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  • 1.
    Bazargan, Mohsen
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Almqvist, Bjarne
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Hieronymus, Christoph F.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Piazolo, Sandra
    School of Earth and Environment, University of Leeds, Leeds, United Kingdom.
    Bahadour Motra, Hem
    Department of Geosciences, Christian-Albrechts-Universität zu Kiel, Kiel, Germany.
    Broumand, Pooyan
    Department of Civil Engineering, Shiraz University, Shiraz, Iran.
    Schmiedel, Tobias
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Numerical, predictive and experimental study on elastic wave propagation in crystalline rocks2021Konferansepaper (Annet vitenskapelig)
    Abstract [en]

    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.

    Fulltekst (pdf)
    fulltext
  • 2.
    Bazargan, Mohsen
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik. Rock Engineering, Geosigma Part of Rejlers; WSL Institute for Snow and Avalanche Research SLF.
    Almqvist, Bjarne S. G.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Motra, Hem Bahadur
    Broumand, Pooyan
    Schmiedel, Tobias
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Mineralogi, petrologi och tektonik.
    Hieronymus, Christoph F.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Elastic Wave Propagation in a Stainless-Steel Standard and Verification of a COMSOL Multiphysics Numerical Elastic Wave Toolbox2022Inngår i: Resources, E-ISSN 2079-9276, Vol. 11, nr 5, artikkel-id 49Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    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.

    Fulltekst (pdf)
    fulltext
  • 3.
    Bazargan, Mohsen
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Almqvist, Bjarne Sven Guestav
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik. Uppsala University.
    Motra, Hem
    Christian-Albrechts-University of Kiel.
    Hieronymus, Christoph F.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Piazolo, Sandra
    University of Leeds.
    Joint pressure and temperature effects on seismic properties of gneisses and amphibolite2021Konferansepaper (Annet vitenskapelig)
    Abstract [en]

    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 600<sup>o</sup>C 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 600<sup>o</sup>C 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.

    Fulltekst (pdf)
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  • 4.
    Bazargan, Mohsen
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Broumand, P.
    Motra, H. B.
    Almqvist, Bjarne
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Hieronymus, Christoph
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Piazolo, S.
    A Numerical Toolbox to Calculate the Seismic Properties of Micro Sized Isotropic and Anisotropic Minerals2020Inngår i: Mineral Exploration Symposium: Conference Proceedings, European Association of Geoscientists and Engineers, 2020, s. 1-3Konferansepaper (Annet vitenskapelig)
    Abstract [en]

    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.

  • 5.
    Bazargan, Mohsen
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Hieronymus, Christoph F.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Vachon, Remi
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Evolution of the statistical distribution of crystal orientations in time- and space-varying viscous flows2019Inngår i: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 218, nr 2, s. 773-786Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Magmas and other viscously deforming fluids in the Earth frequently contain embedded crystals or other solid inclusions. These inclusions generally rotate about their own axis and, under certain conditions, align themselves in a direction dictated by the details of the flow. This rotational behaviour has been studied extensively for homogeneous flows. Here, we couple the crystal rotation dynamics with the fluid mechanical Navier-Stokes equations for the large-scale flow, thus allowing the analysis of crystal rotations in settings that are variable in both space and time. The solution is valid provided that the intercrystal spacing is sufficiently large to preclude interaction between crystals. Additionally, we derive an evolution equation for the probability density function (PDF) of crystal orientations based on the fundamental concept of conservation of generic properties in continuum mechanics. The resulting system of equations is extensively tested against previous analytical and numerical solutions. Given the focus on method validation, we limit the fluid mechanics to simple systems with analytical solutions for the velocity field. Even for the simple examples computed, all of which are characterized by fluid flow that is constant in time, the crystal orientation patterns are spatially complex and change in time. Pressure-driven flow in a channel results in coherent bands of crystal orientations with band thickness decreasing towards the channel walls. In corner flow constrained by two mutually perpendicular walls, the pattern of crystal orientations does not exhibit any significant similarity with the flow field. Given that there is no local one-to-one correspondence between the flow and the PDF pattern, a combined and larger-scale solution of the two systems is generally required. The simple flow examples shown demonstrate the viability of this new approach. Application to more complex flow geometries which may commonly occur in nature is deferred to future studies.

  • 6.
    Bazargan, Mohsen
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Motra, Hem Bahadur
    Institute for Geosciences, Christian Albert University of Kiel, Kiel, Germany.
    Almqvist, Bjarne
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Piazolo, Sandra
    School of Earth and Environment, Institute of Geophysics and Tectonics, University of Leeds, Leeds, United Kingdom.
    Hieronymus, Christoph
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Pressure, temperature and lithological dependence of seismic and magnetic susceptibility anisotropy in amphibolites and gneisses from the central Scandinavian Caledonides2021Inngår i: Tectonophysics, ISSN 0040-1951, E-ISSN 1879-3266, Vol. 820, artikkel-id 229113Artikkel i tidsskrift (Annet vitenskapelig)
    Abstract [en]

    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.

