| 1. |
- Almqvist, Bjarne S.G., et al.
(författare)
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Seismic anisotropy of mid crustal orogenic nappes and their bounding structures : An example from the Middle Allochthon (Seve Nappe) of the Central Scandinavian Caledonides
- 2021
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Ingår i: Tectonophysics. - : Elsevier. - 0040-1951 .- 1879-3266. ; 819
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Tidskriftsartikel (refereegranskat)abstract
- We report compositional, microstructural and seismic properties from 24 samples collected from the Middle Allochthon (Seve Nappe) of the central Scandinavian Caledonides, and its bounding shear zones. The samples stem both from field outcrops and the continental drilling project COSC-1 and include quartzofeldspathic gneisses, hornblende gneisses, amphibolites, marbles, calc-silicates, quartzites and mica schists, of medium to high-strain. Seismic velocities and anisotropy of P (AVp) and S (AVs) waves of these samples were calculated using microstructural and crystal preferred orientation data obtained from Electron Backscatter Diffraction analysis (EBSD). Mica-schist exhibits the highest anisotropy (AVp similar to 31%; max AVs similar to 34%), followed by hornblende-dominated rocks (AVp similar to 5-13%; max AVs similar to 5-10%) and quartzites (AVp similar to 6.5-10.5%; max AVs similar to 7.5-12%). Lowest anisotropy is found in calc-silicate rocks (AVp similar to 4%; max AVs similar to 3-4%), where the symmetry of anisotropy is more complex due to the contribution to anisotropy from several phases. Anisotropy is attributed to: 1) modal mineral composition, in particular mica and amphibole content, 2) CPO intensity, 3) crystallization of anisotropic minerals from fluids circulating in the shear zone (calc-silicates and amphibolites), and to a lesser extent 4) compositional banding of minerals with contrasting elastic properties and density. Our results link observed anisotropy to the rock composition and strain in a representative section across the Central Scandinavian Caledonides and indicate that the entire Seve Nappe is seismically anisotropic. Strain has partitioned on the nappe scale, and likely on the microstructural scale. High- strain shear zones that develop at boundaries of the allochthon and internally within the allochthon show higher anisotropy than a more moderately strained interior of the nappe. The Seve Nappe may be considered as a template for deforming, ductile and flowing middle crust, which is in line with general observations of seismic anisotropy in mid-crustal settings.
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| 2. |
- Balic-Zunic, Tonci, et al.
(författare)
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Full analysis of feldspar texture and crystal structure by combining X-ray and electron techniques
- 2013
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Ingår i: American Mineralogist. - : Mineralogical Society of America. - 0003-004X .- 1945-3027. ; 98:1, s. 41-52
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Tidskriftsartikel (refereegranskat)abstract
- Feldspar crystals typically show a range of exsolution and polysynthetic twinning textures that can present problems for their full characterization, but at the same time give important information about their genesis. We present an integrated procedure for the micro-texture analysis, twin law identification plus crystal structure refinement of all components in a feldspar intergrowth. This procedure was applied to perthitic intergrowths in feldspars from two different pegmatites in the Larvik plutonic complex in the southern part of the Oslo region, Norway. It revealed that the two starting high-temperature (HT) feldspars had similar global chemical compositions but underwent significantly different cooling histories, with cooling times probably differing by over an order of magnitude. Powder X-ray diffraction with Rietveld refinement was used for a preliminary identification of the mineral components and concluding quantitative phase analysis. Electron microprobe analysis was used to bracket the chemical compositions of the constituents. Electron backscatter diffraction was used to reveal the texture of the samples, twin laws and spatial distribution and crystallographic orientation of the crystal domains. Single-grain X-ray diffraction recorded by an area detector was applied for a simultaneous integration of reflection intensities for all crystallographic domains with different orientations and severe diffraction overlaps. The crystal structures were refined using the program JANA2006 that allows a simultaneous calculation for structurally different components. Combined results of various methods helped improve accuracy and resolve ambiguities that arise from the application of a single technique. The approach is widely applicable to the study of mineral intergrowths and bridges an existing gap in the routinely accessible data on the structural characteristics of rock constituents.
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| 3. |
- Bazargan, Mohsen, et al.
