The origin of the Naturaliste Plateau,SE Indian Ocean: Implications from dredged basalts

Although a 1972 dredging by USNS Eltanin from the submarine Naturaliste Plateau was reported to yield rocks of continental origin, a re‐examination of the dredge haul shows that the rock clasts are in fact altered tholeiitic basalts. They have affinities both with MOR basalts and, especially, within‐plate basalts. Petrographically they correlate most closely with the Bunbury Tholeiitic Suite on the Australian mainland to the east. The basalts are reworked cobbles in a manganiferous Quaternary slump mass, which contains a quartz‐rich, felsic, detrital mineral suite with a granite‐gneiss provenance. The basalt cobbles were part of a basal conglomerate, which covered large areas of the Plateau. It is suggested that this was laid down from nearby elevated volcanic structures formed during the inception of seafloor spreading and the separation of Greater India from Australia at about 122 Ma BP.     

Nannofossil age constraints for the northern KwaZulu-Natal shelf-edge wedge: Implications for continental margin dynamics, South Africa, SW Indian Ocean

Samples collected from the shelf-edge wedge using surface grab samples and the Jago submersible constrain the KwaZulu-Natal shelf-edge wedge to a late Pliocene age on the basis of the absence of Gephyrocapsa oceanica s.l. and Discoaster brouweri, and the presence of Calcidiscus macintyrei. This correlates with proposed Tertiary sea-level curves for southern Africa and indicates relative sea-level fall during the late Pliocene coupled with hinterland uplift. Exposed failure scarps in the upper portions of submarine canyons yield sediment samples of early Pleistocene ages, indicating the uppermost age of deposition of clinoform topsets exposed in the scarp walls. Partially consolidated, interbedded silty and sandy deposits of similar age outcrop in the thalweg of Leven canyon at a depth of 150 m. These sediments provide an upper age limit of the shelf-edge wedge of early Pleistocene, giving a sedimentation rate of this wedge of 162–309 m/Ma. The distribution of widespread basal-most Pleistocene sediments on the upper slope indicates that these sediments escaped major reworking during sea-level falls associated with Pleistocene glaciations and remain as relict upper slope veneers. The absence of more recent sediments suggests that this area has been a zone of sediment bypass or starvation since the early Pleistocene. Areas where younger sediments mantle deposits of early Pleistocene ages represent areas of offshore bedload parting, re-distributing younger Holocene sediment offshore and downslope.     

Sediment accumulation in the western Gulf of Lions,France: The role of Cap de Creus Canyon in linking shelf and slope sediment dispersal systems

Previous work in the Gulf of Lions (western Mediterranean Sea) has suggested that significant amounts of sediment escape through the western part of this tectonically passive margin, despite it being far removed from the primary sediment source (the Rhone River, ∼160 km to the NE). The primary mechanism behind this export is hypothesized to be the interaction of a regional, southwestward sediment-transport path with a canyon deeply incising the southwestern part of the shelf, Cap de Creus Canyon.     

Storm-driven shelf-to-canyon suspended sediment transport at the southwestern Gulf of Lions

Shelf-to-canyon suspended sediment transport during major storms was studied at the southwestern end of the Gulf of Lions. Waves, near-bottom currents, temperature and water turbidity were measured on the inner shelf at 28-m water depth and in the Cap de Creus submarine canyon head at 300 m depth from November 2003 to March 2004. Two major storm events producing waves H_s>6 m coming from the E–SE sector took place, the first on 3–4 December 2003 (max H_s: 8.4 m) and the second on 20–22 February 2004 (max H_s: 7 m). During these events, shelf water flowed downcanyon producing strong near-bottom currents on the canyon head due to storm-induced downwelling, which was enhanced by dense shelf water cascading in February 2004. These processes generated different pulses of downcanyon suspended sediment transport. During the peak of both storms, the highest waves and the increasing near-bottom currents resuspended sediment on the canyon head and the adjacent outer shelf causing the first downcanyon sediment transport pulses. The December event ended just after these first pulses, when the induced downwelling finished suddenly due to restoration of shelf water stratification. This event was too short to allow the sediment resuspended on the shallow shelf to reach the canyon head. In contrast, the February event, reinforced by dense shelf water cascading, was long enough to transfer resuspended sediment from shallow shelf areas to the canyon head in two different pulses at the end of the event. The downcanyon transport during these last two pulses was one order of magnitude higher than those during the December event and during the first pulses of the February event and accounted for more than half of the total downcanyon sediment transport during the fall 2003 and winter 2004 period. Major storm events, especially during winter vertical mixing periods, produce major episodes of shelf-to-canyon sediment transport at the southwestern end of the Gulf of Lions. Hydrographic structure and storm duration are important factors controlling off-shelf sediment transport during these events.     

