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1.
Critical state (CS) is a physically observed state of granular materials at failure, based on which the critical state theory (CST) was founded. CST attempts to describe analytically the material response when CS failure occurs and constitutes the framework within which constitutive modeling techniques have been developed in the last half-century. The conditions of CST defined as necessary and sufficient to characterize CS do not include fabric orientation of material samples. In the present work, this absence of fabric is discussed in light of a newly developed anisotropic critical state theory (ACST) that enhances the classical CST by introducing an additional condition that fabric must satisfy at CS. The use of ACST framework in constitutive modeling is presented in a generic way, usable in conjunction with various constitutive models that comply with CST. Answers to some fundamental questions are attempted or suggested, e.g., as to whether the conditions of classical CST are necessary and sufficient for CS to occur, whether the ACST is simply a convenient supplement to CST or a necessary enhancement, and if the latter is true, the merits and open questions that arise. The presentation focuses on the justification and thoughts behind the proposed specific features of ACST, provides clarifications not made before, while several suggestions and disclaimers are made, and alternate approaches are proposed for further investigations on the subject matter.  相似文献   

2.
A new computer simulation of the development of grain shape and crystallographic preferred orientations in dynamically recrystallizing aggregates is presented. This model is based on a uniform array of points, each of which represents a small area of crystal. This model combines homogeneous strains, simplified versions of the lattice rotations predicted by Taylor-Bishop-Hill theory, mobile grain boundaries and the nucleation of subgrains. It allows the progressive development of the fabrics and microstructures to be followed. The limitations of the simplifying assumptions used in this model are discussed.  相似文献   

3.
Geological research on the Mediterranean region is presently characterized by the transition from disciplinary to multidisciplinary research, as well as from national to international investigations. In order to synthesize and integrate the vast disciplinary and national datasets which are available, it is necessary to implement maximum interaction among geoscientists of different backgrounds. The creation of project-oriented task forces in universities and other research institutions, as well as the development of large international cooperation programs, is instrumental in pursuing such a multidisciplinary and supranational approach. The TRANSMED Atlas, an official publication of the 32nd International Geological Congress (Florence 2004), is the result of an international scientific cooperation program which brought together for over two years sixtythree structural geologists, geophysicists, marine geologists, petrologists, sedimentologists, stratigraphers, paleogeographers, and petroleum geologists coming from eighteen countries, and working for the petroleum industry, academia, and other institutions, both public and private. The TRANSMED Atlas provides an updated, synthetic, and coherent portrayal of the overall geological-geophysical structure of the Mediterranean domain and the surrounding areas. The initial stimulus for the Atlas came from the realization of the extremely heterogeneous nature of the existing geological-geophysical data about such domain. These data have been gathered by universities, oil companies, geological surveys and other institutions in several countries, often using different procedures and standards. In addition, much of these data are written in languages and published in outlets that are not readily accessible to the general international reader. By synthesizing and integrating a wealth of preexisting and new data derived from surficial geology, seismic sections at various scales, and mantle tomographies, the TRANSMED Atlas provides for the first time a coherent geological overview of the Mediterranean region and represents an ideal springboard for future studies.  相似文献   

4.
Zhou  Wan-Huan  Jing  Xue-Ying  Yin  Zhen-Yu  Geng  Xueyu 《Acta Geotechnica》2019,14(6):1699-1716
Acta Geotechnica - In this study, the effects of particle sphericity and initial fabric on the shearing behavior of soil-structural interface were analyzed by discrete element method (DEM). Three...  相似文献   

5.
The Kristallbrocken are a characteristic centimetre- to decimetre-sized, laminated halite fabric type occurring in the Stassfurt Formation in the Zechstein Basin. Up to now, the nature of the Kristallbrocken, i.e. if they are relics of fine-grained, polycrystalline halite beds or clasts of ‘single crystal-layers’, as well as the deformation mechanisms of this halite type, were not clear from the literature. Drill core material from the salt deposit Teutschenthal at the southern rim of the Zechstein Basin now allowed investigating less intensely deformed samples for the first time. The deformational behaviour of these Kristallbrocken ranges from brittle to ductile, which is evidenced by fractured Kristallbrocken on the one hand and weakly bent or even folded Kristallbrocken on the other hand. Local X-ray texture measurements demonstrated that the Kristallbrocken are definitely single crystals and that they can be regarded as relics of formerly larger ‘single crystal-layers’ of up to several dm2 in size. The folded Kristallbrocken clearly display by their single grain texture characteristics that their crystal lattice is bent, which was most likely enabled by a kind of flexural-shear folding, and did not develop after deformation from a fine-grained aggregate by recrystallisation. Due to their monocrystallinity, their originally large size, and the solid inclusions forming the internal lamination, the Kristallbrocken have clearly stronger rheological properties than the surrounding fine- to coarse-grained polycrystalline rock salt, and thus also deform by fracturing.  相似文献   

