The Afar depression of the Horn of Africa, the Azores Archipelago, and Iceland are zones of plate divergence and mantle plume discharge. Strike-slip focal mechanisms dominate in major earthquakes. Among nodal planes a particular fault plane can be identified by precise epicentral location, from the strike of the aftershock zone, or by relation to field evidence of active faulting. These fault planes are orthogonal to the directions expected in previous models for transform faults along a continuous divergent plate boundary. Recent volcanic features are consistently at small angles with the fault planes. Rotation of blocks within a deforming zone is implied by the direction of fault planes, the sense of motion along them, and their location on a series of parallel features transverse to the deformed zone. The drift of oceanic or continental large lithospheric plates deforms a zone of lithosphere, deriving from both the plume discharge and ridge accretion while the zone is created. ¿ American Geophysical Union 1993