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Orientation of specimens: Essential data for all fields of geology
Abstract
Accurate rock-specimen orientations are a fundamental link between hand-speciman or thin-section features and larger geologic structures. Without field orientation, observations made using petrological or electron microscopes are devalued and may be worthless in terms of regional geometry. Marking and collecting oriented specimens in the field is fast and should be common practice. This paper outlines some uses of oriented specimens and presents a simple method for their collection and laboratory treatment.
... Larger gravel-sized lithoclasts are too static and difficult to move to gain sufficient measurements to be considered as reliable results. The smaller sand-sized particles, however, can only be handled in this scope by universal-rotating-stage methods following laborious sampling, e.g., using oriented lacquer peels and taking an approach similar to that of structural geologists [125]. The semicircle-rose diagrams are selected and transformed into a couple of distance-angle plots and distance-sharpness plots (Figures 17-19). ...
The “natural GMS laboratory” (granulometry-morphometry-situmetry) is located within the Variscan Basement in SE Germany (Fichtelgebirge Mts.), which is uplifted relative to its Permo-Mesozoic foreland along a deep-seated lineamentary fault zone. This transitional study area is crossed by straight to low drainage systems in the basement, turning meandering channel systems into high sinuosity when entering the foreland. Due to its good geological coverage, the entire region is subjected to an advanced-level terrain analysis and completed with a sedimentological study focusing on the GMS tool. Unlike many applications in the past, the three components of the GMS tool that are of almost equal value ought to be used in combination and not as stand-alone procedures so as to be integrated into other near-surface geoscientific methods, e.g., sediment petrography. The strong points of granulometry of coarse-grained/gravel-sized sediments are its extension into the smaller sand and clay grain size intervals using the sorting, mean and/or median values for an environmental analysis. Morphometry can be linked to the compositional geosciences, e.g., mineralogy and geochemistry. The grain shape is intimately connected with the lithology, providing options from triaxial measuring of the lithoclast to the digital image analysis. It is a favorable tool to supplement the provenance of lithoclasts. Situmetry is the key element of hydrodynamic research and directly builds upon its sister methods. Its applications and numerical approaches are useful for the identification and quantification of physical land-forming processes. It is the fan sharpness and the orientation of lithoclasts relative to the direction of the talweg and in relation cross-sectional valley features that integrate the GMS tool into geological and geomorphological mapping, both of which result in a digital terrain model. Horizontal rose diagrams are useful for the upper reaches of drainage systems, be they of alluvial or non-alluvial types, and vertical ones for alluvial channels in the distal and proximal foreland where stacked patterns of depositional terraces are of widespread occurrence. In general, the GMS tool can be applied to sedimentological, geomorphological, petrographic and tectonic objects in basements and foreland basins; in applied geosciences, it is suitable for the identification of mineral resources and of areas vulnerable to geohazards, and in genetic geosciences for the discrimination of supergene chemical and physical depositional and land-forming processes.
... The former is typically used in locations with particular lithologies that are not easy to drill, or where logistic or legal requirements demand the collection of block samples. The orientation of joint blocks should be measured by determining the strike and dip of the surface before they are removed from the outcrop (e.g., Prior et al., 1987). Advantages of block sampling are independence of the technical requirements of coring equipment and the ability to sample lithologies not suitable for coring. ...
Numerous studies have attempted to use boulder deposits in coastal zones for assessing the hazard due to tsunamis and/or storms. One critical problem is still related to the determination of boulder ages. Although the age of wave-emplaced boulders can often be obtained through ¹⁴C and U/Th dating, marine organisms that were killed during transport or shortly afterward are required for these dating methods to provide accurate ages. In addition, these approaches cannot determine ages of multiple movements of single boulders with complex transport histories. Paleomagnetic dating offers the potential to overcome current challenges for dating the dislocation coastal boulders. After dislocation, wave-emplaced boulders successively acquire a viscous remanent magnetization (VRM) that is parallel to the Earth's geomagnetic field. Since the formation of the VRM is a function of site-specific ambient temperature and time, VRM can be used to determine ages for the emplacement of tsunami and storm boulders. This chapter summarizes state of the art sampling, measurement, and analysis strategies for VRM dating of coastal boulders. First case studies from Ishigaki Island, Beppu Bay, and the Sanriku coast (all Japan) illustrate the potential and current limitations of the method when applied to date the dislocation of tsunami and storm boulders.
