首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
《Tectonophysics》1987,140(1):65-79
We present a method for processing three-component digital recordings of microearthquakes to obtain near-vertical reflection profiles in regions of shallow seismicity. The processing includes magnitude and focal depth normalization and event stacking, where stacking is by small localized groups, with ray theoretical time and distance corrections applied to compensate for varying focal depths. In areas with high seismicity, this procedure allows earthquakes to be treated as “controlled” sources to probe layered structures of the deep crust and upper mantle.The validity of our approach is examined using aftershocks of the Borah Peak, Idaho earthquake (Ms = 7.3). Several thousand events occurred in a NNW-trending zone about 10 km wide, 75 km long, and 15 km deep. A small (~ 10 ×10 km) array of nine University of Wisconsin three-component triggered short-period digital seismographs was installed in the region of aftershock activity. Over a 10-day period, about 1000 useable events were recorded, of which about 120 have been used for this study. Hypocenters have been computed using both P- and S-wave arrivals, the latter being essential for stable solutions of events outside the network.The Borah Peak data have been processed to obtain shear-wave reflection profiles for the central station (Station 8) of the digital station array. The stacked shear wave (transverse) record sections reveal coherent reflections from horizons at mid-crustal to Moho depths. The most prominent reflections are from crustal discontinuities in the depth range 18–28 km. Coherent reflections can be obtained only through stacking, which is necessary to improve the signal to noise ratio. The major sources of data scatter, as manifested by “smearing” of reflections on the stacked records, are crustal heterogeneity and errors in the determination of focal depth and origin time.  相似文献   

2.
Coefficients of association have been widely employed in cluster analysis. However, their use has been, for the most part, restricted to binary data. This limitation can be overcome by redefining positive and negative matches and mismatches in terms of minimum and maximum values of paired elements of parallel vector arrays. Rewriting the algorithms of coefficients of association with these new components gives the new “quantified” coefficients general utility for binary, ordered multistate, and quantitative data, while retaining their original analytic properties. Quantified coefficients of association avoid several problems of shape and size that are associated with correlation coefficients and measures of Euclidean distance. However, when measuring similarity, quantified coefficients weight each attribute of an object by that attribute's magnitude. A related set of similarity indices termed “mean ratios” is introduced; these indices give each attribute equal weight in all situations. Both quantified coefficients of association and mean ratios are related to a number of measures of similarity introduced to various fields of scientific research during the past 50 years. A review of this literature is included in an attempt to consolidate methodology and simplify nomenclature.  相似文献   

3.
全新世气候突变事件是全球变化研究的热点,但对其行为特征与动力学机制的认识仍存在不足。本文选取贵州董哥洞2支高分辨率、高精度定年的石笋（DX1和DA）δ18O记录,分析小冰期和“8.2 ka BP”事件期间亚洲夏季风强度变化的结构特征与其驱动机制。通过两个突变事件的精细对比,发现两者在持续时间、振荡幅度和转型特征等方面均有很好的相似性,尤其是均显示出“两谷一峰”的结构性特征。北高纬地区淡水注入导致大西洋经向翻转流（AMOC）的减弱可能是触发“8.2 ka BP”事件的重要驱动力,这暗示具有相似结构特征的小冰期弱季风事件可能也响应于AMOC机制。将小冰期时期石笋δ18O记录与指示AMOC变化的北大西洋放射性碳数据进行对比,发现两者具有相似的变化过程,这表明在早晚全新世不同气候背景条件下,AMOC仍能够通过影响南北半球温度梯度的变化而影响低纬热带辐合带位置的移动,进而调控千-百年尺度低纬季风区水文循环的时空变化。   相似文献   

