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1.
全球火山活动分布特征   总被引：14，自引：0，他引：14  
根据全球活动火山目录 ,分析研究了全球火山分布的特征 ,描述了各区的火山活动分布 ,总结了火山活动强度的时、空分布特征。全球火山活动可分为三大区 ,西太平洋火山活动区 ,主要与太平洋板块向北西西方向的俯冲活动有关 ;东太平洋火山活动区 ,主要与太平洋东面的小板块 (胡安德富卡板块、科科斯、纳斯卡板块 )向美洲板块的俯冲有关 ;大西洋火山活动区 ,与大西洋和非洲的裂开 ,以及地中海带的活动有关。不同火山区带具有各自的最大喷发等级与相应的复发周期。一条火山弧上活动强度的分布往往是不对称的 ,意味着火山弧在整体上有其动力学的控制机理。火山活动显示了随纬度成带状分布。在 - 10～ 0° ,10 2 0° ,30 4 0°,5 0 6 0°分布有高值带。火山喷发活动还与当地的重力势有关 ,重力势正异常可能与高的正压力有关 ,有利于产生特大喷发。火山活动与大角度的正面俯冲带的弧后火山活动最强 ,当板块运动方向与板块边缘走向成小角度相交时 ,缺少正面俯冲的动力 ,火山活动相对平静。  相似文献   

2.
The Syunik rhombus-like structure in the Khanarassar active dextral fault zone of Armenia is a typical pull-apart basin, formed between terminal parts of two adjacent en echelon fault segments. Some component of subsidence associated with the faults of the structure is found between the en echelon segments; nevertheless, the dextral component continues to be predominant even on the boundaries of the pull-apart basin. The late Pleistocene and Holocene lava volcanoes of the basin are also associated with those faults that have a component of extension. The relative ages of fault displacements and volcanic eruptions have been identified by the mutual correlation of lavas, moraines and topographic features and by archaeological and radiocarbon dating. According to the interpretation of rupturing and volcanism, major earthquakes and volcanic eruptions appear inter-related and three pulses of such activity during the earlier and middle Holocene have been identified.  相似文献   

3.
大兴安岭北段诺敏河第四纪火山24座,分布于诺敏河及其支流毕拉河和甘河支流奎勒河,火山岩分布面积约600km2。诺敏河火山岩均属于钾质系列火山岩(K2O含量2%～4%,且K2O>Na2O-2%),类似于邻区五大连池-科洛钾质火山岩,具有来自类似的富钾地幔源区。但诺敏河火山岩的K2O含量明显低于五大连池-科洛火山岩的K2O含量(一般4%～6%)。东北地区和内蒙东部处于中亚造山带东段,从古生代到新生代,多重构造-岩浆活动导致火山岩源区地球化学非均一性和火山岩的多样性。根据本文提出的火山岩K-Ar年龄(2.3～0.128Ma)及火山地质特征,可将诺敏河第四纪火山岩分为四期。早期(早更新世)火山活动主要沿诺敏河和奎勒河流域分布,火山产物多被晚期沉积物或火山产物所覆盖。中更新世保留的火山锥体及熔岩流是诺敏河第四纪火山产物的主体,表明是第四纪火山活动的高潮期。晚更新世-全新世火山活动限于毕拉河流域,典型的四方山火山和马鞍山火山是该时期火山活动的代表作,保留了较完好的火山地质地貌特征。从火山产物的时空展布,推测诺敏河第四纪火山活动有从东向西发展的趋势。  相似文献   

4.
内蒙东部晚第四纪火山活动与新构造   总被引：8，自引：4，他引：4  
本文所指的晚第四纪包括晚更新世和全新世。内蒙东部晚第四纪火山活动强烈,北起大兴安岭北部的鄂伦春诺敏河火山群、经阿尔山-柴河、锡林浩特-阿巴嘎火山群,南抵察右后旗乌兰哈达火山群,断续延伸约1000km,分布着约390余座大小不一、形态各异的火山,构成了内蒙东部壮观的北北东向第四纪火山喷发带。火山类型包括玛珥式、夏威夷式、斯通博利式、亚布里尼式和冰岛式,以斯通博利式最为发育。爆破式火山作用包括射汽、射汽-岩浆爆发和岩浆爆发。火山岩类型主要为碱性玄武岩及其火山碎屑物(岩),火山岩具初期裂谷构造属性。火山活动主体受北北东向基底断裂控制,但就具体火山群而言,又多处于北东和北西向基底深断裂交会处。区内新构造与火山活动密切相关,深部岩浆的上侵,可能是控制本区新构造活动的主因。尤其是全新世火山的空间展布,显示了内蒙东部新构造的活动性。  相似文献   

