| 1. |
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| 2. |
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| 3. |
- Almqvist, Bjarne, et al.
(författare)
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Bulk strain in orogenic wedges based on insights from magnetic fabrics in sandbox models
- 2018
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Ingår i: Geology. - 0091-7613 .- 1943-2682. ; 46:6, s. 483-486
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Tidskriftsartikel (refereegranskat)abstract
- Anisotropy of magnetic susceptibility (AMS) analysis is used as a petrofabric indicator for a set of four identical-setup sandbox models that were shortened by different amounts and simulate contraction in a fold-and-thrust belt. During model shortening, a progressive reorientation of the initial magnetic fabric occurs due to horizontal compaction of the sand layers. At the early stages of shortening, magnetic lineation (k(1) axis) rotates parallel to the model backstop with subhorizontal orientation, whereas the minimum susceptibility (k(3) axis) is subvertical, which indicates a partial tectonic overprint of the initial fabric. With further shortening, the k(3) axis rotates to subhorizontal orientation, parallel to shortening direction, marking the development of a dominant tectonic magnetic fabric. A near-linear transition in magnetic fabric is observed from the initial bedding to tectonic fabric in all four models, which reflects a progressive transition in deformation from foreland toward hinterland. Model results confirm a long-held hypothesis where the AMS pattern and degree of anisotropy have been suggested to reflect the amount of layer-parallel shortening, based on field observations in many mountain belts. Results furthermore indicate that grain rotation may play a significant role in low-grade compressive tectonic regimes. The combination of analogue models with AMS enables the possibility to predict magnetic fabrics in different tectonic settings and to develop quantitative links between AMS and strain.
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| 4. |
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| 5. |
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| 6. |
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| 7. |
- Almqvist, Bjarne, et al.
(författare)
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Magnetic characterisation of magnetite and hematite from the Blötberget apatite-iron-oxide deposits (Bergslagen), south-central Sweden
- 2019
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Ingår i: Canadian journal of earth sciences (Print). - : Canadian Science Publishing. - 0008-4077 .- 1480-3313. ; 56:9, s. 948-957
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Tidskriftsartikel (refereegranskat)abstract
- Rock magnetic measurements were carried out on drill core material and hand specimens from the Blötberget apatite-iron oxide deposit in the Bergslagen ore province, south-central Sweden, to characterise their magnetic properties. Measurements included several kinds of magnetic susceptibility and hysteresis parameters. Petrographic and scanning electron microscopy (SEM) were used to independently identify and quantify the amount and type of magnetite and hematite. Two hematite-rich samples were studied with laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to quantify the trace element chemistry in hematite and investigate the potential influence of trace elements on magnetic properties. Three aspects of this study are noteworthy. 1) Hematite-rich samples display strong anisotropy of magnetic susceptibility, which is likely to affect the appearance and modelling of magnetic anomalies. 2) The magnitude-drop in susceptibility across Curie and Néel temperature transitions show significant correlation with the respective weight percent (wt%) of magnetite and hematite. Temperature dependent magnetic susceptibility measurements can therefore be used to infer the amounts of both magnetite and hematite. 3) observations of a strongly depressed Morin transition at ca -60 to -70 C (200 to 210 K) are made during low-temperature susceptibility measurements. This anomalous Morin transition is most likely related to trace amounts of V and Ti that substitute for Fe in the hematite. When taken together, these magnetic observations improve the understanding of the magnetic anomaly signature of the Blötberget apatite-iron oxide deposits and may potentially be utilised in a broader context when assessing similar (Paleoproterozoic) apatite-iron oxide systems.
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| 8. |
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| 9. |
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| 10. |
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| 11. |
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| 12. |
- 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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| 13. |
- Almqvist, Bjarne, et al.
