Seismic anisotropy in eastern Africa, mantle flow, and the African superplume

The Madeira and Canary archipelagos, located in the eastern North Atlantic, are two of many examples of hotspot surface expressions, but a better understanding of the crust and upper mantle structure beneath these regions is needed to investigate their structure in more detail. With the study of seismic anisotropy, it is possible to assess the rheology and structure of asthenosphere and lithosphere that can reflect a combination of mantle and crustal contributions.Here, as part of the SIGHT project (SeIsmic and Geochemical constraints on the Madeira HoTspot), we present the first detailed study of seismic anisotropy beneath both archipelagos, using data collected from over 60 local three-component seismic land stations. Basing our observations on both teleseismic SKS and local S splitting, we are able to distinguish between multiple layers of anisotropy. We observe significant changes in delay time and fast shear-wave orientation patterns on short length-scales on the order of tens of kilometres beneath the western Canary Islands and Madeira Island. In contrast, the eastern Canary Islands and Porto Santo the pattern is much more uniform. The detected delay time increase and more complex orientation patterns beneath the western Canary Islands and Madeira can be attributed to mantle flow disturbed and diverted on small-length scales by a strong vertical component. This is a clear indication of the existence of a plume at each of those archipelagos, nowadays exerting a strong influence on the western and younger islands. We therefore conclude that a plume-like feature beneath Madeira exists in a similar way to the Canary Island hotspot and that regional mantle flow models for the region should be reassessed.This is a contribution to project SIGHT (Ref. PTDC/CTA-GEF/30264/2017). The authors would like to acknowledge the financial support FCT through project UIDB/50019/2020 &#8211; IDL.

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Seismic anisotropy and mantle flow from the Great Basin to the Great Plains, western United States

Journal of Geophysical Research Atmospheres ◽

10.1029/2000jb900021 ◽

2000 ◽

Vol 105 (B6) ◽

pp. 13715-13734 ◽

Cited By ~ 68

Author(s):

M. K. Savage ◽

A. F. Sheehan

Keyword(s):

United States ◽

Great Basin ◽

Great Plains ◽

Seismic Anisotropy ◽

Western United States ◽

Mantle Flow

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Supplemental Material: Seismic anisotropy in southern Costa Rica confirms upper mantle flow from the Pacific to the Caribbean

10.1130/geol.s.12773969 ◽

2020 ◽

Author(s):

Vadim Levin ◽

et al.

Keyword(s):

Data Analysis ◽

Costa Rica ◽

Upper Mantle ◽

Seismic Anisotropy ◽

Data Sources ◽

Mantle Flow ◽

Analysis Methodology ◽

The Pacific ◽

Wave Splitting ◽

The Caribbean

Data sources, details of data analysis methodology, and additional diagrams and maps of shear wave splitting measurements.

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Seismic anisotropy reveals crustal flow driven by mantle vertical loading in the Pacific NW

Science Advances ◽

10.1126/sciadv.abb0476 ◽

2020 ◽

Vol 6 (28) ◽

pp. eabb0476

Author(s):

Jorge C. Castellanos ◽

Jonathan Perry-Houts ◽

Robert W. Clayton ◽

YoungHee Kim ◽

A. Christian Stanciu ◽

...

Keyword(s):

Pacific Northwest ◽

Upper Mantle ◽

Seismic Anisotropy ◽

Azimuthal Anisotropy ◽

Mantle Flow ◽

Near Surface ◽

Vertical Loading ◽

The Pacific ◽

Anisotropy Model ◽

Lithosphere Dynamics

Buoyancy anomalies within Earth’s mantle create large convective currents that are thought to control the evolution of the lithosphere. While tectonic plate motions provide evidence for this relation, the mechanism by which mantle processes influence near-surface tectonics remains elusive. Here, we present an azimuthal anisotropy model for the Pacific Northwest crust that strongly correlates with high-velocity structures in the underlying mantle but shows no association with the regional mantle flow field. We suggest that the crustal anisotropy is decoupled from horizontal basal tractions and, instead, created by upper mantle vertical loading, which generates pressure gradients that drive channelized flow in the mid-lower crust. We then demonstrate the interplay between mantle heterogeneities and lithosphere dynamics by predicting the viscous crustal flow that is driven by local buoyancy sources within the upper mantle. Our findings reveal how mantle vertical load distribution can actively control crustal deformation on a scale of several hundred kilometers.