... In general, to properly obtain EBSD data from geological samples, one must: (1) correctly determine the structural reference frame (e.g., foliation and lineation) (Prior et al., 1987) and (2) prepare a well-polished sample without surface damages to increase the indexation rate of the EBSD map. After the EBSD data are acquired, great care must be taken in dealing with the raw data (i.e., post-processing procedures, such as noise reduction and the removal of wild spikes). ...
Automated electron backscattered diffraction (EBSD) data yield abundant information on the lattice-preferred orientations (LPOs) and deformation microstructures and mechanisms of minerals in a rock, which aid in our understanding of the tectonic conditions and history of a region. Additionally, this information allows us to interpret seismic anisotropies in the crust and mantle. However, great care must be taken in the collection and production of crystallographic orientation data via automated EBSD to more precisely interpret LPOs, deformation microstructures, and seismic anisotropies because petro-fabrics can be different depending on sampling methods such as all-points-per-grain (whole) data and one-point-per-grain (one orientation per grain) data. Here, we report a detailed comparison of the crystallographic orientation data produced using both all-points-per-grain and one-point-per-grain techniques to analyze eclogites from the Yuka terrane in the North Qaidam ultrahigh-pressure metamorphic belt of northwestern China. By comparing eclogite crystallographic orientations between different sampling methods, we found that there was no marked difference in the LPOs, fabric strengths (J-index and M-index), and seismic anisotropies for relatively small mineral grains (e.g., garnet and omphacite). However, there were large differences in the LPOs, fabric strengths, and seismic anisotropies of relatively large minerals grains (e.g., amphibole). This result can be attributed to the acquisition method of crystallographic orientation data, which can influence the LPO, fabric strength, and seismic anisotropy of minerals and rocks. Therefore, the pole figures of minerals should be constructed from one point per grain data in order to avoid the oversampling of large grains for samples with highly heterogenous grain size distributions, as well as to permit the comparison of crystallographic orientations obtained using different tools and in other studies.
... Amostras representativas e orientadas, conforme técnica de Prior et al. (1987) Os QMF posicionados a sul da zona de máxima deformação da ZCQSE, no interior do GAD, tendem a um alongamento preferencial segundo a direção N 65° E. Nestes, as bandas de quartzo milonito possuem textura mais fina e grãos de quartzo fitados, com foliação contínua e delgada. ...
No embasamento cristalino do estado do Rio Grande do Sul e do Uruguai ocorrem corpos de quartzo milonitos associados a filonitos (QMF) relacionados a movimentações transcorrentes do Ciclo Brasiliano. A Zona de Cisalhamento Quitéria-Serra do Erval (ZCQSE) desenvolveu corpos alongados de QMF paralelos às estruturas magmáticas e miloníticas impressas nos granitoides sintectônicos parentais do Ediacarano, denominados de Arroio Divisa (GAD). Neste trabalho, apresentam-se os resultados de mapeamento geológico detalhado dos corpos de QMF e análise petrográfica com enfoque em microestruturas de quartzo. Grãos de quartzo grossos interlobados, com subgrãos em padrão tabuleiro de xadrez possuem diversos graus de retrabalhamento, como estiramento e redução no tamanho de grão, nos QMF e nos GAD. A semelhança nas microestruturas de quartzo demonstra que a deformação dos QMF e dos granitoides parentais ocorreu em condições termais semelhantes, em temperatura da fácies anfibolito superior a xisto verde inferior. As análises de macro- a micro-escalas indicam que os corpos de QMF da ZCQSE foram gerados durante o resfriamento dos granitoides encaixantes em regime transcorrente sinistral, em condições não-isovolumétricas, com perda de volume, em ambiente tectônico contracional. A deformação que gerou os corpos de QMF ocorreu em condições termais próximas da temperatura solidus do sistema granítico, com reativação em temperaturas mais brandas.
