What is it about?
Earth has an outer layer (crust) comprising rocks that are generally less dense than those of its mantle beneath. The thickness of the crust is important for several reasons. It is used in studies of how that lower-density crust "floats" on the mantle (isostasy) and how the crust gets stretched and thinned when continents break apart. In the case of oceanic crust, its thickness tells us how much the underlying mantle has melted. However, it can be locally difficult to work out accurately. We have studied the results of a seismic experiment carried out more than 30 years ago off the coast of Sudan in the Red Sea. Recordings from seabed-installed seismometers had been used to work out the layering of the crust along three survey lines. Along the lines, the researchers had provided estimates of the speed of the seismic waves within each layer. We have now been able to test those results by first estimating how dense the crustal units would be. That allowed us to predict how the pull of Earth's gravitational field at the surface varied along those lines. Comparing with the known field, we found major discrepancies at several locations, suggesting that the base of the crust (Moho) was poorly recorded by the seismic experiment. A possible cause may be the existence of low-velocity layers, which are hard to detect by the seismic refraction method used. In other areas, only reflected waves had been used (waves can be reflected from structures within the mantle below the Moho).
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Why is it important?
Many of us are told in geophysics classes that depths of surfaces are poorly resolved if estimated using from refraction methods, but sometimes it requires analyses like these to reinforce that message. The result is important for other studies of the Red Sea that have used depths of Moho derived from seismic refraction experiments (those based on inverting gravity anomalies, for example). The discrepancies also turned out interesting in places, because they highlighted areas of denser rocks that may have been inherited from structures of the original Arabian-Nubian continental shield.
Perspectives
The Red Sea is a difficult place to collect geophysical data for international researchers, so these old datasets are extremely valuable. Efforts to archive such legacy data and model outputs would benefit future research on the region. The article can be accessed here or by request from the authors. Okwokwo, O.I. and Mitchell, N.C., 2024. Structure of the continent-ocean transition in the Red Sea from gravity modelling of seismic refraction profiles off Sudan. In: N.M.A. Rasul and I.C.F. Stewart (Editors), Rifting and sediments in the Red Sea and Arabian Gulf regions. Taylor & Francis.
Dr Neil C. Mitchell
University of Manchester
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This page is a summary of: Structure of the continent-ocean transition in the Red Sea from gravity modelling of seismic refraction profiles off Sudan, March 2024, Taylor & Francis,
DOI: 10.1201/9781003321415-8.
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