Plate motion and dragging of the upper mantle: Lateral variations of lithospheric thickness and their implications for intraplate deformation

1992 ◽  
Vol 19 (8) ◽  
pp. 749-752 ◽  
Author(s):  
Roberto Sabadini ◽  
Carlo Giunchi ◽  
Paolo Gasperini ◽  
Enzo Boschi
Keyword(s):  
Upper Mantle ◽  
Plate Motion ◽  
Lithospheric Thickness ◽  
Intraplate Deformation ◽  
Lateral Variations

scholarly journals Reduced ice mass loss and three-dimensional viscoelastic deformation in northern Antarctic Peninsula inferred from GPS

2020 ◽  
Vol 222 (2) ◽  
pp. 1013-1022 ◽  
Author(s):  
Nahidul Hoque Samrat ◽  
Matt A King ◽  
Christopher Watson ◽  
Andrew Hooper ◽  
Xianyao Chen ◽  
...  
Keyword(s):  
Mass Loss ◽  
Antarctic Peninsula ◽  
Upper Mantle ◽  
Three Dimensional ◽  
Plate Motion ◽  
Viscoelastic Deformation ◽  
Lithospheric Thickness ◽  
Isostatic Adjustment ◽  
Wide Range ◽  
Uplift Rates

SUMMARY We consider the viscoelastic rheology of the solid Earth under the Antarctic Peninsula due to ice mass loss that commenced after the breakup of the Larsen-B ice shelf. We extend the previous analysis of nearby continuous GPS time-series to include five additional years and the additional consideration of the horizontal components of deformation. They show strong uplift from ∼2002 to 2011 followed by reduced uplift rates to 2018. Modelling the GPS-derived uplift as a viscoelastic response to ongoing regional ice unloading from a new ice model confirms earlier estimates of low upper-mantle viscosities of ∼0.3–3 × 1018 Pa s in this region but allows a wide range of elastic lithosphere thickness. The observed and modelled north coordinate component shows little nonlinear variation due to the location of ice mass change to the east of the GPS sites. However, comparison of the observed and modelled east coordinate component constrains the upper-mantle viscosity to be less than ∼9 × 1018 Pa s, consistent with the viscosity range suggested by the uplift rates alone and providing important, largely independent, confirmation of that result. Our horizontal analysis showed only marginal sensitivity to modelled lithospheric thickness. The results for the horizontal components are sensitive to the adopted plate rotation model, with the estimate based on ITRF2014 suggesting that the sum of residual plate motion and pre-2002 glacial isostatic adjustment is likely less than ∼±0.5 mm yr−1 in the east component.

scholarly journals Lithosphere and upper-mantle structure of the southern Baltic Sea estimated from modelling relative sea-level data with glacial isostatic adjustment

Solid Earth ◽  
2014 ◽  
Vol 5 (1) ◽  
pp. 447-459 ◽  
Author(s):  
H. Steffen ◽  
G. Kaufmann ◽  
R. Lampe
Keyword(s):  
Baltic Sea ◽  
Sea Level ◽  
Upper Mantle ◽  
Glacial Isostatic Adjustment ◽  
Relative Sea Level ◽  
Southern Baltic Sea ◽  
Lithospheric Thickness ◽  
Isostatic Adjustment ◽  
Mantle Viscosity ◽  
Ice Model

