Search Documents

By N Madani, N Battalgazy

Geostatistical simulation has been widely used in mineral resource classification of metalliferous deposits based on international standards such as JORC code which is one of the most extensively used around the world for reporting the mineral resources and ore reserves alongside with objectives of the companies (Sadeghi, Madani and Carranza, 2015). However, the modeling process becomes challenging whenever the relationship between the variables of interest is complicated and cumbersome to model. This characteristic sometimes manifests itself in complex spatial and global cross-correlation measures, deeply depending on the formation and genesis of the deposit. The objective then is to generate the models that not only reproduces these significant characteristics, but also, it leads to a better understanding of a mineral resource estimation and successive decision-makings. In this respect, there are different approaches taking into account the spatial correlation structures, for which can be classified as independent simulation, co-simulation, and factor-based methodologies. In this paper, three techniques: Turning Bands Independent Simulation (TBSIM), Turning Bands Co-Simulation (TBCOSIM) and Projection Pursuit Multivariate Transform (PPMT); from those aforementioned approaches are selected to model two cross-correlated variables (Iron and Aluminum Oxide) belonging to an Iron ore deposit located in Brazil, in order to examine which methodology better reproduces the corresponding complexity in terms of local correlation measure. CITATION: Battalgazy, N and Madani, N, 2018. Mineral resource assessment in complex ore deposits based on international standards, in Proceedings Complex Orebodies Conference 2018, pp 79–81 (The Australasian Institute of Mining and Metallurgy: Melbourne).

Nov 21, 2018

Exploration risk increases with time, due to a general decrease in prospectivity as the more accessible deposits are found, and be- cause of rising costs and demands due to an improving standard of living. Risk is separated into financial risk, which increases sharply at the development stage, and the risk of failure which is at its peak at the outset of exploration. Risk is also divided into internal and external factors; some of the latter cannot be offset, viz. the systematic overall risk for any enterprise. The recently emergent risk embodied in withdrawal of large areas from exploration access is discussed. An appeal is made for objective multiple land-use policies to be applied so that land remains available at reasonable cost and with appropriate environ- mental safeguard conditions. Risks specific to gold exploration are discussed. As gold now accounts for over 50% of all mineral exploration, and most other metals are depressed, any sharp decline in gold will have grave effects on the ex- ploration industry. Gold exploration will become more difficult and expensive in about five years, after most of the readily accessible deposits in Australia are found. Government's role is important in pro- viding stability and basic data at reasonable cost. Exploration for specific minerals or of particular areas could be stimulated by extending the 150%.tax deductibility of R & D to aspects of exploration of strategic im- portance, particularly in the event of a downturn. Exploration is classic scientific research, at least through to the proving Stages.

Jan 1, 1987

The seminar had its origins in an Inquiry into the Box Flat Disaster and one of the conclusions from this Inquiry was that there should be a better dissemination of technical knowledge and the results of research work. As a result of this Mr. W. Roach, Chief Inspector of Coal Mines in Queensland and Mr. R. Menzies, Chief Inspector of Coal Mines in N.S.W. had a meeting to see how this could be done. They concluded that it would be difficult to have the sort of regular meetings that would be possible where distance was not significant, but a seminar would be a major step toward satisfying this particular conclusion of this Inquiry. As a consequence, Mr. Roach, who is on the Committee of the Southern Queensland Branch of the Institute, suggested when the Branch programme was being planned this year that a meeting of this type should be held, and the Mine Fires Seminar which was held today is the result. We have received a great deal of support in arranging the Seminar. First and foremost I wish to acknowledge support from the National Coal Board in United Kingdom. Not only have they released Dr. Willett for a period of six to seven weeks to visit Australia, conduct the Seminar and give a series of lectures in various areas through the country, they have also borne the brunt of the financial support that has been necessary to make this possible. In addition we have received considerable support from many organisations in Australia. I 'mentioned this

