We demonstrate that the use of long‐offset seismic data allows wide‐angle reflections and diving waves to be recorded, and that these can be used in conjunction with prestack depth migrations to constrain and to image the base of the basalt flows and the underlying structure in the Faeroe‐Shetland Basin. Crustal velocity models are built first by inverting the traveltimes of the recorded reflections and diving waves using ray‐tracing methods. Finer details of the velocity structure can then be refined by analysis of the amplitudes and waveforms of the arrivals. We show that prestack depth migration of selected wide‐angle arrivals of known origin, such as the base‐basalt reflection, using the crustal velocity model, allows us to build a composite image of the structure down to the pre‐rift basement. This has the advantage that the wide‐angle first‐arriving energy must be primary, and not from one of the many multiples or mode‐converted phases that plague near‐offset seismic data. This allows us to ‘tag’ these primary arrivals with confidence and then to identify the same arrivals on higher‐resolution prestack migrations that include data from all offsets. Examples are drawn from the Faeroe‐Shetland Basin, with a series of regional maps of the entire area showing the basalt depths and the thickness of the basalt flows and underlying sediment down to the top of the pre‐rift basement. The maps show how the basalts thin to the southeast away from their presumed source west of the present Faeroe Islands, and also show the extent to which the structure of the pre‐rift basement controls the considerable variations in sediment thickness between the basement and the cap formed by the overlying basalt flows.
We use normal-incidence and wide-angle seismic data recorded on the Faroe Islands to study the crustal structure along two profiles extending east from the islands, across the Faroe shelf and into the Faroe-Shetland Basin. We show that massive basaltic lava flows extend eastward away from the Faroe Islands, having flowed across an older Mesozoic and early Paleocene sedimentary basin, and feathering out near the centre of the Faroe-Shetland Basin. Sediments beneath the basalts reach a thickness of several kilometres in the basin, but do not extend with a resolvable thickness beneath the Faroe Islands. The crustal thickness decreases toward the centre of the Faroe-Shetland Basin, showing that the basement beneath the centre of the basin has been stretched and thinned by a factor of at least two. The Faroe Islands themselves lie on a continental fragment, which had a total thickness of about 10-15 km of igneous rock added as extrusive lavas at the top, and as high-velocity intrusives near the base of the crust at the time of continental break-up in the Paleocene.
The Makran accretionary wedge is one of the largest on Earth. A 7‐km‐thick column of sands and quartzolithic turbidites are incorporated into this wedge in a series of deformed thrust sheets. We present the results of prestack depth migration and focusing‐error analysis (migration velocity analysis) performed on a profile across the Makran wedge. The depth section shows the deformation style of the accreted sediments, and the migration velocities allow us to estimate porosity variations in the sediments. The thrust sheets show evidence of fault‐propagation folding, with a long wavelength of deformation (≈ 12 km) and secondary thrusting in the kink bands of the folds, such that the central part of each thrust sheet is elevated to form an additional ridge. This deformation style and the 15° steep surface slope of the first ridge suggest a high degree of consolidation. Porosities were calculated from the seismic migration velocities and the ratio of fluid pressure to lithostatic pressure λ was estimated for 5 locations along the profile. Rather than being undercompacted and overpressured as in most accretionary wedges, the sedimentary input is normally compacted (exponential porosity decay) throughout almost the whole wedge. However, a slight increase in porosity and λ at depth, with respect to the normal compaction curve indicates, that the turbiditic sequence might be overpressured landward of the deformation front.
Abstract. Subduction accretion and repeated terrane collision shaped the Alaskan convergent margin. The Yakutat Terrane is currently colliding with the continental margin below the central Gulf of Alaska. During the Neogene the terrane's western part was subducted after which a sediment wedge accreted along the northeast Aleutian Trench. This wedge incorporates sediment eroded from the continental margin and marine sediments carried into the subduction zone on the Pacific plate. Prestack depth migration was performed on six seismic reflection lines to resolve the structure within this accretionary wedge and its backstop. The lateral extent of the structures is constrained by high-resolution swath bathymetry and seismic lines collected along strike. Accretionary structure consists of variably sized thrust slices that were deformed against a backstop during frontal accretion and underplating. Toward the northeast the lower slope steepens, the wedge narrows, and the accreted volume decreases notwithstanding a doubling of sediment thickness in the trench. In the northeastemmost transect, near the area where the terrane's trailing edge subducts, no frontal accretion is observed and the slope is eroded. The structures imaged along the seismic lines discussed here most likely result from progressive evolution from erosion to accretion, as the trailing edge of the Yakutat Terrane is subducting.
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