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Seismic anisotropy in southern Costa Rica confirms upper mantle flow from the Pacific to the Caribbean

Geology ◽

10.1130/g47826.1 ◽

2020 ◽

Vol 49 (1) ◽

pp. 8-12 ◽

Cited By ~ 1

Author(s):

Vadim Levin ◽

Stephen Elkington ◽

James Bourke ◽

Ivonne Arroyo ◽

Lepolt Linkimer

Keyword(s):

Costa Rica ◽

Upper Mantle ◽

Seismic Anisotropy ◽

Volcanic Eruptions ◽

Central American ◽

Mantle Flow ◽

Dynamic Topography ◽

The Pacific ◽

The Caribbean ◽

Surrounding Areas

Abstract Surrounded by subducting slabs and continental keels, the upper mantle of the Pacific is largely prevented from mixing with surrounding areas. One possible outlet is beneath the southern part of the Central American isthmus, where regional observations of seismic anisotropy, temporal changes in isotopic composition of volcanic eruptions, and considerations of dynamic topography all suggest upper mantle flow from the Pacific to the Caribbean. We derive new constraints on the nature of seismic anisotropy in the upper mantle of southern Costa Rica from observations of birefringence in teleseismic shear waves. Fast and slow components separate by ∼1 s, with faster waves polarized along the 40°–50° (northeast) direction, near-orthogonally to the Central American convergent margin. Our results are consistent with upper mantle flow from the Pacific to the Caribbean and require an opening in the lithosphere subducting under the region.

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An investigation of seismic anisotropy in the lowermost mantle beneath Iceland

Geophysical Journal International ◽

10.1093/gji/ggz312 ◽

2019 ◽

Vol 219 (Supplement_1) ◽

pp. S152-S166 ◽

Cited By ~ 2

Author(s):

Jonathan Wolf ◽

Neala Creasy ◽

Angelo Pisconti ◽

Maureen D Long ◽

Christine Thomas

Keyword(s):

Seismic Anisotropy ◽

Horizontal Flow ◽

Mantle Flow ◽

Small Data ◽

Vertical Flow ◽

Low Velocity ◽

Data Set ◽

Study Region ◽

Deep Mantle ◽

Lowermost Mantle

SUMMARY Iceland represents one of the most well-known examples of hotspot volcanism, but the details of how surface volcanism connects to geodynamic processes in the deep mantle remain poorly understood. Recent work has identified evidence for an ultra-low velocity zone in the lowermost mantle beneath Iceland and argued for a cylindrically symmetric upwelling at the base of a deep mantle plume. This scenario makes a specific prediction about flow and deformation in the lowermost mantle, which can potentially be tested with observations of seismic anisotropy. Here we present an investigation of seismic anisotropy in the lowermost mantle beneath Iceland, using differential shear wave splitting measurements of S–ScS and SKS–SKKS phases. We apply our techniques to waves propagating at multiple azimuths, with the goal of gaining good geographical and azimuthal coverage of the region. Practical limitations imposed by the suboptimal distribution of global seismicity at the relevant distance ranges resulted in a relatively small data set, particularly for S–ScS. Despite this, however, our measurements of ScS splitting due to lowermost mantle anisotropy clearly show a rotation of the fast splitting direction from nearly horizontal for two sets of paths that sample away from the low velocity region (implying VSH > VSV) to nearly vertical for a set of paths that sample directly beneath Iceland (implying VSV > VSH). We also find evidence for sporadic SKS–SKKS discrepancies beneath our study region; while the geographic distribution of discrepant pairs is scattered, those pairs that sample closest to the base of the Iceland plume tend to be discrepant. Our measurements do not uniquely constrain the pattern of mantle flow. However, we carried out simple ray-theoretical forward modelling for a suite of plausible anisotropy mechanisms, including those based on single-crystal elastic tensors, those obtained via effective medium modelling for partial melt scenarios, and those derived from global or regional models of flow and texture development in the deep mantle. These simplified models do not take into account details such as possible transitions in anisotropy mechanism or deformation regime, and test a simplified flow field (vertical flow beneath the plume and horizontal flow outside it) rather than more detailed flow scenarios. Nevertheless, our modelling results demonstrate that our ScS splitting observations are generally consistent with a flow scenario that invokes nearly vertical flow directly beneath the Iceland hotspot, with horizontal flow just outside this region.

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

10.5194/egusphere-egu2020-11368 ◽

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

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. 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.

