GLOBAL HEAT FLOW SIMULATION BASED ON A KINEMATIC MODEL OF MANTLE FLOW

Abstract

The global heat flow is the surface representation of thermal processes within the earth's mantle. The long-wavelength pattern of observed heat flow closely resembles the plate tectonics and its most prominent feature is higher values along ocean ridge systems. Theoretically, to determine the thermal state of the Earth's mantle, the heat transfer problem and the mantle convection problem have to be solved simultaneously since they are coupled with each other. However, the development of global seismic tomography provides us with a possibility that, at least under certain assumptions, these problems can be decoupled from each other and solved separately. This allows us to calculate mantle flow velocities first based on the internal loading theory and then use the velocity field as the input to solve the thermal problem. In addition to the internal density anomalies, surface plate movements also excite mantle circulations and, under certain circumstance, they may dominate the structure of the mantle flow. In this study, using a kinematic model of mantle convection and heat transfer, we investigated the underlying processes that generated the observed global heat flow. Buoyancy from the density anomalies and the coupling from the overlying plates are treated as the mantle flow driving force. Both advection and conduction heat transfers are included in the energy equation. Results show that calculated depth derivatives of the near surface temperature are closely correlated to the observed surface heat flow pattern. Higher heat flow values around mid-ocean ridge systems can be reproduced very well. The predicted average temperature as a function of depth reveals that there are two thermal boundary layers, one is close to the surface and another is close to the core-mantle boundary. The rest of the mantle is nearly isothermal. Although, in most of the mantle, advection dominates the heat transfer, the conductive heat transfer is still locally important in the boundary layers and plays an important role for the surface heat flow pattern. The existence of surface plates is responsible for the long wavelength surface heat flow pattern.