美国Chesapeake湾撞击坑研究进展及其启示

位于美国弗吉尼亚东海岸直径85 km的Chesapeake湾撞击坑,是十几年前发现的由一颗陨星撞击形成的一个复杂撞击坑. 该坑的研究经历四个阶段：地下水调查、撞击坑的发现、美国多学科多部门的综合研究和即将进行的国际钻探取心项目. 钻井岩心中的角砾成份和微体化石,提示撞击坑的存在,并确定撞击发生在35 Ma前,即始新始晚期. 地震反射剖面资料帮助寻找到撞击坑的具体位置,确定撞击坑的结构和形态特征. Chesapeake湾撞击坑埋藏在新生界沉积层之下,是全球已知最大的、保存最好的撞击坑之一. Chesapeake湾撞击坑主要形态像一顶倒置的宽边大草帽,包括外缘、环状洼地、峰环（内缘）、内盆和中央峰. 撞击坑的形成破坏了原来的含水层,撞击坑当时即被富含咸水的抛射角砾岩和海啸角砾岩充填,再被后来的沉积层覆盖. Chesapeake湾撞击坑导致地面沉降、河流变向、海岸含水层的中断、内陆咸水楔的出现、地震,决定Chesapeake湾本身的位置,至今仍然影响当地居民的生活. 了解Chesapeake湾撞击坑对我国撞击坑研究具有借鉴作用.     

Messier撞击坑辐射纹形成及斜撞击研究

具有辐射纹的撞击坑是一种特殊的撞击构造,只形成于某些区域一定规模的撞击事件.由于大部分撞击事件都是斜撞击,本文以Messier复合撞击坑为例研究并模拟了斜撞击抛射和辐射纹形成物理过程.Messier复合撞击坑包含Messier撞击坑和Messier A撞击坑,是月球上典型的小角度斜撞击形成的撞击坑.对Messier复合撞击坑的对称性分析发现,Messier A撞击坑对称轴相对于Messier撞击坑对称轴存在一个逆时针小角度偏转,这个小角度偏转说明Messier复合撞击坑起源于一次非理想斜撞击.基于对辐射纹形成过程的认识,本文建立了斜撞击抛射模型,合理解释了Messier复合撞击坑的南北方向辐射纹以及朝西方向辐射纹成因;并利用蒙特卡洛方法数值模拟了斜撞击产生的辐射纹分布,为Messier复合撞击坑的形成提供了一种新的合理解释.     

岫岩陨石撞击坑结构高精度地震探测研究

我国的岫岩陨石撞击坑位于辽东半岛北部低山丘陵地区,直径1.8 km,保存完好,已被多方面的证据证实为陨石撞击坑.陨石的撞击和此后的沉积作用在坑内形成了特殊的地球物理场,使坑内与坑外的介质在速度、密度等方面存在差异.本次通过采用反射和折射地震相结合的探测方法,利用陨石撞击所形成的岩石的地震波速度和波阻抗差异,获得了陨石坑...     

地球上形态特征最显著的20个陨石坑

陨石坑或撞击坑(impact crater)是小行星、彗星和流星体等小天体超高速撞击行星及其卫星表明形成的凹坑或环状地质构造。由于大气层的保护,和其他星球相比,地球遭受到陨石撞击的可能性要低得多。可即便如此,据估计,在地球形成演化的过程中,出现过直径大于10 km的陨石撞击构造不少于1500个,而直径更小的陨石坑数量就更多了!     

月球撞击坑的动力学研究

月球探测和月球资源的合理利用引起了广泛关注,月球撞击坑的研究可以提供月球的重要信息.统计研究表明,撞击坑的直径和大于该直径的撞击坑数目之间满足分形分布N(≥D)∝r-FD,撞击坑的深度和撞击坑的直径满足多项式关系d=kdDn.简化月球表面撞击过程为一个简单的完全非弹性碰撞,可以得到撞击的动力学微分方程;对撞击过程的阻力项做非线性近似并忽略非齐次项,可以得到撞击方程的解析解:y=dtanh(kdt).由此得到撞击物的撞击速率随撞击深度二次方减小:v=kd2-ky2,撞击坑形貌方程满足方程:lnr=1/nln(kd2)/(vt)-β/nr.理论推导撞击坑的深度和撞击坑的直径满足多项式关系,这和实际撞击坑的统计结果相符合.在无标度区域,撞击物的速率和大于该撞击速率的撞击物数目之间满足分形关系:N(≥v)∝v-FD,速率分布为:f(v)∝v-(FD+1).     