Abstract. During the last glacial maximum, a large ice sheet covered Scandinavia, which depressed the earth's surface by several 100 m. In northern central Europe, mass redistribution in the upper mantle led to the development of a peripheral bulge. It has been subsiding since the begin of deglaciation due to the viscoelastic behaviour of the mantle. We analyse relative sea-level (RSL) data of southern Sweden, Denmark, Germany, Poland and Lithuania to determine the lithospheric thickness and radial mantle viscosity structure for distinct regional RSL subsets. We load a 1-D Maxwell-viscoelastic earth model with a global ice-load history model of the last glaciation. We test two commonly used ice histories, RSES from the Australian National University and ICE-5G from the University of Toronto. Our results indicate that the lithospheric thickness varies, depending on the ice model used, between 60 and 160 km. The lowest values are found in the Oslo Graben area and the western German Baltic Sea coast. In between, thickness increases by at least 30 km tracing the Ringkøbing-Fyn High. In Poland and Lithuania, lithospheric thickness reaches up to 160 km. However, the latter values are not well constrained as the confidence regions are large. Upper-mantle viscosity is found to bracket [2–7] × 1020 Pa s when using ICE-5G. Employing RSES much higher values of 2 × 1021 Pa s are obtained for the southern Baltic Sea. Further investigations should evaluate whether this ice-model version and/or the RSL data need revision. We confirm that the lower-mantle viscosity in Fennoscandia can only be poorly resolved. The lithospheric structure inferred from RSES partly supports structural features of regional and global lithosphere models based on thermal or seismological data. While there is agreement in eastern Europe and southwest Sweden, the structure in an area from south of Norway to northern Germany shows large discrepancies for two of the tested lithosphere models. The lithospheric thickness as determined with ICE-5G does not agree with the lithosphere models. Hence, more investigations have to be undertaken to sufficiently determine structures such as the Ringkøbing-Fyn High as seen with seismics with the help of glacial isostatic adjustment modelling.

scholarly journals Seismic anisotropy: an original tool to understand the geodynamic evolution of the Italian peninsula

1997 ◽  
Vol 40 (3) ◽  
Author(s):  
L. Margheriti ◽  
C. Nostro ◽  
A. Amato ◽  
M. Cocco
Keyword(s):  
Upper Mantle ◽  
Common Property ◽  
Seismic Waves ◽  
Seismic Anisotropy ◽  
Structural Features ◽  
Relative Strength ◽  
Plate Motion ◽  
Mantle Minerals ◽  
Anisotropic Bodies ◽  
Shallow Crust

Anisotropy is a common property of the Earth's crust and the upper mantle; it is related to the strain field of the medium and therefore to geodynamics. In this paper we describe the different possible origins of anisotropic behavior of the seismic waves and the seismological techniques used to define anisotropic bodies. In general it is found that the fast polarization direction is parallel to the absolute plate motion in cratonic areas, to the spreading direction near rifts or extensional zones, and to the main structural features in transpressive regimes. The delay times between fast and slow waves reflect the relative strength and penetration at depth of the deformation field. The correspondence between surface structural trends and anisotropy in the upper mantle, found in many regions of the world, strongly suggest that orogenic processes involve not only the shallow crust but the entire lithosphere. Recently in Italy both shear wave splitting analysis and Pn inversion were applied to define the trend of seismic anisotropy. Along the Northern Appeninic arc fast directions follow the strike of the arc (i.e., parallel to the strike of the Miocene-Pleistocene compressional features), whereas in the Tyrrhenian zone fast directions are about E-W SW-NE; parallel to the post-Miocene extension that is thought to have reoriented the mantle minerals fabric in the astenosphere.

No signal of a plume push force in Indo-Atlantic plate speeds before, during, or after Deccan plume arrival

2021 ◽  
Author(s):  
Graeme Eagles ◽  
Lucía Pérez Díaz ◽  
Karin Sigloch
Keyword(s):  
Late Cretaceous ◽  
Upper Mantle ◽  
Plate Boundary ◽  
Plate Motion ◽  
Driving Mechanism ◽  
African Plate ◽  
Plate Motions ◽  
Driving Mechanisms ◽  
High Resolution Models ◽  
Spreading Rates

