1,721,032 research outputs found
Using surface waves to image melt migration pathways and storage beneath the northern East African Rift
No full textContinental rifting is thought to develop from a combination of mechanical stretching and magma assisted rifting. The northern East African Rift is a unique location where we can observe subaerially the initial stages of rifting through to incipient seafloor spreading, as well as the spatial extent of extensional processes away from the rift valley. Multiple models have been proposed to understand the evolution of lithospheric stretching and magmatism in the northern East African Rift, however previous seismic studies are not directly comparable for areas on and off rift due to variations in method, resolution, and scale. It is vital for our understanding of magma assisted rifting processes to have one model that allows comparisons laterally and in depth, a goal that can be achieved in this environment. Here, I invert surface waves from ambient noise and tele seismic Rayleigh waves extracted from 269 seismic stations present between1999 and 2017 to image the Earth’s velocity structure beneath the northern East African Rift System from 5 – 210 km depth. I then use Love waves from ambient noise data to investigate radial anisotropy at crustal depths to determine crustal layering and the depth and shape of magma storage. Shear velocities are everywhere slow in the mantle, with velocities in the rift slow enough (<4.10 ± 0.04 km/s) to require pervasive partial melt. At asthenospheric depths slow velocity anomalies (<4.15 km/s ± 0.04 km/s, 80 – 130 km depth) are not directly beneath melt-rich crustal regions, suggesting mantle melt is ephemeral and/or melt migrates laterally during ascent. Furthermore, the anomalies are segmented along rift, existing in areas that have not undergone significant crustal thinning (segments ∼110 x 80 km wide, ∼60 – 120 km deep),suggesting segmented melt supply starts prior to significant plate deformation. Off rift a fast lid is present at depths of 60 - 80 km but is obscured within the rift suggesting melt is infiltrating the lithosphere. At crustal depths velocities are laterally heterogenous and some of the changes can be attributed to variations in crustal thickness. However, velocities beneath the Main Ethiopian Rift (MER) and the off rift Ethiopian Plateau are slow enough to require melt which I interpret as ongoing magmatic emplacement both on and off rift. The MER is significantly slower than Afar suggesting crustal thickness may be a factor in melt residence time. Anisotropy is required from 5 – 30 km depth suggesting the crust is inherently layered. Effective medium theory suggests thin compositional layering of felsic and mafic intrusions can account for anisotropy up to 4%,however to reconcile the highest observed anisotropy (7%) and lowest velocities we require 2 -4% partial melt oriented in sills. Along rift, horizontally aligned radial anisotropy (VSH > VSV )gets progressively weaker northwards until VSV > VSH, suggesting melt reorients from sills to dykes as rifting progresses. This thesis indicates there can be significant melt accumulation in the crust both on and off rift. Furthermore, melt supply starts early in the breakup process which in turn informs our understanding of how magma assists rifting
Shear Velocity (Vsv and Vsh) and radial anisotropy model from Inversion of Rayleigh and Love dispersion curves
No full textThis dataset has been updated. Please use the version at http://doi.org/10.5258/SOTON/D1689
VERSION 1
A shear velocity model for Journal article: Chambers et al. Imaging the seismic velocity structure of the crust and upper mantle in the northern East African Rift using Rayleigh wave tomography, Geophysical Journal International.
The dataset contains longitude, latitude, depth and shear velocity data.
E.L.C acknowledges funding from NERC studentship NE/L002531/1. C.A.R. and N.H. acknowledge funding from NERC grants NE/M003507/1 and NE/K010654/1 and ERC grant GA 638665. D.K. is supported by NERC grant NE/L013932 and by MiUR through PRIN grant 2017P9AT72.</span
Shear Velocity model from Joint Inversion of Rayleigh waves
No full textThis dataset has been updated. Please use the version at https://doi.org/10.5258/SOTON/D1408
VERSION 1
A shear velocity model for Journal article: Chambers et al. Imaging the seismic velocity structure of the crust and upper mantle in the northern East African Rift using Rayleigh wave tomography, Geophysical Journal International.
The dataset contains longitude, latitude, depth and shear velocity data.
E.L.C acknowledges funding from NERC studentship NE/L002531/1. C.A.R. and N.H. acknowledge funding from NERC grants NE/M003507/1 and NE/K010654/1 and ERC grant GA 638665. D.K. is supported by NERC grant NE/L013932 and by MiUR through PRIN grant 2017P9AT72.</span
Shear Velocity (Vsv and Vsh) and radial anisotropy model from Inversion of Rayleigh and Love dispersion curves
No full textShear velocity and radial anisotropy model for Journal article: Chambers et al. 2021, Variations in melt emplacement beneath the northern East African Rift from radial anisotropy, Earth and Planetary Science Letters. The dataset contains longitude, latitude, depth, shear velocity (Vsv and Vsh) and radial anisotropy data.
