Sector collapse,sedimentation and clast population evolution at an active island-arc volcano: Stromboli,Italy   总被引：1，自引：0，他引：1  

P. Kokelaar  C. Romagnoli 《Bulletin of Volcanology》1995,57(4):240-262

Sciara del Fuoco is the subaerial part of a partially filled sector-collapse scar that extends to 700 m below sea level on Stromboli volcano. The collapse occurred <5000 years ago, involved 1.81 km³ of rock and is the latest of a series of major collapses on the north-west flank of Stromboli. A north-east trending arc-axial fault system channels magmas into the volcano and has caused tilting and/or downthrow to the north-west. The slope of the partial cone constructed between the lateral walls of the collapse scar acts as a channelway to the sea for most eruptive products. From 700 m below sea level and extending to >2200 m and >10 km from the shore to the NNW, a fan-shaped mounded feature comprises debris avalanche deposits (>4 km³) from two or more sector collapses. Volcaniclastic density currents originating from Sciara del Fuoco follow the topographic margin of the debris avalanche deposits, although overbank currents and other unconfined currents widely cover the mounded feature with turbidites. Historical (recorded) eruptive activity in Sciara del Fuoco is considerably less than that which occurred earlier, and much of the partial fill may have formed from eruptions soon after the sector collapse. It is possible that a mass of eruptive products similar to that in the collapse scar is dispersed as volcanogenic sediment in deep water of the Tyrhennian basin. Evidence that the early post-collapse eruptive discharge was greater than the apparent recent flux (2kg/s) counters suggestions that a substantial part of Stromboli's growth has been endogenous. The partial fill of Sciaria del Fuoco is dominated by lava and spatter layers, rather than by the scoria and ash layers classically regarded as main constituents of Strombolian (cinder) cones. Much of the volcanic slope beneath the vents is steeper than the angle of repose of loose tephra, which is therefore rapidly transported to the sea. Delicate pyroclasts that record the magmatic explosivity are selectively destroyed and diluted during sedimentary transport, mainly in avalanches and by shoreline wave reworking, and thus the submarine deposits do not record well the extent and diversity of explosive activity and associated clast-forming processes. Considerable amounts of dense (non-vesicular) fine sand and silt grains are produced by breakage and rounding of fragments of lava and agglutinate. The submarine extension of the collapse scar, and the continuing topographic depression to >2200 m below sea level, are zones of considerable by-passing of fine sand and silt, which are transported in turbidity currents. Evidently, volcanogenic sediments dispersed around island volcanoes by density currents are unlikely to record well the true spectrum and relative importance of clast-forming processes that occurred during an eruption. Marine sedimentary evidence of magmatic explosivity is particularly susceptible to partial or complete obliteration, unless there is a high rate of discharge of pyroclastic material into the sea.  相似文献   

午后多重极光弧观测研究          下载免费PDF全文

温艳波  杨惠根  韩德胜  T.Araki  艾勇  刘瑞源  刘勇华  胡泽骏  陈卓天  邓忠新  刘嵘  王晶 《极地研究》2005,17(4):255-263

本文利用2001-2003年南极中山站175天全天空摄像机观测,对午后多重极光弧的出现率及其与Kp指数的关系进行了统计分析,结果表明午后多重极光弧出现率呈一单峰分布,最大发生率出现在1445UT(1645MLT),其位置在1500MLT极光热点(1300-1700MLT)近夜侧的部分。与地磁活动指数Kp的相关统计分析表明,Kp值为2-3之间时多重极光弧有较大的出现率,这说明中等地磁活动情形下午后多重极光弧有较高的出现率。事件分析表明多重极光弧的强度变化与地磁Pc5脉动具有较高的相关性,并且有类似的频谱特征,这说明午后多重极光弧可能与同时出现的Pc5地磁脉动有关。  相似文献   

