory paleogeographic reconstructions of Zealandia cannot be made without the use of substantial regional scale, non-rigid intracontinental deformation. © 2013 Elsevier Ltd.


Reyners M.,Institute of Geological & Nuclear Sciences
Earth and Planetary Science Letters | Year: 2013

Recent work involving relocation of New Zealand seismicity using a nationwide 3-D seismic velocity model has located the subducted western edge of the Hikurangi Plateau. Both the thickness (ca. 35km) and the area of the plateau subducted in the Cenozoic (ca. 287,000km2) are much larger than previously supposed. From ca. 45Ma, the westernmost tip of the plateau controlled the transition at the Pacific/Australia plate boundary from subduction to the north to Emerald Basin opening to the south. At ca. 23Ma, curvature of the subduction zone against the western flank of the buoyant plateau became extreme, and a Subduction-Transform Edge Propagator (STEP fault) developed along the western edge of the plateau. This STEP fault corresponds to the Alpine Fault, and the resulting Pacific slab edge is currently defined by intermediate-depth seismicity under the northernmost South Island. Alpine STEP fault propagation was terminated at ca. 15Ma, when the western edge of the plateau became parallel to the trench, and thus STEP fault formation was no longer favoured. Wholesale subduction of the plateau at the Hikurangi subduction zone began at ca. 10Ma. The development of a subduction décollement above the plateau mechanically favoured deformation within the overlying Australian plate continental crust. This led to inception of the Marlborough fault system at ca. 7Ma, and the North Island fault system at 1-2Ma. At ca. 7Ma, the western edge of the converging plateau again became more normal to the trench, and there is evidence supporting the development of a second STEP fault beneath the Taupo Volcanic Zone until ca. 3Ma. Both episodes of STEP fault development at the plateau edge led to rapid slab rollback, and correspond closely with episodes of backarc basin opening to the north in the wider Southwest Pacific. The Cenozoic tectonics of New Zealand and the Southwest Pacific has been strongly influenced not only by the resistance to subduction of the buoyant Hikurangi Plateau, but also by the shape of its western edge and changing angle of attack of this edge at the plate boundary. © 2012 Elsevier B.V.


Uruski C.I.,Institute of Geological & Nuclear Sciences
Marine and Petroleum Geology | Year: 2010

The islands of New Zealand cover an area of approximately 250,000 km2, but the New Zealand Exclusive Economic Zone (EEZ) extends to around 4 million km2 and recent confirmation of New Zealand's Extended Continental Shelf (ECS) has added a further 1.7 million km2 to the country's marine estate. Within the 5.7 million km2 of New Zealand's marine territory, approximately 1.2 million km2 are underlain by sedimentary accumulations which may be thick enough to expel petroleum. New Zealand is almost entirely surrounded by sedimentary basins and all that have been explored for petroleum have deepwater extensions. In addition, several deepwater basins have no onshore or shallow water portions (Fig. 1). The northeastern seaboard of the country was once part of the Eastern Gondwana margin. A series of basins that developed along that margin may be related to the Gondwana trench system. They include the Northland Slope Basin to the northeast of the Northland Peninsula, probably the East Coast Basin of North Island and the Chatham Slope Basin (Figs. 1 and 2). The eastern seaboard of North Island is occupied by the East Coast Basin, a fold and thrust belt above the subducting slab of the Pacific plate. Despite more than 100 years of exploration, the East Coast Basin remains a frontier basin and deepwater extensions of the East Coast Basin, including the Raukumara Sub-basin to the north of the Raukumara Peninsula and the Pegasus Sub-basin to the east of Cook Strait (Fig. 1) have only recently begun to be investigated. Northwest of the country, three belts of basins are known; the Northland and Reinga basins occupy the zone closest to the Gondwana margin's volcanic arc and may have originated as fore-arc basins. The head of the New Caledonia Basin contains deepwater parts of the currently productive Taranaki Basin and this basin may have originated in Mesozoic time as a back-arc rift basin. The Northland/Reinga and Deepwater Taranaki sedimentary accumulations are separated by the buried West Norfolk basement ridge and both have extensions beyond New Zealand's jurisdiction and into Australian territory. The western side of the Challenger Plateau may also contain thick sedimentary accumulations. In particular, the Bellona Gap between Challenger Plateau and the Lord Howe Rise and the Monawai Basin along the western flank of the Lord Howe Rise both contain thick sedimentary accumulations, and may have originated as intra-continental basins. To the east of New Zealand, the pattern continues. Basins associated with the fore-arc and trench zone of Gondwana include the Pegasus and Chatham Slope basins. The Chatham Rise basins may have originated as part of the fore-arc, while the Canterbury Basin and the Bounty Trough occupy the back-arc location. The submerged continental mass of New Zealand extends east and southeast of South Island as the Campbell Plateau. The Great South Basin occupies a large area of the Campbell Plateau, where four other named sedimentary accumulations; the Pukaki, Outer Pukaki, Campbell and Outer Campbell basins remain largely unexplored. To the west of Stewart Island, the Solander Basin is the offshore continuation of the onshore Te Anau and Waiau basin system. These southern basins were all created by rift faulting prior to, and during Late Cretaceous opening of the Tasman Sea and Southern Ocean. More than 50 oil and gas seeps are known from the West Coast region of South Island, some associated with faults and others in coal exploration wells. To date, no commercial quantities of petroleum have been discovered within the region, although coal-bearing sedimentary basins extend offshore. Although the petroleum histories of most of the onshore and nearshore areas are considered to begin with Late Cretaceous rifting leading to break-up of Gondwana and basin formation, new data suggests that the rifting history of the 500 km wide Gondwana margin zone that we now know as the New Zealand mini-continent extended further back into the Mesozoic. Basins that were previously considered to be Cretaceous rift basins appear to have developed across older features, probably exploiting pre-existing faults and regions of thinned crust. In onshore New Zealand and on the continental shelf, many of the source rocks are coaly and were deposited during the rifting period associated with Late Cretaceous plate separation and formation of the Tasman Sea and Southern Ocean. During basin formation, the earliest sediments to be deposited were commonly fluvial, lacustrine, deltaic and nearshore facies with an increasing marine influence as the region foundered through the latest Cretaceous and Paleogene. The exceptions to this pattern are the basins of the East Coast of North Island which developed near the Gondwana margin and appear to be almost entirely marine. The present plate boundary was initiated near the start of the Neogene as the New Zealand landmass emerged in response to plate collision. Many of the more spectacular structures in the New Zealand sedimentary basins were formed during the Neogene. Meanwhile, the deepwater basins away from the plate margin continued a quieter development. Some inversion occurred, but generally not to the extent of the nearshore and onshore regions. The relatively gentle structural evolution of the deepwater basins increases the likelihood of discovering large hydrocarbon fields in their intact structural traps. © 2010 Elsevier Ltd.