    Fulltekst (pdf)
    fulltext
  • 7.
    Bergbauer, S.
    et al.
    University of Hawaii.
    Martel, S.J.
    University of Hawaii.
    Hieronymus, C.F.
    University of Hawaii.
    Thermal stress evolution in cooling pluton environments of different geometries1998Inngår i: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 25, nr 5, s. 707-71-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Thermoelastic displacement potentials and fast Fourier transforms can be combined to rapidly calculate the thermal stresses in 2-D for plutons that cool by conduction. First, temperature distributions over time are computed by solving the diffusion equation. Thermal stresses are then obtained using thermoelastic stress potentials. This method can be applied to a broad range of pluton geometries and initial conditions, and requires far less computation time than finite difference or finite element analyses. Results of 2-D analyses show that pluton geometry strongly influences the thermal stresses that occur in a cooling pluton. Thermal stresses of several tens of MPa arise during cooling and are highest at the corners or where the intrusion is thin. The most tensile stress is greater inside a pluton than in the host rock. Moreover, the orientation of the most tensile stress in a cooling pluton generally changes over time. This could result in multiple fracture sets, which would significantly affect the mechanical and hydraulic behavior of a pluton.

  • 8.
    Eken, Tuna
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Shomali, Zaher Hossein
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Roberts, Roland
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Hieronymus, Christoph F.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Bödvarsson, Reynir
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    S and P velocity heterogeneities within the upper mantle below the Baltic Shield2008Inngår i: Tectonophysics, ISSN 0040-1951, E-ISSN 1879-3266, Vol. 462, nr 1-4, s. 109-124Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Upper mantle structure beneath the Baltic (Fennoscandian) Shield is investigated using non-linear tomographic inversion of relative arrival-time residuals. 52 selected teleseismic earthquakes recorded by 45 broadband stations of the Swedish National Seismological Network (SNSN) provide 1532 good quality S-wave relative arrival times. SV and SH arrival-time residuals were initially analyzed independently, providing two separate models. These reveal several consistent major features, many of which are also consistent with P-wave results. Lateral velocity variations of ± 3–4% are observed to depths of at least 470 km. The correlation between the SH and SV models is investigated and shows a pattern of minor but significant differences down to around 150–200 km depth, below which the models are essentially similar. Direct cell by cell comparison of the model velocities reveals a similar pattern, with velocity differences between the models of up to 4%. Numerical tests show that differences in the two S-wave models can only be partially attributed to noise and limited resolution, and some features are attributed to the effect of large scale anisotropy. One of the significant and sharp discrepancies between the S models coincides with a presumed boundary between Archean and Proterozic domains, suggesting different anisotropic characteristics in the two regions.

  • 9.
    Hieronymus, C. F.
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Goes, S.
    Department of Earth Science and Engineering, Imperial College, London, UK.
    Sargent, M.
    Max-Planck Institut für Astronomie, Heidelberg, Germany.
    Morra, G.
    Department of Earth Sciences, Institute of Geophysics, ETH Zürich, Zürich, Switzerland.
    A dynamical model for generating Eurasian lithospheric stress and strain rate fields: Effect of rheology and cratons2008Inngår i: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 113, nr B7, s. B07404-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    For most continents, stress models driven by plate boundary forces have successfully reproduced the main characteristics of the stress field. However, Eurasia has remained a challenge due to its large areas of intraplate deformation. We present a set of three-dimensional models of the upper mantle lithosphere system for a simplified geometry of the Eurasian plate where we try to match the first-order characteristics of the stress and strain rate fields simultaneously. For typical elastic, viscous, or plastic rheologies, high stress levels are required in order to produce realistic convergence rates between India and Asia. Our models show robustly that such stresses are transmitted throughout most of the plate, dominating locally generated stresses even in distal regions such as Europe in a manner that is not compatible with observations. Cratons with roots that extend deep into the mantle are unable to provide a significant stress-shielding effect unless the viscosity contrast between the asthenosphere and the underlying mantle is around 100 or greater. A damage rheology for the lithosphere with history-dependent behavior and material softening by a viscosity reduction of several orders of magnitude is shown to eliminate this conundrum. Continental convergence at high velocity but low stress is facilitated by the formation of long-lived shear zones similar to those observed north of the Himalayas. The low stress associated with the collision, together with the decoupling effect of the shear zones, causes the distal stress field in Europe to be controlled by the effects of the neighboring boundaries in agreement with observations.

  • 10.
    Hieronymus, C.F.
    et al.
    Danish Lithosphere Centre.
    Bercovici, D.
    University of Hawaii.
    A theoretical model of hotspot volcanism: Control on volcanic spacing and patterns via magma dynamics and lithospheric stresses2001Inngår i: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 106, nr B1, s. 683-702Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Many linear island chains are thought to be the result of the steady motion of the lithospheric plates over stationary hotspots. The occurrence of discrete, nearly regularly spaced volcanoes, rather than continuous ridges, are assumed to be caused by the interaction of flexural stresses (due to the volcanic loads) with magma percolation. A parameter study is performed on a simple model that incorporates these effects in addition to dike wall erosion. It is demonstrated that the strength of the erosional feedback determines whether the model generates discrete volcanoes or a continuous ridge. The intervolcanic spacing depends not only on the elastic thickness of the lithosphere but also on the magma pressure at the base of the lithosphere. The size of the eruptive region of the individual volcanoes is controlled by the elastic response of the lithosphere to magma overpressurization. If an initial off-axis edifice is introduced, the model is able to preserve this asymmetry and produce an alternating series of volcanoes. A small initial perturbation grows over time, resulting in double lines or wider patterns depending on the width of the magma source region. Single lines of volcanoes therefore indicate very narrow magma source regions.

