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851.
通过对东经90°海岭ODP758钻孔岩心3.5Ma以来的浮游有孔虫远洋记录的研究,结合岩性地层、稳定同位素地层、碳酸盐地层等资料,探讨了喜马拉雅山加速隆升、"中更新世气候转型"、"中布容溶解"事件等重大构造、环境事件在研究区的表现及相关机制。通过对各项指标的分析和对比,根据ODP758岩心的有孔虫组合面貌、古温度、古盐度和古温跃层的变化情况,推断研究区在1.4~0.7Ma期间的气候变化可能是在"中更新世气候转型"条件下发生的。碳酸钙、碳酸钙沉积通量和非碳酸钙沉积通量曲线在1.7Ma时快速增长,与现有资料显示的青藏高原的隆升历史相符,分别与青藏高原隆升过程中的青藏运动A幕、B幕、C幕(1.7Ma)和昆仑-黄河运动(1.2~0.6Ma)相当。碳酸钙含量、大于150μm的粗组分含量、浮游有孔虫沉积通量和浮游有孔虫生物地层揭示出在0.5~0.4Ma期间研究区深海碳酸盐遭受了强烈溶解。此事件与大西洋、太平洋、印度洋及南沙海区沉积记录中的"中布容溶解"事件一致。 相似文献
852.
Recent high-resolution observations of crustal movements have revealed silent slip events (SSEs) with propagation velocities of around 10–15 km d−1 and with intervals of 3–14 months along the deeper parts of the Cascadia and Nankai subduction zones. This study develops 2-D and 3-D models of these short-interval SSEs considering the frictional behaviour that was confirmed experimentally by Shimamoto for the unstable–stable transition regime. To represent this frictional behaviour, a small cut-off velocity to an evolution effect is introduced in a rate- and state-dependent friction law. When the cut-off velocity to the evolution effect is significantly smaller than that to a direct effect, steady-state friction exhibits velocity weakening at low slip velocities and velocity strengthening at high slip velocities. At the deeper Cascadia and Nankai subduction interfaces, the pore pressure is inferred to be high because of the dehydration of materials in the descending plate. Under conditions where the pore-fluid pressure is nearly equal to the lithostatic pressure and the critical weakening displacement is very small, short-interval SSEs with propagation velocities and slip velocities of 4–8 km d−1 and 2 − 4 × 10−7 m s−1 , respectively, can be reproduced. The propagation velocity of short-interval SSEs is in proportion to the slip velocity. The results also show that during the nucleation process of large earthquakes, the occurrence of short-interval SSEs becomes irregular because of the accelerated slips that occur at the bottom of the seismogenic zone. Our results suggest that monitoring of short-interval SSEs might be useful for forecasting the main earthquakes. 相似文献
853.
The temperature (T) evolution of the barium carbonate (BaCO3) structure was studied using Rietveld structure refinements based on synchrotron X-ray diffraction and a powdered synthetic
sample. BaCO3 transforms from an orthorhombic, Pmcn, α phase to a trigonal, R3m, β phase at 811°C. The orthorhombic BaCO3 structure is isotypic with aragonite, CaCO3. In trigonal R3m BaCO3, the CO3 group occupies one orientation and shows no rotational disorder. The average <Ba–O> distances increase while the <C–O> distances
decrease linearly with T in the orthorhombic phase. After the 811°C phase transition, the <Ba–O> distances increase while C–O distances decrease.
There is also a significant volume change of 2.8% at the phase transition. 相似文献
854.
