Collision-related fore-arc and back-arc evolution of the Northern Sunda ARC

Comparison with other arc systems suggests that in the fore-arc area of the northern Sunda Arc emplacement of chromite and jadeite-bearing serpentinite melange diapirs and deposition of olistostromes were caused by Campanian collision with a continental fragment since underthrust eastwards beneath the arc. The age and position of east-directed thrusts and associated tin granites in the continental back-arc area implies that thrusting and generation of granites were genetically related to the collision, and that the back-thrusts were coupled to the fore-arc thrusts by a decollement beneath the fore-arc basin and magmatic arc. Subsidence and sedimentation in the fore-arc basin succeeded collision, and rise of the back-arc Shan Plateau possibly resulted from late Cenozoic relaxation of collision-related compressive stress.     

川西松潘-甘孜弧前盆地的形成及演化

地处柴南缘昆中蛇绿杂岩带与羌塘地块北缘可可西里-金沙江古缝合线之间的松潘-甘孜褶皱带(包括东昆仑构造带),其主体应属古特提斯洋晚石炭世-晚三叠世时期向其北侧的柴达木古陆南缘俯冲过程中在活动陆缘弧-沟间隙之间增生形成的一个大型弧前构造带。具有由弧前盆地沉积楔和基底增生杂岩构成的双重结构特点,其形成与冈瓦纳大陆北缘若尔盖"三角"地块的楔入及俯冲带向南迁移有关。大致经历了晚石炭世-早三叠世狭窄弧前盆地和中晚三叠世宽阔弧前盆地两个主要演化阶段。     

A model of the Cenozoic evolution of northern New Zealand and adjacent areas of the southwest Pacific

New information from the southwest Pacific indicates that earlier attempts at formulating the evolution of the area assuming a single Upper Cenozoic magmatic arc are untenable. It now appears that there were two arcs during the Miocene and Pliocene, a western Northland/Three Kings Rise arc, and an eastern Tonga-Lau/Kermadec-Colville arc. Both appear to have developed above west-dipping subduction zones. It is suggested that the Norfolk- and Reinga basins formed as back-arc basins to the western arc, and that eastern North Island lay adjacent to Northland and formed the accretionary prism to that arc. Upper Cenozoic evolution of the region involved the simultaneous opening of the Norfolk/Reinga basins, consumption of the western portion of the Oligocene South Fiji Basin by subduction beneath the western arc, and eastwards movement of New Zealand/Three Kings Rise towards the Tonga/Kermadec arc. When the Kermadec and Hikurangi trenches came into line late in the Pliocene, the Tonga/Kermadec arc was able to propagate rapidly southwards into North Island; simultaneously the western arc became extinct, and the tectonic tempo and strike-slip faulting accelerated markedly throughout New Zealand. Eastern North Island was moved dextrally an uncertain distance relative to the western North Island, and rotated 25°–30° clockwise. This accounts for the paradox of a 22-m.y. old accretionary margin lying adjacent to a 2-m.y. old arc (Taupo Volcanic Zone) at the present day.     

The sedimentological evolution of a Lower Palaeozoic accretionary fore-arc in the Southern Uplands of Scotland

