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1.
The state of stress around cavities is heterogeneous. Consequently, the state of fracture may also vary from point to point. Under compressive loading, cavity may be in one of four possible states: pre-fracture (pre-microfracture initiation), microfracture propagation (initiation to the onset of dilata dilatancy to failure) and post-failure. These four states are separated by the crack initiation stress, the crack damage stress and the failure stress. fracture events with confining pressure is examined with reference to three intact rocks: a brittle granite, a semi-brittle limestone and a ductile sal The maximum principal stress (σ1) at crack initiation, crack damage (onset of dilatancy), yielding and failure are established as a function stress (σ3). For a single intact rock, all four fracture events can be represented using one function (the Rocker function) with a single fac various fracture states. The proposed fracture criteria, based on the experimental data, are combined with the existing state of stress to prepare a fracture map around an elli intact Lac du Bonnet granite. The state of stability is expressed through a newly defined stability factor, the unconfined strength ratio (USR), wh to the traditional safety factor. In contrast to the conventional safety factor in rock mechanics (SFstrength/σ1), which is σ3 3–σ1, space of the stability diagram. 相似文献
2.
This paper reports the integrated application of petrographic and Sm–Nd isotopic analyses for studying the provenance of the Neoproterozoic Maricá Formation, southern Brazil. This unit encompasses sedimentary rocks of fluvial and marine affiliations. In the lower fluvial succession, sandstones plot in the “craton interior” and “transitional continental” fields of the QFL diagram. Chemical weathering probably caused the decrease of the 147Sm/ 144Nd ratios to 0.0826 and 0.0960, consequently lowering originally > 2.0 Ga TDM ages to 1.76 and 1.81 Ga. 143Nd/ 144Nd ratios are also low (0.511521 to 0.511633), corresponding to negative εNd present-day values (− 21.8 and − 19.6). In the intermediate marine succession, sandstones plot in the “dissected arc” field, reflecting the input of andesitic clasts. Siltstones and shales reveal low 143Nd/ 144Nd ratios (0.511429 to 0.511710), εNd values of − 18.1 and − 23.6, and TDM ages of 2.16 and 2.37 Ga. Sandstones of the upper fluvial succession have “dissected arc” and “recycled orogen” provenance. 143Nd/ 144Nd isotopic ratios are also relatively low, from 0.511487 to 0.511560, corresponding to εNd values of − 22.4 and − 21.0 and TDM of 2.07 Ga. A uniform granite–gneissic basement block of Paleoproterozoic age, with subordinate volcanic rocks, is suggested as the main sediment source of the Maricá Formation. 相似文献
3.
This review, in honor of Ilmari Haapala's retirement, reflects on lessons learned from studies of three granitic systems in western North America: (1) Mesoproterozoic samples from west Texas and east New Mexico; (2) Laramide granitic systems associated with porphyry-copper deposits in Arizona; and (3) granites of the Colorado Mineral Belt. The studies elucidate relationships amongst tectonic setting, source material, and magma chemistry. Mesoproterozoic basement samples are from two different felsic suites with distinct elemental and isotopic compositions. The first suite, the “plutonic province”, is dominantly magnesian, calc-alkalic to alkali-calcic, and metaluminous. It has low K2O/Na2O and Rb/Sr, and Nd model ages of 1.56 to 1.40 Ga. The second suite, the “Panhandle igneous complex”, is magnesian, metaluminous, alkalic, and is part of the Mesoproterozoic belt of magmatism that extends from Finland to southwestern United States. Samples from the Panhandle igneous complex demonstrate three episodes of magmatism: the first pulse was intrusion of quartz monzonite at 1380 to 1370 Ma; the second was comagmatic epizonal granite and rhyolite at 1360 to 1350 Ma. Both of these rock types are high-K to slightly ultra-high-K. The third pulse at 1338 to 