Variscan Orogeny

Question 1

Which areas of Britain experienced most folding due to the Variscan Orogeny, give reasons why.

Answer 1

Folding started in Southern Europe in Carboniferous times. The Southern basin (Rheic Sea) was destroyed due to continental collision and folded into a mountain chain. Devon and Cornwall show complex structures including isoclinal folding and major northward thrusting

Question 2

Name areas which show evidence of subduction, metamorphic and igneous activity.

Answer 2

• Volcanic Ash Notes A thin layer of volcanic ash indicating the turbulenthistory and formation of this whole area.
• Dollar Cove The slates have been deformed and weakened by the intense
  pressure involved and now form a series of coves and inlets with low cliffs.
• At Jangye-Ryn, slate strata can be seen drastically folded and contorted;
  however, the further away from the metamorphic zone the less evidence of
  deformation there is.
• Porthallow Breccia Jumbled rock debris- the ‘waste’ bulldozed up ahead of an advancing continent.
• Polurrian Cove Hornblende Schists This metamorphic rock contains micas
  and other platy minerals, aligned to give a foliation. Some schists ‘garnet
  mica schists also contain the semi-precious mineral garnet.
• Mullion Island Pillow Lavas Notes Rapidly cooled, hence fine-grained
  igneous volcanic rock from the mantle erupting onto the ancient sea floor.

Question 3

What is a recumbant fold?

Answer 3

A recumbent fold is one in which the axial plane is essentially horizontal, with the limit of variation of axial-planar dip, and the resulting limit of plunge, being 10°. It is a sideways-closing neutral structure that is neither a synformal nor an antiformal fold.

Question 4

What is an isoclinal fold?

Answer 4

A very tight fold, in which the limbs are parallel or nearly parallel to one another is called an isoclinal fold.

Question 5

Describe the trending of the folds created by the orogeny.

Answer 5

Due to the crustal shortening being caused by compression from N-S a series of asymmetric folds were formed which trended E-W. The
steepest fold limbs are found further towards the south of Britain in Cornwall and Devon. More open synclines are found in South Wales.
The steepest limbs are found on the southern limb of the fold which shows that the most pressure was applied from a southerly direction.

Question 6

Describe the events during the Devonian Period.

Answer 6

Following the Caledonian Orogeny, at the end of the Silurian Period, the erosion of the Caledonian Mountains produced large amounts of clastic sediment which was deposited in South Wales and South West England. Cornwall and South Devon were experiencing a marine environment.

Question 7

What was the effect of the Variscan folding on the metamorphic activity of Britain?

Answer 7

The Variscan folding was closely followed by granitic intrusions particularly in SW England. These intrusions led to mineralisation from hydrothermal activity leading to the formation of Tin, Copper and Lead.

Question 8

Describe the three classes of hydrothermal activity.

Answer 8

Epithermal - <1000 km depth 50-200 degrees celsius
Mesothermal - 1500-4000 km depth 200-400 degrees celsius
Hypothermal - >4000km depth 400-600 degrees celsius

Question 9

What are Granitic Tors?

Answer 9

Tors are landforms created by the erosion and weathering of rock; in this case granites. Tors are mostly less than 5 meters (16 ft) high. Many hypotheses have been proposed to explain their origin and this remains a topic of discussion among geologists and geomorphologists, and physical geographers. It is considered likely that tors were created by geomorphic processes that differed widely in type and duration according to regional and local differences in climate and rock types.

Question 10

How was South Wales affected by the Variscan Orogeny?

Answer 10

South Wales experienced a series of open folds however no thermal influence due to the
distance from the suture. In Pembrokeshire, Red Sandstone and Carboniferous rocks were intensely folded and deformed but never metamorphosed Low angle thrusting is visible in coastal sections showing compression from
the South.

Question 11

Describe the order of the Variscan Orogeny.

Answer 11

1- Devonian period Gondwana and Laurasia collided leading the closure of the Rheic Sea.

2- Continental collision led to deformation – folding and faulting and metamorphism.

3- Granitic plutons were emplaced in the crust

4- Hydrothermal activity led to deposits of minerals

5- Weathering and erosion of the surface led to the exposure of the Granitic intrusions on the surface creating Tors