Mark S. Rattenbury

Plutonic rocks of the Median Batholith in eastern and central Fiordland, New Zealand: Field relations, geochemistry, correlation, and nomenclature

Abstract This paper provides a comprehensive description of all major plutonic rock units in Fiordland between Lakes Poteriteri and Te Anau, and the heads of Doubtful and George Sounds. Plutonic rocks comprise c. 80% of the basement in the area described, the remainder being metase dim entary and metavolcaniclastic rocks. The plutonic rocks, of which c. 50% are granitoids, were emplaced in three phases—at c. 492 Ma, between c. 365 and 318 Ma, and between 168 and 116 Ma. Correlatives of the Devonian Karamea Suite emplaced between c. 375 and 367 Ma, and the Triassic to Early Jurassic part of the Darran Suite emplaced between c. 230 and 168 Ma, are not present in the area described here. The strongly deformed Late Cambrian to Early Ordovician Jaquiery Granitoid Gneiss is one of the oldest plutonic rocks yet discovered in New Zealand and is of similar age to plutonic rocks within the Ross and Delamerian Orogens of Victoria Land and South Australia. Rocks emplaced between c. 365 and 318 Ma include Ridge Suite S‐type granitoids and closely related S/A‐type plutons, Foulwind Suite A‐type mafic and granitoid plutons, Tobin Suite I‐type granitoids, and several unassigned mafic plutons. Rocks emplaced between 168 and 116 Ma include extensive c. 168–128 Ma old calc‐alkaline LoSY gabbros, diorites, and granitoids of the Darran Suite, c. 165–135 Ma old hypersolvus perthitic syenogranites and peralkaline granitoids, c. 125 Ma gneissic diorite similar to the Western Fiordland Orthogneiss, and c. 123–116 Ma old quartz diorites and granitoids of the HiSY Separation Point Suite. Plutons from each suite tend to be concentrated in distinct NNE‐striking parallel belts up to 20 km wide and 110+ km long. These belts are one of the key features which define the regional structural grain of Fiordland basement geology. Their strike remains constant from the Carboniferous through to the Cretaceous. S, S/A, and A‐type plutons of the Carboniferous Ridge and Foulwind Suites are confined to a 125 km long but discontinuous belt in southern and central Fiordland, wholly within the areal extent of early Paleozoic metase dim entary basement. Volumetrically minor Carboniferous Tobin Suite I‐type granitoids are confined to the area east of exposed early Paleozoic metasedimentary basement. Much of eastern Fiordland is underlain by an extensive belt of heterogeneous Darran Suite rocks. Darran Suite rocks extend from Stewart Island to the Darran Mountains of northern Fiordland, forming a belt c. 15 km wide and 300 km long. Correlative Darran Suite rocks also occur further west where they intrude early Paleozoic metasediments, indicating that Jurassic to Early Cretaceous arc‐related plutonism and volcanism occurred inboard of the edge of early Paleozoic basement in some parts of the Median Batholith. Distinctive Jurassic, pink, hypersolvus syenogranite and alkalic granitoids form a narrow discontinuous belt within the wider calcalkaline Darran Suite. Cretaceous Separation Point Suite plutons form two major belts, one in easternmost Fiordland partially covered by Cenozoic sedimentary rocks, and the other stitching inboard and outboard parts of the Median Batholith in central Fiordland.

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.

A digital rock density map of New Zealand

Digital geological maps of New Zealand (QMAP) are combined with 9256 samples with rock density measurements from the national rock catalogue PETLAB and supplementary geological sources to generate a first digital density model of New Zealand. This digital density model will be used to compile a new geoid model for New Zealand. The geological map GIS dataset contains 123 unique main rock types spread over more than 1800 mapping units. Through these main rock types, rock densities from measurements in the PETLAB database and other sources have been assigned to geological mapping units. A mean surface rock density of 2440kg/m^3 for New Zealand is obtained from the analysis of the derived digital density model. The lower North Island mean of 2336kg/m^3 reflects the predominance of relatively young, weakly consolidated sedimentary rock, tephra, and ignimbrite compared to the South Islands 2514kg/m^3 mean where igneous intrusions and metamorphosed sedimentary rocks including schist and gneiss are more common. All of these values are significantly lower than the mean density of the upper continental crust that is commonly adopted in geological, geophysical, and geodetic applications (2670kg/m^3) and typically attributed to the crystalline and granitic rock formations. The lighter density has implications for the calculation of the geoid surface and gravimetric reductions through New Zealand.