... The samples have to be spatially oriented to provide information. The method used to collect spatially oriented samples is that proposed by Prior et al. (1987) and modified by Capaccioni and Sarocchi (1996). When the deposit is loose, the samples have to be compacted using casting resins. ...
Shape-fabric is a textural parameter related to the spatial arrangement of elongated particles in geological samples. Its usefulness spans a range from sedimentary petrology to igneous and metamorphic petrology. Independently of the process being studied, when a material flows, the elongated particles are oriented with the major axis in the direction of flow. In sedimentary petrology this information has been used for studies of paleo-flow direction of turbidites, the origin of quartz sediments, and locating ignimbrite vents, among others. In addition to flow direction and its polarity, the method enables flow rheology to be inferred. The use of shape-fabric has been limited due to the difficulties of automatically measuring particles and analyzing them with reliable circular statistics programs. This has dampened interest in the method for a long time. Shape-fabric measurement has increased in popularity since the 1980s thanks to the development of new image analysis techniques and circular statistics software. However, the programs currently available are unreliable, old and are incompatible with newer operating systems, or require programming skills. The goal of our work is to develop a user-friendly program, in the MATLAB environment, with a graphical user interface, that can process images and includes editing functions, and thresholds (elongation and size) for selecting a particle population and analyzing it with reliable circular statistics algorithms. Moreover, the method also has to produce rose diagrams, orientation vectors, and a complete series of statistical parameters. All these requirements are met by our new software. In this paper, we briefly explain the methodology from collection of oriented samples in the field to the minimum number of particles needed to obtain reliable fabric data. We obtained the data using specific statistical tests and taking into account the degree of iso-orientation of the samples and the required degree of reliability. The program has been verified by means of several simulations performed using appropriately designed features and by analyzing real samples.
... A amostra é orientada mediante um clinômetro para determinar a inclinação (mergulho) do furo, e bussola para determinar o azimute (Figura 3). 4. Coleta de blocos orientados de rocha fresca (quando possível) para o estudo cinemático de zonas de cisalhamento análise petrográficas e análises geocronológicas U-Pb SHRIMP em zircão. A coleta e orientação dos blocos obedeceram às técnicas usuais descritas em Prior et al., (1987), Hansen (1990) Allmendinger et al., 2012), com a finalidade de realizar refinamento e o tratamento estatístico destes. Figura 3. Procedimento para coleta de amostra para análise de ASM: a) Perfuração da rocha com perfuratriz manual movida à gasolina com broca diamantada e resfriada com agua. ...
The Mucajaí Gneiss, Serra da Prata Charnokites, San Antonio and Pratinha Gabbros units exhibit Sn gneissic and S1 mylonitic foliation, with deformation microstructures generated under medium to high amphibolite facies temperature conditions. Despite the obliteration of primary structures, some igneous microstructures preserved in granitoids and gabbros, such as preferred orientation of feldspar phenocrysts with crystal faces due to magmatic flow, submagmatic microfractures and dike intrusion controlled by pre-existing anisotropy (Pre-S1), suggest syn-magmatic deformation processes.
U-Pb SHRIMP analysis of oscillatory zones of igneous zircons provided Concordia ages of 1916.7+4.5 Ma for a mylonitized granite of Serra da Prata Charnokites, 1905+5.2 Ma for associated gabbros and 1959+5 for orthognesis of Mucajaí Gneiss. These ages are interpreted as the crystallization ages of the protoliths and understood as the age of the tectono-magmatic event that probably controlled the emplacement of most of the granitic rocks of Serra da Prata Suite and associated gabbros. Another U-Pb SHRIMP age of 1863+6.7 Ma recorded in dynamically recrystallized zones of zircons from mylonitized granite is related to the metamorphic-deformational event that generated the foliation S1 and probably related to the generation of the Sn foliation.
Reworking of deformational structures is interpreted by the presence of mylonitic recrystallized bands oriented parallel to S1 and oblique to Sn foliations. Temperature conditions for this deformation event, characterized by grain size reduction of the of the crystals, is assumed equivalent to upper green schists to medium amphibolite facies and interpreted as tectonic reactivation at shallow crustal levels under a regional stress field similar to that responsible for the registration of the foliation S1 and related to K’Mudku event.
