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Ultramafic-mafic (ophiolite) complexes of the kind
discussed in the lecture on the structure of oceanic crust, all lie above
continental crust located adjacent to the continent-ocean interface. Emplacement
of the supposed oceanic material is thought to have resulted from the attempted
of continental crust beneath oceanic crust along slip surfaces dipping
towards the ocean. This process is known as obduction.
Because continental crust is less dense and more buoyant than oceanic crust,
the former will eventually rise leaving the oceanic material stranded on
the continental margin.
In some cases, the oceanic slab may migrate under the influence of gravity towards the continental interior, causing the development of a foreland basin (also known in the older literature as an exogeosyncline) between the obducted oceanic crust and the continental interior as a result of the depression of the continental margin. The sediments deposited in the basin will include turbidite material of both continental (e.g. K-feldspar; muscovite) and oceanic (e.g chromite) derivation. As the ophiolite migrates landward it overrides its own debris, which is converted to an olistostromal melange composed of blocks of sandstone and ophiolite material in a scaly textured fine-grained pelitic material.
A classic Canadian example of obduction and foreland basin development is represented by the Bay of Islands ultramafic - mafic complex and underlying sedimentary units of Western Newfoundland. (see also :
Other regions which contain good examples of obducted oceanic crust include the Canadian Cordillera, Oman in the Persian Gulf area, and Papua-New Guinea in the Pacific. The ophiolites referred to in geosynclinal theory (e.g. the Alpine system) and early plate tectonic scenarios (e.g. Dietz's miogeoclinal model) represent obducted oceanic crust. In this case, eugeosynclines would be foreland basins, and the 'flysch' of eugeosynclines would represent sediment derived from the overriding oceanic material and the older continental-derived slope and rise sediments that were dragged back onto the continent by the migrating oceanic slab. The zwischengebirge zones of geosynclines therefore mark the locus of obduction, and subsequent continent-continent collision.
The Straits of Belle Isle separating Newfoundland from Labrador in the distant background. Rocks in the foreground are undeformed, flat-lying Cambro-Orodovician shelf carbonates. (Bob Stevens is the geologist sitting on the log of wood.)
Cross-bedded Cambrian shelf sandstones (Chickies Formation of Pennsylvania); early passive margin sediments of the Appalachian Iapetus ocean.
Ordovician 'Lime' turbidites: slope and rise deposits dragged over the edge of the continent during obduction of the Bay of Islands ophiolite.
Melange beneath the Bay of Islands ophiolite (Iapetus) of Western Newfoundland.
'Argille scagliose' (scaly shales) melange associated with obducted ophiolite of the Alpine system of Macedonia I.
'Argille scagliose' (scaly shales) melange associated with obducted ophiolite of the Alpine belt of Macedonia II.
Olistostromal melange exposed on the banks of the St. Laurence near Quebec City (St Aubin).
Ordovician melange with blocks of pillow
lava, carbonate and 'eclogite', located below the Ballantrae ophiolite
(Iapetus) of Scotland.
Distribution of ophiolites.
Early explanation for the structural emplacement of Alpine ophiolites (see Ciorneva).
The obduction model.
Geological map of the Persian Gulf region showing the location and size of the Oman ophiolite; thrust southwards out of the Persian Gulf onto the northern edge of the Arabian Shield.
The geology of the northern Canadian Cordillera.
Geological Map of Newfoundland
The Bay of Islands ophiolite of Western Newfoundland.
The rift stage of the Appalachian continental margin (R.K. Stevens, 1969).
The bulldozer obduction model (R.K. Stevens, 1969).
Stratigraphic history of the Western Newfoundland Foreland Basin (R.K. Stevens, 1969).
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