Three-dimensional inverse modelling of the thermal structure and implications for lithospheric strength in Denmark and adjacent areas of Northwest Europe

Abstract

SUMMARY The subsurface thermal structure of Denmark and adjacent areas and lateral variations in lithospheric strength are investigated using a 3-D finite element solution of the steady-state heat equation and the equations of motion of a continuum. The optimum thermal model is determined by least-squares inversion of surface heat flow with prior information on the distribution of crustal radiogenic heat production rate, sediment and crustal thermal conductivity values, and the heat flow from the asthenosphere into the base of the lithosphere. The mechanical model invokes elastic, plastic and viscous deformation, depending on rock type, temperature, confining pressure and deviatoric stress. The thermal structure derived by the inversion is non-unique. A Monte Carlo-based resolution analysis shows that data and parameter uncertainties result in standard deviations in the Moho temperature of 75‐160 uC. The standard deviation increases almost linearly with Moho depth. All admissible thermal structures are characterized by laterally relatively homogeneous temperatures, and it is a robust conclusion that the lowest Moho temperatures occur in areas where the Moho is the shallowest, and the highest temperatures in areas where it is the deepest. Therefore, in the absence of lateral variations of the rheology of the lithospheric layers, and particularly for the upper mantle, the strength of the lithosphere in Denmark and the bordering areas is greatest in the basins where the Moho is the shallowest. This shows that in transition zones from cratons to younger sedimentary basins, lateral variations in lithospheric rheological properties are significant and should be considered in any attempt to understand the geodynamics of such areas.