Stability of Transform Fault as Inferred from Viscous Flow in the Upper Mantle

The most stable flow pattern responsible for a ridge transform fault system is studied based on a mantle model of a layered viscous fluid. If the How beneath the ridge is caused by a passive convection, the stable flow on the earth's urface is most likely perpendicular to the ridge axis only for components of small wavelength, less than about 5.5 times of the thickness of the lithosphere, and oblique to the ridge axis for long wave components. This difference may suggest the reason why transform faults are formed on every oceanic ridge. It is concluded that a very small viscosity in the asthenosphere and negligibly small resistances against sliding motion are required to maintain the system. It is also suggested that the amount of ridge offset depends not only on the lithosphere thickness but also on the total direction of the system with respect o the plate motion. From the characteristic feature of the equatorial mid-Atlantic ridge, the thickness of the viscous lithosphere is estimated to be about 30km or less under the rigid plate and the viscosity in the asthenosphere may be as small as one-hundredth of that in the lithosphere.