4.
This paper on “Temperature changes in earth-history” is an extension of a lecture given as an introduction to a section of equal title on the annual meeting of the Geologische Vereinigung, March 1976, in Hannover. The general development of paleoclimatological research in the last 300 years is represented on two diagrams (fig. 1–2) showing also the part of different climatic indicators. Otherwise, however, mostly new results and problems of the last years are treated (mainly papers since 1973; references of older literature are to be found in the 3rd edition of the author's book on “Climates of the Past” = “Klima der Vorzeit”, Enke/Stuttgart 1974). This paper refers a) to some short comments on certain climatic indicators as diamictites (a similar term isSchermerhorn's “mixtite”, but “diamictite” is 6 years older and has therefore priority to “mixtite”) and “stellate nodules” (in the chapter “Mesozoic”) indicating perhaps cool climate in the Arctic. - b) Some great ice-ages are briefly discussed: Huronian (very important because of its old age); Late Proterozoic (“Eocambrian”) with many problems on account of its pretended worldwide extension. but with many uncertainities (partly pseudotillites, inconsistent paleomagnetic poles, combination of tillites with dolomites etc.); Permo-Carboniferous (many hypothesises up to 1975 try to explain the pretended “equatorial” position of tillites); Cenozoic ice-age (once “Quaternary” ice-age), with table 1 indicating some possibilities to evaluate the beginning of glaciations in Tertiary time (fig. 4). Why does glaciation start in Antarctica in the Tertiary? (Not or not only on account of drift via South Pole, but perhaps because of high relief and changes in global paleogeography). — c) Diagram of the great ice-ages in earth-history (fig. 6 b): it probably shows not all ice-ages but only the known ones indicating their maxima (i. e. times when inlandice extended to middle latitudes). This curve is probably essentially correct back to 300–400 m. y. yet especially the Precambrian time is still mostly paleoclimatic noman's-land. It is not possible to fix beginning and end of the Pre-Tertiary ice-ages exactly but at any rate the “akryogene” climates lasted longer than the “kryogene” ones (“kryogene” defined as climate with “much ice” [“pleistokryogene”], “akryogene” not as climate “without ice” but as climate with “a little ice” [“oligokryogene”]). - d) Periodicities in the temperature history: before exact dates were available (especially for Late Proterozoic and Huronian ice-ages) and before the Sahara glaciation of the Old Paleozoic was known, a periodicity of 250–300 m. y. was likely to exist. Therefore relations to the “Galactic year” were reasonable, stimulating attempts to find out plausible mechanisms for such a relation. But now, such a periodicity seems unlikely to exist (and much more one of 155 m. y., supposed byWilliams). The relative constancy of global earth temperatures over at least more than 2 billion years is more striking than their variations, though regionally the depressions may be very conspicious (in the middle, “sensitive” latitudes). Such depressions, however, are triggered by very small climatic changes on account of the existence of a hydrosphere with temperatures very favorable for a transformation of water into ice and vice versa. No other celestial body of our solar system has these optimal conditions with the consequences of occasional initiation of ice-ages. Ice ages, so to speak, are an inherited pecularity of the earth. The earth is the only “Ice-age Planet”. Under these circumstances, relatively small factors may cause ice-ages: multilateral origin of climatic changes. The most efficient parameters may be paleogeographic variations (relief etc. inclusive continental drift). Some comments are made on the radiation curves reflecting not the direct cause of glacials and interglacials but perhaps shorter climatic variations as they appear possibly in the curves of ocean temperatures (Emiliani etc.). Volcanic ashes seem not to have any farreaching influence on global temperatures; at least it is geologically impossible to support appropriate hypothesises by observations on continental volcanic sequences. The number of ash-layers in deep-sea cores may reveal sounder arguments though much more observations are needed to corroborate this supposition. — Table 2 gives a summary of the primary (planetary), secondary (multilateral) and — in special situations — tertiary “autocyclic” causes of climatic changes. Table 3 focuses on autocycles i. e. mechanisms which run. off automatically and could have caused the regular climatic variations in the Late Pleistocene with the classic glacialinterglacial sequence (not known from the older Quaternary or Pre-Tertiary ice-ages). In my opinion the most probable hypothesises on autocycles are those which were founded on wide extending subarctic continents of the northern hemisphere (qualified for the formation of large inlandice) in combination with mighty oceanic heat storage (Stokes, D. P. Adam, R. E. Newell).  相似文献   

