scholarly journals The evolution of basal mantle structure in response to supercontinent aggregation and dispersal

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Xianzhi Cao ◽  
Nicolas Flament ◽  
Ömer F. Bodur ◽  
R. Dietmar Müller
Keyword(s):  
Flow Model ◽  
Shear Velocity ◽  
Plate Motion ◽  
Mantle Flow ◽  
Mantle Structure ◽  
Exchange Material ◽  
Plate Reconstruction ◽  
Tectonic Terranes ◽  
Over Time ◽  
Low Shear

AbstractSeismic studies have revealed two Large Low-Shear Velocity Provinces (LLSVPs) in the lowermost mantle. Whether these structures remain stable over time or evolve through supercontinent cycles is debated. Here we analyze a recently published mantle flow model constrained by a synthetic plate motion model extending back to one billion years ago, to investigate how the mantle evolves in response to changing plate configurations. Our model predicts that sinking slabs segment the basal thermochemical structure below an assembling supercontinent, and that this structure eventually becomes unified due to slab push from circum-supercontinental subduction. In contrast, the basal thermochemical structure below the superocean is generally coherent due to the persistence of a superocean in our imposed plate reconstruction. The two antipodal basal thermochemical structures exchange material several times when part of one of the structures is carved out and merged with the other one, similarly to “exotic” tectonic terranes. Plumes mostly rise from thick basal thermochemical structures and in some instances migrate from the edges towards the interior of basal thermochemical structures due to slab push. Our results suggest that the topography of basal structures and distribution of plumes change over time due to the changing subduction network over supercontinent cycles.

Coupled evolution of supercontinents and lower mantle structure

2021 ◽  
Author(s):  
Xianzhi Cao ◽  
Nicolas Flament ◽  
Ömer Bodur ◽  
Dietmar Müller
Keyword(s):  
Lower Mantle ◽  
Reference Frames ◽  
Plate Motion ◽  
Motion Path ◽  
Mantle Flow ◽  
Plate Tectonic ◽  
Mantle Structure ◽  
Plate Motions ◽  
Flow Models ◽  
Over Time

<p>The relationships between plate motions and basal mantle structure remain poorly understood, with some models implying that the basal mantle structure has remained stable over time, while others suggest that it could be shaped by the aggregation and dispersal of supercontinents. Here we investigate the plate-basal mantle relationship through 1) building a series of end-member plate tectonic models over one billion years, and 2) creating mantle flow models assimilated by those plate models. To achieve that, we build synthetic plate tectonic models dating from 1 Ga to 250 Ma that we connect to an existing palaeogeographical plate reconstruction from 250 Ma to create a relative plate motion model for the last 1 Gyr, in which supercontinent breakup and reassembly occur via introversion. We consider three distinct reference frames that result in different net lithospheric rotation. We find that the flow models predict a dominant degree-2 lower mantle structure most of the time and that they are in first-order agreement (~70% spatial match) with tomographic models. Model thermochemical structures at the base of the mantle may split into smaller structures when slabs sink onto them, and smaller basal structures may merge into larger ones as a result of slab pushing. The basal thermochemical structure under the superocean is large and continuous, whereas the basal thermochemical structure under the supercontinent is smaller and progressively assembles during and shortly after supercontinent assembly. In the models, plumes also develop preferentially along the edge of the basal thermochemical structures and tend to migrate towards the interior of basal structures over time as they interact with the slabs. Lone plumes can also form away from the main thermochemical structures, often within a small network of sinking slabs. Lone plumes may migrate between basal structures. We analyse the relationship between imposed tectonic velocities and deep mantle flow, and find that at spherical harmonic degree 2, the maxima of lower mantle radial flow and temperature follow the motion path of the maxima of surface divergence. It may take ~160-240 Myr for lower mantle structure to reflect plate motion changes when the lower mantle is reorganised by slabs sinking onto basal thermochemical structures, and/or when slabs stagnate in the transition zone before sinking to the lower mantle. Basal thermochemical structures move at less than 0.6 °/Myr in our models with a temporal average of 0.16 °/Myr when there is no net lithospheric rotation, and between 0.20-0.23 °/Myr when net lithospheric rotation exists and is induced to the lower mantle. Our results suggest that basal thermochemical structures are not stationary, but rather linked to global plate motions and plate boundary reconfigurations, reflecting the dynamic nature of the co-evolving plate-mantle system.</p>

