Caledonian Orogeny Flashcards
What is the evidence to suggest that northern and southern Britain were on seperate continents?
Major differences in the age and
character of the Precambrian (basement)
rocks.
Palaeomagnetism - magnetism trapped in
certain rocks at their time of formation. It
can give an indication of a continent’s
palaeolatitude.)
What evidence is there to suggest there was a widening ocean between the two areas during the Cambrian.
- Palaeomagnetic evidence gives latitude.
- Fossil contrasts, especially trilobites such as Olenellus, found only in Scotland.
- Contrasts in sedimentary environments between the two areas e.g. Durness Lst.
- Absence of andesitic and rhyolitic volcanic lavas suggests a widening ocean and no subduction.
What continents were Northern Britain and Sothern Britain on, give evidence.
Scotland was on the continent of Laurentia- Precambrian rocks as old as 2500 MYA in northern Britain
England and Wales was on Gondwana - Precambrian rocks no older than 700 MYA in most of southern Britain and Ireland
Describe the evidence to suggest the Iapetus Ocean to close during the Ordovician period.
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:
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
What are ophiolites?
Ophiolites are pieces of oceanic plate that have been thrusted (obducted) onto the edge of continental plates. They provide models for processes at mid-ocean ridges. Ophiolites are an assemblage of mafic and ultramafic lavas and hypabyssal rocks found in association with sedimentary rocks like greywackes and cherts.Ophiolites formed at oceanic ridges were
emplaced by tectonic forces at convergent plate
boundaries and then became exposed in highly
deformed orogenic (mountain) belts.
Give examples of tectonic structures of the Caledonian orogeny.
Moine Thrust Belt
Great Glen Fault
Highland Boundary Fault
Southern Uplands Fault
Tay Nappe
What is a nappe?
A nappe is a large body or sheet of rock that has been moved a distance of about 2 km (1.2 miles) or more from its original position by faulting or folding. A nappe may be the hanging wall of a low-angle thrust fault or it may be a large recumbent fold (i.e., an undulation in the stratified rocks of the Earth’s crust having an essentially horizontal axial plane); both processes position older rocks over younger rocks.
What is a suture?
Sutures and suture zones are defined as the sites in orogenic belts wherein ancient oceanic lithosphere was subducted into the mantle, resulting in the closure of ocean basins and final welding of bounding continental masses along fold–fault belts with high strain.
Name the metamorphic grade indicators in order from low grade to high grade
Chlorite
Muscovite
Biotite
Garnet
Staurolite
Sillimanite
Quartz (Continuous)
Feldspar (Continuous)
What is a geosyncline?
A geosyncline was described as a giant downward fold in the Earth’s crust, with associated upward folds called geanticlines (or geanticlinals), that preceded the climax phase of orogenic deformation. Development of a mountain range by sedimentation of a geosyncline and isostatic uplifting.