<p>Observations of the apparent links between plate speeds and the global distribution of plate boundary types have led to the suggestion that subduction may provide the largest component in the balance of torques maintaining plate motions. This would imply that plate speeds should not exceed the sinking rates of slabs into the upper mantle. Instances of this ‘speed limit’ having been broken may thus hint at the existence of driving mechanisms additional to those resulting from plate boundary forces. The arrival and emplacement of the Deccan-Réunion mantle plume beneath the Indian-African plate boundary in the 67-62 Ma period has been discussed in terms of one such additional driving mechanism, leading to the establishment of “plume-push” hypothesis, which in recent years has gained significant traction. We challenge the model-based observations that form the principal evidence in favour of plume-push: a late Cretaceous pulse of anticorrelating accelerations and decelerations in seafloor spreading rates around the African and Indian plates. Using existing and newly-calculated high-resolution models of plate motion, we instead document an increase in divergence rates at 67-64 Ma. Because of its ubiquity, we consider this increase to be the artefact of a timescale error affecting chrons 29-28. Corrected for this artefact, the evolution of plate speeds resembles a smooth continuation of pre-existing late Cretaceous trends, consistent with the idea that the arrival of the Réunion plume did not substantially affect the existing balance of plate boundary forces on the Indian and African plates. </p>

Scaling laws for stagnant-lid convection with a buoyant crust

2021 ◽  
Vol 228 (1) ◽  
pp. 631-663
Author(s):  
Kyle Batra ◽  
Bradford Foley
Keyword(s):  
Heat Flux ◽  
Scaling Laws ◽  
Thermal Evolution ◽  
Crustal Thickness ◽  
Plate Motion ◽  
Crustal Layer ◽  
Lithospheric Thickness ◽  
Thin Crust ◽  
Mantle Temperature ◽  
Thick Crust

SUMMARY Stagnant-lid convection, where subduction and surface plate motion is absent, is common among the rocky planets and moons in our solar system, and likely among rocky exoplanets as well. How stagnant-lid planets thermally evolve is an important issue, dictating not just their interior evolution but also the evolution of their atmospheres via volcanic degassing. On stagnant-lid planets, the crust is not recycled by subduction and can potentially grow thick enough to significantly impact convection beneath the stagnant lid. We perform numerical models of stagnant-lid convection to determine new scaling laws for convective heat flux that specifically account for the presence of a buoyant crustal layer. We systematically vary the crustal layer thickness, crustal layer density, Rayleigh number and Frank–Kamenetskii parameter for viscosity to map out system behaviour and determine the new scaling laws. We find two end-member regimes of behaviour: a ‘thin crust limit’, where convection is largely unaffected by the presence of the crust, and the thickness of the lithosphere is approximately the same as it would be if the crust were absent; and a ‘thick crust limit’, where the crustal thickness itself determines the lithospheric thickness and heat flux. Scaling laws for both limits are developed and fit the numerical model results well. Applying these scaling laws to rocky stagnant-lid planets, we find that the crustal thickness needed for convection to enter the thick crust limit decreases with increasing mantle temperature and decreasing mantle reference viscosity. Moreover, if crustal thickness is limited by the formation of dense eclogite, and foundering of this dense lower crust, then smaller planets are more likely to enter the thick crust limit because their crusts can grow thicker before reaching the pressure where eclogite forms. When convection is in the thick crust limit, mantle heat flux is suppressed. As a result, mantle temperatures can be elevated by 100 s of degrees K for up to a few Gyr in comparison to a planet with a thin crust. Whether convection enters the thick crust limit during a planet’s thermal evolution also depends on the initial mantle temperature, so a thick, buoyant crust additionally acts to preserve the influence of initial conditions on stagnant-lid planets for far longer than previous thermal evolution models, which ignore the effects of a thick crust, have found.

Radial Anisotropy and it's Relation to Fast and Slow Moving Tectonic Plates

2021 ◽  
Author(s):  
Tak Ho ◽  
Keith Priestley ◽  
Eric Debayle
Keyword(s):  
Upper Mantle ◽  
Large Scale ◽  
Oceanic Lithosphere ◽  
Azimuthal Anisotropy ◽  
Plate Motion ◽  
Moho Depth ◽  
Dispersion Characteristics ◽  
Anisotropic Models ◽  
Ocean Ridges ◽  
Average Dispersion

