ALBERT

Description: A deep seismic sounding experiment was performed during the expedition ARKTIS XV/2of the RV Polarstern and the Polish ship Eltanin in 1999 in the continent-ocean transitionzone of northwestern Svalbard, along the 430-km-long profile AWI-99200. The profile runsfrom the Molloy Deep in the vicinity of an active spreading axis in the northern Atlantic toNordaustlandet. Seismic energy (airgun andTNTshots)was recorded by seismic land (onshore)stations, OBSs and hydrophone systems, with airgun shots recorded up to 200 km onshore and50 km offshore. The data recorded along the entire profile provide an excellent database fora detailed seismic modelling of the crustal P-wave velocity field along the profile track. Aminimal depth of about 6 km to the Moho discontinuity was found east of the Molloy Deep.Here, the upper mantle exhibits a P-wave velocity of about 7.9 km s−1, and the crustal thicknessdoes not exceed 4 km. The continent-ocean transition zone to the east is characterized by acomplex seismic velocity structure. The Moho interface reaches a maximum depth of 28 kmbeneath the continental part of the profile, with a P-wave velocity in the upper mantle of8.15 km s−1. The continental crust consists of three layers with P-wave velocities of about5.5, 5.9-6.0 and 6.2-6.6 km s−1, respectively. In addition, we have found two reflectors in themantle lithosphere at depths of 14-42 and 40-50 km dipping NE. The evolution of the regionappears to be within a shear-rift tectonic setting. The continent-ocean transition zone is mostlydominated by extension, so the last stage of the development of this margin can be classifiedas rifting. The uplifted Moho boundary close to the Molloy Deep can be interpreted as thesouthwestern end of the Molloy Ridge.

Description: New seismic refraction data were collected across the western Svalbard continental marginoff Kongsfjorden (Ny ?lesund) during the cruise leg ARK15/2 of RV Polarstern. The use ofon- and offshore seismic receivers and a dense airgun shot pattern provide a detailed viewof the velocity structure of SvalbardÕs continental interior, the continent-ocean transition,and oceanic crust related to the northern Knipovich Ridge and the Molloy Ridge.The Caledonian central and western terranes of Svalbard are not distinguishable on the basisof seismic velocity structure. Below a 7 to 8 km thick Paleozoic sedimentary cover the crystallinecrust reveals a three-layer structure with seismic velocities ranging between 6.1 and 6.9 km/s.The geological suture between the terranes is imperceptible. The middle and upper crust belowthe Tertiary Forlandsundet Graben shows striking low velocities. This can be related to theEarly Paleozoic convergent transpressive movements between Svalbard and northern Greenland,followed by an extensional (relaxing) phase. We argue that a brittle-fractured rock formation ispresent below the graben, which also buries a sedimentary Paleozoic core.The continent-ocean transition can be classified as a sheared margin formed at the continentalpart of the Spitsbergen Fracture Zone. Moderate crustal thinning is achieved only to the westof the low velocity zone below the Forlandsundet Graben. This leads to the assumption thattranstensional rift movements since Oligocene were decoupled from the central terraneof Svalbard. The Moho dips with an angle of 45° eastwards at the continent-ocean transitionthat exhibits higher seismic velocities of 7.2 km/s on the continental side. These can beinterpreted as minor mantle-derived intrusions, probably induced by convection due to thejuxtaposition of cool continental and hot oceanic lithosphere.The oceanic crust generated at the Knipovich Ridge and the Molloy Ridge is thin (2 to 4 km),compared to the global mean, and is characterised by the absence of oceanic layer 3. Theseobservations can be ascribed to conductive cooling of the ascending mantle due to theextremely low divergence rate and the neighbouring cool continental crust. The underlyingmantle is slightly serpentinized below the Knipovich Ridge segment, reflected by low seismicvelocities of ~7.7 km/s. A thicker sequence of syn- and post rift sediments and sedimentaryrocks are observed on the Molloy Ridge oceanic segment, which results from greater subsidencerelative to the Knipovich Ridge segment.

