The Shetland ophiolite was derived from an intra-continental, ocean-floored basin that opened shortly after 600 Ma on the eastern edge of Laurentia, during the tensional opening of the Iapetus Ocean. The precursor sedimentary and volcanic extensional basin is exposed in the south of Shetland and can be followed northeastwards by offshore geophysical anomalies. Eastward subduction of the floor of the basin led to hydration of the descending slab and the overlying mantle wedge. At about 500 Ma pressure from the east caused westward obduction of part of the subducting slab. The obduction thrust originated in the overlying mantle wedge and cut through the descending slab from above into underlying mantle. The result is a nappe cutting steeply through the sub-ocean-floor layers. The obducted nappe over-rode highly metamorphosed Shetland–Dalradian schists and gneisses and now forms the Lower Nappe of the Shetland Ophiolite Complex. The exposed pseudo-stratigraphic succession extends from several kilometres deep in the mantle, as exposed in the west, through the lower crust to the base of the sheeted dykes in the east, before passing beneath the sea; a surface distance of some 6 km. The ophiolitic pseudo-stratigraphy occurs as steeply dipping layers within the nappe, which gravity modelling shows to be about 3 km thick. The section through the ophiolite is lying on its side. After its obduction, erosion of the nappe and nearby metamorphic basement led to deposition of fine-grained, laminated siliceous sediments, locally graphitic and conglomeratic. Resumed pressure from the east during the Scandian orogeny telescoped the Lower Nappe, thereby emplacing an Upper Nappe over the Lower Nappe. The two nappes are separated by an imbricate zone incorporating siliceous sediments, ophiolitic hornblende schist and slices of the ophiolite nappes. The Skaw granite was intruded at about 425.6 Ma during this phase.