Thick turbidites accumulated along the northern margin of the Iapetus Ocean in Britain from mid-Ordovician to late Silurian times. Recent plate tectonic reconstructions hold that, during subduction, packets of these sediments, together with the underlying pelagic facies and thin portions of the uppermost ocean crust, were stripped from the descending plate and accreted to the inner trench wall on the Laurentian (North American) continental margin. The resulting accretionary prism is represented today by the Ordovician and Silurian rocks of the Southern Uplands of Scotland and the Longford-Down massif of Ireland. In these areas major reverse faults separate tracts of steeply dipping greywackes and mudstones with minor amounts of cherts and basalts. These tracts are up to several kilometres wide; their constituent beds face predominantly to the northwest, away from the site of the ancient ocean, while becoming progressively younger in each major fault slice towards the Iapetus suture in the southeast. From the stratigraphic sequences in these fault slices the sedimentary history of a portion of the Iapetus Ocean, and the British sector of its northern margin, can be reconstructed. In the Southern Uplands the earliest turbidites (mid- and late-Ordovician) are preserved in the northernmost fault slices. Regional facies trends, and vertical facies analysis, suggest that they accumulated in a trench dominated by a series of relatively small lower trench slope-derived fans. Pelagic sediments of the same age are found in the fault slices to the south, suggesting that the Ordovician turbidites were confined to the trench. During the lower and middle Llandovery, volcaniclastic trench turbidites were separated from quartz-rich ocean-floor turbidites (represented in the southern fault slices) by an elongate rise, on which pelagic deposits accumulated. This is interpreted as the outer trench high. In late Llandovery times the rise was overwhelmed, and thick laterally derived quartzose turbidites blanketed both the trench and the ocean floor. Sedimentation was strongly influenced by the evolution of the accretionary prism. By Llandovery times a trench slope break had emerged, supplying sediment both south to the trench and north to an upper slope basin in the Midland Valley of Scotland. In this basin early Silurian turbidites were followed by shallow-water and terrestrial sediments. Most of the sediment was derived from the emergent trench slope break: the volcanic arc and the Grampian orogenic belt to the north provided little or no detritus. Throughout the Ordovician and Silurian, sediment in the trench and on the ocean floor was derived from the volcanic arc, from the lower trench slope/trench slope break, from a degrading plutonic/metamorphic terrain (the Grampian Orogen), and locally by a minor amount of submarine sliding from carbonate-capped volcanic seamounts. Progressive elevation of the trench slope break in Silurian (and perhaps late Ordovician) times indicates that sediment from the arc-orogen hinterland must have bypassed the upper slope in the unexposed section of the margin to the northeast of the Southern Uplands, and travelled into the area axially along the trench floor.     

Revolution or evolution? New Zealand agriculture since 1984

Agriculture was the primary target of moves to deregulate the New Zealand economy in 1984. Within twelve-months all production subsidies had been removed, including those for fertiliser and other inputs, as well as funding for drought relief, floods and other natural weather disasters. Whereas at the start of 1984, subsidies were estimated to represent as much as 33% of farm income, by 2003 this had fallen to less than 2% with most of this spent on agricultural research. The anticipated shift of thousands of people off the land did not appear to occur, and by conventional measures at least New Zealand agriculture in 2003 is a major success story. At the core of the changes imposed on agriculture was a commitment to remove all state or government distortions from the system and to fully expose the agricultural sector to market forces. This included wide-ranging and fundamental changes in the broad institutional context within which agriculture must operate. All this was achieved at great social cost and with a significant impact on the environment. In many respects New Zealand agriculture is now very different from that in 1984. Some sectors, such as dairying, have grown and become increasingly industrialised. On the other hand, sheep farming, particularly for wool has struggled to maintain its market share, while other enterprises have emerged as significant sources of income, including horticulture, viticulture and fruit. It is argued here that the trends evident in New Zealand agriculture since 1984 pre-existed the reform period and that the apparent success of the reforms evident at a national scale have not addressed or removed the fundamental problems which face New Zealand agriculture, just as they do modern agricultural systems elsewhere. This revised version was published online in July 2006 with corrections to the Cover Date.     

Kyanite-paragonite-bearing assemblages, northern Fiordland, New Zealand: rapid cooling of the lower crustal root to a Cretaceous magmatic arc

Fiordland, New Zealand exposes the lower crustal root of an Early Cretaceous magmatic arc that now forms one of Earth's most extensive high‐P granulite facies belts. The Arthur River Complex, a dioritic to gabbroic suite in northern Fiordland, is part of the root of the arc, and records an Early Cretaceous history of emplacement, tectonic burial, and high‐P granulite facies metamorphism that accompanied partial melting of the crust. Late random intergrowths of kyanite, quartz and plagioclase partially pseudomorph minerals in the earlier high‐T assemblages of the Arthur River Complex, indicating high‐P cooling of an over thickened crustal root by c. 200 °C. The kyanite intergrowths are themselves partially pseudomorphed by paragonite, commonly in the presence of phengitic white mica. Biotite–plagioclase intergrowths that partially pseudomorph phengitic white mica and diopside–plagioclase intergrowths that partially pseudomorph jadeitic diopside, combined with published thermochronology results, are consistent with later rapid decompression. A short duration anticlockwise P–T path may be explained by the high‐P juxtaposition of comparatively cool upper crustal rocks following their tectonic burial and under thrusting during the waning stages of Early Cretaceous orogenesis. This was then followed by the decompression giving the rapid exhumation within 20 Myr of peak metamorphism, as suggested by the isotopic data.     

