Britain's Geology: Solving the Jigsaw (Lectures 49-53) Flashcards
What is the chronostratigraphic system based on?
Palaeontological time intervals defined by recognised fossil assemblages
How are pre- and post-Permian rocks in the UK different?
Post-Permian are weakly deformed
Pre-Permian rocks are affected by orogonies that caused deformation, metamorphism and magmatism
An orogenic belt passes across what?
A deformation front from the hinterland to the less deformed foreland
Where does the Caledonian Orogen cross?
Which Periods does it comprise of?
What is it associated with?
Between Welsh/English border and NW Scotland and across Ireland
Ordovician to Devonian
The closure of the Iapetus Ocean
Where does the Variscan Orogen cross?
Which Period?
What is it associated with?
Far south of Britain and Ireland
Late Carboniferous
Closure of the Rheic Ocean
Define terrane
A major fragment of continental crust bounded by faults, with a geological history that contrasts with adjacent terranes
What does basement denote?
Older, more deformed, rocks beneath a younger less deformed cover
Name the two major terranes in the UK
Laurentia and Gondwana
What were Laurentia and Gondwana separated by?
The Iapetus Ocean
Describe the Laurentian terrane basement rocks
Much older than most other British/Irish terranes (back to 3Ga)
Describe the Gondwana terrane basement rocks
Mostly Neoproterozoic
What is the boundary between the Laurentian and Gondwanan terranes called?
Iapetus Suture Zone
The Laurentian and Gondwanan terranes accreted along a series of major faults, name them
Moine thrust
Great Glen fault
Highland boundary fault
Southern Uplands fault
What are the four ways terranes originate?
Volcanic arcs above subducting oceanic lithosphere
Ocean crust, particularly oceanic plateaus
Deep marine sediments scraped off in subduction
Fragments of continental crust at strike-slip faults
How might greenhouse/icehouse Earth states be linked to dispersion of continents?
Clustered continents = few ridges and subduction zones = low CO2 = thin atmosphere = cool Earth
What is the difference between eustatic and isostatic sea-level change?
Eustatic changes are relative to around the globe and controlled by water volume in the oceans
Isostatic changes are caused by local tectonic changes in topography relative to sea-level
What were the three continents that drifted northward and amalgamated to form the British Isles?
When were they amalgamated by?
Laurentian margin (Scotland and NW Ireland)
Avalonia (England, Wales, SE Ireland)
Armorica (S Cornwall and N France)
End Devonian
When did the British Isles cross the equator?
What was the rate of latitudinal drift?
Carboniferous
1 degree per 5Myr
The Caledonian Orogeny is divided into which two phases?
Grampian: Arc-continent collision
Scandian: continent-continent collision
What is subducted in extensional subduction?
Where is there extension?
Where is there shortening?
Old, cool and dense oceanic lithosphere
Trench rollback and extension across the arc and backarc region
Only in the subduction complex of offscraped sediment
What is subducted in contractional subduction?
What is produced by this?
Young, warm and buoyant oceanic lithosphere
A flat slab that induces shortening in the whole upper plate
What does an arc-continent collision produce?
What can the collision induce?
Orogenic shortening in both upper and lower plates
Can induce a polarity flip in the subduction
What does a continent-continent collision produce?
Give an example
Shortening in both upper and lower plates
India colliding into Asia
Subduction and collision are rarely perpendicular to plate boundaries, what is produced by this obliqueness?
Additional boundary-parallel displacements, taken up on large strike-slip faults
How did the Laurentian margin form?
What was deposited on this margin?
Neoproterozoic (580-550 Ma) rifting from west Gondwana
Dalradian Supergroup
Avalonia rifting from Gondwana opened what?
The Rheic Ocean
What occured with the arc-continent collision at the Laurentian margin?
Grampian phase in mid-Ordovician
A flip in subduction polarity
Which continents had docked by early Silurian time?
What is their collision called?
What were their collisions like with Laurentia?
Avalonia and Baltica
Shelveian phase
Baltica: hard collision of the Scandian phase
Avalonia: soft collision
Armorica rifting from Gondwana opened what?
The Theic Ocean
When was the Iapetus closed?
What was the new continent called?
Mid-Silurian
Laurussia
When is it likely Armorica collided with Laurussia?
Which phase might this have given?
When did Gondwana and Laurussia collide?
Mid-Devonian
Acadian phase
Carboniferous
Why are the phases of the Caledonian Orogeny restricted to one or two terranes?
Most terranes were not amalgamated along their bounding faults until the mid-Devonian Acadian phase
When did the Grampian phase occur?
Mid-Ordovician
Before the Iapetus closed
What was the direction of the early Ordovician Laurentian subduction zone?
