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1.
Stresses and effective viscosities in the asthenosphere to a depth of 400 km are calculated on the basis of Weertmans “temperature method” i.e., on relating viscosity to the ratio of the temperature to the melting point (=homologous temperature). Some oceanic and continental geotherms and two melting point—depth curves, the dry pyrolite solidus and the forsterite 90 melting curve are used for the conversion of the homologous temperature to the effective viscosity. Two creep laws are considered, the linear, grain-size-dependent Nabarro—Herring (NH) creep law, and a power creep law, in which the creep rate is proportional to the third power of the stress. A plate tectonic model yields creep rates of 2 · 10 −14 s −1 for the oceanic and 3 · 10 −15 s −1 for the continental asthenosphere. These values are held constant for the calculations and may be valid for regions inside plates.The dry pyrolite mantle model results in high homologous temperatures in the asthenosphere below oceans (0.9), very low stresses (a few bars and lower) and shows a low viscosity “layer” of about 200-km thickness. Below continental shields the homologous temperature has a maximum value of 0.73, stresses are around 5–20 bar and the low-viscosity region is thicker and less pronounced than in the oceanic case. The Fo 90 mantle model generally gives lower homologous temperatures (maximum value below oceans beside active ridges 0.75). The stresses in the asthenosphere beneath oceans vary from a few bars to about 50 bar and below continents to about 100 bar. The low-viscosity region seems to reach great depths without forming a “channel”. The Figs. 1 and 2 show the approximate viscosity—depth distribution for the two mantle models under study.Assuming a completely dry mantle and a mean grain size of 5 mm, power law creep will be the dominating creep process in the asthenosphere. However, grains may grow in a high-temperature—low-stress regime (i.e., below younger oceans), an effect which will further diminish the influence of NH creep. In the upper 100–150 km of the earth some fluid phases may affect considerably creep processes. 相似文献
2.
The deformation of the oceanic lithosphere subducting at the junction of two trenches is studied by means of a three-dimensional finite-element analysis. Results show that the existence of a junction (i.e. a change in trend of the trench axis) yields a specific shape of the outer topographic rise. In a convex junction area—such as the Japan and Kuril trenches, the topographic bulge presents a “dome”, whereas in a concave junction area—such as the Java and Sumatra trenches, this bulge is less pronounced. These theoretical results are confirmed by the bathymetry seaward of the junctions of the Japan and Kuril trenches and of the Peru-Chile trench. Moreover, the existence of the abnormal topographic dome in front of a convex junction contributes to the creation of normal faults which help the subduction of seamounts or of other bathymetric features in such areas. 相似文献
3.
地球上一切物质都是做开合运动的。以热力(热能)驱动物质上浮运动定义为开;以重力(势能)驱动物质下沉的运动定义为合。开合运动具有垂直开合与水平开合的同步统一性和开合相互依存、相互对立、相互转化两个重要特性。因而在旋转运动的统领和调剂下,将所有地球物质、构造、能量等从无序状态转化到有序状态,最后建成了动态稳定平衡的开合旋构造体系。所以,动态稳定平衡体系的形成、破坏、修复不断旋回式演化就是研究地球内部运行的基础和有效方法。为此,我们在垂向上划分了8个开合构造旋回和地壳(球面)开合构造体系,组成了地球四维运行系统。动态稳定平衡体系的建立是遵循重力均衡准则、最小内能原理(结晶化)、几何优选方位(垂直地心)生长和横向物质均匀化4条自然规律的长期作用而实现的,其中重力均衡是主导的。大约40亿年时,地幔岩石圈形成,它像一个球形锅盖盖住热地球,初步建成了稳定的开合旋构造体系,使地球长期进入动态稳定平衡发展阶段。尔后,每次重大地质事件(失衡运动)就是把岩石圈地幔捅个窟窿或撕开一条裂缝,破坏该地区的开合平衡。旋转运动和开合运动迅速将其恢复达到新的平衡,从而构成一个开合旋回,目前地球已经历了8个大旋回。板块构造仅研究最新4个旋回的岩石圈地幔是如何被捅破及被修复的规律并总结相关的理论,因而它在时间上和空间上都有局限性,不属于全球构造。地球类比为一个生命体,其运行系统中的内部结构和功能比较齐全:包含有开合运动、旋转运动和失衡运动组成的运动系统;起“血液”作用的地质流体、贮存和制造“血液”的“心脏”——地核和联系各种“血液”的地幔柱组成的“血液”系统;以重力均衡为准则,由四个重力均衡面、四个开合转换带和球面两条质量均衡线构成的地球平衡系统。由于开与合的转换具有自发形成和自我调剂的功能,所以开与合相互依存、相互对立、相互转化,是地球内部运行体系基本的动力机制和主要的动因。开合转换的本质是热能和势能的转换,驱动失衡运动的主要内因是高温、高压流体超临界爆裂;主要外因是陨击作用。 相似文献
4.
