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Relative continent - mid-ocean ridge elevation: A reference case for isostasy in geodynamics
Earth-Science Reviews ( IF 12.1 ) Pub Date : 2022-08-18 , DOI: 10.1016/j.earscirev.2022.104153
Thomas Theunissen, Ritske S. Huismans, Gang Lu, Nicolas Riel

The choice of crustal and mantle densities in numerical geodynamic models is usually based on convention. The isostatic component of the topography is not calibrated to fit observations resulting in not very well constrained elevations. The density distribution on Earth is not easy to constrain because it involves multiple variables (temperature, pressure, composition, and deformation). We aim in this study to provide a reference case for geodynamic modelling where crustal and mantle densities are calibrated to fit the relative continent/mid-ocean ridge elevation in agreement with observations. We first review observed Earth topography of stable continents and of active mid-ocean ridges and define the characteristic average elevation of these domains. We use self-consistent thermodynamic calculations of dry mantle rocks that include partial melting to calibrate densities of the continental lithospheric mantle and beneath the mid-ocean ridge. The thermodynamic solutions are coupled with a 2-D incompressible plane strain finite element method for viscous-plastic creeping flows to solve for the dynamic evolution during extension from continental rifting to mid-ocean spreading. The combined results from 2-D thermo-mechanical models and 1-D isostatic calculations show that the relative elevation difference between mid-ocean ridges and continents depends on crustal density, mantle composition, and the degree of depletion of the lithospheric mantle. Based on these results we calibrate the reference density that only depends on temperature, which can be used in classic thermo-mechanical models based on the Boussinesq approximation. Finally the model calibration provides a solution that fits (1) the elevation of active mid-ocean ridges far from hotspots (-2750±250 m), (2) the elevation of stable continents far from hotspots (+400±400 m), (3) the average depletion buoyancy of the continental lithospheric mantle (between -20 and -50±15 kg/m3 depending on lithospheric thickness) and (4) the average continental crust density (2835±35 kg/m3 for a 35 km thick crust).



中文翻译:

相对大陆-洋中脊高程:地球动力学中的平衡参考案例

数值地球动力学模型中地壳和地幔密度的选择通常基于惯例。地形的等静压分量未校准以适应观测结果,导致海拔高度受限。地球上的密度分布不​​容易约束,因为它涉及多个变量(温度、压力、成分和变形)。我们的目标是在这项研究中为地球动力学建模提供参考案例,其中地壳和地幔密度被校准以适应与观测一致的相对大陆/大洋中脊高程。我们首先回顾了观测到的稳定大陆和活跃的洋中脊的地球地形,并定义了这些区域的特征平均海拔。我们使用包括部分熔融在内的干地幔岩石的自洽热力学计算来校准大陆岩石圈地幔和大洋中脊下方的密度。热力学解与粘塑性蠕变流的二维不可压缩平面应变有限元方法相结合,以求解从大陆裂谷延伸到洋中扩张过程中的动态演化。二维热力学模型和一维等静压计算的综合结果表明,洋中脊和大陆之间的相对高程差异取决于地壳密度、地幔成分和岩石圈地幔的耗竭程度。基于这些结果,我们校准了仅取决于温度的参考密度,可用于基于 Boussinesq 近似的经典热机械模型。最后,模型校准提供了一个适合(1)远离热点的活动洋中脊海拔(-2750±250m), (2) 远离热点的稳定大陆的海拔 (+400±400m), (3) 大陆岩石圈地幔的平均耗竭浮力(介于-20-50±15kg/m 3取决于岩石圈厚度)和(4)平均大陆地壳密度(2835±35kg/m 3对于 35 km 厚的地壳)。

更新日期:2022-08-18
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