(författare)
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Seismic anisotropy in the Morcles nappe shear zone : Implications for seismic imaging of crustal scale shear zones
- 2013
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Ingår i: Tectonophysics. - : Elsevier BV. - 0040-1951 .- 1879-3266. ; 603, s. 162-178
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Tidskriftsartikel (refereegranskat)abstract
- Microstructures and textures of calcite mylonites from the Morcles nappe large-scale shearzone in southwestern Switzerland develop principally as a function of 1) extrinsic physical parameters including temperature, stress, strain, strain rate and 2) intrinsic parameters, such as mineral composition. We collected rock samples at a single location from this shear zone, on which laboratory ultrasonic velocities, texture and microstructures were investigated and quantified. The samples had different concentration of secondary mineral phases (<5 up to 40 vol.%). Measured seismic P waveanisotropy ranges from 6.5% for polyphase mylonites (similar to 40 vol.%) to 18.4% in mylonites with <5 vol.% secondary phases. Texture strength of calcite is the main factor governing the seismic P wave anisotropy. Measured S wave splitting is generally highest in the foliation plane, but its origin is more difficult to explain solely by calcite texture. Additional texture measurements were made on calcite mylonites with low concentration of secondary phases (<= 10 vol.%) along the metamorphic gradient of the shear zone (15 km distance). A systematic increase in texture strength is observed moving from the frontal part of the shear zone (anchimetamorphism: 280 degrees C) to the higher temperature, basal part (greenschist facies: 350-400 degrees C). Calculated P wave velocities become increasingly anisotropic towards the high-strain part of the nappe, from an average of 5.8%in the frontal part to 13.2% in the root of the basal part. Secondary phases raise an additional complexity, and may act either to increase or decrease seismic anisotropy of shear zone mylonites. Inlight of our findings we reinterpret the origin of some seismically reflective layers in the Grone-Zweisimmen line in southwestern Switzerland (PNR20 Swiss National Research Program). We hypothesize that reflections originate in part from the lateral variation in textural and microstructural arrangement of calcite mylonites in shear zones.
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| 14. |
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| 15. |
- Almqvist, Bjarne, et al.
(författare)
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Seismic properties of the Kohistan oceanic arc root : insights from laboratory measurements and thermodynamic modeling
- 2013
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Ingår i: Geochemistry Geophysics Geosystems. - : American Geophysical Union (AGU). - 1525-2027. ; 14:6, s. 1819-1841
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Tidskriftsartikel (refereegranskat)abstract
- P-wave velocities (Vp) have been measured in the laboratory and calculated using thermodynamic modeling for seven representative rock samples from the lower crust to mantle section of the Kohistan paleo-island arc. Lower crustal rocks comprise plagioclase-rich gabbro, garnet-bearing gabbro, and hornblendite; mantle rocks comprise garnetite, pyroxenite, websterite, and dunite. Measurements were performed at confining pressures up to 0.5GPa and temperatures up to 1200 degrees C. Vp were also calculated using rock major element chemistry with the Perple_X software package. Calculated Vp match closely the laboratory measurements. At depths representative for the arc root, Vp of upper mantle rocks vary from 7.7-8.1km/s, whereas the lower crustal rocks have velocities between 6.9-7.5km/s. P-wave anisotropy is small, with exceptions of sheared gabbros. Measured and calculated seismic properties are consistent with, and complement a growing database of published seismic properties from the Kohistan arc. In the light of such data, we discuss seismic imaging of present-day island arcs. Intermediate Vp (7.4-7.7km/s) in arc roots can be explained by pyroxenites and garnet-bearing mafic rocks. Strong seismic reflectors may be related to garnetites (8.0-8.2km/s).
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| 16. |
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| 17. |
- Andersson, Magnus, 1977-
(författare)
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3D Structure and Emplacement of the Alnö Alkaline and Carbonatite Complex, Sweden : Integrated Geophysical and Physical Property Investigations
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Carbonatites are carbonate-rich magmatic rocks that are rare and of great relevance for our understanding of crustal and mantle processes. Although found on all continents and in settings ranging from Archaean to present-day, their deeper plumbing system is still poorly understood. Therefore, the main goal of this thesis is to broaden the existing knowledge of carbonatite systems, often limited to surface geological observations, by providing depth constraints using a number of geophysical methods and petrophysical measurements. The Alnö alkaline and carbonatite complex in central Sweden was chosen for this purpose. Data from three reflection seismic lines, ground gravity and magnetic measurements are presented. These data are complemented by a series of petrophysical measurements, including ultrasonic velocities, density, magnetic bulk susceptibility, anisotropy of magnetic susceptibility (AMS), and magnetic remanence, to aid in the interpretation of the geophysical data. The reflection seismic data indicate a solidified saucer-shaped fossil magma chamber at about 3 km depth. Caldera-style volcanism, constrained by surface geological observations, provides a plausible scenario to explain the emplacement of the complex, suggesting that carbonatite magmas have been stored, transported and erupted in a similar manner to known emplacement mechanisms for silicic calderas, although these are compositionally different. The AMS data from most of the carbonatite sheets in Alnö show a strong degree of anisotropy and oblate-shaped susceptibility ellipsoids. A set of syn- and post-emplacement processes that may control the AMS signature is evaluated based on the dataset. Overprinting of the primary flow patterns by processes related to sheet closure at the terminal stage of magma transport may explain the AMS observations. A complementary study using 3D inversion of ground gravity and aeromagnetic data was then carried out to better delineate the 3D internal architecture of the complex. Resulting models indicate a depth extent of the complex to about 3-4 km, consistent with the interpretation of the reflection seismic data. The modelling results of a ring-shaped magnetic anomaly observed in the Klingefjärden bay adjacent to Alnö Island further suggest that the complex may extend laterally about 3 km towards the north.
