Variations in the along‐axis segmentation of the Afar Rift system

Abstract

Both oceanic and continental rifts show regular along‐axis segmentation, but the relationship, if any, between the two is poorly understood. The tectonically active East African Rift system encompasses systematic along‐axis variations in extension and magmatism, making it possible to explore the links between strain, magmatism, and the length scales of faulting and depositional systems during rift development. We summarize Quaternary along‐axis segmentation within the Afar Rift system, which is transitional to seafloor spreading, and compare it with segmentation in a “continental” rift sector, the Main Ethiopian Rift (MER) system to the south. We use high‐resolution satellite imagery calibrated by field studies and digital topography data to delineate faults and magmatic centers in unmapped areas and to compare with existing geological reports. From south to north we see a significant and systematic decrease in the lengths of young basin‐bounding normal faults (∼50 km long in MER to ≤ 15 km long in northern Afar), in the lengths and widths of rift basins (from 50 to 100 km long × 30 to 80 km wide in MER to ≤ 20 km long × ≤ 5 km wide in northern Afar), and in the relief of the uplifted rift flanks surrounding the basins (from ≥ 1000 m high in MER to ≤ 100 m high in northern Afar). Erosional escarpments along the older, outer rift margins of Afar reveal long, high border fault segments similar to those found in the less evolved MER, suggesting that the Afar Rift was originally bounded by much longer faults. In northern Afar, there is a drastic increase in the volume of Quaternary basalts, with segmentation now dominated by 50‐ to 80‐km‐long volcanic ridges, which are similar in size, morphology, and spacing to the second order, nontransform offset segmentation of slow‐spreading mid‐oceanic ridges. The along‐axis changes in rift segmentation are concomitant with decreasing crustal thickness (i.e., cumulative strain), decreasing effective elastic thickness estimates, and increasing magma supply. These temporal and spatial patterns suggest that segmented continental rifts can evolve into segmented oceanic rifts dominated by mantle sourced magmatic processes.