  • 11.
    Hieronymus, C.F.
    et al.
    Danish Lithosphere Centre.
    Bercovici, D.
    University of Hawaii.
    Discrete alternating hotspot islands formed by interaction of magma transport and lithospheric flexure1999Inngår i: Nature, ISSN 0028-0836, E-ISSN 1476-4687, Vol. 397, nr 6720, s. 604-607Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The large-scale geometry and age progression of many hotspot island chains, such as the Hawaiian-Emperor chain, are well explained by the steady movement of tectonic plates over stationary hotspots. But on a smaller scale, hotspot tracks are composed of discrete volcanic islands whose spacing correlates with lithospheric thickness(1). Moreover, the volcanic shields themselves are often not positioned along single lines, but in more complicated patterns, such as the dual line known as the Kea and Loa trends of the Hawaiian islands(2,3). Here we make use of the hypothesis that. island spacing is controlled by lithospheric flexure(1) to develop a simple nonlinear model coupling magma flow, which feeds volcanic growth, to the flexure caused by volcanic loads on the underlying plate. For a steady source of melt underneath a moving lithospheric plate, magma is found to reach the surface and build a chain of separate volcanic edifices with realistic spacing. If a volcano is introduced away from the axis of the chain, as might occur following a change in the direction of plate motion, the model perpetuates the asymmetry for long distances and times, thereby producing an alternating: series of edifices similar to that observed in the Kea and Loa trends of the Hawaiian island chain.

  • 12.
    Hieronymus, C.F.
    et al.
    Danish Lithosphere Centre.
    Bercovici, D.
    Yale University.
    Focusing of eruptions by fracture wall erosion2001Inngår i: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 28, nr 9, s. 1823-1826Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Lithospheric flexural stresses beneath volcanic loads are horizontally strongly compressive towards the top of the lithosphere. Thus, while magma transport through the brittle lithosphere occurs via fractures, the fracture paths under the volcanic center are predicted by stress trajectories to be horizontal and thus unable to supply melt to the volcanic edifice where eruptions are observed. Moreover, the magnitude of the compressive stresses under large loads would close down any vertical magma paths. Both problems may be resolved by additional stresses due to melting or thermomechanical erosion of fracture walls developing over the life-span of the volcano. Fractures form and close frequently in the seismogenic zone of the lithosphere, with each fracture eroding away a small amount of material. The total amount of material removed makes the stress field more tensile, thereby facilitating the long-lived and vertically oriented magma pathways necessary to build discrete volcanic structures.

  • 13.
    Hieronymus, C.F.
    et al.
    Danish Lithosphere Centre.
    Bercovici, D.
    University of Hawaii.
    Non-hotspot formation of volcanic chains: control of tectonic and flexural stresses on magma transport2000Inngår i: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 181, nr 4, s. 539-554Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The South Pacific, in the vicinity of both the superswell and the East Pacific Rise, is repleat with volcanic chains that, unlike the Emperor-Hawaiian Chain, defy the hypothesis of formation via the relative motion of plates and hotspots. We propose two nearly identical models for the origin of near-axis and superswell chains, assuming that both regions are underlain by significant quantities of more or less uniformly distributed partial melt. Given an initial volcanic load or a local anomaly in the melt source region, volcanic chains form by magmatic hydrofracture at local tensile maxima of flexural and membrane stresses. Fracture wall erosion by magma flow provides a feedback which results in discrete edifices within the chains. The model predicts island chains aligned with a deviatorically tensile tectonic stress. Near the ridge, the elastic lithosphere is thin, and observations and theoretical considerations indicate a strong deviatorically tensile stress field perpendicular to the ridge axis. Under such conditions, the model results in parallel lines of volcanoes perpendicular to the spreading ridge. Later, interstitial volcanism within the individual chains reduces the average spacing and results in nearly continuous ridges. On the thicker lithosphere of the superswell, membrane stresses are negligible and the model produces chains of much more widely spaced volcanoes. A more isotropic stress field may result in broader chain-like patterns of volcanoes. In both cases, the chains represent self-propagating disturbances; the resulting age progressions are thus independent of plate velocity, but depend only on the dynamics of volcano formation and evolution.

