Thermal and Compositional Structure of the Lithospheric Mantle in the Barents Sea

Laura Marello

We investigate the subcontinental lithospheric mantle (SCLM) of the Barents Sea by performing combined 3D geophysical-petrological forward modelling of the lithosphere and sublithospheric upper mantle using the interactive modelling program LitMod3D. The crustal subsurface geometry is constrained by existing data sets, the composition and thickness of the SCLM is varied to obtain a best fit with the geophysical observables of the geoid, gravity field and S-wave velocities. A regional tomographic data set for the Barents Sea shows a low-velocity domain at the continent-ocean transition and a high-velocity domain under the East Barents Basin, which might be linked to the Eastern Barents mega-basin. The models show that the transition from continental to oceanic mantle can in a first approximation be explained by a homogeneous mantle composition where density and velocity changes are primarily the effect of temperature variations. On the contrary, the velocity anomaly under the eastern Barents Sea cannot be explained by thermal effects alone but requires a change in mantle composition. The compositional change from a young, poorly depleted (Palaeozoic-type) mantle to an old, strongly depleted (Proterozoic-type) mantle has been tested to produce a density decrease of 2.1% and a velocity increase of 2.1%. We relate the high-velocity anomaly under the eastern Barents Sea to the existence of a Palaeoproterozoic depleted lithosphere. This old SCLM is proposed to be a portion of the Baltic plate formed by the accretion and aggregation of microplates during the Timanian event. Additionally, we suggest that there must be a vertical gradient in SCLM composition, here simplified by two layers: a more depleted upper SCLM overlying a more fertile lower SCLM. The latter is proposed to be the reworked portion of the old Baltica lithosphere that has been affected by the thermal anomaly related, for example, to the Devonian rifting. The induced metasomatic interaction between the old SCLM and the sublithospheric mantle produced a compositional change leading to a decrease in the magnesium content and an increase in incompatible elements that coincide with a decrease of velocity. The continental break-up and opening of the North Atlantic event generated a thermal, chemical and mechanical modification in the southwestern Barents Sea SCLM. Today’s SCLM is characterized by thinning toward the margin and a southeast dipping lower mantle anomaly with higher densities and lower seismic velocities. In the northern part of the Barents Sea we map a thinner lithosphere matching the lower seismic velocities, which can be attributed to reworking of the SCLM due to magmatic events related toLarge Igneous Provinces or the evolution of the Amerasian Basin.

AAPG Search and Discovery Article #90177©3P Arctic, Polar Petroleum Potential Conference & Exhibition, Stavanger, Norway, October 15-18, 2013