1330 Ma was intrusion of ultra-high-K quartz syenite. Nd model ages (1.94 to 1.52 Ga) are distinct from those of the “plutonic province” and systematically older than crystallization ages, implying a substantial crustal input to the magmas. At the Sierrita porphyry-copper deposit in the Mazatzal Province of southeastern Arizona, trace element, Sr, and Nd isotopic compositions were determined for a suite of andesitic and rhyolitic rocks (67 Ma) intruded by granodiorite and granite. Isotopic composition and chemical evolution are well correlated throughout the suite. Andesite has the least negative initial εNd (−4.3) and lowest 87Sr/86Sri (0.7069). It is also the oldest and chemically most primitive, having low concentrations of Rb, SiO2, and high concentrations of transition elements. These parameters change through the system to the youngest unit (granite), which has the most negative εNd (−8.5), the highest 87Sr/86Sri (0.7092), and is chemically most evolved. Correlation between chemical and Nd isotopic evolution probably resulted from a continuous process of progressive assimilation, in which mafic magmas invade and incorporate continental crust. Deposits in Arizona with εNd values more negative than the −8.5 of Sierrita lie in the older Yavapai province in the northwestern part of the state. The difference in the most negative epsilon Nd implies that Nd isotopic signature is sensitive to the age of the Precambrian domain. The granites from the Colorado Mineral Belt were emplaced during the transition from Laramide convergence to mid-Tertiary extension. Three different groups of granites are recognized. The first is Laramide and was formed during assimilation-fractional crystallization involving lower crustal mafic source materials; the second and third groups are mid-Tertiary and represent intracrustal melting of heterogeneous sources. This change in source regions and melt regimes in transition from convergence to extension is fundamental to the Mesozoic and Cenozoic evolution of western North America. 相似文献
4.
The present paper provides C- and N-stable isotope characteristics, N-contents and N-aggregation states for alluvial diamonds of known paragenesis from placers along the Namibian coast. The sample set includes diamonds with typical peridotitic and eclogitic inclusions and the recently reported “undetermined” suite of Leost et al. [Contrib. Mineral. Petrol. 145 (2003) 15] which resulted from infiltration of high temperature, carbonate-rich melts. δ13C-values range from −20.3‰ to −0.5‰ ( n=48) for peridotitic diamonds and from −38.5‰ to −1.6‰ ( n=45) for eclogitic diamonds. Diamonds belonging to the “undetermined” suite span a narrower range in δ13C from −8.5‰ to −2.7‰ ( n=13). When compared with previous studies, diamonds from Namibia are characterised by unusually low proportions of N-free (i.e. Type II) peridotitic and eclogitic diamonds (3% and 2%, respectively) and an unprecedented high proportion of N-rich diamonds (15% and 73%, respectively, have N-contents >600 ppm). δ15N-values for diamonds of the peridotitic, eclogitic and “undetermined” suites range from −10‰ to +13‰ without correlations with either N-content or δ13C. The similarity in N-isotopic composition and the N-rich character of diamonds belonging to the eclogitic, peridotitic and “undetermined” suites is striking and suggests a close genetic relationship. We propose that a large part of the diamonds mined in Namibia formed during metasomatic events of similar style that introduced carbon and nitrogen into a range of different host lithologies. 相似文献
5.
为研究循环荷载下原状红黏土路基的动力特性,采用SDT-20型动三轴仪对原状红黏土进行了分级循环动三轴试验,研究了围压、频率及动应力幅值对原状红黏土的动应力、动弹性模量-动应变（ E d-ε d）和动剪切模量-动剪切应变（ G d-γ d）关系曲线的影响规律。结果表明:原状红黏土的动应力-动应变关系曲线发展规律可采用Kondner模型进行描述;动应力随动应变先急剧增大后趋于平稳,并给出了急剧增加时动应变的取值范围,即0%～0.05%;分析了不同围压、振动频率下红黏土的动弹性模量及动剪切模量的变化规律,当动应变小于临界值时,红黏土动弹性模量随动应变的增大而增大;当动应变大于临界值时,红黏土材料动弹性模量随动应变的增大而减小,动剪切模量具有相同变化规律;结合红黏土的动力特性变化规律,利用围压对动弹性模量进行折减,在Darendeli模型的基础上建立了红黏土路基动弹性模量、动剪切模量的分段预测模型,经拟合验证,本文分段模型的适用性较好,可预测分级循环荷载下红黏土的动弹性模量-动应变（E d-ε d）和动剪切模量-动剪切应变（G d-γ d）关系曲线发展趋势。 相似文献
6.