地球陨石坑地球物理探测研究进展

地球物理探测在研究和发现撞击构造方面发挥着重要作用.本文综述了地球陨石坑的重、磁、电、震等常见地球物理特征.陨石坑最明显的地球物理特征是圆形或环形的负重力异常，其主要原因是岩石破裂和角砾化导致岩石密度降低；具有较低的磁异常，细节特征复杂，其主要原因是撞击熔融降低了陨石坑内部岩石的磁化率，陨石撞击后的改造则造成了复杂的细节特征；简单陨石坑具有较高的电导率，复杂陨石坑具有从中央隆起向周缘升高的电导率，其受控于岩石的破碎程度和上覆沉积层的含水量，破碎程度、含水量越高电导率越高；具有低的地震波速，主要原因是破碎的角砾岩和断裂具有相对原岩更低的波速.此外，地震反射波探测发现陨石坑撞击构造有明显凹形特征.国际上已开展了大量陨石坑的地球物理探测研究，而我国现有被发现且证实的陨石坑不仅数量稀少，其相关的地球物理探测研究更是不多见.通过对国内外陨石坑的常见地球物理特征开展综述和总结，不仅可为我国发现更多潜在的陨石坑提供科学参考和依据，同时也为公众认识和了解撞击构造提供可靠的科普素材，进而有效拓展地球陨石坑的科研和人文价值.     

月球南极沙克尔顿撞击坑周围电势和尘埃静电输运的理论分析

近年来,月球南极地区因其特殊的光照条件以及撞击坑永久阴影区中可能存在的水冰而成为未来探测的焦点.沙克尔顿(Shackleton)撞击坑几乎位于月球南极点,已成为包括嫦娥七号任务在内的未来着陆任务最热门的目的地.然而,人们对该撞击坑周围的电势和尘埃环境仍然知之甚少.本文建立了一个理论模型来研究撞击坑周围的月表电势和尘埃的静电输运.研究发现,由于地形遮挡,该撞击坑的底部会充负电,背风侧坑壁表面电位最低可至-175V.相应地,有大量带电尘埃颗粒会从背风侧坑壁发射出来,迁移高度可达10km,水平迁移距离约为40km,这使得撞击坑附近可形成局部尘埃云.通过数值模拟对地形遮挡和局部尘埃云进行了验证,发现撞击坑附近确实产生了局部尘埃云,典型尘埃密度为10⁴～10⁵m^-3.研究结果对未来撞击坑附近月面探测的环境评估具有重要意义.此外,研究结果还有助于理解其他无大气天体的表面充电和尘埃静电输运过程.     

二叠纪生物大灭绝之迷

正地球从形成至今已有46亿年,在这漫长的岁月里,发生过无数次小天体撞击地球的事件。据专家统计,近一亿年来,地球遭到小天体撞击造成直径大于一公里的陨石坑就有一万处之多。由于地球大气层的保护,体积和质量小的小天体往往在坠落地面之     

月球辐射状撞击坑经度方向分布特征与同步旋转轨道状态模拟分析

月球目前的同步旋转轨道状态使得形成的撞击坑分布满足一定的不对称性.本文利用最新的LRO影像和地形数据,结合早期的Clementine影响数据,分析了月球辐射状撞击坑经度方向成坑率分布,结果表明所识别的辐射状撞击坑的.年龄为O.9 Ga内,西一东半球方向存在明显不对称性,比值约为1.35~1.53;同时利用辐射状撞击坑模拟了0.9 Ga内的同步旋转轨道的平均状态,得到该时间内月球的向点-背点为70°W—110°E附近;最后利用撞击坑数据库资料对月球形成以来各地质世纪时间尺度内的撞击坑进行了成坑率分布分析,结论表明月球在大爆炸中后期间可能处于过近似的同步轨道旋转状态,但是与现在的轨道运行状态相反,月球在之后的地质时期内经历了翻转,其诱因可能是雨海和东方海盆地遭受的撞击.     

