ALBERT

Description: The Himalaya and the Tibetan Plateau are uplifted by the ongoing northward underthrusting of the Indian continental lithosphere below Tibet resulting in lithospheric stacking. The layered structure of the Tibetan upper mantle is imaged by seismic methods, most detailed with the receiver function method. Tibet is considered as a place where the development of a future craton is currently under way. Here we study the upper mantle from Germany to northern Sweden with seismic S receiver functions and compare the structure below Scandinavia with that below Tibet. Below Proterozoic Scandinavia, we found two low velocity zones on top of each other, separated by a high velocity zone. The top of the upper low velocity zone at about 100km depth extends from Germany to Archaean northern Sweden. It agrees with the lithosphere-asthenosphere boundary (LAB) below Germany and Denmark. Below Sweden it is known as the 8°discontinuity, or as a mid-lithospheric discontinuity (MLD), similar to observations in North America. Seismic tomography places the LAB near 200km in Scandinavia, which is close to the top of our deeper low velocity zone. We also observed the bottom of the asthenosphere (the Lehmann discontinuity) deepening from 180km in Germany to 260km below Sweden. Remnants of old subduction in the upper about 100km below Scandinavia and Finland are known from controlled source seismic experiments and local earthquake studies. Recent tomographic studies indicate delamination of the lithosphere below southern Scandinavia and northern Germany. We are suggesting that the large scale layered structure in the Scandinavian upper mantle may be caused by processes similar to the ongoing lithospheric stacking in Tibet.

Description: Ultraslow spreadingmid-ocean ridges have a low magma budget and melt is distributed unevenly along the ridge axis. There is little or no basaltic crust between isolated magmatic centers. The processes that focus melts to segments of robust magmatism are not yet understood. During a seismic survey of the ultraslow spreading Knipovich Ridge in the Norwegian-Greenland Sea with ocean bottom seismometers, we discovered a seismic gap in the upper mantle beneath Logachev Seamount, where micro-earthquakes clearly delineate a shallowing of the maximum depth of faulting. A topography of the lithosphere that allows melts to travel laterally along its base and rise in areas of thin lithosphere has been proposed as a possible mechanism to explain the focusing of melts at volcanic centers, but has never been confirmed observationally. Our results are the first geophysical evidence for an along-axis variation of the lithospheric thickness at an ultraslow spreading ridge.

Description: In this study we present a 3D P-wave upper-mantle tomography model of the SW Iberian margin and Alboran Sea based on teleseismic arrival times recorded by Iberian and Moroccan land stations and by a seafloor network deployed for 1 year in the Gulf of Cadiz area during the EC-NEAREST project. The 3D model was computed down to 600 km depth. The tomographic images exhibit significant velocity contrasts, as large as 3%, confirming the complex evolution of this plate boundary region. Prominent high-velocity anomalies are found beneath Betics-Alboran Sea, off-shore SW Portugal, and N Portugal, at sublithospheric depths. The transition zones between high and low velocity anomalies in SW and S Iberia are associated to the contact of oceanic and continental lithosphere. The fast structure below the Alboran Sea-Granada area depicts a L-shaped body steeply dipping from the uppermost mantle to the transition zone where it becomes less curved. This anomaly is consistent with the results of previous tomographic investigations and recent geophysical data such as stress distribution, GPS measurements of plate motion, and anisotropy patterns. In the Atlantic domain, under the Horseshoe Abyssal Plain, the main feature is a high-velocity zone found at uppermost mantle depths. This feature appears laterally separated from the positive anomaly recovered in the Alboran domain by the interposition of low-velocity zones which characterize the lithosphere beneath the SW Iberian peninsula margin, suggesting that there is no continuity between the high velocity anomalies of the two domains west and east of the Gibraltar Strait.