Compressional waves velocity VP was measured during long-term experiments in a high-pressure vessel (in the range [10–75] MPa for confining and pore pressures). Experiments were carried out on a granite specimen prepared by a controlled heating treatment at 510 °C, which generated thermal cracks. Data analysis is proposed by using an effective medium approach based on Kachanov's [Appl. Mech. Rev. 45 (1992)304] model. The elastic behaviour of the cracked rock is controlled by the crack density parameter which varies with confining and pore pressures due to crack closure. In order to model the progressive closure of cracks, we assume elliptical cracks with major axis 2c and aspect ratio . By using a conformal mapping technique, we derive the variation of the crack aspect ratio as a function of effective pressure, the effective pressure coefficient η depending on and Poisson's ratio ν0. As a result, we compute the crack density parameter and the elastic moduli of the cracked rock as a function of confining and pore pressures. To take into account the heterogeneity of the rock sample, a peak-like distribution of crack aspect ratios is introduced, which allows us to calculate the acoustic velocity VP for various effective pressures. Comparison is made between theoretical and experimental values and shows that this simple model captures the essential features of the acoustic velocity variation: an increase of VP when pore pressure is decreased followed by a plateau for a threshold pore pressure. Best consistency between theoretical and experimental velocity values is obtained by introducing a second crack population with a higher mean aspect ratio and an irreversible closure mechanism as effective pressure is cycled. 相似文献
7.
地应力、煤储层渗透率和煤储层压力等是影响煤层气开发的重要因素。通过分析新疆库拜煤田铁列克矿区注入/压降试井及原地应力测试数据,结合铁列克矿区煤层气井日产气量分析,研究了新疆库拜煤田铁列克矿区地应力变化规律及其对煤层气开发的影响,分析了铁西矿区和铁东矿区煤储层地应力特征及其对煤储层物性的影响。结果表明:(1)地应力状态在垂向上发生变化,埋深处于550~650 m、650~850 m和850~1 200 m时,地应力状态类型依次为 σH> σv> σh、 σH≈ σv> σh和 σv> σH> σh;(2)埋深850 m处既是垂直主应力和最大水平主应力的转换点也是渗透率趋势变化点,指示了地应力对渗透率的控制作用;(3)渗透率和煤储层压力与地应力分别呈负相关和正相关关系;(4)地应力对产能的负效应大于地应力对产能的正效应,使典型日产气量随着地应力的增大而减小;(5)铁西矿区和铁东矿区中部煤储层碎粒煤较发育、吸附孔体积和含气量均较大,是煤层气开发的有利区带。研究成果可为库拜煤田下一步煤层气开发提供理论指导。 相似文献
8.