This paper describes the application and characteristics of a new net for crack statistics. The net, here called transmission net or “u-net” (“Übertragungsnetz”, “Ü-Netz”) is used in combination with a transmission table or “u-table” (“Übertragungstabelle”, “Ü-Tabelle”). Its purpose is to obtain a sphere of crack locations from a series of cracks, having been measured according to strike and inclination. The “u-net” is composed of a grid subdivided into degrees or degree-grid (“Gradnetz”) and an equal-area-grid (“flächengleiches Netz”). Having replaced the direction σ of strike by its normal δ, the cracks are registered into the meshes of the degree-grid and are there counted. Then they are transmitted into the equal-area-grid according to the percent values of the u-table. From the pattern of frequency numbers (“Häufigkeitszahlen”) in the equal-area-grid the sphere of crack locations is obtained. Particular specifications regulate the procedure for special measure values, i.e., those of the integral multiples of 5°, especially the angles of inclination τ = 0° and τ = 90° (see 1.4). With greater inaccuracies in measurements, one changes, by means of a given table (see Table V), to a degree-grid of 10°. With very small inaccuracies, on the other hand, the procedure may be simplified, the degree-grid becoming unnecessary (see 1.5). The meshes near the centre, being too long, may be avoided by an additional circle (“Zusatzkreis”—see 1.3). The “u-net” was constructed in such a way that the spheres of crack locations report the real frequency distribution at all times, free from systematic errors. This is achieved by the method that all calculations follow the principle of area equality or area proportionality on the hemisphere (see 2.1). The procedure using the “u-net” can be adapted to differential accuracies of measuring. It is especially suitable for large numbers of cracks; it is simple in calculation and may easily be programmed for digital computers. Thus the “u-net” is advantageous for all applications in which a large number of cracks has to be dealt with. Such applications are very frequent in rock mechanics, in engineering geology for the purpose of foundation of large hydraulic buildings (dams, caverns), in petrography, tectonics and in geophysical investigations such as the determination of crack structures with a view to explaining micromagnetic occurrences, for instance. Furthermore, the “u-net” is applicable not only to crack statistics but also to other similar statistical methods, e.g., to the statistics of cristal axes or to geographic frequency analyses. 相似文献
5.
Multichannel reflection data (Tugolessov et al., 1985) have revealed two deeps in the basement topography of the Black Sea which are filled with sediments from 12 to 15 km thick. The deeps lack the “granitic layer” and are underlain by oceanic-type crust which we assume to be generated by seafloor spreading processes. The age of the deeps was interpreted previously, in a highly controversial manner, as being from the Paleozoic — Early Mesozoic to the Recent. In the paper, age estimations were undertaken using surficial heat flow data, assuming that they are related to deep-seated age-dependent heat flow generated by the cooling oceanic lithosphere, but that they are strongly distorted by the heating of continuously accumulating sediments as well as by additional heat input from radiogenic production within sediments. Using reliable thermophysical parameters of compacting sediments, the distorted heat flow in the sediments was evaluated numerically. This allowed us to estimate the age of the Black Sea deeps floor. The results show that the West Black Sea deep is 130 to 95 m.y. old, and the East Black Sea deep is nearly 110 m.y. old. These figures support an interpretation of the Black Sea deeps as remnants of a Late Mesozoic back-arc basin that evolved behind the Lesser Caucasian — Pontide island arc. The inferred Middle Cretaceous age of the deeps is the first estimate obtained quantitatively, and corresponds well with available heat flow and multichannel reflection data. 相似文献
6.
The estimation of crustal deformations of the Earth from repeated baseline measurements is a singular problem in the absence of prior information. It could be solved by well-known techniques of g-inverse algebra within the framework of a Singular Gauss-Markov Model leading e.g., to BLIMBE (Best Linear Minimum Bias Estimator) or MINOLESS (Minimum NOrm LEast Squares Solution). However, these solutions may not be physically meaningful in view of some independently derived geophysical results. But after introducing these geophysical findings as a-priori information into the linear model, the problem will no longer be singular and can be solved by means of “Improved Linear Estimation” as well as of “Best Linear Prediction”, depending on the way the original model has been expanded. Several alternative estimators and/or predictors are compared with respect to their characterizing properties (homogeneous linear, inhomogeneous linear, biased, unbiased, minimum mean square error, minimum variance, etc.), and the respective gains in efficiency are given. 相似文献
7.
