Lewisian

Dalradian

Grampian Orogeny

Caledonian Orogeny

Devonian

Shetland geology

Regional geology of the Northern Isles

 

Simplified terrane map of Scotland.

 

 

 

 

Geological Evolution

If Greenland were restored to its pre-drift position (i.e. prior to Atlantic opening), then Shetland would be equidistant from Greenland, Scotland and Norway. In fact Shetland is an ‘in-situ’ remnant of the Caledonian Orogenic belt formed by the tectonic plates carrying Laurentia, Baltica, Avalonia and Gondwana as they converged to make up the super-continent of Pangaea. Shetland now forms an important link between the East Greenland, Scottish and Norwegian parts of the Caledonian Orogenic belt.

Continents are seen today to be composed of blocks of crust separated by faults and each block can often be of different ages and composition having had very different histories. These blocks, or terranes, are brought together (accreted) by plate collision, the main rock units forming the British Isles are considered to be terranes. Three of the major terranes that have come together to make up Scotland outcrop in close proximity on Shetland.

Blocks of continental or oceanic crust riding piggy-back on their lithospheric plates do not necessarily collide head on to form a new or larger continent, quite often they collide at an oblique angle.  This means those great slices of crust (terranes) on one plate can be displaced laterally along as well as over the other plate. Displacement of one block of crust relative to another occurs along strike-slip (transcurrent) faults. The famous San Andreas Fault system is an example and is carrying coastal California north relative to the rest of North America at an average rate of several centimetres a year. Shetland (and Scotland) is cut by a fault system of comparable magnitude, the Walls Boundary Fault/Great Glen Fault system, (now thankfully extinct). It was movement along this fault and its splays the Nesting Fault and the Melby (St. Magnus Bay) Fault that brought the terranes that make up Shetland to their present day relative positions.

Like much of Scotland, the cover rocks of Shetland are supported by a platform of Late Archean (2500-3000 Ma) ‘Lewisian’ basement gneiss which is the northward extension of the Hebridean Craton. Historically, the Shetland metamorphic cover is called the East Mainland Succession with the Yell Sound Division (Moine) at its base, overlain by the Scatsta, Whiteness and Clift Hills Divisions (Dalradian). These divisions have been correlated with parts of the metamorphic cover of Scotland. Old Red Sandstone basins have formed in the metamorphic cover and plutonic complexes cut all the cover units.