Ophiolite

An ophiolite is a section of Earth's oceanic crust and the underlying upper mantle that has been uplifted and exposed, and often emplaced onto continental crustal rocks.

Their great significance relates to their occurrence within mountain belts such as the Alps and the Himalayas, where they document the existence of former ocean basins that have now been consumed by subduction.

For this reason, researchers carried out a seismic study on an ophiolite complex (Bay of Islands, Newfoundland) in order to establish a comparison.

[5] The temperature gradients of the metamorphosis of ophiolitic pillow lavas and dykes are similar to those found beneath ocean ridges today.

There is yet no consensus on the mechanics of emplacement, the process by which oceanic crust is uplifted onto continental margins despite the relatively low density of the latter.

[9] The continental margin, promontories and reentrants along its length, is attached to the subducting oceanic crust, which dips away from it underneath the island arc complex.

As subduction takes place, the buoyant continent and island arc complex converge, initially colliding with the promontories.

[9] Based on Sr and Nd isotope analyses, ophiolites have a similar composition to mid-ocean-ridge basalts, but typically have slightly elevated large ion lithophile elements and a Nb depletion.

The collision of the continent and island arc initiates a new subduction zone at the back-arc basin, dipping in the opposite direction as the first.

Often, ore bodies such as iron-rich sulfide deposits are found above highly altered epidosites (epidote-quartz rocks) that are evidence of relict black smokers, which continue to operate within the seafloor spreading centers of ocean ridges today.

[11] Additionally, the crystallization order of feldspar and pyroxene (clino- and orthopyroxene) in the gabbros is reversed, and ophiolites also appear to have a multi-phase magmatic complexity on par with subduction zones.

[12] A fore-arc setting for most ophiolites also solves the otherwise-perplexing problem of how oceanic lithosphere can be emplaced on top of continental crust.

Ophiolites with compositions comparable with hotspot-type eruptive settings or normal mid-oceanic ridge basalt are rare, and those examples are generally strongly dismembered in subduction zone accretionary complexes.

[citation needed] Ophiolites are common in orogenic belts of Mesozoic age, like those formed by the closure of the Tethys Ocean.

Tethyan ophiolites are characteristic of those that occur in the eastern Mediterranean sea area, e.g. Troodos in Cyprus, and in the Middle East, such as Semail in Oman, which consist of relatively complete rock series corresponding to the classic ophiolite assemblage and which have been emplaced onto a passive continental margin more or less intact (Tethys is the name given to the ancient sea that once separated Europe and Africa).

Cordilleran ophiolites are characteristic of those that occur in the mountain belts of western North America (the "Cordillera" or backbone of the continent).

[16] The recognition of the Steinmann Trinity served years later to build up the theory around seafloor spreading and plate tectonics.

[18] He held that Alpine ophiolites were "submarine effusions issuing along thrust faults into the active flank of an asymmetrically shortening geosyncline".

[1] They were recognized as fragments of oceanic lithosphere, and dykes were viewed as the result of extensional tectonics at mid-ocean ridges.