Jan 1, 1973

The purpose of any feasibility study is to determine whether a particular investment or management program will enhance shareholder wealth by, for example, producing an economic return in excess of the cost of equity capital.The threshold rate of return conventionally used in gold feasibility studies is higher than the cost of equity capital, meaning that some viable projects are wrongly described as being sub-economic with their development deferred.Is the industry failing to fully realise its potential? Aside from important technical considerations that determine the cost of exploiting inground inventories, feasibility studies need to incorporate expectations about future gold prices, exchange rates, interest rates and tax rates.Rather than revert to speculation, insights from capital markets give a rigorous framework in which these parameters can be determined for feasibility purposes. Separate questions sometimes, and then inadequately, addressed by feasibility studies include: ò Is it optimum? Many projects are expanded soon after project commissioning - what does this say about the quality of the initial feasibility study? Moreover, does the rate of extraction really matter? ò What is it worth? Although feasibility studies are designed to address viability rather than valuation, such studies are never- theless used by the industry as further evidence that the equity markets have got it wrong again! More likely the problem is with the financial slide rule being used. ò Can it be debt financed? The bankable feasibility study has entered the vocabulary of the gold mining industry but what does it mean? Does it matter anyway how the project is to be Financed?

Jan 1, 1991

New knowledge has no economic value unless it can be applied, or æcapturedÆ, as the R&D jargon has it. Australia probably spends more than $500 m per year on mineral industry R&D, distributed amongst the mining companies (IR&D Board, 1997), CSIRO, the universities and others. The investors in this research (industry, and the community via government) will expect some sort of return from their investment. The return will be poor unless the know-how developed in R&D programs can be exploited. This paper explores some of the issues involved in capturing the outcomes of mineral industry R&D in a form which can be exploited for economic benefit, and suggests ways in which it might be done better. It is based on the experience of the JKMRC in conducting applied research in mining and mineral processing and delivering it to the industry. It may seem presumptuous for a research contractor to be telling its clients how to do their job. However, the process of developing, acquiring and implementing R&D outcomes has to be a partnership between provider and receiver, in order for this process to work efficiently. This applies whether the two parties operate within a single corporation (a corporate laboratory and a business unit, for example) or are two entirely independent organisations (eg a mining company and a university). Both partners have a role to play, and the best examples of realising the benefits of R&D almost always involve the open-hearted and skilful handling of this synergy.

Jan 1, 1998

By C J. Noack, P B. Ganza

The concept of large trucks underground has been around for some time. While the road train concept is gaining momentum, there are underground mining applications where these trucks have not been suitable. Unfortunately few of the ægreenfieldsÆ projects that are most suited to the road train concept have come æon lineÆ since the underground road train was first introduced to Australia in 2001 so that the trucks have generally been adapted to existing operations. As at December 2004, there were 17 road trains operating underground in Australia, one being rebuilt and one under construction. This paper addresses the necessity for the mining industry to have an integrated ore transport system that starts at the underground load haul dump unit (LHD) in the stope production area and finishes at the crusher feed at the processing plant. Chute and orepass designs are investigated, as well as feeders to determine the best load and haul system for large-scale trucking. The paper also discusses potential developments in the large truck scene. The paper highlights the cost and productivity gains by introducing large trucks and associated integrated ore transport systems underground. While 400 vertical metres was seen as the limit of vertical hauling in the 1970s, one kilometre is a common discussion point for trucking limits today. In real terms the cost of underground trucking (including ownership costs) has decreased from $1.50 - $2.00/t.km in the mid-1990s to $1.00 - $1.20/t.km at present.

Jan 1, 2005

By D Laurence

Significant developments in future mining systems, technology and best practices are emerging that are driven by issues relating to human resources, competitiveness, environmental and social responsibility, within the dynamics of globalisation. It is proposed that the definition of the role and attributes of the mining engineer in the mine life cycle needs to be viewed more holistically in the face of increasingly more diverse and complex responsibilities. These developments are motivating the need to consider a fresh approach to mining education and training at all levels for industry. A growing diversity and complexity of professional responsibilities is extending the mining engineerÆs broadly based skill set and reinforcing the need to play a central role within increasingly interdisciplinary teams. On the other hand, the imminent new technologies that will transform the mining process are prompting the assumption that more focused and specialised skill sets will be needed. A way forward is suggested that would first define a blueprint for the future mine, as an attempt to characterise the systems architecture, processes and organisation of the next generation mine. This can be the basis for assessing the nature of the workforce and distribution of responsibilities, from which can be inferred the skills and training requirements. In the context of the mining engineer, the paper explores how understanding such future requirements will enable more proactive development of a strategy for educational development. The paper then concludes by considering issues and lessons learned from some recent mining educational development initiatives underway in Australia and Canada that are shaping significant change.