5.
长白山天池火山双岩浆房岩浆作用与互动式喷发   总被引：6，自引：0，他引：6  
广义的长白山火山在我国境内包括天池火山、望天鹅火山、图们江火山和龙岗火山,是我国最大的第四纪火山岩分布区。长白山各个火山区的火山活动具有此起彼伏的穿时性特征,天池火山之下地壳和地幔两个岩浆房具有上下呼应、互动式喷发之特点。一方面来自地幔的钾质粗面玄武岩浆直接喷出地表,在天池火山锥体内外形成诸多小火山渣锥;另一方面钾质粗面玄武岩浆持续补给地壳岩浆房,发生岩浆分离结晶作用和混合作用,形成双峰式火山岩特征并触发千年大喷发。西太平洋板块俯冲-东北亚大陆弧后引张是长白山天池火山喷发的动力学机制。  相似文献   

6.
Ancient settlement patterns in central Tonga, at the southeastern limit of Lapita expansion into Remote Oceania ˜3 ka, were conditioned by island geomorphology as controlled by spatial geotectonic features and temporal changes in relative sea level on island coasts. Volcanic islands provided lithic resources, but human populations were concentrated on nonvolcanic forearc islands underlain by limestone covered by airfall tephra blankets that weathered to form rich agricultural soils and eroded to provide terrigenous sand for ceramic temper. The forearc islands lie along the Tonga platform, a linear tract of shoals uplifted diachronously by subduction of the buoyant Louisville Ridge at the Tonga Trench. Multiple transverse structural discontinuities break the forearc into discrete structural blocks, some tectonically stable during late Holocene time but others undergoing postuplift subsidence. Understanding the paleoenvironmental settings of Tongan archaeological sites requires reconstructing the contrasting geologic histories of different forearc island clusters. © 2007 Wiley Periodicals, Inc.  相似文献   

7.
The Late Miocene to Pleistocene evolution of the northwestern Iblean plateau (Sicily) is marked by a complex interplay of subaerial and submarine volcanism, subsidence and uplift, eustatic sea-level changes, and shallow-water carbonate and clay sedimentation. Volcanic activity occurred in distinct phases, differing drastically in volume, chemical composition, eruptive and depositional sites, and eruptive mechanisms. Six of the newly defined formations in the northwestern Iblean plateau are either entirely volcanic or contain significant amounts of volcanics. The eastern part of a shallow marine basin was filled completely by Late Pliocene tholeiitic lava flows (Militello Formation) that had advanced subaerially from the south–southeast. Lava deltas advanced southwestward on top of earlier pillow breccia debris flow deposits intertongued with soft Trubi marls and chalks. True submarine eruptions (Monte Caliella Formation) simultaneously produced densely packed pillow piles up to 250?m thick. Inferred water depths based on volcanologic and paleoecologic criteria of interbedded and overlying calcarenites agree well. Subsequent alkalic, more explosive Pleistocene volcanic eruptions (Poggio Vina Formation) changed from initially submarine to late subaerial indicating growth of edifices above sea level, sea-level rise, or land Subsidence by ca. 50?m. They and the latest Militello volcanics are interlayed with minor shallow-water calcarenites. The Poggio Vina volcanics were submerged during a second sea-level rise amounting to up to 100?m. The sea was generally shallow, i.e., <100?m deep, throughout most of the Late Pliocene and early Pleistocene. The Poggio Vina volcanism took place prior to the Emilian transgression. The sea-level rise might represent a continuation of the subsidence trend that caused the Lower Pliocene Trubi marine basin. Subaerial conditions were reached twice in the approximate time interval 1.9–1.6?Ma during phases of voluminous volcanism that outpaced subsidence. Uplift of approximately 600?m (Palagonia) to 950?m (Monte Lauro) occurred subsequent to emplacement of the Pleistocene alkalic volcanics. Bioclastic carbonates deposited concurrently with uplift drape a major fault scarp east of Palagonia with uplift rates in excess of 0.5?mm/a, provided most uplift occurred during ca. 1?Ma. Basinning continued beneath the half graben of the present Piana di Catania where volcanics several hundreds of meters thick – at least some of them alkalic in composition – occur at a depth of approximately 500–1500?m below the present surface. Quaternary uplift of the northwestern Iblean plateau may have been due to a major phase of underplating or rise of partially melted mantle. Composition of the volcanic rocks, total volume, and mass eruptive rates are well-correlated. The volumetrically very minor highly mafic Messinian nephelinites may have formed in response to Messinian lithosphere unloading following draining of the Mediterranean resulting in very low-degree partial melting. The nephelinitic to basanitic Poggio Inzerillo and Poggio Pizzuto pillow lavas may herald a major mantle decompression event, possibly the rise of a mantle diapir. The remarkably homogeneous bronzite-bearing, relatively SiO2-rich Militello tholeiites, representing a very short-lived but voluminous eruptive phase, resemble E-MORB and reflect a major high-degree partial melting event. The Pleistocene Poggio Vina alkali basalts to nephelinites resemble the late-stage alkalic phase in intraplate magmatic systems. The Iblean cycle of a brief but intense phase of widespread tholeiites followed by alkali basaltic volcanism resembles that of Etna Volcano where widespread basal tholeiites erupted at approximately 0.5?Ma and were followed by (evolved) alkali basaltic lavas. The immediate cause-and-effect relationship between volcanism and tectonism remains speculative.  相似文献   