Structural characteristics controlling the seismicity crustal structure of southern Japan Trench fore-arc region, revealed by ocean bottom seismographic data

The Japan Trench is a plate convergent zone where the Pacific Plate is subducting below the Japanese islands. Many earthquakes occur associated with plate convergence, and the hypocenter distribution is variable along the Japan Trench. In order to investigate the detailed structure in the southern Japan Trench and to understand the variation of seismicity around the Japan Trench, a wide-angle seismic survey was conducted in the southern Japan Trench fore-arc region in 1998. Ocean bottom seismometers (15) were deployed on two seismic lines: one parallel to the trench axis and one perpendicular. Velocity structures along two seismic lines were determined by velocity modeling of travel time ray-tracing method. Results from the experiment show that the island arc Moho is 18–20 km in depth and consists of four layers: Tertiary and Cretaceous sedimentary rocks, island arc upper and lower crust. The uppermost mantle of the island arc (mantle wedge) extends to 110 km landward of the trench axis. The P-wave velocity of the mantle wedge is laterally heterogeneous: 7.4 km/s at the tip of the mantle wedge and 7.9 km/s below the coastline. An interplate layer is constrained in the subducting oceanic crust. The thickness of the interplate layer is about 1 km for a velocity of 4 km/s. Interplate layer at the plate boundary may cause weak interplate coupling and low seismicity near the trench axis. Low P-wave velocity mantle wedge is also consistent with weak interplate coupling. Thick interplate layer and heterogeneous P-wave velocity of mantle wedge may be associated with the variation of seismic activity.     

Petrogenesis of fore-arc metabasites from the paleozoic of New England,Eastern Australia

Summary Devonian metabasites in the New England Fold Belt of eastern Australia occur in three fault-bounded slices. Tectonic reconstruction places these and along-strike units in a fore-arc basin bordered to the west by an (eroded) are and to the east by an accretionary subduction complex. All twenty-one samples examined here have undergone prehnite-pumpellyite facies metamorphism and accompanying redistribution of alkaline earth and other LIL elements, although HFS and LRE elements are in magmatic abundances. Relict clinopyroxene compositions indicate crystallization from either MORB or calcalkalic magma, although immobile element variations favour a depleted magma type (e.g. MORB or low-K tholeiite). Fields for boninites, island arc tholeiites and calcalkaline rocks from the Mariana fore-arc have been delineated on a plot of Ce/Zr - Y/Zr. Metabasites from this study plot in the island arc tholeiite field but analyses from along strike (Glenrock) plot in the tholeiite field and closer to the boninite field. This, coupled with the occurrence of calc-alkalic volcanics at Glenrock and MORB and alkalic metabasites further southeast argues for variable loci of magma production and source region inhomogeneity along the fore-arc.As the western magmatic arc was established at the time of fore-arc volcanism, metabasites were not erupted at the inception of subduction as argued for the SW Japan fore-arc. Instead, rifting in the fore-arc could have lowered the pressure in the upper mantle instigating peridotite melting and allowing the passage of magma to the surface. A potential source region for magma is the mantle wedge overlying the subducting plate.

---

Petrogenese von Fore-are Metabasiten aus dem Paläozoikum von New England, Australien