Jan 1, 2008

By B Snook, G Rollinson, R Fairhurst, R Herrington

Nickel laterites constitute a significant proportion of the worldÆs Ni reserves and are considered environmentally friendly alternatives to traditional sulfide resources. Subsequently they are of increasing significance for exploration and production. To be successfully exploited, however, they require an understanding of the relations between mineralogy, texture, grade and recovery. The development of automated quantitative mineralogy can aid in the characterisation of the ores to help improve extraction and processing methods. Recent developments in hydrometallurgy have resulted in relatively straightforward recovery of Ni in silicate (saprolite and smectite) ores. However, the extraction of Ni from limonitic ores is more complex. It requires a detailed understanding of the iron-hydroxide species and other, potentially interfering, oxide and hydroxide compounds. Particularly the relations between the Ni-bearing goethite and Mn, Al and Cr may adversely affect the economic prospects of an ore. Manganese can adversely affect the selective recovery of Ni and Co from solutions but has also been highlighted as a significant potential by-product (cf, Zhang and Cheng, 2007). Aluminium hydroxides can be significant acid consumers leading to an increased expense of extraction (Johnson, Cashmore and Hockridge, 2005). Chromite is resistant to acid attack, but chromium in goethite may adversely affect the dissolution leading to a retention of Ni (McDonald and Whittington, 2008). This study presents an investigation of oxide and hydroxide species in laterites using QEMSCAN« EDS/SEM-based automated quantitative mineralogy. Samples were collected from Turkey (¦aldag), Albania and the Philippines. Particular emphasis has been placed on the development of a quantification for Ni-poor and Ni-rich goethite.

Jan 1, 2008

This paper is essentially an address, rather than a detailed presentation of facts, because the third edition of `Nuclear Electricity - An Australian Perspective' has just been completed and will be available for the seminar. That 96-page booklet contains a lot more information than The AusIMM would be prepared to print in the seminar proceedings. Hence I have kept this paper brief, and ask readers to obtain a (free) copy of Nuclear Electricity at the seminar or from The AusIMM. It can be read as a supplement to the paper (or vice versa). According to one of Australia's well known critics of nuclear power, Professor Ian Lowe, `the pro-nuclear lobby's predictions of the future use of nuclear energy .... have been regularly and consistently wrong by large amounts, always overestimating the future demand for uranium" (Aust. Society, April 1989). While I do not accept this as a bald statement applying to all those in favour of increased use of nuclear power, there is sufficient truth in it for it to be credible. Certainly in the 1950s and the 1960s, when the new technology for producing controlled power from atomic fission was first being commercialised, there were some very exaggerated projections of the role of nuclear power. Many people thought that it would take over from fossil fuel fired electricity generation in much the same way as the motor vehicle took over from horse-drawn transport. And these early projections were not simply concerned with electric power generation, they included excavating harbours and digging canals with nuclear explosives.

Jan 1, 1989

Downstream value-adding is a strategic issue for a country like Australia, which has a resources-driven sector of a scale that has significant impact on the national economy. The question of why producers are not doing more to ævalue-addÆ is often asked. In Australia this debate takes place within the context of: an era of unprecedented growth in demand for resources driven by the rapid urbanisation of China, India and others;impending legislation and uncertainty around carbon pricing;rapidly rising energy costs;significant supply-side shortages including infrastructure, skills and manufacturing capacity; andsignificant industry consolidation of primary resource producers with commensurate improvements in market power.There are several examples of resource-based industries which have developed their business along the complete value-chain; for example large oil and gas producers successfully span upstream and downstream operations including refining and petro-chemicals as well as retail distribution of fuel and other products. Relevant frameworks that provide insight and understanding into the strategic issues around value-adding are reviewed and examples of the application of these frameworks to companies and industries which have successfully adopted a strategy of integrating along the value-chain, as well as those players who have focused on parts of the value-chain are presented. The issues around value-adding in Australia are then reviewed with reference to these frameworks and it is concluded that the issue is a conflict between the objective companies who wish to maximise financial value versus the desire of governments to increase economic benefits. An approach to aligning these objectives is proposed.

