Download 3. Caledonian Orogenesis

3. Caledonian Orogenesis
4. Alpine Orogeny
3. North Atlantic Tertiary
Igneous Province (NATP)
2. Variscan Orogeny
1. Caledonian Orogeny
4. Alpine Orogeny
3. North Atlantic Tertiary
Igneous Province (NATP)
2. Variscan Orogeny
1. Caledonian Orogeny
Pre-Cambrian
Precambrian rocks as old
as 2500 MYA in northern
Britain
Evidence for the interpretation that
northern and southern Britain were on two
different continents:
1. Major differences in the age and
character of the Precambrian (basement)
rocks.
10ºS
2. Palaeomagnetic evidence.
(Palaeomagnetism is magnetism trapped in
certain rocks at their time of formation. It
can give an indication of a continent’s
palaeolatitude.)
Precambrian rocks no older
than 700 MYA in most of
southern Britain and Ireland
Cambrian 544 – 510 MYA – Caledonian Orogenesis
Evidence for the existence
of a widening oceanic area
separating northern and
southern Britain:
1. Palaeomagnetic evidence
gives latitude.
Cambrian
Evidence for the existence
of a widening oceanic area
separating northern and
southern Britain:
1. Palaeomagnetic evidence
gives latitude.
2. Fossil contrasts, especially
trilobites such as Olenellus,
found only in Scotland.
3. Contrasts in sedimentary
environments between the
two areas e.g. Durness Lst.
Cambrian
Evidence for the existence
of a widening oceanic area
separating northern and
southern Britain:
1. Palaeomagnetic evidence
gives latitude.
2. Fossil contrasts, especially
trilobites such as Olenellus,
found only in Scotland.
3. Contrasts in sedimentary
environments between the
two areas e.g. Durness Lst.
4. Absence of andesitic and
rhyolitic volcanic lavas
suggests a widening ocean
and no subduction.
Ordovician 510 - 439MYA •Northern Britain & Southern Britain separated by
the 3,500km Iapetus Ocean
•Laurentia remained stationary approx 10ºS of
Equator whilst Avalonia (containing England & Wales)
drifted away from Gondwana to approx 40ºS.
•Subduction occurring under both continents,
causing the Iapetus to close.
Evidence:
Island-arc volcanism
• Palaeomagnetism
• Distinct trilobite faunal
provinces
• Durness Lst in Scotland
• Volcanic rocks in Lake
District
• Ophiolites at Ballantrae and Arran in Scotland
• Accretionary prism – Southern Uplands in Scotland
Silurian 439 – 409 MYA
Caledonian Orogenesis
Aim:
Show how the study of geology can provide evidence
for the tectonic regime active in Britain during the
Lower Palaeozoic.
1. Name the orogenic belt that formed in Britain during the Lower Palaeozoic.
2. When was this orogenesis completed by?
3. What two continents collided during this orogenesis?
4. What was the ocean that was destroyed?
5. What areas in Britain were affected?
6. What is the general trend of the structures formed in this orogenesis?
7. What is the name of the continent that formed after this collision?
8. What was the name of the mountain chain formed?
9. At what latitude did these two continents collide at?
Cambrian ~550 Ma
Laurentia
(Northern Britain)
Iapetus Suture
Avalonia
(Southern Britain)
Early Ordovician ~490 Ma
Laurentia
(Northern Britain)
Iapetus Suture
Avalonia
(Southern Britain)
Early Silurian ~440 Ma
Laurentia
(Northern Britain)
Iapetus Suture
Avalonia
(Southern Britain)
Devonian ~400 Ma
Laurentia
(Northern Britain)
Iapetus Suture
Avalonia
(Southern Britain)
Subduction Zones Orogenic Belts
Reabsorbed into
mantle by subduction
Cooling, thickening & becomes
denser as it moves away from MOR
Dewatering of slab lowers
MTP of mantle wedge
causing it to partially melt
Does not melt unless young (<3 Ma)
Dehydration of slab cools
it and increases its density
400km phase change olivine to
spinel (10% denser)
Slab pull forces
increase
670km phase change spinel to
perovskite (10% denser)
Caledonian Orogenesis (Lower Palaeozoic)
Foreland Outer Zone
Inner Zone
Hinterland
Show how the study of geology provides evidence for the
tectonic regime active in Britain during the Lower
Palaeozoic.