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MAVEPROS: a new open source software to predict mantle elastic properties and build realistic tomographic models

10.5194/egusphere-egu2020-18884 ◽

2020 ◽

Author(s):

Manuele Faccenda

Keyword(s):

Single Crystal ◽

Open Source ◽

Elastic Properties ◽

Open Source Software ◽

Large Scale ◽

Seismic Anisotropy ◽

Volume Fraction ◽

Dislocation Creep ◽

Mantle Flow ◽

Polycrystalline Aggregates

Coupling large-scale geodynamic and seismological modelling appears a promising methodology for the understanding of the Earth&#8217;s recent dynamics and present-day structure. So far, the two types of modelling have been mainly conducted separately, and a code capable of linking these two methodologies of investigation is still lacking.In this contribution I present MAVEPROS, a new open source software that allows both for the modelling of strain-induced mantle fabrics and seismic anisotropy, and for the generation of realistic synthetic tomographic models.As an input, the software requires the velocity, pressure, temperature (and additionally the fraction of deformation accommodated by dislocation creep) fields (averaged each 100 kyr for typical mantle strain rates) outputted by the large-scale mantle flow models.The strain-induced mantle fabrics are then modelled with D-Rex (Kaminski et al., 2004, GJI), an open source code that has been parallelized and modified to account for fast computation, combined diffusion-dislocation creep (Faccenda and Capitanio, 2012a, GRL; 2013, Gcubed), LPO of transition zone and lower mantle polycrystalline aggregates, P-T dependence of single crystal elastic tensors (Faccenda, 2014, PEPI), advection and non-steady-state deformation of crystal aggregates in 2D/3D cartesian/spherical grids with basic/staggered velocity nodes (Hu et al., 2017, EPSL), homogeneous sampling of the mantle by implementation of the Deformable PIC method (Samuel, 2018, GJI), apparent anisotropy in layered or crack-bearing rocks estimated with the Differential Effective Medium (DEM) (Sturgeon et al., Gcubed, 2019). The new version of D-Rex can solve for the LPO evolution of 100.000s polycrystalline aggregates of the whole mantle in a few hours, outputting the full elastic tensor of poly-crystalline aggregates as a function of each single crystal orientation, volume fraction and P-T scaled elastic moduli.The crystal aggregates can then be interpolated in a tomographic grid for either visual inspection of the mantle elastic properties &#160;(such as Vp and Vs isotropic anomalies; radial, azimuthal, Vp and Vs anisotropies; reflected/refracted energy at discontinuities for different incidence angles as imaged by receiver function studies; ), or to generate input files for large-scale synthetic waveform modelling (e.g., SPECFEM3D format; FSTRACK format to calculate SKS splitting (Becker et al., 2006, GJI)).

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Mantle flow under the Central Alps: Constraints from non-vertical-ray SKS shear-wave splittting

10.5194/egusphere-egu2020-7450 ◽

2020 ◽

Author(s):

Eric Löberich ◽

Götz Bokelmann

Keyword(s):

Shear Wave ◽

Upper Mantle ◽

Seismic Anisotropy ◽

Single Layer ◽

Depth Resolution ◽

Shear Wave Splitting ◽

Mantle Flow ◽

Central Alps ◽

Upper Mantle Anisotropy ◽

Wave Splitting

The association of seismic anisotropy and deformation, as e.g. exploited by shear-wave splitting measurements, provides a unique opportunity to map the orientation of geodynamic processes in the upper mantle and to constraint their nature. However, due to the limited depth-resolution of steeply arriving core-phases, used for shear-wave splitting investigations, it appears difficult to differentiate between asthenospheric and lithospheric origins of observed seismic anisotropy. To change that, we take advantage of the different backazimuthal variations of fast orientation &#966; and delay time &#916;t, when considering the non-vertical incidence of phases passing through an olivine block with vertical b-axis as opposed to one with vertical c-axis. Both these alignments can occur depending on the type of deformation, e.g. a sub-horizontal foliation orientation in the case of a simple asthenospheric flow and a sub-vertical foliation when considering vertically-coherent deformation in the lithosphere. In this study we investigate the cause of seismic anisotropy in the Central Alps. Combining high-quality manual shear-wave splitting measurements of three datasets leads to a dense station coverage. Fast orientations &#966; show a spatially coherent and relatively simple mountain-chain-parallel pattern, likely related to a single-layer case of upper mantle anisotropy. Considering the measurements of the whole study area together, our non-vertical-ray shear-wave splitting procedure points towards a b-up olivine situation and thus favors an asthenospheric anisotropy source, with a horizontal flow plane of deformation. We also test the influence of position relative to the European slab, distinguishing a northern and southern subarea based on vertically-integrated travel times through a tomographic model. Differences in the statistical distribution of splitting parameters &#966; and &#916;t, and in the backazimuthal variation of &#948;&#966; and &#948;&#916;t, become apparent. While the observed seismic anisotropy in the northern subarea shows indications of asthenospheric flow, likely a depth-dependent plane Couette-Poiseuille flow around the Alps, the origin in the southern subarea remains more difficult to determine and may also contain effects from the slab itself.

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