VARIATIONS IN THE ALONG-AXIS SEGMENTATION OF THE AFAR RIFT SYSTEM

Both oceanic and continental rifts show regular along-axis segmentatio n, but the relationship, if any, between the two is poorly understood. The tectonically active East African Rift system encompasses systemat ic along-axis variations in extension and magmatism, making it possibl e to explore the links between strain, magmatism, and the length scale s of faulting and depositional systems during rift development. We sum marize Quaternary along-axis segmentation within the Afar Rift system, which is transitional to seafloor spreading, and compare it with segm entation in a ''continental'' rift sector, the Main Ethiopian Rift (ME R) system to the south. We use high-resolution satellite imagery calib rated by field studies and digital topography data to delineate faults and magmatic centers in unmapped areas and to compare with existing g eological reports. From south. to north we see a significant and syste matic decrease in the lengths of young basin-bounding normal faults (s imilar to 50 km long in MER to less than or equal to 15 km long in nor thern Afar), in the lengths and widths of rift basins (from 50 to 100 km long x 30 to 80 km wide in MER to less than or equal to 20 km long x less than or equal to 5 km wide in northern Afar). and in the relief of the uplifted rift hanks surrounding the basins (from greater than or equal to 1000 m high in MER to less than or equal to 100 m high in northern Afar). Erosional escarpments along the older, outer rift marg ins of Afar reveal long, high border fault segments similar to those f ound in the less evolved MER, suggesting that the Afar Rift was origin ally bounded by much longer faults. In northern Afar, there is a drast ic 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 cha nges in rift segmentation are concomitant with decreasing crustal thic kness (i.e., cumulative strain), decreasing effective elastic thicknes s estimates, and increasing magma supply. These temporal and spatial p atterns suggest that segmented continental rifts call evolve into segm ented oceanic rifts dominated by mantle sourced magmatic processes.