Thermal cracking and the deep hydration of oceanic lithosphere: A key to the generation of plate tectonics?

The development of transient thermal stress in suboceanic mantle is investigated on the basis of two-dimensional thermoviscoelastic models incorporating composite rheology appropriate for dry oceanic lithosphere. Thermal stress is shown to be sufficiently high to deeply fracture the coldest part of lithosphere, e. g., to the depth of at least similar to 30 km ( and possibly down to similar to 50 km) in 100-Ma-old lithosphere. The release of thermal stress by tension cracking is limited to the vicinity of cracks, and the cascade crack system is suggested to be required given the finite fracture strength of mantle materials. Possible physical and chemical consequences of deep thermal cracking are also discussed. The rheological evolution of oceanic lithosphere is likely to be affected by thermal cracking and subsequent serpentinization, which introduces the localized zones of weakness in the otherwise stiffest part of lithosphere. This localized weakening may help to explain why plate tectonic convection, not stagnant lid convection, operates in Earth's mantle.