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201.
Hideo Hoshizumi Kozo Uto Kazunori Watanabe 《Journal of Volcanology and Geothermal Research》1999,89(1-4)
During the past 500 thousand years, Unzen volcano, an active composite volcano in the Southwest Japan Arc, has erupted lavas and pyroclastic materials of andesite to dacite composition and has developed a volcanotectonic graben. The volcano can be divided into the Older and the Younger Unzen volcanoes. The exposed rocks of the Older Unzen volcano are composed of thick lava flows and pyroclastic deposits dated around 200–300 ka. Drill cores recovered from the basal part of the Older Unzen volcano are dated at 400–500 ka. The volcanic rocks of the Older Unzen exceed 120 km3 in volume. The Younger Unzen volcano is composed of lava domes and pyroclastic deposits, mostly younger than 100 ka. This younger volcanic edifice comprises Nodake, Myokendake, Fugendake, and Mayuyama volcanoes. Nodake, Myokendake and Fugendake volcanoes are 100–70 ka, 30–20 ka, and <20 ka, respectively. Mayuyama volcano formed huge lava domes on the eastern flank of the Unzen composite volcano about 4000 years ago. Total eruptive volume of the Younger Unzen volcano is about 8 km3, and the eruptive production rate is one order of magnitude smaller than that of the Older Unzen volcano. 相似文献
202.
Tadahide Ui Norimichi Matsuwo Mari Sumita Akihiko Fujinawa 《Journal of Volcanology and Geothermal Research》1999,89(1-4)
Processes generating block and ash flows by gravitational dome collapse (Merapi-type pyroclastic flow) were observed in detail during the 1990–1995 eruption of Unzen volcano, Japan. Two different types were identified by analysis of video records and observations during helicopter flights. Most of the block and ash flows erupted during the 1991–1993 exogenous dome growth stage initially involved crack propagation due to cooling and flowage of the dome lava lobes. The mass around the crack became unstable, locally decreasing in tensile strength. Finally, a slab separated from the lobe front, fragmented progressively from the base to the top within a few seconds, and became a block and ash flow. Rock falls immediately followed, in response to local instability of the lobe front. Clasts in these rock falls fragmented and merged with the preceding flow. In contrast, block and ash flows during the endogenous dome growth stage in 1994 were generated due to local bulge of the dome. Unstable lava blocks collapsed and subsequently fragmented to produce block and ash flows. 相似文献
203.
Steve J. Chipera Fraser Goff Cathy J. Goff Melissa Fittipaldo 《Journal of Volcanology and Geothermal Research》2008
Quantitative X-ray diffraction analysis of about 80 rhyolite and associated lacustrine rocks has characterized previously unrecognized zeolitic alteration throughout the Valles caldera resurgent dome. The alteration assemblage consists primarily of smectite–clinoptilolite–mordenite–silica, which replaces groundmass and fills voids, especially in the tuffs and lacustrine rocks. Original rock textures are routinely preserved. Mineralization typically extends to depths of only a few tens of meters and resembles shallow “caldera-type zeolitization” as defined by Utada et al. [Utada, M., Shimizu, M., Ito, T., Inoue, A., 1999. Alteration of caldera-forming rocks related to the Sanzugawa volcanotectonic depression, northeast Honshu, Japan — with special reference to “caldera-type zeolitization.” Resource Geol. Spec. Issue No. 20, 129–140]. Geology and 40Ar/39Ar dates limit the period of extensive zeolite growth to roughly the first 30 kyr after the current caldera formed (ca. 1.25 to 1.22 Ma). Zeolitic alteration was promoted by saturation of shallow rocks with alkaline lake water (a mixture of meteoric waters and degassed hydrothermal fluids) and by high thermal gradients caused by cooling of the underlying magma body and earliest post-caldera rhyolite eruptions. Zeolitic alteration of this type is not found in the later volcanic and lacustrine rocks of the caldera moat (≤ 0.8 Ma) suggesting that later lake waters were cooler and less alkaline. The shallow zeolitic alteration does not have characteristics resembling classic, alkaline lake zeolite deposits (no analcime, erionite, or chabazite) nor does it contain zeolites common in high-temperature hydrothermal systems (laumontite or wairakite). Although aerially extensive, the early zeolitic alteration does not form laterally continuous beds and are consequently, not of economic significance. 相似文献
204.
盐岩底辟构造是一类与油气藏关系极为密切的构造类型,是当前国外构造地质研究和油气勘探的热点.目前盐岩底辟构造的研究仍处于描述性阶段,对盐及其相关构造动力学演化的研究还很薄弱.对于油气而言,盐岩底辟构造既提供了油气运移的驱动力和运移通道,又可以产生丰富的圈闭.离散元模拟是构造地质地球物理研究中一种新的动力学模拟方法,作者通过构建离散元模型,模拟了主动底辟盐构造断裂系统的演化过程,结果表明,该断裂系统的发育主要经历了对称变形、不对称变形、整体破裂三个阶段.研究结果揭示了盐岩底辟构造的动力学演化过程,有利于提高对盐岩底辟构造相关油气藏的勘探和预测. 相似文献
205.
