红水河中游地区地质构造稳定性的遥感图像分析

红水河中游地区是以北西向和北东向断裂相交叉为基本构造格局。在新构造运动时期,这两组老构造分段复活,将该区分割成一个相对独立的活动块体。 遥感信息与岩相古地理及汞矿化带相结合的分析结果表明:发震的垭都—紫云深大断裂,在新构造运动时期并未进入断块。遥感信息与历史震中的时空分布和断块结合分析表明:中强震震中绝大多数分布在断块以外,并以右江断裂带以南的频率为最大,由于这里已远在百公里之外,今后若再发生中强震,经过长途衰减,波及坝址的影响有限。 断块内距坝址15—20公里的巴鱼地区,近期发生过4.6级地震。这是一个处在乐业环状构造长轴转折端前沿,也是逻西活动断裂等多组断裂交叉处,小型环状构造也很发育,构造应力容易在此集中,历史震中与小型环状构造同断裂的交叉点关系密切。影像显示该区是相对孤立的局部地段,小震频繁,应力分阶段释放,不具备孕育强震的条件。 龙滩和岩滩电站坝址周围8公里内无大型或活动断裂通过,亦无两组以上断裂交叉点出现,构造稳定性良好,但应注意水库诱发地震的研究。     

恒应变率试验中岩石膨胀与应变能损耗特性的研究

借助广义品质因数和变形的归一化处理方法,分析了房山砂岩在10~(-5)/s恒应变率试验中膨胀和应变能损耗特征.结果表明,房山砂岩的膨胀伴随着应变能的迅速耗散,差应力-体应变曲线在膨胀阶段出现的反转点与应变能损耗曲线的特征点相对应.     

Timing of the late-glacial climate reversal in the Southern Hemisphere using high-resolution radiocarbon chronology for Kaipo bog, New Zealand

The pattern of climate change in the Southern Hemisphere during the Younger Dryas (YD) chronozone provides essential constraint on mechanisms of abrupt climate change only if accurate, high-precision chronologies are obtained. A climate reversal reported previously at Kaipo bog, New Zealand, had been dated between 13,600 and 12,600 cal yr B.P. and appeared to asynchronously overlap the YD chron, but the chronology, based on conventionally radiocarbon-dated bulk sediment samples, left the precise timing questionable. We report a new high-resolution AMS ¹⁴C chronology for the Kaipo record that confirms the original chronology and provides further evidence for a mid-latitude Southern Ocean cooling event dated between 13,800 and 12,400 cal yr B.P. (2σ range), roughly equivalent to the Antarctic Cold Reversal.     

Towards a climate event stratigraphy for New Zealand over the past 30 000 years (NZ‐INTIMATE project)

It is widely recognised that the acquisition of high‐resolution palaeoclimate records from southern mid‐latitude sites is essential for establishing a coherent picture of inter‐hemispheric climate change and for better understanding of the role of Antarctic climate dynamics in the global climate system. New Zealand is considered to be a sensitive monitor of climate change because it is one of a few sizeable landmasses in the Southern Hemisphere westerly circulation zone, a critical transition zone between subtropical and Antarctic influences. New Zealand has mountainous axial ranges that amplify the climate signals and, consequently, the environmental gradients are highly sensitive to subtle changes in atmospheric and oceanic conditions. Since 1995, INTIMATE has, through a series of international workshops, sought ways to improve procedures for establishing the precise ages of climate events, and to correlate them with high precision, for the last 30 000 calendar years. The NZ‐INTIMATE project commenced in late 2003, and has involved virtually the entire New Zealand palaeoclimate community. Its aim is to develop an event stratigraphy for the New Zealand region over the past 30 000 years, and to reconcile these events against the established climatostratigraphy of the last glacial cycle which has largely been developed from Northern Hemisphere records (e.g. Last Glacial Maximum (LGM), Termination I, Younger Dryas). An initial outcome of NZ‐INTIMATE has been the identification of a series of well‐dated, high‐resolution onshore and offshore proxy records from a variety of latitudes and elevations on a common calendar timescale from 30 000 cal. yr BP to the present day. High‐resolution records for the last glacial coldest period (LGCP) (including the LGM sensu stricto) and last glacial–interglacial transition (LGIT) from Auckland maars, Kaipo and Otamangakau wetlands on eastern and central North Island, marine core MD97‐2121 east of southern North Island, speleothems on northwest South Island, Okarito wetland on southwestern South Island, are presented. Discontinuous (fragmentary) records comprising compilations of glacial sequences, fluvial sequences, loess accumulation, and aeolian quartz accumulation in an andesitic terrain are described. Comparisons with ice‐core records from Antarctica (EPICA Dome C) and Greenland (GISP2) are discussed. A major advantage immediately evident from these records apart from the speleothem record, is that they are linked precisely by one or more tephra layers. Based on these New Zealand terrestrial and marine records, a reasonably coherent, regionally applicable, sequence of climatically linked stratigraphic events over the past 30 000 cal. yr is emerging. Three major climate events are recognised: (1) LGCP beginning at ca. 28 000 cal. yr BP, ending at Termination I, ca. 18 000 cal. yr BP, and including a warmer and more variable phase between ca. 27 000 and 21 000 cal. yr BP, (2) LGIT between ca. 18 000 and 11 600 cal. yr BP, including a Lateglacial warm period from ca. 14 800 to 13 500 cal. yr BP and a Lateglacial climate reversal between ca. 13 500 and 11 600 cal. yr BP, and (3) Holocene interglacial conditions, with two phases of greatest warmth between ca. 11 600 and 10 800 cal. yr BP and from ca. 6 800 to 6 500 cal. yr BP. Some key boundaries coincide with volcanic tephras. Copyright © 2007 John Wiley & Sons, Ltd.     