(25 marks)
Geology
= Rock types + geological structures
Igneous Rocks – Andesites &
Rhyolites in Cumbria (BVG) or North
Wales (Snowdonia)
Folds – Harlech Dome, Shap Fell,
Tebay & Tay Nappe
Metamorphic Rocks – Slate in North
Wales & Cumbria, & Schists in
Scotland
Trends -
Sedimentary Rocks – Black Shales
(Skiddaw Slates)
Faults – Moine Thrust
Intrusions – Skiddaw Granite or
Cairngorms
Accretionary Prism – Southern
Uplands
Ophiolites – Ballantrae Complex
With reference to a named orogenic belt in Britain, explain
how a study of the geology enables a reconstruction of the
plate tectonic regime in which it developed.
(25 marks)
Named Orogenic Belt in Britain ?
Era
Period
Orogeny
Cenozoic
Quaternary
Tertiary
ALPINE
(40 MYA)
Mesozoic
Cretaceous
Jurassic
Triassic
Upper
Palaeozoic
Permian
Carboniferous
Devonian
VARISCAN
(300 MYA)
Lower
Palaeozoic
Silurian
Ordovician
Cambrian
CALEDONIAN
(400 MYA)
With reference to a named orogenic belt in Britain, explain
how a study of the geology enables a reconstruction of the
plate tectonic regime in which it developed.
(25 marks)
Geology
= Rock types + geological structures
Sedimentary Rocks – Black Shales
(Skiddaw Slates)
Folds – Harlech Dome, Shap Fell,
Tebay & Tay Nappe
Igneous Rocks – Andesites &
Rhyolites in Cumbria (BVG) or North
Wales (Snowdonia)
Faults – Moine Thrust
Metamorphic Rocks – Slate in North
Wales & Cumbria, & Schists in
Scotland
Intrusions – Skiddaw Granite or
Cairngorms
Accretionary Prism – Southern
Uplands
Ophiolites – Ballantrae Complex
Tectonic Structures of the Caledonian Orogenesis
• Moine Thrust Belt
• Great Glen Fault
• Highland Boundary Fault
• Southern Uplands Fault
• Tay Nappe
Caledonian Faults
Moine Thrust Belt
• NE-SW strike
• 200km long
• 0-12km wide
• up to 150km displacement
• 435-425 Mya
Moine Thrust Belt
Caledonian Faults
Great Glen Fault
• NE-SW strike
• 150km long
• strike-slip fault
• sinistral
• >100km displacement
• 430-425 Mya
Highland Boundary Fault
• NE-SW strike
• Reverse fault
• Allowed Midland Valley
to descend as major rift
by 4,000m
• Sinistral displacement
• 430-424 Mya
Caledonian Faults
Southern Uplands Fault
• NE-SW strike
• Reverse fault
• Allowed Midland Valley
to descend as major rift
by 4,000m
• Sinistral displacement
• 430-424 Mya
Caledonian Faults
Tay Nappe
Tay Nappe
Ballantrae Ophiolite Complex
1. North-west Highlands
Eriboll rocks – 500 million years old
Torridonian rocks – 1 billion – 770 million years old
Fragmental, red-brown in colour.
Coarse-grained sand & pebbles, with some thin layers of
finer grained red mudstone. Grains sub-rounded to
rounded.
Sandstone layers are many metres thick and commonly
show clear cross-bedding.
Horizontal to gently dipping strata.
Lewisian rocks – 3 – 1.8 billion years old
Coarse-grained, crystalline rock, in which the crystals
can easily be seen with the naked eye.
Stripy appearance – with alternating darker and paler
stripes. Darker stripes made of minerals such as
hornblende and biotite, white or pink stripes made up of
quartz and feldspar.
Rocks white to pink on fresh surfaces, but typically
weathers to a grey colour.
Made up of almost entirely of grains of quartz, which
are cemented tightly together to form a very hard rock.
These rocks are broken into loose, angular blocks
forming scree slopes.
The lower layers (oldest) show cross-bedding and
fossilised symmetrical ripples. The upper (younger)
layers contain vertical “pipes” a few centimetres in
length and ½cm to 1 cm wide. Fossil trilobite Olenellus
can also be found.
Durness rocks – 450 million years old
Grey, crystalline rock which is creamy yellow in places.
Dissolves fairly readily in rainwater to produce caves.
Fizzes with HCl acid.
Rock contains stromatolites (sediment mounds bound
together by algae) and ooliths (small-spherical growths
of CaCO3 upto 2mm).
Lewisian rocks – 3 – 1.8 billion years old
Torridonian rocks – 1 billion - 770 million years old
Eriboll rocks – 500 million years old
Lower layers
Eriboll rocks – 500 million years old
Upper layers
Durness rocks – 450 million years old
2. Northern Highlands
Moine rocks – 1 billion – 870 million years old
Crystalline rocks, medium to coarse-grained. Silvery
grey in colour. Abundant flakes of muscovite and biotite
mica, which are aligned into parallel layers (foliated).