5.
《Quaternary Science Reviews》2005,24(10-11):1123-1149
A prominent, abrupt climate event about 8200 years ago brought generally cold and dry conditions to broad northern-hemisphere regions especially in wintertime, in response to a very large outburst flood that freshened the North Atlantic. Changes were much larger than typical climate variability before and after the event, with anomalies up to many degrees contributing to major displacement of vegetative patterns. This “8k” event provides a clear case of cause and effect in the paleoclimatic realm, and so offers an excellent opportunity for model testing. The response to North Atlantic freshening has the same general anomaly pattern as observed for older events associated with abrupt climate changes following North Atlantic freshening, and so greatly strengthens the case that those older events also reflect North Atlantic changes. The North Atlantic involvement in the 8k event helps in estimating limits on climate anomalies that might result in the future if warming-caused ice-melt and hydrologic-cycle intensification at high latitudes lead to major changes in North Atlantic circulation. Few model experiments have directly addressed the 8k event, and most studies of proxy records across this event lack the time resolution to fully characterize the anomalies, so much work remains to be done.  相似文献   

6.
Calcian dolomite from the Devonian Lost Burro formation has been investigated with electron microscopy techniques. Electron diffraction shows evidence for “c” and “d” type reflections which may occur independently and are indicative of ordered superstructures. High resolution electron microscopy combined with selected area optical diffraction is the basis for models to explain the superstructures in calcian dolomite. It is proposed that “c” reflections are due to ordered substitution of Mg by Ca in basal cation layers. “d” reflections result when the rhombohedral stacking of basal layers is interrupted by intercalation of additional Ca layers. During electron irradiation at 1 MeV the Mg-Ca distribution becomes disordered and the crystal structure attains calcite symmetry. The arrangement of CO3 groups remains ordered.  相似文献   

7.
A simple model for the dynamics of stars located in a sphere with a radius of one-tenth of the central parsec, designed to enable estimation of the probability of capture in the close vicinity (r < 10?3 pc) of a supermassive black hole (SMBH) is presented. In the case of binary stars, such a capture with a high probability results in the formation of a hyper-velocity star. The population of stars in a sphere of radius <0.1 pc is calculated based on data for the Galactic rotation curve. To simulate the distortion of initially circular orbits of stars, these are subjected to a series of random shock encounters (“kicks”), whose net effect is to “push” these binary systems into the region of potential formation of hyper-velocity stars. The mean crossing time of the border of the close vicinity of the SMBH (r < 10?3 pc) by the stellar orbit can be used to estimate the probability that a binary system is captured, followed by the possible ejection of a hyper-velocity star.  相似文献   

8.
In the Marvejols area (Southern french Massif Central), the gneissic Marvejols supergroup is overthrust on the metasedimentary “Série du Lot”, deposited in part prior to 540 Ma. The allochtonous terranes are characterized by the occurrence of a leptyno-amphibolitic group, a complex association of mafic and felsic rocks of igneous and sedimentary derivation. A 480±10 Ma age has been obtained by U-Pb dating of zircons, for the crystallization of both mafic and felsic meta-igneous rocks. These rocks were emplaced during an important extensional tectonics. Relics of eclogites, pyrigarnites, coronite gabbros and HP-trondhjemites are clear evidence for a further HP-HT event dated at 415±6 Ma on zircons from a HP trondhjemite. Subsequently, the Marvejols supergroup underwent an amphibolite facies metamorphism with incipient mobilization dated at 345±10 Ma. Rifting and thinning of the continental crust in Cambro-Ordovician times appears to be a major geodynamic feature which could account for the thermal events often referred to the “Caledonian” orogeny. The Silurian (415 Ma) age of the HP episode is clearly older than the main Variscan tectonometamorphic event; it is in agreement with Rb-Sr dating of the Moldanubian granulites and with some radiometric data from Southern Brittany (France). These results point to a compressive phase, probably in a subduction context, in view of the high pressures reached (15–20 Kb), after the Cambro-Ordovician distensive phase. The main final tectono-metamorphic paroxysm (blocking of subduction process and continental collison ?) is not prior to the end of Devonian (340–350 Ma) and is related to the Variscan orogeny s.s  相似文献   