Isostatic response of the Earth to changes in Quaternary Times of ice and water loads is partly elastic, and partly involves viscous mantle flow. The relaxation spectrum of the Earth, critical for estimation of the mantle flow component, is estimated from published determinations of Fennoscandian and Laurentide rebound, and of the nontidal acceleration of the Earth's rotation. The spectrum is consistent with an asthenosphere viscosity around 10 21P, and a viscosity around 10 23P below 400 km depth. Calculation of relaxation effects is done by convoluting the load history with the response function in spherical harmonics for global effects, and in rectangular or cylindrical transforms for smaller regional effects. Broad-scale deformation of the globe, resulting from the last deglaciation and sea level rise, is calculated to have involved an average depression of ocean basins of about 8 m, and mean upward movement of continents of about 16 m, relative to the center of the Earth, in the last 7000 yr. Deflection in the ocean margin “hinge zone” varies with continental shelf geometry and rigidity of the underlying lithosphere: predictions are made for different model cases. The computational methods is checked by predicting Fennoscandian and Laurentide postglacial warping, from published estimates of icecap histories, with good results. The depth variations of shorelines formed around 17,000 BP (e.g., North America, 90–130 m; Australia, 130–170 m), are largely explainable in terms of combined elastic and relaxation isostasy. Differences between Holocene eustatic records from oceanic islands (Micronesia, Bermuda), and continental coasts (eastern North America, Australia), are largely but not entirely explained in the same terms. 相似文献
8.
Along the Japan trench where some Mw8 class interplate earthquakes occurred in the past century such as the 1896 Sanriku tsunami earthquake (M6.8, Mt8.6, 12×10 20 N m) and the 1968 Tokachi-oki earthquake (Mw8.2, 28×10 20 N m), the Pacific plate is subducting under northeast Japan at a rate of around 8 cm/year. The seismic coupling coefficient in this region has been estimated to be 20–40%. In the past decade, three ultra-slow earthquakes have occurred in the Sanriku-oki region (39°N–42°N): the 1989 Sanriku-oki (Mw7.4), the 1992 Sanriku-oki (Mw6.9), and the 1994 Sanriku-oki (Mw7.7) earthquakes. Integrating their interplate moments released both seismically and aseismically, we have the following conclusions. (1) The sum of the seismic moments of the three ultra-slow earthquakes was (4.8–6.6)×10 20 N m, which was 20–35% of the accumulated moment (18.6–23.0)×10 20 N m, in the region (39°N–40.6°N, 142°E–144°E) for the 21–26 years since the 1968 Mw8.2 Tokachi-oki earthquake. This is consistent with the previous estimates of the seismic coupling coefficient of 20–40%. On the other hand, the sum of the interplate moments including aseismic faulting is (11–16)×10 20 N m, leading to a “seismo-geodetic coupling coefficient” of 50–85%, which is an extension of the seismic coupling coefficient to include slow events. (2) The time constants showed a large range from 1 min (10 2 s) for the 1968 Tokachi-oki earthquake to 10–20 min (10 3 s) for the 1896 Sanriku tsunami earthquake, to one day (10 5 s) for the 1992 Sanriku-oki ultra-slow earthquake, to on the order of one year (10 7 s) for the 1994 Sanriku-oki ultra-slow earthquakes. (3) Based on the space–time distribution, three “gaps of moment release,” (40.6°N–42°N, 142°E–144°E) 39°N–40°N, 142°E–143°E) and (39°N–40°N, 142°E–144°E), are identified, instead of the gaps of seismicity. 相似文献
9.