  • 14.
    Hieronymus, Christoph
    Danish Lithosphere Centre, Ostervoldgade 10, L, Copenhagen K, 1350, Denmark.
    Control on seafloor spreading geometries by stress- and strain-induced lithospheric weakening2004Inngår i: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 222, nr 1, s. 177-189Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Seafloor spreading typically occurs along ridge segments oriented at right angles to plate motion and offset by orthogonal transform faults. Few regions exhibit different patterns, such as the East Pacific Rise (EPR), which additionally displays overlapping spreading centres (OSCs) and microplates. We introduce a dynamical model using two independent, scalar types of damage in an elastic plate that generates most observed spreading geometries as natural failure modes, suggesting that the dynamics of the underlying mantle have only a minor influence on accretionary plate margins. The elastic modulus that is affected by the damage determines the type of localized deformation. Damage reducing the bulk modulus tends to result in tensile fractures, while a reduction in shear modulus leads to shear fractures. The damage source determines the fracture orientation. Material weakening in tension results in fractures perpendicular to the most tensile principal stress, while shear weakening results in two conjugate fractures at 45degrees relative to the applied stress. Strain or energy-dependent damage results in propagating, localized fractures. Stress-dependent damage tends to evolve into diffuse regions that may eventually focus into narrow zones. Starting from small perturbations with reduced elastic moduli as nucleation points, all ridge geometries start with ridge propagation caused by tensile energy reducing both elastic moduli by a model of damage caused by tensile energy reducing both moduli. Orthogonal transform faults develop in regions between offset segments if there is an additional reduction in shear modulus due to shear stress. The orthogonality of the transform faults does not derive from the local stress orientation but from the dynamics of damage focusing which causes the fault to converge towards an optimal geometry that concentrates nearly all deformation into damaged zones. OSCs form when the shear damage leading to transform faults is suppressed, while microplate formation requires additional reduction of the shear modulus by tensile energy. Oblique spreading at 45degrees. recently discovered along ultraslow spreading ridges, occurs when both moduli are weakened by shear energy. A parameter regime exists in which ridge-transform patterns develop at low applied tension, while microplates form at higher stresses. These results indicate that at least three different micromechanical processes operate with different evolution rates. OSCs and oblique spreading require different material properties.

  • 15.
    Hieronymus, Christoph F.
    Institute of Geophysics, ETH Zürich, Switzerland.
    Time-dependent strain localization in viscous media with state-dependent viscosity2006Inngår i: Physics of the Earth and Planetary Interiors, ISSN 0031-9201, E-ISSN 1872-7395, Vol. 157, nr 3-4, s. 151-163Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The temporal evolution of viscous rheologies with dependence on an additional state variable is examined. A localization measure is introduced that quantifies the change in the degree of localization in time. Three sample rheologies are analyzed in detail, each representative of a larger class of rheologies: (1) a grain-size dependent viscosity with grain growth and diminution, (2) shear heating with temperature-dependence according to the Arrhenius law, and (3) shear heating with temperature dependence in the Frank-Kamenetzky approximation. All three rheologies display stages of temporal increase and decrease of localization, depending on the initial conditions. This localization behavior is not discernible in plots of strain rate versus strain at constant driving stress which are the typical output of creep experiments. The grain-size dependence of olivine leads to effectively non-Newtonian behavior with a stress exponent of about 5. If the laws describing the grain-size evolution are applicable to the mantle, then the lower mantle and those parts of the upper mantle that are dominated by diffusion creep thus have a stronger stress-dependence than the depth range governed by dislocation creep, provided that the timescale of deformation is greater than 10^5 –10^6 years.

  • 16.
    Hieronymus, Christoph F.
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Shomali, Zaher Hossein
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Pedersen, Laust B.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    A dynamical model for generating sharp seismic velocity contrasts underneath continents: Application to the Sorgenfrei-Tornquist Zone2007Inngår i: Earth and Planetary Science Letters, ISSN 0012-821X, E-ISSN 1385-013X, Vol. 262, nr 1-2, s. 77-91Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    New seismic velocity models based on teleseismic traveltime tomography show a sharp lithospheric boundary at the Sorgenfrei–Tornquist Zone (STZ) between 100 and 250 km depth with P-waves about 4% faster and S-waves 6% faster within the cratonic lithosphere to the north. Experiments and thermodynamic calculations indicate that seismic velocity differences in the shallow mantle down to the transition zone must be mostly of thermal origin as typical mantle rocks are characterized by similar velocities based on composition alone. We propose a dynamical model of convection in the upper mantle that is consistent with rheological data and that satisfies the seismic observations by maintaining an abrupt lateral temperature contrast over hundreds of Myrs. A step-like increase in lithospheric thickness from 100 to 250 km is assumed to have formed in a Triassic rifting event at the STZ (around 220 Ma) and is subsequently exposed to active convection below. A lithosphere that is distinct from the mantle in terms of temperature and composition remains stable against convective erosion. Heat advection to different depth beneath the thin and the thick lithosphere leads to a maximum horizontal contrast of 500 °C at 150 km depth over a lateral transition distance of 100 km, sufficient to generate 5% and 8% in maximum P- and S-wave velocity perturbation, respectively. A purely conductive model under the same conditions yields only Δvp ≈ 1% and Δvs ≈ 2%, while a lithospheric evolution simulation without a compositional effect on the rheology leads to significant thermo-mechanical erosion of the lithosphere giving Δvp ≈ 2% and Δvs ≈ 4%.