Barite occurrences related to the Cenozoic (Late Alpine) low-temperature hydrothermal activity are present in the continental Ohře (Eger) Rift area. A specific, Ra-bearing type of barite has been known under the name “radiobarite” from this area since 1904. Revision of 12 localities revealed the presence of alleged radiobarite only in the Teplice (Lahošť–Jeníkov) and Karlovy Vary areas. Barite from other localities is radium-poor. Barite crystals showing concentric oscillation colour zoning totally prevail. Isomorphous substitution of Sr ( X×10 −1 to X×wt%), Ca ( X×10 −2 wt%) and Fe ( X×10 −1 wt%) for Ba was proved. Average SrO contents of 0.4 wt% are markedly exceeded in some samples from Lahošť–Jeníkov (max. 3.2 wt%) and Karlovy Vary (max. 4.9 wt%). Besides inclusions of stoichiometric iron disulphide, the same samples also contain iron disulphides with unusual high contents of Co (max. 12.2 wt%) and Ni (max. to 8.4 wt%). Specific activity of 238U in the studied barites is very low while that of 226Ra reaches 8 Bq/g in several samples. Therefore, 226Ra is not in equilibrium with its parent uranium. These “radiobarites” or their parts must be therefore relatively young, not older than 10–15 ka. Very low uranium contents (<0.4 ppm) were also confirmed by neutron activation analyses of barite samples. Unit-cell dimensions refined from X-ray powder diffraction data do not show any systematic variation with the measured chemical composition. Their values agree with the data given in the literature. Reflection half-widths, however, seem to correlate with chemistry. Peaks are wider in samples from Lahošť–Jeníkov and Karlovy Vary. Sulphur and oxygen stable isotope compositions of the Cenozoic barite mineralization of Teplice area are very uniform (δ34S values between 3.9‰ and 7.1‰ CDT, and δ18O values between 6.1‰ and 7.7‰ SMOW), while the barites of Děc˘ín area show more variable sulphur sources. Sulphate derived from sediments of the Tertiary Most Basin seems to dominate for the Teplice area, while Cretaceous sediments are a more probable sulphur source in the Děc˘ín area. Calculation of oxygen isotope composition of hydrothermal fluids based on fluid inclusion homogenization temperatures and barite δ18O data shows δ18Ofluid values in the range of meteoric waters or δ18O – shifted deep circulating meteoric or basinal waters. 相似文献
9.
岩体冻融损伤机制为温度降低使岩体中的水发生相变、体积膨胀、产生冻胀力的作用,岩体中的微裂隙在冻胀力的作用下扩展延伸,温度升高时,融化的水进入新的裂隙,冻结成冰再次产生冻胀作用,反复循环使裂隙网络扩展,最终造成岩体的损伤。基于此,从弹塑性力学、断裂力学的角度出发,研究了在冻胀力的作用下单裂隙扩展特性,推导了冻胀力与裂纹扩展长度之间的关系,利用Mori-Tanaka方法建立了岩体宏观损伤量与冻胀力及冻融次数之间的关系式,讨论了岩体弹性模量与冻融次数、冻胀应力以及渗透系数的变化规律,并与试验结果进行了比较分析。结果表明,岩体在冻融循环条件下的弹性模量随冻融次数的增加呈非线性减小;冻胀应力越大,岩体弹性模量衰减越快;岩体的渗透系数越大,弹性模量衰减越慢。 相似文献
10.
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
11.
Experimental studies of synthetic and natural basalt systems suggest that conditions of magma genesis and fractionation depend fundamentally on mantle temperatures and lithospheric stress fields. In general, compressional settings are more conducive to polybaric fractionation than extensional settings and in this regard, the Anatolian magmatic province offers a natural laboratory for comparing near-coeval basalt eruptions as a function of regional tectonics — compressional (collision-related) régimes dominating in eastern Anatolia and extensional tectonics