8.
Fluctuations in relative sea level, tectonic movement, and sedimentation during the late Pleistocene to Holocene in the Hisarönü Gulf (SE Aegean Sea) and surrounding area were investigated with a high‐resolution geophysical survey and underwater archaeological observations. The Hisarönü Gulf has been affected by vertical tectonic movements and rising sea level following the last glacial period (20,000 yr B.P.). High‐resolution seismic data were interpreted to reveal the structure of the late Pleistocene to Holocene deposits and determine the location of the paleoshoreline. In order to describe the relative rise of sea level, principles of sequence stratigraphy were used for the late Pleistocene to Holocene transition, and submerged archaeological remains and bioerosional indicators were used for the late Holocene period. A comparison of archaeological observations in the study area with the known regional sea level curve indicates that the relative rise in sea level for the late Holocene is, for the most part, due to the tectonic subsidence of the coastal plain. © 2012 Wiley Periodicals, Inc.  相似文献   

9.
Samples of volcanic rocks from Alborán Island, the Alboran Sea floor and from the Gourougou volcanic centre in northern Morocco have been analyzed for major and trace elements and Sr–Nd isotopes to test current theories on the tectonic geodynamic evolution of the Alboran Sea. The Alborán Island samples are low-K tholeiitic basaltic andesites whose depleted contents of HFS elements (0.5×N-MORB), especially Nb (0.2×N-MORB), show marked geochemical parallels with volcanics from immature intra-oceanic arcs and back-arc basins. Several of the submarine samples have similar compositions, one showing low-Ca boninite affinity. 143Nd/144Nd ratios fall in the same range as many island-arc and back-arc basin samples, whereas 87Sr/86Sr ratios (on leached samples) are somewhat more radiogenic. Our data point to active subduction taking place beneath the Alboran region in Miocene times, and imply the presence of an associated back-arc spreading centre. Our sea floor suite includes a few more evolved dacite and rhyolite samples with (87Sr/86Sr)0 up to 0.717 that probably represent varying degrees of crustal melting. The shoshonite and high-K basaltic andesite lavas from Gourougou have comparable normalized incompatible-element enrichment diagrams and Ce/Y ratios to shoshonitic volcanics from oceanic island arcs, though they have less pronounced Nb deficits. They are much less LIL- and LREE-enriched than continental arc analogues and post-collisional shoshonites from Tibet. The magmas probably originated by melting in subcontinental lithospheric mantle that had experienced negligible subduction input. Sr–Nd isotope compositions point to significant crustal contamination which appears to account for the small Nb anomalies.

