Beschreibung: In order to study the lithospheric structure in southern Ukraine, a seismic wide-angle reflection/refraction project DOBRE-4 was conducted. The 500-km-long profile starts in the SW from the Alpine/Variscan North Dobrudja Fold-Thrust Belt, being part of the Trans-European Suture Zone. It runs to the NE, mostly along the NW Black Sea coastal plain, towards the centre of the Precambrian Ukrainian Shield. The field acquisition in October 2009 included 13 chemical shot points with charge sizes 600–1000 kg every 35–50 km and 230 recording stations, every ~2.5 km. The high data quality allows modelling of the P - and S -wave velocity structure along the profile. Two methods were used for the modelling of the seismic data. At first, ray tracing trial-and-error modelling was developed using arrivals of major refracted and reflected P - and S -wave phases. Next, the amplitudes of the recorded phases were analysed using finite-difference full waveform method. The resulting velocity model shows fairly homogeneous structure of the middle to lower crust both vertically and laterally. The situation is different in the upper crust, with V p velocities decreasing upwards from ca . 6.35 at 15–20 km to 5.9–5.8 km s –1 at the top of the crystalline basement and to ca . 5.15–3.80 km s –1 in Neoproterozoic and Palaeozoic and to 2.70–2.30 km s –1 in Mesozoic strata. Below the upper crust the V p smoothly increases downward, from ca . 6.50 to 6.7–6.8 km s –1 near the crustal base, making it difficult to differentiate between the middle and lower crust. No V p velocities exceeding 6.80 km s –1 have been recorded even in the lowermost part of the crust, unlike in similar profiles on the East European Craton. There is no clear change in the velocity field when moving laterally from the Precambrian platform into the younger tectonic units to the SW. Therefore, on purely seismic grounds it is not possible to distinguish major tectonic units known from the surface. The Moho is, however, clearly delineated by a velocity contrast of ca . 1.3–1.7 km s –1 . A specific feature of the velocity model are waveform successive changes in Moho depth, corresponding to successive downward and upward bends, with wavelength of the order of 150 km and the amplitude attaining 8–17 km. Similar wavy aspect is shown by the upper mantle and upper crust, with shorter wavelength pattern in the latter. The origin of the undulations is explained by compressional lithospheric-scale buckling and ascribed to Late Jurassic–Early Cretaceous and/or end Cretaceous collision-related tectonic events associated with closing of the Palaeotethys and Neotethys oceans in this part of Europe. To our knowledge, no such spectacular folds deforming the Moho, have been as yet revealed elsewhere by either deep reflection or refraction seismics. The presence of several detachment horizons in the folded crust calculated in the velocity model, is compatible with the existence of fold systems with various dominant wavelengths at different crustal levels. Such a situation is considered as typical of lithospheric-scale folding and reflecting the rheological stratification of the lithosphere.

Beschreibung: The Scythian Platform (ScP) with a heterogeneous basement of Baikalian–Variscan–Cimmerian age is located between the East European Craton (EEC) on the north and the Crimean–Caucasus orogenic belt and the Black Sea (BS) Basin on the south. In order to get new constrains on the basin architecture and crustal structure of the ScP and a better understanding of the tectonic processes and evolution of the southern margin of the EEC during Mesozoic and Cenozoic time, a 630-km-long seismic wide-angle refraction and reflection (WARR) profile DOBRE-5 was acquired in 2011 October. It crosses in a W–E direction the Fore-Dobrudja Trough, the Odessa Shelf of the BS and the Crimean Plain. The field acquisition included eight chemical shot points located every 50 km and recorded by 215 stations placed every ~2.0 km on the land. In addition, the offshore data from existing profile 26, placed in the Odessa Shelf, were used. The obtained seismic model shows clear lateral segmentation of the crust within the study region on four domains: the Fore-Dobrudja Domain (km 20–160), an offshore domain of the Karkinit Trough at the Odessa Shelf of the BS (km 160–360), an onshore domain of the Central Crimean Uplift (Crimean Plain, km 360–520) and the Indolo-Kuban Trough at the Kerch Peninsula (km 520–620) that is the easternmost part of the Crimea. Two contrasting domains of the ScP within the central part of the DOBRE-5 profile, the Karkinit Trough and the Central Crimean Uplift, may represent different stages of the ScP formation. A deep Karkinit Trough with an underlying high-velocity (〉7.16 km s –1 ) lower crust body suggests its rifting-related origin during Early Cretaceous time. The Central Crimean Uplift represents a thick (up to 47 km) crustal domain consisting of three layers with velocities 5.8–6.4, 6.5–6.6 and 6.7–7.0 km s –1 , which could be evidence of this part of the ScP originating on the crust of Precambrian craton (EEC). The thick heterogeneous basement of the Central Crimean Uplift shows inclusions of granitic bodies associated with magmatic activity related with Variscan orogeny within the ScP. General bending and crustal scale buckling of the Central Crimean Uplift with a wavelength of 230 km could be an effect of the Alpine compressional tectonics in the adjacent Crimean Mountains. The extended/rifted continental margin of the ScP (EEC) at the Odessa Shelf and buckling/uplifted domain of the Central Crimean Uplift affected by compressional tectonics, are separated by the N–S oriented Western Crimean Fault. The crust of the southern margin of the EEC is separated from the ScP, which originated on the EEC crust tectonised and reworked during the Palaeozoic–Mesozoic, by the crustal fault of ~W–E orientation, which corresponds with the Golitsyn Fault observed at the surface between the EEC and the ScP. The Fore-Dobrudja Domain with a thick (〉10 km) heterogeneous basement and two subhorizontal layers in the crystalline crust (with velocities 6.2–6.3 and 6.4–6.65 km s –1 ) differs from the ScP crust and its origin could be very similar to that of the Trans-European Suture Zone and Palaeozoic West European Platform.

Beschreibung: Wide angle refraction and reflection measurements were carried out in the passive continental margin zone of the northwestern Svalbard during several expeditions in 1978–1999. Data from a set of 2-D archival and modern seismic profiles recorded in-line and off-line, and from an additional permanent seismic station, were altogether used for seismic modelling of the crustal structure of the study area. Seismic arrivals (airgun and chemical explosive sources) were recorded by land (onshore) seismic stations, ocean bottom seismometers (OBS), and ocean bottom hydrophone stations (OBH). Good quality refracted and reflected P waves have provided an excellent data base for a seismic modelling. Chemical explosive sources were recorded even up to 300 km distances. The 3-D tomographic inversion method was applied. The results are comparable to the earlier 2-D modelling. Additional off-line information allowed to develop a 3-D image of the crustal structure. The continental crust thins to the west and north. A minimum depth of about 6 km to the Moho interface was determined east of the Molloy Deep and in the Knipovich Ridge. The Moho discontinuity deepens down to about 30 km below the continental crust of Spitsbergen.

Beschreibung: 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.

Beschreibung: 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.