(författare)
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An experimental and numerical investigation of grain size effects on ultrasonic wave velocities in gabbro
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The measured seismic velocities of investigating structures are one of the important methods to provide a rough characterization of the earth’s crust. However, since they are non-unique inferences about compositional and non-compositional features drawn from compressional and shear–wave velocities are very difficult in different tectonic environments. In this work, the effect of pressures and temperatures on compressional (Vp), shear wave velocities (Vs) and velocity anisotropy of gabbroitic rocks with different grain sizes were measured from ambient conditions to 400MPa and 600oC which represent the conditions of greater depth. The comparison of the calculated velocities from numerical tools and measured experimentally derived in situ velocities of gabbro rock samples collected from Cyprus give hints that grain sizes have an important bearing on the in situ seismic velocities, velocity anisotropy and the reflectivity observed at relatively shallow depth.Three gabbro samples with different grain sizes are investigated. For this purpose, the mineral composition of the samples is determined, as well as chemical composition and elastic wave propagation tests.
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| 4. |
- bazargan, mohsen, et al.
(författare)
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Elastic wave anisotropy in amphibolites and paragneisses from the Swedish Caledonides measured at high pressures (600 MPa) and temperatures (600 °C)
- 2019
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Konferensbidrag (refereegranskat)abstract
- Among the most important properties of crustal and mantle rocks is seismic anisotropy. Here we present laboratory measurements of directional dependence of elastic waves (velocity anisotropy, shear wave splitting) for eight cubic samples of deformed crustal rocks from central Sweden. The samples originate from a deep drilling investigation in central Sweden, as part of the Collisional Orogeny in the Scandinavian Caledonides (COSC) project and consist of paragneisses and amphibolites from the uppermost ca 1500 m of the borehole. The main aim of the work is to understand how these rocks were emplaced into the middle crust during the Caledonian orogeny, which took place ca 490-390 Ma. The samples were selected because they span a range of deformation conditions in the borehole. Therefore, we seek a better understand of the seismic properties and their relationship to rock microstructure and mineral composition. Two recent studies, by Hedin et al. (2016) and Wenning et al. (2016) provide an initial understanding of the reflection seismic and petrophysical character of the borehole and its surroundings. However, in order to better understand the elastic wave and anisotropy characteristics of the borehole, further laboratory investigations are desired. Such investigations should integrate laboratory measurements with petrological and textural analysis. In this study, we address the contribution of microstructural parameters (crystallographic preferred orientation, shape preferred orientation, microcracking) to the elastic anisotropy of different amphibolites and paragneisses under true triaxial experimental condition. Experimental data include the measurements of elastic wave velocities (Vp, and polarized shear wave Vs1 and Vs2) and densities at confining pressures up to 600 MPa and temperatures up 600°C. The results include the determination of temperature and pressure derivatives of density, velocities, and the calculation of velocity anisotropy. Measurements confirm strong relations of velocity anisotropy, shear wave splitting and shear wave polarisation to the structural frame of the rocks (foliation, lineation). Importantly, microcrack closure is closely linked to linear strain and our measurements illustrate how micro cracks affect the inelastic deformation of samples as well as enhancing anisotropy at low confining pressures. Above ca 150 MPa the effect of cracks is almost eliminated, due to progressive closure of microcracks. Amphibolites are moderate to highly anisotropic, with P and S wave anisotropies exceeding 10 %. Paragneisses are much less anisotropic, with P and S waves anisotropies <10 %. Measurements presented in this study provide constraints on simultaneous pressure and temperature effects on Vp and Vs, in different sample directions.
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| 5. |
- bazargan, mohsen, et al.
(författare)
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Experimental investigation and numerical modelling of elastic wave propagation in metamorphic rocks
- 2019
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Konferensbidrag (refereegranskat)abstract
- Calculating anisotropy and a better understanding of this physical effect is one of the main challenges in geophysics, whether in the size of the field or micro scale. one of the very common methods of calculating seismic anisotropy is to take into account the bulk properties of the material, in microscale, this is based on the average value of Crystallographic preferred orientation measurements by using, for example, Scanning Electron Microscopy to operate - Electron backscatter diffraction method. In this study, we demonstrate a new method to predict seismic anisotropy and comparing our results with magnetic susceptibility anisotropy to figure how does the relationship between these two value can form a better understanding on this method. We have investigated the existence of anisotropy, its effect on different parameters numerically and experimentally.
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| 6. |
- Bazargan, Mohsen, et al.