The unmistakable supra-subduction zone (SSZ) signature shown by our Alboran basalts and basaltic andesite samples refutes geodynamic models that attribute all Neogene volcanism in the Alboran domain to decompression melting of upwelling asthenosphere arising from convective thinning of over-thickened lithosphere. Our data support recent models in which subsidence is caused by westward rollback of an eastward-dipping subduction zone beneath the westernmost Mediterranean. Moreover, severance of the lithosphere at the edges of the rolling-back slab provides opportunities for locally melting lithospheric mantle, providing a possible explanation for the shoshonitic volcanism seen in northern Morocco and more sporadically in SE Spain.  相似文献   


10.
Speculations on the nature and cause of mantle heterogeneity   总被引：8，自引：0，他引：8  
Hotspots and hotspot tracks are on, or start on, preexisting lithospheric features such as fracture zones, transform faults, continental sutures, ridges and former plate boundaries. Volcanism is often associated with these features and with regions of lithospheric extension, thinning, and preexisting thin spots. The lithosphere clearly controls the location of volcanism. The nature of the volcanism and the presence of ‘melting anomalies’ or ‘hotspots’, however, reflect the intrinsic chemical and lithologic heterogeneity of the upper mantle. Melting anomalies—shallow regions of ridges, volcanic chains, flood basalts, radial dike swarms—and continental breakup are frequently attributed to the impingement of deep mantle thermal plumes on the base of the lithosphere. The heat required for volcanism in the plume hypothesis is from the core. Alternatively, mantle fertility and melting point, ponding and focusing, and edge effects, i.e., plate tectonic and near-surface phenomena, may control the volumes and rates of magmatism. The heat required is from the mantle, mainly from internal heating and conduction into recycled fragments. The magnitude of magmatism appears to reflect the fertility, not the absolute temperature, of the asthenosphere. I attribute the chemical heterogeneity of the upper mantle to subduction of young plates, aseismic ridges and seamount chains, and to delamination of the lower continental crust. These heterogeneities eventually warm up past the melting point of eclogite and become buoyant low-velocity diapirs that undergo further adiabatic decompression melting as they encounter thin or spreading regions of the lithosphere. The heat required for the melting of cold subducted and delaminated material is extracted from the essentially infinite heat reservoir of the mantle, not the core. Melting in the upper mantle does not requires the instability of a deep thermal boundary layer or high absolute temperatures. Melts from recycled oceanic crust, and seamounts—and possibly even plateaus—pond beneath the lithosphere, particularly beneath basins and suture zones, with locally thin, weak or young lithosphere. The characteristic scale lengths—150 to 600 km—of variations in bathymetry and magma chemistry, and the variable productivity of volcanic chains, may reflect compositional heterogeneity of the asthenosphere, not the scales of mantle convection or the spacing of hot plumes. High-frequency seismic waves, scattering, coda studies and deep reflection profiles are needed to detect the kind of chemical heterogeneity and small-scale layering predicted from the recycling hypothesis.  相似文献   

11.
Archaeological sites in the northern Ha'apai Group of central Tonga occur on small islands within the uplifted forearc belt of the Tonga-Kermadec arc-trench system. The present inland positions of occupation sites that probably once occupied coastal settings imply significant expansion of some island shorelines during late Holocene time (ca. 3250 B.P. to present). Geologic processes leading potentially to enlargement of the islands include continuing forearc uplift, eustatic or glacio-hydro-isostatic fall in sea level following a mid-Holocene highstand, and progressive accretion of beach ridges to island coasts, with or without changes in relative sea level. Radiometric dates for uplifted coral terraces in Tonga indicate that forearc uplift has been negligible during Holocene time. By contrast, theoretical considerations, regional analysis of shoreline indicators throughout the South Pacific, and limited empirical data from Tonga itself all imply that regional sea level has declined locally by 1–2 m since a mid-Holocene highstand (ca. 6000-3000 B.P.), which was a hydro-isostatic response to transfer of water mass from Pleistocene ice caps to the ocean basins. Emergence of originally coastal sites is thus expected since initial settlement of the islands by Lapita peoples. Accretionary coastal flats composed of multiple beach ridges are 250–500 m wide on favorable leeward shores and the flanks of sand cays, but some presently unknown proportion of this incremental island growth may have occurred prior to the post-mid-Holocene decline in relative sea level. Ash falls from tephra eruptions at Tongan volcanoes also modified island environments through Quaternary time. Evidence for significant change in the configuration and morphology of islands in Ha'apai during the period of human settlement highlights the need for systematic interdisciplinary archaeological and geological research in the study of Pacific prehistory. © 1994 John Wiley & Sons, Inc.  相似文献   