太阳活动与地震有关吗?

一般认为,其它天体的活动,主要是太阳喷射粒子到达地球引起电离层浓度的变化及月球对地球大气的潮汐作用,是产生地磁日变、季节变化和11年左右周期变化的原因。在地震前后,人们经常观测到地磁场的异常变化。这种与地震有关的地磁异常变化,来源于何处呢?会不会同样是太阳等天体活动引起地磁的异常变化,而这种地磁的异常变化,又促使地震的发生     

Dynamic modeling suggests terrace zone asymmetry in the Chicxulub crater is caused by target heterogeneity

We investigate the cause of terrace zone asymmetry in the Chicxulub impact crater using dynamic models of crater formation. Marine seismic data acquired across the crater show that the geometry of the crater's terrace zone, a series of sedimentary megablocks that slumped into the crater from the crater rim, varies significantly around the offshore half of the crater. The seismic data also reveal that, at the time of impact, both the water depth and sediment thickness varied with azimuth around the impact site. To test whether the observed heterogeneity in the pre-impact target might have affected terrace zone geometry we constructed two end-member models of upper-target structure at Chicxulub, based on the seismic data at different azimuths. One model, representing the northwest sector, had no water layer and a 3-km thick sediment layer; the other model, representing the northeast sector, had a 2-km water layer above a 4-km sediment layer. Numerical models of vertical impacts into these two targets produced final craters that differ substantially in terrace zone geometry, suggesting that the initial water depth and sediment thickness variations affected the structure of the terrace zone at Chicxulub. Moreover, the differences in terrace zone geometry between the two numerical models are consistent with the observed differences in the geometry of the terrace zone at different azimuths around the Chicxulub crater. We conclude that asymmetry in the pre-impact target rocks at Chicxulub is likely to be the primary cause of asymmetry in the terrace zone.     

Possible relations between meteorite impact and igneous petrogenesis,as indicated by the Sudbury structure,Ontario, Canada

Recent investigations indicate the importance of meteorite impact as a process which has operated throughout geologic time to produce numerous originally circular structures as much as 50 km in diameter. One such structure, at Sudbury, Ontario, is associated with large volumes of internally derived igneous rock. Geological and experimental studies have demonstrated that rocks subjected to intense shock waves produced by hypervelocity meteorite impacts and by nuclear or chemical explosions develop distinctive and uniqueshock-metamorphic features, including: (1) high-pressure minerals such as coesite and stishovite; (2) crystal lattice deformation features such as isotropic feldspar (maskelynite) and « planar features » (shock lamellae) in quartz; (3) ultra-high-temperature reactions not produced by normal geological processes, such as decomposition of zircon to baddeleyite and melting of quartz to lechatelierite. These petrographic features, currently regarded as unequivocal evidence for meteorite impact, can be preserved and recognized even in very old and deeply eroded structures. Such features have now been observed in more than 50 « crypto-explosion » structures ranging in size from 2 km to more than 60 km in diameter. The recent discovery of shock-metamorphic features in rocks of the Sudbury structure, Ontario, indicates that this old and complex structure was also produced by a large meteorite impact. Petrographic shock effects are widespread in inclusions of « basement » rock in the Onaping « tuff », a unit now regarded as afallback breccia deposited in the original crater immediately after impact. Similar shock effects also occur in the footwall rocks around the basin, associated with shatter cones and unusual Sudbury-type breccias. Study of Sudbury specimens has establishedgrades of progressive shock metamorphism comparable to those recognized at younger impact structures (Brent, Ontario; Ries basin, Germany). Igneous activity associated with known meteorite impact structures takes two forms:

1. direct production of impact melt. At many structures (e.g., Brent, Ontario; Lake Mien, Sweden; Clearwater Lakes and Manicouagan, Quebec), breccias containing shock-metamorphic features occur with «sills» and «dikes» of fine- to medium- grained crystalline igneous rock. Such units, previously regarded as internal volcanic products, now appear to have been formed by complete fusion, injection, and rapid crystallization of large volumes of target rock during the impact event.
2. emplacement of internally derived magma. The presence of the clearly internally-derived Nickel Irruptive within the Sudbury basin indicates that large meteorite impacts may also control the emplacement of internally-generated magmas through « unroofing » or by the production of deeply-extending zones of weakness below the crater.

