We present analysis of new gravity data to produce a 2D crustal and upper mantle density model across the northern Main Ethiopian Rift (NMER). The magmatic NMER is believed to represent the transitional stage between continental and oceanic tiffing. We conclude that beneath our profile, magma emplacement into the upper crust occurs in the form of a 20 kmwide body beneath the axis of the rift, and a 12 km-wide off-axis body beneath the NW margin of the rift. These are coincident with Quaternary volcanic chains, anomalies in seismic velocity and conductivity identified by the Ethiopia Afar Geoscientific Lithospheric Experiment (EAGLE) along the same profile. We also identify a shallow, high-density body beneath the axial Boset volcano interpreted as either a dyke zone or a magma reservoir that may have fed Quaternary felsic volcanism. Our results provide supporting evidence for a c. 15 km-thick mafic underplate layer beneath the northwestern rift flank, imaged by the EAGLE controlledand passive-source seismic data. A relatively low-density upper mantle is required beneath the underplate and the rift to produce the long wavelength features of the gravity anomaly. The resulting model suggests that the lithosphere to the SE of the rift is unaffected by rifting processes. Our results combined with those from other EAGLE studies show that magmatic processes dominate rifting in the NMER.

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The South Falkland basin is a partially filled Cenozoic foreland basin located south of the Falkland Plateau. It was formed by flexure of this southern edge of the South American plate when the load represented by Burdwood Bank collided. This continental fragment belongs to the predominantly oceanic Scotia plate. Flexure probably started in early Cenozoic times and has continued to the present day. The whole region is submarine and so the detailed stratigraphy and structure of the basin has been well imaged by seismic reflection profiling. The clarity of this imagery has made analysis of structures within the collision zone possible. The plate boundary itself is an active oblique thrust fault which has controlled the growth of a frontal fold. There is evidence for older phases of thrusting and folding further south. Within and beneath the sediments which blanket the flexed South American plate, normal faulting occurs on a variety of scales. Episodes of stratigraphic growth associated with the largest of these faults demonstrates that they were active during flexural bending. We have modeled the development of the South Falkland basin using two different approaches, both of which are based upon the simplest elastic model. Inverse modeling of free‐air gravity and bathymetric profiles suggest that the elastic thickness of the loaded crust is 5–20 km. A complementary approach based upon the spectral analysis of free‐air gravity and bathymetry shows that the elastic thickness is 15 ± 5 km. Both techniques indicate that the flexed continental lithosphere is weak, a conclusion supported by the presence of normal faults within the flexed plate. A small increase in elastic thickness from west to east appears consistent with a change in the density and penetration of normal faulting.

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Two wide‐angle seismic lines located in the northern Rockall Trough were acquired in May 2000. One line (line E) crosses the trough from the continental shelf off Lewis to normal oceanic crust west of Lousy Bank in NW‐SE direction. The other line (line D) intersects with line E, crosses the Wyville‐Thomson Ridge in a SW‐NE direction and ends in the Faeroe‐Shetland Basin. Sonobuoy data and expanding spread profiles acquired in the same area have been remodeled. Analysis of the seismic data using travel times and amplitudes reveals an up to 5 km thick sedimentary basin including an up to 1.5 km thick basaltic layer which is present in most of the trough. Further conclusions of this study are that the Rockall Trough is underlain by highly stretched continental crust of ∼13 km thickness. The crust thickens to ∼24 km beneath Lousy Bank, which is interpreted to be of continental nature. Beneath the Hebrides continental shelf a three‐layer continental crust of 26 km is modeled. An up to 12 km thick high‐velocity layer is observed underneath the ocean‐continent boundary and is interpreted as magmatic underplating resulting from excess volcanism during rifting. No evidence for an underplate layer could be distinguished beneath the trough area. Modeling of the structure of the Wyville‐Thomson Ridge revealed no existing igneous core of the ridge confirming existing theories, that it is a compressional structure.