  • 17.
    Hieronymus, Christoph
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Goes, S.
    Complex cratonic seismic structure from thermal models of the lithosphere: effects of variations in deep radiogenic heating2010Inngår i: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 180, nr 3, s. 999-1012Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Cratons are the long-term tectonically stable cores of the continents. Despite their thermal stability they display substantial seismic complexity with lateral and vertical lithospheric anomalies of up to several percent in both V(S) and V(P). Although some of these anomalies have been correlated with compositional variations, others are too large to be explained with any common mantle lithosphere compositions ranging from fertile peridotites to highly melt-depleted dunites, under the assumption that thermal perturbations are negligible. To test whether temperature anomalies could contribute to seismic complexity, we performed a set of 2-D thermal calculations for a range of cratonic tectonic models and converted them into seismic structure, accounting for variations in phase and elastic and anelastic response to pressure and temperature. With the long thermal equilibration time in cratonic settings, even relatively mild variations in concentrations of radioactive elements can leave long-lasting lithospheric thermal anomalies of 100-300 degrees C. Concentrations of radioactive elements decrease with increasing melt depletion ( or decreasing metasomatic refertilization), resulting in lower temperatures and increased seismic velocities. This thermal seismic effect enhances the intrinsic velocity-increasing compositional seismic signature of melt depletion. The joint thermochemical effects can leave cratonic seismic anomalies of up to 3-4.5 per cent in V(S) and up to 2.5-4 per cent in V(P), with gradients sometimes as sharp as a few kilometre in width. Thus the variations in major and minor element mantle lithosphere composition commonly seen in mantle samples can account for much of the variability in imaged seismic structure of cratonic lithosphere.

  • 18.
    Hieronymus, Christoph
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Goes, Saskia
    Department of Earth Science and Engineering, Imperial College London, London, UK.
    Complex cratonic seismic structure from thermal models of the lithosphere: effects of variations in deep radiogenic heating2010Inngår i: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 180, nr 3, s. 999-1012Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Cratons are the long-term tectonically stable cores of the continents. Despite their thermal stability they display substantial seismic complexity with lateral and vertical lithospheric anomalies of up to several percent in both  VS and  VP . Although some of these anomalies have been correlated with compositional variations, others are too large to be explained with any common mantle lithosphere compositions ranging from fertile peridotites to highly melt-depleted dunites, under the assumption that thermal perturbations are negligible. To test whether temperature anomalies could contribute to seismic complexity, we performed a set of 2-D thermal calculations for a range of cratonic tectonic models and converted them into seismic structure, accounting for variations in phase and elastic and anelastic response to pressure and temperature. With the long thermal equilibration time in cratonic settings, even relatively mild variations in concentrations of radioactive elements can leave long-lasting lithospheric thermal anomalies of 100–300 °C. Concentrations of radioactive elements decrease with increasing melt depletion (or decreasing metasomatic refertilization), resulting in lower temperatures and increased seismic velocities. This thermal seismic effect enhances the intrinsic velocity-increasing compositional seismic signature of melt depletion. The joint thermochemical effects can leave cratonic seismic anomalies of up to 3–4.5 per cent in  VS and up to 2.5–4 per cent in  VP , with gradients sometimes as sharp as a few kilometre in width. Thus the variations in major and minor element mantle lithosphere composition commonly seen in mantle samples can account for much of the variability in imaged seismic structure of cratonic lithosphere.

  • 19.
    Krumbholz, Michael
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Berggrundsgeologi.
    Hieronymus, Christoph
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Burchardt, Steffi
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Berggrundsgeologi.
    Troll, Valentin
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Berggrundsgeologi.
    Tanner, David
    Leibniz Institute of Applied Geophysics.
    Friese, Nadine
    Wintershall Norge AS.
    Weibull-distributed dyke thickness reflects probabilistic character of host-rock strength2014Inngår i: Nature Communications, E-ISSN 2041-1723, Vol. 5, s. 3272-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Magmatic sheet intrusions (dykes) constitute the main form of magma transport in the Earth’s crust. The size distribution of dykes is a crucial parameter that controls volcanic surface deformation and eruption rates and is required to realistically model volcano deformation for eruption forecasting. Here we present statistical analyses of 3,676 dyke thickness measurements from different tectonic settings and show that dyke thickness consistently follows the Weibull distribution. Known from materials science, power law-distributed flaws in brittle materials lead to Weibull-distributed failure stress. We therefore propose a dynamic model in which dyke thickness is determined by variable magma pressure that exploits differently sized host-rock weaknesses. The observed dyke thickness distributions are thus site-specific because rock strength, rather than magma viscosity and composition, exerts the dominant control on dyke emplacement. Fundamentally, the strength of geomaterials is scale-dependent and should be approximated by a probability distribution.

