高黎贡山脉南部的晚新生代构造-重力垮塌及其成因   总被引：2，自引：0，他引：2  

王刚  万景林  王二七 《地质学报》2006,80(9):1262-1273

云南西部高黎贡山脉构成青藏高原东南边界的一部分,平均海拔3000m以上,山脉南部高度突然下降到海拔2000m左右,线性延伸的山体变为平缓起伏的面状高地。野外工作发现,这一地貌突变是由于伸展变形和重力垮塌造成的。晚新生代以来高黎贡山脉中段和南段东西两侧均发育伸展构造,其中分隔盆地与山脉地貌单元的主要正断层被称为高黎贡山脉东坡和西坡主边界正断层,它们控制了山脉两侧盆地沉积及腾冲地块内部的火山喷发作用,造成山脉与盆地的地势差距、山体的构造剥蚀以及水系的下切。高黎贡山脉及其西侧的腾冲地块构成青藏高原周边唯一的伸展边界。这些伸展构造是畹町、瑞丽、梁河等左行走滑断裂的尾端转换构造。畹町断裂的左行走滑运动将上部地壳物质向东、向西拉开,使得高黎贡山脉南部发生构造-重力垮塌变形,形成三角形的构造窗,暴露出由变质结晶岩系构成的保山地块基底。磷灰石裂变径迹年龄表明,大约距今14Ma开始的构造-重力垮塌去顶作用首先发生在山脉南部,并逐渐向北发展,高黎贡山脉中部在距今8~5Ma进入伸展构造的强烈活动期。  相似文献   

采场坍塌空区地球物理探测及矿山恢复生产设计与安全评价   总被引：3，自引：3，他引：0       下载免费PDF全文

王育忠  高谦  王正辉 《地球物理学进展》2008,23(3):903-908

采用空场法开采的某金矿Au3号矿体于2006年12月4日发生了冒落垮塌事故,迫使矿山停产.为了尽快恢复矿山生产,确保今后矿山生产安全,有必要进行采场坍塌原因分析,了解采场塌埋填实情况,并根据实际塌落情况,进行安全评价和恢复生产的初步设计.针对这一要求,本文首先简要介绍了Au3号矿体的地质、采矿条件,并采用了地球物理勘探方法,进行了塌落采场的空区探测,揭示了采场的空区填埋情况,并进行采场稳定性评价;在此基础上,提出了恢复矿山生产的采矿设计.目前,该矿山正在基于本文的研究成果和提出的初步设计方案进行恢复性生产.  相似文献   

地壳应变场对气压短周期变化的响应   总被引：5，自引：0，他引：5       下载免费PDF全文

周龙寿  邱泽华  唐磊 《地球物理学进展》2008,23(6):1717-1726

大气压力变化可分为由日、月引力和太阳热辐射产生的潮汐变化和非潮汐频段的短周期气压变化,其引起的地壳应变可达10-9量级.我们选用分钟采样的观测精度达10-9量级的钻孔应变仪,对山东省泰安台2O02～2005年气压及体应变分钟值资料进行深入分析,研究了地壳应变场对短周期气压变化的响应规律.频谱分析结果表明,短周期气压变化对体应变的影响主要集中在周期大于2000 s的低频段.利用小波分解方法,得出了短周期气压系数随不同频段和不同时间的变化规律:高频段(周期小于960 s)气压系数数值较小,随频率变化幅度较大;低频段(周期大于960 s)气压系数数值较大,但变化趋于平稳;在频段一定的情况下,短周期气压系数随时间变化较平稳.从钻孔应变仪观测原理和按余弦函数分布的载荷作用于地壳表面的应力解出发,得出了地壳不同深度对不同频率短周期气压场的响应,理论计算与实际观测基本相符.研究结果表明,短周期气压变化对低频信号(周期大于2000 s)的影响较大,提取低频地球自由振荡信号时要消除气压影响,对苏门答腊大地震激发的地球球型自由振荡的功率谱分析验证了这一结论.  相似文献   

Permeability and pore-connectivity variation of pumices from a single pyroclastic flow eruption: Implications for partial fragmentation     

Michihiko Nakamura  Keiichi Otaki  Shingo Takeuchi 《Journal of Volcanology and Geothermal Research》2008

