5:15 PM - 6:45 PM
[SCG48-P10] The effects of coordinate systems on mantle convection and the cooling of oceanic lithosphere
Keywords:mantle convection simulation, heat flow, isostatic topography, coordinate system, small-scale convection
Spreading of the lithosphere at the mid-ocean ridges is a fundamental geodynamical process, yet it is not fully understood. The half-space cooling (HSC) model assumes a purely conductive half-space with a cool surface. Its prediction agrees well with the observed heat flow and subsidence data at young ages but is systematically lower at older ages. An alternative, the plate cooling model, fits the observed data well, but lacks a clear physical basis. Based on a series of numerical simulations, Huang and Zhong (2005) attributed this discrepancy to trapped heat and small-scale convection beneath older lithosphere, but their simulations were in the Cartesian coordinate system and yielded a mantle potential temperature higher than observation.
Our study explores the effects of coordinate system on mantle convection, specifically in the 2D spherical annulus and the conventional Cartesian coordinates. In the spherical annulus, plumes form beneath intermediate-age regions and recurrently migrate towards the ridge until their heads hit the surface. Notably, under conditions of low internal heating and high viscosity, excess heat accumulates beneath the relatively older oceanic plate without small-scale convection. Increasing internal heating rate or decreasing viscosity, can intensify convection and initiate small-scale convection under old plates, while preserving the plume’s recurrent generation-migration patterns. When realistic material properties are assumed, the 2D Cartesian coordinate cases yield an average mantle potential temperature 600-700℃ higher than observation and deviate significantly from the predictions of half-space cooling model. On the contrary, the spherical annulus cases predict a cooler mantle, whose potential temperature is closer to the observation value of 1300-1400℃. These differences are inherent to the geometry. Overall, factors that elevate mantle potential temperature, including the choice of coordinate system, can intensify convection, cause plumes migrate for shorter distances, induce small-scale convection, and further distort the system away from the purely conductive state. In preferred cases, surface heat flow and isostatic topography calculated from the numerical simulations indeed show trends that are flatter than the HSC model at older ages, aligning with analysis of observational data by Hillier (2010). However, the absolute values are not closely related, possibly due to the simplicity of our model setup, such as the imposed uniform horizontal velocity at the top, the reflective boundaries at the two side walls, and a homogeneous temperature structure applied at the bottom of the mantle.
Our study explores the effects of coordinate system on mantle convection, specifically in the 2D spherical annulus and the conventional Cartesian coordinates. In the spherical annulus, plumes form beneath intermediate-age regions and recurrently migrate towards the ridge until their heads hit the surface. Notably, under conditions of low internal heating and high viscosity, excess heat accumulates beneath the relatively older oceanic plate without small-scale convection. Increasing internal heating rate or decreasing viscosity, can intensify convection and initiate small-scale convection under old plates, while preserving the plume’s recurrent generation-migration patterns. When realistic material properties are assumed, the 2D Cartesian coordinate cases yield an average mantle potential temperature 600-700℃ higher than observation and deviate significantly from the predictions of half-space cooling model. On the contrary, the spherical annulus cases predict a cooler mantle, whose potential temperature is closer to the observation value of 1300-1400℃. These differences are inherent to the geometry. Overall, factors that elevate mantle potential temperature, including the choice of coordinate system, can intensify convection, cause plumes migrate for shorter distances, induce small-scale convection, and further distort the system away from the purely conductive state. In preferred cases, surface heat flow and isostatic topography calculated from the numerical simulations indeed show trends that are flatter than the HSC model at older ages, aligning with analysis of observational data by Hillier (2010). However, the absolute values are not closely related, possibly due to the simplicity of our model setup, such as the imposed uniform horizontal velocity at the top, the reflective boundaries at the two side walls, and a homogeneous temperature structure applied at the bottom of the mantle.