Magnetic mineralogy studies indicate that the principal magnetic marker of magnetic sub-fabrics in granitoids and gabbros is magnetite multidomain. At Sierra da Prata Charnokites magnetic sub-fabrics display a magnetic foliation oriented preferentially NE-SW. On the other hand, the Pratinha Gabbros display a random orientation of the magnetic foliation that is related to the heterogeneous distribution of deformation, concentrated along the margins with high dip angles and high values of magnetic anisotropy Pj (high strain). Less intense deformation was found at the center of the gabbroic body that displays sub-horizontal magnetic foliation with low values of magnetic anisotropy Pj (low strain). Given the nature of the internal deformation and the arrangement of the magnetic sub-fabrics, it is interpreted that S1 and Sn rock fabrics were produced by deformation under medium to high amphibolite facies temperature conditions related to dextral transpressional regime induced by NW-SE shortening active during the Paleoproterozoic.
Key Words: Charnockite-Gabbro association, Serra da Prata Pluton, internal deformation mechanism, anisotropy magnetic susceptibility, U-Pb SHRIMP geocronology.
... Although different conditions exists within a flow (Jeffery, 1922; Lindsay, 1968) a common process is that particles arrange themselves with the long axes (a) parallel to the flow direction (Sestini and Pranzini, 1964; Best, 1992; Capaccioni and Sarocchi, 1996). In order to obtain fabric information we collect an oriented sample of the outcrop (Prior et al., 1987) that is cut in horizontal slices through the middle part of the sample. In these surfaces, the axes (a) of the elongated particles are measured with respect to the north by means of image analysis. ...
Experimentally derived particle size distribution often shown multimodal shape and this
characteristic is usually interpreted as a mixture of two or more populations. The origin of these
mixture has been commonly interpreted as due to he complex processes linked to the origin of
the sediment ad clasts , to the transport and final deposition, or in other terms, the geological cycle
of sediment transport and evolution, the weathering and pedogenenetic process may affect the final
distribution of particles present in the sampled deposit.
The basic idea that the all the processes responsible of the deposit leave some trace of them in the
special characteristics of the mixture and their population. We assume that the mixture maintain
encoded in its global distribution
Aim of DECOLOG software is develop a solution to decode the information present in the natural
mixture of particles/sediments using, as paradigm, the log-normal distribution and particularly a
defined mixture of these distributions.
DECOLOG performs this operation using innovative techniques of optimization and in automatic
way without needs of special efforts from user as the initial guessing of Peaks of the observed
distribution ... the easiness of use is one of the most innovative and appreciated characteristics of
current version of DECOLOG
This software is released as FREEWARE for the scientific community. This imply that is released
and downloadable for free, but without warranties .
The authors of this software want acknowledge the people that with their testing activities and
suggestions help us to improve the performance of DECOLOG. Suggestions from the future users
are welcomed and greatly appreciated.
This chapter details the types of observation that are made on sedimentary rocks in the field, and in the subsurface, including drill cuttings and cores. Field observations include lithology, petrology, bedding types, sedimentary structures, and trace and body fossils. Petrophysical (wireline) logging techniques are also described. Numerous photographs are used to illustrate the field characteristics of sedimentary rocks.
A successful basin analysis requires the collection and integration of several, perhaps many, different kinds of data. Direct observation of the rocks may or may not be fundamental to the study. In the case of a surface geological project, they will be preeminent, though perhaps supplemented by geochemical and geophysical information, plus laboratory analysis of collected samples. For subsurface petroleum studies, actual rock material available for examination may be very limited, consisting of well cuttings from rotary drilling, plus a few short cores. Petrophysical well logs and regional seismic lines may provide at least as important a part of the total data base. Investigations for stratabound ores and minerals typically employ networks of diamond drill holes from which a continuous core is normally available.
