Formation of submarine flat-topped volcanic cones in Hawai'i     

David A. Clague  James G. Moore  Jennifer R. Reynolds 《Bulletin of Volcanology》2000,62(3):214-233

 High-resolution bathymetric mapping has shown that submarine flat-topped volcanic cones, morphologically similar to ones on the deep sea floor and near mid-ocean ridges, are common on or near submarine rift zones of Kilauea, Kohala (or Mauna Kea), Mahukona, and Haleakala volcanoes. Four flat-topped cones on Kohala were explored and sampled with the Pisces V submersible in October 1998. Samples show that flat-topped cones on rift zones are constructed of tholeiitic basalt erupted during the shield stage. Similarly shaped flat-topped cones on the northwest submarine flank of Ni'ihau are apparently formed of alkalic basalt erupted during the rejuvenated stage. Submarine postshield-stage eruptions on Hilo Ridge, Mahukona, Hana Ridge, and offshore Ni'ihau form pointed cones of alkalic basalt and hawaiite. The shield stage flat-topped cones have steep (∼25°) sides, remarkably flat horizontal tops, basal diameters of 1–3 km, and heights <300 m. The flat tops commonly have either a low mound or a deep crater in the center. The rejuvenated-stage flat-topped cones have the same shape with steep sides and flat horizontal tops, but are much larger with basal diameters up to 5.5 km and heights commonly greater than 200 m. The flat tops have a central low mound, shallow crater, or levees that surrounded lava ponds as large as 1 km across. Most of the rejuvenated-stage flat-topped cones formed on slopes <10° and formed adjacent semicircular steps down the flank of Ni'ihau, rather than circular structures. All the flat-topped cones appear to be monogenetic and formed during steady effusive eruptions lasting years to decades. These, and other submarine volcanic cones of similar size and shape, apparently form as continuously overflowing submarine lava ponds. A lava pond surrounded by a levee forms above a sea-floor vent. As lava continues to flow into the pond, the lava flow surface rises and overflows the lowest point on the levee, forming elongate pillow lava flows that simultaneously build the rim outward and upward, but also dam and fill in the low point on the rim. The process repeats at the new lowest point, forming a circular structure with a flat horizontal top and steep pillowed margins. There is a delicate balance between lava (heat) supply to the pond and cooling and thickening of the floating crust. Factors that facilitate construction of such landforms include effusive eruption of lava with low volatile contents, moderate to high confining pressure at moderate to great ocean depth, long-lived steady eruption (years to decades), moderate effusion rates (probably ca. 0.1 km³/year), and low, but not necessarily flat, slopes. With higher effusion rates, sheet flows flood the slope. With lower effusion rates, pillow mounds form. Hawaiian shield-stage eruptions begin as fissure eruptions. If the eruption is too brief, it will not consolidate activity at a point, and fissure-fed flows will form a pond with irregular levees. The pond will solidify between eruptive pulses if the eruption is not steady. Lava that is too volatile rich or that is erupted in too shallow water will produce fragmental and highly vesicular lava that will accumulate to form steep pointed cones, as occurs during the post-shield stage. The steady effusion of lava on land constructs lava shields, which are probably the subaerial analogs to submarine flat-topped cones but formed under different cooling conditions. Received: 30 September 1999 / Accepted: 9 March 2000  相似文献   