6.
A direct correlation was shown for the first time between mosaic irisation patterns in synthetic and natural precious opals (from Australia, Ethiopia, Honduras, Slovakia, and Russia) and their frustumational (lump or mosaic–cluster) inner structure by means of photoluminescence, X-ray phase analysis, IR and Raman spectroscopy, and scanning electron microscopy.  相似文献   

7.
Methane seepage leads to Mg-calcite and aragonite precipitation at a depth of 4,850 m on the Aleutian accretionary margin. Stromatolitic and oncoid growth structures imply encrustation of microorganisms (microbial mats) in the host sediment with a unique growth direction downward into the sediment, forming crust-shaped lithologies. Biomarker investigations of the residue after carbonate dissolution show strong enrichments in crocetane and archaeol, which contain extremely low '13C values. This indicates the presence of methane-consuming archaea, and '13C values of -42 to -51‰ PDB indicate that methane is the carbon source for the carbonate crusts. Thus, it appears that stromatolitic encrustations of methanotrophic anaerobic archaea probably occurs in a consortium with sulphate-reducing bacteria and that carbonate precipitation proceeds downward into the sediment, where ascending cold fluids provide a methane source. Strontium and oxygen isotope analyses as well as 14C ages of the carbonates suggest that the fluids come from deep within the sediment and that carbonate precipitation began about 3,000 years ago.  相似文献   

8.
The recently identified Guanzizhen ophiolitic mélange belt (GOMB) in the West Qinling orogen is regarded, by many geochemical and geochronological works, as the westward extension of the Shangdan belt in the East Qinling that separated the North and South China blocks. The GOMB trends generally NW–SE and comprises ophiolites and forearc volcanic–sedimentary complex (the Liziyuan Group). Ductile shear zones with various widths are a striking structural feature of the GOMB and studying them could contribute to a better understanding of the tectonic processes of the collisional orogeny in this area. Here we characterize the deformation style of these shear zones by combining the low-field anisotropy of magnetic susceptibility (AMS) data, microstructural and field observations. The new dataset demonstrates that the deformation of these ductile shear zones is dominated by dextral transpressional shearing under low amphibolite grade condition. Combining the dextral transpression of the GOMB and the coeval sinistral transpression of the Shangdan belt in the East Qinling, we propose a Late Triassic bilateral extrusion tectonics of the Qinling orogen in response to the collision between the Ordos and Sichuan blocks.  相似文献   

9.
Anisotropy of magnetic susceptibility (AMS) and U–Pb (SHRIMP) zircon ages in the eastern part of the Três Córregos batholith (Ribeira belt, SE Brazil) indicate a well-defined fabric pattern acquired between 600 and 595 Ma. The batholith consists mostly of porphyritic granites distributed in the Ribeirão Branco, Barra do Chapéu and Itaóca plutons. Late fluorite-bearing alkaline granites, some containing Sn-polymetallic greisen-type deposits, intruded the Ribeirão Branco pluton and the low-grade metasedimentary host rocks. The magnetic fabric of the Ribeirão Branco granite is dominantly oblate and oblique to the pluton elongation while that in Barra do Chapéu is mostly concentric. On both plutons, AMS records the preferred orientation of coarse, homogeneous Ti-poor magnetite grains. Such fabric patterns indicate a partitioned strain field dominated by strike-slip left-lateral shear deformation in the Ribeirão Branco and extension in the Barra do Chapéu pluton. The zircon ages of these plutons are 600 ± 6 Ma and 595 ± 4 Ma. Likewise, the ages of alkaline plutons were in the range of 597 and 595 Ma, registering a fast transition between the typical syntectonic batholithic magmatism to the late, highly evolved and specialized magmas. These results indicate that the Ribeira belt was deformed by transtension in the Middle Ediacaran. The geological setting is consistent with a continental arc with the Três Córregos batholith emplacing at the middle-upper crust.  相似文献   

