Lecture 7: Dreaming of Avalon: the assembly of Britain

Question 1

The British Isles:

a big jigsaw

Answer 1

Geologically divided into geological terranes ~NE-SW

Each own characteristic rocks, fossils and structures

Question 2

Southern Uplands, Scotland

Why is it strange…?

Answer 2

A whole series of fault bound slices of rocks of different ages- complex Palaeozoic sediments separated by NE-SW trending faults

Ultrabasic rocks at Ballantrae don’t fit into general picture at all – all difficult to interpret

Question 3

Ireland jigsaw

Answer 3

Also different stripes of terranes in NE-SW direction.
Strange rocks in west (South Mayo ultrabasics)…
…and east (Grangegeeth volcanic’s)

Question 4

Western Europe: an even bigger jigsaw

Answer 4

Again, large, distinct geological terranes

Question 5

The starting point: Rodinia 700 Ma

Answer 5

By 600 Ma everything was weirdly centred on the South Pole

towards end of Precambrian began to break up

Question 6

How did Rodinia begin to break up?

The plate tectonic engine…

Answer 6

By creating new ocean crust at mid ocean ridges, new ocean basins form and widen
By destroying crust (subducting it) down ocean trenches, ocean basins narrow and close

Question 7

Earth’s magnetics

The polarity of the Earth’s magnetic field is not constant

Answer 7

‘Flips’ over geological time periods
These magnetic reversals
Magnetic minerals can record magnetic field orientation when rocks ‘formed’.

Question 8

Older stratigraphic sequences on land record reversals of Earth’s magnetic signal further back in time

Can locate where rocks formed using:

Answer 8

Magnetic inclination = palaeolatitude
magnetic declination = ‘longitude’
(somewhat less accurate method of reconstruction)

Question 9

Why is this signal of magnetic field orientation preserved in the rocks?

Answer 9

Magnetic minerals (e.g. magnetite) can crystallise or be deposited in certain rocks
The minerals will align to orientation of magnetic field lines at the time rock forms.

Can use this to track where rocks have formed originally…

Question 10

Ocean crust created at mid ocean ridges produce…

And can be used to…

Answer 10

Characteristic rocks, incl. gabbros, sheeted dykes and pillow basalts
Ocean crust records magnetic polarity reversals
Thus we can use reversal record to date ocean crust

Question 11

Reversing the record

Can use ages to determine how plates have moved
however…

Answer 11

All ocean crust older than Jurassic has been subducted
So can’t use ocean crust to reconstruct pre-Jurassic plate movements
need other methods…

Question 12

Geology tell tales signs of plate movements…

Mountains

Answer 12

Tectonic plates collide, new mountain ranges form, then eroded
We can find the eroded roots of mountain ranges and their eroded debris in younger sediments

Question 13

Geology tell tales signs of plate movements…

Cold rocks, hot rocks

Answer 13

Climatically sensitive sediments

Cold: tillites, glacial dropstones
Warm: coals, evaporites, red desert sandstones, bauxites

Question 14

One and the same…

Durness Limestone, Cambrian of NW Scottish Highlands
Cambrian Limestone, East Greenland

Answer 14

These units contain the same fossils – areas were once joined
Both deposited in warm, low latitude waters

Question 15

(palaeo-)biogeography tell tales signs of plate movements…

Fossils

Answer 15

Distribution of once living organisms

Similarities in fossil assemblages found on now separated land masses indicate they were once connected

Question 16

What are homeomorphs?

Answer 16

In different geographical areas species evolve which develop similar morphologies to fill the same ecological niche

Sn. hemisphere: penguins
Nn. hemisphere: puffins
(extinct Great Auk)

Question 17

How was the British Isles separated in the Cambrian: ~510 Ma?

How can we tell they were geographically seperated?

Answer 17

Nn. Scotland part of Laurentia

rest of British Isles was part of Avalonia

Same trilobites in Avalonia, Gondwana, Baltica and Armorica
Different but homeomorphic species in Siberia and Laurentia
Laurentia began to move north (continents drift apart) as Iapetus Ocean began to open

Question 18

How was the British Isles separated in the Early Ordovician: ~470 Ma?

Why did new trilobites evolve?

Answer 18

Avalonian trilobites still like Gondwana
However different ones evolving in isolation.

This was because a new mid-ocean ridge opened up the Tornquist Sea = Baltica becomes more isolated.

Iapetus Ocean began to close…

Question 19

Early Ordovician:

evidence of Iapetus closure

Answer 19

Shelf faunas were provincial

HOWEVER things in water (pelagic faunas) became found in more places.
Land masses became closer together.
Pelagic organisms could drift across the narrowing oceans

Question 20

What happened in the Mid Ordovician: ~450 Ma

Answer 20

Iapetus continues to close
another MOR opens Rheic Ocean: Avalonia splits from Gondwana, moves north
Not only planktonic can drift now between communities but benthic also (planktonic larvae).

Question 21

What happened in the Ordovician-Silurian: ~440 Ma?

Answer 21

Rheic ocean expands even more and the Iapetus continues to shrink.
Avalonia and Baltica continue to move north.
shelf faunas are now pretty much the same everyone due to little ocean between continental land masses.

Question 22

What happened in the Mid Silurian:

Closing up

Answer 22

Subduction along southern edge of Laurentia is closing Iapetus
Benthic shelf faunas are now cosmopolitan
Subduction along southern margin of Avalonia begins to close Rheic Ocean

Question 23

Collision!

Answer 23

Baltica and Avalonia collide obliquely with Laurentia
Western edge of Eastern Avalonia collides first, rotates anti-clockwise
Iapetus closes from SW to NE (‘zips up’)

Question 24

When plates collide

Accretionary prism

Answer 24

Sediment scraped off subducting plate = an accretionary prism
Each slice youngs away from trench
Whole prism youngs towards trench

Question 25

Back to front in the Southern Uplands

Answer 25

Accretionary prism

Young to South East

Question 26

When plates collide

Crash and weld

Answer 26

volcanic island chains, continental fragments, oceanic plateaux are too buoyant to subduct
they ‘weld’ onto the edge of continental margins as “exotic terranes”

Question 27

When plates collide

Slice and splice

Answer 27

as plates collide some slivers of ocean crust trapped along colliding margins
ultrabasic rocks known as ophiolites

Question 28

Over time Avalonia and Baltica come together

Answer 28

Mountain building episode
forms the Caledonides, Appalachians and Acadian montain ranges, etc.
the ‘join’ (suture) between these previously separate plates is known as the Iapetus Suture

Question 29

As the Rheic Ocean starts to close…

Answer 29

Armorica collides with Avalonia
the Variscan Orogeny, Late Devonian-Carboniferous ~380-320 Ma
Lizard Peninsula (Cornwall) is ancient Rheic ocean crust

Question 30

British Isles: the sum of the parts

Answer 30

to NW of Iapetus Suture terranes have geological properties more like Greenland and USA
to SE of suture terranes are more similar to Scandinavia and Europe
all the result of millions of years of plate movements…

Question 31

What was the original huge supercontinent called?

Answer 31

Rodinia

Question 32

What is magnetic inclination?

Answer 32

The angle made with the horizontal by the Earth’s magnetic field lines

Question 33

What is magnetic declination?

Answer 33

The angle that is produced between geographic north and magnetic north.