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1A Earth > Britain's Geology: Solving the Jigsaw (Lectures 49-53) > Flashcards

Britain's Geology: Solving the Jigsaw (Lectures 49-53) Flashcards

1
Q

What is the chronostratigraphic system based on?

A

Palaeontological time intervals defined by recognised fossil assemblages

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2
Q

How are pre- and post-Permian rocks in the UK different?

A

Post-Permian are weakly deformed

Pre-Permian rocks are affected by orogonies that caused deformation, metamorphism and magmatism

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3
Q

An orogenic belt passes across what?

A

A deformation front from the hinterland to the less deformed foreland

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4
Q

Where does the Caledonian Orogen cross?
Which Periods does it comprise of?
What is it associated with?

A

Between Welsh/English border and NW Scotland and across Ireland
Ordovician to Devonian
The closure of the Iapetus Ocean

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5
Q

Where does the Variscan Orogen cross?
Which Period?
What is it associated with?

A

Far south of Britain and Ireland
Late Carboniferous
Closure of the Rheic Ocean

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6
Q

Define terrane

A

A major fragment of continental crust bounded by faults, with a geological history that contrasts with adjacent terranes

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7
Q

What does basement denote?

A

Older, more deformed, rocks beneath a younger less deformed cover

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8
Q

Name the two major terranes in the UK

A

Laurentia and Gondwana

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9
Q

What were Laurentia and Gondwana separated by?

A

The Iapetus Ocean

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10
Q

Describe the Laurentian terrane basement rocks

A

Much older than most other British/Irish terranes (back to 3Ga)

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11
Q

Describe the Gondwana terrane basement rocks

A

Mostly Neoproterozoic

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12
Q

What is the boundary between the Laurentian and Gondwanan terranes called?

A

Iapetus Suture Zone

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13
Q

The Laurentian and Gondwanan terranes accreted along a series of major faults, name them

A

Moine thrust
Great Glen fault
Highland boundary fault
Southern Uplands fault

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14
Q

What are the four ways terranes originate?

A

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

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15
Q

How might greenhouse/icehouse Earth states be linked to dispersion of continents?

A

Clustered continents = few ridges and subduction zones = low CO2 = thin atmosphere = cool Earth

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16
Q

What is the difference between eustatic and isostatic sea-level change?

A

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

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17
Q

What were the three continents that drifted northward and amalgamated to form the British Isles?
When were they amalgamated by?

A

Laurentian margin (Scotland and NW Ireland)
Avalonia (England, Wales, SE Ireland)
Armorica (S Cornwall and N France)
End Devonian

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18
Q

When did the British Isles cross the equator?

What was the rate of latitudinal drift?

A

Carboniferous

1 degree per 5Myr

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19
Q

The Caledonian Orogeny is divided into which two phases?

A

Grampian: Arc-continent collision
Scandian: continent-continent collision

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20
Q

What is subducted in extensional subduction?
Where is there extension?
Where is there shortening?

A

Old, cool and dense oceanic lithosphere
Trench rollback and extension across the arc and backarc region
Only in the subduction complex of offscraped sediment

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21
Q

What is subducted in contractional subduction?

What is produced by this?

A

Young, warm and buoyant oceanic lithosphere

A flat slab that induces shortening in the whole upper plate

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22
Q

What does an arc-continent collision produce?

What can the collision induce?

A

Orogenic shortening in both upper and lower plates

Can induce a polarity flip in the subduction

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23
Q

What does a continent-continent collision produce?

Give an example

A

Shortening in both upper and lower plates

India colliding into Asia

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24
Q

Subduction and collision are rarely perpendicular to plate boundaries, what is produced by this obliqueness?

A

Additional boundary-parallel displacements, taken up on large strike-slip faults

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25
Q

How did the Laurentian margin form?

What was deposited on this margin?

A

Neoproterozoic (580-550 Ma) rifting from west Gondwana

Dalradian Supergroup

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26
Q

Avalonia rifting from Gondwana opened what?

A

The Rheic Ocean

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27
Q

What occured with the arc-continent collision at the Laurentian margin?

A

Grampian phase in mid-Ordovician

A flip in subduction polarity

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28
Q

Which continents had docked by early Silurian time?
What is their collision called?
What were their collisions like with Laurentia?

A

Avalonia and Baltica
Shelveian phase
Baltica: hard collision of the Scandian phase
Avalonia: soft collision

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29
Q

Armorica rifting from Gondwana opened what?

A

The Theic Ocean

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30
Q

When was the Iapetus closed?

What was the new continent called?

A

Mid-Silurian

Laurussia

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31
Q

When is it likely Armorica collided with Laurussia?
Which phase might this have given?
When did Gondwana and Laurussia collide?

A

Mid-Devonian
Acadian phase
Carboniferous

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32
Q

Why are the phases of the Caledonian Orogeny restricted to one or two terranes?