Zusammenfassung Die devonischen Metabasite aus dem New-England-Faltengürtel von Ostaustralien kommen in drei störungsgebundenen Schuppen vor. Tektonische Rekonstruktion ordnet diese und auch parallel streichende Gesteinseinheiten in ein Fore-arc-Becken ein, welches im Westen von einem (erodierten) Inselbogen und im Osten von einem Subduktions-Komplex begrenzt wird. Die 21 hier untersuchten Proben sind durch Metamorphose in der Prehnit-Pumpellyit-Fazies gekennzeichnet, mit der Umlagerung der Alkalien und anderer LIL-Elemente einherging, obwohl die HFS- und LRE-Elemente primär magmatische Gehalte zeigen. Reliktische Klinopyroxen-Zusammensetzungen deuten die Kristallisation aus einem MORB- oder Kalkalkali-Magma an, während immobile Elementverteilungen eher auf einen verarmten Magmentyp hinweisen (MORB oder low-K tholeiite). Als Vergleichsbasis wurden die Felder für Boninite, Inselbogentholeiite und Kalkalkali-Gesteine vom Mariana Fore-arc in ein Ce/Zr - Y/Zr-Diagramm eingetragen. Die Metabasite fallen in das Feld der Inselbogentholeiite. Die Analysen der parallel streichenden Gesteinseinheiten (Glenrock) fallen jedoch in das Tholeiit-Feld, in unmittelbarer Nähe zu Boniniten. Dies spricht für eine Magmenproduktion an verschiedenen Orten und für eine Inhomogenität des Ausgangsmaterials entlang des Fore-arcs, auch auf Grund des Vorkommens von Kalkalkali-Vulkaniten am Glenrock bzw. von MORB und alkalischen Metabasiten weiter im Südosten. Während der Entstehung des westlichen magmatischen Inselbogens, zur Zeit des Forearc-Vulkanismus, wurden am Beginn der Subduktion keine Metabasite eruptiert, wie sie für den SW-Japanischen Fore-arc angenommen werden. Rifting im Fore-arc hat möglicherweise den Druck im oberen Mantel gesenkt. Dies hat Aufschmelzung von Peridotit begünstigt und das Aufdringen des Magmas an die Erdoberfläche ermöglicht. Der über der subduzierenden Platte liegende Mantelkeil gilt als potentieller Herkunftsort für das Magma.

---

---

With 6 Figures     

Development of a palaeotsunami database for New Zealand

A New Zealand palaeotsunami database has been developed. The philosophy has been to include as much tsunami-related data as possible. Most of the events recorded are true palaeotsunamis that occurred prior to the historical record or have no written observations. Some are hybrids that are in some manner poorly recorded historical events. Data include physical evidence from geological, archaeological and geomorphological sources and cultural information from anthropological research and prehistorical Māori oral recordings. Each line of data represents a summary of one piece of evidence containing key details listed under a series of headings. The estimated veracity of each line item is based upon the sum of the information contained in the linked reference(s). The palaeotsunami database contains approximately 300 line items and describes between 35 and 40 palaeotsunamis. This wealth of data helps to improve our understanding of tsunami sources, event its magnitude and frequency.     

川甘青复理石盆地地壳结构与演化

：本文讨论川甘青复理石盆地的地质概况、地壳属性、形成机制和资源前景。这个地区是中央造山带 中未发生陆陆碰撞的构造单元,包括甘南、青海东及川西北的倒三角形地区,过去称为“松潘甘孜造山带”。地壳波 速结构的特点包括：①上地壳分两层,上层的波速较低的沉积岩层,厚度可达７ｋｍ,波速小于６．０ｋｍ／ｓ;下层为结晶 基底,波速６．０～６．２ｋｍ／ｓ。②中地壳为正常波速,但厚度较大,可达２０ｋｍ 以上。③下地壳波速从６．６ｋｍ／ｓ随深度 上升到７．３ｋｍ／ｓ,厚度亦增加到２０ｋｍ 以上。上述特点的产生与复理石盆地形成时洋壳的俯冲有关。构造演化可 分四个阶段,①洋壳俯冲阶段;②深水浊流沉积及等深流沉积阶段;③ 陆坡凸起沉积加剧阶段;④ 古特提斯洋封闭 以后,地壳推挤成高原阶段。川甘青复理石盆地也许是中国大陆浅层油气勘探的最后希望。在盆地边部及西秦岭 西延余脉,有良好的金矿资源开发前景。     

Fluid composition in the anhydrous C-O-S system under lower crustal P-T conditions according to thermodynamic modeling