    Fulltekst (pdf)
    fulltext
  • 20.
    Lund, Björn
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Schmidt, Peter
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Hieronymus, Christoph
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Stress evolution and fault stability during the Weichselian glacial cycle2009Rapport (Annet vitenskapelig)
  • 21.
    Rhodes, Emma
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Mineralogi, petrologi och tektonik. Uppsala Univ, Ctr Nat Hazards & Disaster Sci, Uppsala, Sweden..
    Barker, Abigail
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Mineralogi, petrologi och tektonik. Uppsala Univ, Ctr Nat Hazards & Disaster Sci, Uppsala, Sweden..
    Burchardt, Steffi
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Mineralogi, petrologi och tektonik. Uppsala Univ, Ctr Nat Hazards & Disaster Sci, Uppsala, Sweden..
    Hieronymus, Christoph F.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Rousku, S. N.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    McGarvie, D. W.
    Univ Lancaster, Lancaster Environm Ctr, Lancaster, England..
    Mattsson, T.
    Univ St Andrews, Sch Earth & Environm Sci, St Andrews, Fife, Scotland.;Stockholm Univ, Dept Geol Sci, Stockholm, Sweden..
    Schmiedel, Tobias
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Mineralogi, petrologi och tektonik.
    Ronchin, E.
    Sapienza Univ Rome, Dept Earth Sci, Rome, Italy..
    Witcher, Taylor
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Mineralogi, petrologi och tektonik.
    Rapid Assembly and Eruption of a Shallow Silicic Magma Reservoir, Reyðarártindur Pluton, Southeast Iceland2021Inngår i: Geochemistry Geophysics Geosystems, E-ISSN 1525-2027, Vol. 22, nr 11, artikkel-id e2021GC009999Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Although it is widely accepted that shallow silicic magma reservoirs exist, and can feed eruptions, their dynamics and longevity are a topic of debate. Here, we use field mapping, geochemistry, 3D pluton reconstruction and a thermal model to investigate the assembly and eruptive history of the shallow Reyoarartindur Pluton, southeast Iceland. Primarily, the exposed pluton is constructed of a single rock unit, the Main Granite (69.9-77.7 wt.% SiO2). Two further units are locally exposed as enclaves at the base of the exposure, the Granite Enclaves (67.4-70.2 wt.% SiO2), and the Quartz Monzonite Enclaves (61.8-67.3 wt.% SiO2). Geochemically, the units are related and were likely derived from the same source reservoir. In 3D, the pluton has a shape characterized by flat roof segments that are vertically offset and a volume of >2.5 km(3). The pluton roof is intruded by dikes from the pluton, and in two locations displays depressions associated with large dikes. Within these particular dikes the rock is partially to wholly tuffisitic, and rock compositions range from quartz monzonite to granite. We interpret these zones as eruption-feeding conduits from the pluton. A lack of cooling contacts throughout the pluton indicates rapid magma emplacement and a thermal model calculates the top 75 m would have rheologically locked up within 1,000 years. Hence, we argue that the Reyoarartindur Pluton was an ephemeral part of the wider plumbing system that feeds a volcano, and that timeframes from emplacement to eruption were rapid.

    Fulltekst (pdf)
    fulltext
  • 22.
    Rhodes, Emma
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Mineralogi, petrologi och tektonik.
    Barker, Abigail
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Mineralogi, petrologi och tektonik.
    Burchardt, Steffi
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Mineralogi, petrologi och tektonik.
    Hieronymus, Christoph F.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Rousku, Sabine
    McGarvie, Dave
    Mattsson, Tobias
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Mineralogi, petrologi och tektonik.
    Schmiedel, Tobias
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Mineralogi, petrologi och tektonik.
    Ronchin, Erika
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Mineralogi, petrologi och tektonik.
    Witcher, Taylor
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Mineralogi, petrologi och tektonik.
    Rapid formation and eruption of a silicic magma chamber2022Konferansepaper (Annet vitenskapelig)
    Abstract [en]

    Shallow magmatic reservoirs have been identified at many volcanoes worldwide. However, questions still remain regarding their size, dynamics and longevity. The Reyðarártindur Pluton exposed in Southeast Iceland provides a superb example to investigate the above questions. Here, we use field mapping, sampling, geochemistry, 3D pluton shape modelling and a numerical thermal model to reconstruct the assembly and eruptive history of the shallow magma body.

    In 3D, the c. 2.5 km3 pluton has a castle-like shape characterised by flat roof segments that are vertically offset along steep faults. The exposed pluton is constructed largely of a single rock unit, the Main Granite (69.9 to 77.6 wt.% SiO2). Two additional units occur only as enclaves: the Granite Enclaves (67.4 to 70.2 wt.% SiO2), and the Quartz Monzonite Enclaves (61.8 to 67.3 wt.% SiO2). However, geochemistry clearly indicates that the units are related and hence were likely derived from the same source reservoir. 

    In two locations, the pluton roof displays depressions associated with large dykes. Within these two dykes the rock is partially to wholly tuffisitic, and geochemical compositions range from quartz monzonite to granite. We interpret these dykes as eruption-feeding conduits from the pluton. Additionally, we speculate that the mingling of magmatic units with compositional ranges from quartz monzonite to granite within the conduits indicates that injection of new magma into the reservoir triggered eruption. 

    Rapid pluton construction is indicated by ductile contacts between units in the pluton and a thermal model calculates the top 75 m would have rheologically locked up within 1000 years. Hence, we argue that the pluton was a short-lived part of the wider magmatic system that fed the associated volcano, and that timeframes from emplacement to eruption were limited to 1000 years.

