| 41. |
- Eshagh, Mehdi, 1977-
(författare)
-
On the approximations in formulation of the Vening Meinesz-Moritz theory of isostasy
- 2017
-
Ingår i: Geophysical Journal International. - : Oxford University Press (OUP). - 0956-540X .- 1365-246X. ; 210:1, s. 500-508
-
Tidskriftsartikel (refereegranskat)abstract
- Different approximations are used in Moho modelling based on isostatic theories. The well-known approximation is considering a plate shell model for isostatic equilibrium, which is an oversimplified assumption for the Earth’s crust. Considering a spherical shellmodel, as used in the Vening Meinesz-Moritz (VMM) theory, is a more realistic assumption, but it suffers from different types of mathematical approximations. In this paper, the idea is to investigate such approximations and present their magnitudes and locations all over the globe. Furthermore, we show that the mathematical model of Moho depth according to the VMM principle can be simplified to that of the plate shell model after four approximations. Linearisation of the binomial term involving the topographic/bathymetric heights is sufficient as long as their spherical harmonic expansion is limited to degree and order 180. The impact of the higher order terms is less than 2 km. The Taylor expansion of the binomial term involving the Moho depth (T) up to second order with the assumption of T-2 = TT0, T-0 is the mean compensation depth, improves this approximation further by up to 4 km over continents. This approximation has a significant role in Moho modelling over continents; otherwise, loss of frequency occurs in the Moho solution. On the other hand, the linear approximation performs better over oceans and considering higher order terms creates unrealistic frequencies reaching to a magnitude of 5 km in the Moho solution. Involving gravity data according to the VMM principle influences the Moho depth significantly up to 15 km in some areas.
|
|
| 42. |
- Eshagh, Mehdi, Professor, 1977-, et al.
(författare)
-
The temporal viscoelastic model of flexural isostasy for estimating the elastic thickness of the lithosphere
- 2021
-
Ingår i: Geophysical Journal International. - : Oxford University Press. - 0956-540X .- 1365-246X. ; 227:3, s. 1700-1714
-
Tidskriftsartikel (refereegranskat)abstract
- The (effective) elastic thickness of the lithosphere defines the strength of the lithosphere with respect to a load on it. Since the lithosphere is buoyant on a viscous mantle, its behaviour with respect to a load is not fully elastic, but rather viscoelastic. Fennoscandia is a well-known area in the world where the lithosphere has not yet reached its isostatic equilibrium due to the ongoing uplift after the last glacial period at the end of the Pleistocene. To accommodate for this changing property of the lithosphere in time, we present the flexural model of isostasy that accommodates temporal variations of the lithospheric flexure. We then define a theoretical model for computing the elastic thickness of the lithosphere based on combining the flexural and gravimetric models of isostasy. We demonstrate that differences between the elastic and viscoelastic models are not that significant in Fennoscandia. This finding is explained by a relatively young age of the glacial load when compared to the Maxwell relaxation time. The approximation of an elastic shell is then permissible in order to determine the lithospheric structure and its properties. In this way, the elastic thickness can be estimated based on combining gravimetric and flexural models of isostasy. This approach takes into consideration the topographic and ocean-floor (bathymetric) relief as well as the lithospheric structural composition and the post-glacial rebound. In addition, rheological properties of the lithosphere are taken into consideration by means of involving the Young modulus and the Poisson ratio in the model, both parameters determined from seismic velocities. The results reveal that despite changes in the Moho geometry attributed to the glacial isostatic adjustment in Fennoscandia are typically less than 1 km, the corresponding changes in the lithospheric elastic thickness could reach or even exceed ±50 km. The sensitivity analysis confirms that even small changes in input parameters could significantly modify the result (i.e. the elastic thickness estimates). The reason is that the elastic thickness estimation is an inverse problem. Consequently, small changes in input parameters can lead to large changes in the elastic thickness estimates. These findings indicate that a robust estimation of the elastic thickness by our method is possible if comprehensive information about structural and rheological properties of the lithosphere as input parameters are known with a relatively high accuracy. Otherwise, even small uncertainties in these parameters could result in large errors in the elastic thickness estimates.
|
|
| 43. |
|
|
| 44. |
- Fuchs, Lukas, et al.
