Sublithospheric small‐scale convection and its implications for the residual topography at old ocean basins and the plate model

Abstract

[1] The seafloor topography and heat flux differ significantly from the predictions of the half-space cooling (HSC) model at old ocean basins. Understanding the deviations has important implications for thermal evolution of oceanic lithosphere and large-scale mantle dynamics. A widely used model that explains significant fraction of the deviations is the plate model, but the dynamical feasibility of the plate model has never been demonstrated. In this study, we investigated the effects of sublithospheric small-scale convection (SSC) and of internal heating on seafloor heat flux and topography and mantle thermal structure, and we examined the dynamic feasibility of the plate model by formulating high-resolution two-dimensional numerical models of mantle convection with strongly temperature-and depth-dependent rheology. We found that mantle convection with tectonic plates often leads to formation of a broad thermal anomaly below old lithosphere where the mantle is not cooled by subducted slabs and heat transfer is less efficient because of thick lithosphere, especially when significant internal heating is present. This trapped heat may exist in the middle mantle when the SSC is absent, and it may also be redistributed by the SSC to shallow depths to reheat the lithosphere and to homogenize mantle temperature. When internal heating accounts for >$60% of the total heat output, the trapped heat may provide sufficient heat supply to preferentially reheat old lithosphere via SSC while maintaining uniform mantle temperature. We suggest that the trapped heat and the SSC are responsible for the residual heat flux and topography at old ocean basins relative to the HSC model predictions. Our models also show that for the plate model to be dynamically viable, both the SSC and significant internal heating (>60%) are necessary. This is because only the SSC in a mantle with significant internal heating can erode and reheat the lithosphere while maintaining a nearly constant mantle temperature below lithosphere, which is the basic assumption of the plate model. With the viscosity structure and internal heating rate for the present-day mantle, we think that the plate model is dynamically viable.Citation: Huang, J., and S. Zhong (2005), Sublithospheric small-scale convection and its implications for the residual topography at old ocean basins and the plate model,

New evidence for dislocation creep from 3-D geodynamic modeling of the Pacific upper mantle structure

“…Such cooling is observed in earlier studies [24,39,40], although adding internal heating to the model might reduce or even completely compensate for this cooling [40]. This cooling increases the local viscosity, which delays the formation of TBI, but the amount of erosion by TBI is not significantly affected, as discussed above on the basis of two cases with different rates of accumulation of cold downwellings in the upper mantle due to different ∆η values.…”

“…This may lead to increased heat flux and decreased topographic subsidence (i.e., topographic flattening) at relatively old seafloor, compared to the purely conductive cooling model predictions [18]. Although O'Connell and Hager [39] and Davies [44] suggested that by enhancing the cooling of the mantle the TBI may lead to deepened seafloor topography, Huang and Zhong [40] recently demonstrated using mantle convection models with a reasonable internal heating rate that the TBI produces topographic flattening at the surface, supporting the original suggestion by Parsons and McKenzie [18].…”

Small-scale gravitational instabilities under the oceans: Implications for the evolution of oceanic lithosphere and its expression in geophysical observables

“…Similar values have been extensively used in earlier studies on SSC (e.g. [11][12][13][14]), allowing qualitative comparisons between these and our models. We emphasise, however, that these activation energy and pre-exponential factor values are too low to be consistent with currently available laboratory experiments on diffusion creep (e.g.…”

“…To avoid these undesired effects we use a free-slip condition in regions near both corners, which leads to the generation of a smoother corner flow. For similar reasons, we further introduce two small weak zones near the upper corners (see Figure 1a), as commonly done in similar studies [12]. The viscosity in these regions is divided by 10, 100 or 1000 depending on the model.…”

Basin formation by thermal subsidence of accretionary orogens

“…van Hunen et al, 2005). The plate model does not explicitly describe the physics of this transition, but provides a good fit to the bathymetry and heatflow data (Huang and Zhong, 2005). Gravitational instabilities of the thickening lithosphere are not explicitly included in our model, but the average reduction in lithospheric thickening of any detaching or delaminating lithosphere is accounted for by the plate model.…”