9.
European geologists, in general, have tended to favor a “short” chronology for the glacial Pleistocene, with four major glacial cycles in the past 500,000 or 600,000 yr. Interpretation of ocean floor sediments by Emiliani and others has accorded with this view, in contrast to the “long” chronology of Ericson and Wollin and their followers, who spread the four North American glacial episodes over a 2-m.y. period. An examination of the available radiometric dates and age estimates from paleomagnetic polarity zones serves to confirm Richmond's view that the four major European glacials do not equate with the four North American glacials in a simple one-to-one manner, but that the Illinoian matches the Elster (Mindel) rather than the Saale (Riss). The Alpine Günz is then equated broadly with the Kansan and overlaps in time with the Jaramillo normal polarity event at about 900,000 y.a. The Nebraskan is older than 1.2 m.y. and is thus coeval with the European Upper Villafranchian, within which the Donau and Biber glacial events may be traced. Montane glaciation certainly extended back into the Tertiary but cold pulses of sufficient duration to produce continental glaciation were more marked through the past 1.5 m.y. More critical studies of the terrestrial record are needed before firm correlations can be made.  相似文献   

10.
初论地质信息有序系列   总被引：3，自引：0，他引：3  
信息有序系列（ＩＯＳ）是笔者提出的一个新概念，是指具有非随机性定义的信息、非周期性、存在于大量无序中的有序部分特征的有序性。ＩＯＳ的理论是应用基于较少假设的、较为简单的数字或几何模式去研究地质体客观存在的复杂性，减少类似概率分布、周期性、平稳性、遍历理论等假设要求。ＩＯＳ中的一组现象，表面上看类似于周期，但被广义地定义，它们不能被谱分析和统计分析发现。“系列”一词用以与“模型”相区别，其特点是仅在有限范围和短时间内存在，是不平稳和遍历的。“有序”一词是强调在混沌中存在的有序部分，可应用于预测目的。所以，ＩＯＳ是有序性中一部分。大地震、热点、超大型矿床、节理等时空分布表明了ＩＯＳ的客观存在。  相似文献   

11.
《Geodinamica Acta》1998,11(1):33-49
The “Zone Houillère Briançonnaise” (ZHB) is the westernmost continuous unit of the Pennine domain that overlies the Pennine Front. The ZHB comprises Namurian and Westphalian formations locally overlain by Permian and Triassic cover rocks. As with other basements of the Pennine domain (Briançonnais in France. Grand Saint Bernard in Italy and Switzerland), the ZHB comprises gneissic units, whose age, origin and tectonicsignificance have been discussed for a long time. This study deals with the Sapey gneisses, defined near Modane (Savoie), and the Costa Citrin metagranites (Valle d'Aosta. Italy). The alpine tectonic evolution of the ZHB involves an early thrusting event that produced a non-penetrative foliation in the Carboniferous formations. This foliation occurs only in the vicinity of tectonic contacts between individual slices. The thrusting event is overprinted by a regional large-scale, eastward-verging, deformation event and then by a partly extensional event. Pre-alpine relict amphibolilcfacics mineral assemblages found in the Sapey gneisses confirm that they belong to a poly-metamorphic basement. Ages from U-Pb geochronology on zircons (three samples of different lithology and from different regions) confirm this interpretation. Concordia diagrams evidence that zircons have a Proterozoic inheritance and that minimum ages are older than 360 Ma (lower intercepts with the concordia). By contrast, two samples of the Costa Citrin metagranite yielded well-defined Visean-Namurian ages at 324±17 MA and 323±8 Ma. This old basement age (early Hercynian or older) associated with evidences for a Visean-Namurian magmatic event in the ZHB are consistent with the recent reappraisal of the age of the sedimentary formations of the ZHB (now dated of Namurian to Westphalian). These data support the possible “exotism” of a “Briançonnais” terrane with respect to external domains of the Alps. The ages of the basement (pre-Carboniferous), of the sediments (Visean-Namurian) and of the recorded magmatic event (Visean-Namurian) of the ZHB cannot be related to the late orogenic basin formation and magmatic evolution of the Eastern Massif Central or of the External Basement Massifs during Stephanian times. The significance of the Visean-Namurian plutonism of the Costa Citrin is discussed with respect to other parts of the european hercynian belt. It is interpreted either to be a consequence of a late Hercynian “eastern” subduction or rather, to relate to an early extensional stage during the orogeny.  相似文献   

