Baba, Kiyoshi, Chen, Jin, Sommer, Malte, Utada, Hisashi, Geissler, Wolfram H., Jokat, Wilfried and Jegen, Marion (2017) Marine magnetotellurics imaged no distinct plume beneath the Tristan da Cunha hotspot in the southern Atlantic Ocean. Tectonophysics, 716 . pp. 52-63. DOI 10.1016/j.tecto.2016.09.033.

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Abstract

Highlights

• The electrical structure beneath the Tristan da Cunha (TDC) hotspot was investigated.
• Plume-like structure was not imaged beneath TDC by 3-D inversion analysis.
• The plume may be small and/or weak or take place elsewhere outside of the study area.
• Conductivity and bathymetry anomalies show a contrast across the TDC fracture zone.
• Mantle temperature and melting process at ridge may cause the conductivity anomaly.

Abstract

The Tristan da Cunha (TDC) is a volcanic island located above a prominent hotspot in the Atlantic Ocean. Many geological and geochemical evidences support a deep origin of the mantle material feeding the hotspot. However, the existence of a plume has not been confirmed as an anomalous structure in the mantle resolved by geophysical data because of lack of the observations in the area. Marine magnetotelluric and seismological observations were conducted in 2012–2013 to examine the upper mantle structure adjacent to TDC. The electrical conductivity structure of the upper mantle beneath the area was investigated in this study. Three-dimensional inversion analysis depicted a high conductive layer at ~ 120 km depth but no distinct plume-like vertical structure. The conductive layer is mostly flat independently on seafloor age and bulges upward beneath the lithospheric segment where the TDC islands are located compared to younger segment south of the TDC Fracture Zone, while the bathymetry is rather deeper than prediction for the northern segment. The apparent inconsistency between the absence of vertical structure in this study and geochemical evidences on deep origin materials suggests that either the upwelling is too small and/or weak to be resolved by the current data set or that the upwelling takes place elsewhere outside of the study area. Other observations suggest that 1) the conductivity of the upper mantle can be explained by the fact that the mantle above the high conductivity layer is depleted in volatiles as the result of partial melting beneath the spreading ridge, 2) the potential temperature of the segments north of the TDC Fracture Zone is lower than that of the southern segment at least during the past ~ 30 Myr.

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