大陆地壳活动带片麻岩穹隆构造与分层流变     

刘俊来  陈小宇  张健  周保军  樊文魁  闫佳鑫 《地质学报》2022,96(9):3158-3181

大陆克拉通化与造山带形成后的活化与再造机理和条件，是板块构造理论登陆面对的重要课题，针对广泛发育于陆缘、陆内环境中的大陆地壳活动带开展的深入研究为此提供了重要约束。以中下地壳深变质岩为核，中浅变质岩为幔部的穹隆构造是大陆地壳活动带最为典型特征构造样式之一。本文基于对古元古代—新生代不同时期典型大陆地壳活动带内片麻岩穹隆构造的分析，总结出以下共性特点：① 发育厚皮构造，强烈的热异常与高应变使得活动带中保存着下地壳基底岩石卷入地壳变形过程的痕迹;② 核部往往由高级变质岩石（通常伴有花岗岩或混合岩）组成，变质程度主体为低角闪岩相到高角闪岩相，局部可以达到麻粒岩相,从核部向幔部变质程度逐渐降低; ③ 分层（或层状）流变是地壳活动带变形的重要表现形式; ④ 中、深层次的岩石共同遭受了强烈剪切变形作用的改造，不同构造层次（核部与幔部）岩石中的构造具有几何学、运动学和动力学上的一致性,拉伸线理和不同尺度的a型褶皱广泛发育，伴随着区域尺度的a型或b型穹隆构造; ⑤ 幔部岩系与核部岩系具有特征的运动学上的耦合关系而流变学上的解耦, 二者之间及内部常发育不同尺度的剪切不连续面（Tectonic discontinuity contact, TDC）。基于上述特点分析，本文提出切向（近水平）剪切流动与多流变层分层流动是大陆地壳活动带中、下地壳流动一致性的体现。多种变形体制叠加，包括近水平切向流动作用的主要贡献并辅以垂向运动的叠加或递进变形，造就了现今大陆地壳活动带中广泛发育的片麻岩穹隆构造，它们递进演化成为线性a型穹隆群、b型穹隆或演变为变质核杂岩构造。  相似文献   

地震作用下单层球面网壳结构的动力稳定性   总被引：19，自引：0，他引：19  

郭海山  钱宏亮  沈世钊 《地震工程与工程振动》2003,23(1):31-37

本文以具有实际工种意义的40m跨度K8型单层球面网壳结构为研究对象，研究了其在地震作用下的动力失稳特点，提出了动力稳定性判别方法。系统分析了各种因素对网壳结构动力稳定性临界荷载的影响，其中包括：水平地震作用、竖向地震作用和三向地震作用的影响；考虑材料弹塑性的影响；不同地震输入的影响；初始几何缺陷的影响；不同矢跨比和不同杆件截面的影响。  相似文献   

Radiometric dating of three large volume flank collapses in the Lesser Antilles Arc     

A. Samper  X. Quidelleur  G. Boudon  A. Le Friant  J.C. Komorowski 《Journal of Volcanology and Geothermal Research》2008

It is now recognised that flank collapses are a recurrent process in the evolution of the Lesser Antilles Arc volcanoes. Large magnitude debris-avalanche deposits have been identified off the coast of Dominica, Martinique and St. Lucia, with associated volumes up to 20 km³ [Deplus, C., Le Friant, A., Boudon, G., Komorowski, J.-C., Villemant, B., Harford, C., Ségoufin, J., Cheminée, J.-L., 2001. Submarine evidence for large-scale debris avalanches in the Lesser Antilles Arc. Earth Planet. Sci. Lett., 192: 145–157.]. We present new radiometric dating of three major events using the K–Ar Cassignol–Gillot technique. In the Qualibou depression of St. Lucia, a collapse has been constrained by dome emplacement prior to 95 ± 2 ka. In Dominica, where repetitive flank collapse events have occurred [Le Friant, A., Boudon, G., Komorowski, J.-C., Deplus, C., 2002. L'île de la Dominique, à l'origine des avalanches de débris les plus volumineuses de l'arc des Petites Antilles. C.R. Geoscience, 334: 235–243], the Plat Pays event probably occurred after 96 ± 2 ka. Inside the depression caused by this event, Scotts Head, which is interpreted as a proximal megabloc from the subsequent Soufriere avalanche event has been dated at 14 ± 1 ka, providing an older bound for this event. On Martinique three different domes within the Carbets structure dated at 337 ± 5 ka constrain the age of this high magnitude event. Finally, these results obtained from three of the most voluminous flank collapses provide constraints to estimate the recurrence of these events, which represent one of the major hazards associated with volcanoes of the Lesser Antilles Arc.  相似文献   