A

Most terranes were not amalgamated along their bounding faults until the mid-Devonian Acadian phase

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33
Q

When did the Grampian phase occur?

A

Mid-Ordovician

Before the Iapetus closed

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34
Q

What was the direction of the early Ordovician Laurentian subduction zone?

A

Outboard

Dipped SE, away from the margin rather than beneath it

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35
Q

What formed above the Laurentian subduction zone?

Where did it move towards?

A

A volcanic arc (Taconic arc)

Towards the Laurentian margin

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36
Q

When the Taconic arc collided with the Laurentian margin, what happened?
After this, what happened to the subduction?

A

Overthrusted the Laurentian crust

Subduction resumed but dipped NW, towards Laurentia (polarity flip)

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37
Q

The Grampian deformation can be summarised as the main event in which supergroups and which terranes?

A

Dalradian and Moine supergroups

Highland terranes

38
Q

What is the first continental collision preserved in the British sector of the Caledonian belt?
Why?

A

Baltica and Laurentia

Sectors NW of the Highland Boundary Fault slid into their current position along large sinistral strike-slip faults.

39
Q

When did the Scandian phase occur?

Where is it focused along?

A

Early Silurian

Moine Thrust Zone, which dips gently SE beneath the Highlands

40
Q

What happened at the Moine Thrust Zone?

A

Thrusts the deformed Moine rocks over the undeformed Laurentian shelf sediments

41
Q

What is the closure of the Iapetus Ocean marked by?

A

End of the Southern Uplands accretionary prism

First southward supply of Laurentian sediment onto Avalonian crust

42
Q

Why was the Avalonia-Laurentia collision not marked my major deformation?

A

Highly oblique to the Laurentian margin

Mostly involved margin-parallel strike-slip

43
Q

What was the Acadian phase marked by?

When did it occur?

A

Main deformation on Avalonia

Mid-Devonian, post-dates Iapetus closure by 25 Myr

44
Q

What are the options for the causes of the Acadian phase if not the Iapetus closure?

A

Continental collision with part of Armorica/Iberia

Avalonia over-rode a hot, buoyant sector of the Rheic Ocean, producing flat-slab subduction

45
Q

When was the Theic Ocean seaway blocked by?

When were Gondwana and Laurussia fully joined?

A 
Early Carboniferous (360 Ma)
End-Carboniferous (300 Ma)
46
Q

Which continent was the last to be added before Pangaea was assembled?
When was this?

A

Siberia

End-Permian (250 Ma)

47
Q

Relatively, where was Britain and Ireland in Pangaea?

A

Eastern side

Nearest ocean being the Tethys that separated Asia from Africa/Arabia

48
Q

When was Pangaea intact until?

A

Mid-Jurassic (170 Ma)

49
Q

What was all across Pangaea by end-Triassic (200 Ma)?

What did they develop into?

A

Rift zones

Mesozoic oceans

50
Q

In the Early Carboniferous, what occurred behind the Theic Ocean subduction zone?
Why?
What did this give?

A

Back-arc extension
Down going lithosphere was old and cold, causing slab roll-back
Fault-bounded extensional basins and associated rift volcanics

51
Q

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?

A

Thermal subsidence of the rift basins after active extension
Deltas on the former carbonate shelf
Coal deposits of the UK

52
Q

When was the Variscan Orogeny?

What caused it?

A

Mid-Devonian to Early Permian

Successive Gondwanan fragments attaching to Laurussia

53
Q

When was the climax of the Variscan Orogeny in Britain and Ireland?
What occurred?

A

Late Carboniferous

Crustal shortening and uplift causing emergence and a regional unconformity

54
Q

How did the Variscan front affect southern Britain and Ireland?
What occurred north of the front in the Variscan Foreland?

A

Intense folds and cleavage

Old faults reactivated, high level intrusions formed, gentle folding, no cleavage.

55
Q

During the Variscan Orogeny, what was intruded in SW England?

A

Large granitic Cornubian Batholith

56
Q

Why did flexural subsidence occur ahead of the Variscan Front?
What caused the shortening?

A

Loading of Avalonia by the overthrusting Armorican lithosphere
Collision of Gondwana with Armorica/Iberia in late Carboniferous

57
Q

What happened to the foreland basin when Gondwana collided in the late Carboniferous?

A

Migrated northwards ahead of the thrust front

58
Q

Where did the flexural basin develop?

What was it filled with?

A

Across SW England (ahead of Variscan mountain front)

Basinal sediments - shales and turbidites - from north and south

59
Q

Which global event affected the fill of the Carboniferous basins?

A

Permo-Carboniferous Pangaean ice age

60
Q

What is likely to have caused the Permo-Carboniferous Pangaean ice age?

