Slip accumulation and lateral propagation of active normal faults in Afar

We investigate fault growth in Afar, where normal fault systems are known t o be currently growing fast and most are propagating to the northwest. Usin g digital elevation models, we have examined the cumulative slip distributi on along 255 faults with lengths ranging from 0.3 to 60 km. Faults exhibiti ng the elliptical or "bell-shaped" slip profiles predicted by simple linear elastic fracture mechanics or elastic-plastic theories are rare. Most slip profiles are roughly linear for more than half of their length, with overa ll slopes always <0.035. For the dominant population of NW striking faults and fault systems longer than 2 km, the slip profiles are asymmetric, with slip being maximum near the eastern ends of the profiles where it drops abr uptly to zero, whereas slip decreases roughly linearly and tapers in the di rection of overall Aden rift propagation. At a more detailed level, most fa ults appear to be composed of distinct, shorter subfaults or segments, whos e slip profiles, while different from one to the next, combine to produce t he roughly linear overall slip decrease along the entire fault. On a larger scale, faults cluster into kinematically coupled systems, along which the slip on any scale individual fault or fault system complements that of its neighbors, so that the total slip of the whole system is roughly linearly r elated to its length, with an average slope again <0.035. We discuss the or igin of these quasilinear, asymmetric profiles in terms of "initiation poin ts" where slip starts, and "barriers" where fault propagation is arrested. In the absence of a barrier, slip apparently extends with a roughly linear profile, tapered in the direction of fault propagation.