Jan 1, 2008

By N Kreutzman, A Moilanen, V Palosaari

Carboxymethyl cellulose, CMC, is widely used in mineral processing as a flotation depressant, mainly for the depression of carbonate and talcaceous gangue in the flotation of Cu-Ni sulfide and PGM ores. Although CMC is a common depressant that can be used in the flotation of different ores, understanding of the interaction mechanisms between CMC and mineral particles in different flotation circuits and different pulp conditions is still limited. Electrokinetic measurements, microflotation and laboratory-scale flotation tests were carried out to optimise the action of CMC depressants in the flotation of a PGM ore. Standard and new CMC brands produced and characterised by Noviant were used in the study. Pure talc and chlorite minerals and a low-grade PGM ore composite were used as test materials. A laboratory flotation test evaluation was performed using the recovery-grade relationship and double value index (DVI) calculation to estimate the effects of both valuable and harmful components on the product value. Solution conditions play an important role in the depression of gangue minerals such as talc. The most favourable conditions for MgO depression were found to be mildly acidic in which conditions the electrostatic repulsion between the CMC molecule and talc is minimised. Similar effects were obtained with electrolytes that affect the conformation of the CMC molecule and the mineral surfaces. In addition to the control of chemical conditions in the pulp, CMC molecules were modified to optimise structural variables. The new CMC brands developed in this work showed significant potential to improve flotation performance, particularly when pulp conditions are controlled.

Jan 1, 2005

By J J. Cilliers, K Hadler

Froth structure and stability are known to play important roles in determining mineral flotation recovery and selectivity. However, measuring froth stability in a consistent manner remains a significant challenge, especially at an industrial scale. Following preliminary tests on a copper concentration plant, a quantitative dynamic stability measure is investigated in this study in order to extend the results over a wider range of conditions and on a different orebody. The technique is based on the Bikerman foam test and uses a non-overflowing froth column to quantify froth stability. Experiments were carried out using an automated version of the froth stability column under different operating conditions. Air flowrate was the key operating variable. Tests were reproduced on a single flotation cell of a Platinum Group Metals concentrator. The froth stability factor, , was measured for each operating condition, and compared with the air recovery in the cell, , which was measured using image analysis. The froth stability column results gave the same trends as image analysis. In particular the froth stability factor was found to be linearly related to the actual fraction of air overflowing the cell. The metallurgical results clearly indicated that changes in air flowrate result in variations in flotation performance that can be attributed to changes in froth stability. The results showed that high froth stability conditions occur at intermediate air flowrates, and result in improved flotation performance. It is found that the froth stability column is a simple, cost-effective and reliable method for quantifying froth stability, and for indicating changes in flotation performance.

Jan 1, 2005

By C Rogers

"The Coal Seam Gas (CSG) industry has seen a rapid increase in activity over a 30-year periodfrom the early 1990s within Australia. Despite public concerns over environmental issues, theCSG industry has increased in Queensland to the point where more than 90 per cent of naturalgas consumed in Queensland is CSG. The New South Wales (NSW) CSG industry has been slowerto respond for many reasons. Ninety-five per cent of natural gas consumed in NSW comes frominterstate. However, CSG activity in New South Wales is increasing notwithstanding publicconcerns. The CSG potential of three Petroleum Exploration Licenses (PELs) are being assessedwithin the Southern Sydney Basin that provides a unique opportunity to develop resourcesstrategically around many abandoned coal mines. The question has been asked - what do theseabandoned coal mines mean to the project? Are they good or bad? Are they friend or foe? Thispaper att empts to answer these questions and in doing so describes the coal mine ventilation andgas management philosophies and practices used to assess gas production potential. The coalmining philosophies not only have the potential to assist in the construction of a highly successfulCSG project, but they also potentially add signifi cant pre production value by re defi ning how theresources and reserves are assessed and therefore valued. The behaviour of abandoned coal minesand the goaves within the PELs are not well understood by the petroleum industry. However, in the Illawarra region there is an abundance of coal mining knowledge and expertise that if properly focused can greatly increase the chances of project success."