The inferred development of the Sudbury structure was a complex process involving: (1) impact of an asteroidal body, forming a large (100-km) diameter crater with a central uplift; (2) subsidence of the central uplift and simultaneous emplacement of the Nickel Irruptive; (3) metamorphism, deformation, and erosion to its present appearance. The post-impact history of the Sudbury structure thus corresponds closely to that established for many ring-dike complexes and caldera subsidences. Similar compound impact-igneous structures, in which internal igneous activity is superimposed on a large impact crater, probably exist on both the earth and the moon. Future examination of « roofed lopoliths » and « ring-dike structures » for shock-metamorphic effects, combined with serious consideration of the geophysical effects produced by large-energy meteorite impacts, will be a productive field for cooperative studies by astrogeologists and igneous petrologists.     

Calculations of Asteroid Impacts into Deep and Shallow Water

Contrary to received opinion, ocean impacts of small (<500?m) asteroids do not produce tsunamis that lead to world-wide devastation. In fact the most dangerous features of ocean impacts, just as for land impacts, are the atmospheric effects. We present illustrative hydrodynamic calculations of impacts into both deep and shallow seas, and draw conclusions from a parameter study in which the size of the impactor and the depth of the sea are varied independently. For vertical impacts at 20?km/s, craters in the seafloor are produced when the water depth is less than about 5?C7 times the asteroid diameter. Both the depth and the diameter of the transient crater scale with the asteroid diameter, so the volume of water excavated scales with the asteroid volume. About a third of the crater volume is vaporised, because the kinetic energy per unit mass of the asteroid is much larger than the latent heat of vaporisation of water. The vaporised water carries away a considerable fraction of the impact energy in an explosively expanding blast wave which is responsible for devastating local effects and may affect worldwide climate. Of the remaining energy, a substantial portion is used in the crown splash and the rebound jet that forms as the transient crater collapses. The collapse and rebound cycle leads to a propagating wave with a wavelength considerably shorter than classical tsunamis, being only about twice the diameter of the transient crater. Propagation of this wave is hindered somewhat because its amplitude is so large that it breaks in deep water and is strongly affected by the blast wave??s perturbation of the atmosphere. Even if propagation were perfect, however, the volume of water delivered per metre of shoreline is less than was delivered by the Boxing Day 2004 tsunami for any impactor smaller than 500?m diameter in an ocean of 5?km depth or less. Near-field effects are dangerous for impactors of diameter 200?m or greater; hurricane-force winds can extend tens of kilometers from the impact point, and fallout from the initial splash can be extremely violent. There is some indication that near-field effects are more severe if the impact occurs in shallow water.     

小行星撞击对地球的上地幔对流的影响

本文分别基于数值结果和地质学模型,在假定地球的上地幔存在稳定Rayleigh-Bénard对流的基础上,模拟了直径为10 km（陨石坑直径约180 km,以Chicxulub为例）和直径为100 km（陨石坑直径约1000 km）的小行星撞击对地球的上地幔对流格局的影响.本文将直径10 km小行星的撞击效果等效为热异常,将直径100 km小行星的撞击效果等效为热异常和速度异常（主要指陨石坑底部的回弹）的叠加.计算结果表明,当小行星的直径在10 km左右时,撞击对上地幔对流的影响十分微弱,热扰动时间仅2—3 Ma;而当小行星的直径达到100 km时,撞击就会对上地幔对流产生强烈影响.这时,对流从扰动到新的稳态有一定模式可循（依次为：调整、多个对流环、调整、稳定）,扰动的持续时间受黏度和撞击点位置影响,同时稳定后地幔热柱会向着撞击点的方向产生一定的位移.     

Crater ejecta: Markers of impact catastrophes

The role of impact crater ejecta in the evolution of the Earth and Solar System planets is considered. Using the methods of numerical modeling, the accretion and erosion of planets, the redistribution of planetary material due to impacts of large cosmic bodies and related geospheric interactions, the asymmetrical deposition of ejecta around the crater rim, and the possibility of nonballistic transportation of ejecta are analyzed. The influence of impact events on the Earth’s envelopes is compared with the effects of volcanic eruptions.     