10.
Geological mapping coupled with structural investigations carried out in the Voltri Massif (eastern Ligurian Alps, Italy) provide new data for the interpretation of the tectonic context controlling main fabric development during exhumation of its high-pressure core. The Voltri Massif is here interpreted as a c. 30 km-long eclogite-bearing, asymmetric dome formed by the progressive verticalisation of the regional, second-phase mylonitic foliation developed during retrogressive greenschist metamorphic conditions. In this light, the exhumation history is driven by a ductile-to-brittle extensional process, operating through low-angle, top-to-the-W multiple detachment systems. A Late Eocene–Early Oligocene age for this extensional episode is proposed on the basis of structural correlations, stratigraphic and radiometric constraints. In this scenario, the Voltri Massif is interpreted as an extensional domain developed to accommodate the Late Eocene–Early Oligocene arching of the Western Alps–Northern Apennines orogenic system.  相似文献   

11.
《Applied Geochemistry》2003,18(10):1653-1662
The through- and out-diffusion of HTO, 36Cl− and 125I− in Opalinus Clay, an argillaceous rock from the northern part of Switzerland, was studied under different confining pressures between 4 and 15 MPa. The direction of diffusion and the confining pressure were perpendicular to the bedding. Confining pressure had only a small effect on diffusion. An increase in pressure from 4 to 15 MPa resulted in a decrease of the effective diffusion coefficient of ∼20%. Diffusion accessible porosities were not measurably affected. The values of the effective diffusion coefficients, De, ranged between (5.6±0.4)×10−12 and (6.7±0.4)×10−12 m2 s−1 for HTO, (7.1±0.5)×10−13 and (9.1±0.6)×10−13 m2 s−1 for 36Cl− and (4.5±0.3)×10−13 and (6.6±0.4)×10−13 m2 s−1 for 125I−. The rock capacity factors, α, measured were circa 0.14 for HTO, 0.040 for 36Cl− and 0.080 for 125I−. Because of anion exclusion effects, anions diffuse slower and exhibit smaller diffusion accessible porosities than the uncharged HTO. Unlike 36Cl−, 125I− sorbs weakly on Opalinus Clay resulting in a larger rock capacity factor. The sorption coefficient, Kd, for 125I− is of the order of 1–2×10−5 m3 kg−1. The effective diffusion coefficient for HTO is in good agreement with values measured in other sedimentary rocks and can be related to the porosity using Archie's Law with exponent m=2.5.  相似文献   

12.
The present day South Tibetan Detachment (STD) of Higher Himalaya is a system of low-angle normal faults. In the Himachal High Himalaya, the STD hanging wall is characterized by the presence of S-type per-aluminous Paleozoic (~475 Ma) granite called the Kinnaur Kailash Granite (KKG). This granite is later intruded by Cenozoic leucogranites (~18 Ma) in vicinity of the STD zone. In this work, microstructures, anisotropy of magnetic susceptibility (AMS), and U–Pb geochronology were carried out on the KKG and the leucogranites with an aim to (a) understand the conditions of fabric development and (b) decipher the tectonic relationship between deformation along the STD and the evolution of these granites. Microstructural features and magnetic anisotropy indicate that the granites are intensely deformed in vicinity of the STD and preserve their emplacement-related fabric in the interior parts. It is inferred that close to the STD zone, fabrics of both the KKG and the leucogranite are tectonic and are modified by the Cenozoic (~20 Ma) right-lateral slip and extensional tectonics. Magnetic fabric in the interior parts of the KKG is related to its emplacement indicating that original fabric was preserved. U–Pb geochronology of zircons from two samples of the KKG yields crystallization age of 477.6 ± 3.4 and 472 ± 4 Ma. The leucogranite gives a crystallization age of 18.5 ± 0.6 Ma. Zircons from the KKG also reveal signatures of a deformation event (20.6 ± 2.3 Ma) at its rim. It is inferred that deformation of the external rim of the KKG and crystallization of the leucogranites are synchronous and triggered by ductile deformation along the STD.  相似文献   

13.
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