Are transform faults thermal contraction cracks?

The oceanic lithosphere cools as it moves away from the ocean ridge where it was formed. Thermal stresses will be associated with this cooling unless the stresses are relieved by plastic flow. Studies of the bending of the lithosphere at Hawaii and at ocean trenches show that the upper 25 km of the lithosphere behaves elastically on geological time scales. It is concluded that thermal stresses will develop at temperatures below 300°C. It is determined that the thermal stresses in the cooling oceanic lithosphere are as high as 4 kbar. These stresses are sufficiently large to fracture the lithosphere. The thermal stresses in the oceanic lithosphere give a bending moment. For a ridge segment with a length of less than about 100 km the thermal stresses can be relieved by bending of the lithosphere; for longer ridge segments the hydrostatic restoring force restricts bending. Transform faults and associated fracture zones are universal features of the ocean ridge system. They exhibit the graben structure associated with crustal extension. It is suggested that transform faults and fracture zones are contraction cracks that relieve the thermal stresses in the cooling lithosphere.