characterizing a western province related to Aegean Sea opening. Projection of Plio-Quaternary basalt normative compositions from the Western Anatolia Extensional Province (WAEP), the Central Anatolian ‘Ova’ Province (CAOP), and Eastern Anatolia Compressional Province (EACP) are projected onto Ol–Ne–Cpx and Pl–Cpx–Ol planes in the simplified basalt system (Ne–Cpx–Ol–Qz), each showing distinctive liquid lines of descent. WAEP basalts are mostly constrained by low-pressure (< 0.5 GPa) cotectics while CAOP and EACP compositions conform to moderate and/or high-pressure (0.8–3.0 GPa) cotectics. Overall, a quasi-linear shift from moderate and/or high-pressure to low-pressure equilibria matches the westward transgression from compressional east Anatolia to the extensional west Anatolian–Aegean region. Comparison of their respective primary (mantle-equilibrated) magmas–simulated by normalizing their compositions to MgO = 15 wt.% (Mg-15)–with parameterized anhydrous and H 2O-undersaturated experimental melts suggests they segregated from spinel- to garnet-lherzolite mantle facies at pressures between c. 2 and 3 GPa (c. 70–100 km depth) under H 2O-undersaturated conditions. Interpolated potential temperatures ( Tp) and lithospheric stretching factors ( β) range as follows: (1) eastern Anatolian basalts associated with the Arabian foreland show Tp varying between 1250 and 1400 °C (except for the Karacalidag alkali basalts, south of the Bitlis–Zagros fracture zone, for which Tp ranges up to 1450 °C), for β values of 1.2–1.8. Tp values for central Anatolia (e.g. Sivas) range between 1300 and 1375 °C (except for Karapinar, Egrikuyu and Hasandag, which show < 1150 °C), and β values of 1.3–1.4. For western Anatolian basalts, Tp range mostly between 1250 and 1330 °C, except for a single value for Canakkale of 1400 °C and Kula sample showing Tp < 1200 °C, and β values of 1.3–2.0. Variation of these conditions is as great or greater than that between provinces, although there are clearly significant constraints on the inferred polybaric to low-pressure isobaric fractionation régimes. Covariation of total FeO, TiO 2, La/Yb, Ce/Sm, Zr/Y and Zr/Nb reflects small but significant differences in bulk composition and ambient melt fraction while the covariance of Ce/Sm and Sm/Yb is consistent with the segregation of primitive melts at the spinel- to garnet-lherzolite transition. 相似文献
12.
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
13.
大光包滑坡是2008年汶川地震触发的最大规模滑坡, 平硐揭示该滑坡滑带形成于糜棱质化、泥化的层间错动带, 且地下水从该错动带渗出。为分析强震过程饱水错动带材料动力特性, 本文作者在平硐内取不同层位错动带材料C 1(粉粒为主)、C 2(粉黏粒为主)和C 3(砂粒为主), 室内进行了系列动三轴试验, 得出如下结论:(1)在设计动荷载(40 kPa、60 kPa和80 kPa)下, C 1、C 2孔压增长缓慢, 没有液化; 而C 3孔压增长迅速, 5%应变时动孔压增长到围压值, 试样液化。(2)试样动剪应力值与振动次数关系较大; 随着振次增加C 3动剪应力值明显减小, 而C 1、C 2动剪应力小幅度减小; (3)在相同动应变下, C 3动应力和动弹性模量最大, 动阻尼比最小; 而C 2动应力和弹性模量最小, 动阻尼比最大。结果表明, 大光包层间错动带在地震中可能发生液化, 从而导致错动带抗剪强度骤然降低, 滑坡快速启动。 相似文献
14.
采用自主研发的新型固化剂对天津城市污泥进行固化处理,通过GCTS真三轴仪对污泥固化土进行不固结不排水试验,探讨其在干湿循环作用下的应力-应变特征和强度指标变化规律。试验结果表明:污泥固化土应力-应变曲线在初始阶段近似表现为线性关系,同等条件下,破坏应力随中主应力比b的增大而增大;相同b值下,破坏应力随干湿循环次数的增大而逐渐减小。经过干湿循环1,3,5,7,10次之后,不同围压下污泥固化土的破坏应力值均呈现下降趋势。当循环次数超过5次后,其降低幅度趋于平缓。在b值较小、循环次数小于3时,应力-应变曲线产生应变软化现象,随着围压和b值的增大表现为硬化型。污泥固化土c、φ值随干湿循环次数的增大呈现出降低趋势,并最终趋于稳定。在此基础上,对不同中主应力比条件下的c、φ值变化规律进行分析,分别建立其与干湿循环次数和中主应力比之间的关系式,并构建出能够考虑不同围压及中主应力比影响的初始弹性模量E i和主应力差渐近值（σ 1?σ 3） ulti预测公式。 相似文献
15.
