Spectral modelling of mantle convection in a non orthogonal geometry: application to subduction zones

The lithospheric sinking along subduction zones is part of the mantle convection. Therefore, computing the volume of lithosphere recycled within the mantle by subducting slabs quantifies the equivalent amount of mantle that should be displaced, for the mass conservation criterion. Starting from the analysis of the subduction hinge kinematics, that could either move towards (H-convergent) or away (H-divergent) with respect to the fixed upper plate, we compute the amount of lithosphere currently subducting below 31 subduction zones worldwide. Our results show that ~190 km3/yr and ~88 km3/yr of lithosphere are currently subducting below H-divergent and H-convergent subduction zones, respectively. This volume discrepancy is principally due to the difference in the two end-members subduction rate, that takes into account the hinge kinematics. We also propose supporting numerical models providing asymmetric volumes of subducted lithosphere, using the subduction rate,&#160;instead of plate convergence, as boundary condition. Subduction zones show a worldwide asymmetry from geological and geophysical observations, such as slab dip, structural elevation, gravity anomalies, heat flow, metamorphic evolution, subsidence and uplift rates or depth of the d&#233;collement planes. This asymmetry is expressed also in the behaviour of the subduction hinge, so that H-divergent subduction zones appears to be coincident with subduction zones having &#8220;westward&#8221;-directed slabs, whereas H-convergent are compatible with those that have &#8220;eastward-to-northeastward&#8221;-directed slabs. On the basis of this geographical polarity of subducting slabs, the obtained lithospheric volume estimation gives ~214 km3/yr and ~88 km3/yr of subducting lithosphere for subduction zones with W-directed and E-to-NE-directed slabs, respectively. This imply that W-directed subduction zones contribute more than twice in lithospheric sinking into the mantle with respect to E-to-NE-directed ones. In accordance with the conservation of mass principle, this volumetric asymmetry in the mantle suggests a displacement of ~120 km3/yr of mantle material from the west to the east, providing a constrain for a global asymmetric mantle convection.

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Dynamics of upper and lower mantle subduction and its effects on the amplitude and pattern of mantle convection.

10.5194/egusphere-egu21-11502 ◽

2021 ◽

Author(s):

Erik van der Wiel ◽

Cedric Thieulot ◽

Wim Spakman ◽

Douwe van Hinsbergen

Keyword(s):

Tectonic Evolution ◽

Lower Mantle ◽

Mantle Convection ◽

Subduction Zones ◽

Morphological Evolution ◽

Phase Changes ◽

Plate Motion ◽

Plate Tectonic ◽

Model Setup ◽

Modeling Strategy

Long-lived, Mesozoic-Cenozoic subduction zones such as the Pacific slab under the Americas and the Tethyan slab under Eurasia consumed thousands of kms of lithosphere of which remnants are detected in today&#8217;s mantle by seismic tomography. Major differences, however, in subduction zone evolution occurred between these systems which include strong variations in subduction rate, slab morphological evolution, and trench motion, which all appear mostly to be accommodated in the upper 1000 km of the mantle (van der Meer et al. 2018). Furthermore, sinking rates of slabs below this zone tend to be similar for different subduction systems and an order of magnitude smaller than their plate/subduction velocities. Working from the premise that the mantle rheology that accommodated these subduction systems is basically similar, although still poorly constrained, we test the hypothesis that the contrasting evolution of these subduction systems is primarily tied in with the global plate tectonic forcing of subduction.It is generally accepted that plate motion is primarily driven by slab pull with contributions from ridge push, rather than the drag of the underlying mantle. If correct, numerical subduction models should be able to obtain upper as well as lower mantle subduction velocities and sinking rates similar to those reconstructed from geological records. We are at the start of this investigation and will present the numerical model setup, modeling strategy, and preliminary results of a 2-D subduction modelling experiment. We implement a 2D-cylindrical model setup for solving the conservation of momentum, mass and energy with the open-source geodynamics code ASPECT (Kronbichler et al. 2012) using a nonlinear visco-plastic rheology and including the major phase changes. Our focus is on the possible role of the absolute motion of the subducting and overriding plates in concert with slab pull variation reconstructed from plate tectonic evolution models, while in both subduction cases the same (partly nonlinear) mantle rheological processes are required to accommodate slab morphology change and slab sinking. Kinematic modelling constraints are derived from global plate tectonic evolution models, while the tomographically inferred present-day stage provides the end-stage geometry of slabs.van der Meer, D. G., Van Hinsbergen, D. J., & Spakman, W. (2018). Atlas of the underworld: Slab remnants in the mantle, their sinking history, and a new outlook on lower mantle viscosity.&#160;Tectonophysics,&#160;723, 309-448.Kronbichler, M., Heister, T., & Bangerth, W. (2012). High accuracy mantle convection simulation through modern numerical methods.&#160;Geophysical Journal International,&#160;191(1), 12-29.