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| 18. |
- Andersson, Magnus, et al.
(författare)
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Magma transport in sheet intrusions of the Alnö carbonatite complex, central Sweden
- 2016
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 6
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Tidskriftsartikel (refereegranskat)abstract
- Magma transport through the Earth's crust occurs dominantly via sheet intrusions, such as dykes and cone-sheets, and is fundamental to crustal evolution, volcanic eruptions and geochemical element cycling. However, reliable methods to reconstruct flow direction in solidified sheet intrusions have proved elusive. Anisotropy of magnetic susceptibility (AMS) in magmatic sheets is often interpreted as primary magma flow, but magnetic fabrics can be modified by post-emplacement processes, making interpretation of AMS data ambiguous. Here we present AMS data from cone-sheets in the Alno carbonatite complex, central Sweden. We discuss six scenarios of syn- and post-emplacement processes that can modify AMS fabrics and offer a conceptual framework for systematic interpretation of magma movements in sheet intrusions. The AMS fabrics in the Alno cone-sheets are dominantly oblate with magnetic foliations parallel to sheet orientations. These fabrics may result from primary lateral flow or from sheet closure at the terminal stage of magma transport. As the cone-sheets are discontinuous along their strike direction, sheet closure is the most probable process to explain the observed AMS fabrics. We argue that these fabrics may be common to cone-sheets and an integrated geology, petrology and AMS approach can be used to distinguish them from primary flow fabrics.
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| 19. |
- Balashova, Anna, et al.
(författare)
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The Lake Natron Footprint Tuff (northern Tanzania) : volcanic source, depositional processes and age constraints from field relations
- 2016
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Ingår i: Journal of Quaternary Science. - : Wiley. - 0267-8179 .- 1099-1417. ; 31:5, s. 526-537
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Tidskriftsartikel (refereegranskat)abstract
- The Engare Sero Footprint Site, situated on the southern shore of Lake Natron in northern Tanzania, has been reported to host one of the best preserved sets of fossilized hominid footprints in the world. However, until now there has been no detailed characterization and age determination of the footprint-bearing strata (the Footprint Tuff). Here, we combine field observations with geochemical and mineralogical analyses and measurements of the anisotropy of magnetic susceptibility to constrain depositional processes, the role of reworking and the volcanic source for the Footprint Tuff. We find that the footprint-bearing horizon consists of volcanic ash-fall that has been slightly reworked by water, and that this was produced during a voluminous eruption of the Oldoinyo Lengai volcano. The unit, which covered the footprints and helped to preserve them, consists of the wind-blown material from the same eruption, mixed together with locally derived detrital material. We can constrain the ash horizon to be of Holocene age, based on: (i) the location of the Footprint Tuff within the regional stratigraphy, (ii) previous age determination of an ash layer that can be correlated with the Footprint Tuff and (iii) the regional climatological history of the area. The ash horizon was probably deposited around 11000-10500 years ago, but could potentially be even younger than this.
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| 20. |
- Bazargan, Mohsen, et al.
(författare)
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A Numerical Toolbox to Calculate the Seismic Properties of Micro Sized Isotropic and Anisotropic Minerals
- 2020
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Ingår i: Mineral Exploration Symposium. - : European Association of Geoscientists and Engineers. ; , s. 1-3
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Seismology offers valuable information about the nature of lithosphere and asthenosphere. Seismic waves are used to investigation the Earth’s crust and upper mantel and to understand their placement and depth. In this respect, outcrop samples’ thin sections provide a wealth of information about the rocks seismic properties. A numerical toolbox is presented to investigate the seismic properties of rock samples. For this purpose, the toolbox makes use of image processing capabilities of MATLAB combined with computational power of FEM based COMSOL multiphysics. The toolbox provides variety of studies and analyses and it is specifically used to investigate the wave speed velocities in a Gabbro sample thin-section. There are several software packages in the technical community which can calculate the seismic velocities analytically by employing the effective medium theory. The toolbox is benchmarked against the existing software packages and additional features are discussed. An objective of the numerical measurements would be the investigate of the influence of grain sizes on elastic wave velocities and potential scattering due to the wavelength effects.
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| 21. |
- 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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| 22. |
- bazargan, mohsen, et al.
