Dynamical models for sea floor spreading

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Abstract
Fluid dynamic models of the lithosphere‐asthenosphere system are studied. These models provide a basis for understanding driving mechanisms involved in sea floor spreading. The asthenosphere is represented as a highly viscous Newtonian fluid that is nonrotating, Boussinesq, incompressible, and nondissipative. A sequence of two‐dimensional model problems is considered in an effort to understand the dynamic role of various energy sources available in the upper mantle. The first models investigate the role of vertical temperature gradients and find these incapable of generating flows in the asthenosphere that move overlying lithospheric plates by viscous traction. By considering the combined effect of horizontal and vertical temperature gradients, a mechanism for effecting the original breakup of large continental masses is suggested. The dynamic role of phase changes is investigated in a series of finite‐amplitude model problems. The olivine‐spinel phase transition is found to increase the amplitude of convection by a factor of about 2, and the depth at which the phase change occurs is influenced by the convection. A final class of problems explicitly consider the lithosphere coupled to the underlying asthenosphere and the mass flux into and out of the asthenosphere implied by moving surface plates. It is found that if the subducted lithosphere has a negative buoyancy of the order of present estimates, the lithospheric plates will move at velocities of 1–10 cm yr−1.

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