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The Role of Crustal Strength in Controlling Magmatism and Melt Chemistry During Rifting and Breakup

Abstract : The strength of the crust has a strong impact on the evolution of continental extension and breakup. Strong crust may promote focused narrow rifting, while wide rifting might be due to a weaker crustal architecture. The strength of the crust also influences deeper processes within the asthenosphere. To quantitatively test the implications of crustal strength on the evolution of continental rift zones, we developed a 2-D numerical model of lithosphere extension that can predict the rare Earth element (REE) chemistry of erupted lava. We find that a difference in crustal strength leads to a different rate of depletion in light elements relative to heavy elements. By comparing the model predictions to rock samples from the Basin and Range, USA, we can demonstrate that slow extension of a weak continental crust can explain the observed depletion in melt chemistry. The same comparison for the Main Ethiopian Rift suggests that magmatism within this narrow rift zone can be explained by the localization of strain caused by a strong lower crust. We demonstrate that the slow extension of a strong lower crust above a mantle of potential temperature of 1,350 degrees C can fit the observed REE trends and the upper mantle seismic velocity for the Main Ethiopian Rift. The thermo-mechanical model implies that melt composition could provide quantitative information on the style of breakup and the initial strength of the continental crust. Plain Language Summary There are various regions within the continents today that appear to be extending and breaking apart, known as rifts. There are examples of regions where the extension is distributed over hundreds of kilometres, such as the Basin and Range in North America. There are also regions of narrow focused extension such as the Main Ethiopian Rift. Both these example regions also have a history of magmatism, with basaltic rocks outcropping at the surface. The width of a rift can be related to the strength of the continental crust, weak crust gives a wide rift and vica-versa, yet such relations are subjective. In this study we explore if the chemical composition of the erupted basaltic rocks is a function of the crustal strength. We developed a numerical model that approximates the extension of the upper mantle, including the continental crust. Our model finds that the magmatism in the wide Basin and Range can be explained by the break-up of a weak continental crust. The same model would suggest that the magmatism observed in Ethiopia is due to extension of a strong continental crust. Such a result is contentious, as previous studies have suggested that both regions have weak continental crust.
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John Armitage, Kenni Petersen, Marta Pérez-Gussinyé. The Role of Crustal Strength in Controlling Magmatism and Melt Chemistry During Rifting and Breakup. Geochemistry, Geophysics, Geosystems, AGU and the Geochemical Society, 2018, 19 (2), pp.534-550. ⟨10.1002/2017GC007326⟩. ⟨insu-02920041⟩



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