The deep structure of the Scandes and its relation to tectonic history and present-day topography

Abstract

We review the results of the TopoScandiaDeep project, a component of the TOPOEUROPE project, in which we have studied the crustal and upper mantle structures of southern Norway in relation to its high topography. The Scandinavian Mountain Chain (the Scandes) is an intracontinental mountain chain at the western edge of the Baltic shield, and has its southern part located in southern Norway. The timing as well as the processes causing the formation of the Scandes are disputed. We bring new geophysical constraints to this issue by providing crustal and mantle seismic models for the area and by integrated modeling of the lithosphere and its potential deformation. New maps of Moho depth and crustal seismic velocities have been compiled using data from refraction lines, P-receiver functions and noise cross-correlation. These results show a thickening of the crust from southwest to northeast and a small crustal root not directly located below the topographic high. P-, S- and surface wave tomography infer seismic mantle velocities lower than in normal shield structure, with a possible sharp boundary close to the Oslo Graben. These low velocities are imaged in the lithosphere and in the underlying mantle down to the 410 km discontinuity. Integrated modeling of seismic models and gravity data shows that the low velocities below southern Norway are compatible with a change in lithosphere thickness from c. 100 km under southern Norway to nearly 200 km under southern Sweden, with possible additional differences in composition. The study also indicates that the topography can be isostatically sustained by the density distribution in the crust and lithospheric mantle. We argue that the lithospheric lateral variation has been present for at least 300 My and has had a significant influence on the localization of the topography, independently of the mechanism for uplift.