A Comprehensive review of modern stratigraphic methods. The stratigraphic record is the major repository of information about the geological history of Earth, a record stretching back for nearly 4 billion years. Stratigraphic studies fill out our planet’s plate-tectonic history with the details of paleogeography, past climates, and the record of evolution, and stratigraphy is at the heart of the effort to find and exploit fossil fuel resources. Modern stratigraphic methods are now able to provide insights into past geological events and processes on time scales with unprecedented accuracy and precision, and have added much to our understanding of global tectonic and climatic processes. It has taken 200 years and a modern revolution to bring all the necessary developments together to create the modern, dynamic science that this book sets out to describe. Stratigraphy now consists of a suite of integrated concepts and methods, several of which have considerable predictive and interpretive power. The new, integrated, dynamic science that Stratigraphy has become is now inseparable from what were its component parts, including sedimentology, chronostratigraphy, and the broader aspects of basin analysis.
Six stages of crenulation cleavage development during a single deformation event (D2) can be recognised in both matrix and porphyroblast inclusion trails in the Robertson River Formation, N.E. Australia. These stages progress from undeformed to crenulation and differentiated crenulation of S1, through development of differentiated crenulation cleavage (S2) and differentiated schistosity (S2), to homogeneous foliation (S2).These rocks underwent prograde metamorphism during D2 and chloritoid, garnet, staurolite, andalusite, and sillimanite isograds have been mapped. Most rocks are rich in porphyroblasts, which commonly contain well-defined inclusion trails. The geometry of these trails varies from one mineral type to the next, depending on the timing of porphyroblast growth relative to the stage of crenulation cleavage development in the schistose matrix. The deformation history involved progressive, bulk, inhomogeneous shortening and the strain is very heterogeneous on all scales, partly as a consequence of this. Hence each of the six stages of crenulation cleavage development formed locally at various times during D2. Therefore, if a porphyroblast grew late in the deformation, it could overgrow all stages of crenulation cleavage development, including the very early ones.The temperature increased during metamorphism in any one locality and consequently some porphyroblasts overgrew others that were unstable in the new conditions, preserving them from destruction. Hence the precise timing of mineralogical reactions (deduced from isograds and mineral chemistry) relative to the stages in schistosity development can also be determined. In the andalusite zone the sequence of porphyroblasts, in decreasing order of age, is biotite-garnet-biotite-staurolite-andalusite. Dissolution of garnet occurred during growth of staurolite and especially andalusite, and dissolution of staurolite accompanied andalusite growth.
A paleomagnetic study of eleven sites of Devonian limestones sampled along the flanks and hinge of a polyphase Hercynian recumbent fold in the Montagne Noire reveals well defined directions of characteristic remanent magnetization (ChRM). The directions have good within-site precision but a large between-site scatter, which fails to be reduced by a classical tilt correction. This is readily understood if widespread remagnetization occurred between the two major Hercynian phases of folding (A and B), which are known to have shaped the Montagne Noire. This remagnetization appears to be an instance of a continent-wide chemical event that wiped out much of the magnetic memory of Devonian and lower Carboniferous rocks in Laurasia, Baltica and northern Gondwana, possibly due to groundwater circulation following the major Hercynian tectonic phases. Comparison of observed and predicted Hercynian magnetic directions allows one to determine, through a fairly simple geometric construction, the tectonic elements of the later phase B. It is found that different rotations affected the flanks and hinge zone of the folds generated by phase A, thus demonstrating how a pre-existing structure can control the development of deformation in a later tectonic phase. The effects of the non cylindrical phase B can next be removed to reveal the broad tectonic features of phase A deformations. The reversed (Kiaman) polarity of the ChRM allows a determination of the age of phase B, which is found to be about 300–310 Ma and of the age of the remagnetization itself at about 310–320 Ma. The search for the age of remagnetization included paleomagnetic measurements at two lower Permian sites from the nearby Lodève basin. These provide new data for the Permian pole of Europe and point out the limitations of some earlier studies, due to insufficient magnetic cleaning. Finally, the principal shortening directions of the two phases can be determined from the magnetic data and are found to have rotated counterclockwise by 30° between phase A and phase B. This feature, already observed in the collision of India against Eurasia, may be related to the progressive suturing of former subduction zones during the Hercynian collision of Gondwana against Laurasia. This study demonstrates the potential of remagnetizations for the solution of rather complex tectonic problems. We propose the term “magnetotectonics” for this rapidly expanding field of studies, in which the methods of paleomagnetism and structural analysis are closely associated.