西藏班公湖-怒江缝合带西段特提斯洋晚二叠世(Ca.252Ma)洋内俯冲的地球化学和年代学证据     

韦少港  宋扬  唐菊兴 《地质论评》2016,62(S1):194-196

多龙蛇绿混杂岩是班公湖-怒江蛇绿岩带的重要组成部分,位于西藏阿里地区改则县北西约120 km的多龙矿集区内,大地构造位置处于班公湖-怒江缝合带中西段,南羌塘板块南缘。多龙蛇绿混杂岩主要分布在多龙矿区中部及东北部。矿区中部蛇绿岩主要由辉长岩、辉绿(玢)岩、枕状玄武岩、气孔杏仁状玄武岩、玄武质岩屑凝灰岩及硅质岩等组成,东西向延伸约35 km,南北宽3~7 km,出露面积约180 km2;该蛇绿岩残片的组成单元(包括基性岩单元以及硅质岩单元等)多被构造肢解,整体表现为不规则透镜体,以构造岩片的形式断续分布于侏罗系次深海陆棚-盆地斜坡复陆碎屑岩-类复理石建造内的断层带中,构成典型的网结状构造。矿区东北部蛇绿岩主要由含铁斜方辉石橄榄蛇纹岩、玻基玄武岩、碳酸盐化角闪辉长岩、微纹层状硅质岩等组成,该蛇绿混杂岩带沿北西-南东向断裂展布,延伸约12 km,宽1~3 km,出露面积约30 km2;该蛇绿岩残片组成单元(包括超基性岩单元、基性岩单元以及硅质岩单元等)均呈构造岩片的形式产出在三叠系灰岩地层内的断层带中。  相似文献   

Large-scale deformation associated with ridge subduction   总被引：1，自引：0，他引：1  

Eric L. Geist  Michael A. Fisher  David W. Scholl 《Geophysical Journal International》1993,115(2):344-366

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Early Cretaceous volcanic rocks and Early Cenozoic extrusions of Cape Mary, Schmidt Peninsula, north Sakhalin: Geochemical study     

V. P. Simanenko  S. V. Rasskazov  T. A. Yasnygina  A. I. Malinovskii  A. A. Chashchin 《Russian Journal of Pacific Geology》2007,1(3):265-275

The Early Cretaceous volcanic rocks of the Mariisky sequence and Early Cenozoic extrusive-vent rocks of Cape Mary are exposed at the northwestern extremity of Schmidt Peninsula, north Sakhalin. In chemical composition, all the rocks are subdivided into four groups. Three groups include the volcanic rocks of the Mariisky sequence, which consists, from bottom to top, of calc-alkaline rocks, transitional calc-alkaline-tholeiite rocks, and incompatible element-depleted tholeiites. These rocks show subduction geochemical signatures and are considered as a fragment of the Moneron-Samarga island arc system. Trace-element modeling indicates their derivation through successive melting of garnet-bearing mantle and garnet-free shallower mantle sources containing amphibole; pyroxene; and, possibly, spinel. The mixed subduction and within-plate characteristics of the extrusive vent rocks of Cape Mary attest to their formation in a transform continental margin setting.  相似文献   

Geochemical Evidence for Slab Melting in the Trans-Mexican Volcanic Belt   总被引：3，自引：0，他引：3  

Gomez-Tuena  Arturo; Langmuir  Charles H.; Goldstein  Steven L.; Straub  Susanne M.; Ortega-Gutierrez  Fernando 《Journal of Petrology》2007,48(3):537-562

Geochemical studies of Plio-Quaternary volcanic rocks from theValle de Bravo–Zitácuaro volcanic field (VBZ) incentral Mexico indicate that slab melting plays a key role inthe petrogenesis of the Trans-Mexican Volcanic Belt. Rocks fromthe VBZ are typical arc-related high-Mg andesites, but two differentrock suites with distinct trace element patterns and isotopiccompositions erupted concurrently in the area, with a traceelement character that is also distinct from that of other Mexicanvolcanoes. The geochemical differences between the VBZ suitescannot be explained by simple crystal fractionation and/or crustalassimilation of a common primitive magma, but can be reconciledby the participation of different proportions of melts derivedfrom the subducted basalt and sediments interacting with themantle wedge. Sr/Y and Sr/Pb ratios of the VBZ rocks correlateinversely with Pb and Sr isotopic compositions, indicating thatthe Sr and Pb budgets are strongly controlled by melt additionsfrom the subducted slab. In contrast, an inverse correlationbetween Pb(Th)/Nd and ¹⁴³Nd/¹⁴⁴Nd ratios, which extend to lowerisotopic values than those for Pacific mid-ocean ridge basalts,indicates the participation of an enriched mantle wedge thatis similar to the source of Mexican intraplate basalts. In addition,a systematic decrease in middle and heavy rare earth concentrationsand Nb/Ta ratios with increasing SiO₂ contents in the VBZ rocksis best explained if these elements are mobilized to some extentin the subduction flux, and suggests that slab partial fusionoccurred under garnet amphibolite-facies conditions. KEY WORDS: arcs; mantle; Mexico; sediment melting; slab melting  相似文献   