Illawarra Reversal: The fingerprint of a superplume that triggered Pangean breakup and the end-Guadalupian (Permian) mass extinction

The Permian magnetostratigraphic record demonstrates that a remarkable change in geomagnetism occurred in the Late Guadalupian (Middle Permian; ca. 265 Ma) from the long-term stable Kiaman Reverse Superchron (throughout the Late Carboniferous and Early-Middle Permian) to the Permian–Triassic Mixed Superchron with frequent polarity changes (in the Late Permian and Triassic). This unique episode called the Illawarra Reversal probably reflects a significant change in the geodynamo in the outer core of the planet after a 50 million years of stable geomagnetism. The Illawarra Reversal was likely led by the appearance of a thermal instability at the 2900 km-deep core–mantle boundary in connection with mantle superplume activity. The Illawarra Reversal and the Guadalupian–Lopingian boundary event record the significant transition processes from the Paleozoic to Mesozoic–Modern world. One of the major global environmental changes in the Phanerozoic occurred almost simultaneously in the latest Guadalupian, as recorded in 1) mass extinction, 2) ocean redox change, 3) sharp isotopic excursions (C and Sr), 4) sea-level drop, and 5) plume-related volcanism. In addition to the claimed possible links between the above-listed environmental changes and mantle superplume activity, I propose here an extra explanation that a change in the core's geodynamo may have played an important role in determining the course of the Earth's surface climate and biotic extinction/evolution. When a superplume is launched from the core–mantle boundary, the resultant thermal instability makes the geodynamo's dipole of the outer core unstable, and lowers the geomagnetic intensity. Being modulated by the geo- and heliomagnetism, the galactic cosmic ray flux into the Earth's atmosphere changes with time. The more cosmic rays penetrate through the atmosphere, the more clouds develop to increase the albedo, thus enhancing cooling of the Earth's surface. The Illawarra Reversal, the Kamura cooling event, and other unique geologic phenomena in the Late Guadalupian are all concordantly explained as consequences of the superplume activity that initially triggered the breakup of Pangea. The secular change in cosmic radiation may explain not only the extinction-related global climatic changes in the end-Guadalupian but also the long-term global warming/cooling trend in Earth's history in terms of cloud coverage over the planet.     

