encompasses the final closure of Iapetus and several phases of thrusting,
deformation, magmatism and strike-slip faulting due to oblique multiple plate
collision from the Early Silurian through to the Late Devonian, c.
440-360 Ma. It was during this period that the fault bounded terranes
of the British Isles, each carrying its own distinctive geological
inheritance, came together although strike-slip faulting probably continued
through until Jurassic times.
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.
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.
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.
Shetland the plutonic complexes form two groups; those east of the Walls
Boundary Fault and those west of the Fault, (Fig. 2). 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.