<p>We present a new radially anisotropic (<strong>ξ)</strong> tomographic model for the upper mantle to transition zone depths derived from a large Rayleigh (~4.5 x 10<sup>6 </sup>paths) and Love (~0.7 x 10<sup>6</sup> paths) wave path average dispersion curves with periods of 50-250 s and up to the fifth overtone. We first extract the path average dispersion characteristics from the waveforms. Dispersion characteristics for common paths (~0.3 x 10<sup>6</sup> paths) are taken from the Love and Rayleigh datasets and jointly inverted for isotropic V<sub>s </sub>and <strong>ξ</strong>. CRUST1.0 is used for crustal corrections and a model similar to PREM is used as a starting model. V<sub>s</sub> and <strong>ξ</strong> are regionalised for a 3D model. The effects of azimuthal anisotropy are accounted for during the regionalisation. Our model confirms large-scale upper mantle features seen in previously published models, but a number of these features are better resolved because of the increased data density of the fundamental and higher modes coverage from which our <strong>ξ</strong>(z) was derived. Synthetic tests show structures with radii of 400 km can be distinguished easily. Crustal perturbations of +/-10% to V<sub>p</sub>, V<sub>s</sub> and density, or perturbations to Moho depth of +/-10 km over regions of 400 km do not significantly change the model. The global average decreases from <strong>ξ~</strong>1.06 below the Moho to <strong>ξ</strong>~1 at ~275 km depth. At shallow depths beneath the oceans <strong>ξ</strong>>1 as is seen in previously published global mantle radially anisotropic models. The thickness of this layer increases slightly with the increasing age of the oceanic lithosphere. At ~200 km and deeper depths below the fast-spreading East Pacific Rise and starting at somewhat greater depths beneath the slower spreading ridges, <strong>ξ</strong><1. At depths ≥200 km and deeper depths below most of the backarc basins of the western Pacific <strong>ξ</strong><1. The signature of mid-ocean ridges vanishes at about 150 km depth in V<sub>s</sub> while it extends much deeper in the <strong>ξ</strong> model suggesting that upwelling beneath mid-ocean ridges could be more deeply rooted than previously believed. The pattern of radially anisotropy we observe, when compared with the pattern of azimuthal anisotropy determined from Rayleigh waves, suggests that the shearing at the bottom of the plates is only sufficiently strong to cause large-scale preferential alignment of the crystals when the plate motion exceeds some critical value which Debayle and Ricard (2013) suggest is about 4 cm/yr.</p>

The formation of viscous anisotropy in the asthenosphere and its effect on plate tectonics

2020 ◽  
Author(s):  
Agnes Kiraly ◽  
Clinton P. Conrad ◽  
Lars N. Hansen ◽  
Menno Fraters
Keyword(s):  
Upper Mantle ◽  
Effective Viscosity ◽  
Plate Tectonics ◽  
Large Scale ◽  
Plate Motion ◽  
Shear Direction ◽  
Viscosity Change ◽  
Macro Scale ◽  
Geodynamic Processes ◽  
Anisotropic Viscosity