Lead angles and emitting electron energies of Io-controlled decameter radio arcs     

S.L.G. Hess  A. Pétin  B. Bonfond 《Planetary and Space Science》2010,58(10):1188-1198

The Io-controlled radio arcs are emissions in the decametric radio range which appear arc shaped in the time-frequency plane. Their occurrence is controlled by Io's position, so it has been for long inferred that they are powered by the Io-Jupiter electrodynamic interaction. Their frequency ranges correspond to the electron cyclotron frequencies along the Io Flux tube, so they are expected to be generated by cyclotron maser instability (CMI). The arc shape was proposed to be a consequence of the strong anisotropy of the decametric radio emissions beaming, combined with the topology of the magnetic field in the source and the observation geometry. Recent papers succeeded at reproducing the morphologies of a few typical radio arcs by modeling in three dimensions the observation geometry, using the best available magnetic field model and a beaming angle variation consistent with a loss-cone driven CMI. In the continuation of these studies, we present here the systematic modeling of a larger number of observations of the radio arcs emitted in Jupiter's southern hemisphere (including multiple arcs or arcs exhibiting abrupt changes of shape), which permits to obtain a statistical determination of the emitting field line localization (lead angle) relative to the instantaneous Io field line, and of the emitting particle velocities or energies. Variations of these parameters relative to Io's longitude are also measured and compared to the location of the UV footprints of the Io-Jupiter interaction. It is shown that the data are better organized in a reference frame attached to the UV spot resulting from the main Alfvén wing resulting from the Io-Jupiter interaction. It is proposed that the radio arcs are related to the first reflected Alfvén wing rather than to the main one.  相似文献   

How many sutures in the southern Central Asian Orogenic Belt：Insights from East Xinjiang-West Gansu（NW China）？   总被引：1，自引：0，他引：1  

Wenjiao Xiao  ;Chunming Han  ;Wei Liul  ;Bo Wan  ;Ji'en Zhang  ;Song  jian Ao  ;Zhiyong Zhang  ;Dongfang Song  ;Zhonghua Tian  ;Jun Luo 《地学前缘(英文版)》2014,5(4):525-536

How ophiolitic mèlanges can be defined as sutures is controversial with regard to accretionary orogenesis and continental growth.The Chinese Altay,East junggar,Tianshan,and Beishan belts of the southern Central Asian Orogenic Belt（CAOB） in Northwest China,offer a special natural laboratory to resolve this puzzle.In the Chinese Altay,the Erqis unit consists of ophiolitic melanges and coherent assemblages,forming a Paleozoic accretionary complex.At least two ophiolitic melanges（Armantai,and Kelameili） in East Junggar,characterized by imbricated ophiolitic melanges,Nb-enriched basalts,adakitic rocks and volcanic rocks,belong to a Devonian-Carboniferous intra-oceanic island arc with some Paleozoic ophiolites,superimposed by Permian arc volcanism.In the Tianshan,ophiolitic melanges like Kanggurtag,North Tianshan,and South Tianshan occur as part of some Paleozoic accretionary complexes related to amalgamation of arc terranes.In the Beishan there are also several ophiolitic melanges,including the Hongshishan,Xingxingxia-Shibangjing,Hongliuhe-Xichangjing,and Liuyuan ophiolitic units.Most ophiolitic melanges in the study area are characterized by ultramafic,mafic and other components,which are juxtaposed,or even emplaced as lenses and knockers in a matrix of some coherent units.The tectonic settings of various components are different,and some adjacent units in the same melange show contrasting different tectonic settings.The formation ages of these various components are in a wide spectrum,varying from Neoproterozoic to Permian.Therefore we cannot assume that these ophiolitic melanges always form in linear sutures as a result of the closure of specific oceans.Often the ophiolitic components formed either as the substrate of intra-oceanic arcs,or were accreted as lenses or knockers in subduction-accretion complexes.Using published age and paleogeographic constraints,we propose the presence of （1） a major early Paleozoic tectonic boundary that separates the Chinese Altay-East Junggar multiple subduction system  相似文献   

