INFLUENCE OF CURRENT MANTLE STRUCTURES ON THE TOPOGRAPHY OF THE SCANDES

The Scandinavian Mountain Chain (the Scandes) exhibits characteristics that are unusual for an old, Palaeozoic mountain belt. These include the lack of an isostatically compensating shape of the Moho and renewed Neogene uplift in a passive margin setting. Several mantle processes have been invoked to explain a post-rift uplift of the Scandes, and their respective durations and wavelengths are strongly controlled by the rheology and thermal structure of the lithosphere.

The TopoScandiaDeep project investigates the potential influence of current mantle structures on the topography of the Scandes and associated uplift processes. The first part of the project focussed on a refined model of crustal thickness and mantle structure in the area. A number of results are the outcome of the analysis of seismological data acquired in Southern Norway from 2006-2008.

A new map of Moho depth and a new crustal seismic model have been compiled using data from three refraction lines, P-receiver function analysis and noise cross-correlation. These results confirm and precise previous crustal thickness estimates, but the P-wave residuals, tomography, as well as surface wave analysis show relatively low seismic mantle velocities below southern Norway compared to Southern Sweden, with a sharp boundary close to the Oslo Graben.

Integrated 3D modelling of the lithospheric subsurface and the available geophysical data sets better characterises the transition zone of the lithospheric mantle (its thermal, compositional and geometric nature). The increase in seismic mantle velocities from Norway towards Sweden reflects a transition towards a much thicker (by several tens of kilometers) and colder mantle. Differences in mineral composition (owing to stronger depletion) may contribute to the velocity contrast. The study also indicates that the topography can be isostatically sustained without invoking additional thermal mantle anomalies.

The strong differences in thickness and in composition of the sub-continental lithospheric mantle in western Scandinavia indicate major differences in the evolution prior to the Caledonian orogeny. Furthermore, it indicates that steps in the lithosphere can be long-lived features, not easily destroyed by a convecting mantle but in turn influencing the convection pattern on a regional scale.