A

Closure of the Theic seaway between Gondwana and Laurentia

61
Q

What ended the Permo-Carboniferous Pangaean ice age?

A

Migration of Gondwana away from the South Pole

62
Q

How did the Permo-Carboniferous ice age affect sedimentation in Britain? (2)

A

No direct glacial sedimentation

Sea level changes induced sedimentary cycles from deeper to shallower deltaic and marine facies

63
Q

What does a deltaic cycle record?

Why are they coarsening-up?

A

Advance or progradation of one delta lobe onto a marine shelf or lake floor
Deeper facies are overlain by shallower facies

64
Q

What are the hypotheses that explain deltaic cycle? (2)

A

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

65
Q

What allowed for sedimentation to resume in Britain in the Early Permian? (2)

A

Post-orogenic collapse

Thermal subsidence

66
Q

What were the influences on sedimentation in Britain in the Early Permian? (3)

A

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

67
Q

What are the phases by which the Atlantic Ocean opened? (3)

A

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

68
Q

Which two promontories of Africa collided with Eurasia?
When?
What did they cause?

A

Italy and Iberia
Paleogene and Neogene
Pyrenean and Alpine Orogonies

69
Q

How did the North Atlantic struggle to rift? (2)

A

Failed to rift along a single clean break

Atlantic margins have many failed rifts

70
Q

How do continents on a sphere tend to split? (2)

Give two Atlantic examples

A

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

71
Q

What is an active example of a three-armed rift? (2)

A

In Afar, with Arabia rifting from Africa

A failed rift is left in the East African Rift Valley

72
Q

How does extended lithosphere deform? (2)

A

Brittle faulting in the upper crust

Ductile flow in the lower crust and lithospheric mantle

73
Q

During extension, what happens to rift basins?

Why might rift shoulders be uplifted?

A

Rift basins subside and accumulate sediment

The zone of ductile thinning and lithosphere heating is wider than the rift zone.

74
Q

What can produce rift volcanism?

A

Decompression melting of asthenosphere

75
Q

When extension of lithosphere ends, what continues above the old rift? (2)

A

Thermal subsidence as the stretched lithosphere/asthenosphere column cools and contracts
A sag basin results

76
Q

What do the main post-Variscan faults around Britain and Ireland delimit?

A

The rift basins developed during the Mesozoic

77
Q

How do Mesozoic rift basins differ w.r.t to the continental margin of the Atlantic? (2)

A

Rockall and Faeroe troughs are parallel

Central North Sea Graben and Western Approaches Basin are at a high angle

78
Q

How do North Sea rift basins differ from the other rift basins around Britain and Ireland? (2)

A

North Sea ones are overlain by more extensive sag basins

The others don’t have much of a thermal subsidence sag phase

79
Q

Which part of Britain wasn’t separated from Europe by a rift? (2)

A

SE England

Structurally continuous with Belgium

80
Q

Outline the two rifting episodes that affected North Sea sedimentation (2)

A

Triassic extension gave way to Early Jurassic thermal subsidence
Late Jurassic to Early Cretaceous extension gave way to Late Cretaceous thermal subsidence

81
Q

Outline the North Sea sedimentation during the basin cycles (1)
Why? (1)

A

Progression from shallower to deeper facies

Sediment supply was slower than subsidence = underfilled basin

82
Q

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

A period of Mid-Jurassic uplift
Onset of a mantle plume
An unconformity and a switch from marine shelt to deltaic deposition

83
Q

When and where did the Thulean plume take place? (2)

A

Beneath Iceland

Late Cretaceous to now

84
Q

Where did the Thulean igneous province develop? (1)

What did it comprise of? (3)

A

NW Britain and Ireland
Basic and acid intrusive centres with originallhy overlying volcanoes
Extensive basaltic lava flows
Suite of NW-SE striking basic dykes

85
Q

What effect did uplift have on Britain and Ireland from the Thulean plume? (2)

A

Emergence

Denudation in onland areas and redeposition offshore as turbidite fans in the North Sea and Faeroe Basins

86
Q

Why was much of the uplift from the Thulean plume permanent? (2)

A

Magmatic underplating

Addition of bodies of basic intrusive rocks, in or at the base of the lower crust

87
Q

What is taken as the base of the Quaternary? (1)

A

Major northern hemisphere ice sheets at about 2.4 Ma

88
Q

What are the four main glacial event in Britain in the Quaternary? (4)

A

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

89
Q

Maximum ice cover in Britain in the Quaternary correlates very closely with what?
What conclusion can be drawn from this?

A

The Thulean uplift

Extent of Quaternary glaciation links indirectly to Atlantic opening

90
Q

How is ice melting still affecting the British and Irish lithosphere?
What is this superimposed on?

A

Isostatic rebound is still occurring

Continued thermal subsidence of the North Sea basin

1A Earth (15 decks)

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