The general solution of the HENON–HEILES problem     

K.?E.?PapadakisEmail author  C.?L.?Goudas  G.?A.?Katsiaris 《Astrophysics and Space Science》2005,295(3):375-396

The general solution of the Henon–Heiles system is approximated inside a domain of the (x, C) of initial conditions (C is the energy constant). The method applied is that described by Poincaré as ‘the only “crack” permitting penetration into the non-integrable problems’ and involves calculation of a dense set of families of periodic solutions that covers the solution space of the problem. In the case of the Henon–Heiles potential we calculated the families of periodic solutions that re-enter after 1–108 oscillations. The density of the set of such families is defined by a pre-assigned parameter ε (Poincaré parameter), which ascertains that at least one periodic solution is computed and available within a distance ε from any point of the domain (x, C) for which the approximate general solution computed. The approximate general solution presented here corresponds to ε = 0.07. The same solution is further improved by “zooming” into four square sub-domain of (x, C), i.e. by computing sufficient number of families that reduce the density parameter to ε = 0.003. Further zooming to reduce the density parameter, say to ε = 10⁻⁶, or even smaller, although easily performable in both areas occupied by stable as well as unstable solutions, was found unnecessary. The stability of all members of each and all families computed was calculated and presented in this paper for both the large solution domain and for the sub-domains. The correspondence between areas of the approximate general solution occupied by stable periodic solutions and Poincaré sections with well-aligned section points and also correspondence between areas occupied by unstable solutions and Poincaré sections with randomly scattered section points is shown by calculating such sections. All calculations were performed using the Runge-Kutta (R-K) 8th order direct integration method and the large output received, consisting of many thousands of families is saved as “Atlas of the General Solution of the Henon–Heiles Problem,” including their stability and is available at request. It is concluded that approximation of the general solution of this system is straightforward and that the chaotic character of its Poincaré sections imposes no limitations or difficulties.  相似文献   

大洋岛弧的前世今生   总被引：2，自引：2，他引：0  

吴福元  王建刚  刘传周  刘通  张畅  纪伟强 《岩石学报》2019,(1):1-15

根据板块构造理论,板块的边界是地质作用最为强烈的地区,因而它们是当今固体地球科学研究的重点。依据应力性质的不同,地球上板块的边界类型有扩张的洋中脊、汇聚的俯冲带和调节板块运动差异的转换断层三种。就汇聚型板块边界而言,它又可进一步划分为洋-洋俯冲的大洋或洋内岛弧带(Intra-oceanic arc)、洋-陆俯冲的安第斯型活动大陆边缘带和陆-陆接触的大陆碰撞带三种。相对而言,大洋岛弧的研究程度最低。传统认为最典型的大洋岛弧——日本诸岛,已不再被认为是洋-洋俯冲的产物,因为已有研究显示它是从亚洲大陆裂解的碎块。根据目前的调查,现今的大洋岛弧主要集中在西太平洋地区,以太平洋与菲律宾板块间的Izu-Bonin-Mariana弧和太平洋-澳大利亚间的西南太平洋岛弧为代表。大洋岛弧研究的最重要问题是,洋洋之间如何产生了俯冲。目前多倾向于认为:大洋中的转换断层可使不同时代的大洋岩石圈相互接触,在这种情况下,较老的岩石圈由于冷却时间较长而密度相对较大,因而可下沉而俯冲到较年轻的岩石圈之下。这一模型也被誉为蛇绿岩形成的初始俯冲定律(Subduction Initial Rule,简称SIR)。但存在的问题是,目前全球还没发现有转换断层转变为俯冲带的实例。更何况,全球大洋中发育如此众多的转换断层,但为何只在西太平洋发育大洋岛弧?本文通过对资料的总结还发现,这些大洋岛弧基本都是从亚洲或者澳大利亚大陆东部边缘裂解的碎块,只是后期的弧后扩张作用使裂解的碎块发生强烈的改造,形成具有类似大洋岩石圈的特点。目前提出的洋-洋自发形成俯冲带的模型并没有理论基础,也没有实际地质事实的支持。但在加勒比海、斯科舍海和阿留申地区,大洋岛弧的出现与洋底高原诱发的俯冲带跃迁或俯冲极性反转有关。因此,板块构造理论中的洋洋初始俯冲模式需要进一步资料的验证。  相似文献