Late Devonian (385−370 Ma) granitoid intrusions in the Meguma Zone of southwestern Nova Scotia represent two geographically separate magmatic suites that show subtly different lithological, geochemical and isotopic characteristics. “Central intrusions” crop out with satellite mafic-intermediate intrusions, range in composition from granodiorite to leucogranite, contain two micas, have exclusively peraluminous compositions (molar A/CNK 1.1-1.3), variably high values for FeO T (0.4–6.0 wt.%), Ba (5–980 ppm), Y (6–50 ppm), Pb (2–50 ppm), Ga (11–53 ppm), 87Sr/ 86Sri (0.7081-0.7130), δ 18O (9.8–13.0) and δ 34S (4.5–11.9), in conjunction with low values for εNd (−1 to −6.5). In contrast, “peripheral plutons” crop out with synplutonic mafic-intermediate intrusions, range in composition from tonalite to leucogranite, may contain minor hornblende, have dominantly peraluminous compositions (molar A/CNK 0.9-1.3), variably high concentrations of TiO 2 (0.1-1.1 wt.%), Al 2O 3 (12.0–19.7 wt.%), CaO (0.2–4.9 wt.%), Sr (7–720 ppm), Cr (3–111 ppm) and V (1–136 ppm), higher εNd values (−2.0 to 3.2), and lower values for 87Sr/ 86Sri (0.7040-0.7079), δ 188O (7.6–10.5) and δ 34S (0–4.6). Such regional diversity is explained by inferring that upper crustal contamination dominated the central granitoid compositions and mixing with mantle-derived mafic-intermediate magmas dominated peripheral granitoid compositions. However, additional contributions from heterogeneous lower crust cannot be excluded. 相似文献
16.
The Main Central Thrust (MCT) is a tectono-metamorphic boundary between the Higher Himalayan crystallines (HHC) and Lesser Himalayan metasediments (LHS), reactivated in the Tertiary, but which had already formed as a collisional boundary in the Early Paleozoic. To investigate the nature of the MCT, we analyzed whole-rock Nd isotopic ratios of rocks from the MCT and surrounding zones in the Taplejung–Ilam area of far-eastern Nepal, Annapurna–Galyang area of central Nepal, and Maikot–Barekot area of western Nepal. We define the MCT zone as a ductile–brittle shear zone between the upper MCT (UMCT) and lower MCT (LMCT). The protoliths of the MCT zone may provide critical constraints on the tectonic evolution of the Himalaya. The LHS is lithostratigraphically divided into the upper and lower units. In the Taplejung–Ilam area, different lithologic units and their εNd (0) values are as follows; HHC (− 10.0 to − 18.1), MCT zone (− 18.5 to − 26.2), upper LHS unit (− 17.2), and lower LHS unit (− 22.0 to − 26.9). There is a distinct gap in the εNd (0) values across the UMCT except for the southern frontal edge of the Ilam nappe. In the Annapurna–Galyang and Maikot–Barekot areas, different lithologic units and their εNd (0) values are as follows; HHC (− 13.9 to − 17.7), MCT zone (− 23.8 to − 26.2 except for an outlier of − 12.4), upper LHS unit (− 15.6 to − 26.8), and lower LHS unit (− 24.9 to − 26.8). These isotopic data clearly distinguish the lower LHS unit from the HHC. Combining these data with the previously published data, the lowest εNd (0) value in the HHC is − 19.9. We regard rocks with εNd (0) values below − 20.0 as the LHS. In contrast, rocks with those above − 19.9 are not always the HHC, and some parts of them may belong to the LHS due to the overlapping Nd isotopic ratio between the HHC and LHS. Most rocks of the MCT zone have Nd isotopic ratios similar to those of the LHS, but very different from those of the HHC. The spatial patterns in the distribution of εNd (0) value around the UMCT suggest no substantial structural mixing of the HHC and LHS during the UMCT activities in the Tertiary. A discontinuity in the spatial distribution of εNd (0) values is laterally continuous along the UMCT throughout the Himalayas. These facts support the theory that the UMCT was originally a material boundary between the HHC and LHS, suggesting the MCT zone was mainly developed with undertaking a role of sliding planes during overthrusting of the HHC in the Tertiary. 相似文献
17.
