Magnetotelluric image of a hyperextended and serpentinized rift system in the SW Barents Sea

Abstract

Magnetotellurics (MT) image the Earth’s electrical resistivity down to the mantle but is rarely used for investigation of offshore rifted margins. In such setting, the stretched lithosphere is altered by distinct tectono-thermal processes such as partial melt, hydration, or chemical alteration. However, lower crustal and upper mantle often display similar seismic velocities, density, and magnetic properties. Integration of resistivity models from MT data can lift this ambiguity in a hyper-extended rift system. In this project, we use an extensive marine MT database consisting of 337 receivers located along seven regional transects, emanating from ~70 000 km2 of 3-D CSEM surveys acquired for hydrocarbon exploration in the SW Barents Sea. 3D inversion of long period, marine MT data (1 – 3000 s) is performed on 104 receivers located along two, ~300 km long transects located over the Bjørnøya Basin, located ~300 km offshore northern Norway. We also apply 1D Bayesian inversion to a selection of receivers as an alternative to the deterministic approach for a more comprehensive exploration of the solution space.The resolving power of MT data is assessed with synthetic tests in an archetypal rift system where ample crustal thickness variation occurs. The results highlight that our MT data sense the transition from necking to hyper-extended domain where the crust (<~10 km) is not reconstructed by 3D inversion. In the Bjørnøya Basin – the northernmost member of a hyper-extended Cretaceous basin chain in the North Atlantic – seismic interpretation is used to assign a stratigraphic level to two prominent conductors in the resistivity models: (1) 0.1-1 Ω.m within Lower Cretaceous marine shales buried at 10-15 km depth (2) 1-10 Ω.m within the uppermost mantle. The bulk resistivity of a 2-phase, fluid-rock model emphasizes that seawater as a sole pore fluid phase is not conductive enough to explain the magnitude of the two conductors. A 25% serpentinization of mantle rocks can account for a fivefold rise in salinity of the residual fluid and is compatible with density and seismic velocities in the Bjørnøya Basin. Samples from fossil margins in the Alps and Pyrenes shows contamination of post-rift sediments pore fluids by mantle elements, highlighting the mobility of mantle-reacted fluids in hyper-extended systems. High-salinity fluid can ascend and mix with seawater in pore spaces of the sediments, supporting our proposed model of saline fluid circulation in hyper-extended basins.