The rocks that form Shetland built up as separate blocks of the Earth’s crust over a period of almost three billion years. These blocks, known as terranes, formed perhaps hundreds of kilometres apart and under different conditions. The terranes were brought together in their present position by vertical and lateral (sideways) movement along faults as continents collided and mountains were built up and then pulled apart.
The Caledonian Mountain Chain, which included the Shetland ophiolite, was partly built up by compression forces acting along a series of low angle reverse or ‘thrust’ faults that pushed rocks of one terrane up and over the rocks of another.
Once the mountain chain had been built up the direction of the tectonic plate movements beneath changed and the mountain chain began to be pulled apart along normal faults. Wide valleys, known as pull-apart basins, formed between the mountains as rocks slipped vertically down past each other due to the tensional forces pulling on the crust at either side of the faults. These basins quickly filled with sediment eroded from the mountains on either side and are often known as 'Old Red Sandstone Basins'.
These tensional forces also forced blocks of crust to slide horizontally past each other for many hundreds of kilometres along a series of strike-slip faults that were active from about 435 to about 175 million years ago. The major strike slip fault system that brought the terranes of Shetland together is the Great Glen/Walls Boundary Fault that slices through Scotland and Shetland. On Shetland there are two major off-shoots or ‘splays’ of this fault - the Melby Fault and the Nesting Fault – although the most recent movement along the Melby Fault may have been reverse.
Periodic movement along the faults as strain built up and released triggered earthquakes and crushed or deformed and altered rocks along the fault zones. The fault zones cut deep into the crust beneath the mountains, and as lines of weakness acted as conduits for the rising magma that formed the intrusive rocks within the mountains or volcanoes in the sedimentary basins.