The nature of the crust beneath the Afar triple junction: Evidence from receiver functions

Afar depression is an ideal locale to study the role of extension and magmatism as rifting progresses to seafloor spreading. Here we present receiver function results from new and legacy experiments. Crustal thickness ranges from similar to 45 km beneath the highlands to similar to 16 km beneath an incipient oceanic spreading center in northern Afar. The crust beneath Afar has a thickness of 20-26 km outside the currently active rift segments and thins northward. It is bounded by thick crust beneath the highlands of the western plateau (similar to 40 km) and southeastern plateau (similar to 35 km). The western plateau shows V-P/V-S ranging between 1.7-1.9, suggesting a mafic altered crust, likely associated with Cenozoic flood basalts, or current magmatism. The southeastern plateau shows VP/VS more typical of silicic continental crust (similar to 1.78). For crustal thicknesses <26 km, high V-P/V-S (>2.0) can only be explained by significant amounts of magmatic intrusions in the lower crust. This suggests that melt emplacement plays an important role in late stage rifting, and melt in the lower crust likely feeds magmatic activity. The crust between the location of the Miocene Red Sea rift axis and the current rift axis is thinner (<22 km) with higher VP/VS (>2.0) than beneath the eastern part of Afar (>26 km, V-P/V-S < 1.9). This suggests that the eastern region contains less partial melt, has undergone less stretching/extension and has preserved a more continental crustal signature than west of the current rift axis. The Red Sea rift axis appears to have migrated eastward through time to accommodate the migration of the Afar triple junction.