(författare)
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Joint pressure and temperature effects on seismic properties of gneisses and amphibolite
- 2021
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Pressure and temperature change simultaneously in the Earth’s crust from surface to depth. Joint pressure and temperature changes influence many different physical properties. There are many studies on samples at elevated pressure, where the influence of open cracks, fractures, voids and pores have been studied. Applying confining pressure has a direct influence on crack closure, and this influence on dynamic properties (density and elastic modulus, bulk, shear and young’s) of rocks above 200 MPa is assumed linear with the linear increase in wave speed. This is because it is generally assumed that most cracks are closed above 200 MPa, which in nature would correspond to a depth of ~7-8 km. However, from the KTB deep drilling well in Germany, it is known that fluid-filled fractures and pores can remain open until 8 to 9 km depth. Applying temperature can affect the dynamic properties of rock by thermal expansion, possibly reopening cracks that were closed at pressures >200 MPa, and thermally expanding grains. This influence is also assumed to be linear at a temperature below partial melting, and in the absence of phase transitions. A similar effect has been observed by a number of research groups during laboratory experiments and calculating seismic velocity results under 600 MPa confining pressure and 600oC temperature. In this work, an effort has been made to mathematically investigate the influence of temperature and pressure on the seismic properties (velocity of pressure and shear waves, density and Poisson’s ratio) of crystalline rocks, measured during laboratory experiments. Elastic wave speeds, moduli and density are increasing as a function of pressure and decreasing as a function of temperature. However, these pressure and temperature-related changes are shown to be nonlinear from room conditions up to 600oC and 600 MPa. In this presentation, we focus on non-linear changes mainly in the high-pressure portion of the velocity as a function of pressure (>200 MPa). When confining pressure is applied, measured P- and S- waves show an increase in velocity and decrease in anisotropy. However, the effect of temperature on measured P- and S- waves show a decrease in velocity and increases in anisotropy. These changes are not very different from linear, but it is not possible to fit velocity as a function of pressure or temperature with linear mathematical functions. The implications of non-linear relationships between pressure, temperature and elastic wave speeds are discussed in this presentation.
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| 7. |
- bazargan, mohsen, et al.
(författare)
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Numerical and experimental investigations of elastic wave anisotropy in monomineral and polymineral rocks
- 2020
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Konferensbidrag (refereegranskat)abstract
- Seismic anisotropy is a key property to understand the structure of the crust and mantle. In this contribution, we investigate the influence of shape (morphological) preferred orientation (SPO), crystallographic preferred orientation (CPO) and the spatial distribution of grains on seismic anisotropy in rocks (Bazargan et al., 2018). A numerical toolset has been developed with COMSOL to investigate these effects numerically, which has been benchmarked analytically and against other numerical models. Numerical samples modelled in 2D and 3D can determine anisotropy, by measurements along different sample axes, using different geometrical setups and mineral compositions. This numerical tool can include a variety of mineral arrangements and propagate P and S waves from different directions to calculate anisotropy. Current numerical results confirm directly the relations between the structural framework of the rocks (foliation, lineation) and velocity anisotropy, shear wave splitting and shear wave polarisation. This has been proven numerically with the effects of layering, which represents foliation and lineation in 2D. One of the aims of this work is to apply the fundamental results and effects of an effective medium to improve our finite element method (FEM) toolbox to provide a numerical modelling tool for seismic data that have been collected in the field. Since the numerical and laboratory measurements are worked on together to verify the numerical results, to compare the models and explain the laboratory measurements have been conducted. Here we also present laboratory measurements of directional dependence of elastic waves velocity and shear wave splitting to the internal rock structure. In the experimental part of this study, we illustrate the contribution of microstructural parameters (grain sizes, SPO and microcracks) to the elastic anisotropy of relatively similar quartzites and granites. An objective with the laboratory measurements is to investigate the effect of grain size and its possible influence on elastic wave speed and potential scattering effects due to wavelength effects. Granites are the ones we use to investigate anisotropy related to SPO and CPO. Our experimental data consist of the measurements of elastic wave velocities (Vp, Vs 1 and Vs 2) at confining pressures up to 600 MPa (Bazargan et al., 2019). numerical modelling together with laboratory measurements are used to obtain a better understanding of the role of microstructures in elastic wave propagation and its anisotropy
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| 8. |
- Bazargan, Mohsen, et al.