Intensely deformed
and folded dating to
450 million years.
Lower layer of Moine rocks
Hard, crystalline & fine-grained rock. Light
grey in colour with a streaked out texture
due to the elongation of the minerals.
Moine rocks – 1 billion to 870 million years old
Moine rocks – 1 billion to 870 million years old
Lower layer
3. Grampian Highlands
Cairngorm rocks – 400 million years old
Dalradian rocks – 750 – 480 million years old
A mixture of rocks which have been intensely folded and metamorphosed.
Crystalline rocks, interlocking and coarse-grained.
Mineralogically composed of quartz and feldspar, which
are randomly orientated. Relatively undeformed and
approximately 400 mya.
Repeated fining-upward sequences, with coarse-grained sands at the base often
with flute casts and tool marks, and fine-grained silts and clays at the top.
Hard and re-crystalline in places.
Lochranza, Isle of Arran – Greenish tinge
to rocks. Fine-grained and re-crystalline.
Chlorite mica minerals which are clearly
aligned.
Ballachulish, Glencoe – dark grey and black in colour,
very fine-grained. Re-crystalline muscovite mica
minerals.
Fine layering visible throughout rock.
Tay Nappe
Complex overfolds and huge nappes.
Locally geology has become inverted.
Trend of fold axis is NE to SW, with
fold amplitudes of up to 10km from
trough to crest.
Cairngorm rocks – 400 million years old
Dalradian rocks – 400 million years old
5. Southern Uplands
Southern Uplands – 470 -420 million years old
Rocks have been weakly metamorphosed. Great majority of older rocks
are coarse-grained greywackes with smaller amounts of finer
siltstones, mudstones, shale, volcanic ash and lavas.
Metamorphism of fine-grained rocks (mudstone and shale) has
produced slate.
Greywacke, a grey, black, dark-green or deep-purplish hard rock, is
coarse-grained and poorly sorted. It contains angular fragments of
quartz, feldspar, ferromagnesian minerals and igneous and
metamorphic rock fragments held together in a dark mud or clay
matrix.
Low grade metamorphism has re-crystallised the cement to produce a
tough hard rock which often looks like an igneous rock.
Southern Upland greywackes are divided into 3 main
zones, separated by faults that run NE-SW, parallel to
the strike of the beds.
Overall the beds get younger to the SE.
The many NE-SW trending faults all have their
downthrow side on the SE. Within each of the 30 or so
fault blocks, the oldest beds are on the SE side, but
the blocks get progressively older towards the NW.
These faults dip towards the NE and get progressively
steeper towards the NE.
Fossils are restricted to the fine sediments, where
dead animal remains would have been preserved by slow
accumulation of mud in an oxygen-deficient
environment. The most abundant fossils are the
graptolites. No fossils occur in the greywackes.
6. Lake District
Skiddaw Group – 470 million years old
Weakly metamorphosed greywackes, sandstones, siltstones and
mudstones. Fine-grained rocks clearly foliated.
Uniform grey colour.
Approx. 3km thick. Very few fossils. Intensely folded.
Borrowdale Volcanic Group – 450 million years old
6km thick sequence of volcanic rocks – mostly
andesite and tuffs, with ignimbrites and welded
tuffs.
Windermere Supergroup – 440 - 420 million years old
Limestones and thick turbidite sequences of
sandstones, siltstones and mudstones, weakly
metamorphosed and folded.
e.g. Shap Fell and Jeffrey’s Mount, Tebay – evidence
of tectonic folding, with fold axes trending NE-SW.
Igneous Intrusions – 400 million years old
Undeformed batholith emplaced at a depth of 5kn. Now exposed due
to erosion and uplift. Coarse-grained and felsic in colour. Found at
Shap. Skiddaw, Ennerdale and Eskdale. Gravity anomaly data suggests
that these intrusions all join up to form one large batholith.
7. North Wales & Anglesey
Anglesey Rocks – 580 – 550 million years old
Schists with metamorphic minerals indicating high pressure and low
temperature conditions. These schists were orinally fine-grained
shales and medium-grained sandstones.
Cambrian Rocks – 500 million years old
Thick sequences of marine sandstones and shales, weakly
metamorphosed. Many of the sandstones such as the Rhinog Grits are
greywackes.
Above this the sediments grade into fine-grained shales now forming
the Llanberis Slates.
Ordovician Rocks – 450 million years old
Volcanic rocks containing rhyolites and andesites, and tuffs. Outcrops
trend in a NE to SW direction. In some areas many of the volcanic
rocks are welded ash flows.
All sediments have been gently folded into large open antiforms and
synforms. E.g Harlech Dome. The strike of the axial planes is NE-SW.