Strain transfer between disconnected, propagating rifts in Afar

We showed before that both the Aden and Red Sea plate boundaries are curren tly rifting and propagating along two distinct paths into Afar through the opening of a series of disconnected, propagating rifts. Here we use new geo chronological, tectonic, and paleomagnetic data that we acquired mostly in the southeastern part of Afar to examine the geometry, kinematics, and time -space evolution of faulting related to strain transfer processes. It appea rs that transfer of strain is accommodated by a bookshelf faulting mechanis m wherever rifts or plate boundaries happen to overlap without being connec ted. This mechanism implies the rotation about a vertical axis of small rig id blocks along rift-parallel faults that are shown to slip with a left-lat eral component, which is as important as their normal component of slip (ra tes of similar to2-3 mm/yr). By contrast, where rifts do not overlap, eithe r a classic transform fault (Maskali) or an oblique transfer zone (Mak'arra sou) kinematically connects them. The length of the Aden-Red Sea overlap ha s increased in the last similar to0.9 Myr, as the Aden plate boundary propa gated northward into Afar. As a consequence, the first-order blocks that we identify within the overlap did not all rotate during the same time-span n or by the same amounts. Similarly, the major faults that bound them did not necessarily initiate and grow as their neighboring faults did. Despite the se variations in strain distribution and kinematics, the overlap kept accom modating a constant amount of strain (7 to 15% of the extension amount impo sed by plate driving forces), which remained distributed on a limited numbe r (seven or eight) of major faults, each one having slipped at constant rat es (similar to3 and 2 mm/yr for vertical and lateral rates, respectively). The fault propagation rates and the block rotation rates that we either mea sure or deduce are so fast (30-130 mm/yr and 15-38 degrees /Myr, respective ly) that they imply that strain transfer processes are transient, as has be en shown to be the case for the processes of tearing, rift propagation, and strain jumps in Afar.