(författare)
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Numerical, predictive and experimental study on elastic wave propagation in crystalline rocks
- 2021
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Calculating anisotropy and better understanding this physical effect is one of the main challenges in geophysics, whether in the size of the field or micro scale. one of the very common methods of calculating seismic anisotropy is to take into account the bulk properties of the material, in a micro-scale, this is based on the average value of Crystallographic preferred orientation measurements by using, for example, Scanning Electron Microscopy to operate - Electron backscatter diffraction method.
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| 9. |
- Bazargan, Mohsen, et al.
(författare)
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Pressure, temperature and lithological dependence of seismic and magnetic susceptibility anisotropy in amphibolites and gneisses from the central Scandinavian Caledonides
- 2021
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Ingår i: Tectonophysics. - : Elsevier. - 0040-1951 .- 1879-3266. ; 820
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- As a petrofabric indicator, anisotropy of magnetic susceptibility (AMS) can potentially be used to infer seismic properties of rocks, and in particular seismic anisotropy. To evaluate the link between AMS and seismic anisotropy we present laboratory measurements of elastic wave velocities and anisotropy of magnetic susceptibility (AMS) for eight samples from the deep drilling investigation forming a part of the Collisional Orogeny in the Scandinavian Caledonides (COSC) project. The samples consist of a representative suite of mid crustal, deformed rock types, namely felsic and biotite-rich gneisses, and amphibolites (mafic gneisses). Compressional (P) and shear (S) waves were measured at confining pressures from ambient to 600 MPa and temperature from room condition to 600 °C. Seismic anisotropy changes with increasing temperature and pressure, where the effect of pressure is more significant than temperature. Increasing pressure results in an increase in mean wave speed values from 4.52 to 7.86 km/s for P waves and from 2.75 to 4.09 km/s for S waves. Biotite gneiss and amphibolite exhibit the highest anisotropy with P wave velocity anisotropy (AVp) in the ranges of ~9% to ~20%, and maximum S- wave anisotropy exceeds 10%. In contrast, Felsic gneisses are significantly less anisotropic, with AVp of <7% and AVs of <6%. Up to 20% anisotropy may be generated by microcracks at 600 MPa and 600 °C, which is likely originating from thermal expansion of anisotropic minerals. An agreement is found between AMS and seismic anisotropy, although this is only a case if mean magnetic susceptibility (kmean) ranges between ~1 × 10−5 to ~1 × 10−3 [SI]. Such kmean values are common in rocks dominated by paramagnetic matrix minerals. Based on our results we propose that such samples are the most likely to be useful for the prediction of seismic anisotropy based on their AMS data.
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| 10. |
- Bergman, Helena, et al.
(författare)
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The recognition of multiple magmatic events and pre-existing deformation zones in metamorphic rocks as illustrated by CL signatures and numerical modelling : examples from the Ballachulish contact aureole, Scotland
- 2012
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Ingår i: International journal of earth sciences. - : Springer Science and Business Media LLC. - 1437-3254 .- 1437-3262. ; 101:5, s. 1127-1148
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Tidskriftsartikel (refereegranskat)abstract
- The combination of cathodoluminescence (CL) analysis, temperature and temperature-time calculations, and microstructural numerical modelling offers the possibility to derive the time-resolved evolution of a metamorphic rock. This combination of techniques is applied to a natural laboratory, namely the Ballachulish contact aureole, Scotland. Analysis of the Appin Quartzite reveals that the aureole was produced by two distinct magmatic events and infiltrated by associated fluids. Developing microstructures allow us to divide the aureole into three distinct regions. Region A (0-400 m, 663A degrees C < T (max) < 714A degrees C) exhibits a three-stage grain boundary migration (GBM) evolution associated with heating, fluid I and fluid II. GBM in region B (400-700 m, 630A degrees C < T (max) < 663A degrees C) is associated with fluid II only. Region C (> 700 m of contact, T (max) < 630A degrees C) is characterised by healed intragranular cracks. The combination of CL signature analysis and numerical modelling enables us to recognise whether grain size increase occurred mainly by surface energy-driven grain growth (GG) or strain-induced grain boundary migration (SIGBM). GG and SIGBM result in either straight bands strongly associated with present-day boundaries or highly curved irregular bands that often fill entire grains, respectively. At a temperature of similar to 620A degrees C, evidence for GBM is observed in the initially dry, largely undeformed quartzite samples. At this temperature, evidence for GG is sparse, whereas at similar to 663A degrees C, CL signatures typical for GG are commonplace. The grain boundary network approached energy equilibrium in samples that were at least 5 ka above 620A degrees C.
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