Description: The increasingly dense coverage of Europe with broad-band seismic stations makes it possible to image its lithospheric structure in great detail, provided that structural information can be extracted effectively from the very large volumes of data. We develop an automated technique for the measurement of interstation phase velocities of (earthquake-excited) fundamental-mode surface waves in very broad period ranges. We then apply the technique to all available broad-band data from permanent and temporary networks across Europe. In a new implementation of the classical two-station method, Rayleigh and Love dispersion curves are determined by cross-correlation of seismograms from a pair of stations. An elaborate filtering and windowing scheme is employed to enhance the target signal and makes possible a significantly broader frequency band of the measurements, compared to previous implementations of the method. The selection of acceptable phase-velocity measurements for each event is performed in the frequency domain, based on a number of fine-tuned quality criteria including a smoothness requirement. Between 5 and 3000 single-event dispersion measurements are averaged per interstation path in order to obtain robust, broad-band dispersion curves with error estimates. In total, around 63,000 Rayleigh- and 27,500 Love-wave dispersion curves between 10 and 350 s have been determined, with standard deviations lower than 2 per cent and standard errors lower than 0.5 per cent. Comparisons of phase-velocity measurements using events at opposite backazimuths and the examination of the variance of the phase-velocity curves are parts of the quality control. With the automated procedure, large data sets can be consistently and repeatedly measured using varying selection parameters. Comparison of average interstation dispersion curves obtained with different degrees of smoothness shows that rough perturbations do not systematically bias the average dispersion measurement. They can, therefore, be treated as random but they do need to be removed in order to reduce random errors of the measurements. Using our large new data set, we construct phase-velocity maps for central and northern Europe. According to checkerboard tests, the lateral resolution in central Europe is ≤150 km. Comparison of regional surface-wave tomography with independent data on sediment thickness in North-German Basin and Polish Trough confirms the high-resolution potential of our phase-velocity measurements. At longer periods, the structure of the lithosphere and asthenosphere around the Trans-European Suture Zone (TESZ) is seen clearly. The region of the Tornquist-Teisseyre-Zone in the southeast is associated with a stronger lateral contrast in lithospheric thickness, across the TESZ compared to the region across the Sorgenfrei-Tornquist-Zone in the northwest.

Description: A 410 km long Ocean Bottom Seismometer profile spanning from the Bear Island, Barents Sea to oceanic crust formed along the Mohns Ridge has been modelled by use of ray-tracing with regard to observed P-waves. The northeastern part of the model represents typical continental crust, thinned from ca. 30 km thickness beneath the Bear Island to ca. 13 km within the Continent–Ocean-Transition. Between the Hornsund FZ and the Knølegga Fault, a 3–4 km thick sedimentary basin, dominantly of Permian/Carboniferous age, is modelled beneath the ca. 1.5 km thick layer of volcanics (Vestbakken Volcanic Province). The P-wave velocity in the 3–4 km thick lowermost continental crust is significantly higher than normal (ca. 7.5 km s–1). We interpret this layer as a mixture of mafic intrusions and continental crystalline blocks, dominantly related to the Paleocene-Early Eocene rifting event. The crystalline portion of the crust within the south-western part of the COT consists of a ca. 30 km wide and ca. 6 km thick high-velocity (7.3 km s–1) body. We interpret the body as a ridge of serpentinized peridotites. The magmatic portion of the ocean crust accreted along the Knipovich Ridge from continental break-up at ca. 35 Ma until ca. 20 Ma is 3–5 km thicker than normal. We interpret the increased magmatism as a passive response to the bending of this southernmost part of the Knipovich Ridge. The thickness of the magmatic portion of the crust formed along the Mohns Ridge at ca. 20 Ma decreases to ca. 3 km, which is normal for ultra slow spreading ridges.

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: Wide-angle seismic data acquired by use of air-guns and Ocean Bottom Seismometers (OBS) contain strong direct water arrivals and multiples, generally considered as noise and thus not included in the modelling. However, a recent study showed that standard ray-tracing modelling of the water multiples recorded off the Bear Island, North Atlantic, provided a reliable estimate of the velocity distribution in the water layer. Along the profile, the water velocity is found to change from about 1450 to approximately 1490 m/s. In the uppermost 400 m the velocities are in the range of 1455-1475 m/s, corresponding to the oceanic thermocline. In the deep ocean there is a velocity decrease with depth, and a minimum velocity of about 1450 m/s is reached at about 1.5 km depth. Below that, the velocity increases to about 1495 m/s at approximately 2.5 km depth. Here, we demonstrate that including the amplitudes in the modelling provide valuable information about the VP contrast at the seafloor, as well as the VP/VS ratio and attenuation (QP) of the uppermost sediments. The VP contrast at the seafloor is estimated at about 250 m/s, within a precision of approximately ±30 m/s. The VP/VS ratio in the uppermost sedimentary layer is modeled in the range 2.25-2.50, and the QP factor is estimated at 1000 for the water and 30-50 and 40-50 for the uppermost sediments. The values obtained for the sediments suggest a lithology dominated by silty clays, with porosity below average.