Lowermost mantle flow at the eastern edge of the African Large Low Shear Velocity Province

2015 ◽  
Vol 420 ◽  
pp. 12-22 ◽  
Author(s):  
Heather A. Ford ◽  
Maureen D. Long ◽  
Xiaobo He ◽  
Colton Lynner
Keyword(s):  
Shear Velocity ◽  
Mantle Flow ◽  
Eastern Edge ◽  
Low Shear ◽  
Lowermost Mantle

How thermochemical piles initiate plumes at their edges

2020 ◽  
Author(s):  
Björn Heyn ◽  
Clinton Conrad ◽  
Reidar Trønnes
Keyword(s):  
Numerical Models ◽  
Shear Velocity ◽  
Viscous Drag ◽  
Mantle Flow ◽  
Core Mantle Boundary ◽  
The Core ◽  
Plate Velocity ◽  
Igneous Provinces ◽  
Suggested Link ◽  
Low Shear

<p>Deep-rooted mantle plumes are thought to originate from the margins of the Large Low Shear Velocity Provinces (LLSVPs) at the base of the mantle. Visible in seismic tomography, the LLSVPs are often numerically modeled as dense and viscous thermochemical piles. Although the piles force lateral mantle flow upwards at their edges, it is not clear if, and how, plumes are predominantly initiated at the pile margins. In this study, we develop numerical models that show a series of plumes periodically rising from the margin of an approximately 300 km thick dense thermochemical pile, with each plume temporarily increasing the pile’s local thickness to almost 370 km due to upward viscous drag from the rising plume. When the plume is pushed towards the pile center by the lateral mantle flow, the viscous drag on the dense material at the pile margin decreases and the pile starts to collapse back towards the core-mantle boundary (CMB). This causes the dense pile material to extend laterally along the CMB (about 150 km), locally thickening the lower thermal boundary layer on the CMB next to the pile, which initiates a new plume. The resulting plume cycle is reflected in both the thickness and lateral motion of the local pile margin within a few degrees of the pile edge, while the overall thickness of the pile is not affected. The frequency of plume generation is mainly controlled by the rate at which slab material is transported to the CMB, and thus depends on the plate velocity and the sinking rate of slabs in the lower mantle. Within Earth, this mechanism of episodic plume initiation may explain the suggested link between the positions of hotspots and Large Igneous Provinces (LIPs) and the LLSVP margins. Moreover, a collapse of the southeastern corner of the African LLSVP, and subsequent triggering of plumes around the spreading pile material, may explain the observed clustering of LIPs in that area between 95 and 155 Ma.</p>

scholarly journals Improving global paleogeography since the late Paleozoic using paleobiology

2017 ◽  
Vol 14 (23) ◽  
pp. 5425-5439 ◽  
Author(s):  
Wenchao Cao ◽  
Sabin Zahirovic ◽  
Nicolas Flament ◽  
Simon Williams ◽  
Jan Golonka ◽  
...  
Keyword(s):  
Ocean Circulation ◽  
Plate Motion ◽  
Dynamic Topography ◽  
Map Projections ◽  
Motion Models ◽  
Tectonic Reconstruction ◽  
Isotope Record ◽  
America South ◽  
Regional Sea Level ◽  
Plate Reconstruction

