The upper-mantle low-velocity anomaly beneath Ethiopia, Kenya, and Tanzania: Constraints on the origin of the African superswell in eastern Africa and plate versus plume models of mantle dynamics

2011 ◽  
pp. 37-50 ◽  
Author(s):  
Andrew A. Nyblade
Keyword(s):  
Upper Mantle ◽  
Eastern Africa ◽  
Plate Versus ◽  
Low Velocity ◽  
Mantle Dynamics ◽  
Velocity Anomaly

The upper mantle beneath the South Atlantic Ocean, South America and Africa from waveform tomography with massive data sets

2020 ◽  
Vol 221 (1) ◽  
pp. 178-204 ◽  
Author(s):  
N L Celli ◽  
S Lebedev ◽  
A J Schaeffer ◽  
M Ravenna ◽  
C Gaina
Keyword(s):  
South America ◽  
Upper Mantle ◽  
High Velocity ◽  
South Atlantic ◽  
The South ◽  
Low Velocity ◽  
The West ◽  
Velocity Anomaly ◽  
Archean Crust ◽  
Velocity Anomalies

SUMMARY We present a tomographic model of the crust, upper mantle and transition zone beneath the South Atlantic, South America and Africa. Taking advantage of the recent growth in broadband data sampling, we compute the model using waveform fits of over 1.2 million vertical-component seismograms, obtained with the automated multimode inversion of surface, S and multiple S waves. Each waveform provides a set of linear equations constraining perturbations with respect to a 3-D reference model within an approximate sensitivity volume. We then combine all equations into a large linear system and solve it for a 3-D model of S- and P-wave speeds and azimuthal anisotropy within the crust, upper mantle and uppermost lower mantle. In South America and Africa, our new model SA2019 reveals detailed structure of the lithosphere, with structure of the cratons within the continents much more complex than seen previously. In South America, lower seismic velocities underneath the transbrasilian lineament (TBL) separate the high-velocity anomalies beneath the Amazon Craton from those beneath the São Francisco and Paraná Cratons. We image the buried portions of the Amazon Craton, the thick cratonic lithosphere of the Paraná and Parnaíba Basins and an apparently cratonic block wedged between western Guyana and the slab to the west of it, unexposed at the surface. Thick cratonic lithosphere is absent under the Archean crust of the São Luis, Luis Álves and Rio de La Plata Cratons, next to the continental margin. The Guyana Highlands are underlain by low velocities, indicating hot asthenosphere. In the transition zone, we map the subduction of the Nazca Plate and the Chile Rise under Patagonia. Cratonic lithosphere beneath Africa is more fragmented than seen previously, with separate cratonic units observed within the West African and Congo Cratons, and with cratonic lithosphere absent beneath large portions of Archean crust. We image the lateral extent of the Niassa Craton, hypothesized previously and identify a new unit, the Cubango Craton, near the southeast boundary of the grater Congo Craton, with both of these smaller cratons unexposed at the surface. In the South Atlantic, the model reveals the patterns of interaction between the Mid-Atlantic Ridge (MAR) and the nearby hotspots. Low-velocity anomalies beneath major hotspots extend substantially deeper than those beneath the MAR. The Vema Hotspot, in particular, displays a pronounced low-velocity anomaly under the thick, high-velocity lithosphere of the Cape Basin. A strong low velocity anomaly also underlies the Cameroon Volcanic Line and its offshore extension, between Africa and the MAR. Subtracting the global, age-dependent VS averages from those in the South Atlantic Basins, we observe areas where the cooling lithosphere is locally hotter than average, corresponding to the location of the Tristan da Cunha, Vema and Trindade hotspots. Beneath the anomalously deep Argentine Basin, we image unusually thick, high-velocity lithosphere, which suggests that its anomalously great depth can be explained, at least to a large extent, by isostatic, negative lithospheric buoyancy.

Integrated geophysical-petrological modelling of the Eifel region

2020 ◽  
Author(s):  
Agnes Wansing ◽  
Jörg Ebbing ◽  
Eva Bredow
Keyword(s):  
Mantle Plume ◽  
Upper Mantle ◽  
West Germany ◽  
Small Scale ◽  
Low Velocity ◽  
Crust And Upper Mantle ◽  
Quaternary Volcanism ◽  
Velocity Anomaly ◽  
Type Composition ◽  
Eifel Region

<p>We present an integrated geophysical-petrological model of the Eifel region. The Eifel is a volcanic active region in West Germany that exhibits Tertiary as well as Quaternary volcanism. One suggestion for the source of this volcanism is a small-scale upper mantle plume.</p><p>The 3D model includes the crust and upper mantle and was generated by combined modelling of topography and the gravity field with constraints from seismology and geochemistry. In the best-fit model, the subcontinental lithospheric mantle is associated with a Phanerozoic-type composition, resulting in a depth of 80 km for the lithosphere-asthenosphere boundary (LAB) beneath the Eifel and in comparison 110 - 130 km beneath the Paris basin. A Proterozoic-type composition in contrast results in a LAB depth of 120 km in the Eifel. While the model fits the geophysical observables and features a thin lithosphere, it does not lead to a plume-like structure and does not feature a seismic low-velocity anomaly.</p><p>The measured low-velocity anomaly can be reproduced by introducing (1) an even thinner lithosphere or (2) a plume-like body above the thermal LAB with a composition based on data from Eifel xenoliths, which have a mainly basanitic composition. This additional structure results in a thermal anomaly and has an effect on the isostatic elevation of c. 360 m, but it does not result in a significant signal in the gravity anomalies. Further modelling showed how crustal intrusions could additionally mask the gravitational effect from such a small-scale upper mantle plume.</p><p>The model does not conclusively explain the source of the Eifel volcanism, but the models and the calculation of synthetic dispersion curves help to assess the possibility to resolve a small-scale upper mantle plume with joint inversion in future analysis.</p>