(författare)
-
A new numerical method to calculate inhomogeneous and time dependent large deformations of two-dimensional geodynamic flows with application to diapirism
- 2013
-
Ingår i: Geophysical Journal International. - : Oxford University Press (OUP). - 0956-540X .- 1365-246X. ; 194:2, s. 623-639
-
Tidskriftsartikel (refereegranskat)abstract
- A key to understand many geodynamic processes is studying the associated large deformation fields. Finite deformation can be measured in the field by using geological strain markers giving the logarithmic strain f = log 10(R), where R is the ellipticity of the strain ellipse. It has been challenging to accurately quantify finite deformation of geodynamic models for inhomogeneous and time-dependent large deformation cases. We present a new formulation invoking a 2-D marker-in-cell approach. Mathematically, one can describe finite deformation by a coordinate transformation to a Lagrangian reference frame. For a known velocity field the deformation gradient tensor, F, can be calculated by integrating the differential equation DtFij = LikFkj, where L is the velocity gradient tensor and Dt the Lagrangian derivative. The tensor F contains all information about the minor and major semi-half axes and orientation of the strain ellipse and the rotation. To integrate the equation centrally in time and space along a particle's path, we use the numerical 2-D finite difference code FDCON in combination with a marker-in-cell approach. For a sufficiently high marker density we can accurately calculate F for any 2-D inhomogeneous and time-dependent creeping flow at any point for a deformation f up to 4. Comparison between the analytical and numerical solution for the finite deformation within a Poiseuille–Couette flow shows an error of less than 2 per cent for a deformation up to f = 1.7. Moreover, we determine the finite deformation and strain partitioning within Rayleigh–Taylor instabilities (RTIs) of different viscosity and layer thickness ratios. These models provide a finite strain complement to the RTI benchmark of van Keken et al. Large finite deformation of up to f = 4 accumulates in RTIs within the stem and near the compositional boundaries. Distinction between different stages of diapirism shows a strong correlation between a maximum occurring deformation of f = 1, 3 and 4, and the early, intermediate and late stages of diapirism, respectively. Furthermore, we find that the overall strain of a RTI is concentrated in the less viscous regions. Thus, spatial distributions and magnitudes of finite deformation may be used to identify stages and viscosity ratios of natural cases.
|
|
| 45. |
- Fuchs, Lukas, 1984-, et al.
(författare)
-
Numerical models of salt diapir formation by down-building : the role of sedimentation rate, viscosity contrast, initial amplitude and wavelength
- 2011
-
Ingår i: Geophysical Journal International. - 0956-540X .- 1365-246X. ; 186:2, s. 390-400
-
Tidskriftsartikel (refereegranskat)abstract
- Formation of salt diapirs has been described to be due to upbuilding (i. e. Rayleigh-Taylor like instability of salt diapirs piercing through a denser sedimentary overburden) or syndepositional down-building process (i. e. the top of the salt diapir remains at the surface all the time). Here we systematically analyse this second end-member mechanism by numerical modelling. Four parameters are varied: sedimentation rate nu(sed), salt viscosity eta(salt), amplitude delta of the initial perturbation of the sedimentation layer and thewavenumber k of this perturbation. The shape of the resulting salt diapirs strongly depends on these parameters. Small diapirs with subvertical side walls are found for small values of nu(sed) and eta(salt) or large values of delta, whereas taller diapirs with pronounced narrow stems build for larges values of nu(sed) and eta(salt) or small values of delta. Two domains are identified in the four-parameter space, which separates successful down-building models from non-successful models. By applying a simple channel flow law, the domain boundary can be described by the non-dimensional law nu(sedcrit)' = C(1)1/2 delta(0)'rho(sed)'k'(2/)k'(2) + C2, where rho(sed)' is the sediment density scaled by the density contrast Delta rho between sediment and salt, the wavelength is scaled by the salt layer thickness h(salt), and velocity is scaled by eta(salt)/(h(salt)(2)Delta rho g), where eta(salt) is the salt viscosity and g is the gravitational acceleration. From the numerical models, the constants C(1) and C(2) are determined as 0.0283 and 0.1171, respectively.
|
|
| 46. |
- Fuchs, Lukas, et al.