SUMMARY This study images upper‐mantle structure beneath different tectonic and geomorphological provinces in southern Scandinavia by P‐wave traveltime tomography based on teleseismic events. We present results using integrated data from several individual projects (CALAS, MAGNUS, SCANLIPS, CENMOVE and Tor) with a total of 202 temporary seismological stations deployed in southern Norway, southern Sweden, Denmark and the northernmost part of Germany. These stations, together with 18 permanent stations, yield a high density data coverage and enable presentation of the first high resolution 3D seismic velocity model for the upper mantle for this region, which includes the entire northern part of the prominent Tornquist Zone and the Southern Scandes Mountains. P‐wave arrival time residuals of up to ±1 s are observed indicating large seismic velocity contrasts at depths. Relative regional as well as absolute global tomographic inversion is carried out and consistently show upper‐mantle velocity variations relative to the ak135 global reference model of up to ±2–3 per cent corresponding to P‐wave velocity differences of 0.4–0.5 km s–1 from depths of about 100 km to more than 300 km. High upper‐mantle velocities are observed to great depth to the east in Baltic Shield areas of southwestern Sweden suggesting the existence of a deep lithosphere keel. Lower velocities are found to the west and southwest beneath the Danish and North German sedimentary basins and in most of southern Norway. A well defined, generally narrow and deep boundary is observed between areas of contrasting upper‐mantle seismic velocity. In the southern part of the study area, this boundary is localized along and east of the Sorgenfrei–Tornquist Zone. It seems to follow the eastern boundary of a zone of significant Late Carboniferous–Permian volcanic activity from southwestern Sweden to the Oslo Graben area. To the north, it crosses shield units, Caledonides as well as areas of high topography. Supported by independent results of surface wave studies, we interpret this velocity boundary as a first order lithosphere boundary representing the southwestern edge of thick shield lithosphere. In basin areas to the southwest, low upper‐mantle velocities are associated with asthenosphere beneath thinned lithosphere and velocity contrasts are likely to arise mainly from temperature differences. To the north structural and geodynamic relations are more complex and both temperature and compositional differences may play a part. Reduced upper‐mantle velocity beneath southern Norway also seems, despite relatively low heat flow, to be associated with areas of thinned lithosphere, pointing towards increased temperatures and reduced density in the upper mantle. This feature extends over large areas and seems not directly correlated to the shorter wavelength high topography of the Scandes Mountains, but may contribute with some isostatic buoyancy on a regional scale. For this northern area, there is no obvious geodynamic explanation to reduced upper‐mantle velocity. A...

S U M M A R YA regional model of the 3-D variation in seismic P-wave velocity structure in the crust of NW Europe has been compiled from wide-angle reflection/refraction profiles. Along each 2-D profile a velocity-depth function has been digitised at 5 km intervals. These 1-D velocity functions were mapped into three dimensions using ordinary kriging with weights determined to minimise the difference between digitised and interpolated values. An analysis of variograms of the digitised data suggested a radial isotropic weighting scheme was most appropriate. Horizontal dimensions of the model cells are optimised at 40 × 40 km and the vertical dimension at 1 km.The resulting model provides a higher resolution image of the 3-D variation in seismic velocity structure of the UK, Ireland and surrounding areas than existing models. The construction of the model through kriging allows the uncertainty in the velocity structure to be assessed. This uncertainty indicates the high density of data required to confidently interpolate the crustal velocity structure, and shows that for this region the velocity is poorly constrained for large areas away from the input data.

The post-Caledonian development of the West Orkney Basin is regularly cited as a classic example of basement-influenced rifting. This paper presents the first detailed multidisciplinary analysis of the three-dimensional (3D) geometries and distribution of post-Caledonian faults in onshore northernmost Scotland, examining their relationships to basement fabrics and comparing them to rift-related structures developed offshore in the West Orkney Basin. Two phases of rift-related faulting are distinguished: 1) Devonian ENE-WSW extension localized in the east of the basin and related to regional sinistral transtension along the Great Glen Fault; and 2) Permo-Triassic NW-SE extension focused to the west of the basin and probably contemporaneous with movements along the Minch Fault.