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The Western Isles–North Channel (‘WINCH’) deep seismic reflection profile runs through the North Channel across the extension of the Midland Valley into the Firth of Clyde.A variety of shallow Carboniferous and younger sedimentary basins dominates the upper crustal structure. The deep structure includes a reflective lower crust, bounded downwards by the Moho, and dipping ‘thrusts’. No margins to the Midland Valley are observed equivalent to the bounding faults on land. Thrusts below both the Highlands and the Southern Uplands have a variety of dip directions indicative of zig-zag crustal shortening of 100 km in the Highlands and 60 km in the Southern Uplands.The Moho varies only a little in depth, despite the gross crustal deformation in the Caledonides. Isostatic recovery of the Moho on unlocking of collided crustal blocks may explain the formation of some sedimentary basins.

BIRPS deep seismic refection data collected on the western United Kingdom continental shelf show the existence of a wide variety of sedimentary basins, most of which originated during Palaeozoic-Mesozoic crustal extension. Symmetrical interior-fracture basins are numerous, but more complex basins are also common and show the importance of fault reactivation and the influence of pre-existing structures on basin development. All of the major basement-penetrating faults are interpreted as having been reactivated. They do not visibly cut through the entire crust and into the upper mantle. The crystalline crust thins dramatically beneath the basins indicating local crustal extensions of up to 60%, although regional extension is less than 30%.The typical BIRPS profile shows a highly reflective lower crust sandwiched between an unreflective upper crust and upper mantle. This pattern of reflectivity appears to be characteristic of deep seismic data collected from within extended regions. The relatively small amount of extension which has affected this region suggests that the highly reflective lower crust is more likely to be due to lithological variation formed by mafic igneous intrusion and underplating during crustal extension than to-extensional ductile fabrics within the lower crust. =SWAT profiles 2 to 7 and 9. Stipple pattern marks the extent of the post-orogenic basins crossed by the profiles: CB =Colonsay basin; CBB = Cardigan Bay basin; CISB = Central Irish Sea basin; ECB = Eastern Channel basin; FCB = Firth of Clyde basin; IHB =Inner Hebrides basin; LIB = Loch Indaal basin; MB = Malin basin; McB = Minch basin; NCB = North Channel basin; NCSB = North Celtic Sea basin; NLB = North Lewis basin; OHB = Outer Hebrides basin; PeB = Peel basin; PB = Portpatrick basin; PBB = Plymouth Bay basin; RT =Rathlin Trough; SB = Solway basin; SCSB = South Celtic Sea basin; SGB =St George's Channel basin; SHB = Sea of Hebrides basin; ST = Stanton Trough; WAB =Western Approaches basin; WCB =Western Channel basin; WOB=Western Orkneys basin. CP=Carnsore Point. Major faults shown: BFS =Bala fault system;