Models for the origin of accretionary lapilli   总被引：1，自引：0，他引：1  

R. Schumacher  H. -U. Schmincke 《Bulletin of Volcanology》1995,56(8):626-639

Binding between initially cohesionless ash particles to form concentric accretionary lapilli is provided primarily by the capillary forces of liquid bridges from condensed moisture and by electrostatic attraction. Capillary forces are strong bonds if the particles are in close contact, but they decrease rapidly with increasing particle spacing. Electrostatic attraction between charged ash particles is much weaker but effective over larger distances, increasing the frequency of collision between them.Experimental results of liquid film binding of volcanic ash showed that agglomeration was most successful between 15 and 25 wt.%, defining the agglomeration window for the formation of accretionary lapilli. Below 5–10 wt.% and above about 25–30 wt.% of water, concentric agglomeration was inhibited. Particles <350 m could be selected from a wider particle population in the experiments using only small amounts of water, which can explain the growth of accretionary lapilli in pyroclastic surges around agglomeration nuclei. Experiments testing the behavior of volcanic ash in electric fields showed that ash clusters formed instantaneously when the ash entered the field between a corona discharge gun and a grounded metal plate. The maximum grain size incorporated into the artificial clusters was about 180 m but >90 wt.% of ash was <45 m.Accretionary lapilli form in turbulent ash clouds when particles carrying liquid films of condensed moisture collide with each other and when the binding forces exceed the grain dispersive forces. Larger particles >500 m act as agglomeration nuclei in surges, accreting ash <350 m around them. In pyroclastic flows the aggregates are thought to originate from already size-sorted ash at the interface between the lower avalanche part of the flow and its overriding elutriation cloud. The fine-grained rims around accretionary lapilli found close to source are interpreted to be accreted dominantly by electrostatic attraction of very fine ash similar to clustering in elutriation clouds.  相似文献   

Response of coastal plain rivers to falling relative sea-level: allogenic controls on the aggradational phase     

JOHN B. SWENSON  TETSUJI MUTO† 《Sedimentology》2007,54(1):207-221

This study combines mathematical modelling and supporting flume experiments to address the problem of how coastal plain rivers respond to a steady fall in relative sea-level. The theoretical component of the study focuses on the development of a moving boundary model of fluviodeltaic progradation that treats rigorously the dynamics of the shoreline and alluvial–basement transition (the upstream limit of the alluvial river system). Dimensional analysis and numerical solutions to the model governing equations together suggest that, at first order, coastal plain rivers will remain aggradational on a timescale that varies with allogenic sediment and water supply and the fall rate of relative sea-level. In natural fluviodeltaic systems, this intrinsic timescale is likely to vary by several orders of magnitude, suggesting that the aggradational phase of river response can be geologically long-lived. At second order, the duration of alluvial aggradation is controlled by two dimensionless numbers that embody system geometry and the kinematics of alluvial sediment transport. Model predictions were tested in a series of carefully scaled flume experiments. The level of agreement between predicted and measured trajectories for the shoreline and alluvial–basement transition strongly suggests that the moving boundary theory developed here successfully captures the response of small-scale fluviodeltaic systems to falling sea-level. The results of this study have several sequence-stratigraphic implications: a fall in relative sea-level at the shoreline is not a sufficient condition for river incision; the onset of alluvial degradation and sequence-boundary formation need not coincide with a maximum in the rate of sea-level fall; and the onset of sequence-boundary formation is sensitive to allogenic sediment supply.  相似文献   

Formation of the Yakuno ophiolite; accretionary subduction under medium-pressure-type metamorphic conditions     

Soichi Osozawa  Hiroshi Takeuchi  Toru Koitabashi 《Tectonophysics》2004,393(1-4):197