Zoning patterns in a plagioclase porphyroblast in a blastomylonite derived from Lewisian gneiss near Loch Eriboll, northwest Scotland, suggest that the crystal grew during deformation. Discontinuities in the concentrations of CaO, K2O, and FeO mark the boundaries of overgrowths and define a core-overgrowth structure. The simultaneous changes in concentration of CaO and K2O are used to construct a qualitative chemical potential diagram that reflects changes in aCa++/a2H+ and aK+/aH+ in the coexisting fluid during growth. Increases in these activity ratios are believed to have caused supersaturation of the grain boundary fluids with respect to plagioclase. The repeated periods of growth indicate repeated overstepping of equilibrium, which could have been caused by ion exchange on surfaces newly exposed during deformation. These data fit a model that relates cyclic variations of strain rate in fault zones with cyclic periods of crystal growth, which in turn could have a major influence on fault rock rheology.
Strain distribution across the high-pressure-type Sambagawa regional metamorphic belt was measured by studies of shape change of initially spherical radiolaria. Strains change from 5 to 2000% with increasing metamorphic temperatures. Aburpt strain increase apparently occurs in the intermediate metamorphic grades of the terrane. Deformation stresses during metamorphism were inferred using the grain-sized quartz piezometer method, which is applicable to dynamically recrystallized quartzose rocks. Stress distribution across the terrane was composed of two levels; the stress level in the lower-grade zones was about 130 MPa but in the higher-grade zones was about 50 MPa. Combining stress distribution, strain distribution, and temperatures of the studied metamorphic terrane, the activation energy of the deformation of these quartzose rocks is estimated to have been about 80–90 kJ/mol. The huge amount of deformation of the deep-seated Sambagawa metamorphic terrane, which was formed probably at the lower crustal to uppermost mantle levels over the subducting lithosphere, was sufficient to bring about the uplift of parts of the terrane to the earth's surface.
The most useful criteria used for the determination of the sense of shear can be found in the huge Himalayan ductile shear zone known as M.C.T. They allow the determination of the sense of shear in relation with an unequivocal southward-directed intracontinental thrust. The various criteria analyzed at different scales of observation show the permanence of the shear flow regime throughout the metamorphic evolution. However, most of the observed shear-criteria are due to late nappe emplacement postdating the climax of the metamorphism. The most striking feature is the stretching lineation which is roughly normal to the thrust trace all along the belt. It parallels everywhere the shear direction and thus indicates the transport direction of the nappes. The observed radial pattern of stretch trajectories implies that the local motion does not correspond to the average plate convergence.
Contrary to recently proposed classifications of fault-related rocks (esp. Wise et al., 1984), cataclasis associated with brittle faulting can produce well-foliated fault gouge. Naturally foliated gouge associated with the Punchbowl fault, Los Angeles Co., California is reproduced in experiments in which only brittle conditions and cataclastic deformation mechanisms prevailed. Moreover, only a brittle regime of physical conditions is inferred for the Punchbowl faulting. Classifications of fault-related rocks must accommodate foliated cataclasites.
Rotated helicitic garnets, syntectonically overgrowing axial plane crenulation cleavage on the limbs of folds, show the opposite sense of rotation to that undergone by the folded foliation during the development of the fold. The garnet rotation is inconsistent with simple minded shear models, for the development of the crenulation cleavage, and it is useful to consider this problem in order to understand the development of crenulation cleavage better.Various models are considered in an attempt to explain both garnet rotation and cleavage morphology. Earlier models were based on the assumption that garnet rotation is driven by shear parallel to S2. They were mostly unsatisfactory, but one, involving migration of the crenulation hinges, is viable when combined with other models described here.The new models proposed here are based on the assumption that garnet rotation is driven by shear on the crenulated surface (S1); the new foliation (S2) is considered to be a passive structure, at least in the early stages of its development. These models satisfactorily explain garnet rotation and development of the cleavage, including differentiation and they can be used to predict the morphological evolution of the cleavage microstructure as it should be preserved in garnets. Comparison of observed and predicted helicitic structures strongly supports the proposed models.Evidence is presented to show that where a garnet has an Se that is a schistosity and an Si that is a crenulation cleavage, the schistosity can have developed by unfolding of the crenulations.
