DEEP SEISMIC-REFLECTION STRUCTURE OF THE CALEDONIAN OROGENIC FRONT WEST OF SHETLAND

Although the Moine Thrust has been well studied on land in northwester n Scotland, the subject of its deep crustal structure and regional con tinuation offshore remains controversial. Here, we use post-stack migr ation and filtering of deep seismic reflection profiles crossing the p rojection of the Moine Thrust and coincident Caledonian orogenic front around the western margin of the Shetland Platform in order to charac terize the crustal structure of the thrust in an area not substantiall y affected by basin formation and extension, and where onshore geologi cal constraints can be more readily applied. This study provides for t he first time seismic reflection constraints on middle and lower crust al structure down to the level of the Moho discontinuity underlying th e thrust. The seismic data reveal a thick wedge of east-dipping reflec tions in the lower crust capped by a highly coherent reflection which can be correlated to the postulated Moine Thrust as previously mapped on Shetland. The continuation of this wedge from the middle crust down to the Moho, against which individual reflections are abruptly trunca ted, suggests a 'thick-skinned' interpretation for the Caledonian orog en beneath the western flank of the Shetland Platform. The available s eismic data from around Shetland imply that the Moine Thrust/Caledonia n front forms a boundary between an upper plate which is relatively fe atureless structurally and a lower plate composed of interpreted north west-verging thrusts, possibly a crustal duplex, structurally below th e level of the Moine nappe. Our observations are thus in contrast with previous interpretations made from seismic data in the West Orkney Ba sin north of the Northwest Highlands that describe a highly reflective Moinian upper plate.