The petrographic and chemical composition of magmatic rocks generated during the Archaean appears to be different from that of post-Archaean rocks. Komatiites are widespread before 2.5 Ga and rarely occur afterwards. In addition the Archaean continental crust is primarily TTG (Tonalitic, Trondhjemitic and Granodioritic) in composition, exhibiting typical trondhjemitic differentiation trends; whereas modern equivalents are granodioritic to granitic following classical calc-alkaline differentiation trends. This distinction becomes more prominent when rare-earth elements (REE) are taken into account: Archaean TTG are Yb-poor (Yb N < 8.5) and have high (La/Yb) ratios (5 < (La/Yb) N < 150), in comparison, the post-2.5 Ga granitoids, emplaced in subduction-zone geodynamic environments have high Yb content (4.5 N<20) with very low (La/Yb)N ratios ( 20). Theoretical calculations and experimental petrology have shown that the TTG can be produced by partial melting of an Archaean tholeiite transformed into garnet-bearing amphibolite. Consequently, the low heavy REE content of the TTG is explained by the influence of both residual garnet and hornblende in their source. After 2.5 Ga the role of these minerals in calc-alkaline magma genesis becomes progressively less important, which is interpreted in terms of a cooling Earth model.In modern subduction zone environments the subducted oceanic slab is relatively “old and cold” and the geothermal gradient along the Benioff plane in low (ca. 10°C/km). Consequently, the down-going lithosphere undergoes dehydration before partial melting is able to occur. The liberated fluids are light REE and LILE-enriched and ascend into the overlying mantle wedge where they induced partial fusion. The produced magmas separate from their mantle source region leaving a residue mainly composed of olivine and pyroxenes. Mantle derived magmas typically exhibit high Yb contents due to low KDYb values for olivine and pyroxenes. During the Archaean, the subducted lithosphere was relatively “young and hot” providing high geothermal gradients along the Benioff zone. Thus, partial melting of the subducted slab was possible at lower temperatures before dehydration would take place. Garnet and hornblende are the main residual phases accounting for the low Yb contents of the Archaean TTG. This model can be tested using a modern analogue of Archaean-like subduction processes. In south Chile an oceanic ridge has subducted and all thermodynamic calculations indicate that this creates locally high geothermal gradients along the Benioff zone. Thus in very small areas, Archaean-like environments may be simulated in modern subduction zones. The modern andesites produced in this environment show Archaean geochemical characteristics with low YbN (<5), whereas the majority of andesites along the Andean arc have modern patterns with YbN ranging from 8 to more than 17. This conclusion was generalised to all young subducted lithospheres all over the world. In conclusion, it appears that since the Archaean there has been a change in the site of continental crust genesis. The location of calc-alkaline magma source in subduction-zone environments has migrated through time from the subducted slab to the mantle wedge. This is a direct consequence of the progressive cooling of the Earth. 相似文献
10.
The Slave craton in northwestern Canada, a relatively small Archean craton (600×400 km), is ideal as a natural laboratory for investigating the formation and evolution of Mesoarchean and Neoarchean sub-continental lithospheric mantle (SCLM). Excellent outcrop and the discovery of economic diamondiferous kimberlite pipes in the centre of the craton during the early 1990s have led to an unparalleled amount of geoscientific information becoming available. Over the last 5 years deep-probing electromagnetic surveys were conducted on the Slave, using the natural-source magnetotelluric (MT) technique, as part of a variety of programs to study the craton and determine its regional-scale electrical structure. Two of the four types of surveys involved novel MT data acquisition; one through frozen lakes along ice roads during winter, and the second using ocean-bottom MT instrumentation deployed from float planes. The primary initial objective of the MT surveys was to determine the geometry of the topography of the lithosphere–asthenosphere boundary (LAB) across the Slave craton. However, the MT responses revealed, completely serendipitously, a remarkable anomaly in electrical conductivity in the SCLM of the central Slave craton. This Central Slave Mantle Conductor (CSMC) anomaly is modelled as a localized region of low resistivity (10–15 Ω m) beginning at depths of 80–120 km and striking NE–SW. Where precisely located, it is spatially coincident with the Eocene-aged kimberlite field in the central part of the craton (the so-called “Corridor of Hope”), and also with a geochemically defined ultra-depleted harzburgitic layer interpreted as oceanic or arc-related lithosphere emplaced during early tectonism. The CSMC lies wholly within the NE–SW striking central zone defined by Grütter et al. [Grütter, H.S., Apter, D.B., Kong, J., 1999. Crust–mantle coupling; evidence from mantle-derived xenocrystic garnets. Contributed paper at: The 7th International Kimberlite Conference Proceeding, J.B. Dawson Volume, 1, 307–313] on the basis of garnet geochemistry (G10 vs. G9) populations. Deep-probing MT data from the lake bottom instruments infer that the conductor has a total depth-integrated conductivity (conductance) of the order of 2000 Siemens, which, given an internal resistivity of 10–15 Ω m, implies a thickness of 20–30 km. Below the CSMC the electrical resistivity of the lithosphere increases by a factor of 3–5 to values of around 50 Ω m. This change occurs at depths consistent with the graphite–diamond transition, which is taken as consistent with a carbon interpretation for the CSMC. Preliminary three-dimensional MT modelling supports the NE–SW striking geometry for the conductor, and also suggests a NW dip. This geometry is taken as implying that the tectonic processes that emplaced this geophysical–geochemical body are likely related to the subduction of a craton of unknown provenance from the SE (present-day coordinates) during 2630–2620 Ma. It suggests that the lithospheric stacking model of Helmstaedt and Schulze [Helmstaedt, H.H., Schulze, D.J., 1989. Southern African kimberlites and their mantle sample: implications for Archean tectonics and lithosphere evolution. In Ross, J. (Ed.), Kimberlites and Related Rocks, Vol. 1: Their Composition, Occurrence, Origin, and Emplacement. Geological Society of Australia Special Publication, vol. 14, 358–368] is likely correct for the formation of the Slave's current SCLM. 相似文献
11.
