ALBERT

Description: The European Plate has a 4.5 Gy long and complex tectonic history. This is reflected in the present-day large-scale crustal structures. A new digital Moho depth map is compiled from more than 250 data sets of individual seismic profiles, 3-D models obtained by body and surface waves, receiver function results and maps of seismic and/or gravity data compilations. We have compiled the first digital, high-resolution map of the Moho depth for the whole European Plate, extending from the mid-Atlantic ridge in the west to the Ural Mountains in the east, and from the Mediterranean Sea in the south to the Barents Sea and Spitsbergen in the Arctic in the north. In general, three large domains within the European Plate crust are visible. The oldest Archean and Proterozoic crust has a thickness of 40–60 km, the continental Variscan and Alpine crust has a thickness of 20–40 km, and the youngest oceanic Atlantic crust has a thickness of 10–20 km.

Description: A seismic velocity cross-section down to 700 km depth beneath the Tibetan Plateau has been constructed. Beneath the cover layer, felsic rocks rich in a quartz exist down to 15–25 km depth. Beneath these depths, temperatures are probably high enough for ductile flow and partial melting to occur. The velocity increase across the boundary at 30–40 km depth marks the interface between felsic upper crust and more mafic lower crust. Crustal thickness is greatest (c. 74 km) south of c. 31.5°N, where Indian lower crust forms the basal layer. Northwards, crustal thickness decreases to c. 66 km around 33°N, before increasing to c. 70 km beneath northern Tibet. Crossing the Kunlun, the crust thins to c. 54 km beneath the Qaidam basin. High-velocity, dense, cold Indian lithospheric mantle extends northwards until about the Banggong-Nujiang suture, where it downwells to 350–400 km depth. The lithosphere–asthenosphere boundary occurs at 160–225 km depth. The apparent northwards deepening of the 410 and 660 km discontinuities implies that the upper mantle beneath northern Tibet is slower, less dense and warmer than under southern Tibet which, in turn, could provide some of the isostatic support for the high elevations in northern Tibet where the crust is thinner than under southern Tibet.

Description: Using active and passive seismology data we derive a shear ( S ) wave velocity model and a Poisson's ratio () model across the Chilean convergent margin along a profile at 38°15'S, where the M w 9.5 Valdivia earthquake occurred in 1960. The derived S -wave velocity model was constructed using three independently obtained velocity models that were merged together. In the upper part of the profile (0–2 km depth), controlled source data from explosions were used to obtain an S -wave traveltime tomogram. For the middle part (2–20 km depth), data from a temporary seismology array were used to carry out a dispersion analysis. The resulting dispersion curves were used to obtain a 3-D S -wave velocity model. In the lower part (20–75 km depth, depending on the longitude), an already existent local earthquake tomographic image was merged with the other two sections. This final S -wave velocity model and already existent compressional ( P ) wave velocity models along the same transect allowed us to obtain a Poisson's ratio model. The results of this study show that the velocities and Poisson's ratios in the continental crust of this part of the Chilean convergent margin are in agreement with geological features inferred from other studies and can be explained in terms of normal rock types. There is no requirement to call on the existence of measurable amounts of present-day fluids, in terms of seismic velocities, above the plate interface in the continental crust of the Coastal Cordillera and the Central Valley in this part of the Chilean convergent margin. This is in agreement with a recent model of water being transported down and released from the subduction zone.

Description: For a period of about 1 yr between the summers of 2010 and 2011, 25 broad-band seismographs were deployed in a roughly linear array across the eastern end of the Qaidam basin and the Qilian Shan in the northeastern Tibetan plateau. This region is probably the most suitable place to study the ongoing convergence interaction between the high Tibetan plateau and the main Asian continental plate. Low-frequency P receiver function analysis of the data provides an image of the crust and mantle down to 700 km depth. In addition to the Moho at 45–65 km depth beneath the profile, the 410 and 660 km discontinuities bounding the mantle transition zone can be identified at 400–410 and 650–660 km depths, respectively. A possible increase in temperature in the upper mantle thought to exist beneath the northern part of the high Tibetan plateau is thus confined to this part of the plateau and lower upper-mantle temperatures similar to those beneath southern Tibet occur beneath the Qaidam basin and Qilian Shan. When higher frequencies are included in the P receiver function analysis, a positive Ps converter dipping down to the south from 70–75 km depth at 37.9°N to about 110 km depth at 36°N is imaged. As this feature is only seen in high-frequency images and not in the low-frequency image, it is modelled as the positive Ps conversion from the base of an approximately 5-km-thick anisotropic layer at the top of the Asian mantle lithosphere which is currently subducting. This south-dipping converter continues to the south on the INDEPTH IV profile. S receiver function analysis completes the image of the structure below the Qilian Shan profile with the identification of the lithosphere–asthenosphere boundary (LAB). The LAB of the Asian Plate is identified for a reference slowness of 6.4 s deg –1 at 12–14 s (105–125 km depth) between 38 and 41°N below the northern part of the S receiver function profile. To the south it increases in depth such that it is at about 19 s (170 km depth) between 34 and 35°N at the southern end of the profile. The LAB of the Asian Plate occurs at similar depths on the INDEPTH IV profile at the latitudes where the INDEPTH IV and Qilian Shan profiles overlap. As on the INDEPTH IV profile to the south, between 34 and 35°N at the southern end of the Qilian Shan profile there is evidence from the S receiver functions for the LAB of a separate Tibetan Plate.