Structural setting of the Globe‐Progress and Blackwater gold mines, Reefton goldfield, New Zealand

Abstract Globe‐Progress and Blackwater mines, the two largest mines in the Reefton goldfield of North Westland, have been placed into a regional structural context dominated by upright, tight folding of the host‐rock Ordovician Greenland Group turbidites. The Greenland Group lacks distinctive lithostratigraphic marker horizons or bio‐stratigraphic control, but mapping out areas of common structural facing from the determination of bedding‐cleavage vergence along two transects has revealed a series of north‐trending, generally shallow plunging folds. Undisrupted fold hinge closure is rarely seen, but the bedding‐cleavage vergence boundaries are inferred to be fold hinge regions or in some instances post‐folding faults. Vergence boundaries are often accompanied by changes in sedimentological younging. The technique defines the location of fold hinges, faults and, potentially, because of the structural control on mineralisation, gold‐bearing quartz veins. The Globe‐Progress gold‐bearing quartz vein system occurs in a zone of relatively tight and shorter wavelength folds within a wider framework of more open, longer wavelength, east‐verging folds. On the western flanks of Globe Hill, the Globe‐Progress shear separates east‐verging folded strata in the hanging wall from more symmetrically folded, commonly overturned strata in the footwall. The shear curves from an easterly strike to a southerly strike and steepens from 30–40° dips at depth to near‐surface dips of 70–80°. Minor structures in the hanging wall indicate west over east‐directed movement of the Globe‐Progress shear. The Blackwater quartz vein system occurs parallel to steeply west‐dipping, west‐facing strata in a section of tight upright folds. The folds occur within the hinge area of a regional synclinal structure, which in turn is one of a series of 4–5 km long wavelength folds.

A 3D Geological Model for Christchurch City (New Zealand): A Contribution to the Post-earthquake Re-build

Geological maps for areas of high density population play an important role in the design and planning of building and network infrastructure as well as management of subsurface resource such as groundwater and aggregate. The Christchurch urban area, New Zealand, experienced high ground shaking accelerations during the 2010–2011 Canterbury earthquake sequence (CES) and the built environment suffered serious damage from shaking and consequent liquefaction. Damage from the earthquakes focused efforts to improve understanding of materials and their properties beneath that city. This paper presents a summary of work undertaken to build a high resolution 3D geological model of these materials, including geomorphological mapping and analysis of materials using a large number of drillhole logs and digital cone penetration tests. An integrated 3D geological model and 3D numerical geotechnical models will contribute to the re-building of Christchurch and its infrastructure following the destructive earthquakes. Potential uses include planning optimal routing for major horizontal infrastructure (sewer, drainage and water supplies) where some fore-knowledge of foundation conditions will allow re-alignment to easier substrates, more accurate cost estimates and more focused site specific foundation testing.

Pember Diorite—an Early Jurassic intrusion in the Rakaia Terrane, Puketeraki Range, Canterbury, New Zealand

Abstract Biotite‐hornblende‐augite microdiorite within the Puketeraki Range and Lees Valley of inland North Canterbury forms small stocks and dikes intruding Rakaia Terrane rocks and is here named the Pember Diorite. Geochemically the Pember Diorite has medium to high‐K tholeiitic characteristics. Ar‐Ar dating of amphibole gives a crystallisation age of 185.6±3.3 Ma (2σ) (Early Jurassic), somec. 25 m.y. younger than the Late Triassic Rakaia Terrane rocks it intrudes. The Pember Diorite has undergone prehnite‐pumpellyite facies metamorphism, indicating that at least some of the regional low‐grade metamorphism in the Rakaia Terrane is younger than c. 186 Ma.

The aeromagnetic expression of New Zealand’s Alpine Fault: regional displacement and entrainment of igneous rock

ABSTRACT The Alpine Fault is very well delineated by recent high-resolution aeromagnetic data sets over much of its length in New Zealand’s South Island. Aeromagnetic data acquired over parts of the West Coast, Tasman and Marlborough regions for the New Zealand Government reveal different types of total magnetic intensity anomalies associated with the fault that can be characterised as truncated, step, ridge, trough and broad anomalies. Fault-terminated Permian ultramafic rocks, Late Cretaceous basalt and some Early Cretaceous intrusions are attributed to the more prominent truncated anomalies. The pre-eminent step anomaly is associated with relatively magnetic Alpine Schist juxtaposed against variable but generally less magnetic Western Province rocks. Strongly positive ridge anomalies occurring in the Alpine Fault zone are associated with Mesozoic mafic metavolcanic and ultramafic rocks. A trough anomaly associated with the Alpine Fault occurs in the southernmost survey and may reflect some hydrothermal demagnetisation. Broad, low-amplitude anomalies underlying the hanging wall are attributed in one instance to the extension of the magnetic Hohonu Range granitoids and dikes extending southeast in the footwall and in other places as magnetic schist rocks in the hanging wall. The apparent great-circle displacement of the Alpine Fault based on offset of magnetic anomalies associated with the Dun Mountain Ultramafics Group rocks is c. 461 km, in close agreement with the more precise 459 km inferred from geological map offset.

Geochemical baseline soil surveys for understanding element and isotope variation across New Zealand

ABSTRACT Systematic grid-based soil geochemical baseline surveys have been undertaken in the South Island since 2013 at regional scales in Southland, Otago, Buller, Nelson and Marlborough and more detailed scales such as in Dunedin City and Nelson’s Richmond Range. More than 1300 sites and 3000 samples have now been analysed for their chemical element composition, supported by rigorous quality control procedures. The initial data interpretation shows strong underlying rock control on soil chemistry and localised anthropogenic input, principally in urban areas and through fertiliser application in higher intensity farming areas. The technical success and potential multi-use application of these surveys provide an argument for undertaking a national systematic geochemical baseline survey designed around international best practice.