喜马拉雅造山带东段错那洞片麻岩穹窿的变质作用及构造意义     

丁慧霞  李文坛  江媛媛 《岩石学报》2019,35(2):312-324

错那洞穹窿是喜马拉雅造山带北部发育的一系列片麻岩穹窿之一,因其赋存有超大型稀有金属矿床而倍受关注。本文对错那洞穹窿核部产出的石榴石十字石蓝晶石白云母片岩进行了岩石学、相平衡模拟和锆石U-Pb年代学研究,为揭示穹窿的成因和成矿作用提供了重要限定。岩石学研究表明,石榴石蓝晶石十字石白云母片岩的共生矿物组合是石榴石+蓝晶石+十字石+白云母+斜长石+石英+钛铁矿+金红石,为典型的中压角闪岩相变质岩。相平衡模拟表明岩石的变质温压条件为670℃和9. 0kbar,并未经历部分熔融。锆石U-Pb定年结果表明,片岩的变质作用发生在47～29Ma,即经历了一个较长期(～20Myr)的变质演化过程。结合现有研究成果,我们认为错那洞片麻岩穹窿具有与喜马拉雅造山带北部发育的其它片麻岩穹窿相同的成因,穹窿核部的中级变质岩为高喜马拉雅结晶岩系的上部构造层位,其变质作用发生在印度大陆向拉萨地体之下低角度俯冲过程中;穹窿核部淡色花岗岩是高分异的异地花岗岩,是高喜马拉雅结晶岩系下部高温高压麻粒岩部分熔融所形成的熔体经历高程度分离结晶产物。此外,本文研究成果为印度与亚洲大陆的碰撞时间和性质提供了进一步约束。  相似文献   

Re-folded structure of syn-orogenic granitoids(Padrón dome,NW Iberia):Assessing rheological evolution of cooling continental crust in a collisional setting     

Rubén Díez Fernández  Francisco J.Rubio Pascual  Luis Miguel Martín Parra 《地学前缘(英文版)》2019,10(2):651-669

This contribution discusses about the rheological, kinematic and dynamic frameworks necessary to produce recumbent and upright folds from syn-orogenic granitic massifs that were formed during an early stage of magma genesis related to the onset of a migmatitic dome. Syn-kinematic granitoids occurring within the high-grade infrastructure of the Padron migmatitic dome(NW Iberia) are deformed into largescale recumbent folds(D_2) that are later affected by upright folds(D_3). Petrostructural analysis of a selected area of this dome reveals that after a period of crustal thickening(D_1), NNW-directed extensional flow gave way to recumbent folds and penetrative axial plane foliation(S_2). Superimposed subhorizontal compression resulted in upright folds(D_3). A closer view into the dynamics of the dome allows exploring the factors that may condition the nucleation of folds with contrasting geometries during progressive deformation of molten continental crust. The formation of folds affecting syn-kinematic granitoids suggests a cooling metamorphic path in migmatitic domes. Active and passive folding mechanisms require a crystallizing(cooling) magma to nucleate folds. A more competent metamorphic host inhibits fold nucleation from much less competent magmas. As it crystallizes, magma becomes more rigid(competent),and approaches viscosity values of its host. Passive folding is favored when no significant competence contrast exists between magma and host, so this folding mechanism is more likely shortly after magma genesis and emplacement. In such conditions, and under dominant subhorizontal flow accompanied by flattening(D_2),passive folding would produce isoclinal recumbent geometries. Further magma cooling introduces a shift into the rheological behavior of partially molten crust. Thereon, crystallizing magma bodies would represent significant competence contrasts relative to their host. At this point, buckling is a more likely folding mechanism, and more regular, buckle folds re-fold previous structures after significant cooling. The geometry of resulting folds is upright due to dominant subhorizontal compression(D_3) at this stage.  相似文献   

单层球面网壳在地震下的响应特性与失效机理   总被引：3，自引：0，他引：3  

支旭东  范峰  沈世钊 《世界地震工程》2007,23(4):88-94

针对单层球面网壳在地震作用下的响应进行了大量算例分析,探讨了结构失效特点,从宏观和微观综合多项指标给出结构强震失效的判别方法。在频域响应分析的基础上阐述了网壳结构动力作用的特点,总结了单层球面网壳的地震下的破坏特征及失效机理。  相似文献