12.
Early Miocene (ca.?21–18 Ma) volcanism in the Karacada? area comprises three groups of volcanic rocks: (1) calcalkaline suite (andesitic to rhyolitic lavas and their pyroclastics), (2) mildly-alkaline suite (alkali basalt, hawaiite, mugearite, benmoreite and trachydacite), and (3) a single trachyandesitic flow unit. Field observations, 40Ar/39Ar ages and geochemical data show that there was a progressive temporal transition from group 1 to 3 in a post-collisional tectonic setting. The calcalkaline suite rocks with medium-K in composition resemble those of subduction-related lavas, whereas the mildly-alkaline suite rocks having a sodic tendency (Na2O/K2O=1.5–3.2) resemble those of within-plate lavas. Incompatible element and Sr-Nd isotopic characteristics of the suites suggest that the lithospheric mantle beneath the Karacada? area was heterogeneously enriched by two processes before collision: (1) enrichment by subduction-related processes, which is important in the genesis of the calcalkaline volcanism, (2) enrichment by small degree melts from the astenosphere, which dominates the mildly alkaline volcanism. Perturbation of the enriched lithosphere by either delamination following collision and uplift or removal of the subducted slab following subduction and collision (i.e., slab breakoff) is the likely mechanism for the initiation of the post-collision volcanism.  相似文献   

13.
西昆仑阿什库勒火山群地质特征和活动分期   总被引：3，自引：2，他引：1  
阿什库勒火山群作为青藏高原内最新期的北部昆仑火山群的典型代表,长期以来一直受到国内外地学界的广泛关注。我们通过对阿什库勒盆地火山区的野外地质地貌考察,研究了阿什库勒盆地内第四纪火山活动历史,完成了阿什库勒火山群重要火山锥体的1∶2000火山机构图。同时,对系统采集于不同火山机构和部位的20个火山岩样品进行了系列Ar-Ar同位素年代测试分析,获得了翔实的研究区不同火山岩石单元的绝对年龄数据。结合野外地质地貌考察结果,讨论了阿什库勒盆地的火山活动的第四纪时期活动历史,并将主要活动时期进一步划分为早更新世早期、早更新世中期、中更新世早期、中更新世晚期、晚更新世期和全新世期六期,取得了与前人有关该研究区火山活动历史和活动期次类似的划分方案,并补充了新的年代学证据。研究成果弥补了藏北高原西部由于恶劣工作环境而造成的第四纪火山学研究的不足。  相似文献   

14.
Ataúro is a key to understanding the late stage volcanic and subduction history of the Banda Arc to the north of Timor. A volcanic history of bi-modal subaqueous volcanism has been established and new whole rock and trace element geochemical data show two compositional groups, basaltic andesite and dacite–rhyolite. 40Ar/39Ar geochronology of hornblende from rhyo-dacitic lavas confirms that volcanism continued until 3.3 Ma. Following the cessation of volcanism, coral reef marine terraces have been uplifted to elevations of 700 m above sea level. Continuity of the terraces at constant elevations around the island reflects regional-scale uplift most likely linked to sublithospheric processes such as slab detachment. Local scale landscape features of the eastern parts of Ataúro are strongly controlled by normal faults. The continuation of arc-related volcanism on Ataúro until at least 3.3 Ma suggests that subduction of Australian lithosphere continued until near this time. This data is consistent with findings from the earthquake record where the extent of the Wetar seismic gap to a depth of 350 km suggests slab breakoff, as a result of collision, commenced at ∼4 Ma, leading to subsequent regional uplift recorded in elevated terraces on Ataúro and neighbouring islands.  相似文献   

