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)