We present the results of a thrust fault reactivation study that has been carried out using analogue (sandbox) and numerical modelling techniques. The basement of the Pannonian basin is built up of Cretaceous nappe piles. Reactivation of these compressional structures and connected weakness zones is one of the prime agents governing Miocene formation and Quaternary deformation of the basin system. However, reactivation on thrust fault planes (average dip of ca. 30°) in normal or transtensional stress regimes is a problematic process in terms of rock mechanics. The aim of the investigation was to analyse how the different stress regimes (extension or strike-slip), and the geometrical as well as the mechanical parameters (dip and strike of the faults, frictional coefficients) effect the reactivation potential of pre-existing faults. Results of analogue modelling predict that thrust fault reactivation under pure extension is possible for fault dip angle larger than 45° with normal friction value (sand on sand) of the fault plane. By making the fault plane weaker, reactivation is possible down to 35° dip angle. These values are confirmed by the results of numerical modelling. Reactivation in transtensional manner can occur in a broad range of fault dip angle (from 35° to 20°) and strike angle (from 30° to 5° with respect to the direction of compression) when keeping the maximum horizontal stress magnitude approximately three times bigger than the vertical or the minimum horizontal stress values. Our research focussed on two selected study areas in the Pannonian basin system: the Danube basin and the Derecske trough in its western and eastern part, respectively. Their Miocene tectonic evolution and their fault reactivation pattern show considerable differences. The dominance of pure extension in the Danube basin vs. strike-slip faulting (transtension) in the Derecske trough is interpreted as a consequence of their different geodynamic position in the evolving Pannonian basin system. In addition, orientation of the pre-existing thrust fault systems with respect to the Early to Middle Miocene paleostress fields had a major influence on reactivation kinematics. As part of the collapsing east Alpine orogen, the area of the Danube basin was characterised by elevated topography and increased crustal thickness during the onset of rifting in the Pannonian basin. Consequently, an excess of gravitational potential energy resulted in extension (σv > σH) during Early Miocene basin formation. By the time topography and related crustal thickness variation relaxed (Middle Miocene), the stress field had rotated and the minimum horizontal stress axes (σh) became perpendicular to the main strike of the thrusts. The high topography and the rotation of σh could induce nearly pure extension (dip-slip faulting) along the pre-existing low-angle thrusts. On the contrary, the Derecske trough was situated near the Carpathian subduction belt, with lower crustal thickness and no pronounced topography. This resulted in much lower σv value than in the Danube basin. Moreover, the proximity of the retreating subduction slab provided low values of σh and the oblique orientation of the paleostress fields with respect to the master faults of the trough. This led to the dominance of strike-slip faulting in combination with extension and basin subsidence (transtension). 相似文献
18.
The δD SMOW values of sedimentary kaolins from the western border of the Bohemian Massif, northeast Bavaria, that did not suffer a deep burial (less than 1000 m) nor a hydrothermal overprint, change systematically from Late Triassic (−50‰) to Mid-Jurassic and Late Cretaceous (−56‰ to −66‰) to Upper Oligocene–Mid-Miocene (−77‰ to −90‰). All analyzed clays are far from hydrogen isotope equilibrium with present-day meteoric waters. Combined oxygen and hydrogen isotope data of selected samples indicate low temperatures of formation (<30°C) and no evidence for preferential D/H exchange with younger waters. The hydrogen isotopic evolution of kaolins is interpreted as reflecting a systematic isotopic change of paleo-meteoric waters in that region. This can be related mainly to the northward drift of stable Europe after the break-up of Gondwana. Increasing continentality, surface uplift and global cooling are additional factors responsible for decreasing δD SMOW values since the Mid-Cretaceous. Kaolinite hydrogen isotope ratios of two large residual economic deposits (Tirschenreuth: δDSMOW=−80‰ to −76‰; Hirschau–Schaittenbach: δDSMOW=−70‰ to −63‰) can be used in combination with additional geological evidence to constrain the timing of weathering in these areas. A late Early Cretaceous kaolinization age is suggested for the Early Triassic sandstone-hosted deposits near Hirschau–Schnaittenbach, whereas a Late Oligocene to Mid-Miocene age is indicated for the Carboniferous granite-hosted Tirschenreuth deposits. 相似文献
19.
