The Role of Strike-Slip Fault Systems at Plate Boundaries

A new analysis shows that most (59%) plate boundaries have a relative velocity vector that is markedly oblique (greater than 22 degrees) to the boundary normal. A significant proportion (14%) have vectors that are nearly (± 22 degrees) parallel to the boundary. Accommodation of the oblique motion usually involves strike-slip faulting, but the kinematic role of these faults differs at divergent and convergent boundaries. Four main types of plate-boundary related strike-slip faults are distinguished: ridge transforms, boundary transforms, trench-linked strike-slip faults and indent-linked strike-slip faults. Discrimination of the four types should be possible in ancient orogenic belts, but is complicated by the common reactivation of the strike-slip zones in other roles. Plate-boundary related strike-slip faults form major lineaments at the present day. Ridge transforms have a low preservation potential in continents. Boundary transforms and indent-linked faults often re-use old lineaments, but trench-linked strike-slip faulting is an effective method of forming new lineaments in continental crust. Strike-slip faulting in general is less commonly recognized in ancient orogenic belts than its abundance in present plate-boundary orogens requires. This under-recognition results both from poor understanding of strike-slip kinematics and from deeper prejudices about the way in which orogenic belts form.