(författare)
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Crystal rotations and alignment in spatially varying magma flows: Two-dimensional examples of common subvolcanic flow geometries
- 2019
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Konferensbidrag (refereegranskat)abstract
- Elongate inclusions immersed in a viscous flow generally rotate at a rate that is different from the local angular velocity of the fluid. As a result, the inclusions tend to orient themselves into a coherent pattern commonly referred to as shape-preferred orientation (SPO). The SPO of the particles can be used as a strain marker that allows the reconstruction of the fluid's velocity field. Moreover, the SPO of elongate crystals is one of the main mechanisms that lead to the anisotropy of magnetic susceptibility (AMS), which allows rapid measurement of average crystal orientations. A robust model is therefore highly desirable that can link the AMS measurements of the exposed flow to the complete velocity field when the flow was active. Much of the previous work on the dynamics of flow-induced particle rotations has been focused on spatially homogenous flows with large-scale tectonic deformations as the main application. Here, we show how the method can be extended to cover flows that vary in space as well as time, such as magma with embedded crystals moving through the volcanic plumbing system. Additionally, we introduce an evolution equation for the probability density function (PDF) of crystal orientations. The PDF evolution is coupled with the Navier-Stokes equation governing the flow, thus yielding a PDF of crystal orientations at every position. The overall correctness of the evolution equation as well as the accuracy of the numerical method are demonstrated by rigorous testing. We then apply this new theory to a number of simple, two-dimensional flow geometries commonly encountered in magmatic intrusions, such as flow from a dike into a reservoir and from a reservoir into a dike, isothermal flow inside an inflating or deflating reservoir, and thermal convection in a magma chamber. We confirm some of the previous general findings. For example, near channel walls, the flow is often parallel to the bonding surface, and the resulting simple shear flow causes preferred crystal orientations that parallel the boundary. On the other hand, where pure shear deformation dominates, there is a tendency for crystals to orient themselves in the direction of the most tensile strain rate. This occurs not only near the centre of certain flows but also along the boundaries of a deflating magma chamber and in thermal convection where the convective currents turn away from the chamber walls, thus leading to crystal orientations perpendicular to the bounding surface. Flows impinging on a boundary are instead characterizing by a most tensile strain rate that is parallel to the surface, thus leading to boundary-parallel crystal orientations. In the field, this local pattern due to an impinging flow may be difficult to distinguish from that due to simple shear flow along the wall. In other words, while a given flow will always result in a unique pattern of crystal orientations, it is generally impossible to infer the local flow field from only a local observation of the crystal PDF. Overall, we find that the crystal orientation patterns agree well with results from analogue experiments where similar geometries are available.
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| 23. |
- 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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| 24. |
- Bazargan, Mohsen, et al.
(författare)
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Elastic Wave Propagation in a Stainless-Steel Standard and Verification of a COMSOL Multiphysics Numerical Elastic Wave Toolbox
- 2022
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Ingår i: Resources. - : MDPI AG. - 2079-9276. ; 11:5
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Tidskriftsartikel (refereegranskat)abstract
- Laboratory-based elastic wave measurements are commonly used to quantify the seismic properties of Earth’s crust and upper mantle. Different types of laboratory apparatuses are available for such measurements, simulating seismic properties at different pressure and temperature. To complement such laboratory measurements, we present a numerical toolbox to investigate the seismic properties of rock samples. The numerical model is benchmarked against experimental results from a multi-anvil apparatus, using measurements of a stainless steel calibration standard. Measured values of the mean compressional- and shear-wave velocities at room conditions of the steel block were 6.03 km/s and 3.26 km/s, respectively. Calculated numerical results predicted 6.12 km/s and 3.30 km/s for compressional and shear-wave velocities. Subsequently, we measured Vp and Vs up to 600 MPa hydrostatic confining pressure and 600 °C. These measurements, at pressure and temperature, were then used as the basis to predict numerical wave speeds. There is, in general, good agreement between measurement and predicted numerical results. The numerical method presented in this study serves as a flexible toolbox, allowing for the easy setup of different model geometries and composite materials.
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| 25. |
- Bazargan, Mohsen, et al.
(författare)
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Evaluating seismic anisotropy from sources of compositional layering and crystallographic preferred orientation
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The continental collision of Laurentia and Baltica in the Paleozoic Caledonian orogeny and subsequent exhumation and erosion of the orogen exposes the mid to deep crustal structure at or near the surface of the mountain belt in Norway and western Sweden (the Scandinavian Caledonides). The Collisional Orogeny in the Scandinavian Caledonides (COSC) project focuses on understanding mountain-building processes. As part of the COSC project, scientific drilling was carried out in the summer of 2014 (COSC-1), through the amphibolite grade Lower Seve Nappe Complex (SNC). High-resolution full-waveform sonic (FWS) logging data and X-ray fluorescence (XRF) measurements on core sections from the 2.5 km deep COSC-1 borehole reveal a strong correlation between seismic properties and major element chemistry. Numerical modelling and laboratory investigations are also suggesting the strong effects of anisotropy in the seismic wave speed. The ratio between compressional- and shear wave velocity (Vp/Vs), and CaO-, and SiO2-content reflect felsic and mafic layering in the Lower Seve nappe, the effect of anisotropy and the thickness of these layers were investigated by numerical models. Integrating the Vp/Vs ratio and chemical data with seismic reflectivity and seismic anisotropy provides insight into the deformation that took place in the middle crust of the Scandinavian Caledonides.