The relationship between permeability and vesicularity in volcanic rocks has been used to infer the degassing behavior of hydrous magma. Recent data on natural samples from various eruptions show a wide variation, fitting a power–law relationship of the percolation models with low (< 30%) critical vesicularity (Ф_C). In this study, we present data on permeability and pore-connectivity of juvenile rhyolitic pumice clasts in a pyroclastic flow around Onikobe volcano, NE Japan, and investigate their relationship with vesicularity developed in a single eruption event. The permeability of the pumices having a relatively low abundance of microlites and microphenocrysts shows a trend increasing by 4 orders of magnitude (from 10^− 13.8 to 10^− 10.1 m²) in a high and narrow vesicularity range (from 72 to 80%). This trend intersects at a high angle with the fit to the permeability–vesicularity data in the previous studies that has a low Ф_C, and is located on the extension of the trend for the products of isotropic decompression experiments. The two-dimensional (2D) connectivities of pores for the pumices were also measured from thin sections. From the point of view of percolation theory, connectivity provides information about the probability of percolation. They showed a steep increase from ca. 0 to 0.7 in an almost similar vesicularity range, as compared to their permeabilities. We attribute the increase in 2D connectivity to the increasing amount of ruptured bubble walls, which might have provided less-tortuous paths through larger apertures for gas flow. This, in turn, would cause an effective increase in the permeability. Aggregates of bubble-wall-shaped glass shards were found in the pumices, and their amount and degree of welding are higher in the pumices that have a higher abundance of microlites and microphenocrysts. These pumices have relatively high permeability and 2D connectivity at low vesicularity, which is accounted for by the existence of large irregularly shaped pores. These textural characteristics suggest that a series of partial fragmentation processes, including local rupturing of bubble walls and subsequent foam-collapse with permeable gas flow, might have occurred before the ultimate bulk fragmentation, thus resulting in the increase in permeability. We suggest that the 2D connectivity of pores is a useful parameter to quantify the degree of fragmentation of bubble walls and has the potential for use to assess their permeability.  相似文献   

Voluminous lava flows at Oldoinyo Lengai in 2006: chronology of events and insights into the shallow magmatic system     

Matthieu Kervyn  Gerald G. J. Ernst  Jurgis Klaudius  Jörg Keller  François Kervyn  Hannes B. Mattsson  Frederic Belton  Evelyne Mbede  Patric Jacobs 《Bulletin of Volcanology》2008,70(9):1069-1086

The largest natrocarbonatite lava flow eruption ever documented at Oldoinyo Lengai, NW Tanzania, occurred from March 25 to April 5, 2006, in two main phases. It was associated with hornito collapse, rapid extrusion of lava covering a third of the crater and emplacement of a 3-km long compound rubbly pahoehoe to blocky aa-like flow on the W flank. The eruption was followed by rapid enlargement of a pit crater. The erupted natrocarbonatite lava has high silica content (3% SiO₂). The eruption chronology is reconstructed from eyewitness and news media reports and Moderate Resolution Imaging Spectroradiometer (MODIS) satellite data, which provide the most reliable evidence to constrain the eruption’s onset and variations in activity. The eruption products were mapped in the field and the total erupted lava volume estimated at 9.2 ± 3.0 × 10⁵ m³. The event chronology and field evidence are consistent with vent construct instability causing magma mixing and rapid extrusion from shallow reservoirs. It provides new insights into and highlights the evolution of the shallow magmatic system at this unique natrocarbonatite volcano.  相似文献   

A note on classical damping matrices     

J. Enrique Luco 《地震工程与结构动力学》2008,37(4):615-626

An alternative damping matrix that leads to classical normal modes and that depends explicitly on a set of prescribed modal damping ratios is presented. The alternative damping matrix can be thought of as a factorized Caughey series that allows for a simple explicit solution for the coefficients of the series and thus avoids the need to solve a potentially ill‐conditioned system of algebraic equations. The relation between the proposed damping matrix and Rayleigh, Caughey and modal damping matrices is examined. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

Transient 3D numerical simulations of column collapse and pyroclastic density current scenarios at Vesuvius     

