The significance of shear heating in continental delamination

Most numerical models of continental delamination processes, up to now, have not considered the effects of viscous heating in the thermo-mechanics of a geological system with both thermal and compositional buoyancies. We have studied the influence of viscous heating in the continental delamination problem using a two-dimensional finite-difference model with markers to delineate the distribution of crustal and mantle rheologies and the compositional buoyancy forces. This upper-mantle model consisting of a two-component crust and an olivine mantle is driven by both thermal and compositional buoyancy forces. For the advection of the different crust and mantle components, we have used 180 000 markers for the lower resolution, and up to 1.5 million markers for the highest resolution. A quasi-brittle rheology is employed in the lithosphere. The ductile rheology is non-Newtonian in the crust and mantle. We have employed a high resolution grid of up to 201 X 401 finite difference points for the momentum equation in order to resolve the fine features associated with viscous heating. The spatial resolution for the temperature grid is four times higher, being as fine as 840 m, while the densest grid for the streamfunction is 3.35 km. The effective thermal Rayleigh number hovers around O(10⁶) and the chemical Rayleigh number is slightly larger. Comparison has been conducted between models with and without shear heating. Noticeable differences in the thermal fields, up to several hundred degrees, are found in certain localized areas, adjacent to the descending continental root, which can go down to around 300 km depth. There are greater thermal fluctuations in the regions with pronounced amounts of viscous dissipation. We found that there is a spatial correlation between the regions with a significant release of gravitational potential energy due to the varying large undulations of the crust-mantle interface and regions with significant viscous heating in the crust. The timescales of the dynamics, O(10⁴a), are considerably reduced during the intense shear-heating episodes, as revealed by the dramatic increase in the number of time-steps needed for maintaining a proper temporal resolution. (C) 2000 Elsevier Science B.V. All rights reserved.