PREDICTION OF IMPACT OF THE THREE GORGES PROJECT ON ESTUARY BEACH OF THE YANGTZE RIVER

ＰＲＥＤＩＣＴＩＯＮＯＦＩＭＰＡＣＴＯＦＴＨＥＴＨＲＥＥＧＯＲＧＥＳＰＲＯＪＥＣＴＯＮＥＳＴＵＡＲＹＢＥＡＣＨＯＦＴＨＥＹＡＮＧＴＺＥＲＩＶＥＲＷｅｉｂｉｎｇＦＥＮＧ１ＹｉｇａｎｇＷＡＮＧ２ａｎｄＸｉｕｃｈｅｎｇＺＨＯＮＧ３ＡＢＳＴＲＡＣＴＡｌｔｈｏ...     

Upper Eocene ejecta of the New Jersey continental margin reveal dynamics of Chesapeake Bay impact

Evidence for the Chesapeake Bay Crater as the source for New Jersey continental margin ejecta is provided by fine-grained tektites and coarse-grained unmelted ejecta. The Upper Eocene ejecta deposit, now demonstrated to be part of the North American strewn field, occurs on the New Jersey continental margin at Ocean Drilling Program (ODP) Sites 904 and 903. The mineralogy, major oxide composition of the ejecta materials, and biostratigraphic age of the enclosing sediments link the origin of these ejecta to the recently recognized Chesapeake Bay impact crater, located only 330 km away. Sediments associated with the ejecta provide information about the dynamics of impact events. The 35-cm-thick ejecta-bearing layer can be subdivided into three subunits that indicate a sequence of events. Bottom subunit III documents sediment failure and deposition of gravel-sized fragments, middle subunit II records deposition of abundant sand-sized ejecta by gravity settling, and upper subunit I contains a 12-cm-thick sedimentary deposit containing rare silt-sized tektites and evidence of waning currents. These events are interpreted by linking sediment deposition to seismic ground motion and subsequent tsunami waves triggered by both the Chesapeake Bay impact and slope failures.     

Comparing efficacy of different taxonomic resolutions and surrogates in detecting changes in soft bottom assemblages due to coastal defence structures

Sandy shores on the West coast of the North Adriatic Sea are extensively protected by different types of defence structures to prevent coastal erosion. Coastal defence schemes modify the hydrodynamic regime, the sediment structure and composition thus affecting the benthic assemblages. This study examines the effectiveness in detecting changes in soft bottom assemblages caused by coastal defence structures by using different levels of taxonomic resolution, polychaetes and/or bivalves as surrogates and different data transformations. A synoptic analyses of three datasets of subtidal benthic macrofauna used in studies aimed at assessing the impact of breakwaters along the North Adriatic coast has been done. Analyses of similarities and correlations between distance matrices were done using matrices with different levels of taxonomic resolution, and with polychaetes or bivalves data alone. Lentidium mediterraneum was the most abundant species in all datasets. Its abundance was not consistently related to the presence of defence structures. Moreover, distribution patterns of L. mediterraneum were masking the structure of the whole macrofaunal assemblages. Removal of L. mediterraneum from the datasets allowed the detection of changes in benthic assemblages due to coastal defences. Analyses on different levels of taxonomic resolution showed that the level of family maintained sufficient information to detect the impacts of coastal defence structures on benthic assemblages. Moreover, the outcomes depended on the transformation used. Patterns of distribution of bivalves, used as surrogates, showed low correlations with the patterns of the total macrofaunal species assemblages. Patterns of polychaetes, if identified to the species or genus level showed higher correlations with the whole dataset. However, the identification of polychaetes to species and genus level is as costly as the identification of all macrobenthic taxa at family level.This study provided additional evidences that taxonomic sufficiency is a useful tool in environmental monitoring, also in investigations on the impacts of coastal defence structures on subtidal macrofauna. The use of coarser taxonomic level, being time-efficient, would allow improving sampling designs of monitoring programs by increasing replication in space and time and by allowing long term monitoring studies.