This paper presents field, petrographic–structural and geochemical data on spinel and plagioclase peridotites from the southern domain of the Lanzo ophiolitic peridotite massif (Western Alps). Spinel lherzolites, harzburgites and dunites crop out at Mt. Arpone and Mt. Musinè. Field evidence indicates that pristine porphyroclastic spinel lherzolites are transformed to coarse granular spinel harzburgites, which are in turn overprinted by plagioclase peridotites, while strongly depleted spinel harzburgite and dunite bands and bodies replace the plagioclase peridotites. On the northern flank of Mt. Arpone, deformed, porphyroclastic (lithospheric) lherzolites, with diffuse pyroxenite banding, represent the oldest spinel-facies rocks. They show microstructures of a composite subsolidus evolution, suggesting provenance from deeper (asthenospheric) mantle levels and accretion to the lithosphere. These protoliths are locally transformed to coarse granular (reactive) spinel harzburgites and dunites, which show textures reminiscent of melt/rock reaction and geochemical characteristics suggesting that they are products of peridotite interaction with reactively percolating melts. Geochemical data and modelling suggest that <1–5% fractional melting of spinel-facies DMM produced the injected melts. Plagioclase peridotites are hybrid rocks resulting from pre-existing spinel peridotites and variable enrichment of plagioclase and micro-gabbroic material by percolating melts. The impregnating melts attained silica-saturation, as testified by widespread orthopyroxene replacement of olivine, during open system migration in the lithosphere. At Mt. Musinè, coarse granular spinel harzburgite and dunite bodies replace the plagioclase peridotites. Most of these replacive, refractory peridotites have interstitial magmatic clinopyroxene with trace element compositions in equilibrium with MORB, while some Cpx have REE-depleted patterns suggesting transient geochemical features of the migrating MORB-type melts, acquired by interaction with the ambient plagioclase peridotite. These replacive spinel harzburgite and dunite bodies are interpreted as channels exploited for focused and reactive migration of silica-undersaturated melts with aggregate MORB compositions. Such melts were unrelated to the silica-saturated melts that refertilized the pre-existing plagioclase peridotites. Finally, MORB melt migration occurred along open fractures, now recorded as gabbroic dikes. Our data document the complexity of rock-types and mantle processes in the South Lanzo peridotite massif and describe a composite tectonic and magmatic scenario that is not consistent with the “asthenospheric scenario” proposed by previous authors. We envisage a “transitional scenario” in which extending subcontinental lithospheric mantle was strongly modified (both depleted and refertilized) by early melts with MORB-affinity formed by decompression partial melting of the upwelling asthenosphere, during pre-oceanic rifting and lithospheric thinning in the Ligurian Tethys realm. 相似文献
12.
“Lower-crustal suite” xenoliths occur in “on-craton” and “off-craton” kimberlites located across the south-western margin of the Kaapvaal craton, southern Africa. Rock types include mafic granulite (plagioclase-bearing assemblages), eclogite (plagioclase-absent assemblages with omphacitic clinopyroxene) and garnet pyroxenite (“orthopyroxene-bearing eclogite”). The mafic granulites are subdivided into three groups: garnet granulites (cpx + grt + plag + qtz); two pyroxene garnet granulites (cpx + opx + grt + plag); kyanite granulites (cpx + grt + ky + plag + qtz). Reaction microstructures preserved in many of the granulite xenoliths involve the breakdown of plagioclase by a combination of reactions: (1) cpx + plag → grt + qtz; (2) plag → grt + ky + qtz; (3) plag → cpx (jd-rich) + qtz. Compositional zoning in minerals associated with these reactions records the continuous transition from granulite facies mineral assemblages and pressure (P) — temperature (T) conditions to those of eclogite facies. Two distinct P-T arrays are produced: (1) “off-craton” granulites away from the craton margin define a trend from 680 °C, 7.5 kbar to 850 °C, 12 kbar; (2) granulite xenoliths from kimberlites near the craton margin and “on-craton” granulites produce a trend with similar geothermal gradient but displaced to lower T by ˜ 100 °C. Both P-T fields define higher geothermal gradients than the model steady state conductive continental geotherm (40 mWm2) and are not consistent with the paleogeotherm constructed from mantle-derived garnet peridotite xenoliths. A model involving intrusion of basic magmas around the crust/mantle boundary followed by isobaric cooling is proposed to explain the thermal history of the lower crust beneath the craton margin. The model is consistent with the thermal evolution of the exposed Namaqua-Natal mobile belt low-pressure granulites and the addition of material from the mantle during the Namaqua thermal event (c. 1150 Ma). The xenolith P-T arrays are not interpreted as representing paleogeotherms at the time of entrainment in the host kimberlite. They most likely record P-T conditions “frozen-in” during various stages of the tectonic juxtaposition of the Namaqua Mobile Belt with the Kaapvaal craton. 相似文献
13.