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| 26. |
- 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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| 27. |
- 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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| 28. |
- 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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| 29. |
- Bazargan, Mohsen
(författare)
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Numerical and laboratory studies of seismic properties and elements of rock fabric from the microscale to the macroscale
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Physical properties of rocks studied in the laboratory are useful to provide constraints on the dynamics of Earth’s interior. This may include direct constraints on in-situ seismic properties, such as elastic wave velocity measurements that can be compared to seismological data, or petrofabric indicators such as anisotropy of magnetic susceptibility (AMS). Another approach that provides predictive insight into the physical properties of Earth’s interior are computer models. Numerical modelling, in particular, can be used to investigate the dynamic propagation of elastic waves or the flow of a material to generate a fabric or texture (i.e., petrofabric in rocks). This thesis focuses on an integrative approach, utilizing both laboratory measurements and numerical modelling, to understand physical properties and petrofabric development in rocks originating in Earth’s crust. The physical properties of rocks are affected by both intrinsic sources (e.g., inherent to crystals) and extrinsic sources (e.g., layering, microcracks, shape preferred orientation of crystals, grain size, presence of geological fluids). A versatile numerical elastic wave propagation model is constructed with COMSOL Multiphysicsâ and benchmarked against a stainless-steel standard used for laboratory elastic wave measurements. The numerical model is flexible and enables setup of composite materials with different sample geometries, which is of importance when modeling the physical properties of rocks in realistic geological scenarios. Using the elastic wave propagation model, this thesis explores different scenarios and their influence on seismic properties, including the effect of grain size on bulk elastic wave speed and compositional layering on seismic anisotropy. The first application focuses on a joint laboratory and numerical study of similar composition gabbro samples, with distinctly different grain size. The numerical model is used to evaluate the relationship between wavelength and grain size. The second application utilizes laboratory measurements as input data for the models, to determine seismic properties of compositionally layered materials. It is shown that the seismic properties, and in particular anisotropy, of a layered material depends on 1) the combination of the inherent rock properties and layering and 2) the wavelength (l) to layer thickness ratio (d). Importantly, independent of scale, the physical properties are wavelength dependent, showing a decrease (apparent dispersion) in velocity, when transitioning from a ray medium (l/d < 1) to an effective medium (l/d > 10). In the second part of this thesis, another COMSOL Multiphysics modelling approach is used to investigate how crystals rotate in a magmatic flow, and how a petrofabric in different magmatic rocks may develop. A set of different magmatic flow scenarios are explored, with direct application to natural examples of dykes and magma chambers. These numerical models may serve as useful predictors of petrofabric in igneous rocks where determination of flow direction of magma is of interest.
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| 30. |
- 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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| 31. |
- 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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| 32. |
- Bender, Hagen, 1987-
(författare)
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Assembly of the Caledonian Orogenic Wedge, Jämtland, Sweden
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Collisional orogeny creates the largest mountain belts on Earth. The Caledonides of Scandinavia are a deeply eroded, ancient mountain belt, which today exposes a deep section through the former orogenic interior. The orogenic internides hold important geological information necessary to understand the geodynamic processes shaping collisional plate boundaries. This thesis explores the kinematics and timing of orogenic wedge formation in Jämtland, central Sweden. An integrated approach of structural field mapping, microstructural analysis, Rb–Sr radiogenic dating and rock magnetism yielded new and comprehensive tectonochronologic data. A regionally extensive network of kinematic field data demonstrated pervasive ductile top-to-the-ESE shearing across the entire tectonostratigraphy. Rb–Sr multi-mineral isochron ages constrained the absolute timing of ductile deformation to c. 430 Ma and c. 415 Ma. Local structural and magnetic data showed that final nappe emplacement and exhumation had occurred before extensional deformation initiated. The new data presented in this thesis contradicted a tectonic model previously proposed for Caledonian nappe stacking. These findings were used to develop an alternative tectonic model consistent with both the new and other available structural, petrological and chronological data. The new model for orogenic wedge assembly comprises three stages of foreland-directed, top-to-the-ESE thrusting. It reflects the complex interactions caused by the merging of two subduction zones accommodating Baltica–arc–Laurentia collisions during Ordovician to Devonian time.