Abstract. Paleogeographic reconstructions are important to understand Earth's tectonic evolution, past eustatic and regional sea level change, paleoclimate and ocean circulation, deep Earth resources and to constrain and interpret the dynamic topography predicted by mantle convection models. Global paleogeographic maps have been compiled and published, but they are generally presented as static maps with varying map projections, different time intervals represented by the maps and different plate motion models that underlie the paleogeographic reconstructions. This makes it difficult to convert the maps into a digital form and link them to alternative digital plate tectonic reconstructions. To address this limitation, we develop a workflow to restore global paleogeographic maps to their present-day coordinates and enable them to be linked to a different tectonic reconstruction. We use marine fossil collections from the Paleobiology Database to identify inconsistencies between their indicative paleoenvironments and published paleogeographic maps, and revise the locations of inferred paleo-coastlines that represent the estimated maximum transgression surfaces by resolving these inconsistencies. As a result, the consistency ratio between the paleogeography and the paleoenvironments indicated by the marine fossil collections is increased from an average of 75 % to nearly full consistency (100 %). The paleogeography in the main regions of North America, South America, Europe and Africa is significantly revised, especially in the Late Carboniferous, Middle Permian, Triassic, Jurassic, Late Cretaceous and most of the Cenozoic. The global flooded continental areas since the Early Devonian calculated from the revised paleogeography in this study are generally consistent with results derived from other paleoenvironment and paleo-lithofacies data and with the strontium isotope record in marine carbonates. We also estimate the terrestrial areal change over time associated with transferring reconstruction, filling gaps and modifying the paleogeographic geometries based on the paleobiology test. This indicates that the variation of the underlying plate reconstruction is the main factor that contributes to the terrestrial areal change, and the effect of revising paleogeographic geometries based on paleobiology is secondary.

scholarly journals Ice-dynamic projections of the Greenland ice sheet in response to atmospheric and oceanic warming

2015 ◽  
Vol 9 (3) ◽  
pp. 1039-1062 ◽  
Author(s):  
J. J. Fürst ◽  
H. Goelzer ◽  
P. Huybrechts
Keyword(s):  
Sea Level Rise ◽  
Sea Level ◽  
Flow Model ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Water Runoff ◽  
Strong Impact ◽  
Ice Dynamics ◽  
Over Time ◽  
Loss Rates

Abstract. Continuing global warming will have a strong impact on the Greenland ice sheet in the coming centuries. During the last decade (2000–2010), both increased melt-water runoff and enhanced ice discharge from calving glaciers have contributed 0.6 ± 0.1 mm yr−1 to global sea-level rise, with a relative contribution of 60 and 40% respectively. Here we use a higher-order ice flow model, spun up to present day, to simulate future ice volume changes driven by both atmospheric and oceanic temperature changes. For these projections, the flow model accounts for runoff-induced basal lubrication and ocean warming-induced discharge increase at the marine margins. For a suite of 10 atmosphere and ocean general circulation models and four representative concentration pathway scenarios, the projected sea-level rise between 2000 and 2100 lies in the range of +1.4 to +16.6 cm. For two low emission scenarios, the projections are conducted up to 2300. Ice loss rates are found to abate for the most favourable scenario where the warming peaks in this century, allowing the ice sheet to maintain a geometry close to the present-day state. For the other moderate scenario, loss rates remain at a constant level over 300 years. In any scenario, volume loss is predominantly caused by increased surface melting as the contribution from enhanced ice discharge decreases over time and is self-limited by thinning and retreat of the marine margin, reducing the ice–ocean contact area. As confirmed by other studies, we find that the effect of enhanced basal lubrication on the volume evolution is negligible on centennial timescales. Our projections show that the observed rates of volume change over the last decades cannot simply be extrapolated over the 21st century on account of a different balance of processes causing ice loss over time. Our results also indicate that the largest source of uncertainty arises from the surface mass balance and the underlying climate change projections, not from ice dynamics.

scholarly journals Effective buoyancy ratio: a new parameter to characterize thermo-chemical mixing in the Earth's mantle