T. Esposti Ongaro  A. Neri  G. Menconi  M. de'Michieli Vitturi  P. Marianelli  C. Cavazzoni  G. Erbacci  P.J. Baxter 《Journal of Volcanology and Geothermal Research》2008

Numerical simulations of column collapse and pyroclastic density current (PDC) scenarios at Vesuvius were carried out using a transient 3D flow model based on multiphase transport laws. The model describes the dynamics of the collapse as well as the effects of the 3D topography of the volcano on PDC propagation. Source conditions refer to a medium-scale sub-Plinian event and consider a pressure-balanced jet. Simulation results provide new insights into the complex dynamics of these phenomena. In particular: 1) column collapse can be characterized by different regimes, from incipient collapse to partial or nearly total collapse, thus confirming the possibility of a transitional field of behaviour of the column characterized by the contemporaneous and/or intermittent occurrence of ash fallout and PDCs; 2) the collapse regime can be characterized by its fraction of eruptive mass reaching the ground and generating PDCs; 3) within the range of the investigated source conditions, the propagation and hazard potential of PDCs appear to be directly correlated with the flow-rate of the mass collapsing to the ground, rather than to the collapse height of the column (this finding is in contrast with predictions based on the energy-line concept, which simply correlates the PDC runout and kinetic energy with the collapse height of the column); 4) first-order values of hazard variables associated with PDCs (i.e., dynamic pressure, temperature, airborne ash concentration) can be derived from simulation results, thereby providing initial estimates for the quantification of damage scenarios; 5) for scenarios assuming a location of the central vent coinciding with that of the present Gran Cono, Mount Somma significantly influences the propagation of PDCs, largely reducing their propagation in the northern sector, and diverting mass toward the west and southeast, accentuating runouts and hazard variables for these sectors; 6) the 2D modelling approximation can force an artificial radial propagation of the PDCs since it ignores azimuthal flows produced by real topographies that therefore need to be simulated in fully 3D conditions.  相似文献   

Caldera rim collapse: A hidden volcanic hazard     

Olivier Merle  Laurent Michon  Patrick Bachèlery 《Journal of Volcanology and Geothermal Research》2008

Following the emblematic flank collapse of Mount St Helens in 1981, numerous models of flank sliding have been proposed. These models have allowed to largely improve the understanding of mechanisms involved in such landslides, which represent a tremendous risk for populations living around volcanoes. In this article, a new mode of landslide formation, related to buried calderas, is described. The model emphasizes the paramount importance of the hidden ring fault that, even when the caldera is buried, still remains a plane of weakness in the core of the edifice. Under certain conditions, this plane of weakness becomes activated as the upper part of a pre-existing critical slip surface and is used in the emplacement of huge landslides which travel downslope at a very high velocity. A natural example is taken from Piton de la Fournaise Volcano (La Réunion Island, Indian Ocean). It reveals that the primary cause triggering caldera rim collapse is partial unbuttressing of the flank of the volcano. In the natural example, this occurs through regressive erosion that excavates deep canyon in the direction of the buried caldera but other mechanisms may exist. On account of the large volumes of material involved in caldera rim collapse as well as their long runout distances, such a volcanic hazard should be taken into account on every volcano where buried calderas are suspected.  相似文献   

Management of coral reefs: where we have gone wrong and what we can do about it   总被引：1，自引：0，他引：1  

Sale PF 《Marine pollution bulletin》2008,56(5):805-809

Globally, our current management of coral reefs is inadequate and becoming more so as we place new and greater stresses on these ecosystems. The future looks very dim, and yet we have the capacity to do a far more effective job of reef management if we want to. Making substantial improvements to the condition of these enormously valuable coastal marine ecosystems does not require new scientific discoveries, but a new commitment to apply the knowledge we already possess to manage our impacts so that sustainability becomes possible.  相似文献   

浅埋偏压连拱隧道中导洞坍方数值分析与处治     

李志勇  晏莉  阳军生 《岩土力学》2007,28(1):102-106

某连拱隧道在中导洞施工时出现坍方。针对该隧道的工程实际情况,建立了数值分析模型,采用FLAC软件模拟了隧道施工力学行为,从围岩塑性区分布、位移情况以及锚杆和混凝土衬砌内力分布情况分析了隧道变形和坍塌发生的原因,并且结合工程实际提出了有效地处治方法。  相似文献