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We use broadband seismic data acquired by the St. Elias Erosion/Tectonics Project (STEEP) and the Alaska Earthquake Information Center to image the geometry of the subducting Yakutat Block in southeast Alaska. We combine results for both P-and S-wave receiver functions. P-wave to S-wave data were imaged with a fully three-dimensional wavefi eld imaging method centered on the STEEP region. The S-wave to P-wave data were imaged using a simpler common conversion point stacking method at two scales: a regional scale covering all of southeast Alaska and a smaller scale identical to the P-wave data. Our data confi rm that the southeastern Alaska subduction zone extends from the eastern end of the Aleutian Trench an additional 300 km to the Fairweather-Queen Charlotte fault system. We also locate the boundary between the Yakutat Block and North American Plate. We fi nd direct evidence that the subducted Yakutat Block and Pacifi c plate slabs are continuous and that Yakutat Block subduction extends from Prince William Sound to the east at least as far as Icy Bay. The dip angle of the slab ranges from 11° to 16° with a gradual increase from west to east across this region. Our data show a clear separation between the subducted Yakutat Block and the North American Plate under the Alaska Range, suggesting that deformation along the Denali fault and interior Alaska is not the product of coupling between North America and the subducted Yakutat Block. This dip angle also places the subducted Yakutat Block at the proper depth to produce arc magmatism found in the Wrangell volcanic fi eld. Modeling the geometry of the system suggests that sedimentary cover is being stripped at the western side of the Yakutat Block and the lower crust of the Yakutat Block is involved in the subduction. The system transitions from a single dipping megathrust on the western side of the Yakutat Block to intense shortening in the vicinity of Mount Saint Elias, where we suggest that the lower crust is likely undergoing ductile deformation and has thickened more than 60 km under the Peninsular terrane.

Summary VARNET, an international multidisciplinary project, was designed to examine the ‘Variscan Front’ in the southwest of Ireland. As part of the seismic experiment a 200 km long wide‐angle seismic profile (Line A) was recorded from the Old Head of Kinsale in the south of Ireland to Galway Bay on the west coast. Along Line A, 170 seismic stations were deployed at approximately 1 km intervals. Results from ray trace modelling indicate a multilayered crust. The upper crust, extending to a depth of about 14 km, is laterally variable. South of the Shannon Estuary sedimentary basins alternate with uplifted basement (5.8–6.0 km s‐1). This correlates well with the surface geology. The larger sedimentary basin in the south reveals a shallowly south‐dipping structural feature that coincides with the Killarney–Mallow Fault Zone. North of the Shannon Estuary surprisingly high velocities (6.4 km s‐1) at shallow depths were observed. Between the upper and middle crust there is a transition layer about 2–4 km thick. The middle crust thickens towards the Shannon Estuary. These structural changes coincide with the proposed location of the Iapetus Suture Zone. The middle crust is underlain by a lower crust with a variable velocity structure, and the total crustal thickness varies from 29–32 km. Detailed traveltime investigations have also revealed a reflector in the upper mantle at depths between 39 and 44 km. The seismic model shows that there are no deep crustal variations beneath the Killarney–Mallow Fault Zone, indicating a thin‐skinned mechanism of deformation.

This paper reviews the complex crustal and upper-mantle seismic velocity structure of Ireland and surrounding seas. Data from 11 seismic refraction profiles reveal that onshore Ireland mean crustal velocities range between 6.25 and 6.5 km s )1 with crustal thickness of 28.5-32 km. Superimposed on a threelayer crust, the sedimentary layer has a thickness of approximately 6-8 km at the southern coastline, but only 3-4 km in the vicinity of the Shannon Estuary in western Ireland. The lateral heterogeneity of the upper-crustal layer is pervasive throughout Ireland, with velocities of 5.7-6.2 km s )1 and a layer thickness of 3-10 km. A low-velocity zone is found in the south-east which is interpreted as the buried south-western extension of the Leinster Granite. The mid-crustal layer (6.3-6.7 km s )1 ) is between 8 and 16 km thick. Significant changes occur in the vicinity of the Shannon Estuary, around the location of the Iapetus Suture Zone. The lower crust is fairly uniform with velocities of 6.8-7.2 km s )1 and a thickness of approximately 8-10 km except towards the south of Ireland where the Moho appears as a transition zone. Offshore Ireland, a twolayer crust with a thickness of 24-26 km beneath the North Celtic Sea Basin and only 14-15 km beneath the Rockall Trough prevails.Irish crust reviewed • M. Landes et al.Irish crust reviewed • M. Landes et al.

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