Jun 1, 2012

Mineral resource classification is essential because it is required for public disclosure. The resources must be binned by a qualified person into measured, indicated and inferred categories using an appropriate methodology. The classification methodologies used have long been a topic of research and discussion. There are various traditional and geostatistical methods used in practice. Some are more popular than others. Because of the uniqueness of each ore deposit, it seems almost impossible to have a single industry-accepted norm for mineral resource classification. Therefore, some experts have adapted the methodologies tailored for the deposits they are working on, but many simply use the methods they and their clients understand and are comfortable with. In this paper, a new approach is presented to classify the mineral resources using a data geometry index. This index combines both traditional and geostatistical aspects of resource classification based on the data configuration around the block interpolated. It is a function of several variables such as kriging variance calculated for a block, distance from the block to the nearest sample and the average distance of all samples used to interpolate the block. Thus the approach using this index takes into account the spatial data configuration around the block being estimated, as well as the traditional distance criteria. The data geometry index and how it is calculated is explained. A case study for a gold deposit is presented to see how the classification based on this index compares to the existing methodology used in this deposit for mineral resource classification.

Sep 26, 2011

By Fountain C. R

In April 1987, Mount Isa Mines Limited (MIM) commissioned an ISASMELT reactor to smelt 15t/h of copper concentrate at the Mount Isa Copper Smelter. This unit was based on the Sirosmelt lance technology and incorporated features proven in the Lead ISASMELT plant which has been operating in the MIM Lead Smelter since 1983. From April 1987 to June 1989, the copper smelting plant treated 167000 tonnes. Concentrate, coal and fluxes are fed continuously to the furnace. Oxygen enriched air is injected through a lance which is submerged in the slag bath. Matte and slag are tapped together through a single taphole into a reverberatory furnace, and from there are treated with normal reverberatory furnace production. Advantages of the process include its ability to eliminate most lead and arsenic in feed, its flexibility with respect to feed composition, sizing and moisture, and the plant's low capital cost. Acceptable refractory life has also been demonstrated. The commissioning of a 70 t/day oxygen plant has rerated the plant from 15t/h to 18t/h while allowing a reduction in fuel usage of one third. A trial using 150 t/day oxygen was carried out in May 1989, and showed that the furnace could treat 48 t/h chalcopyrite concentrate while maintaining a very low fuel requirement. The oxygen trial results have given considerable confidence for plant scale-up. Detailed project evaluation is currently underway for the replacement of the roaster and reverberatory furnaces at Mount Isa with a single 110 t/h ISASMELT reactor.

Jan 1, 1989

By Lawson F

The leaching behaviour of BHAS copper matte in oxygenated acidic sulphate chloride solutions at 85¦C has been studied using samples containing 34% copper, 41% lead and 12% sulphur. Copper sulphide in the matte, essentially Cu2S was found to leach in two distinct stages. Similar to the behaviour of pure chalcocite, copper dissolution in the first stage was affected by the availability of oxygen in solution. The presence of chloride ions in the leachant was essential if there was to be any leaching in the second stage (which usually commenced after about 40% of the copper had been taken into solution). Lead sulphide in the matte was oxidised to lead sulphate and formed a shell around the partially leached matte particles. Electrochemical studies showed that this lead sulphate and/or sulphur produced during leaching can form a passivating layer around the copper sulphide. However, this layer was disrupted when chloride ions were present. A Plackett-Burman analysis was used to determine the species in the recycled leach solution that affect the leaching of copper. Under the experimental conditions used, in the first stage the rate of copper dissolution was influenced by the concentrations of sulphuric acid, Zn(II), Cu(II), Fe(III) and Cd(II). In the second stage, the rate was futher modified by the concentrations of Cl(I) and As(V) ions. In solutions of low acidity and high As(V) concentration, insoluble copper arsenate formed which coated the partially leached matte particles and acted as a barrier to futher reaction.

Jan 1, 1989

By C V. Putnis, T Geisler, A Putnis

In nature, ilmenite (FeTiO3) undergoes weathering through oxidation and removal of Fe to form an apparently continuous series of compositions from ilmenite to pseudorutile (ideally Fe2Ti3O9), and with further weathering, to leucoxene. To determine the mechanism of acid-leaching of ilmenite to ultimately form rutile, we have carried out an experimental study of ilmenite alteration in autoclaves at 150¦C in HCl solutions. Resulting products were studied by X-ray diffraction, scanning electron microscopy, electron microprobe and Raman spectroscopy. In some experiments the solution was initially enriched in 18O and the distribution of the isotope in the alteration products mapped from the peak shift in the Raman spectra. The results indicate that the alteration may proceed in two stages, each with a sharp interface between the parent phase and the product. The alteration begins at the original ilmenite crystal surface and along cracks through which the fluid can migrate. The first alteration product is pseudorutile û no phases intermediate between ilmenite and pseudorutile were detected. The textural relationship between ilmenite and pseudorutile suggests a coupled dissolution-reprecipitation mechanism rather than a solid state continuous oxidation and Fe diffusion mechanism. The second stage involves a further dissolution-reprecipitation step to form rutile. Raman spectroscopy shows that the 18O is incorporated in the rutile during the recrystallisation. Throughout the alteration process the original morphology of the ilmenite is preserved although the product is highly porous. The rutile inherits crystallographic information from the parent ilmenite, resulting in a triply-twinned rutile microstructure.