E.L.C acknowledges funding from NERC studentship NE/L002531/1. C.A.R. and N.H. acknowledge funding from NERC grants NE/M003507/1 and NE/K010654/1 and ERC grant GA 638665. D.K. is supported by NERC grant NE/L013932 and by MiUR through PRIN grant 2017P9AT72.
This version of the dataset, http://doi.org/10.5258/SOTON/D1689v2, was updated on 2022/11/04. The previous version is available at http://doi.org/10.5258/SOTON/D1689v1 </span
Shear Velocity model from Joint Inversion of Rayleigh waves
No full textA shear velocity model for Journal article: Chambers et al. Imaging the seismic velocity structure of the crust and upper mantle in the northern East African Rift using Rayleigh wave tomography, Geophysical Journal International.
The dataset contains longitude, latitude, depth and shear velocity data.
E.L.C acknowledges funding from NERC studentship NE/L002531/1. C.A.R. and N.H. acknowledge funding from NERC grants NE/M003507/1 and NE/K010654/1 and ERC grant GA 638665. D.K. is supported by NERC grant NE/L013932 and by MiUR through PRIN grant 2017P9AT72.
This version of the dataset, https://doi.org/10.5258/SOTON/D1408v2, was updated on YYYY/MM/DD. The previous version is available at https://doi.org/10.5258/SOTON/D1408v1</span
Anisotropic seismic structure of the northern East African Rift system and Red Sea from surface waves
No full textContinental rifting is a fundamental process of plate tectonics that has been shaping our planet for billions of years. The northern East African Rift system, including the Gulf of Aden and the Red Sea, presents an excellent opportunity to study this process in locations sub-aerially prior to continental break-up, through to full seafloor spreading. We present results from anisotropic surface wave imaging of the region’s crust and uppermost mantle. Anisotropic structures provide additional information about the form of structures at depth and deformation in the region. We find low seismic velocities within the Main Ethiopian Rift (MER), the Red Sea and Gulf of Aden that likely represent melt emplaced in the crust and uppermost mantle. Radial anisotropy, defined as a difference in wave speed of vertically versus horizontally polarized seismic waves, is observed across the region but is strongest within the rift. The strength of radial anisotropy in the MER suggests that horizontally layered melt intrusions ar
Variations in melt emplacement beneath the northern East African Rift from radial anisotropy
Get PDFWhere and how melt is stored in the crust and uppermost mantle is important for understanding the dynamics of magmatic plumbing systems and the evolution of rifting. We determine shear velocity and radial anisotropy in the magmatically rifting northern East African Rift to determine the locus and orientation of melt, both on and off-rift. Love and Rayleigh fundamental modes are extracted from ambient noise data from 9-26 s period and then inverted for shear velocity. V
SV is 0.15 ± 0.03 km/s lower than V
SH from 5-30 km depth on average. V
SH>V
SV across most of the study region suggests the crust is inherently horizontally layered, with the largest anisotropy in the upper 5-15 km. Effective medium theory suggests thin compositional layering of felsic and mafic intrusions can account for anisotropy up to 4%. However, to reconcile the largest observed anisotropy (6.5%), and lowest velocities, we require 2-4% partial melt oriented in sills. Along the rift, horizontally aligned radial anisotropy gets weaker north-eastwards, suggesting sills become less dominant with progressive rifting. The Erta Ale magmatic segment is the only location where V
SV>V
SH, suggesting the crust contains vertical micro-cracks and dykes. Overall, the results suggest during early continental breakup when the rift is narrow, sill formation is the dominant storage mechanism. As a rift widens, vertical dyke intrusion becomes dominant and is likely controlled by variations in crustal thickness and stress state.
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Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Using ambient noise to image the Northern East African Rift
Get PDFThe northern East African Rift (EAR) is a unique location where we observe continental rifting in the Main Ethiopian Rift (MER) transitioning to incipient seafloor spreading in Afar. Here we present a 3‐D absolute shear wave velocity model of the crust and uppermost mantle of the northern EAR generated from ambient noise tomography. We generate 4820 station pair correlation functions, from 170 stations (present over 12 years), which were inverted for phase velocity from 8–33s period and finally for 3‐D absolute shear velocity structure to 60 km depth. Everywhere in the uppermost mantle, shear velocity is slower than expected for a mantle peridotite composition (<4.1 km/s). This suggests the presence of pervasive partial melt, with focused upwelling and melt storage beneath the MER, where the slowest velocities (3.20 km/s ±0.03) are observed. Average crustal shear velocity is faster beneath Afar (3.83 km/s ±0.04) than the MER (3.60 km/s ±0.04), albeit Afar has localized slow velocities beneath active volcanic centers. We interpret these slow velocity regions (including the MER) as magmatic intrusions and heating of the crust. Beneath the northwestern plateau, crustal velocities are laterally heterogeneous (3.3 – 3.65 ±0.05 km/s at 10 km), suggesting a complex geological history and inhomogeneous magma distribution during rift development. Comparison between the MER and Afar allows us to draw conclusions between different stages of rifting. In particular, the MER has the slowest crustal velocities, consistent with longer magma residence times in the crust, early during the breakup process
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