Large-scale mass wasting in the western Indian Ocean constrains onset of East African rifting

Faulting and earthquakes occur extensively along the flanks of the East African Rift System, including an offshore branch in the western Indian Ocean, resulting in remobilization of sediment in the form of landslides. To date, constraints on the occurrence of submarine landslides at margin scale are lacking, leaving unanswered a link between rifting and slope instability. Here, we show the first overview of landslide deposits in the post-Eocene stratigraphy of the Tanzania margin and we present the discovery of one of the biggest landslides on Earth: the Mafia mega-slide. The emplacement of multiple landslides, including the Mafia mega-slide, during the early-mid Miocene is coeval with cratonic rifting in Tanzania, indicating that plateau uplift and rifting in East Africa triggered large and potentially tsunamigenic landslides likely through earthquake activity and enhanced sediment supply. This study is a first step to evaluate the risk associated with submarine landslides in the region

Metamorphic density changes as key process to form anorogenic plateaus

International audienceAnorogenic plateaus are those topographic barriers that reach medium elevations of approximately 1500 m, e.g. South African-, East African- or Mongolian Plateau. They are inferred to be closely link to mantle plumes away from plate boundaries. Actually, plateau formation processes in geodynamic settings outside of orogens have not been unambiguously established. Recently, Wichura et al. [1] have clearly shown a pre-rift uplift of the East African plateau. They suggested pre-rift topographic variations by lithospheric thermal expansion, due to mantle plume-lithosphere heat interactions. Following their assumption, we developed an one-dimensional model, which calculates density as a function of pressure, temperature, and chemical composition, based on the fact that heat variations in the continental lithosphere and crust influences metamorphic density. Thus, we present a new petrologic aspect for plateau uplift [2], because models on plateau uplift generally do not take into account the effects of metamorphic phase transitions and ignore the fact that chemical reactions influence both the stability of mineral assemblages and rock density. Our model underscores how metamorphic density of the lithosphere varies with depth and reveals how combination of chemical composition of rocks, mineralogy, and geothermal gradient all have significant effects on the density distribution within the lithosphere and ultimately the evolution of anorogenic plateaus. Thus, we show that metamorphic phase transitions in crust and lithospheric mantle due to heating at the lithosphere-asthenosphere boundary by a mantle plume are key processes that drive significantly uplift and the generation of long-wavelength topography. Furthermore, in order to better understand the temporal characteristics of mantle plume related topography we calculated the timing to generate significant topographic uplift. Our results are very instructive and suggest considerable primary thermal uplift of approximately 1400 m as a viable mechanism for anorogenic plateau formation. In this way, our model may help to explain pre-rift topography of the East-African Plateau, related to heat generated and transferred by the activity of a mantle plume. In addition, we show that density-change models that ignore metamorphic processes and/or mineral reactions will result in a reduced amount of uplift or may require inadequate temperatures to explain uplift scenarios

The BAYSOFI Campaign - Measurements Carried out during the Total Solar Eclipse of August 11, 1999

The total solar eclipse of August 11, 1999 provided a unique opportunity to observe the input of fast day-night and night-day transitions, under high solar elevation around noon, on the earth-atmosphere-biosphere system. Within the interdisciplinary field campaign BAYSOFI, measurements of radiation, boundary layer micrometeorology and photochemistry, photosynthesis and transpiration were carried out at Freising-Weihenstephan and several locations nearby focusing on short-term effects of the eclipse. Although the overall grosswetterlage on August 11 was not favourable for viewing the eclipse, with clouds covering most of central Europe, observational conditions at Weihenstephan were fair due to a large hole in the cloud layer which appeared just half hour before totality lasting for more than one hour. Thus significant effects of the eclipse on radiation, photolysis rates, OH, the temperature, wind, turbulence structure and stratification, ozone and CO2 fluxes, photosynthesis, transpiration and sap flow of trees could be observed which are reported and discussed in the following sequence of papers