The recent recognition that long period (i.e., of the order of hours) electromagnetic induction studies could play a major role in the detection of the asthenosphere has led to much interest amongst the geophysical and geological communities of the geomagnetic response functions derived for differing tectonic environments. Experiments carried out on the ocean bottom have met with considerable success in delineating the “electrical asthenosphere”, i.e., a local maximum in electrical conductivity (minimum in electrical resistivity) in the upper mantle.In this paper, observations of the time-varying magnetic field recorded in three regions of Scandinavia, northern Sweden (Kiruna—KIR), northern Finland/northeastern Norway (Kevo—KEV) and southern Finland (Sauvamaki—SAU), are analysed in order to obtain estimates of the inductive response function, C( ω), for each region. The estimated response functions are compared with one from the centre of the East European Platform (EEP), and it is shown that the induced eddy currents, at periods of the order of 10 3–10 4 s, in the three regions flow much closer to the surface than under the platform centre. Specifically, at a period of ~3000 s, these currents are flowing at depths of the order of: KEV—120 km; KIR—180 km; SAU—210 km; EEP—280 km; implying that the transition to a conducting zone, of σ -0.2 S/m, occurs at around these depths. One-dimensional inversion of
and
shows that there must exist a good conducting zone, of σ = 0.1–1.0 S/m, under each of the two regions, of 40 km minimum thickness, at depths of: KEV 105–115 km; KIR 160–185 km. This is to be contrasted with EEP, where the ρ- d profile displays a monotonically decreasing resistivity with depth, reaching σ~0.1 S/m at > 300 km.Finally, a possible temperature range for the asthenosphere, consistent with the deduced conducvitity, is discussed. It is shown that, at present, there is insufficient knowledge of the conditions (water content, melt fraction, etc.) likely to prevail in the asthenosphere to narrow down the probable range of 900°–1500°C. 相似文献
14.
Miyake, N. and Denda, A., 1993. Utilization of underground spaces in urban areas: Urban geo-grid plan. In: M. Langer, K. Hoshino and K. Aoki (Editors). Engineering Geology in the Utilization of Underground Space. Eng. Geol., 35: 175–181. The Urban Geo-grid Plan aims for a systematic and better coexistence of above-ground and underground areas through the utilization of underground space presently not in use, without interfering with existing urban functions. There would be “base points” supporting regional functions which are laid out in a grid connected by “lines”. Base points would consist of “grid points” and “grid stations”, which serve as control centers for grid points, the grid points and grid stations being interconnected by tunnels to form an “underground network”. Normally, a base point would be used for multiple purposes to supplement regional functions which are inadequate, as well as urban functions, and in emergencies, such as earthquake disasters, it would demonstrate information and evacuations functions. 相似文献
15.