The Pengguan tectonic dome of Longmen Mountains, Sichuan Province: Mesozoic denudation of a Neoproterozoic magmatic arc-basin system 总被引:3,自引:0,他引:3
DanPing Yan MeiFu Zhou GuoQing Wei JianFeng Gao ShaoFeng Liu Ping Xu XiaoYing Shi 《中国科学D辑(英文版)》2008,51(11):1545-1559
Neoproterozoic igneous and metamorphic complexes occur as tectonic domes in the Longmen Mountains of the western margin of the Yangtze Block, and are important in reconstructing the Rodinian supercontinent and constraining the timing and mechanism of tectonic denudational processes. The Pengguan dome consists of granitic intrusions and metamorphic rocks of the Huangshuihe Group and is tectonically overlain by ductilly deformed Sinian to Paleozoic strata. The plutonic intrusions consist of granites with abundant amphibolite enclaves. New LA-ICP-MS zircon U-Pb dating yielded an emplacement age of 809±3 Ma and a protolith age of 844±6 Ma for the granite. The granitic rocks have geochemical signatures typical of A-type granites, indicating their formation under an extensional environment, by melting of newly formed tonalite-trondhjemite-granodiorite (TTG) rocks. A detachment fault, characterized by variable ductile shear deformation of S-C fabric and ESE-ward kinematics, separates the Pengguan dome from the Sinian-Paleozoic cover. 40Ar/39Ar dating of muscovite from the mylonite in the detachment fault of the dome demonstrates that ductile deformation occurred at ~160 Ma. This study indicates the existence of a Neoproterozoic magmatic arc-basin system, which was denudated by a Jurassic middle crustal ductile channel flow along the Longmenshan thrust belt. 相似文献
206.
207.
R. J. Dixon 《Geological Journal》1990,25(1):35-46
The Moel-y-Golfa Andesite is a discordant intrusive body of porphyritic pyroxene andesite, 1 km across and at least 200 m thick. Textural variation defines a central facies and a marginal facies, the latter displaying well-formed lobes, tubes, and pillows. The vertically stacked lobes, brecciated tubes, and elongate pillows of andesite intrude and locally enclose enclaves of altered host sediment and are cut by steeply dipping slide scars. The host sediments are locally strongly silicified and cut by numerous, anastomosed vertical and subvertical fissures. Fissures are filled by disrupted host sediment and fragments of brecciated andesite. The formation and filling of each fissure system was probably concomitant with the brecciation of an intrusive magma lobe. The complex relationship of igneous and sedimentary material indicates that the Moel-y-Golfa Andesite was emplaced into the host sediments whilst they were unconsolidated or only partly lithified. 相似文献
208.
锂- 铍等稀有金属是中国乃至世界上重要的战略性关键金属矿产,美国、澳大利亚和日本等国早已将其列为禁止出口矿产名录,其战略地位非同一般。中国锂- 铍矿产资源丰富,成矿背景优越,但地表第四纪覆盖严重,且分布不均。近年甜水海- 松潘- 甘孜造山带花岗伟晶岩型锂矿资源逐步被勘探发现,引起国内外学者广泛关注。川西石渠县扎乌龙锂矿作为该造山带中部重要的花岗伟晶岩型锂矿点,厘清该矿床锂资源聚集主控因素及其稀有金属富集规律将为甜水海- 松潘- 甘孜造山带下一步战略找矿行动提供指导和重要决策依据。本文对扎乌龙地区锂辉石钠长石伟晶岩脉、白云母钠长伟晶岩脉、电气石钠长石伟晶岩脉、卡吉亚二云母花岗岩体以及岩体边部的含电气石细晶岩、十字石黑云母片岩等进行系统采样分析,利用LA- ICP- MS独居石U- Th- Pb同位素定年技术,确定扎乌龙卡吉亚二云母花岗岩形成于晚三叠世213~211 Ma,含电气石细晶岩约形成于210 Ma,白云母钠长石伟晶岩约形成于210~200 Ma,富锂伟晶岩可能在205 Ma集中产出。结合前人年代学资料,综合分析认为扎乌龙矿区由二云母花岗岩、电气石细晶岩到白云母钠长石伟晶岩和锂辉石钠长石伟晶岩形成时代逐步年轻。 相似文献
209.
Hongwei Yin Jie Zhang Lingsen Meng Yuping Liu Shijing Xu 《Journal of Structural Geology》2009,31(9):989-995
Geological mapping, seismic analyses, and analogue experiments show that active salt diapirism results in significant faulting in the overburden strata. Faults associated with active diapirism generally develop over the crest of the dome and form a radial pattern. In this study, we have created a 3-D discrete element model and used this model to investigate the fault system over active diapirs. The model reproduces some common features observed in physical experiments and natural examples. The discrete element results show that most faults initiate near the model surface and have displacement decreasing downward. In addition, model results indicate that the earliest fault, working as the master fault, has a strong influence on the subsequent fault pattern. The footwall of the master fault is mainly deformed by arc-parallel stretching and develops a subradial fault pattern, whereas the hanging wall is deformed by both arc-parallel stretching and gliding along the master fault and top of salt, and hence develops both parallel and oblique faults. Model results replicate the fault pattern and deformation mechanism of the Reitbrook dome, Germany. 相似文献
210.