Lecture notes for Ireland trip: a global perspective
on the Caledonides
Can sometimes divide the geology of an area into:
- basement rocks: pervasively deformed and metamorphosed in last
- cover sequence: sediments rocks deposited above the eroded remnants
of the basement.
- surficial geology: recent sediments and landscapes formed within
the basement and cover.
For the northern half of Ireland the basement rocks gained
their character in the Caledonide orogeny. I will leave elucidation
of the specifics of the Ireland Caledonides to Mark Gutchewsky (are resident
expert), and to your reading, and to the field trip itself. What I hope
to do here is provide a bit of a global framework for understanding the
Caledonides = extensive and long mountain belt that is part of
assembly of Pangea super continent primarily from Ordovician through Silurian
times. It can be found in the following areas:
- British Island Caledonides
- Norwegian and East Greenland Caledonides
- Svalbard Caledonides
- North Greenland Caledonides
- Taconic and Acadian phases of Appalachians
- Laurentia: continent composed of North America and Greenland
- Baltica: continent composed of Northern Europe to the Urals of Russia
- Gondwana Land: southern landmass with South American and Africa as
- Avalonia: large extensive arc terrane, with local continental fragments.
- Iapetus: intervening ocean basin closed during the Caledonide orogeny.
- many other smaller exotic terranes that used to reside in Iapetus ocean
Types of events involved:
- subduction related igneous activity and metamorphism associated with
closure of Iapetus basin. 480-460 Ma.
- accretion of suspect terranes - local, diachronous.
- climatic collision and suturing of N America and Euarasia, starting
at 440 Ma. - Iapetan suture runs through British Isles.
- granites associated with crustal thickening.
- sinistral strike-slip movement (Acadian): circa 420 Ma
- collapse and formation of Devonian basins.
- polyphase deformation history due to: subduction, outboard collision,
accretion, final collision, dispersal, collapse.
General structural style and elements:
- older basement terranes with their own cover and history.
- fold and thrust nappes and tectonic zones: higher nappes from deeper,
higher metamorphic grade. Larger nappes associated with final suturing.
- pervasive metamorphosism and deformation.
- hinterland strike-slip fault zones.
- stitching granites.
- common: Ireland, Britain, Southwest Norway, Svalbard
- rift basins filled with terrestrial sediments: known since 1800s as
Old Red Sandstone: well known for its fish fossils.
- why are the Devonian basins localized along the Caledonide axis? -
gravitational collapse, delamination.
- Wilson cycle and birth of North Atlantic along the Caledonides.
Continental stability followed with deposition of platform cover sequence.
See very similar history in Svalbard.
Pickering, K. T. & Smith, A. G., 1997, 19/6 The Caledonides; in van
der Pluijm, B & Marshak, S., Earth Structure - An Introduction to Structural
Geology and Tectonics, McGraw Hill, p. 435-444
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