Thermodynamic modeling was used to show that the composition of fluids in the anhydrous subsystem of the C-O-S-H system is dominated by COS, CO, and CS₂ under reducing conditions ( $f_{O_2 } $ < NNO). At an increase in oxygen fugacity up to NNO to NNO + 1 at T = 900–1100°C and lower crustal pressures, the composition of model fluid reaches the maximum content of elemental sulfur. In the presence of water (more than a few mole percent), the compounds of sulfur and carbon are decomposed to form hydrogen sulfide, this is why their content in volcanic gases is negligible. Fluid dominated by carbon dioxide is characteristic of some basic magmas of subduction zones and flood basalt provinces related to superplumes. It is suggested that, under such geodynamic conditions, the COS, CO, and CS₂ components of the fluid dissolve, concentrate, and transport nickel and, possibly, platinum group metals.     

A reactive flow model of the geothermal reservoir Waiwera, New Zealand

A numerical model of the complex interaction between fluid flow, heat transfer, and chemical reactions of the shallow, coastal, hydrothermal system of Waiwera (New Zealand) is used to test the proposed conceptual model of the field.Due to declining water levels, resulting from over-exploitation during the 1970s, the objective here was to set up a coupled fluid flow and heat transfer model to help in enabling a predictive and sustainable use of the resource. The presented conceptual model of the area is based on hydraulic, thermal, and chemical field observations, which date back as far as 1863. The numerical simulations were carried out with the reactive transport code SHEMAT.The inflow of geothermal water at the bottom of the reservoir prevents seawater from entering the aquifer. If seawater intrusion occurs, it is due to over-exploitation and happens in the upper parts of the aquifer. This is in contrast to common seawater-freshwater interfaces where seawater intrudes at the bottom. The numerical investigations emphasize that, after modifications of the production regime in the late 1980s, the system is recovering again.Additionally, geochemical calculations have been conducted to answer the question if dissolution or precipitation reactions might change the hydraulic properties of the geothermal aquifer. Mixing of fresh, geothermal, and seawater, although each of them in thermodynamic equilibrium with calcite, lead to calcite precipitation or dissolution. Nevertheless, the simulations show that the hydraulic properties of the aquifer are not significantly affected by dissolution or precipitation.

---

Resumen Se ha utilizado un modelo numérico de la interacción compleja entre el flujo de fluidos, la transferencia de calor y las reacciones químicas del sistema hidrotermal costero poco profundo de Waiwera (Nueva Zelanda) con el objeto de probar el modelo conceptual del área propuesto. Debido a los niveles de agua decrecientes, el objetivo de este trabajo es establecer un modelo pareado del flujo de aguas y de la transferencia de calor para colaborar en el uso predecible y sostenible del recurso. El modelo conceptual del área presentado está basado en observaciones de terreno hidráulicas, termales y químicas que datan de 1863. Las simulaciones se llevaron a cabo con el código de transporte reactivo SHEMAT. El influjo de agua geotermal al fondo del reservorio impide que el agua de mar entre en el acuífero. Si la intrusión de agua de mar se presenta, esto sucede a causa de la sobreexplotación y ocurre en las partes superiores del acuífero. Esto contrasta con las interfaces comunes de agua de mar-agua dulce donde la intrusión del agua de mar se da al fondo. Las investigaciones numéricas enfatizan que el sistema se está recuperando de nuevo luego de las modificaciones al regimen de producción a fines de los 80. Adicionalmente, los cálculos geoquímicos están dirigidos a contestar si las reacciones de disolución o precipitación pueden cambiar las propiedades hidráulicas del acuífero geotérmico. La mezcla de agua dulce, geotermal y de mar causa precipitación o disolución de calcita a pesar de que cada una de ellas se encuentra en equilibrio termodinámicao con la calcita. Sin embargo, las simulaciones muestran que las propiedades hidráulicas del acuífero no son afectadas siginificativamente por la disolución o precipitación.