Integrated “plume winter” scenario for the double-phased extinction during the Paleozoic–Mesozoic transition: The G-LB and P-TB events from a Panthalassan perspective

The event across the Paleozoic–Mesozoic transition involved the greatest mass extinction in history together with other unique geologic phenomena of global context, such as the onset of Pangean rifting and the development of superanoxia. The detailed stratigraphic analyses on the Permo-Triassic sedimentary rocks documented a two-stepped nature both of the extinction and relevant global environmental changes at the Guadalupian–Lopingian (Middle and Upper Permian) boundary (G-LB, ca. 260 Ma) and at the Permo-Triassic boundary (P-TB, ca. 252 Ma), suggesting two independent triggers for the global catastrophe. Despite the entire loss of the Permian–Triassic ocean floors by successive subduction, some fragments of mid-oceanic rocks were accreted to and preserved along active continental margins. These provide particularly important dataset for deciphering the Permo-Triassic paleo-environments of the extensive superocean Panthalassa that occupied nearly two thirds of the Earth’s surface. The accreted deep-sea pelagic cherts recorded the double-phased remarkable faunal reorganization in radiolarians (major marine plankton in the Paleozoic) both across the G-LB and the P-TB, and the prolonged deep-sea anoxia (superanoxia) from the Late Permian to early Middle Triassic with a peak around the P-TB. In contrast, the accreted mid-oceanic paleo-atoll carbonates deposited on seamounts recorded clear double-phased changes of fusuline (representative Late Paleozoic shallow marine benthos) diversity and of negative shift of stable carbon isotope ratio at the G-LB and the P-TB, in addition to the Paleozoic minimum in ⁸⁷Sr/⁸⁶Sr isotope ratio in the Capitanian (Late Guadalupian) and the paleomagnetic Illawarra Reversal in the late Guadalupian. These bio-, chemo-, and magneto-stratigraphical signatures are concordant with those reported from the coeval shallow marine shelf sequences around Pangea. The mid-oceanic, deep- and shallow-water Permian records indicate that significant changes have appeared twice in the second half of the Permian in a global extent. It is emphasized here that everything geologically unusual started in the Late Guadalupian; i.e., (1) the first mass extinction, (2) onset of the superanoxia, (3) sea-level drop down to the Phanerozoic minimum, (4) onset of volatile fluctuation in carbon isotope ratio, 5) ⁸⁷Sr/⁸⁶Sr ratio of the Paleozoic minimum, (6) extensive felsic alkaline volcanism, and (7) Illawarra Reversal.The felsic alkaline volcanism and the concurrent formation of several large igneous provinces (LIPs) in the eastern Pangea suggest that the Permian biosphere was involved in severe volcanic hazards twice at the G-LB and the P-TB. This episodic magmatism was likely related to the activity of a mantle superplume that initially rifted Pangea. The supercontinent-dividing superplume branched into several secondary plumes in the mantle transition zone (410–660 km deep) beneath Pangea. These secondary plumes induced the decompressional melting of mantle peridotite and pre-existing Pangean crust to form several LIPs that likely caused a “plume winter” with global cooling by dust/aerosol screens in the stratosphere, gas poisoning, acid rain damage to surface vegetation etc. After the main eruption of plume-derived flood basalt, global warming (plume summer) took over cooling, delayed the recovery of biodiversity, and intensified the ocean stratification. It was repeated twice at the G-LB and P-TB.A unique geomagnetic episode called the Illawarra Reversal around the Wordian–Capitanian boundary (ca. 265 Ma) recorded the appearance of a large instability in the geomagnetic dipole in the Earth’s outer core. This rapid change was triggered likely by the episodic fall-down of a cold megalith (subducted oceanic slabs) from the upper mantle to the D″ layer above the 2900 km-deep core-mantle boundary, in tight association with the launching of a mantle superplume. The initial changes in the surface environment in the Capitanian, i.e., the Kamura cooling event and the first biodiversity decline, were probably led by the weakened geomagnetic intensity due to unstable dipole of geodynamo. Under the low geomagnetic intensity, the flux of galactic cosmic radiation increased to cause extensive cloud coverage over the planet. The resultant high albedo likely drove the Kamura cooling event that also triggered the unusually high productivity in the superocean and also the expansion of O₂ minimum zone to start the superanoxia.The “plume winter” scenario is integrated here to explain the “triple-double” during the Paleozoic–Mesozoic transition interval, i.e., double-phased cause, process, and consequence of the greatest global catastrophe in the Phanerozoic, in terms of mantle superplume activity that involved the whole Earth from the core to the surface biosphere.     

