Abstract
AbstractThe Red Sea arm of the triple junction in northeastern Ethiopia provides an opportunity to investigate rift-forming processes at divergent boundaries. In an attempt to study the subsurface, especially the distribution and role of melt in the rifting process, we carried out a high-precision gravity survey with a mean-square error of 0.011 mgal, assisted by differential global positioning system measurements. The profile is 162 km long and strikes ENE–WSW across the southern part of the Red Sea rift at a latitude of approximately 11.75° N. Modelling of the Bouguer anomaly, constrained by a priori information, showed detailed in-rift variations in the crustal structure and the distribution of melt beneath the rift axis. Our interpretation suggested that the process of continental break-up is governed by crustal stretching and rifting accompanied by the emplacement of melt into the lower crust above a lower density upper mantle. In addition, we interpreted the thickness of the crust beneath this part of the rift axis to be 25 km. The subsurface distribution of density beneath the profile shows that the south-central part of the Red Sea rift has modified thinned crust, intruded by high-density material, which resembles the crust formed during seafloor spreading.
Highlights
Structure and to refine our understanding of the role of melt in the rifting process at divergent boundaries (Fig. 1)
Afar marks a triple junction between the Nubian, Somalian and Arabian plates (Fig. 1), which are diverging as a result of extension in the Red Sea, Gulf of Aden and East African rifts (e.g. McKenzie et al 1970; Mohr 1970; Tazieff et al 1972; Ghebreab 1998; Beyene & Abdelsalam 2005)
Border faults on the SE and SW flanks of the Afar depression mark the abrupt transition from the rift valley floor to the 2–3 kmhigh Ethiopian and Southeastern plateaus
Summary
Afar marks a triple junction between the Nubian, Somalian and Arabian plates (Fig. 1), which are diverging as a result of extension in the Red Sea, Gulf of Aden and East African rifts (e.g. McKenzie et al 1970; Mohr 1970; Tazieff et al 1972; Ghebreab 1998; Beyene & Abdelsalam 2005). Since the Oligocene, extension has migrated away from the border faults towards the current southern Red Sea rift axis in the Dabbahu–Manda Harraro rift and in the Tendaho Graben. The synrift volcanic geology of the western Afar margin shows that during the period 27 –25 Ma, extension and volcanism in the southern Red Sea rift migrated 40–50 km westwards of the main border fault. The locus of extension and magmatism has migrated eastwards in the Red Sea rift system (Wolfenden et al 2005; Ayalew et al 2006; Keir et al 2011), leaving a suite of near-surface volcanic rocks and sedimentary basins that young towards the Quaternary–Recent axial magmatic segments, where faulting and magmatism are localized The locus of extension and magmatism has migrated eastwards in the Red Sea rift system (Wolfenden et al 2005; Ayalew et al 2006; Keir et al 2011), leaving a suite of near-surface volcanic rocks and sedimentary basins that young towards the Quaternary–Recent axial magmatic segments, where faulting and magmatism are localized (e.g. Barberi & Varet 1977; Manighetti et al 2001)
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