D. Suetsugu H. Shiobara H. Sugioka Y. Fukao T. Kanazawa 《Physics of the Earth and Planetary Interiors》2007,160(3-4):310-318
We analyzed seismic waveforms recorded by the broadband ocean bottom seismographs deployed in the South Pacific superswell to determine the depths of the mantle discontinuities using a receiver function method. We estimated the thickness of the mantle transition zone (MTZ) to be 245 km on average beneath the superswell region, which is close to the global average. The MTZ is found to be thinned locally beneath the Society hot spot by 30 km. Temperature anomalies computed from the thinned transition zone and the Clapeyron slope of the olivine phase transformations are 150–200 K beneath the Society hot spot. Previous studies, using land-based data, suggested the presence of a hot MTZ beneath the Pitcairn hot spot. The locally hot transition zone beneath the hot spots is attributed to narrow mantle plumes probably rising from the lower mantle. The normal average thickness obtained from the present study indicates that there is no broad upwelling across the transition zone beneath the entire superswell region at present. 相似文献
855.
OM (optical microscope)/TEM (transmission electron microscope) micro- and submicrostructural analysis of hornblende rocks sheared at high temperatures from the Diancangshan area, western Yunnan reveals evidence for deformation in the brittle-ductile transition of hornblende at middle crustal level (about 637℃ and 0.653 GPa) and mechanisms of deformation in the transitional regime are further discussed. Sheared hornblende rocks at middle crustal level have typical mylonitic microstructures, shown by coarse porphyroclasts and fine matrix grains. Different mineral phases in the rocks show distinct deformation characteristics. Hornblende and feldspar grains are intensely deformed with ob- vious grainsize reduction, but quartz grains are recrystallized dominantly by grain growth. Hornblende grains show typical brittle-ductile transition nature. Initial crystallographic orientations of porphyro- clasts have strong effects on the behavior of grains during deformation. There are mainly two types of porphyroclasts, type I "hard" porphyroclasts and type II "soft" porphyroclasts, with [001] perpendicular and parallel to external shear stresses respectively. "Hard" porphyroclasts generally occur as compe- tent grains that are rarely deformed or sometimes deformed by fracturing and dislocation tangling. "Soft" porphyroclasts are highly deformed primarily by dislocation tangling (as shown in the cores of the porphyroclasts), but twinning, dislocation glide and climb probably due to hydrolytic weakening also contribute to dynamic recrystallization of the porphyroclasts into fine grains in the matrix. The micro- and submicrostructures of the two types of porphyroclasts and fine-grained matrix provide powerful evidence for the behavior of brittle-ductile transition of hornblende grains. It is concluded that twinning nucleation is one of the most important processes that operate during dynamic recrystalliza- tion of hornblende crystals at the brittle-ductile transition. (100) [001] twin gliding and dislocation creep (dislocation glide and climb) are mutually enhanced during twinning nucleation. As a newly discovered mechanism of dynamic recrystallization, it may have played more important roles than ever recognized during dynamic recrystallization of crystals with twins in the brittle-ductile transition. 相似文献
856.
The theory of an earthquake source nucleation is discussed. Based on the assumption that self organization of damage process takes place in the zone of an earthquake source nucleation the theory incorporates the damage rheology framework of Lyakhovsky et al. and the approach of phenomenology theory of second-order phase transition. Namely, the free energy governing the process of an earthquake source nucleation depends on two collective variables α and φ in addition to the strain tensor εij. The variable α quantifies the fracture of the medium and the variable φ quantifies the interaction effect of cracks. The region ΩS is associated with a potential source of an earthquake if the damage variable α exceeds the critical value αcr(1) inside ΩS. The important feature of a potential source is that interaction of fractures causes acceleration of damage process inside the region of potential source and the material should lose stability primarily in this region. Interaction of fractures results also in occurrence of a residual stress caused by nonuniformity of fracture density. The appearance and development of the potential source result in increase of intensity of damage process not only in the region of potential source but also in a certain neighborhood of the last. It is compatible with such observed effects as acceleration of seismic energy release and growth of correlation length of weak seismicity before large earthquake. Transition of potential source to the stage of avalanche-unstable fracturing is associated with instability generated by explosive increase of interaction of fractures when the damage variable α exceeds the second critical value αcr(2) inside ΩS. 相似文献
857.