2014 ◽  
Vol 6 (2) ◽  
pp. 2675-2697
Author(s):  
A. Galsa ◽  
M. Herein ◽  
L. Lenkey ◽  
M. P. Farkas ◽  
G. Taller
Keyword(s):  
Mixing Time ◽  
Initial Density ◽  
Shear Velocity ◽  
Dense Layer ◽  
Earth’S Mantle ◽  
A Value ◽  
Short Time ◽  
Chemical Mixing ◽  
Buoyancy Ratio ◽  
Low Shear

Abstract. Numerical modeling has been carried out in a 2-D cylindrical shell domain to quantify the evolution of a primordial dense layer around the core mantle boundary. Effective buoyancy ratio, Beff was introduced to characterize the evolution of the two-layer thermo-chemical convection in the Earth's mantle. Beff decreases with time due to (1) warming the compositionally dense layer, (2) cooling the overlying mantle, (3) eroding the dense layer by thermal convection in the overlying mantle, and (4) diluting the dense layer by inner convection. When Beff reaches the instability point, Beff = 1, effective thermo-chemical convection starts, and the mantle will be mixed (Beff = 0) during a short time. A parabolic relation was revealed between the initial density difference of the layers and the mixing time. Morphology of large low shear velocity provinces as well as results from seismic tomography and normal mode data suggest a value of Beff ≥ 1 for the mantle.

Geological signatures of slab shear-off during ongoing India-Asia convergence

2021 ◽  
Author(s):  
Abdul Qayyum ◽  
Nalan Lom ◽  
Eldert L Advokaat ◽  
Wim Spakman ◽  
Douwe J.J van Hinsbergen
Keyword(s):  
Numerical Models ◽  
Leading Edge ◽  
Plate Motion ◽  
Indian Plate ◽  
Mantle Structure ◽  
Continental Lithosphere ◽  
Common View ◽  
Plate Convergence ◽  
Collision Zone ◽  
The Himalaya

<p>Much of our understanding of the dynamics of slab break-off and its geological signatures rely on numerical models with a simplified set-up, in which slab break-off follows arrival of a continent in a mantle-stationary trench, the subsequent arrest of plate convergence, and after a delay time of 10 Ma or more, slab break off under the influence of slab pull. However, geological reconstructions show that plate tectonic reality deviates from this setup: post-collisional convergence is common, trenches are generally not stationary relative to mantle, neither before nor after collision, and there are many examples in which the mantle structure below collision zones is characterized by more, or fewer slabs than collisions.</p><p>A key example of the former is the India-Asia collision zone, where the mantle below India hosts two major, despite the common view of a single collision. Kinematic reconstructions reveal that post-collisional convergence amounted 1000s of kms, and was associated with ~1000 km of trench/collision zone advance. Collision between India-Asia collision zone may provide a good case study to determine the result of post-collisional convergence and absolute lower and upper plate motion on mantle structure, and to evaluate to what extent commonly assumed diagnostic geological phenomena of slab break-off apply.</p><p>In addition to the previously identified major India, Himalaya, and Burma slabs, we here map smaller slabs below Afghanistan and the Himalaya that reveal the latest phases of break-off. We show that west-dipping and east-dipping slabs west and east of India, respectively, are dragged northward parallel to the slab, slabs subducting north of India are overturned, and that the shallowest slab fragments are found in the location where the horizontally underthrust Indian lithosphere below Tibet is narrowest. Our results confirm that northward Indian absolute plate motion continued during two episodes of break-off of large (>1000 km wide) slabs, and decoupling of several smaller fragments. These slabs are currently found south of the present day trench locations. The slabs are located even farther south (>1000 km) of the leading edge of the Indian continental lithosphere, currently underthrust below Tibet, from which the slabs detached, signalling ongoing absolute Indian plate motion. We conclude that the multiple slab break-off events in this setting of ongoing plate convergence and trench advance is better explained by shearing off of slabs from the downgoing plate, possibly at a depth corresponding to the base of the Indian continental lithosphere, are not (necessarily) related to the timing of collision. A recently proposed, detailed diachronous record of deformation, uplift, and oroclinal bending in the Himalaya that was liked to slab break-off fits well with our kinematically reconstructed timing of the last slab shear-off, and may provide an important reference geological record for this process. We find that the commonly applied conceptual geological signatures of slab break-off do not apply to the India-Asia collision zone, or to similar settings and histories such as the Arabia-Eurasia collision zone. Our study provides more realistic boundary conditions for future numerical models that aim to assess the dynamics of subduction termination and its geological signatures.</p>

scholarly journals Competing Deformation Mechanisms in Periclase: Implications for Lower Mantle Anisotropy