Zhao J.X.,Institute of Geological & Nuclear Sciences
Bulletin of the Seismological Society of America | Year: 2010

Attenuation models derived from recorded ground motions are still important elements of probabilistic seismic hazard studies. Engineers use empirical attenuation models to derive the displacement demand for a site of interest from an earthquake at a given location. Many attenuation models have been published for different parts of the world and for different types of earthquakes. Most models have a simple function of constant or magnitude-dependent geometric spreading, and seldom consider well-known seismological effects such as Moho reflection for shallow crustal earthquakes, multiple travel paths and constructive interference for subduction earthquakes, and special characteristics of volcano zones. The reason for not accounting for such effects may be the desire for simplicity in the attenuation functional forms for engineering applications and a lack of records from which to reliably identify these effects quantitatively. In this article, a large set of strong-motion records obtained from dense recording networks in Japan is used to derive geometric attenuation functional form and a possible manner to model the effect of volcanic zones. A liberal approach is taken to introduce a relatively large number of parameters that can account for known seismological effects while retaining a fairly simple attenuation functional form, based on analyses of residuals from simple models similar to those published previously. Preliminary results are reported here, together with the proposed geometric attenuation function forms and plausible explanation of the physical process that leads to the proposed geometric attenuation functions. The proposed model shows a large increase in the maximum likelihood from the random effects methodology, the elimination of bias in the distribution of residuals with respect to source distance, and much improved fitting for well-recorded earthquakes.


Reyners M.,Institute of Geological & Nuclear Sciences
Journal of Volcanology and Geothermal Research | Year: 2010

We use first motion polarity data from small earthquakes recorded by a recent dense seismograph deployment to determine 16 new focal mechanisms at the southern termination of the Taupo Volcanic Zone (TVZ). These crustal earthquakes are between 11 km and 41 km deep, and have focal mechanisms which range from normal to strike-slip. We then combine these focal mechanism data with previously determined focal mechanisms to investigate earthquake strain directions. A prominent feature of the combined data is the consistency of the T-axes, which indicates that subhorizontal, ~ NNW extensional strain predominates at all depths in the crust at the southern termination of the TVZ. We then use all currently available first motion data to invert for the stress tensor orientation. First motion data from earthquakes throughout the crust show a consistent minimum principal stress (σ3) orientation, which is subhorizontal and oriented NW. The σ3 azimuth that we observe (316°) is perpendicular to the strike of recent aligned vents in the southern TVZ, very similar to the extension direction in the Ruapehu segment of the TVZ determined from structural data, and consistent with the σ3 direction determined from a stress inversion using crustal earthquakes in the central TVZ north of Lake Taupo. This suggests that the stresses driving the opening of the TVZ continue to be active at its southern termination. The stress and strain results, combined with the earthquake distribution, are consistent with a model where the TVZ is only the present day surface expression of an extensive region of active mantle corner flow that is progressively eroding thicker crust to the south. We also investigate the temporal evolution of stress around Mt Ruapehu, the southernmost large andesitic volcano in the TVZ, given previous suggestions that this may change during the eruption cycle. We find no major change in the stress tensor orientation at the time of the 1995 eruption, using first motion data from earthquakes 15-20 km from the volcano. © 2009 Elsevier B.V. All rights reserved.

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