The gravity field of a dipping plate in Greece

Summary. The surface gravity anomalies for a three-dimensional density model of a dipping lithospheric plate under the Aegean island arc and the Aegean Sea in Greece in the eastern Mediterranean Sea have been calculated. Such a dipping plate has been suggested in geophysical investigations. The calculated gravity maximum over the dipping plate is located in the area of the largest observed Bouguer anomalies inside the island arc. It is suggested that this should be taken into account in studies of crustal structure and gravity in the area.     

Summary of project TOR: delineation of a stepwise, sharp, deep lithosphere transition across Germany–Denmark–Sweden

Project Tor (Teleseismic Tomography across the Tornquist Zone in Germany–Denmark–Sweden) is now producing results. We are able to detect very significant deep lithosphere differences, and we can now discuss the sharpness laterally. In 1996–1997, our 120 seismographs constituted the largest seismic antenna ever in Europe. The Tor area was chosen along a well-studied crustal profile of an earlier project, and the inversion efforts are concentrated on the deep lithosphere and asthenosphere differences to depths around 300 km. The Tor investigation can be called two-and-a-half-dimensional, as it has a 900-km profile length with 100 km width plus a few seismographs off the profile. The Tor data have been subjected to P-wave travel time tomography, surface wave and receiver function analysis as well as anisotropy and scattering measurements. Through ray tracing in a compiled crustal model and subtraction of the modelled travel time anomalies, the influence of the lower lithosphere/asthenosphere on the seismic rays from distant earthquakes is being established. Travel time tomography results confirm very large lateral lithosphere differences of 4–6% in P-wave velocity. For several events of the large data base, it is demonstrated that the observed P-wave travel time anomalies of 1–2 s can be divided almost equally between known crustal effects and lower lithosphere/asthenosphere differences, which then must account for about 1 s of the travel time differences. The transition is interpreted to be sharp and steep in two places. It goes all the way through the lithosphere at the northern rim of the Tornquist Zone near the border between Sweden and Denmark, and here the lithosphere difference is large. A smaller lithosphere difference is found near the southern edge of the Ringkøbing-Fyn High just north of the border between Denmark and Germany. Also, this transition is sharp and steep, and goes all through the lithosphere. These two sharp transitions divide the Tor region into three different lithosphere structures distinguishable in P-wave travel time tomography, surface wave dispersion, P- and S-wave anisotropy, and partly in P-wave scattering.     

Stratigraphy and facies distribution of the Utsira formation and the Pliocene sequences in the northern North Sea

The Upper Cenozoic deposits of the northern North Sea have been analysed in order to establish a regional and detailed stratigraphy. The Utsira Formation is subdivided into four log-units and mapped, and two main depocentres are outlined. The lower part of the Utsira Formation consists of thick marine, mounded sand bodies, interpreted as overall stacked lowstand fan deposits, while the upper part of the formation consists of more clayey-silty intervals, indicating increased relative sea level. The succeeding progradational Pliocene deposits are subdivided into 13 high-frequency depositional sequences and are mapped. The sequences are grouped into four composite sequences. Each of the four Pliocene composite sequences is composed of one or two rather locally distributed, prograding sequences (lowstand sequence set), succeeded by one or two more widely distributed aggrading-prograding sequences (transgressive-highstand sequence set). Boundaries between the composite sequences are recorded as marked changes in distribution of depocentres and sequence architecture. The regional uplift of Scandinavia is believed to be the main control on sediment input, feeding the succeeding general prograding Pliocene sequences. Oscillations of the eustatic sea level punctuated the tectonically controlled progradation and affected variations in the accommodation space, and thus created the high-frequency sequences.