(författare)
-
Thermo-mechanical modelling of progressive deformation and seismic anisotropy at the lithosphere-asthenosphere boundary
-
Ingår i: Geophysical Journal International. - 0956-540X .- 1365-246X.
-
Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Deformation at the lithosphere-asthenosphere boundary is strongly governed by its effective viscosity, which depends on temperature, strain rate, and grain size. Moreover, deformation can cause lattice preferred orientation resulting in seismic anisotropy and shear wave splitting. We used a 1D model approach to calculate shear strain and characteristic depths for an oceanic plate as a function of age. We assume a composite rheology (dislocation and diffusion creep) in combination with a half-space cooling model temperature field for constant and variable thermal parameters, and different potential mantle temperatures. Systematically, sensitivity of characteristic depths, deformation pattern, and seismic delay times δt on temperature, plate velocity, steady state grain size, and rheology have been analyzed. Model results show that the characteristic depths are only affected by local variations in the temperature field or a shift in the dominant deformation mechanism. The other parameters, however, do strongly affect the maximum total shear strain. Due to a continuous simple shear of the upper mantle governed by the motion of the plate, anisotropy, thickness of the anisotropic layer, and δt reach relatively large values in comparison to observed data. However, a small amount of dislocation creep (25-40 %), due to a modified rheology or small grain sizes, leads to a significantly thinner anisotropic layer. As a result, δt is reduced by 50 % or more. The change of the characteristics of the anisotropic layer and degree of its anisotropy may reflect and be of significance for the viscous (de)coupling between the lithosphere and asthenosphere.
|
|
| 47. |
|
|
| 48. |
- Greve, Annika, et al.
(författare)
-
The geomagnetic field intensity in New Zealand : Palaeointensities from Holocene lava flows of the Tongariro Volcanic Centre
- 2017
-
Ingår i: Geophysical Journal International. - 0956-540X. ; 211:2, s. 814-830
-
Tidskriftsartikel (refereegranskat)abstract
- Very few absolute palaeointensity data exist from Holocene-aged rocks in New Zealand. Here we present a new suite of high-quality palaeointensities, supported by detailed rock magnetic investigations. Samples from 23 sites representing 10 distinct eruptive units of the Tongariro Volcanic Centre, Taupo Volcanic Zone, New Zealand, were studied. Both traditional double heating and microwave palaeointensity methods were employed. The reliability of the palaeointensity data varies with rock magnetic properties of the samples, corresponding, in particular, to their positions within the lava flows. The highest success rates are from samples obtained from near the flow tops where a significant proportion of the remanence unblocked at intermediate temperatures (200-350 °C). By contrast, samples from flow centres, particularly the parts showing platey fracturing, have the lowest success rates. Reliable, high-quality palaeointensity results ranging from 32.4 ± 5.1 μT to 72.1 ± 4.7 μT were obtained from six flows with ages between c. 12 800 yr BP and the present. These correspond to virtual dipole moments that increase from 52 ± 10 ZAm2 in the early Holocene and peak at 112 ± 14 ZAm2 about 300 yr ago. The data agree well with calibrated relative palaeointensities from New Zealand lake sediments. The volcanic and sedimentary data together yield a Holocene virtual axial dipole moment curve that fits the global average variation well in the early Holocene, but which differs significantly in recent millennia. This difference is associated with recent migration of the southern high latitude core-mantle boundary flux lobe towards New Zealand, as is seen in global field models.
|
|
| 49. |
- Hagos, Lijam, et al.
