Description: Tristan da Cunha Island is one of the hot spots in the Atlantic Ocean. The discussion about its source have not reached consensus yet whether it is in shallow asthenosphere or deeper mantle, because of lack of the geophysical observations in the area. A marine magnetotelluric (MT) experiment was conducted together with seismological observations in the area in 2012–2013 by collaboration between Germany and Japan, in order to give further constraints on the physical state of the mantle beneath the area. A total of 26 seafloor stations were deployed around the Tristan da Cunha islands and available data were retrieved from 23 stations. The MT responses were estimated for those available sites. The detailed data processing will be presented by Chen et al. in this meeting. In this study, we report on the topographic effect on the observed MT responses. During the cruises for seafloor instruments deployment and recovery, detailed bathymetry data were collected around the stations by onboard multi-narrow beam echo sounding (MBES) system. We compiled the MBES data and ETOPO1 data to incorporate the local and regional topography. Then, we applied iterative topographic effect correction and one-dimensional (1-D) conductivity structure inversion. The MT responses of each station were simulated by three-dimensional (3-D) forward modeling. Preliminary results show the overall feature of the observed MT responses at some stations were qualitatively well explained by the seafloor topography included in the conductivity structure model over the 1-D mantle structure. An extreme example is the station near the Tristan da Cunha Island. The impedance phases varies ~300 degrees in shorter period range which is reconstructed by the 3-D forward modeling. Some implications on the lateral variation in the conductivity of the upper mantle will be discussed by demonstrating the residuals between the MT responses corrected for the topographic effect and the 1-D forward response.

Description: The Bransfield Basin is a back-arc basin located in Western Antarctica between the South Shetland Islands and Antarctic Peninsula. Although the subduction of the Phoenix plate under the South Shetland block has ceased, extension continues through a combination of slab rollback and transtensional motions between the Scotia and Antarctic plates. This process has created a continental rift in the basin, interleaved with volcanic islands and seamounts, which may be near the transition from rifting to seafloor spreading. In the framework of the BRAVOSEIS project (2017–2020), we deployed a dense amphibious seismic network in the Bransfield Strait comprising 15 land stations and 24 ocean-bottom seismometers, as well as a network of 6 moored hydrophones; and acquired marine geophysics data including multibeam bathymetry, sub-bottom profiler, gravity & mag-netics, multi-channel seismics, and seismic refraction data. The experiment has collected a unique, high quality, and multifaceted geophysical data set in the Central Bransfield Basin, with a special focus on Orca and Humpback seamounts. Preliminary results confirm that the Bransfield region has slab-related intermediate depth seismicity, with earthquake characteristics suggesting distributed extension across the rift. Gravity and magnetic highs delineate a segmented rift with along-axis variations that are consistent with increased accumulated strain to the northeast. Orca volcano shows evidences of an active caldera and magma accumulation at shallow depths, while Humpback volcano has evolved past the caldera stage and is currently dominated by rifting structures. These differences suggest that volcanic evolution is influenced by the position along the rift. Although a lot of analysis remains, these results provide useful constraints on the structure and dynamics of the Bransfield rift and asso-ciated volcanoes.

