The Walls Boundary Fault is a major transcurrent fault that forms a shallow ‘S’ bend as it extends from south of Fair Isle, through Mainland, to continue northward along Yell Sound. This fault has a long and complex history, at times linking to the Great Glen and Helmsdale Faults of Scotland. Movements along the WBF may have occurred from the Devonian through to the Carboniferous producing perhaps as much as 170km of sinistral displacement. Later, possibly during the Jurassic, there may have been 65km dextral displacement on the WBF. About this time a splay of the WBF, the Nesting Fault, formed producing 16km of dextral displacement.
A major control on the understanding of Caledonian events is the nature of the Highland Boundary Fault and the Great Glen - Walls Boundary Fault systems. The postulated large sinistral displacements along these faults place the Northern Highland and Grampian terranes well to the north-east to lie opposite Scandinavia in the Early Silurian. Here the collision of Baltica with Laurentia caused the thrusting to the north-west of Moine rocks over the Lewisian basement, characterised by the Moine Thrust Zone of Scotland.
The Lewisian basement in north-west Mainland is in sheared contact to the east with an imbricate stack of interleaved metasediments, Lewisian basement and inlier type gneisses and garnet mica-schists. This is believed to be the manifestation of the Moine Thrust in Shetland, although it is possible that this is an older structure than the Moine Thrust and the true thrust lies off-shore to the west of Shetland.
Continued sinistral strike-slip movements through the later Silurian to Devonian times juxtaposed the terranes of the Northern and Central Highlands with those south of the Highland Boundary fault. These fault systems extended deep into the crust so also acted as zones of weakness to become an important control on the siting and ascent of magmas that were to become the Caledonian ‘Newer Granites’. These ‘newer granites’ have a calc-alkaline geochemistry indicative of subduction related volcanic arcs, e.g. magma generation by the subduction of Iapetus oceanic crust beneath Laurentia. These magmas probably ascended the fault zones as dykes and coalesced in mid to high crustal levels to form characteristic sub-rounded to elongate steep-sided plutons that we now see exposed by erosion.
In Shetland the plutonic complexes form two groups; those east of the Walls Boundary Fault and those west of the Fault. The older group, the eastern group c. 400 Ma, is cut by the Walls Boundary and Nesting Faults and was intruded into the rocks of the East Mainland Succession at the end of the Caledonian Orogeny but before the deposition of the Old Red Sandstone (ORS) intermontane basins. The western group is post-orogenic (Early Carboniferous. c. 350 Ma) and occupies much of the area west of the Walls Boundary Fault and is truncated to the east by this fault.
suites of dykes, mainly lamprophyre, occur east of the WBF but, apart from an
isolated trachytic dyke, do not cut the ORS. West of the WBF, swarms of basic
and acid dykes occur widely through the North and West Mainland. Here most of
the dykes are younger than the granites they cut, but some basic dykes are older
than the granites.