2D crust-upper mantle velocity structure along a seismic section in Nanling, South China

2020 ◽  
Author(s):  
Jia-ji xi ◽  
Guo-ming jiang ◽  
Gui-bin zhang ◽  
Xiao-long he
Keyword(s):  
Upper Mantle ◽  
Velocity Structure ◽  
Ore Deposits ◽  
Low Velocity ◽  
Seismic Section ◽  
Late Mesozoic ◽  
Velocity Anomaly ◽  
Metallogenic Belt ◽  
Velocity Anomalies ◽  
Mantle Velocity

<p>    There exists an important polymetallic ore belt in Nanling of the southeastern China. Previous studies suggest that the mineralization of Nanling is probably related to the bottom intrusion of magmatic rocks in the late Mesozoic. In this study, a natural seismic section was installed by using 81 portable stations with an interval of 5 km from July 2017 to August 2019, which runs across the Nanling belt in the south of Fujian and Jiangxi provinces. As a result, we have picked up 3,818 relative residual data from 215 teleseismic events with magnitude greater than 5.5. And we have applied the teleseismic full-waveform tomography and the teleseismic travel-time tomography to study the crust and the mantle velocity structure beneath the Nanling metallogenic belt, respectively. Our preliminary results show that: (1) a clear low-velocity anomaly exists in the crust beneath the Zhenghe-Dapu fault and its east side, which might be related to the rich ore deposits in Nanling; (2) some high-velocity anomalies in the uppermost mantle beneath the Wuyi metallogenic belt may be relevant to the igneous rock cooling and the lithospheric thickening; (3) there are obvious low-velocity anomalies at the upper mantle beneath the Wuyi and Nanling metallogenic belts, which are speculated to be hot materials from asthenosphere upwelling into the bottom of the lithosphere. Our results provide a new insight for investigating the deep structures and deep dynamic processes of Nanling tectonic belt.</p>

scholarly journals Upper mantle seismic structure beneath the Ethiopian hot spot: Rifting at the edge of the African low-velocity anomaly

2008 ◽  
Vol 9 (12) ◽  
pp. n/a-n/a ◽  
Author(s):  
I. D. Bastow ◽  
A. A. Nyblade ◽  
G. W. Stuart ◽  
T. O. Rooney ◽  
M. H. Benoit
Keyword(s):  
Upper Mantle ◽  
Hot Spot ◽  
Seismic Structure ◽  
Low Velocity ◽  
Velocity Anomaly

scholarly journals Lithospheric dynamics in the vicinity of the Tengchong volcanic field (southeastern margin of Tibet): an investigation using P receiver functions

2020 ◽  
Vol 224 (2) ◽  
pp. 1326-1343
Author(s):  
Hengchu Peng ◽  
José Badal ◽  
Jiafu Hu ◽  
Haiyan Yang ◽  
Benyu Liu
Keyword(s):  
Upper Mantle ◽  
Lower Crust ◽  
Volcanic Field ◽  
Receiver Functions ◽  
Indian Plate ◽  
Fast Wave ◽  
Low Velocity ◽  
Crust And Upper Mantle ◽  
Velocity Anomaly ◽  
Lateral Extrusion

SUMMARY Tengchong volcanic field (TVF) in the northern Indochina block lies in a critical area for understanding complex regional dynamics associated with continent–continent convergence between the Indian and Eurasian plates, including northeastward compression generated by subduction of the Indian Plate beneath the Burma Arc, and southeastward lateral extrusion of the crust from below central Tibet. We gathered 3408 pairs of P receiver functions with different frequencies and calculated the splitting parameters of the Moho-converted Pms phase. An anisotropic H-κ stacking algorithm was used to determine crustal thickness and Vp/Vs ratios. We also inverted for the detailed S-velocity structure of the crust and upper mantle using a two-step inversion technique. Finally, we mapped the topography of the lithosphere–asthenosphere boundary. Results show fast-wave polarization directions with a dominant NE–SW orientation and delay times varying between 0.19 and 1.22 s, with a mean of 0.48 ± 0.07 s. The crustal Vp/Vs ratio varies from 1.68 to 1.90 and shows a maximum value below the central part of the TVF, where there is relatively thin crust (∼35–39 km) and a pronounced low-velocity anomaly in the middle–lower crust. The depth of the lithosphere–asthenosphere boundary ranges from 53 to 85 km: it is relatively deep (∼70–85 km) in the vicinity of the TVF and relatively shallow in the south of the study area. In the absence of low shear wave velocity in the upper mantle below the TVF, we propose that the low-velocity anomaly in the lower crust beneath the TVF derives from the upper mantle below the neighbouring Baoshan block.