    Rhodes, E. Barker, A. K. Burchardt, S. et al. (2021). Rapid assembly and eruption of a shallow silicic magma reservoir, Reyðarártindur Pluton, Southeast Iceland. G-Cubed. DOI: 10.1029/2021GC009999

  • 23.
    Schmidt, Peter
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Lund, Björn
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Hieronymus, Christoph
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Implementation of the glacial rebound pre-stress advection correction in general-purpose finite element analysis software: Springs versus foundations2012Inngår i: Computers & Geosciences, ISSN 0098-3004, E-ISSN 1873-7803, Vol. 40, s. 97-106Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    When general-purpose finite element analysis software is used to model glacial isostatic adjustment (GIA), the first-order effect of prestress advection has to be accounted for by the user. We show here that the common use of elastic foundations at boundaries between materials of different densities will produce incorrect displacements, unless the boundary is perpendicular to the direction of gravity. This is due to the foundations always acting perpendicular to the surface to which they are attached, while the body force they represent always acts in the direction of gravity. If prestress advection is instead accounted for by the use of elastic spring elements in the direction of gravity, the representation will be correct. The use of springs adds a computation of the spring constants to the analysis. The spring constant for a particular node is defined by the product of the density contrast at the boundary, the gravitational acceleration, and the area supported by the node. To be consistent with the finite element formulation, the area is evaluated by integration of the nodal shape functions. We outline an algorithm for the calculation and include a Python script that integrates the shape functions over a bilinear quadrilateral element. For linear rectangular and triangular elements, the area supported by each node is equal to the element area divided the number of defining nodes, thereby simplifying the computation. This is, however, not true in the general nonrectangular case, and we demonstrate this with a simple 1-element model. The spring constant calculation is simple and performed in the preprocessing stage of the analysis. The time spent on the calculation is more than compensated for by a shorter analysis time, compared to that for a model with foundations. We illustrate the effects of using springs versus foundations with a simple two-dimensional GIA model of glacial loading, where the Earth model has an inclined boundary between the overlying elastic layer and the lower viscoelastic layer. Our example shows that the error introduced by the use of foundations is large enough to affect an analysis based on high-accuracy geodetic data.

  • 24.
    Schmidt, Peter
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Lund, Björn
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Hieronymus, Christoph
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Maclennan, John
    Department of Earth Sciences, University of Cambridge.
    Árnadóttir, Thora
    Nordic Volcanological Center, Institute of Earth Sciences, University of Iceland.
    Pagli, Carolina
    School of Earth and Environment, University of Leeds.
    Effects of present day deglaciation on melt production rates beneath Iceland2013Inngår i: Journal of Geophysical Research-Solid Earth, ISSN 2169-9313, Vol. 118, nr 7, s. 3366-3379Artikkel i tidsskrift (Annet vitenskapelig)
    Abstract [en]

    Ongoing deglaciation in Iceland not only causes uplift at the surface but also increases magma production at depth due to decompression of the mantle. Here we study glacially induced decompression melting using 3-D models of glacial isostatic adjustment in Iceland since 1890. We find that the mean glacially induced pressure rate of change in the mantle increases melt production rates by 100–135%, or an additional 0.21–0.23 km3 of magma per year beneath Iceland. Approximately 50% of this melt is produced underneath central Iceland. The greatest volumetric increase is found directly beneath Iceland's largest ice cap, Vatnajökull, colocated with the most productive volcanoes. Our models of the effect of deglaciation on mantle melting predict a significantly larger volumetric response than previous models which only considered the effect of deglaciation of Vatnajökull, and only mantle melting directly below Vatnajökull. Although the ongoing deglaciation significantly increases the melt production rate, the increase in melt supply rate at the base of the lithosphere is delayed and depends on the melt ascent velocity through the mantle. Assuming that 25% of the melt reaches the surface, the upper limit on our deglaciation-induced melt estimates for central Iceland would be equivalent to an eruption the size of the 2010 Eyjafjallajökull summit eruption every seventh year.

  • 25.
    Vachon, Remi
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Hieronymus, Christoph F.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Effect of host-rock rheology on dyke shape, thickness and magma overpressure2017Inngår i: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 208, nr 3, s. 1414-1429Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The size and thickness of dykes is of fundamental importance for volcano dynamics because dykes are the primary path for magma transport, and because large numbers of dykes often comprise a major proportion of the volcanic edifice and of the underlying crust. Standard elastic models predict dyke geometry to be elliptic in cross-section for constant overpressure and uniform host-rock properties, whereas observations show that dyke thickness is typically more nearly constant with a sharp taper at the ends. Moreover, the predicted overpressures required to inflate dykes in a purely elastic medium are often significantly higher (> 150 MPa and up to 2 GPa) than those estimated by other means (about 1-50 MPa). In this study, we use 2-D finite element models to test whether other host-rock rheologies lead to more realistic dyke shapes and overpressures. We examine three different rheologies, each of which is affected by the presence of the dyke itself: (1) elasticity with reduced moduli in regions of low pressure or tension; (2) elastoplasticity with plastic failure in the high-stress regions surrounding the dyke tips; (3) viscoelasticity with a viscosity decrease due to heating by the dyke. We use rheological parameters obtained from laboratory experiments whenever possible, and assume static conditions for the final dyke shape. We find that all three rheologies tend to make the dyke more rectangular relative to the elliptical dykes of the linearly elastic models. The change in shape is due to enhanced deformation in the high-stress zone surrounding the dyke tip. We also find that the overpressure required to inflate an initially thin dyke to a given thickness is reduced for all three rheologies. The greatest decrease in overpressure by a factor of about 0.1 is observed for the elastoplastic model, and for the viscoelastic model if the dyke intrudes into moderately pre-heated host-rock. We discuss our results with respect to dyke observations from Rum Island (Scotland) and use these as a guide to evaluate our models.