Mineral inclusions recovered from 100 diamonds from the A154 South kimberlite (Diavik Diamond Mines, Central Slave Craton, Canada) indicate largely peridotitic diamond sources (83%), with a minor (12%) eclogitic component. Inclusions of ferropericlase (4%) and diamond in diamond (1%) represent “undetermined” parageneses. Compared to inclusions in diamonds from the Kaapvaal Craton, overall higher CaO contents (2.6 to 6.0 wt.%) of harzburgitic garnets and lower Mg-numbers (90.6 to 93.6) of olivines indicate diamond formation in a chemically less depleted environment. Peridotitic diamonds at A154 South formed in an exceptionally Zn-rich environment, with olivine inclusions containing more than twice the value (of 52 ppm) established for normal mantle olivine. Harzburgitic garnet inclusions generally have sinusoidal rare earth element (REEN) patterns, enriched in LREE and depleted in HREE. A single analyzed lherzolitic garnet is re-enriched in middle to heavy REE resulting in a “normal” REEN pattern. Two of the harzburgitic garnets have “transitional” REEN patterns, broadly similar to that of the lherzolitic garnet. Eclogitic garnet inclusions have normal REEN patterns similar to eclogitic garnets worldwide but at lower REE concentrations. Carbon isotopic values (δ13C) range from − 10.5‰ to + 0.7‰, with 94% of diamonds falling between − 6.3‰ and − 4.0‰. Nitrogen concentrations range from below detection (< 10 ppm) to 3800 ppm and aggregation states cover the entire spectrum from poorly aggregated (Type IaA) to fully aggregated (Type IaB). Diamonds without evidence of previous plastic deformation (which may have accelerated nitrogen aggregation) typically have < 25% of their nitrogen in the fully aggregated B-centres. Assuming diamond formation beneath the Central Slave to have occurred in the Archean [Westerlund, K.J., Shirey, S.B., Richardson, S.H., Gurney, J.J., Harris, J.W., 2003b. Re–Os systematics of diamond inclusion sulfides from the Panda kimberlite, Slave craton. VIIIth International Kimberlite Conference, Victoria, Canada, Extended Abstracts, 5p.], such low aggregation states indicate mantle residence at fairly low temperatures (< 1100 °C). Geothermometry based on non-touching inclusion pairs, however, indicates diamond formation at temperatures around 1200 °C. To reconcile inclusion and nitrogen based temperature estimates, cooling by about 100–200 °C shortly after diamond formation is required. 相似文献
16.
The potassic igneous rock suite (with molar K 2O/Na 2O > 1) can be divided into an “orogenic” subgroup that occur in subduction-related tectonic settings and an “anorogenic” sub-group that are confined to stable continental settings. Representatives of both sub-groups possess trace element and isotopic features consistent with the contamination of their magma sources by incompatible element rich and isotopically evolved “metasomatic” components. It is argued here that these metasomatic components are principally derived from subducted lithosphere, including subducted sediments. Most examples of orogenic potassic magmatism (e.g. Italian potassic rocks, Spanish lamproites, Sunda arc leucitites) have trace-element and Sr, Nd and Pb isotopic characteristics consistent with the contamination of their mantle sources by a component derived from marine sediments. Anorogenic sub-group potassic magmas have generally similar incompatible trace element and Sr and Nd isotopic characteristics to those of orogenic potassic magmas, but many examples have unusual Pb isotopic compositions with unradiogenic 206Pb/ 204Pb. Modern marine sediments characteristically have low U/Pb ratios and the unradiogenic 206Pb/ 204Pb of anorogenic potassic magmas may have evolved during long-term storage of subducted sediments (or components derived from them) within the subcontinental lithosphere. These unusual Pb isotopic compositions require substantial time periods (> 1 Ga) to have elapsed between the fractionation events lowering the U/Pb ratio (i.e. erosion and sedimentation at the Earth's surface) and subsequent potassic magmatism and it is therefore not surprising that most examples of anorogenic potassic magmatism are not associated with recent subduction processes. Although the eruption of potassic magmas is commonly related to rifting or hotspot activity, these processes do not necessarily play an important role in the genesis of the unusual sources from which potassic magmas are derived. 相似文献
17.
对山西隆起区中新生代构造演化的认识,是探讨华北陆块演化、破坏等科学问题的基础。在中新生代期间,不论是岩石圈的差异演化、构造体制转换、岩浆活动,还是地貌反转,位于华北陆块中心部位的山西地区总是处于过渡带的位置。该地理位置说明了该区在华北陆块演化研究中的重要性。在近年1∶5万、1∶25万区域地质调查成果及以往研究资料的基础上,以中生代区域性断裂、新生代汾渭裂谷及山体隆升为主要研究内容,探讨山西隆起区中新生代岩石圈从增厚到减薄、构造体制从挤压到伸展转换,以及随华北陆块一起经历地貌格局从东高西低到西高东低转换的构造演化进程。研究认为,山西地区中新生代构造体系的发育从属统一的区域动力学环境,周边板块之间的相互作用引起深部软流圈变化: 软流圈下沉,岩石圈则相对增厚,地壳垂向伸展,横向收缩; 软流圈上涌,岩石圈相应减薄,地壳水平伸展。软流圈的变化对该地区中新生代构造变形、沉积格局、岩浆活动等方面起着控制作用。 相似文献
18.
