The Cathles Parameter (Ct): A Geodynamic Definition of the Asthenosphere and Implications for the Nature of Plate Tectonics

A weak asthenosphere, or low‐viscosity zone (LVZ), underlying Earth's lithosphere has played an important role in interpreting isostasy, postglacial rebound (PGR), and the seismic LVZ, as well as proposed mechanisms for continental drift, plate tectonics, and postseismic relaxation. Consideration of the resolving power of PGR, postseismic relaxation, and geoid modeling studies suggests a sublithospheric LVZ perhaps ~100–200 km thick with a viscosity contrast of ~100–1,000. Ab initio numerical models of plate‐like boundary layer motions in mantle convection also suggest a key role for the LVZ. Paradoxically, a thinner LVZ with a strong viscosity contrast is most effective in promoting plate‐like surface motions. These numerical results are explained in terms of the reduction in horizontal shear dissipation due to an LVZ, and a simple scaling theory leads to somewhat nonintuitive model predictions. For example, an LVZ causes stress magnification at the base of the lithosphere, enhancing plate boundary formation. Also, flow within the LVZ may be driven by the plates (Couette flow), or pressure‐driven from within the mantle (Poiseuille flow), depending upon the degree to which plates locally inhibit or drive underlying mantle convection. For studies of the long‐wavelength geoid, PGR, and mantle convection, a simple dimensionless parameter controls the effect of the LVZ. This “Cathles parameter” is given by Ct = η*(D/λ)3, where η* is the viscosity contrast, D is the thickness of the LVZ, and λ is the flow wavelength, emphasizing the tightly coupled trade‐off between LVZ thickness and viscosity contrast. Plain Language Summary The Earth's global system of tectonic plates move over a thin, weak channel (“low‐viscosity zone”) in the mantle immediately underlying the plates. This weak channel is commonly referred to as the asthenosphere, and its presence accounts for a number of important Earth observations, including isostasy (e.g., support for the uplift of large mountain ranges), the shape of the Earth's gravity field, the response of the Earth's surface to the removal of large ice sheets (“postglacial rebound”), and the relationship between plate motions and underlying thermal convection in the mantle. In this paper, we show that these phenomena can be understood in terms of a single unifying parameter consisting of the viscosity contrast between the asthenosphere and the underlying mantle, and the cube of the thickness of the asthenosphere. We propose to call