The notion that the Yakuno ophiolite and overlying Maizuru Group represents an accretionary prism formed during the Permian evolution of Japan on the Yakuno eruptive sequence, association of hemipelagic mudstone with silicic tuff, exotic fossiliferous limestones derived from previously accreted sea-mounts, upward coarsening of sequences terrigenous sandstone and conglomerate, and mildly deformed Permian and Triassic forearc basin formations. The most important indicator, however, is the seaward imbrication and repetition observed in both the Maizuru Group and the ophiolite itself. D1 deformation structures include axial–planar foliations (pressure-solution cleavage for the Maizuru Group and granulite–amphibolite metamorphic layering in the ophiolite), flattening type strain, symmetric pressure shadows and fringes, and isoclinal folds showing axial–planar foliations and thrust faulting at their overturned limb. The exceptional asymmetry observed indicates seaward-directed shearing near the thrust, while D1 structures in the Maizuru zone are explained by off-scraping, above the basal decollement. The later Jurassic D2 kink fold structure includes a first-order asymmetric kink with a brittle thrust at its overturned limb, more-or-less coeval with M2 retrograde metamorphism. Medium-pressure M1 prograde metamorphism in the Yakuno ophiolite produced layering of granulite and amphibolite, and in the Maizuru Group, formation of illite along pressure-solution cleavage of mudstones. The metamorphic grade is controlled by the stratigraphic relationships and appears typical of that in ocean floor regions. However, there was only one episode of M1 prograde metamorphism which occurred contemporaneously with D1 off-scraping. Given that subduction zones are normally characterized by high P/T metamorphic regimes, the observed P/T history appears to reflect relatively unusual conditions. Such high thermal gradients may plausibly reflect the approach of a young, hot oceanic plate which continued subducting beneath the Japanese arc. Accordingly, the Yakuno ophiolite was probably formed at the trench–trench–ridge triple junction.  相似文献   

高精度重力测量在香港石硖尾滑坡治理区的应用     

许德树  卢景奇  李元厚  李晓昌  于德武 《物探与化探》2004,28(4):345-348

介绍了采用5m网度的高精度重力测量进行工程勘查的方法技术，包括测地技术、重力测量技术和三维反演技术。通过对已知山顶建筑物工程设计参数进行重力影响的消除，实现了人文干扰的改正。采用精确的直立棱柱体理论重力异常正演公式进行正演，并采用广义逆的方法进行反演，获得了香港九龙石硖尾滑坡治理区新鲜花岗岩基顶面起伏的构造特征。与钻探结果对比表明，该花岗岩基顶面深度图结果可靠。  相似文献   

Evolution of an accretionary complex along the north arm of the Island of Sulawesi, Indonesia     

Yusuf Surachman Djajadihardja  Asahiko Taira  Hidekazu Tokuyama  Kan Aoike  Christian Reichert  Martin Block  Hans U. Schluter  Sonke Neben 《Island Arc》2004,13(1):1-17

Abstract Seismic reflections across the accretionary prism of the North Sulawesi provide excellent images of the various structural domains landward of the frontal thrust. The structural domain in the accretionary prism area of the North Sulawesi Trench can be divided into four zones: (i) trench area; (ii) Zone A; (iii) Zone B; and (iv) Zone C. Zone A is an active imbrication zone where a decollement is well imaged. Zone B is dominated by out‐of‐sequence thrusts and small slope basins. Zone C is structurally high in the forearc basin, overlain by a thick sedimentary sequence. The subducted and accreted sedimentary packages are separated by the decollement. Topography of the oceanic basement is rough, both in the basin and beneath the wedge. The accretionary prism along the North Sulawesi Trench grew because of the collision between eastern Sulawesi and the Bangai–Sula microcontinent along the Sorong Fault in the middle Miocene. This collision produced a large rotation of the north arm of Sulawesi Island. Rotation and northward movement of the north arm of Sulawesi may have resulted in southward subduction and development of the accretionary wedge along North Sulawesi. Lateral variations are wider in the western areas relative to the eastern areas. This is due to greater convergence rates in the western area: 5 km/My for the west and 1.5 km/My for the east. An accretionary prism model indicates that the initiation of growth of the accretionary prism in the North Sulawesi Trench occurred approximately 5 Ma. A comparison between the North Sulawesi accretionary prism and the Nankai accretionary prism of Japan reveals similar internal structures, suggesting similar mechanical processes and structural evolution.  相似文献   