Rebecca S. Anderson Lin Huang Richard Iannone Jochen Rudolph 《Journal of Atmospheric Chemistry》2007,56(3):275-291
The carbon kinetic isotope effects (KIEs) in the reactions of several unsaturated hydrocarbons with chlorine atoms were measured
at room temperature and ambient pressure using gas chromatography combustion isotope ratio mass spectrometry (GCC-IRMS). All
measured KIEs, defined as the ratio of the rate constants for the unlabeled and labeled hydrocarbon reaction k
12/k
13, are greater than unity or normal KIEs. The KIEs, reported in per mil according to Cl
ɛ = (k
12/k
13−1) × 1000‰ with the number of experimental determinations in parenthesis, are as follows: ethene, 5.65 ± 0.34 (1); propene,
5.56 ± 0.18 (2); 1-butene, 5.93 ± 1.16 (1); 1-pentene, 4.86 ± 0.63 (1); cyclopentene, 3.75 ± 0.14 (1); toluene, 2.89 ± 0.31
(2); ethylbenzene, 2.17 ± 0.17 (2); o-xylene, 1.85 ± 0.54 (2). To our knowledge, these are the first reported KIE measurements for reactions of unsaturated NMHC
with Cl atoms. Relative rate constants were determined concurrently to the KIE measurements. For the reactions of cyclopentene
and ethylbenzene with Cl atoms, no rate constant has been reported in refereed literature. Our measured rate constants are:
cyclopentene (7.32 ± 0.88) relative to propene (2.68 ± 0.32); ethylbenzene (1.15 ± 0.04) relative to o-xylene (1.35 ± 0.21), all × 10−10 cm3 molecule−1 s−1. The KIEs in reactions of aromatic hydrocarbons with Cl atoms are similar to previously reported KIEs in Cl-reactions of
alkanes with the same numbers of carbon atoms. Unlike the KIEs for previously studied gas-phase hydrocarbon reactions, the
KIEs for alkene–Cl reactions do not exhibit a simple inverse dependence on carbon number. This can be explained by competing
contributions of normal and inverse isotope effects of individual steps in the reaction mechanism. Implications for the symmetries
of the transition state structures in these reactions and the potential relevance of Cl-atom reactions on stable carbon isotope
ratios of atmospheric NMHC are discussed. 相似文献
858.
The Shatter Belt and the European Core - A Geopolitical Discussion on the Untypical Case of Slovenia 总被引:1,自引:0,他引:1
Anton Gosar 《GeoJournal》2000,52(2):107-117
Europe in general is in a great state of change. States uniting with difficulty, states collapsing in pain, newly freed states struggling for new political, economic, and social identities - it is a region in a true transition. The paper is focussing primarily on the European Shatter Belt, formerly known as Eastern Europe. This is in fact a subset of new and old nation-states in the region of Central and Eastern Europe or, to use an older, but increasingly popular term, Mitteleuropa (Ruppert, 1997). Mitteleuropa is reemerging from the commotion resulting from the collapse of the old political order - communism. Our focus in this presentation is on personal experiences and observations. We will note the progress and positive dimensions of the change (transition) in the light of Europe's and NATO expansion. We will address the issues at two scales- at the international/national level and then, at the level of a Slovenian case study. This will enable us to discuss the natural system of the process of enlargement and incorporate supporting material of geographical, historical and social nature as envisioned by Hartshorne. Since the collapse of communism, the region has experienced some serious traumas that vary in intensity across the region. Probably the greatest pain has been experienced in the former Yugoslavia - an estimated 150,000 have perished and about 2.5 millions have been displaced within and outside the country. Chaos still exists despite the protectorate arrangement in Bosnia. The hot spot Kosovo has yet to be calmed down. In parts of the region economic pain has been considerable. Every state of Mitteleuropa experienced absolute economic decline from 1990–1995; high inflation rates, increased economic inequality, high levels of poverty, increase of crime and more. Economic and psychological stress effect demographic processes. Things have partly changed to the better in the most recent years. The adoption of capitalism and a democratic political system has been painful, disorienting, and is still in progress. The democratization process varies considerably across the region and in some states is in jeopardy. Reformed communist parties had staged a restoration in nearly all of the states - in some winning the control of the government, in others gaining strong and influential positions, or both. But, two major processes promise future democracy, rule of law, wealth and stability in general. This is the decision of the member states of the European Union and NATO to incorporate some/several states of the Shatter Belt into their own zone of protection and/or federation. Nation-states of the region tend to follow the EU Agenda 2000 rules with Slovenia one of the first! 相似文献
859.
