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The Tristan da Cunha islands are located 450 km east of the Mid-Atlantic Ridge and 60 km north of the Tristan da Cunha transform fault, and fracture zone system in the Atlantic Ocean. The islands are products of intraplate hotspot volcanism. However, several controversies are associated with theories for the origin of the Tristan da Cunha hotspot. Previous magnetotelluric data 3-D inversion results based on finite-difference algorithm couldn’t image plume-like vertical structure, unlike many geological and geochemical studies (Baba et al., 2017). In this study, we revisit the electrical structure of the region by inverting combined vertical magnetic transfer functions and the MT responses from seafloor stations around the islands and those newly obtained on the Tristan da Cunha and Nightingale islands. The topographic effect is one of the keys to obtaining a more reliable subsurface structure, and therefore, we use two finite-element-based inversion methods of Usui (2021) and Grayver (2015). For inversion analysis, we compared the topographic responses from these methods and a finite-difference-based forward modeling method (Baba & Seama, 2002; Baba & Chave, 2005) used in the previous study (Baba et al., 2017). The responses for sites with minimum topography effects are almost identical from all the three methods except for some sites where difference is observed in shorter periods. This could be due to the difference in the mesh resolution around the sites. The responses for sites with significant topography effects show a reasonable match for the finite-element methods in longer periods. In contrast a significant difference is observed in responses from all three methods for short periods. The comparison of modeled responses proves the applicability of Usui’s inversion scheme to MT data from this region. In the workshop, the inversion results from Usui’s inversion method will be presented.

The Rio Grande Rise (RGR) isone of the most prominent bathymetric features in the South Atlantic Ocean.It has long been assumed that this massive oceanic plateau is a late-Cretaceous Large Igenous Province related to the Tristan-Gough mantle plume. But recent studies suggest that it might be a fragment of detached continental marginthathas been preserved asa'microcontinent'embedded in plume-influenced oceanic crust[1, 2].Here we present the first geochemical results from a combined geophysical and petrological cruise(MSM-82)carried out in spring 2019. We measured major and traceelement concentrations of 47whole rock samplesdredged from the flanks of a long rift valleythat cuts throughthe eastern (E) and western (W) parts of the RGR. Our preliminary data showthat theERGR differs in compositionfrom the WRGR. The latter rangesfrom trachybasaltto trachyandesite, while the former mainly consists of alkalinebasalt.Geophysical results show that the WRGR is underlain by thicker crust so the more evolved compositionsmight be due to longer residence times for melts in the associated magma chambers. Light rare earth elementenrichment[(La/Sm)N: 1.5 to 6.5]in all samples indicatesan enrichedmagmasource, consistent with the involvment of a mantle plume during the formation of the RGR. However, variable ratios ofimmobileincompatible trace elements (e.g. Nb/Zr) imply aheterogenoussource region.All samples have high Nb/Th (6.4 –14.3) similar to oceanic basalts, indicatingthat they have not undergone significant contamination bya continental component with low Nb/Th.Thus, our initial findingsare consistent withaplume-related magmatic origin for the RGRratherthana microcontinent.

New marine geophysical data acquired across the partly ice‐covered northern East Greenland continental margin highlight a complex interaction between tectonic and magmatic events. Breakup‐related lava flows are imaged in reflection seismic data as seaward dipping reflectors (SDRs), which are found to decrease in size both northwards and southwards from a central point at 75° N. We provide evidence that the magnetic anomaly pattern in the shelf area is related to volcanic phases and not to the presence of oceanic crust. The remnant magnetization of the individual lava flows is used to deduce a relative timing of the emplacement of the volcanic wedges. We find that the SDRs have been emplaced over a period of 2‐4 Ma progressively from north to south and from landward to seaward. The new data indicate a major post‐middle Eocene magmatic phase around the landward termination of the West Jan Mayen Fracture Zone. This post‐40 Ma volcanism likely was associated with the progressive separation of the Jan Mayen microcontinent from East Greenland. The break‐up of the Greenland Sea started at several isolated seafloor spreading cells whose location was controlled by rift structures and led to the present‐day segmentation of the margin. The original rift basins were subsequently connected by steady‐state seafloor spreading that propagated southwards, from the Greenland Fracture Zone to the Jan Mayen Fracture Zone. Key Points Polyphase Cenozoic volcanic rifting and consecutive emplacement of breakup‐related lava flows units along the northern East Greenland margin Breakup along restricted margin segments is followed by north to south directed progressive opening of the Greenland Sea Widespread post‐middle Eocene (< 40 Ma) offshore magmatism, associated with the breakup of the Jan Mayen microcontinent from East Greenland

According to classical plume theory, the Tristan da Cunha hotspot is thought to have played a major role in the rifting of the South Atlantic margins and the creation of the aseismic Walvis Ridge during and after the breakup of the South Atlantic. Between February 2012 and January 2013 a network of 24 broadband ocean-bottom seismometers was in operation around the volcanic archipelago of Tristan da Cunha. Ambient noise data from the OBS and a seismic station on Nightingale Island were used to constrain the crustal and uppermost structure around the island. From the vertical and hydrophone recordings of more than 300 days we could reconstruct the ambient noise Green's functions by cross-correlation. The dispersion curves of Rayleigh/Scholte waves could be derived from the cross-correlations in the period range from 2 to 32 seconds. Group velocity maps were determined for each individual period using travel time tomography. These group velocity maps were converted to depth by dispersion curve inversion to construct a 3D S-wave velocity model of the crust and uppermost mantle in the region. This model shows a strong velocity anomaly beneath the Tristan da Cunha archipelago. The influence of the water depth on the inversion is discussed.