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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 orogeny.
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.

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

Players involved:

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 major portions.
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.

Devonian basins:

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.

Reading:

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