12.
Security and the future: Anticipating the event of terror   总被引：1，自引：0，他引：1  
Ben Anderson 《Geoforum》2010,41(2):227-235
This paper explores the relation between processes of security and futurity in the context of efforts to govern the complexity and contingency of events of terror. It argues that processes of securing function by generating a dangerous or promissory supplement to the present that thereafter propels the extension of forms of security. The paper develops this argument through an example of how an event of terror was anticipated: a RAND exercise into the aftermath of a ‘ground burst’ nuclear explosion in Long Beach, California on March 14th 2005. It argues that exercises (in)secure through three quasi-causal operations, each of which render events of terror actionable and result in specific relations between the present and future. First, ‘hypothetically possible’ generic events are named. The future takes place as a threatening horizon. Second, the defined phases of an event’s happening are staged (an advent, its multiplication into a crisis in the context of a milieu, and a response/recovery phase). The here and now is suspended between an ‘as if’ future and the present. Third, the consequences of the event are played. The future is both an intensified ‘practical’ presence embodied by exercise participants and an outside that exceeds attempts to definitively know it. The conclusion summarises the implications of the paper for work on futurity, security and the event.  相似文献   

13.
Proposed igneous and metamorphic ages of the shergottites, a suite of achondritic basaltic meteorites, are examined in terms of the shergottites' petrography and mineral chemistry. No metamorphic or hydrothermal event appears capable of resetting the refractory Sr and Nd isotopic systems of the shergottites since the time of their igneous crystallization. The approximately concordant Rb-Sr (180 ± 20 m.y.) and U-Th-Pb (190 ± 30 m.y.) internal “isochrons” are thus interpreted as representing the time of igneous crystallization. The Sm-Nd internal “isochrons” which yield older apparent ages also show large scatter and are regarded as artifacts of the shergottites' complex igneous history. Rubidium-Sr, Sm-Nd and Pb-Pb wholerock “isochrons” are interpreted as mixing lines and are reasonably attributed to igneous processes such as wall-rock assimilation and magma mixing. If the shergottites are indeed less than 200 m.y. old, they represent the youngest known extraterrestrial basalts. This conclusion is supportive of the hypothesis that the SNC meteorites are samples of the planet Mars.  相似文献   