In a deformed polymictic conglomerate from the Hill End area, New South Wales, Australia, almost all the strain is confined to fine-grained, quartz-rich rock fragments. Coarse quartz and feldspar clasts generally show little evidence of intracrystalline plasticity. The most strongly elongated rock fragments are finer grained and richer in layer silicates relative to quartz and feldspar, indicating that both grain size and composition have influenced finite strain, and thus strain rate. The relationship between grain size and strain rate is broadly consistent with a dominance of grain-boundary deformation mechanisms over intracrystalline dislocation mechanisms at finer grain-sizes. However, strong quartz c-axis preferred orientations have been measured in the most highly strained clasts, implying a significant component of dislocation glide. Close examination of the microfabric, mineralogy and petrology of the fine-grained clasts shows that other factors are likely to have influenced their mechanical behaviour and led to enhancement of strain rate. These factors include the trace-element chemistry of the phases (especially quartz), the role of a mobile fluid phase, the presence of poorly-bonded mica (001) interfaces and the evolutionary nature of microfabric and mineralogy, all of which are relevant to the bulk of crustal metamorphic rocks and should be taken into account in developing mechanical models of metamorphic belts.
Upper and Middle Austro-Alpine nappes of the Eastern Alps, the structural uppermost subunits of the Alps, suffered two major superposed deformations (D1, D2) during the eo-Alpine (Cretaceous) orogeny. D1 is characterized by NW- to W-directed thrusting and nappe-internal rock flow, D2 by large-scale folding and imbrication toward the north and northeast. Structures, strain, metamorphic history, microfabrics, and stretching trajectories indicate that the superposition of D2 over D1 structures can be interpreted as a progressive deformation. It is concluded that the emplacement of the nappes follows a curved translation path. A model which emphasizes non-coaxial deformation structures explains the detachment of the nappes, the configuration of the separating thrust planes, and the distribution of intense deformation zones. The overall kinematic history of the Austro-Alpine nappes fits a model in which strike-slip shearing, horizontal shortening, and vertical lengthening act together continuously over a finite deformation zone (transpressional model). This model, emphasizing a gradually changing strain regime, eliminates the space problems encountered in models which consider simple south over north imbrication during the Alpine orogeny. The study stresses two interpretations: 1.(1) stretching lineations indicate nappe-transport directions.2.(2) transpression may be a common feature of orogenic belts.
As a consequence of the India—Asia collision, a deeply rooted crustal slice has been moved southwards over the northern boundary of the Indian plate. At the present level of observation the Main Central Thrust pile, a rock-pile of several kilometers thickness, is affected by shear deformation which imprinted a flat foliation and a constant N—S trending lineation. These tectonites were deformed under a wide range of metamorphic conditions grading from lower greenschists up to the sillimanite field. The M.C.T. shear zone is studied from the view-point of the quartz-rich rocks: over 100 samples helped to define a microstructural zonation, roughly parallel to the thermal one; 35 samples, particularly rich in quartz, have been used for the preferred orientation study of their c-axes, and a few of them for their 〈a〉 axes. Strong lattice orientations, when undisturbed by (mainly) secondary recrystallization, attest to large plastic strains in quartz. The c-axes are mostly distributed along unequally populated crossed-girdles which may evolve into an inclined single girdle. The 〈a〉 axes concentrate perpendicular to the more populated girdle.Concerning quartz the following is suggested: 1.(1) the deformation has a dominant shear component responsible for the dominant stretching lineation. It could be simple shear in case of inclined single girdles2.(2) one set of 〈a〉 axes line up parallel to the shear direction. Slip on various planes in the 〈a〉 direction may be an important deformation mechanism3.(3) asymmetry in the lattice preferred orientations could be used to predict the sense of shear for the Himalayan thrusting in more than 80% of cases.