History of Britain Flashcards
Key events in Britain’s geological history
- Closure of Iapetus
- Formation of Pangaea
- Opening of the Atlantic
Secular changes in Earth’s environment
Glaciation
* Neoproterozoic, snowball Earth (dropstones in Namibia)
* Carboniferous
* Holocene - boulder clays at Ketton Quarry
Temperature
* No overall trend
CO2
* Down from Devonian to Carboniferous, increase to Triassic, decrease to today
Sea level
* High stands in Silurian and Cretaceous, low stands in Permian
Rock sequence of Arran
Dalradian (pre-Cambrian)
* Sedimentary, deformed by Caledonian orogeny
* Deposited in growing Iapetus Ocean basin
Old Red Sandstone (Devonian)
* Southern Hemisphere subtropical arid belt
* Lack of fossils, calcite cement in terrestrial rock suggests dry environment
Limestones and shales (Carboniferous)
* Limestone rich in fossils, shallow marine environment
* Gigantoproductus brachiopod thrived in warm shallow sea
* Coals suggests extensive plants
* Seen in tropical regions today
New Red Sandstone (Permian)
* Alternating fine-grained, well-sorted and coarse-grained, poorly-sorted layers
* Fine grain shows crossbedding - aeolian environment
* Coarse grains deposited by flash flood events - wadi (intermittent river) found in desert environment
Granite
* Associated with rifting during the opening of the Atlantic - North Atlantic Igneous Province
* Also associated with NW-SE striking dyke swarm
* Driven by Thulean plume igneous province
* Melting due to subduction in closure of Iapetus as well
Linked Earth system
- Idea that atmosphere, biosphere and tectonics are all linked
- Continents clustered leads to less volcanism, less degassing, less CO2, icehouse
- Opposite for dispersed continents
- More continents displaces seawater, leading to increased sea level
Continental drift of the UK
- UK has drifted northwards over the Phanerozoic
- Evidence: composition of sedimentary rocks over time, palaeomagnetism
Orogenic mechanisms
Extensional subduction
* Subduction of old, cold oceanic lithosphere causes rollback (steepening angle of trench)
* Pulls and stretches continental lithosphere causing rifting, volcanic arc
Contractional subduction
* Subduction of new, hot, buoyant oceanic lithosphere compresses continental plate, causing mountains to build in folds
Arc-continent collision
* Arcs above subduction zones dipping away from continents will eventually collide with them, causing shortening
* This collision can cause the direction of subduction to reverse
Continent-continent collision
* Collision causes shortening in upper and lower crust
Geology of UK
- Iapetus Suture - above is Laurentian terranes, below is Gondwanan terranes
- Lies roughly along England-Scotland border
- Major faults (N to S) in Scotland: Moine thrust, Great Glen fault, Highland boundary fault, Southern Uplands fault
- Western rocks older than Eastern (pre-Carboniferous)
Closure of Iapetus
Silurian
* Associated with the deformation event known as the Caledonian orogeny
Caledonian Orogeny
Grampian Phase
* Arc-continent collision
* Two subduction zones dipping away from Laurentia. Taconic Arc over margin.
* Sea between retreats, collision with Taconic Arc
* Polarity of subduction reverses
* Associated with deformation in Dalradian
Scandian Phase
* Continent-continent collision
* Subducting plate now dipping under Laurentia, Baltica moves towards Laurentia and collides
* Avalonia also collides but not much deformation - soft collision
* Iapetus now closed
* Continent of Laurussia formed
Acadian phase
* True cause unknown
* 25Ma after Iapetus closed (Devonian)
* Possibly due to collision of Armorica into Laurussia further south, or hot-slab subduction of Rheic Ocean plate
Formation of Pangaea
Permian
* Collision of Gondwana, Siberia, Laurussia
* Associated with the Variscan orogeny
* Climate: Pangean Ice age (closure of water passages restricts circulation, Carb), Permo-Triassic deserts (arid inland climate)
Variscan orogeny
- Continent-continent collision
- Collision of Gondwana into Laurussia, closure of the Theic Ocean (Carboniferous)
- Back-arc extension during closure of Theic Ocean created extensional sedimentary basins, flexural basin due to weight of mountains
- Thermal subsidence of rift basins created extra accommodation space
- Period marked by abundance of sedimentary rocks
- Folds and cleavange in Southern UK
Opening of the Atlantic
- Central Atlantic - Jurassic
- South Atlantic - Cretaceous
- North Atlantic - late Cret
- Rifting started at three-armed rifts: one arm fails, creating basin (North Sea)
- Rift basin created through faulting, sag basin when thermal subsidence
- Two main rifting events followed by thermal subsidence: Triassic and late Jurassic
- Triassic: small plume
- late Jurassic: Thulean Plume (Iceland)
- North Atlantic Igneous Province, NW/SE dykes on Arran, granite plutons
Pyrenean orogeny
- Collision of Africa into Asia
- Deformed southern Britain in Palaeogene
Quaternary ice sheets
Northern UK is rising due to isostatic rebound from weight of ice being removed