Outboard
Dipped SE, away from the margin rather than beneath it
What formed above the Laurentian subduction zone?
Where did it move towards?
A volcanic arc (Taconic arc)
Towards the Laurentian margin
When the Taconic arc collided with the Laurentian margin, what happened?
After this, what happened to the subduction?
Overthrusted the Laurentian crust
Subduction resumed but dipped NW, towards Laurentia (polarity flip)
The Grampian deformation can be summarised as the main event in which supergroups and which terranes?
Dalradian and Moine supergroups
Highland terranes
What is the first continental collision preserved in the British sector of the Caledonian belt?
Why?
Baltica and Laurentia
Sectors NW of the Highland Boundary Fault slid into their current position along large sinistral strike-slip faults.
When did the Scandian phase occur?
Where is it focused along?
Early Silurian
Moine Thrust Zone, which dips gently SE beneath the Highlands
What happened at the Moine Thrust Zone?
Thrusts the deformed Moine rocks over the undeformed Laurentian shelf sediments
What is the closure of the Iapetus Ocean marked by?
End of the Southern Uplands accretionary prism
First southward supply of Laurentian sediment onto Avalonian crust
Why was the Avalonia-Laurentia collision not marked my major deformation?
Highly oblique to the Laurentian margin
Mostly involved margin-parallel strike-slip
What was the Acadian phase marked by?
When did it occur?
Main deformation on Avalonia
Mid-Devonian, post-dates Iapetus closure by 25 Myr
What are the options for the causes of the Acadian phase if not the Iapetus closure?
Continental collision with part of Armorica/Iberia
Avalonia over-rode a hot, buoyant sector of the Rheic Ocean, producing flat-slab subduction
When was the Theic Ocean seaway blocked by?
When were Gondwana and Laurussia fully joined?
Early Carboniferous (360 Ma) End-Carboniferous (300 Ma)
Which continent was the last to be added before Pangaea was assembled?
When was this?
Siberia
End-Permian (250 Ma)
Relatively, where was Britain and Ireland in Pangaea?
Eastern side
Nearest ocean being the Tethys that separated Asia from Africa/Arabia
When was Pangaea intact until?
Mid-Jurassic (170 Ma)
What was all across Pangaea by end-Triassic (200 Ma)?
What did they develop into?
Rift zones
Mesozoic oceans
In the Early Carboniferous, what occurred behind the Theic Ocean subduction zone?
Why?
What did this give?
Back-arc extension
Down going lithosphere was old and cold, causing slab roll-back
Fault-bounded extensional basins and associated rift volcanics
How was further accomodation space provided during the Late Carboniferous?
What was formed by an influx of clastic sediments?
What do these deposits host now?
Thermal subsidence of the rift basins after active extension
Deltas on the former carbonate shelf
Coal deposits of the UK
When was the Variscan Orogeny?
What caused it?
Mid-Devonian to Early Permian
Successive Gondwanan fragments attaching to Laurussia
When was the climax of the Variscan Orogeny in Britain and Ireland?
What occurred?
Late Carboniferous
Crustal shortening and uplift causing emergence and a regional unconformity
How did the Variscan front affect southern Britain and Ireland?
What occurred north of the front in the Variscan Foreland?
Intense folds and cleavage
Old faults reactivated, high level intrusions formed, gentle folding, no cleavage.
During the Variscan Orogeny, what was intruded in SW England?
Large granitic Cornubian Batholith
Why did flexural subsidence occur ahead of the Variscan Front?
What caused the shortening?
Loading of Avalonia by the overthrusting Armorican lithosphere
Collision of Gondwana with Armorica/Iberia in late Carboniferous
What happened to the foreland basin when Gondwana collided in the late Carboniferous?
Migrated northwards ahead of the thrust front
Where did the flexural basin develop?
What was it filled with?
Across SW England (ahead of Variscan mountain front)
Basinal sediments - shales and turbidites - from north and south
Which global event affected the fill of the Carboniferous basins?
Permo-Carboniferous Pangaean ice age
What is likely to have caused the Permo-Carboniferous Pangaean ice age?
Closure of the Theic seaway between Gondwana and Laurentia
What ended the Permo-Carboniferous Pangaean ice age?
Migration of Gondwana away from the South Pole
How did the Permo-Carboniferous ice age affect sedimentation in Britain? (2)
No direct glacial sedimentation
Sea level changes induced sedimentary cycles from deeper to shallower deltaic and marine facies
What does a deltaic cycle record?
Why are they coarsening-up?
