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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.
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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
Precambrian rocks as old as 2500 MYA in northern Britain Pre-Cambrian Evidence for the interpretation that northern and southern Britain were on two different continents: 10ºS 1. Major differences in the age and character of the Precambrian (basement) rocks. 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 10ºSof Equator whilst Avalonia (containing England & Wales) drifted away from Gondwana to approx 40ºS. • Subduction occurring under both continents, causing the Iapetus to close. Island-arc volcanism Evidence: • 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
Caledonian Orogenesis Show how the study of geology can provide evidence for the tectonic regime active in Britain during the Lower Palaeozoic. Aim: 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 Cooling, thickening & becomes denser as it moves away from MOR Reabsorbed into mantle by subduction 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 Inner Zone Hinterland Outer Zone
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) Metamorphic Rocks – Slate in North Wales & Cumbria, & Schists in Scotland Sedimentary Rocks – Black Shales (Skiddaw Slates) Folds – Harlech Dome, Shap Fell, Tebay & Tay Nappe Faults – Moine Thrust Trends - 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 ?
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) Igneous Rocks – Andesites & Rhyolites in Cumbria (BVG) or North Wales (Snowdonia) Metamorphic Rocks – Slate in North Wales & Cumbria, & Schists in Scotland Folds – Harlech Dome, Shap Fell, Tebay & Tay Nappe Faults – Moine Thrust 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
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
1. North-west Highlands Eriboll rocks – 500 million years old 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. 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. 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 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.
Eriboll rocks – 500 million years old Lower layers
Eriboll rocks – 500 million years old Upper layers
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 Lower layer
3. Grampian Highlands Cairngorm rocks – 400 million years old Crystalline rocks, interlocking and coarse-grained. Mineralogically composed of quartz and feldspar, which are randomly orientated. Relatively undeformed and approximately 400 mya. Dalradian rocks – 750 – 480 million years old A mixture of rocks which have been intensely folded and metamorphosed. 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.
5. Southern Uplands 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. 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.
