Thomas Theunissen

Relative continent/mid-ocean ridge elevation: a reference case for isostasy in geodynamics

The choice on crustal and mantle densities in numerical geodynamics model is usually based on common habits. However, lithospheric deformation involving regional or fault-related flexure or involving strong change in elevation as distal rifted margin formation or long-term erosion of mountain belt require good constrain on densities to provide a reasonable estimation of isostatic response. Accommodation space for sediments, absolute elevation (gravity potential and conditions for erosion and deposition) and long term survival of relief depends on isostasy.

We use 2-D incompressible plane strain finite element method for viscous-plastic creeping flows coupled with a melt prediction routine to solve for the dynamic evolution during extension from continental rifting to mid-ocean spreading. Two independent melting prediction procedure to compute density structure below the mid-ocean ridge are compared in order to evaluate sensitivity to phase changes on solution. The first approach considers the use of P-T density maps and melt prediction from thermodynamic solution. A simple conversion from batch to incremental melting is proposed in order to use only one thermodynamic solution. The second approach considers a simpler approach that uses approximate mantle solidus to compute melt fraction and degree of depletion.

Both approaches provide a solution to fit mid-ocean ridge elevation. Crustal and mantle densities and mid-ocean ridge elevation are calibrated based on observations. We use thermodynamic solution using characteristic bulk mantle compositions to estimate the average degree of depletion of the lithospheric mantle repectively -15 kg/m3, -28.5 kg/m3 and -35 kg/m3 respectively for 125 km, 180 km and 240 km thick lithospheres. The average crustal density is 2850 kg/m3 in this reference model in agreement with global and regional estimations. Finally the reference model fits the characteristic mid-ocean ridge elevation of -3000 m +/- 100 m and continental elevation of 300 m +/- 100 m obtained from a global analysis. This study provides a reference case for isostasy in geodynamics and we discuss the importance of such reference model to use geodynamic numerical models to calculate subsidence and melting histories which could be compared with observation.