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| 33. |
- Bender, Hagen, et al.
(författare)
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Metamorphic Zonation by Out‐of‐Sequence Thrusting at Back‐Stepping Subduction Zones : Sequential Accretion of the Caledonian Internides, Central Sweden
- 2018
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Ingår i: Tectonics. - : John Wiley & Sons. - 0278-7407 .- 1944-9194. ; 37:10, s. 3545-3576
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Tidskriftsartikel (refereegranskat)abstract
- Exhumation of the high‐grade metamorphic Seve Nappe Complex and its emplacement between lower‐grade nappes has been related to wedge extrusion in the central Scandinavian Caledonides. To test this hypothesis, the kinematic evolution of the Caledonian nappe pile is studied by systematic structural mapping in central and northern Jämtland, Sweden. Structural data, combined with petrological and quartz microstructure observations, document pervasive top‐to‐the‐ESE, foreland‐directed shearing under progressively decreasing metamorphic grade across the entire nappe pile. Mylonitic foliation, foliation‐parallel boudinage, and abundant top‐to‐the‐ESE and rare, scattered top‐to‐the‐WNW shear‐sense indicators imply foreland‐directed general shear. This deformation regime caused exhumation by concurrent thrusting and vertical ductile thinning. We propose a specific succession of in‐ and out‐of‐sequence thrusts that generated the metamorphic zonation. Our model envisions in‐sequence propagation of thrusts during exhumation of the Seve Nappe Complex, related to subduction of Baltica beneath a volcanic arc within Iapetus. Concurrently, Iapetus subducted beneath Laurentia farther to the west. When Iapetus was closed, Baltica subduction stepped westward and continued beneath Laurentia. The back stepping of subduction at the onset of continental collision caused out‐of‐sequence propagation of the orogenic wedge. Thrusting cut downsection across the existing tectonostratigraphy, emplacing units of lower metamorphic grade above the high‐grade Seve Nappe Complex. This imbrication generated the present metamorphic zonation of the Caledonian nappe pile during sustained convergence between Laurentia and Baltica.
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| 34. |
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| 35. |
- Bender, Hagen, 1987-
(författare)
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Testing Tectonic Concepts in the Seve Nappe Complex, Jämtland, Sweden
- 2016
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- When continental plates collide, one of the involved continents is subducted beneath the other one. As a consequence, the lithosphere thickens along the convergent plate boundary and causes the formation of a mountain belt. High-grade metamorphic rocks, formerly parts of the subducted continent, can be exhumed back to the surface and are commonly found in the central domain of mountain belts. The leucogranite- bearing Seve Nappe Complex in the central part of the Scandinavian Caledonides is a good example of such commonly migmatic rock units, which hold the key to understanding the tectonic evolution of a mountain range. This study aims to develop a tectonic model for the Swedish Caledonides, which integrates new structural data, collected during extensive fieldwork, and new geochronological constraints. Here we present the results of the first part of this study. We show how top-to-the-foreland directed shearing affected the migmatic part of the Seve Nappe Complex from bottom to top at amphibolite-facies conditions. Subsequently, the entire Caledonian nappe stack underwent a greenschist-facies overprint, associated with pervasive, again, top- to-the-foreland directed shearing. This last tectonic event resulted in the assembly of the presently observed nappe architecture, which is characterized by the excision of large sections of the lithosphere.
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| 36. |
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| 37. |
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| 38. |
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| 39. |
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| 40. |
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| 41. |
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| 42. |
- Bukala, Michal, et al.
