Tristan da Cunha hotspot : Mantle plume or shallow plate tectonics?

Abstract

Tristan da Cunha is a small volcanic island in the South Atlantic Ocean close to the Mid- Atlantic Ridge. It is part of an area, which is characterized by widely scattered seamounts and small islands at the western end of the Walvis Ridge - Tristan/Gough hotspot track. Tristan da Cunha represents the end member of a classical hotspot track with an underlying plume: The active volcanic island Tristan da Cunha at the youngest end of the track is linked to the Cretaceous Etendeka flood basalt province in northwestern Namibia at its oldest end. But the genesis of the island itself has so far been puzzling. It is hotly debated if the island sits actually above a deep-seated mantle plume or if it is caused by shallow plate tectonics. To understand the Tristan da Cunha hotspot, a multi-disciplinary geophysical study has been conducted in 2012 and 2013 on board the German research vessel Maria S. Merian to acquire marine magnetotelluric and seismological data. The aim was to reveal the upper mantle structure with electrical density and velocity perturbations. Within this study I focused on the seismological dataset. At first, I performed a P-wave finite-frequency tomography with cross-correlated travel time residuals of teleseismic earthquakes. This allows to resolve the upper mantle structure beneath the island in terms of velocity perturbations and clarifies the existence of a mantle plume. I also investigated the local seismicity in the Tristan region to identify tectono-magmatic processes at the Mid-Atlantic Ridge and Close to the islands and seamounts. Moreover, I compared and combined my tomographic results with electromagnetic results to identify zones of partial melt and to understand plume processes in the upper mantle beneath the Tristan da Cunha hotspot. The tomographic results provide evidence for the existence of the Tristan conduit southwest of the archipelago. Its shape is cylindrical with a radius ca. 100 km down to a depth of 250 km. The structure ramifies in narrow veins below that depth. A recent link from the conduit to a seamount chain shows, that melt is channelled towards seamounts and islands in the study area. High seismicity within an oceanic plate segment north of Tristan da Cunha can be related to the internal stresses of the fragment. Differently directed forces act at the northern boundary of this plate. An earthquake free zone coincides spatially with the location of the Tristan mantle plume. This indicates a ductile regime in the lithosphere above the plume. Furthermore, hints for an incipient ridge jump towards a parallel line to the actual location of the Tristan plume were found. Several earthquakes were localised close to the archipelago of Tristan da Cunha. The locations of these earthquakes are related to young surface eruptions like small volcanic cones or seamounts.