循环往复加载的地下结构Pushover分析方法及其在地震损伤分析中的应用

考虑到地震作用下地下结构往往受到双向往复荷载作用,本文提出了循环往复加载的地下结构Pushover分析方法.介绍了该方法的实施步骤、基本功能与特点.该方法考虑了地震作用下地下结构双向受力的特点,利用多点位移控制的推覆分析算法进行地震作用下正向加载-卸载-反向再加载的全过程分析.该方法将一次循环加载过程近似看作一次地震作用过程,提出了基于循环往复加载Pushover分析的损伤模型,避免了对土-结构整体模型进行复杂的动力相互作用分析;通过一次循环往复加载的Pushover分析,根据结构构件刚度的改变对结构损伤进行有效评估.结合实际工程进行算例分析初步验证了循环往复加载Pushover分析及地震损伤模型的有效性.     

Reversal sequence in a multiple scale dynamo mechanism

We investigate the temporal evolution of the magnetic dipole field intensity of the Earth through a multiscale dynamo mechanism. On a large range of spatio-temporal scales, the helical motions of the fluid flow are given by a schematic model of a fully developed turbulence. The system construction is symmetric with respect to left-handed and right-handed cyclones. The multiscale cyclonic turbulence coupled with a differential rotation (schematic ω dynamo) or alone (schematic ² dynamo) is the ingredient of the loop through which poloidal and toroidal fields are built from one another. Two kinds of reaction of the magnetic field on the flow are considered: the presence of a magnetic field first favours a larger-scale organization of the flow, and, second, impedes this flow by the effect of growing Lorentz forces. We obtain the general features of the geomagnetic field intensity observed over geological times and describe a general mechanism for reversals, excursions and secular variation. The mechanism happens to keep a memory during the chrons and loose it during the events (excursions and inversions).     

Late-glacial and Holocene vegetation and climatic history of the Cass Basin, central South Island, New Zealand

Lithology, pollen, macrofossils, and stable carbon isotopes from an intermontane basin bog site in southern New Zealand provide a detailed late-glacial and early Holocene vegetation and climate record. Glacial retreat occurred before 17,000 cal yr B.P., and tundra-like grassland–shrubland occupied the basin shortly after. Between 16,500 and 14,600 cal yr B.P., a minor regional expansion of forest patches occurred in response to warming, but the basin remained in shrubland. Forest retreated between 14,600 and 13,600 cal yr B.P., at about the time of the Antarctic Cold Reversal. At 13,600 cal yr B.P., a steady progression from shrubland to tall podocarp forest began as the climate ameliorated. Tall, temperate podocarp trees replaced stress-tolerant shrubs and trees between 12,800 and 11,300 cal yr B.P., indicating sustained warming during the Younger Dryas Chronozone (YDC). Stable isotopes suggest increasing atmospheric humidity from 11,800 to 9300 cal yr B.P. Mild (annual temperatures at least 1°C higher than present), and moist conditions prevailed from 11,000 to 10,350 cal yr B.P. Cooler, more variable conditions followed, and podocarp forest was completely replaced by montane Nothofagus forest at around 7500 cal yr B.P. with the onset of the modern climate regime. The Cass Basin late-glacial climate record closely matches the Antarctic ice core records and is in approximate antiphase with the North Atlantic.     

古新世—始新世最热事件对地球表层循环的影响及其触发机制

古新世—始新世最热事件（PETM, Paleocene Eocene Thermal Maximum）是发生在古新世—始新世交界时的一次全球性的气候突变事件。它造成了大洋环流模式的突然倒转和海水盐度、大气湿度的迅速上升。海洋表层生态系统和陆地生态系统生产力迅速上升,许多属种的植物、动物、微生物生活范围向高纬区扩大;大洋底栖微生物发生集群灭绝。现代哺乳动物的主要属种（灵长类、奇蹄类及偶蹄类）产生,哺乳动物演化进程发生重大改变。地球表层碳循环系统发生不同程度的碳同位素负偏移,全球碳循环系统发生大规模搅动。对于PETM的触发机制,主流的观点认为是海底天然气水合物突然释放造成巨量甲烷迅速进入表层系统引发的碳循环系统内部反馈。而对于甲烷释放的原因,又存在着减压释放和热释放两种解释;此外还有科学家用岩浆作用和地外星体撞击来解释PETM的发生。