User:Marcelo.silka/sandbox

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Competing theories on geologic evolution:[edit]

Over time, many theories have tried to clarify the evolution of the East African Rift. In 1972 it was proposed that the EAR was not caused by tectonic activity, but rather by differences in crustal density.[8] Since the nineties, evidence has been found in favor of mantle plumes beneath the EAR.[1] Others proposed an African superplume causing mantle deformation.[9][10][2] The question is still debated.

The conceptual extensional difference between plume models and the superplume model placed beneath the East African Rift. Modified from Hansen et al. 2012.
Maps of four different depth slices of the Shear-velocity (Vs) model developed by Emry et al. 2018 (https://doi.org/10.17611/DP/EMCAFANTEM18, modified from the original using the IRIS DMC application https://doi.org/10.1785/0220120032). Depths are 105 km, 165 km, 235 km and 424 km. The lower Vs (colors toward red) represent hotter structures in the mantle. The fourth map depicts a depth below the 410 km discontinuity where average Vs is significantly lighter (getting overall bluer), but it still displays the signature of a linear plume upwelling along the East African Rift. In the white box, the Vs vertical profile at 10°N, 40°E illustrates the increase of velocity with depth including the effect of the 410 km discontinuity.

The most accepted review of the theories was put forth in 2009: that magmatism and plate tectonics have a feedback with one another, controlled by oblique rifting conditions. At that time it was suggested that lithospheric thinning generated volcanic activity, further increasing the magmatic processes at play such as intrusions and numerous small plumes. These processes further thin the lithosphere in saturated areas, forcing the thinning lithosphere to behave like a mid-ocean ridge.[10].

Although reasonably considered, the exact conformation of deep-rooted mantle plumes is still a matter of active research.[11] Studies that contribute to the broader understanding on the evolution of rifts can be grouped into the techniques of isotope geochemistry, seismic tomography and geodynamical modeling.

Isotope Geochemistry[edit]

The varying geochemical signatures of a suite of Ethiopian lavas suggest multiple plume sources: at least one of deep mantle origin, and one from within the subcontinental lithosphere.[3] In accordance, a study of Halldórsson et al. in 2014 compare the geochemical signature of rare earth’s isotopes from Xenolith and lava samples collected in the EAR. The results corroborate the coexistence of a superplume “common to the entire rift” with another mantle material source being either of subcontinental type or of mid-ocean ridge type.[4]

Seismic Tomography[edit]

The geophysical method of Seismic_tomography is a suitable tool to investigate Earth’s subsurface structures deeper than the crust. It is an inverse problem technique that models which are the seismic velocities of the inner Earth that reproduce seismographic data recorded around the world. Recent improvements of tomographic Earth models of P-wave and S-wave velocities suggest that a superplume upwelling from the lower mantle at the northeastern EAR feeds plumes of smaller scale into the upper mantle.[5][6]

Geodynamical Modeling[edit]

Parallel to geological and geophysical measures (e.g. isotope ratios and seismic velocities) it is constructive to test hypotheses on computer based geodynamical models. A 3D numerical geodynamical model of the plume-crust coupling was capable of reproducing the lateral asymmetry of the EAR around the Tanzania craton.[7] Numerical modeling of plume-induced continental break-up shows two distinct stages, crustal rifting followed by lithospheric breakup, and the upwelling between stages of an upper mantle plume.[8]

References:[edit]

  1. ^ Montelli, R.G.; et al. (2006). "A catalogue of deep mantle plumes: New results from finite‐frequency tomography". Geochem. Geophys. Geosyst. 7: 1–69 – via agupubs.onlinelibrary.wiley.com. {{cite journal}}: Explicit use of et al. in: |first= (help)
  2. ^ Hansen, S.E.; et al. (2012). "Mantle structure beneath Africa and Arabia from adaptively parameterized P-wave tomography: Implications for the origin of Cenozoic Afro-Arabian tectonism". Earth and Planetary Science Letters. 319–320: 23–34 – via www.sciencedirect.com. {{cite journal}}: Explicit use of et al. in: |first= (help)
  3. ^ Furman, Tanya (2007). "Geochemistry of East African Rift basalts: An overview". Journal of African Earth Sciences. 48: 147–160 – via www.sciencedirect.com.
  4. ^ Halldórsson, S.A.; et al. (2014). "A common mantle plume source beneath the entire East African Rift System revealed by coupled helium-neon systematics". Geophys. Res. Lett. 41: 2304–2311 – via AGU.org. {{cite journal}}: Explicit use of et al. in: |first= (help)
  5. ^ Civiero, C.; et al. (2015). "Multiple mantle upwellings in the transition zone beneath the northern East-African Rift system from relative P-wave travel-time tomography". Geochemistry, Geophysics, Geosystems. 16: 2949–2968 – via AGU.org. {{cite journal}}: Explicit use of et al. in: |first= (help)
  6. ^ Emry, E.L.; et al. (2019). "Upper Mantle Earth Structure in Africa From Full‐Wave Ambient Noise Tomography". Geochemistry, Geophysics, Geosystems. 20: 1–28 – via AGU.org. {{cite journal}}: Explicit use of et al. in: |first= (help)
  7. ^ Koptev, A.; et al. (2016). "Contrasted continental rifting via plume-craton interaction: Applications to Central East African Rift". Geoscience Frontiers. 7: 221–236 – via www.sciencedirect.com. {{cite journal}}: Explicit use of et al. in: |first= (help)
  8. ^ Koptev, A.; et al. (2018). "Plume-induced continental rifting and break-up in ultra-slow extension context: Insights from 3D numerical modeling". Tectonophysics. 746: 121–137 – via www.sciencedirect.com. {{cite journal}}: Explicit use of et al. in: |first= (help)


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