Integrated geophysical modelling of a lateral transition zone in the lithospheric mantle under Norway and Sweden

The Scandinavian Mountain Chain (the Scandes) exhibits characteristics that are unusual for an old, Palaeozoic mountain belt. These include renewed Neogene uplift in a passive margin setting and the lack of a pronounced crustal root. We investigate the influence of present-day thermal, compositional and geometric structures in the crust and mantle on the topography of the southern Scandes and the resulting implications for the tectonic history of Fennoscandia. A self-consistent 3-D subsurface model of southern Norway and Sweden is constructed from recent geophysical data sets that constrain the crustal architecture, lithosphere geometry, density distributions and thermal properties. Recent seismological studies show evidence for a relatively abrupt transition from seismically slower lithospheric mantle underneath southern Norway to seismically faster lithospheric mantle underneath southern Sweden. We find that a transition from thin subcontinental lithospheric mantle (SCLM) underneath southern Norway to a significantly thicker SCLM beneath southern Sweden can explain this velocity trend. A difference in composition between the two SCLM domains (owing to different depletion/refertilization) is required to satisfy the gravity field and isostatically compensated topography, but contributes only slightly to the observed velocity contrast. Such a lateral transition zone, constituting a major change in the lithospheric structure in southwestern Fennoscandia, can be related to several major Proterozoic and Phanerozoic tectonic events such as the Sveconorwegian orogeny, the Permian Oslo Rift and perhaps even the opening of the Atlantic.