Walvis Ridge Passive-Source Seismic Experiment (WALPASS) - Cruise No. MSM20/1 - January 06 - January 15, 2012 - Cape Town (South Africa) - Walvis Bay (Namibia) (English)

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Continental break-up is closely related to the question regarding the driving forces behind the associated processes. Following a well-accepted hypothesis, mantle plumes are the most important triggers for the separation of continents. Around the head of a plume the upper mantle temperatures are strongly increased. After weakening of the continental crust by horizontal extension the high temperatures cause extensive volcanism, forming flood basalts as parts of so-called Large Igneous Provinces (LIP) on the continents, and in some cases also in the newly formed oceanic basin. Following this hypothesis, in the ideal case flood basalts are deposited on the continents before break-up. After the start of the drifting stage with formation of oceanic crust between the continents, the mantle plume should create an aseismic ridge due to its continuing activity. The aseismic ridge in the adjacent oceanic basin will mark the plume position through space and time. However, such a clear relationship between volcanism on the continents and the adjoining ocean basins is rarely observed on a global scale. Among the best examples on Earth for such a relationship are the Parana/Etendeka flood basalts and the Rio Grande Rise/Walvis Ridges on both margins of the South Atlantic. Expeditions MSM17/1+2 and MSM20/1 were dedicated to investigate critical parameters, like crustal thickness and structure of the underlying Earths mantle offshore northern Namibia. Our work was led by the assumption that in the uppermost Earths mantle and in the overlying crust structures relating to fossil plume and/or deformation processes are frozen in and can be visualized by geophysical methods. During MSM 20/1 we recovered twelve broadband ocean-bottom seismometers (BBOBS) deployed in 2011 during MSM 17/2 (Jokat, 2012). The acquired passive seismological data will hopefully shed light into the deep lithospheric structure of the study area. Additionally, leg MSM20/1 was a floating university teaching Master students the acquisition, processing and interpretation of hydro-acoustic data gathered with the on-board systems EM120, EM1002 and PARASOUND. Transits to and between the stations were used for running the hydro-acoustic systems in different water depths (from upper slope down to the deep sea basin) and environments (slope, abyssal plain, Walvis-Ridge). After the successful recovery of all BBOBS about two days were available for detailed surveys. A nearby sediment wave field on the slope and some structures on the shallow shelf were chosen as targets.