南海西沙海域永乐海底峡谷形成演化过程分析

远海孤立碳酸盐台地周缘发育了碳酸盐岩峡谷,对其开展研究有助于深刻理解碳酸盐碎屑沉积物的“源-汇”体系及深水油气成藏等方面。文章利用多波束测深、高分辨率二维多道地震等数据,精细刻画南海西沙海域永乐海底峡谷的地貌形态及内部充填特征,揭示该峡谷沉积演化过程,分析峡谷成因控制因素及稳定性。永乐海底峡谷形成演化可分为萌芽、汇聚和拓展3个阶段,随着演化过程的发展,峡谷规模及对沉积物输运作用增加。永乐海底峡谷形成及演化主要受古地貌隆起形成的负地形和沉积物重力流侵蚀作用影响。峡谷在第四纪以后仍有较明显的活动迹象。分析显示永乐海底峡谷是西北次海盆的重要物质输送通道,其沉积演化过程及稳定性对研究碳酸盐台地沉积物输运等深水沉积过程及岛礁工程建设具有一定参考意义。     

海底热液成矿活动研究的进展、热点及展望

海底热液活动研究进展迅速,热液矿化作用已成为当前国际地球科学最为活跃的研究领域之一。本文叙述了:(1)热液调查新技术、新方法、矿床赋存状态、主要类型以及热液矿化作用的研究进展:(2)当前国际研究的热点,包括海底热液矿床和构造岩浆背景、热液流体形成和金属浸析作用、热液沉淀过程中的金属分馏作用、热液排放与大洋的质重平衡等。最后强调加强海洋地球科学国内外合作的必要性。     

扬子东南大陆边缘晚前寒武纪古海洋演化的稀土元素记录

本文采用中子活化方法分析了晚震旦～早寒武纪１９件样品的稀土元素组成,发现硅质岩石ＲＥＥ配分形式有着复杂的变化,初步可以划分为五种类型,包括“台地型”、“红海型”、“海水型”及两种特殊的配分形式。剖面上Ｃｅ异常由小到大的演变反映了古海洋底层水由缺氧环境向氧化条件的逐渐转化。正Ｅｕ异常的出现代表了热水沉积事件的存在。     

Submarine hydrothermal mineralization in the Okinawa Trough, SW of Japan: an overview

The Okinawa Trough is a heavily sedimented, rifted back-arc basin formed in an intracontinental rift zone. Submarine hydrothermal activity is located within the six back-arc rifts located in the middle and southern Okinawa Trough and its distribution is controlled principally by tectonic factors. Subduction of the Daito and Gagua Ridges beneath the Ryukyu Arc has resulted in fracturing of the brittle lithosphere beneath the Okinawa Trough. Hydrothermal activity is strongest in the volcanic arc-rift migration phenomenon (VAMP) area plus the JADE site and Southernmost Part of the Okinawa Trough (SPOT) area which form the prolongation of these two ridges. These areas are characterized by extremely high heat flow locally. Submarine hydrothermal fluids from the Okinawa Trough tend to be strongly influenced by interaction of the hydrothermal fluids with organic matter in the sediment resulting in high alkalinity and NH₄⁺ concentrations of the fluids. The fluids also contain high concentrations of CO₂ of magmatic origin. Submarine hydrothermal mineralization in the trough is diverse. The CLAM site consists principally of carbonate chimneys. Interaction of the hydrothermal fluid with organic matter in the sediment is particularly strong at this site. This is most probably a sediment-hosted deposit in which sulphide minerals have deposited within the sediment column leaving ‘spent ore-fluids’ to emerge at the seafloor. The JADE site consists of active and inactive sulphide–sulphate chimneys and mounds. The Zn–Pb-rich sulphides at this site contain the highest concentrations of Pb, Ag and Au so far recorded in submarine hydrothermal sulphide deposits. At Minami-Ensei Knoll and Hatoma Knoll, active and inactive chimneys consist principally of anhydrite and barite as a result of phase separation of the hydrothermal fluids beneath the seafloor. An intense black smoker has recently been discovered at Yonaguni Knoll in the SPOT area. If it is confirmed that sulphide mineralization is dominant at this site, this could be a highly prospective area. The most prospective areas for economic-grade minerals in the Okinawa Trough appear to be the JADE site and the SPOT area.     