Minerals ◽  
2019 ◽  
Vol 9 (11) ◽  
pp. 650 ◽  
Author(s):  
Feng Lin ◽  
Samantha Couper ◽  
Mike Jugle ◽  
Lowell Miyagi
Keyword(s):  
Lower Mantle ◽  
Seismic Anisotropy ◽  
Shear Velocity ◽  
Deformation Mechanisms ◽  
Effect Of Temperature ◽  
Lattice Strains ◽  
Slip Activity ◽  
Consistent Method ◽  
Mantle Anisotropy ◽  
Low Shear

Seismic anisotropy is observed above the core-mantle boundary in regions of slab subduction and near the margins of Large Low Shear Velocity Provinces (LLSVPs). Ferropericlase is believed to be the second most abundant phase in the lower mantle. As it is rheologically weak, it may be a dominant source for anisotropy in the lowermost mantle. Understanding deformation mechanisms in ferropericlase over a range of pressure and temperature conditions is crucial to interpret seismic anisotropy. The effect of temperature on deformation mechanisms of ferropericlase has been established, but the effects of pressure are still controversial. With the aim to clarify and quantify the effect of pressure on deformation mechanisms, we perform room temperature compression experiments on polycrystalline periclase to 50 GPa. Lattice strains and texture development are modeled using the Elasto-ViscoPlastic Self Consistent method (EVPSC). Based on modeling results, we find that { 110 } ⟨ 1 1 ¯ 0 ⟩ slip is increasingly activated with higher pressure and is fully activated at ~50 GPa. Pressure and temperature have a competing effect on activities of dominant slip systems. An increasing { 100 } ⟨ 011 ⟩ : { 110 } ⟨ 1 1 ¯ 0 ⟩ ratio of slip activity is expected as material moves from cold subduction regions towards hot upwelling region adjacent to LLSVPs. This could explain observed seismic anisotropy in the circum-Pacific region that appears to weaken near margins of LLVSPs.

scholarly journals Velocity and density characteristics of subducted oceanic crust and the origin of lower-mantle heterogeneities

2020 ◽  
Vol 11 (1) ◽  
Author(s):  
Wenzhong Wang ◽  
Yinhan Xu ◽  
Daoyuan Sun ◽  
Sidao Ni ◽  
Renata Wentzcovitch ◽  
...  
Keyword(s):  
Phase Transition ◽  
Oceanic Crust ◽  
Seismic Tomography ◽  
Lower Mantle ◽  
Shear Velocity ◽  
Calcium Ferrite ◽  
Velocity Anomaly ◽  
Mantle Heterogeneities ◽  
Subducted Oceanic Crust ◽  
Low Shear

AbstractSeismic heterogeneities detected in the lower mantle were proposed to be related to subducted oceanic crust. However, the velocity and density of subducted oceanic crust at lower-mantle conditions remain unknown. Here, we report ab initio results for the elastic properties of calcium ferrite‐type phases and determine the velocities and density of oceanic crust along different mantle geotherms. We find that the subducted oceanic crust shows a large negative shear velocity anomaly at the phase boundary between stishovite and CaCl2-type silica, which is highly consistent with the feature of mid-mantle scatterers. After this phase transition in silica, subducted oceanic crust will be visible as high-velocity heterogeneities as imaged by seismic tomography. This study suggests that the presence of subducted oceanic crust could provide good explanations for some lower-mantle seismic heterogeneities with different length scales except large low shear velocity provinces (LLSVPs).

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