角锥棱镜有效使用面积的计算   总被引：1，自引：1，他引：1  

李松  刘博 《测绘信息与工程》2002,27(4):25-27

从理论上推导出角锥棱镜的主要光学性质，以及一定形状的棱镜有效面积随口径、入射角大小之间的变化规律，给出反映这种变化规律的图线，对正确使用角锥棱镜具有一定意义。  相似文献   

中国大地构造的动力学新理论   总被引：2，自引：0，他引：2  

杨志华  苏生瑞  李勇  周义 《矿物岩石》2001,21(3):160-164

在全球范围内流行的按密度差、温度差建立的地幔对流、拆沉作用和地幔柱（羽）理论，阐明不了中国大陆岩石圈、大陆造山带的组成、结构与演化。从我们对中国秦岭及西部造山带的研究中，发现和提出的在地球自转速度变化制约下的多层扭动涡旋甩出说－核幔壳“风暴”所引发的热核反应是地球发展与演化唯一的、统一的“动力”来源，是中国大陆岩石圈、大陆造山带形成、发展与演化的动力学理论。  相似文献   

吉林省伊通火山群   总被引：5，自引：0，他引：5  

王振中 《吉林地质》1994,13(2):29-35,41

伊通火山群形成于第三纪，玄武岩来自上地幔，受断裂控制。火山机构多是由未喷出地表的玄武岩、地幔岩捕获体及少许隐爆角砾岩组成，发育奇特有规律排列的节理柱组合系统展现了潜火山生成在近地表下环境。  相似文献   

Metallogeny of accretionary orogens — The connection between lithospheric processes and metal endowment     

Frank P. Bierlein  David I. Groves  Peter A. Cawood 《Ore Geology Reviews》2009,36(4):282-292

Accretionary orogens throughout space and time represent extremely fertile settings for the formation and preservation of a wide variety of mineral deposit types. These range from those within active magmatic arcs, either in continental margin or intra-oceanic settings, to those that develop in a variety of arc-flanking environments, such as fore-arcs and back-arcs during deformation and exhumation of the continental margin. Deposit types also include those that form in more distal, far back-arc and foreland basin settings. The metallogenic signature and endowment of individual accretionary orogens are, at a fundamental level, controlled by the nature, composition and age of the sub-continental lithosphere, and a complex interplay between formational processes and preservational forces in an evolving Earth. Some deposit types, such as orogenic gold and volcanic massive sulfide (VMS) deposits, have temporal patterns that mimic the major accretionary and crustal growth events in Earth history, whereas others, such as porphyry Cu–Au–Mo and epithermal Au–Ag deposits, have largely preservational patterns. The presence at c. 3.4 Ga of (rare) orogenic gold deposits, whose formation necessitates some form of subduction–accretion, provides strong evidence that accretionary processes operated then at the margins of continental nuclei, while the widespread distribution of orogenic gold and VMS deposits at c. 2.7–2.6 Ga reflects the global distribution of accretionary orogens by this time.  相似文献   

Basalts of the Pantalassa ocean in the Samarka terrane, Central Sikhote Alin     

V. P. Simanenko  A. N. Filippov  A. A. Chashchin 《Russian Journal of Pacific Geology》2009,3(3):220-233

Basalts developed on the right bank of the Matai River belong to the Samarka terrane (Central Sikhote Alin), which is a fragment of the Jurassic accretionary prism. They associate with Carboniferous-Permian reef limestones, Permian pelagic cherts, Jurassic hemipelagic cherty-clayey deposits, and terrigenous rocks of the near-continental sedimentation area. The petrogeochemical features of the basalts provide insight into the character of the volcanism in different settings of the ancient Pantalassa ocean. In terms of chemistry, the Carboniferous-Permian basalts are similar to the within-plate ocean-island basalts related to plume mantle sources. They were presumably formed in an oceanic area with numerous islands and seamounts. The Permian basalts associated with cherts are tholeiitic in composition and were formed from depleted mantle in a spreading center located in the pelagic area. The Jurassic basalts are of plume origin and, in terms of geochemistry, occupy an intermediate position between OIB and E-MORB. They were presumably formed in a convergent zone in a geodynamic setting of rapid oblique subduction.  相似文献