860.
Neotethyan suprasubduction zone ophiolites represent anomalous oceanic crust developed in older host basins during trench rollback cycles and later entrapped in orogenic belts as a result first of trench-passive margin and then continent–continent collisions. The Middle Jurassic Mirdita zone ophiolites in northern Albania constitute a critical transition between the dominantly mid-ocean ridge basalt (MORB)-related Early Jurassic Alpine–Apennine ophiolites in the west and supra-subduction zone (SSZ)-generated Cretaceous Eastern Mediterranean ophiolites in the east. The previously recognized Western- and Eastern-type ophiolites in the Mirdita zone display significant differences in their internal structure and pseudostratigraphy, but their geochemical affinities are more gradational in contrast to the earlier claims that these ophiolites may have formed in different tectonic settings at different times. Crosscutting relations of dike intrusions in the Eastern-type ophiolites indicate changes in the chemistry of magmatic plumbing systems from basaltic to andesitic, dacitic, rhyodacitic, and boninitic compositions through time and from west to east. The chemostratigraphy of the extrusive sequence in the Western-type ophiolites shows that the MORB-like tholeiitic rocks display a significant decrease in their TiO2 contents and Zr concentrations stratigraphically upward, although their εNd(T) values (+ 7.3 to + 6.9) show minor variation. The basaltic andesites in the upper 100 m of the Western extrusive sequence have island arc tholeiite (IAT)-like chemical characteristics (low-Ti, lower HFSE and HREE distribution, significant LREE depletion and higher Co, Ni, and Cr contents) that signify increased subduction influence in magma/melt evolution. The Eastern-type extrusive rocks range in composition from basaltic andesite to andesite, dacite and rhyodacite stratigraphically upward mimicking the temporal changes in the sheeted dikes, and they display constant Zr ( 50 ppm) but significantly varying Cr contents. The TiO2 contents of their pyroxenes are < 0.3 wt.%, and their εNd(T) values decrease from + 6.5 in the lower parts to + 3.1 in the uppermost section of the sequence. Farther east in the extrusive sequence the youngest boninitic lavas and dikes have εNd(T) values between − 1.4 and − 4.0. These chemical variations through time point to a mantle source increasingly contaminated by subduction-derived aqueous fluids and sediments, which were incorporated into the melt column beneath an extending protoarc–forearc region. Slab retreat and sinking played a major role in establishing asthenospheric upwelling and corner flow beneath the forearc mantle that in turn facilitated shallow partial melting of highly depleted harzburgitic peridotites, producing boninitic magmas. This chemical progression in the Mirdita zone ophiolite volcanism is similar to the temporal variations in magma chemistry documented from very young intraoceanic arcs built on recently generated backarc crust (i.e., South Sandwich arc). The Western and Eastern-type ophiolites in the Mirdita zone are therefore all subduction-related with the subduction zone influence in the lavas increasing stratigraphically upward as well as eastwards, suggesting a west-dipping slab geometry. The Mirdita zone and the Western Hellenic ophiolites in the Balkans were produced within a marginal basin that had evolved between the Apulian and Pelagonian microcontinents, and were subsequently emplaced onto their passive margins diachronously through different collisional processes. 相似文献