Faults

Diagram to illustrate a strike slip faultZoom Diagram to illustrate a reverse faultZoomThe 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. ?These forces caused 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. The most recent movement along the Melby Fault may have been reverse (vertical).