    Fulltekst (pdf)
    fulltext
  • 26.
    Vachon, Remi
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Hieronymus, Christoph F.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Mechanical energy balance and apparent fracture toughness for dykes in elastoplastic host rock with large-scale yielding2019Inngår i: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 219, nr 3, s. 1786-1804Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The dynamics of dyke emplacement are typically modelled by assuming an elastic rheology for the host rock. However, the resulting stress field predicts significant shear failure in the region surrounding the dyke tip. Here, we model the dyking process in an elastic-perfectly plastic host rock in order to simulate distributed shear fracturing and subsequent frictional slip on the fracture surfaces. The fluid mechanical aspects of the magma are neglected as we are interested only in the fracture mechanics of the process. Magma overpressure in dykes is typically of the same order of magnitude as the yield stress of the host rock in shear, especially when the pressure effect of volatiles exsolving from the magma is taken into account. Under these conditions, the plastic deformation zone has spatial dimensions that approach the length of the dyke itself, and concepts based on linear elastic fracture mechanics (LEFM) no longer apply. As incremental plasticity is path dependent, we describe two geologically meaningful endmember cases, namely dyke propagation at constant driving pressure, and gradual inflation of a pre-existing crack. For both models, we find that plastic deformation surrounding the fracture tip enhances dyke opening, and thus increases the energy input into the system due to pressure work integrated over the fracture wall. At the same time, energy is dissipated by plastic deformation. Dissipation in the propagation model is greater by about an order of magnitude than it is in the inflation model because the propagating dyke tip leaves behind it a broad halo of deformation due to plastic bending and unbending in the relict process zone. The net effect is that plastic deformation impedes dyke growth in the propagation model, while it enhances dyke growth in the inflation model. The results show that, when the plastic failure zone is large, a single parameter such as fracture toughness is unable to capture the physics that underpin the resistance of a fracture or dyke against propagation. In these cases, plastic failure has to be modelled explicitly for the given conditions. We provide analytical approximations for the propagation forces and the maximum dyke aperture for the two endmember cases, that is, the propagating dyke and the dyke formed by inflation of a crack. Furthermore, we show that the effect of plasticity on dyke energetics, together with an overestimate of magma pressure when interpreting dyke aspect ratios using elastic host rock models, offers a possible explanation for the long-standing paradox that laboratory measurements of fracture toughness of rocks consistently indicate values about two orders of magnitude lower than those derived from dyke observations.

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  • 27.
    Vachon, Rémi
    et al.
    Universitetet i Tromsø.
    Bazargan, Mohsen
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Hieronymus, Christoph F.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Ronchin, Erika
    Sapienza University of Rome.
    Almqvist, Bjarne
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Geofysik.
    Crystal rotations and alignment in spatially varying magma flows: 2-D examples of common subvolcanic flow geometries2021Inngår i: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 226, nr 1, s. 709-727Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Elongate inclusions immersed in a viscous fluid generally rotate at a rate that is different from the local angular velocity of the flow. Often, a net alignment of the inclusions develops, and the resulting shape preferred orientation of the particle ensemble can then be used as a strain marker that allows reconstruction of the fluid’s velocity field. Much of the previous work on the dynamics of flow-induced particle rotations has focused on spatially homogeneous flows with large-scale tectonic deformations as the main application. Recently, the theory has been extended to spatially varying flows, such as magma with embedded crystals moving through a volcanic plumbing system. Additionally, an evolution equation has been introduced for the probability density function of crystal orientations. Here, we apply this new theory to a number of simple, 2-D flow geometries commonly encountered in magmatic intrusions, such as flow from a dyke into a reservoir or from a reservoir into a dyke, flow inside an inflating or deflating reservoir, flow in a dyke with a sharp bend, and thermal convection in a magma chamber. The main purpose is to provide a guide for interpreting field observations and for setting up more complex flow models with embedded crystals. As a general rule, we find that a larger aspect ratio of the embedded crystals causes a more coherent alignment of the crystals, while it has only a minor effect on the geometry of the alignment pattern. Due to various perturbations in the crystal rotation equations that are expected in natural systems, we show that the time-periodic behaviour found in idealized systems is probably short-lived in nature, and the crystal alignment is well described by the time-averaged solution. We also confirm some earlier findings. For example, near channel walls, fluid flow often follows the bounding surface and the resulting simple shear flow causes preferred crystal orientations that are approximately parallel to the boundary. Where pure shear deformation dominates, there is a tendency for crystals to orient themselves in the direction of the greatest tensile strain rate. Where flow impinges on a boundary, for example in an inflating magma chamber or as part of a thermal convection pattern, the stretching component of pure shear aligns with the boundary, and the crystals orient themselves in that direction. In the field, this local pattern may be difficult to distinguish from a boundary-parallel simple shear flow. Pure shear also dominates along the walls of a deflating magma chamber and in places where the flow turns away from the reservoir walls, but in these locations, the preferred crystal orientation is perpendicular to the wall. Overall, we find that our calculated patterns of crystal orientations agree well with results from analogue experiments where similar geometries are available.

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