Description: Cratons with their thick lithospheric roots can influence the thermal structure, and thus the convective flow, in the surrounding mantle. As mantle temperatures are hard to measure directly, depth variations in the mantle transition zone (MTZ) discontinuities are often employed as a proxy. Here, we use a large new data set of P-receiver functions to map the 410 km and 660 km discontinuities beneath the western edge of the East European Craton and adjacent Phanerozoic Europe across the most fundamental lithospheric boundary in Europe, the Trans-European Suture Zone (TESZ). We observe significantly shorter travel times for conversions from both MTZ discontinuities within the craton, caused by the high velocities of the cratonic root. By contrast, the differential travel time across the MTZ is normal to only slightly raised. This implies that any insulating effect of the cratonic keel does not reach the MTZ. In contrast to earlier observations in Siberia, we do not find any trace of a discontinuity at 520 km depth, which indicates a rather dry MTZ beneath the western edge of the craton. Within most of covered Phanerozoic Europe, the MTZ differential travel time is remarkably uniform and in agreement with standard Earth models. No widespread thermal effects of the various episodes of Caledonian and Variscan subduction that took place during the amalgamation of the continent remain. Only more recent tectonic events, related to Alpine subduction and Quarternary volcanism in the Eifel area, can be traced. While the East European craton shows no distinct imprint into the MTZ, we discover the signature of the TESZ in the MTZ in the form of a linear region of about 350 km width with a 1.5 s increase in differential travel time, which could either be caused by high water content or decreased temperature. Taking into account results of recent S-wave tomographies, raised water content in the MTZ cannot be the main cause for this observation. Accordingly, we explain the increase, equivalent to a 15 km thicker MTZ, by a temperature decrease of about 80 K. We discuss two alternative models for this temperature reduction, either a remnant of subduction or an indication of downwelling due to small-scale, edge-driven convection caused by the contrast in lithospheric thickness across the TESZ. Any subducted lithosphere found in the MTZ at this location is unlikely to be related to Variscan subduction along the TESZ, though, as Eurasia has moved significantly northward since the Variscan orogeny.

Description: In 2004 and 2005 a passive seismic experiment was carried out in the northern and northeastern part of the Bohemian Massif (Sudetes) to study the lithospheric structure. We present results from Ps and Sp receiver function analyses. With one exception, Moho depth at stations in the northwestern part of the study area varies between 28 and 32 km. Thicker crust up to 35 km was mapped toward the south (Moldanubian unit) and toward the east (Moravo–Silesian and Brunovistulian units) confirming results from previous active seismic measurements. There exists a relatively sharp step in Moho depth between units of the central Sudetes (~ 30 km) and the Moravo–Silesian unit (~ 35 km). The vp/vs ratios inverted from primary and multiple Moho Ps conversions hint for different crustal compositions of the units. Toward the Carpathian thrust we have no clear indications for any crustal root or slab beneath the western Carpathians. However, our data suggests a deepening of the Moho or at least a complicated crust–mantle transition in this area. Additional Ps phases were observed between 6 and 10 s delay time in the Sudetes. These phases cannot be explained by Moho reverberations, but are most probably caused by low velocity zones in the middle crust or lithospheric mantle as shown by modeling of theoretical receiver functions. The stations showing these abnormal phases are located in the area of Permo-Carboniferous basins on probably Teplá–Barrandian crust. Therefore we assume that the phases hint at a mid-crustal low velocity zone between 16 and 20 km depth, which is interpreted as a felsic solidified magma reservoir of the Permo-Carboniferous volcanism beneath the Sudetic Basins. Sp receiver functions show phases with negative polarity at 9 to 12 s lead time on average, which we interpret as lithosphere–asthenosphere boundary at about 80 to 110 km depth.

Description: A prominent continental rift is underlying the western part of the Antarctic continent. The current stretching is accompanied by active volcanism at the rim of the Ross Sea as well as underneath the thick ice sheet. Airborne radar measurements have detected active volcanoes south of the Ross Sea. Korea Polar Research Institute (KOPRI) and Alfred Wegener Insitute have deployed 4 long-term broadband Ocean Bottom Seismometers (OBSs) in the Ross Sea near the Jang Bogo Antarctic station during 2011-2012 KOPRI’s Antarctic expedition. It is a pilot research project aiming to better understanding the current seismicity of the West Antarctic Rift System. To accomplish it, we are going to investigate local seismicity and ambient noise around Frankin Island to estimate possible magmatic activity around a seamount. Acoustic noise from glaciers nearby and T-phase propagation study would be conducted in parallel. In addition, we will observe teleseismic events to deteremine the lithospheric structure, and examine shear wave splitting using the OBS data.