Mesozoic alkaline intrusive complexes are widespread in the southern portion of the North China Craton and can provide some important constraints on the evolution of the Mesozoic lithosphere beneath the region. Three selected intrusive complexes (Tongshi, Hongshan, and Longbaoshan) are generally high in alkalis (K 2O+Na 2O=913 wt.%) and Al 2O 3 (1421.6 wt.%) and low in CaO and TiO 2 (<0.6 wt.%), with high and variable SiO 2 contents. Rocks from these complexes are all enriched in LREE and LILE (Cs, Rb, Ba, U, Th), depleted in Nb and Ti, have a highly positive Pb anomaly, and are characterized by lack of a clear Eu anomaly despite trace element abundances and isotopic ratios that vary greatly between complexes. The Tongshi complex has high Cs (2.68.5 ppm) and REE abundances (∑REE=112.6297 ppm, (La/Yb) N=13.130.9) and MORB-like Sr–Nd–Pb isotopic ratios (( 87Sr/ 86Sr) i<0.704; εNd>0; ( 206Pb/ 204Pb) i>18). The Hongshan complex has low REE concentrations (∑REE=28.2118.7 ppm, (La/Yb) N=4.614.7) and is moderately enriched as demonstrated by their Sr–Nd isotopic ratios (( 87Sr/ 86Sr) i>0.706; εNd<−7). The Longbaoshan complex is extremely REE enriched (∑REE=211.3392.6 ppm, (La/Yb) N=32.460.9) and has an EM2-like Sr–Nd isotopic character (( 87Sr/ 86Sr) i>0.7078; εNd<−11). We suggest that the Tongshi complex originated from the asthenosphere and the Hongshan complex and the Longbaoshan complex were derived from the partial melting of previously subduction-modified lithospheric mantle, in response to post-collisional lithospheric extension and asthenospheric upwelling. The occurrence of these alkaline intrusive complexes demonstrates that the lithosphere beneath the region must have been considerably thinned at the time of intrusion of these complexes. This study also shed light on the temporal evolution of the Mesozoic lithosphere and the timing of the lithospheric thinning. 相似文献
20.
震旦纪—寒武纪转折期是地球演化的关键节点,这一时期的碳-氧同位素记录在塔里木保留完整却关注较少。选取苏盖特布拉克露头震旦系与寒武系交界地层实测采样,并开展了古生物、镜下鉴定、碳同位素漂移事件的综合分析,探讨了N1(BACE)、P1(ZHUCE)等碳同位素漂移事件的成因机制。样品的 δ13C carb和 δ18O carb相关系数( R2=0.05)、 δ18O carb和Mn/Sr相关系数( R2=0.09)及岩石学特征表明,后期成岩改造并未导致碳-氧同位素的显著分馏,原始碳同位素特征得以基本保留。由下至上,在该剖面识别出P-1事件( δ13C carb峰值1.9‰2.4‰)、N1事件(-6.8‰-10.3‰)、P1事件(1.4‰4.1‰)、N2a-c事件(-0.4‰-2.8‰)、P2a-c事件(0.2‰0.6‰)和N3事件(-3.4‰)。综合碳同位素演化特征与古生物、年代学数据,确认了玉尔吐斯组底部硅质页岩与奇格布拉克组顶部藻云岩的分界面为塔里木震旦系与寒武系的分界线,并实现了该露头与老林、肖滩、三峡、西伯利亚、阿曼和摩洛哥剖面的地层对比。分析认为,塔里木北缘震旦纪—寒武纪转折期的碳漂移事件,更多受控于古海洋氧气含量变化引起的固碳率 forg的波动。玉尔吐斯组早期海侵背景下的大规模缺氧事件导致的初级生产力和固碳率降低,是N1负漂移事件(BACE)的主因;随后海退中氧气含量的增加引起生产力重建和固碳率增加,形成了P1正漂移事件(ZHUCE)。这一成果性认识有助于塔里木盆地寒武纪古环境研究与深层超深层油气远景资源评价。 相似文献
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