(författare)
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Brittle deformation during eclogitization of early Paleozoic blueschist
- 2020
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Ingår i: Frontiers in Earth Science. - LAUSANNE SWITZERLAND : Frontiers Media SA. - 2296-6463. ; 8
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Tidskriftsartikel (refereegranskat)abstract
- The Tsakkok Lens of the Scandinavian Caledonides represents the outermost Baltican margin that was subducted in late Cambrian/Early Ordovician time during closure of the Iapetus Ocean. The lens predominantly consists of metasedimentary rocks hosting eclogite bodies that preserve brittle deformation on the mu m-to-m scale. Here, we present a multidisciplinary approach that reveals fracturing related to dehydration and eclogitization of blueschists. Evidence for dehydration is provided by relic glaucophane and polyphase inclusions in garnet consisting of clinozoisite + quartz +/- kyanite +/- paragonite that are interpreted as lawsonite pseudomorphs. X-Ray chemical mapping of garnet shows a network of microchannels that propagate outward from polyphase inclusions. These microchannels are healed by garnet with elevated Mg relative to the surrounding garnet. Electron backscatter diffraction mapping revealed that Mg-rich microchannels are also delimited by low angle (<3 degrees) boundaries. X-ray computed microtomography demonstrates that some garnet is transected by up to 300 mu m wide microfractures that are sealed by omphacite +/- quartz +/- phengite. Locally, mesofractures sealed either by garnet- or omphacite-dominated veins transect through the eclogites. The interstices within the garnet veins are filled with omphacite + quartz + rutile + glaucophane +/- phengite. In contrast, omphacite veins are predominantly composed of omphacite with minor apatite + quartz. Omphacite grains are elongated along [001] crystal axis and are preferably oriented orthogonal to the vein walls, indicating crystallization during fracture dilation. Conventional geothermobarometry using omphacite, phengite and garnet adjacent to fractures, provides pressure-temperature conditions of 2.47 +/- 0.32 GPa and 620 +/- 60 degrees C for eclogites. The same method applied to a mesoscale garnet vein yields 2.42 +/- 0.32 GPa at 635 +/- 60 degrees C. Zirconium-in-rutile thermometry applied to the same garnet vein provides a temperature of similar to 620 degrees C. Altogether, the microchannels, microfractures and mesofractures represent migration pathways for fluids that were produced during glaucophane and lawsonite breakdown. The microfractures are likely precursors of the mesoscale fractures. These dehydration reactions indicate that high pore-fluid pressure was a crucial factor for fracturing. Brittle failure of the eclogites thus represents a mechanism for fluid-escape in high-pressure conditions. These features may be directly associated with seismic events in a cold subduction regime.
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| 43. |
- Burchardt, Steffi, 1982-, et al.
(författare)
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Progressive growth of the Cerro Bayo cryptodome, Chachahuén volcano, Argentina : implications for viscous magma emplacement
- 2019
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Ingår i: Journal of Geophysical Research - Solid Earth. - 2169-9313 .- 2169-9356. ; 124, s. 7934-7961
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Tidskriftsartikel (refereegranskat)abstract
- Cryptodome and dome collapse is associated with volcanic hazards, such as, explosive eruptions, pyroclastic density currents, and volcanic edifice collapse. The study of the growth and evolution of volcanic domes provides vital information on the link between dome growth and the development of weakness zones that may cause collapse. The Cerro Bayo cryptodome is superbly exposed in the eroded Miocene Chachahuén volcano in the Neuquén basin, Argentina. Cerro Bayo is a >0.3 km3 trachyandesitic cryptodome that intruded within the uppermost kilometer of the Chachahuén volcano. Here we investigate the emplacement of the Cerro Bayo cryptodome using structural mapping, photogrammetry, 3D structural modelling and measurement of magma flow indicators, brittle deformation features and magnetic fabrics with anisotropy of magnetic susceptibility (AMS). Magma flow fabrics near the margin are concentric and indicate contact-parallel flow and internal inflation of the body. Magmatic and magnetic fabrics and fracture patterns in the interior of the cryptodome are more complex and outline several structural domains. These domains are separated by magmatic shear zones that accommodated intrusion growth. The shear zones locally overprint the earlier formed concentric fabric. The nature of the structural domains shows that emplacement of Cerro Bayo occurred in three stages that resemble the endogenous to exogenous growth of volcanic domes. The formation of magmatic shear zones during cryptodome formation may have a profound effect on cryptodome stability by creating weakness zones that increase the risk of collapse.
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| 44. |
- Burchardt, Steffi, et al.
(författare)
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Progressive growth of the Cerro Bayo cryptodome, Chachahuén volcano, Argentina – implications for viscous magma emplacement
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Cryptodome and dome collapse is associated with volcanic hazards, such as, explosive eruptions, pyroclastic flows, and volcanic edifice collapse. Study of the growth and evolution of volcanic domes provides vital information on the link between dome growth and the development of weakness zones that may cause collapse. The Cerro Bayo cryptodome is superbly exposed in the eroded Miocene Chachahuén volcano in the Neuquén basin, Argentina. Cerro Bayo is a >0.3 km3 trachyandesitic cryptodome that intruded within the uppermost kilometer of the Chachahuén volcano. Here we investigate the emplacement of the Cerro Bayo cryptodome using structural mapping, photogrammetry, 3D structural modelling and measurement of magma flow indicators, brittle deformation features and magnetic fabrics with anisotropy of magnetic susceptibility (AMS). Magma flow fabrics near the margin are concentric and indicate contact-parallel flow and internal inflation of the body. Magmatic and magnetic fabrics and fracture patters in the interior of the cryptodome are more complex and outline several structural domains. These domains are separated by magmatic shear zones that accommodated intrusion growth. The shear zones locally overprint the earlier formed concentric fabric. The nature of the structural domains shows that emplacement of Cerro Bayo occurred in three stages that resemble the endogenous to exogenous growth of volcanic domes. The formation of magmatic shear zones during cryptodome formation may have a profound effect on cryptodome stability by creating weakness zones that increase the risk of collapse.