柴北缘托莫尔日特滩间山群碎屑岩组的沉积环境与构造背景分析SCIEI北大核心CSCD

柴北缘构造带断续出露的寒武-奥陶纪滩间山群火山-沉积岩系是早古生代时期原特提斯洋俯冲造山作用的产物,也是中国西部块状硫化物和造山型金矿床的重要含矿岩系。前人已对滩间山群火山岩组开展了大量的地球化学和同位素年代学研究,而碎屑岩组的沉积序列和沉积相研究相对薄弱。我们在柴北缘托莫尔日特地区地质填图过程中,对该地区的滩间山群碎屑岩组开展了系统的沉积组合序列和沉积相研究。托莫尔日特地区滩间山群碎屑岩组是由富含火山碎屑物质的含砾砂岩、砂岩、粉砂岩、泥岩、沉凝灰岩、硅质凝灰岩、硅质岩及少量砾岩共同组成。可分为下岩性段和上岩性段,其中下岩性段为一套形成于海底扇外扇环境以沉凝灰岩为主的沉积组合;上岩性段包含海底扇内扇、中扇和外扇沉积,且以中扇环境的沉积为主。垂向上,它们呈现出粒度向上逐渐变细特征;空间上,具有向北西方向火山物质成分逐渐增多、砾岩和砂岩厚度变薄且砾岩消失的变化特征。砂岩主要为岩屑长石杂砂岩,其中岩屑主要是安山岩和少量玄武岩、英安岩、凝灰岩;长石主要为斜长石;石英碎屑相对缺失。砾岩中砾石以安山岩和硅质岩为主,并含有少量灰岩和凝灰岩砾石。底冲刷面、正粒序、平行层理和波纹层理等沉积构造普遍发育,呈现出典型的浊流沉积特征;同时在局部露头见有滑塌构造和波痕。区域上,这套沉积组合序列位于寒武-奥陶纪岛弧火山岩的南侧;古水流分析显示,它们的碎屑物质主要来自于其北东方向。这些结果表明,柴北缘构造带东段的滩间山群碎屑岩组是一套成熟度极低且与寒武-奥陶纪岛弧火山活动密切相关的沉积组合。     

Development of a geotechnical and geophysical database for seismic zonation of the Ankara Basin,Turkey

The Kocaeli earthquake (M _w = 7.4) of 17 August 1999 occurred in the Eastern Marmara Region of Turkey along the North Anadolu Fault and resulted in a very serious loss of life and property. One of the most important geotechnical issues of this event was the permanent ground deformations because of both liquefaction and faulting. These deformations occurred particularly along the southern shores of ?zmit Bay and Sapanca Lake between the cities of Yalova and Adapazar? in the west and east, respectively. In this study, three sites founded on delta fans, namely De?irmendere Nose, Yeniköy tea garden at Seymen on the coast of ?zmit Bay, and Vak?f Hotel site on the coast of Sapanca Lake were selected as typical cases. The main causes of the ground deformations at these sites were then investigated. Geotechnical characterization of the ground, derivation of displacement vectors from the pre- and post-earthquake aerial photographs, liquefaction assessments based on field performance data, and analyses carried out using the sliding body method have been fundamental in this study. The displacement vectors determined from photogrammetric evaluations conducted at De?irmendere and Seymen showed a combined movement of faulting and liquefaction. But except the movements in the close vicinity of shorelines, the dominant factor in this movement was faulting. The results obtained from the analyses suggested that the ground failure at De?irmendere was a submarine landslide mainly because of earthquake shaking rather than liquefaction. On the other hand, the ground failures at the Yeniköy tea garden on the coast of Seymen and the hotel area in Sapanca town resulted from liquefaction-induced lateral spreading. It was also obtained that the ground deformations estimated from the sliding body method were quite close to those measured by aerial photogrammetry technique.