Geological structure of Great Britain

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The geological structure of Great Britain is complex, resulting as it does from a long and varied geological history spanning more than two billion years. This piece of the Earth's crust has experienced several episodes of mountain building or 'orogenies', each of which has added further complexity to the picture.

A wide range of geological structures occur across Britain and include examples at a variety of scales of:

Our understanding of Britain's large-scale structure has been gained over many decades by simple

gravity surveys, seismic surveys, aeromagnetic surveys and other forms of remote sensing
.

Terranes

A useful approach to considering Britain's geological structure is to examine the various terranes from which it is composed. These are essentially continental fragments whose boundaries are generally defined by faults. Individual terranes typically contain suites of structures, the histories and form of which differ from those of neighbouring terranes.

Terranes of Scotland

The

Moine Thrust, beyond which lies the Northern Highlands Terrane. This area in turn abuts against the Central Highlands (or Grampian) Terrane along the Great Glen Fault. Similarly the Highland Boundary Fault separates the Central Highlands Terrane from the Midland Valley Terrane, which is itself separated from the Central - Southern Uplands Terrane by the Southern Uplands Fault. Each of these terranes form a part of the former continent of Laurentia whose southeastern margin is defined by the Iapetus Suture
Zone, which runs parallel to the English-Scottish border, though some miles to its south.

Terranes of England and Wales

The Central - Southern Uplands Terrane extends across the northernmost part of England. Its southern margin is the Iapetus Suture, south of which lies the

Lleyn peninsula are deemed to be composed of numerous micro-terranes, known collectively as the Rosslare - Monian Terranes.[1]

Pennine Block & Basin Province

Northern England is characterised by a series of fault-bound blocks separated by sedimentary basins whose origin can be traced back to the Carboniferous period. The North Pennines are formed on the Alston Block which is defined to the west by the Pennine Fault and to the north by the Stublick and Ninety Fathom Faults. It is separated from the Askrigg Block to the south by the Stainmore Trough. This latter block, coincident with the Yorkshire Dales, is defined to the west by the Dent Fault and to the south by the Craven Fault System. The Northumberland and Gainsborough Troughs lie to the north and south of these two blocks.

Structural legacy of mountain-building episodes

Another approach to the study of the geological structure of the area is through consideration of the variety of structures resulting from each of several orogenies (or 'mountain-building' episodes) which have taken place over geological history. Structures originating in one event may play a part in subsequent orogenic events and be modified by them. Thus lines of crustal weakness commonly associated with, for example, the Caledonian Orogeny will often predate this particular mountain-building period, much as some of those created during this phase were reactivated during later events.

The Caledonian Orogeny

Main article: Caledonian orogeny

The

Shetland Islands
. Key structures include:

Each of these structures is aligned northeast - southwest, albeit with the more northerly of them trending closer to NNE - SSW. A map or satellite photo readily reveals these major trends. There are hundreds of other lesser faults and folds which follow a similar alignment - a trend known as the Caledonoid trend.

The Variscan Orogeny

Main article: Variscan orogeny

The

Old Red Continent), followed by a further collision between Gondwana and the enlarged Laurussia. In Britain it resulted in a variety of geological structures across the southwest from Pembrokeshire and South Glamorgan in Wales to Devon and Cornwall
. Structures include:

Post-Variscan crustal extension

An east-west extensional regime affecting the crust of England led to the formation of a series of structural basins developing through England between the English Channel and the Irish Sea during Permian and Triassic times. The Worcester Basin links to the Stafford Basin which in turn links to the Cheshire Basin and thence to the East Irish Sea Basin. The Needwood and Knowle basins are two smaller basins in the North Midlands associated with this rift complex.[2]

The Alpine Orogeny

North-south cross-section through southern England, showing the Weald Basin, which was inverted during the Alpine Orogeny forming the Wealden Anticline. Vertical exaggeration 1:5.
Main article: Alpine orogeny

The

Carpathians and other mountain ranges of southern Europe
, but the northernmost 'ripples' of this event have affected the structure of southern England. Structures include;

See also

References

  1. ^ British Geological Survey 1996 Tectonic map of Britain, Ireland and adjacent areas Pharaoh, T.C. et al 1:1,500,000 (Keyworth, Notts, BGS)
  2. ISBN 0852723334
    .
  3. ^ British Geological Survey. 1996, Tectonic Map of Britain, Ireland & adjacent areas, Pharaoh et al. 1:1500 000 (Keyworth, BGS)
  4. ^ Trewin N.H. (ed) 2002, The Geology of Scotland. The Geological Society, London
  5. ^ Brenchley P.J & Rawson P.F. (eds) 2006, The Geology of England and Wales. The Geological Society, London
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