Magnetotelluric Constraints on the Temperature, Composition, Partial Melt Content, and Viscosity of the Upper Mantle Beneath Svalbard

Abstract

Long-period magnetotelluric (MT) data can be used to interpret upper mantle temperature, hydrogen content, and the presence of partial melt, all of which strongly influence mantle viscosity. We have collected the first long-period MT data in Svalbard and have combined them with preexisting broadband MT data to produce a model of the electrical resistivity of Svalbard's upper mantle. Asthenospheric resistivities are low compared to stable continental settings but more comparable to young oceanic asthenosphere, suggesting that the physical state of Svalbard's upper mantle is controlled by its proximity to the Mid-Atlantic Ridge. Interpretation of the MT model using a petrologically constrained genetic algorithm approach shows that partial melt is present in the uppermost asthenosphere beneath Svalbard. This is the first direct evidence of partial melt in Svalbard's asthenosphere from deep geophysical soundings. Viscosities calculated from the geophysical data show a low viscosity layer (~10 18 Pa s) coincident with the partial melt layer, underlain by a higher viscosity layer (~10 20 Pa s) extending to the transition zone. Viscosities calculated from glacial isostatic adjustment (GIA) data in Svalbard show a considerable range due mainly to uncertainties in past ice sheet models. Improved constraints on Svalbard's mantle viscosity from geophysical data may help to improve these GIA models.

Glacially Induced Stress Across the Arctic From the Eemian Interglacial to the Present—Implications for Faulting and Methane Seepage

“…Interestingly, our models and previous GIA studies show that there is a general underestimation of the uplift rate in the Svalbard region (Auriac et al., 2016 and references therein). We argue that the mismatch between the computed present‐day uplift and GPS measurement in that area is most likely due to a combination of mechanisms that are generally not considered in GIA models: Temperature anomaly: Interpretation of seismic tomography models and magneto telluric investigations have revealed a strong temperature anomaly in the upper mantle of the North Atlantic (Minakov, 2018 and reference within; Selway et al., 2020), probably linked with the Icelandic and Jan Mayen hotspots. Results indicate an average excess mantle temperature under Svalbard (40°C–100°C) compared to the central Barents Sea.…”

“…Results indicate an average excess mantle temperature under Svalbard (40°C–100°C) compared to the central Barents Sea. That would correspond to a layer of viscosity ∼10 18 Pa.s in the uppermost mantle and a layer of viscosity ∼10 20 Pa.s in the underlying lower mantle (Selway et al., 2020). These values are more than two orders of magnitude lower than what is typically used in GIA studies (including ours) for the Svalbard area (Auriac et al., 2016; Patton et al., 2017).…”

“…Temperature anomaly: Interpretation of seismic tomography models and magneto telluric investigations have revealed a strong temperature anomaly in the upper mantle of the North Atlantic (Minakov, 2018 and reference within; Selway et al., 2020), probably linked with the Icelandic and Jan Mayen hotspots. Results indicate an average excess mantle temperature under Svalbard (40°C–100°C) compared to the central Barents Sea.…”

Probabilistic Linear Inversion of Satellite Gravity Gradient Data Applied to the Northeast Atlantic

“…The negative density anomaly beneath Reykjanes ridge is not well expressed in the shear velocity image as it can be related to limited resolution at the edges of the regional tomography model. A deep-seated density anomaly north of Iceland might extend over large distance in the shallow asthenosphere toward Svalbard margin where a thin lithosphere is predicted using various geophysical data (Minakov, 2018;Selway et al, 2020;Vågnes & Amundsen, 1993).…”

Constraining Upper Mantle Viscosity Using Temperature and Water Content Inferred From Seismic and Magnetotelluric Data

“…Hence, other sources of information, independent from any GIA observables and seismic events, that can place additional constraints on mantle viscosity are important. For instance, localized geophysical measurements, such as seismic and magnetotelluric (MT) observations, can be used to infer variations in mantle structure that also relate to mantle viscosity (e.g., Ivins et al., 2022; Liu & Hasterok, 2016; O’Donnell et al., 2017; Selway et al., 2020). Because such geophysical measurements scan the subsurface of the Earth, they can provide good depth and lateral resolution for upper mantle structure.…”