This paper investigates the effect of shear heating in the asthenosphere on the thermal structure of the upper mantle. Equations describing the motion of the lithosphere over the asthenosphere in the presence of a strongly temperature-dependent stress-strain rate relation are derived and solved with the help of several approximations. These approximations are shown to be valid under conditions appropriate for the earth.Two sets of solutions are found. For one set (the “subcritical” solutions) a normal shear stress—velocity relation is found for small stresses. The velocity increases as the stress increases, reaching a maximum velocity σ c for a critical stress σ c. The subcritical solutions have a negligible effect on the thermal structure of the earth, even at the critical stress. The other set of solutions (the “supercritical” solutions) has the bizarre property that a decrease of applied shear stress leads to an increase of velocity. Thus, as the shear stress goes to zero, the velocity becomes infinite. At larger shear stresses the velocity decreases until it reaches σc at a stress σc (the two sets of solutions share this point in common). There are no steady solutions of any kind for shear stresses in excess of σc. We discard the supercritical solutions as candidates for the thermal structure of the earth on the basis of their instability to small perturbations of applied stress and temperature.The realm of subcritical solutions (stress less than σc, velocity less than σc) thus defines a regime of plate motion in which the thermal effects of shear heating are negligible. If the shear stresses acting on plates exceed σc, however, new physical processes must come into play to dissipate the excess heat generated. Assuming that the velocities of plates on the earth today are less than σc, relative to the deep mantle, a strict upper limit of a few tens of bars can be derived for σc, corresponding to effective viscosities of ca. 10 19 poise in the asthenosphere. 相似文献
19.
The relationship between the breakup of eastern Gondwanaland and the Kerguelen plume activity is a subject of debate. The Cona mafic rocks are widely exposed in the Cona area of the eastern Himalaya of south Tibet, and are studied in order to evaluate this relationship. Cona mafic rocks consist predominantly of massive basaltic flows and diabase sills or dikes, and are divided into three groups. Group 1 is composed of basaltic flows and diabase sills or dikes and is characterized by higher TiO 2 and P 2O 5 content and OIB-like trace element patterns with a relatively large range of Nd( T) values (+ 1.84 to + 4.67). A Group 1 diabase sill has been dated at 144.7 ± 2.4 Ma. Group 2 consists of gabbroic sills or crosscutting gabbroic intrusions characterized by lower TiO 2 and P 2O 5 content and “depleted” N-MORB-like trace element patterns with relatively higher, homogeneous Nd( T) values (+ 5.68 to + 6.37). A Group 2 gabbroic diabase dike has been dated at 131.1 ± 6.1 Ma. Group 3 basaltic lavas are interbedded with the Late Jurassic–Early Cretaceous pelitic sediments; they have compositions transitional between Groups 1 and 2 and flat to slightly enriched trace element patterns. Sr–Nd isotopic data and REE modeling indicate that variable degrees of partial melting of distinct mantle source compositions (enriched garnet–clinopyroxene peridotite for Group 1 and spinel-lherzolite for Group 2, respectively) could account for the chemical diversity of the Cona mafic rocks. Geochemical similarities between the Cona mafic rocks and the basalts probably created by the Kerguelen plume based on spatial–temporal constraints seem to indicate that an incubating Kerguelen plume model is more plausible than a model of normal rifting (nonplume) for the generation of the Cona mafic rocks. Group 1 is interpreted as being related to the incubating Kerguelen plume–lithosphere interaction; Group 2 is likely related to an interaction between anhydrous lithosphere and rising depleted asthenosphere enriched by a “droplet” originating from the Kerguelen plume, while Group 3 may be attributed to thermal erosion resulting in the partial melting of lithosphere during the long-term incubation of a magma chamber/pond at a shallow crustal level. The Cona mafic rocks are probably related to a progressively lithospheric thinning beneath eastern Gondwanaland from 150–145 Ma to 130 Ma. Our new observations seem to indicate that the Kerguelen plume may have started its incubation as early as the latest Jurassic or earliest Cretaceous period and that the incubating Kerguelen plume may play an active role in the breakup of Greater India, eastern India, and northwestern Australia. 相似文献
20.
Recently completed investigations of the crustal structure on ancient shields of the East European platform carried out with the method of “deep seismic sounding” (D.S.S.) have drastically changed the previous notions about the deep structure of shields in general. In the upper crust, in the so-called “granitic” layer, complex anticlinal and synclinal structures as well as numerous faults, thrusts, etc., have been identified. A flattening of steeply dipping seismic interfaces with depth is observed. The crustal thickness in different tectonic zones ranges from 30 to 60 km. It is shown that the M-structure correlates with the sub-surface tectonics in the Ukrainian Shield. 相似文献
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