<p>Developing an appropriate characterization of upper mantle viscosity structure presents one of the biggest challenges for understanding geodynamic processes in the upper mantle. This is because different creep mechanisms become activated depending on depth, accumulated strain, and applied stress, and other factors such grain size and anisotropic fabric can change as the deformation develops, changing the effective viscosity. Here we focus on the relationship between anisotropic fabric development and viscous anisotropy.</p><p>Under applied shear, olivine crystals, which form a large proportion of the asthenosphere, rotate towards the shear direction and accumulate a lattice preferred orientation (LPO) parallel to the macroscopic deformation. On a large scale, LPO can be observed through the propagation of seismic waves because of the anisotropic elastic properties of olivine. As olivine is anisotropic in its viscous properties, this developing texture within the asthenosphere can affect the macro-scale viscosity of the asthenosphere. This behavior has been detected in rock mechanics measurements on pure olivine aggregates, showing more than an order magnitude of viscosity change between shear parallel to the olivine aggregate’s LPO versus shear across this fabric (Hansen et al., EPSL 2016a, JGR 2016b).</p><p>Here, we use numerical models developed first in MATLAB and then implemented into the mantle convection code ASPECT. These models incorporate both anisotropic fabric development and anisotropic viscosity, both calibrated according to laboratory measurements on slip system activities of olivine aggregates (Hansen et al., JGR 2016b), to better understand the coupling between the large-scale formation of LPO textures and changes in asthenospheric viscosity.</p><p>The modeling results allows us to discuss the role of anisotropic viscosity on the processes of plate tectonics. An asthenosphere with a well-developed LPO becomes weak parallel to its texture, allowing for increasing plate velocity at the surface, for a given applied driving force.  On the other hand, this fabric resists abrupt changes in the direction of plate motion because the effective viscosity is elevated for shear perpendicular to the developed LPO. This increased resistance to fabric-perpendicular shear also decreases strain rates, which slows texture development. This means that asthenospheric fabric can impede changes in plate motion direction for periods of over 10 Myrs. However, the same well-developed texture in the asthenosphere could enhance the initiation of subduction or lithospheric gravitational instabilities as vertical deformation is favored across a sub-lithospheric olivine fabric, and the sheared fabric can quickly rotate into a vertical LPO. These end-member cases examining shear-deformation across a formed asthenospheric fabric illustrate the importance of olivine fabrics, and their associated viscous anisotropy, for a variety of geodynamic processes.</p>

Seismic anisotropy and mantle deformation in NW Iran through splitting measurements of SKS and direct S phases

2020 ◽  
Author(s):  
Shiva Arvin ◽  
Farhad Sobouti ◽  
Keith Priestley ◽  
Abdolreza Ghods ◽  
Seyed Khalil Motaghi ◽  
...  
Keyword(s):  
Flow Field ◽  
Upper Mantle ◽  
Seismic Anisotropy ◽  
Central Iran ◽  
Plate Motion ◽  
Propagation Path ◽  
Mantle Flow ◽  
South Caspian Basin ◽  
Nw Iran ◽  
S Waves

<p><span>The present tectonics of Iran has resulted from the continental convergence of the Arabian and Eurasian plates. Our study area, in NW Iran comprises a part of this collision zone and consists of an assemblage of distinct lithospheric blocks including the central Iranian Plateau, the South Caspian Basin, and the Talesh western Alborz Mountains. A proper knowledge of mantle flow field is required to bettwer constrain mantle kinematics in relation to the dynamics of continental deformation in NW Iran. To achieve this aim, we examined splitting of teleseismic shear waves (e.g. SKS and S) arriving with steep arrival angles beneath the receiver, which provide excellent lateral resolution in the upper mantle. We used data from 68 temporary broadband stations with varying operation periods (4 to 31 months) along 3 linear profiles. We perfomed splitting analyses on SK(K)S and direct S waves. </span>Resultant splitting parameters obtained from both shear phases exhibit broad similarities. Relatively large time delays observed for direct S-waves, however, are anticipated since these waves travel longer than SKS along a non-vertical propagation path in an anisotropic layer. Overall, the fast polarization directions (FPDs) in the Alborz, Talesh, Tarom Mountain and in NW Iran indicate a strong consistency with NE-SW anisotropic orientations. Besides, we observe a good accordance between S and SKS results. A comparison of splitting parameters with the absolute plate motion (APM) vector and structural trends in Iran and eastern Turkey suggests asthenospheric flow field as the dominant source for observed seismic anisotropy. The lithospheric layer beneath these regions is relatively thin (compared to the adjacent Zagros region), explaining why it appears to only make a partial contribution to the observed anisotropy. The stations located in central Iran just southwest of the Alborz yield angular deviations from the general NE-SW trend as this may be explained by changing style of deformation across the different tectonic blocks. These stations indicate significant misfit between SK(K)S and direct S-waves that could be caused by local heterogeneities developed due to a diffuse boundary from the flow organization in the upper mantle of central Iran. Another possibility for large differences between two types of waves might be reflect the anisotropic structure of a remnant slab segment or a foundered lithospheric root beneath central Iran with a volume small enough to be detected by SKS phases, but not by the direct S waves.</p>

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