Advance or progradation of one delta lobe onto a marine shelf or lake floor
Deeper facies are overlain by shallower facies
What are the hypotheses that explain deltaic cycle? (2)
Sequences form when global sea levels fall, then end and restart in the next sea level rise
Lobe switching on the delta, as lobes are abandoned and restarted
What allowed for sedimentation to resume in Britain in the Early Permian? (2)
Post-orogenic collapse
Thermal subsidence
What were the influences on sedimentation in Britain in the Early Permian? (3)
Closure of seaways by the Variscan Orogeny and global eustatic sea-level minimum
Isolation of NW Europe within the supercontinent
Increasingly arid climate as it drifted in the northern hemisphere
What are the phases by which the Atlantic Ocean opened? (3)
Began to open in the Jurassic along lines created during Triassic rifting. Gondwana rotation narrowed Tethys Ocean
South Atlantic began to open in Cretaceous, Africa rotation accelerated w.r.t Eurasia
Atlantic ridge system propagated north into North Atlantic in Late Cretaceous and then Arctic Ocean, separating Eurasia from NA
Which two promontories of Africa collided with Eurasia?
When?
What did they cause?
Italy and Iberia
Paleogene and Neogene
Pyrenean and Alpine Orogonies
How did the North Atlantic struggle to rift? (2)
Failed to rift along a single clean break
Atlantic margins have many failed rifts
How do continents on a sphere tend to split? (2)
Give two Atlantic examples
Along three-armed rifts
If two develop into ocean, the third is likely abandoned on one or other continental margin
Benue Trough and the North Sea
What is an active example of a three-armed rift? (2)
In Afar, with Arabia rifting from Africa
A failed rift is left in the East African Rift Valley
How does extended lithosphere deform? (2)
Brittle faulting in the upper crust
Ductile flow in the lower crust and lithospheric mantle
During extension, what happens to rift basins?
Why might rift shoulders be uplifted?
Rift basins subside and accumulate sediment
The zone of ductile thinning and lithosphere heating is wider than the rift zone.
What can produce rift volcanism?
Decompression melting of asthenosphere
When extension of lithosphere ends, what continues above the old rift? (2)
Thermal subsidence as the stretched lithosphere/asthenosphere column cools and contracts
A sag basin results
What do the main post-Variscan faults around Britain and Ireland delimit?
The rift basins developed during the Mesozoic
How do Mesozoic rift basins differ w.r.t to the continental margin of the Atlantic? (2)
Rockall and Faeroe troughs are parallel
Central North Sea Graben and Western Approaches Basin are at a high angle
How do North Sea rift basins differ from the other rift basins around Britain and Ireland? (2)
North Sea ones are overlain by more extensive sag basins
The others don’t have much of a thermal subsidence sag phase
Which part of Britain wasn’t separated from Europe by a rift? (2)
SE England
Structurally continuous with Belgium
Outline the two rifting episodes that affected North Sea sedimentation (2)
Triassic extension gave way to Early Jurassic thermal subsidence
Late Jurassic to Early Cretaceous extension gave way to Late Cretaceous thermal subsidence
Outline the North Sea sedimentation during the basin cycles (1)
Why? (1)
Progression from shallower to deeper facies
Sediment supply was slower than subsidence = underfilled basin
What separates the two tectonic cycles in the North Sea? (1)
What is the hypothesis for this event? (1)
How is this event recorded? (2)
A period of Mid-Jurassic uplift
Onset of a mantle plume
An unconformity and a switch from marine shelt to deltaic deposition
When and where did the Thulean plume take place? (2)
Beneath Iceland
Late Cretaceous to now
Where did the Thulean igneous province develop? (1)
What did it comprise of? (3)
NW Britain and Ireland
Basic and acid intrusive centres with originallhy overlying volcanoes
Extensive basaltic lava flows
Suite of NW-SE striking basic dykes
What effect did uplift have on Britain and Ireland from the Thulean plume? (2)
Emergence
Denudation in onland areas and redeposition offshore as turbidite fans in the North Sea and Faeroe Basins
Why was much of the uplift from the Thulean plume permanent? (2)
Magmatic underplating
Addition of bodies of basic intrusive rocks, in or at the base of the lower crust
What is taken as the base of the Quaternary? (1)
Major northern hemisphere ice sheets at about 2.4 Ma
What are the four main glacial event in Britain in the Quaternary? (4)
Anglian ice sheets, 450,000 years, most extensive in the onland record
Wolstonian ice sheets, 250,000 years, poorly recorded
Devensian ice sheets, 20,000 years, dominate the later record
Loch Lomond ice, 10,000 years, restricted to upland areas
Maximum ice cover in Britain in the Quaternary correlates very closely with what?
What conclusion can be drawn from this?
The Thulean uplift
Extent of Quaternary glaciation links indirectly to Atlantic opening
How is ice melting still affecting the British and Irish lithosphere?
What is this superimposed on?
Isostatic rebound is still occurring
Continued thermal subsidence of the North Sea basin
