Lithosphere-Asthenosphere Interactions Below Rifts

Abstract

Quantitative model results of flow within a viscous lower lithosphere and asthenosphere are presented, which result from the thermal and mechanical consequences of extension within the solid upper lithosphere. The flow is calculated using a two-dimensional, time-dependent formulation of the Navier -Stokes equations for an incompressible viscous fluid. The viscosity depends on temperature, pressure and strain-rate. Velocity boundary conditions on the upper surface of the fluid simulate extension in the overlying solid lid. Calculations were made for different viscosity distributions and for different rates and spatial distributions of extension.

Significant small-scale convection develops below the solid upper lithosphere for some model input parameters. The spatial distribution of extension in the overlying solid layer defines the lateral thermal gradients, the primary driving force of the convective flow. The most vigorous flows occur when the rift is narrow and the transition from extended to unextended regions is sharp. The rate of extension is relatively unimportant for the parameter ranges explored here. The strain-rate dependence of viscosity is an important factor in determining the mode of convection.

Small-scale convection will affect both the subsidence history and, in some cases, the volumes of melt produced during extension. The effect on subsidence is potentially significant, as a perturbation to the larger effects of thermal changes within the lithosphere and crustal thinning. Time dependent subsidence is particularly striking on the rift shoulders. Decompression melting of upwelling mantle is shown to deliver significantly more melt to crustal levels due to small-scale convection, and provides a means of producing excess melt volumes without anomalously high mantle temperatures. These results are important for understanding a wide variety of rifted terranes, including rifted continental margins, and they supplement and largely confirm earlier work on rift-generated small scale convection.