Jan 1, 2008

By Bhattacharyya AK

With increasing demand for more efficient production from the underground coal mines in the Southern Coalfields- of New South Wales, their design and operational aspects are constantly being reviewed and improved. One key area in such pursuits is the reliable design of stable and yielding pillars. The current designs of pillars based mostly on experience leads to unnecessary sterilisation of coal or strata control problems. To explore such mine design problems, computer based modelling is increasingly being applied. In the study described in this paper, the object was to determine the particular width of pillars separating three roadways in a panel at which the pillars yielded completely and their loads largely transferred to the unmined coal at the flanks of the panel. This was approached through a computer simulation of the behaviour of the strata around a panel, in which the three roadways were separated by pillars of progressively decreasing width. A computer program called FLAC based on the finite difference method was used to simulate the relevant strata behaviour in vertical sections. The appropriate in situ values of the deformability parameters relating to the coal seam, roof and floor were determined from the back analysis of some displacement measurements in such a panel. The process involved successive modelling of a vertical section through such a panel,starting with realistic values of the parameters and varying them until the measured and the modelled displacements agreed closely. Based on the results of the numerical modelling, the width of a yielding pillar for the given conditions is Suggested.

Jan 1, 1992

Exploration risk increases with time, due to a general decrease in prospectivity as the more accessible deposits are found, and be- cause of rising costs and demands due to an improving standard of living. Risk is separated into financial risk, which increases sharply at the development stage, and the risk of failure which is at its peak at the outset of exploration. Risk is also divided into internal and external factors; some of the latter cannot be offset, viz. the systematic overall risk for any enterprise. The recently emergent risk embodied in withdrawal of large areas from exploration access is discussed. An appeal is made for objective multiple land-use policies to be applied so that land remains available at reasonable cost and with appropriate environ- mental safeguard conditions. Risks specific to gold exploration are discussed. As gold now accounts for over 50% of all mineral exploration, and most other metals are depressed, any sharp decline in gold will have grave effects on the ex- ploration industry. Gold exploration will become more difficult and expensive in about five years, after most of the readily accessible deposits in Australia are found. Government's role is important in pro- viding stability and basic data at reasonable cost. Exploration for specific minerals or of particular areas could be stimulated by extending the 150%.tax deductibility of R & D to aspects of exploration of strategic im- portance, particularly in the event of a downturn. Exploration is classic scientific research, at least through to the proving Stages.

Jan 1, 1987

By Davey FJ

The Hikurangi Margin of eastern North Island New Zealand represents the feather edge of the over-riding Indo-Australian Plate at its convergent boundary with the subducting Pacific Plate. Towards the south, convergence becomes slower and increasingly oblique, and sediment supply from the Southern Alps becomes increasingly rapid. In this environment a 150 km wide, imbricate thrust, accretionary prism has grown since the Miocene, uplifting slope ridges and basins and a range of coastal hills. Offscraping of thick, axially transported, trench-fill turbidites begins in a zone of proto- thrusts, which extends up to 25 km beyond the principle, slope-toe, deformation front. On the slope, landward tilted, trench-slope basins are separated by ridges. The ridges ride up on thrusts that sole out at a major decollement. The decollement dips landwards at 3 degrees to a depth of 14 km at the coast, where it coincides with an onshore zone of high seismicity. Beneath the decollement, about 1 km of consolidated, oceanic sediment is going under the margin with the underlying oceanic basement. Some of these sediments may underplate the frontal ranges to the west and geochemical evidence of subducted sediment is found in the volcanic zone beyond. A rapid decrease in the width of the accretionary prism to the north is associated with a decrease in sediment supply and perhaps with a dislocation in the downgoing plate. To the south, the accretionary prism narrows, swings westward, and merges into the zone of very oblique collision that marks the transform between mirror image subduction to north and south of New Zealand.

Jan 1, 1987