Imaging the East Asia using waveform tomography with massive datasets 

2021 ◽  
Author(s):  
Hui Dou ◽  
Sergei Lebedev ◽  
Bruna Chagas de Melo ◽  
Baoshan Wang ◽  
Weitao Wang
Keyword(s):  
East Asia ◽  
Mantle Plume ◽  
Upper Mantle ◽  
High Velocity ◽  
Deep Structure ◽  
Indian Plate ◽  
Low Velocity ◽  
Velocity Anomaly ◽  
The Pacific ◽  
Velocity Anomalies

<p>We present a new shear-wave velocity model of the upper mantle beneath the East Asia, ASIA2021, derived using the Automatic Multimode Inversion technique. We use waveform fits of over 1.3 million seismograms, comprising waveforms of surface waves, S and multiple S waves.  In total, data from 9351 stations and 23344 events constrain ASIA2021, which maps in detail the structure of the lithosphere and underlying mantle beneath the region. Our model reveals deep structure beneath the tectonic units that make up East Asia. It shows agreement with previous models at larger scales and, also, sharper and stronger velocity anomalies at smaller regional scales. High-velocity continent roots are mapped in detail beneath the Sichuan Basin, Tarim Basin, Ordos Block, and Siberian Craton, extending to over 200 km depths. The lack of a high-velocity continental root beneath the Eastern North China Craton (ENCC), underlain, instead, by a low-velocity anomaly, is consistent with the destruction of this Archean nucleus. Strong low-velocity anomalies are mapped within the top 100 km beneath Tibet, Pamir, Altay-Sayan area, and back-arc basins. At greater depths, ASIA2021 shows high-velocity anomalies related to the subducted and underthrusted lithosphere of India beneath Tibet and the subduction of the Pacific and other plates in the upper mantle. In the mantle transition zone (MTZ), we find high-velocity anomalies probably related to deflected subducted slabs or detached portions of ancient continent cratons. In particularly, ASIA2021 reveals separate bodies, probably originating from the Indian Plate lithosphere beneath central Tibet, with one at 100-200 km beneath Songpan-Ganzi Block (SGFB) and the other in the MTZ. A strong low-velocity anomaly extending from the surface to the lower mantle beneath Hainan volcano and South China Sea is consistent with the hypothesis of the Hainan mantle plume. The high-velocity anomaly beneath ENCC in MTZ can be interpreted as a detached Archean continent root. The Pacific Plate subducts beneath the eastern margin of Asia into the MTZ and appears to deflect and extend horizontally as far west as the Songliao Basin. The absence of major gaps in the stagnant slab is consistent with the origin of Changbaishan volcano above being related to the Big Mantle Wedge, proposed previously. The low-velocity anomalies down to ~ 700 km depth beneath the Lake Baikal area suggest a hot upwelling (mantle plume) feeding the widely distributed Cenozoic volcanoes in central and western Mongolia.</p>

scholarly journals Feedbacks between a non-Newtonian upper mantle, mantle viscosity structure and mantle dynamics

2020 ◽  
Vol 224 (2) ◽  
pp. 961-972
Author(s):  
A G Semple ◽  
A Lenardic
Keyword(s):  
Upper Mantle ◽  
Lower Mantle ◽  
Mantle Convection ◽  
Mantle Flow ◽  
Mantle Dynamics ◽  
Plate Motions ◽  
Low Viscosity ◽  
Mantle Viscosity ◽  
Flow Feature ◽  
Mantle Rheology

SUMMARY Previous studies have shown that a low viscosity upper mantle can impact the wavelength of mantle flow and the balance of plate driving to resisting forces. Those studies assumed that mantle viscosity is independent of mantle flow. We explore the potential that mantle flow is not only influenced by viscosity but can also feedback and alter mantle viscosity structure owing to a non-Newtonian upper-mantle rheology. Our results indicate that the average viscosity of the upper mantle, and viscosity variations within it, are affected by the depth to which a non-Newtonian rheology holds. Changes in the wavelength of mantle flow, that occur when upper-mantle viscosity drops below a critical value, alter flow velocities which, in turn, alter mantle viscosity. Those changes also affect flow profiles in the mantle and the degree to which mantle flow drives the motion of a plate analogue above it. Enhanced upper-mantle flow, due to an increasing degree of non-Newtonian behaviour, decreases the ratio of upper- to lower-mantle viscosity. Whole layer mantle convection is maintained but upper- and lower-mantle flow take on different dynamic forms: fast and concentrated upper-mantle flow; slow and diffuse lower-mantle flow. Collectively, mantle viscosity, mantle flow wavelengths, upper- to lower-mantle velocities and the degree to which the mantle can drive plate motions become connected to one another through coupled feedback loops. Under this view of mantle dynamics, depth-variable mantle viscosity is an emergent flow feature that both affects and is affected by the configuration of mantle and plate flow.

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