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Definition | Ophiolite | Futura Planet

Definition | Ophiolite | Futura Planet

Ophiolites represent fragments of ancient lithosphères oceanic vehicles carried on the continental crust by the game of tectonic plates. They allow the detailed observation of the structure and composition of a sample of oceanic crust difficult to access in normal times. Ophiolites allow, among other things, to understand the formation processes of the oceanic crust at the ridge axis. They are therefore considered to be valuable analogues testifying to processes currently taking place at the bottom of the oceans.

Ophiolites, a sample of open-cast oceanic crust

The typical architecture of an ophiolite is therefore the same as that of a current oceanic crust. It is generally composed, from bottom to top, of ultramafic rocks (peridotites), gabbros, a vein complex and basalts. But the architectural and compositional diversity of ophiolites testifies to the variability of the processes of creation of the oceanic crust. This variability has led them to be classified into two large groups according to the composition of their coat residual. A distinction is thus made between lherzolitic type ophiolites (LOT) and harzburgitic type ophiolites (HOT). LOT type ophiolites come from a mantle that has undergone a weak partial melting and often have a reduced, very fine, or even absent crust, testifying to a low magmatic budget. This type of ophiolite is mainly composed of fresh and serpentinized peridotites (altered by sea water). On the contrary, the HOT type ophiolites show a greater partial melting and a strong magmatic production. They consist of a thick crust with the typical succession of peridotite, gabbros, vein complex, basalts.

There are over 150 ophiolitic complexes around the world. The most famous ophiolites are the ophiolites of Oman, the Alps, California, Turkey … Oman ophiolite represents the typical reference of an ophiolite of the HOT type. It was she who served as a model to define the architecture of a typical oceanic crust, during the Penrose conference in 1972. The ophiolite des Alpes (at Chenaillet for example) represents for its part the reference of the LOT type.

Formation of an ophiolitic complex

Ophiolites are generally witnesses to the closing of an ocean, a stage preceding the formation of a continental chain when two continental plates collide. This process of throwing relics of the floor oceanic is called obduction. The presence of these rocks in the mountain ranges remained an enigma for a long time, until the elaboration of plate tectonics. If the study of ophiolites has made it possible to better understand the genesis of the oceanic crust, their study nevertheless remains complex. Indeed, the obduction process can lead to a significant transformation of these rocks initially created at the level of ocean ridges : metamorphism, dislocation, tectonic folding, scaling, alteration … The tectonic processes leading to the formation of mountain ranges mean that the ophiolitic series are rarely complete and reconstructing their original architecture represents an important work.

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