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| 45. |
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| 46. |
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| 47. |
- Cyprych, Daria, et al.
(författare)
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Seismic anisotropy from compositional banding in granulites from the deep magmatic arc of Fiordland, New Zealand
- 2017
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Ingår i: Earth and Planetary Science Letters. - : Elsevier BV. - 0012-821X .- 1385-013X. ; 477, s. 156-167
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Tidskriftsartikel (refereegranskat)abstract
- We present calculated seismic velocities and anisotropies of mafic granulites and eclogites from the Cretaceous deep lower crust (similar to 40-65 km) of Fiordland, New Zealand. Both rock types show a distinct foliation defined by cm-scale compositional banding. Seismic properties are estimated using the Asymptotic Expansion Homogenisation - Finite Element (AEH-FE) method that, unlike the commonly used Voigt-Reuss-Hill homogenisation, incorporates the phase boundary network into calculations. The predicted mean P- and S-wave velocities are consistent with previously published data for similar lithologies from other locations (e.g., Kohistan Arc), although we find higher than expected anisotropies (AV(P) similar to 5.0-8.0%, AV(S) similar to 3.0-6.5%) and substantial S-wave splitting along foliation planes in granulites. This seismic signature of granulites results from a density and elasticity contrast between cm-scale pyroxene +/- garnet stringers and, plagioclase matrix rather than from crystallographic orientations alone. Banded eclogites do not show elevated anisotropies as the contrast in density and elastic constants of garnet and pyroxene is too small. The origin of compositional banding in Fiordland granulites is primarily magmatic and structures described here are expected to be typical for the base of present day magmatic arcs. Hence, we identify a new potential source of anisotropy within this geotectonic setting.
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| 48. |
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| 49. |
- Elger, Judith, et al.
(författare)
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Core‐log‐seismic integration in metamorphic rocks and its implication for the regional geology : A case study for the ICDP drilling project COSC‐1, Sweden
- 2021
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Ingår i: Geochemistry Geophysics Geosystems. - : American Geophysical Union (AGU). - 1525-2027. ; 22:3
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Tidskriftsartikel (refereegranskat)abstract
- Continental collision causes deformation in the crust along shear zones. However, the physical and chemical conditions at which these zones operate and the deformation processes that enable up to hundreds of km of tectonic transport are still unclear because of the depth at which they occur and the challenges in imaging them. Ancient exhumed collision zones allow us to investigate these processes much better, for example at the COSC‐1 borehole in the central Scandinavian Caledonides. This study combines data from the COSC‐1 borehole with different seismic measurements to provide constraints on the spatial lithological and textural configuration of the Seve Nappe Complex. This is one of the few studies that shows that core‐log‐seismic integration in metamorphic rocks allows to identify the spatial distribution of major lithological units. Especially gamma ray logs in combination with density data are powerful tools to distinguish between mafic and felsic lithologies in log‐core correlation. Our results indicate that reflections along the borehole are primarily caused by compositional rather than textural changes. Reflections in the Seve Nappe Complex are not as distinct as in greater depths but continuous and several of them can be linked to magmatic intrusions, which have been metamorphically overprinted. Their setting indicates that the Seve Nappe Complex consists of the remnants of a volcanic continental margin. Our results suggest that ductile‐deformed middle crustal reflectivity is primarily a function of pre‐orogenic lithological variations which has to be considered when deciphering mountain building processes.Plain Language SummaryAreas where continents collide experience different kind of deformation. However, these processes and the conditions at which they take place are difficult to study because of the great depth at which they occur. Former collision zones that are closer to the surface these days allow the investigation of these processes much better, for example at the COSC‐1 borehole in the central Scandinavian Caledonides. The challenge remains to image the remnant of these processes in high detail but at the same time over a large area. This study combines data from the COSC‐1 borehole with different geophysical measurements to better understand the lithology and structure of the Seve Nappe Complex. We show that the combination of these data allows us to distinguish between rocks from mafic and sedimentary origin. Our results indicate that the geophysical data along the borehole image the change of the composition of the rocks which probably originates from magmatic intrusions and have been overprinted by geological processes, rather than from fracture zones.
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| 50. |
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