15.
长白山天池火山粗面玄武岩的喷发历史与演化   总被引：18，自引：11，他引：18  
本文新提出的年代学和岩石化学结果，进一步从天池火山与区域火山活动的关系，论述了天池火山造盾、造锥历史和岩浆结晶分异转型的时间约束，早更新世早期（2Ma前）开始粗面玄武岩的造盾，早更新世晚期（约1Ma）粗面玄武岩向粗安岩、粗面岩演化，中更新世是粗面岩造锥的主阶段，到了晚更新世（约0．1Ma）粗面岩向碱流岩演化。在中-晚更新世来自地壳岩浆房的粗面岩、碱流岩造锥过程中，来自地幔的粗面玄武岩浆喷发活动始终没有间断过。由于来自地幔粗面玄武质岩浆持续向地壳岩浆房补给，所以天池火山是一座长寿命的火山。岩浆的结晶分异作用和混合作用是天池火山岩浆演化的两个最重要过程，前者形成天池火山双峰式火山岩分布特征，后者成为天池火山喷发的触发机制。天池火山在晚更新世-全新世碱流质岩浆主喷发期兼有少量玄武质粗安岩、粗安岩或粗面质岩浆的交替喷出，揭示了天池火山的地壳岩浆房熔体的分层结构特点，由于来自地幔粗面玄武质岩浆注入地壳岩浆房，导致不同层位岩浆的扰动和混合作用，触发天池火山的喷发。  相似文献   

16.
活火山是指1万年来有过喷发历史的全新世火山。火山的高分辨年代学对火山灾害评估和火山分类具有重要意义。对于缺乏历史记载的全新世火山,直接对火山岩进行同位素定年很困难。本文利用具有高时间分辨率的镭-钍-铀非平衡确定中国东部年轻火山的年龄。根据镭-钍-铀同位素,海南岛的马鞍岭和雷虎岭是全新世火山(马鞍岭:4.3ka;雷虎岭:4.7ka);镜泊湖火山(4.9ka)也是全新世火山;龙岗火山存在晚更新世和全新世活动(7.0ka,15.0ka);大兴安岭阿尔山和诺敏河Ra/Th非平衡消失但~(230)Th/~(238)U非平衡显著,属于晚更新世喷发(阿尔山:63ka;诺敏河:71ka)。海南岛的马鞍岭火山、雷虎岭火山和东北地区的龙岗火山、镜泊湖火山,是4座活火山。至于东北地区的阿尔山和诺敏河火山是否是活火山,有待测试更多样品的Ra/Th同位素。五大连池老黑山和火烧山有历史喷发记录,这与它们都存在显著Ra/Th非平衡一致。五大连池老黑山和火烧山的岩浆滞留年龄分别小于4.2ka和3.2ka,岩浆上升速率 18～23m/y。  相似文献   

17.
龙岗火山群单成因火山作用   总被引：17，自引：0，他引：17  
长白山区龙岗火山群内分布有100多座由玄武质岩渣锥、熔岩流、低平火山口等火山结构组成的新生代火山。单成因火山作用决定着龙岗火山区内火山结构与规模,这对指导当地减轻火山灾害工作也有一定意义。  相似文献   

18.
This paper demonstrates the decisive role of natural preconditions on the formation of large late Holocene sea ingressions in peaty coastal plains along the North Sea’s southern shores. Geological and archaeological evidence shows that these sea ingressions (expansion of new tidal systems) were mainly caused by land subsidence, which occurred due to intensified agricultural use of artificially drained peatlands since the Late Iron Age (250–12 BC). This made the coastal plain sensitive to storm-surge ingression through weak spots, e.g., at the location of existing creeks, in the coastline.Using The Netherlands as a case study, we show that natural preconditions (i.e., the geological setting at the time of ingression) played a key role in the pacing and extent of tidal area expansion. Ingressive tidal systems eventually reached most far inland in coastal segments with wide peaty back-barrier plains. In contrast, sea ingression formation was hampered in coastal segments with well-developed natural ingression-protecting geomorphic features (e.g., beach-barriers, supratidal levees). Feedback mechanisms, such as additional peat subsidence by loading of sediment imported into the new tidal area, caused further tidal volume increase and created accommodation space for tidal deposits. These combined effects caused irreversible sea ingression over large areas that consequently became unsuitable for habitation for many centuries.Improved understanding of such sea-ingression mechanisms and their facilitating conditions are essential for the assessment of the sensitivity of many densely populated coastal plains, which experience major human-induced subsidence, eventually leading to coastal plain drowning.  相似文献   