14.
Enthalpies of solution in lead borate at ~700°C have been measured for 36 natural and heat treated plagioclase feldspars. The samples made up two series, as characterised by TEM and XRD. A “low” series contained the natural ordered material and a “high” series the same samples annealed at high temperatures to induce cation disorder. Enthalpy of solution differences between the two series give the enthalpy changes associated with the disordering reactions: low albite → high albite: ~3 kcal/mole “e” structure → Cī high albite structure: ~ 1.4-2.8 kcal/mole Il? structure →- Cl? high albite structure: ~0.7-1.9 kcal/mole Il? structure equilibrated at low temperature → Il? structure equilibrated at high temperature: ~ 1.8?0.8 kcal/mole.ΔHsoln data for the high series overlap with the data of Newtonet al. (1980) for synthetic high structural state plagioclases except in the composition range ~An90–An100. They are consistent with an interpretation of the solid solution as being composed, at high temperatures, of two ideal (zero heat of mixing) segments, one with Cl? symmetry and one with Il symmetry, and having a non-first order (continuous) order/disorder transformation between them. The low series can also be separated into two distinct trends, for Il? and “e” structures.Values of the enthalpy change due to disordering (ΔHord) also show a number of systematic trends. Firstly, the values for e → Cl? are larger than for Il? → Cl? in the composition range where both e and Il? structures are observed (~An65-An72). Secondly, the enthalpy change on disordering the most ordered e structures at An-rich compositions is larger than for Ab-rich e structures. The apparent change in ΔHord, which occurs at ~An50, may be important for the origin of the Bøggild miscibility gap. Thirdly, the large enthalpy change of the e structure, due to ordering, may be sufficient to stabilise it relative even to a mixture of low albite plus anorthite. Values for the enthalpy change on disordering Il? anorthites and bytownites to a Cl? structure have been estimated by assuming that the Cl? solid solution is ideal (non-enthalpic) and then extrapolating a straight line through the data for Ab-rich compositions to pure anorthite.  相似文献   

15.
N.M.S. Rock 《Earth》1977,13(2):123-169
Lamprophyres are porphyritic dyke-rocks, readily recognisable in the field, which seemed to have become enveloped in petrological mystery because of their scattered and often contradictory literature. Important characteristics common to all members of this extremely heterogeneous group are suggested to include their intrusion at a late stage in any igneous event, the presence of essential amphibole or biotite coupled with peculiar chemistry, and their lack of felsic phenocrysts. Panidiomorphic texture is usual, though by no means exclusive to lamprophyres.At least four lamprophyre subgroups may be recognised; these are quite distinct in petrology, chemistry, paragenesis and petrogenesis although individual members of each subgroup are closely similar. Thealkaline lamprophyre subgroup is considered in detail: new definitions ofcamptonite andmonchiquite are proposed which are believed to reconcile majority usage whilst accomodating existing analytical data; the termouachitite is reassessed andfourchite suggested to be superfluous;sannaite, an alkali feldspar-rich alkaline lamprophyre, may be more abundant than its frequency within the literature implies — some “minettes” and “vogesites” are more correctly termed sannaite, particularly those occurring in carbonatite complexes.Camptonites most probably represent alkali basalt magmas which became unusually hydrous through influence of an alkaline pluton, and then evolved at relatively low pressures. Varied evidence is considered to suggest that the nearly isotropic base in monchiquites typically consists of altered glass rather than analcime; where analcime is present it probably formed along with zeolites by secondary processes. “Monchiquitic” is not an appropriate synonym for “analcime-bearing”, as in some literature. Field, chemical and statistical evidence is taken to suggest that many monchiquites are either chilled heteromorphs of camptonites or camptonitic magmas which crystallised under high CO2 activities. Liquid immiscibility seems to be more extensively developed in camptonites and monchiquites than in any other igneous rocks, and is probably governed by their hydrous state.The genetic connotations of lamprophyre names, asserted early in their history but later questioned, are here modified but reaffirmed: broadly minettes and related lamprophyres connote post-orogenic granites or mildly potassic (shoshonitic) alkaline rocks, alkaline lamprophyres connote alkali basalts or sodic alkaline rocks, and alnoites connote carbonatites.  相似文献   

