ALBERT

Description: According to classic plume theory, the Tristan da Cunha mantle plume played a major role in the rifting of the South Atlantic margins by impinging at the base of the continental lithosphere shortly before or during the breakup of the South Atlantic. Onshore the Tristan mantle plume is associated with the emplacement of the Parana-Etendeka flood basalt province ~132 Ma, and offshore with the age-progressive Walvis Ridge and associated young seamount province. The global data is lacking coverage and therefore resolution around the Tristan da Cunha for shallower mantle depths. The search for deep-reaching geophysical/thermal anomalies beneath Tristan da Cunha was the starting point for a multi-disciplinary geophysical experiment to acquire passive marine electromagnetic, seismic and bathymetric data within the framework of the SPP-1375 “South Atlantic Margin Processes and Links with onshore Evolution” (SAMPLE) funded by the German Science foundation. The experiment included two expeditions with the German research vessel MARIA S. MERIAN in 2012 and 2013. Between February 2012 and January 2013, a network of 24 ocean bottom seismometers (OBS) from the German DEPAS pool (Deutscher Geräte-Pool für Amphibische Seismologie) and 26 ocean-bottom magneto-telluric stations from GEOMAR Kiel and the University of Tokyo were deployed around the archipelago of Tristan da Cunha. The seismological stations are equipped with a Güralp CMG-40T wideband seismometer (60 s) incorporated in a titanium pressure housing, a hydrophone, and a GEOLON MCS (Marine Compact Seismocorder) data logger from SEND GmbH Hamburg, Germany. Each sensor channel is sampled at 50 Hz (2 OBS at 100 Hz), preamplifier gain of the hydrophone channel is 4 and 1 for the three seismometer components. The seismometers are connected to a cardanic levelling mechanism, which was activated every 21 days to level the sensors. Based on previous experiments and triangulation of some stations, the accuracy of the positions is estimated to 500 m. To complement the ocean-bottom network and the existing observatory on Tristan da Cunha (TRIS), two land seismometers and one magnetotelluric station were installed on Nightingale Island located southwest of the main island each of which was equipped with a Güralp-3ESP seismometer (60 s) and an EarthData data logger. As recording parameters, we chose 100 Hz sampling rate and low preamplifier gain (0.4). One of the stations (NIG01) recorded earthquake data for the entire year, while the second station failed after few days due to water damage. Unfortunately, the permanent station TRIS failed also during the experiment.