19.
Ambitle Volcano (new name) is the most recently active of four eruptive centres that make up the mainly Pliocene–Pleistocene Tabar–Lihir–Tanga–Feni (TLTF) alkalic volcanic province, located in the New Ireland Basin, Papua New Guinea. Ambitle Volcano is a submarine and subaerial stratovolcano occupying all of Ambitle Island. The volcano rises 2500 m above the surrounding sea floor to sea level and, with a maximum elevation of 479 m above sea level, indicates a structure nearly 3000 m high. Volcanic deposits rest unconformably on Oligocene basement rocks of the New Ireland Basin. The cone of Ambitle Volcano is constructed mainly of lavas and pyroclastic and epiclastic rocks; lavas are commonly vesiculated. These lavas are strongly undersaturated and intermediate in composition (phonolitic tephrite and tephritic phonolite) with alkali basalt, tephrite and basanite and trachybasalt and trachyandesite also present. Syenite porphyry and monzonite stocks intrude the cone-forming mafic–intermediate sequence at Kabang–Matangkaka and in the upper Nanum River. The central part of the Ambitle Volcano is now modified as a prominent semi-circular topographic rim around the Nanum Valley. The Nanum Valley Crater (new name) is the product of large-scale summit failure of the SW flanks of the summit of the Ambitle Volcano. This event is dated no younger than 0.68–0.49 Ma. The Ambitle Crater (new name), the product of Late Quaternary resurgence of volcanism following sector collapse of Ambitle Volcano, is located in the NE portion of the Nanum Valley Crater. The crater is elongated NNE and measures 900 m × 550 m at its widest development. The strong NNE–SSW linearity of the western rim of Ambitle Crater is structurally controlled by the Kabang Fault. Tephra was erupted from the Ambitle Crater at 2300 ± 100 a and is widely dispersed throughout the Nanum Valley Crater and beyond. This is the youngest volcanic event in the TLTF volcanic province. The Niffin graben is a major NW–SE-trending structural corridor that transects Ambitle Island. The structural corridor is parallel to the NW–SE strike of the TLTF volcanic province suggesting it has been an important control on magmatism and volcanism. Presently active geothermal systems are located along Niffin graben structures in the western valleys of the island and in the Nanum Valley Crater. The volcanic rocks of Ambitle Volcano host porphyry Cu–Au style mineralisation and epithermal Ladolam-type Au mineralisation. Extensive exploration including surface sampling and subsurface drilling completed since 1983 on many prospects has not defined an economic resource.  相似文献   

20.
The West Siberian Basin (WSB) records a detailed history of Permo-Triassic rifting, extension and volcanism, followed by Mesozoic and Cenozoic sedimentation in a thermally subsiding basin. Sedimentary deposits of Permian age are absent from much of the basin, suggesting that large areas of the nascent basin were elevated and exposed at that time. Industrial seismic and well log data from the basin have enabled extension and subsidence modelling of parts of the basin. Crustal extension (β) factors are calculated to be in excess of 1.6 in the northern part of the basin across the deep Urengoy graben. 1-D backstripping of the Triassic to Cenozoic sedimentary sequences in this region indicates a period of delayed subsidence during the early Mesozoic. The combination of elevation, rifting and volcanism is consistent with sublithospheric support, such as a hot mantle plume.

This interpretation accords with the geochemical data for basalts from the Siberian Traps and the West Siberian Basin, which are considered to be part of the same large igneous province. Whilst early suites from Noril'sk indicate moderate pressures of melting (mostly within the garnet stability field), later suites (and those from the West Siberian Basin) indicate shallow average depths of melting. The main region of magma production was therefore beneath the relatively thin (ca. 50–100 km) lithosphere of the basin, and not the craton on which the present-day exposure of the Traps occurs. The indicated uplift, widespread occurrence of basalts, and short duration of the volcanic province as a whole are entirely consistent with published models involving a mantle plume. The main argument against the plume model, namely lack of any associated uplift, appears to be untenable.  相似文献   


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