---

Résumé Un modèle numérique de linteraction complexe entre lécoulement de leau, le transfert de chaleur et les réactions chimiques du système hydrothermal côtier peu profond de Waiwera (Nouvelle-Zélande) est utilisé afin de tester le modèle conceptuel. Lobjectif est de bâtir un modèle couplé découlement et de transfert de chaleur afin dencourager une approche prévisioniste et une utilisation durable de la ressource en réponse à des niveaux deau en déclin qui résultent dune surexploitation depuis les années 1970. Le modèle conceptuel présenté est basé sur des observations de terrain concernant lhydraulique, la température et la composition chimique de laquifère, des observations qui peuvent dater daussi loin que 1863. Les simulations numériques ont été réalisées avec le modèle de transport SHEMAT. Lentrée deau géothermale à la base du réservoir préviens lintrusion deau de mer dans laquifère. Si lintrusion deau de mer a lieu, elle est due à la surexploitation qui a lieu dans la partie supérieure de laquifère. Cette situation est lopposée des interfaces eau de mer - eau douce traditionnelles où leau de mer pénètre par la base. Les simulations numériques suggèrent que suite aux modifications du régime de production à la fin des années 1980, le système récupère. De plus, des calculs géochimiques ont été effectués afin de répondre à la question à savoir si les réactions de dissolution et de précipitation changent les propriétés hydrauliques de laquifère géothermal. Le mélange de leau douce, leau géothermale et leau de mer, même si elle sont tous en équilibre thermodynamique avec la calcite, provoque la dissolution ou précipitation de la calcite. Néanmoins, les simulations montrent que les propriétés hydrauliques de laquifère ne sont pas significativement affectées par la dissolution ou la précipitation.

     

Geochemistry of loess,continental crustal composition and crustal model ages

Isotopic, major and trace element studies of loess deposits from America, China, Europe and New Zealand show general uniformity of composition. Silica, Zr and Hf are enriched relative to estimates of bulk composition of the upper continental crust. The REE data are indistinguishable from those of average shales, confirming the concept that these REE patterns ($\text{La}{}_{\mathrm{N}}\text{/}\text{Yb}{}_{\mathrm{N}}\text{= 9.5}\text{Eu}\text{/}\text{Eu}{}^1\text{= 0.66}$) represent the upper crustal average. Sm-Nd model ages are variable but <1700 m.y. They reflect derivation from younger elevated erogenic areas subject to Pleistocene glaciation. Although Sm-Nd model ages vary by a factor of two, the REE patterns remain constant. This indicates that processes responsible for formation of the upper crust have produced no secular change in composition since the mid-Proterozoic.     

Morphological evolution of gold nuggets in proximal sedimentary environments,southern New Zealand

A rare collection of centimetre-scale proximal gold nuggets has been examined in the context of the Late Pleistocene to Holocene eluvial and colluvial sediments in which they occur, in a range of environments from arid temperate to periglacial. Liberation of supergene nuggets from their basement source in fault zones occurred progressively by physical erosion as the host basement rocks were uplifted between middle Miocene and Holocene. The host sediments are made up of poorly sorted angular greenschist facies schist debris. Some nuggets were recycled through several generations of these colluvial deposits, and additional nuggets were liberated to younger sediments during this time. The nuggets did not migrate laterally more than a few tens of metres during this repeated recycling, and were passively elevated vertically with the rising basement on to the crest and slopes of a 1600 m high mountain range. The most recently liberated nuggets retain almost all the morphological features of their supergene origin, including coarse (cm scale) crystal shapes, delicate crystalline internal structure, and imprints of oxidised pyrite crystals. Minor transport in colluvium has caused some abrasion and rounding of gold crystals. Repeated recycling progressively obscured the crystal shapes, although relict crystals are still recognisable on parts of most nuggets. Differential timing of liberation from basement has resulted in a wide range of rounding effects in groups of closely-coexisting nuggets. Variably crystalline gold overgrowths (micron scale) coat abraded surfaces of all or part of most nuggets, commonly intergrown with authigenic smectite clay minerals. These overgrowths developed from alkaline groundwater (pH 7–9) that had undergone extensive chemical interaction with labile minerals in the host colluvium. Abrasion during recycling removed some of the overgrowths, but this was replaced in the new sedimentary hosts. Physical and chemical processes affecting nuggets in these sediments have similarly affected eluvial and colluvial nuggets in a wide range of settings around the world, including Yukon (Canada), California (USA) and arid parts of Australia.