(författare)
-
Re-evaluation of focal depths and source mechanisms of selected earthquakes in the Afar depression
- 2006
-
Ingår i: Geophysical Journal International. - 0956-540X .- 1365-246X. ; 167:1, s. 297-308
-
Tidskriftsartikel (refereegranskat)abstract
- We present a stepwise inversion procedure to assess the focal depth and model earthquake source complexity of seven moderate-sized earthquakes (6.2 > M-w > 5.1) that occurred in the Afar depression and the surrounding region. The Afar depression is a region of highly extended and intruded lithosphere, and zones of incipient seafloor spreading. A time-domain inversion of full moment tensor was performed to model direct P and SH waves of teleseismic data. Waveform inversion of the selected events estimated focal depths in the range of 17-22 km, deeper than previously published results. This suggests that the brittle-ductile transition zone beneath parts of the Afar depression extends more than 22 km. The effect of near-source velocity structure on the moment tensor elements was also investigated and was found to respond little to the models considered. Synthetic tests indicate that the size of the estimated, non-physical, non-isotropic source component is rather sensitive to incorrect depth estimation. The dominant double couple part of the moment tensor solutions for most of the events indicates that their occurrence is mainly due to shearing. Parameters associated with source directivity (rupture velocity and azimuth) were also investigated. Re-evaluation of the analysed events shows predominantly normal faulting consistent with the relative plate motions in the region.
|
|
| 50. |
- Hall, Stuart, et al.
(författare)
-
Palaeomagnetic palaeolatitudes of the Ontong Java Plateau from 120 to 55 Ma: implications for the apparent polar wander path of the Pacific Plate
- 2007
-
Ingår i: Geophysical Journal International. - 0956-540X. ; 169:2, s. 455-470
-
Tidskriftsartikel (refereegranskat)abstract
- We present palaeomagnetic inclinations from Late Cretaceous-Early Tertiary deep-water carbonates obtained during ODP Leg 192 drilling at the Ontong Java Plateau (OJP) for the following periods: magnetochrons C33n (73.6-79.1 Ma), C32 (71.1-73.6 Ma), C27r-C31n (61.3-68.7 Ma) and C25-C26 (55.9-60.9 Ma). Compaction-induced inclination shallowing is considered to be negligible for the OJP sediments examined here because: (i) their palaeomagnetic inclinations are in excellent accord with those recently obtained from underlying OJP basement rocks, (ii) the studied sediments have anisotropy of anhysteretic remanent magnetization fabrics that appear poorly correlated with individual characteristic remanent magnetization inclinations and (iii) in the few cases where we observe a significant difference between our new OJP sedimentary palaeomagnetic data and data from other parts of the Pacific Plate, obtained mainly from seamount magnetization studies and skewness analyses, the sedimentary inclinations are not systematically lower, and therefore, cannot be explained in terms of inclination shallowing. Combining our new data with existing OJP palaeomagnetic data we obtain an internally consistent data set that we interpret to indicate northward motion of the plateau from its formation at similar to 120 Ma until 55 Ma. At 120 Ma the central plateau was located at 24 degrees +/- 4 degrees S. Approximately 45 Myr later (similar to 76 Ma) the plateau is located near 21 degrees S. Our data, therefore, indicate that the OJP was either stationary or exhibited a slow northward latitudinal drift during this interval. In contrast, from similar to 76 Ma until 68 Ma the plateau moved rapidly through approximately 10 degrees of latitude to similar to 10 degrees S. These intervals of slow and rapid motion track similar motions previously proposed for the entire Pacific Plate during the Cretaceous but extends the interval of slow motion into the Late Cretaceous. From 68 Ma until 56 Ma the plateau was again almost latitudinally stationary before moving slowly northward. More direct palaeomagnetic data are necessary to better define Pacific Plate motions during this time interval.
|
|
