ALBERT

Description: Seismic reflection data from the Danish North Sea are interpreted to map the structure of the Palaeozoic basement in the area of the MONA LISA deep seismic lines. Based on a characteristic near-basement reflection, the upper crystalline crust of Baltica of offshore Denmark is traced to the south into the southern Horn-Graben, and to the west to the eastern shoulder of the Central Graben. A two-way-traveltime map of the near-basement horizon and several interpreted seismic sections reveal that three main tectonic events influenced the topography of the basement: (1) a compressional event which could be Caledonian in age; (2) a Palaeozoic extensional event postdating the compressional deformation and expressed in a system of WSW-ESE to W-E striking Palaeozoic half-grabens; and (3) the Permo-Triassic rifting that led to the formation of NNW-SSE to NNE-SSW trending Mesozoic faults of the Horn Graben and the Central Graben which are oriented sub-perpendicular to the Palaeozoic system. Compressive deformation is localized in a narrow zone around and south of the hitherto interpreted Caledonian Deformation Front and foreland deformation on Baltica is suggested as its origin. The seismic image of the Palaeozoic halfgrabens indicates that the East North Sea High is an inverted Palaeozoic rift which subsequently was cut by younger Late Palaeozoic to Mesozoic rifts of the Horn and Central Grabens. The timing of the first extensional phase remains speculative, but it predates the Rotliegend unconformity. Some of the older Palaeozoic normal faults may have been reactivated as transfer zones between the different graben segments during the Permo-Mesozoic extension.

Description: Industry seismic reflection data, oil test well data, interpretation of gravity and magnetic data, and seismic refraction deep-crustal profiles provide new perspectives on the subsurface geology of San Fernando Valley, home of two of the most recent damaging earthquakes in southern California. Seismic reflection data provide depths to Miocene-Quaternary horizons; beneath the base of the Late Miocene Modelo Formation are largely nonreflective rocks of the Middle Miocene Topanga and older formations. Gravity and seismic reflection data reveal the North Leadwell fault zone, a set of down-to-the-north faults that does not offset the top of the Modelo Formation; the zone strikes northwest across the valley, and may be part of the Oak Ridge fault system to the west. In the southeast part of the valley, the fault zone bounds a concealed basement high that influenced deposition of the Late Miocene Tarzana fan and may have localized damage from the 1994 Northridge earthquake. Gravity and seismic refraction data indicate that the basin underlying San Fernando Valley is asymmetric, the north part of the basin (Sylmar subbasin) reaching depths of 5-8 km. Magnetic data suggest a major boundary at or near the Verdugo fault, which likely started as a Miocene transtensional fault, and show a change in the dip sense of the fault along strike. The northwest projection of the Verdugo fault separates the Sylmar subbasin from the main San Fernando Valley and coincides with the abrupt change in structural style from the Santa Susana fault to the Sierra Madre fault. The Simi Hills bound the basin on the west and, as defined by gravity data, the boundary is linear and strikes [~]N45{degrees}E. That northeast-trending gravity gradient follows both the part of the 1971 San Fernando aftershock distribution called the Chatsworth trend and the aftershock trends of the 1994 Northridge earthquake. These data suggest that the 1971 San Fernando and 1994 Northridge earthquakes reactivated portions of Miocene normal faults.

Description: 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.

Description: New insights in crustal structure in southern Norway are given by combining stacking techniques and traveltime tomography of 3-D wide-angle reflection/refraction data. The Magnus Rex crustal scale wide-angle refraction/reflection data set in Southern Norway covers an area of 400 km 430 km where 716 receivers on three profiles recorded seismic waves from 26 explosive sources. Previous data analysis focused on 2-D interpretation along the profiles. Here we extract additional P -wave velocity information by inverting inline and cross-line data simultaneously. We combine stacking routines, traveltime tomography, and interpolation algorithms to the high quality inline and cross-line data. A smooth 3-D crustal velocity model is inverted from traveltimes of diving Pg waves with similar results for two initial models. Initial models include a 1-D average model and an interpolated 3-D model based on robust, local 1-D velocity-depth functions derived from CMP-sorted and stacked records. The depth to Moho is determined from reflected waves ( PmP ) by traditional exploration seismology processing routines (CMP sorting, NMO correction, stacking, depth conversion). We find that this combination of stacking methods and traveltime tomography is well suited to exploit sparse 3-D wide-angle data. The results along the profiles are similar to the earlier 2-D models and the 3-D velocity model shows little lateral variation. The crust in SW Norway is generally 35–40 km thick and has relatively low average velocity, as it lacks the characteristic high-velocity lower crust, otherwise observed in the Baltic Shield. However, the Oslo Graben is characterized by high crustal velocities and a slightly elevated Moho. Our results suggest that this crustal structure continues towards the north along the strike of the graben.