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Estimation of Secular Change in the Size of Continents for Understanding Early Crustal Development

Frontiers in Earth Science ◽

10.3389/feart.2020.541094 ◽

2020 ◽

Vol 8 ◽

Author(s):

Hikaru Sawada

Keyword(s):

Continental Crust ◽

Mantle Convection ◽

Subduction Zones ◽

Hot Spot ◽

Oceanic Islands ◽

Secular Change ◽

Island Arcs ◽

Secular Changes ◽

Clastic Rocks ◽

Essential Parameter

The size of continents is an essential parameter to understand the growth of the continental crust and the evolution of the solid Earth because it is subject to tectonism and mantle convection and affects the preservation of the crust. This article reviews the secular change in the size of continents on the early Earth, focusing on terrigenous clastic rocks, especially quartzose sandstones occurring on relatively large continents. The earliest continental crust in the Hadean or early Archean was produced with a width of ∼200–500 km, similar to modern oceanic island arcs along subduction zones or oceanic islands in hot spot regions by mantle plume heating. Through the collision and amalgamation of such primitive continental crusts, continental blocks over 500 km in width and length evolved and appeared by ca. 3.5 Ga. Through further amalgamation, during ca. 3.3–2.5 Ga, the Archean continents emerged with widths and lengths greater than 1,000 km, which were still smaller than those of modern continents. Continents with widths and lengths of nearly 10,000 km have existed since ca. 2.4 Ga (early Proterozoic). Further analyses of the composition and formation mechanism of clastic rocks will help reveal more quantitative secular changes in the sizes of continents.

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Controls on the present-day dynamic topography predicted from mantle flow models since 410 Ma

Geophysical Journal International ◽

10.1093/gji/ggab052 ◽

2021 ◽

Author(s):

An Yang ◽

Ting Yang

Keyword(s):

Rayleigh Number ◽

Mantle Convection ◽

Subduction Zones ◽

Surface Heat ◽

Heat Fluxes ◽

Phase Changes ◽

Plate Motion ◽

Dynamic Topography ◽

Convergence Region ◽

Temperature Dependent Viscosity

Summary Mantle convection induces dynamic topography, the lithosphere's surface deflections driven by the vertical stresses from sub-lithospheric mantle convection. Dynamic topography has important influences on a range of geophysical and geological observations. Here, we studied controls on the Earth's dynamic topography through three-dimensional spherical models of mantle convection, which use reconstructed past 410 Myr global plate motion history as time-dependent surface mechanical boundary condition. The numerical model assumes the extended-Boussinesq approximation and includes strongly depth- and temperature-dependent viscosity and phase changes in the mantle. Our results show that removing the chemical layer above the CMB and including depth-dependent thermal expansivity have both a limited influence on the predicted present-day dynamic topography. Considering phase transitions in our models increases the predicted amplitude of dynamic topography, which is mainly influenced by the 410 km exothermic phase transition. The predicted dynamic topography is very sensitive to shallow temperature-induced lateral viscosity variations (LVVs) and Rayleigh number. The preservation of LVVs significantly increases the negative dynamic topography at subduction zones. A decrease (or increase) of Rayleigh number increases (or decreases) the predicted present-day dynamic topography considerably. The dynamic topography predicted from the model considering LVVs and with a Rayleigh number of 6 × 108 is most compatible with residual topography models. This Rayleigh number is consistent with the convective vigor of the Earth as supported by generating more realistic lower mantle structure, slab sinking rate, and surface and CMB heat fluxes. The evolution of the surface heat flux pattern is similar to the long-term eustatic sea-level change. Before the formation of Pangea, large negative dynamic topography formed between the plate convergence region of Gondwana and Laurussia. The predicted dynamic topography similar to that of present day has already emerged by about 262 Ma. Powers for degrees 1–3 dynamic topography at 337 Ma and 104 Ma which correspond to times of higher plate velocities and higher surface heat fluxes are larger.

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Simulation and Contrast Analysis of tem Images of Cracks

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100181658 ◽

1990 ◽

Vol 48 (1) ◽

pp. 578-579

Author(s):

D. Goyal ◽

A. H. King

Keyword(s):

Displacement Vector ◽

Crack Tip ◽

Perfect Crystal ◽

Operating Conditions ◽

Displacement Fields ◽

Fringe Contrast ◽

Orthogonal Geometry ◽

Moire Fringe ◽

Moiré Fringe

TEM images of cracks have been found to give rise to a moiré fringe type of contrast. It is apparent that the moire fringe contrast is observed because of the presence of a fault in a perfect crystal, and is characteristic of the fault geometry and the diffracting conditions in the TEM. Various studies have reported that the moire fringe contrast observed due to the presence of a crack in an otherwise perfect crystal is distinctive of the mode of crack. This paper describes a technique to study the geometry and mode of the cracks by comparing the images they produce in the TEM because of the effect that their displacement fields have on the diffraction of electrons by the crystal (containing a crack) with the corresponding theoretical images. In order to formulate a means of matching experimental images with theoretical ones, displacement fields of dislocations present (if any) in the vicinity of the crack are not considered, only the effect of the displacement field of the crack is considered.The theoretical images are obtained using a computer program based on the two beam approximation of the dynamical theory of diffraction contrast for an imperfect crystal. The procedures for the determination of the various parameters involved in these computations have been well documented. There are three basic modes of crack. Preliminary studies were carried out considering the simplest form of crack geometries, i. e., mode I, II, III and the mixed modes, with orthogonal crack geometries. It was found that the contrast obtained from each mode is very distinct. The effect of variation of operating conditions such as diffracting vector (), the deviation parameter (ω), the electron beam direction () and the displacement vector were studied. It has been found that any small change in the above parameters can result in a drastic change in the contrast. The most important parameter for the matching of the theoretical and the experimental images was found to be the determination of the geometry of the crack under consideration. In order to be able to simulate the crack image shown in Figure 1, the crack geometry was modified from a orthogonal geometry to one with a crack tip inclined to the original crack front. The variation in the crack tip direction resulted in the variation of the displacement vector also. Figure 1 is a cross-sectional micrograph of a silicon wafer with a chromium film on top, showing a crack in the silicon.

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