16.
In 2009 two wells were drilled with 100% core recovery at Roquefort-La Bédoule (Bouches-du-Rhône, SE France), the historical Bedoulian stratotype. Here we present holostratigraphic results based on a detailed study of the cored sediments. Our work confirms that the La Bédoule area offers one of the best records for the period spanning the late Bedoulian, the anoxic event OAE1a and the Bedoulian/Gargasian (lower-upper Aptian substages) transition. New data provide a refined succession of micropaleontogical events already well correlated with ammonites from previous fieldwork and, thus, improve the cross-calibration of bioevents with high-resolution isotope stratigraphy. Methods of the quantitative micropalaeontology applied on benthic foraminifera such as tritaxias help testing their probable orbitally triggered cyclicity, which might be used to precise estimates of duration of events such as OAE1a, the Dufrenoya furcata ammonite Zone, the Globigerinelloides ferreolensis planktonic foraminiferal zone and the C7 isotopic stage.The lithologic, biotic and possibly isotopic changes seen at the level of and around bed 170 (top of “Niveau Blanc” sensu auctorum) are strong arguments to use this key-level as the boundary between the two Aptian substages (or stages in an alternative classification) and to support the proposal of La Bédoule as a potential locality for the GSSP of the Gargasian Substage (or of historical Aptian sensu stricto, in the alternative classification).  相似文献   

17.
The curvature of alumino-silicate structures related to imogolite and kaolinite is investigated using techniques derived from differential geometry and topology. It is demonstrated that the curvatures of these structures can be varied to give a limited range of “modulated” relatives, of both hyperbolic and elliptic curvature. These modulated structures may account for a number of soil minerals, the “poorly ordered non-crystalline aluminosilicates” (Wilson et al. 1988), including protoimogolite and allophane.  相似文献   

18.
We present the results of population syntheses for binary stars carried out using the “Scenario Machine” code with the aim of analyzing events that may result in long gamma-ray bursts. We show that the observed distribution of morphological types of the host galaxies of long gamma-ray bursts can be explained in a model in which long gamma-ray bursts result from the core collapse of massive Wolf-Rayet stars in close binaries. The dependence of the burst rate on galaxy type is associated with an increase in the rate of stellar-wind mass-loss with increasing stellar metallicity. The separation of binary components at the end of their evolution increases with the stellar-wind rate, resulting in a reduction of the number of binaries that produce gamma-bursts.  相似文献   

19.
中国现代人类起源:来自南方化石地点的年代学证据   总被引：11，自引：0，他引：11  
沈冠军 《地学前缘》2004,11(2):543-548
现代人类起源的时间和地点是当前古人类学研究和争议的热点。文中评述了 2 0多年来第四纪测年技术取得的进展及其对现代人类起源研究的影响。基于新生碳酸盐岩铀系测年 ,研究了中国南方柳江人洞等多个晚期人类化石地点的年代 ,结果远大于原认为的 2 0～ 30ka。晚期智人在中国出现的时间很可能不晚于西亚和南非 ,中国 4 0～ 1 0 0ka间人类化石的“缺环” ,很可能只是测年技术系统误差造成的假象。上述年代学新证据不支持中国现代人类全部来自非洲的假说。对岩溶洞穴遗址进一步的综合研究 ,有可能为破解有关现代人类起源的诸多疑点提供重要的以至一锤定音的证据  相似文献   

20.
Since the 1960s, an early explosive activity in the Mont-Dore Massif is associated with a major pyroclastic rhyolitic eruption (5–7 km3) known as the “Grande Nappe” (GN). This event, linked to the formation of a 6-km-diameter cryptic caldera named “Haute Dordogne”, was before our investigation dated by 40Ar/39Ar at 3.07 ± 0.04 Ma. Our new single-crystal laser fusion 40Ar/39Ar dates obtained on two outcrops of the GN (Rochefort-Montagne and Ludières) questioned several hypotheses made concerning this “landmark” event of the Mont-Dore Massif history. We demonstrate that: (1) the GN rhyolitic eruption has occurred much later than previously estimated (i.e. 2.77 ± 0.02–0.07 Ma full external uncertainties); (2) the correlation made between the Vendeix rhyolitic complexes (intra-caldera position) dated back to 2.74 ± 0.04 Ma and the GN is proposed; (3) xenocryst contamination could be very high (i.e. 70% for the Rochefort-Montagne GN outcrop) and explains the noticeable older age obtained previously; (4) a link between the GN eruption and the formation of a caldera is questionable; the hypothesis of a northward-oriented blast channeled eastward toward the paleo-Allier River is thus proposed.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号