Ophiolites are considered to be masses of oceanic crust and upper mantle thrust onto the edge of a continental or incorporated in mountain belts, where they are tectonically disrupted and now exposed by erosion.
A typical Ophiolite, in this case based primarily on the Semail Ophiolite in Oman on the Arabian peninsula, a particularly well exposed and relatively intact ophiolite.
A brief description of the layers, beginning at the top as follows –
Layer -1 :
A thin layer of pelagic sediments that absent on the newly generated crust at ridge axes and thickness away from the axes as sediment accumulates on progressively older crust.
Layer -2 :
It is basaltic that can be subdivided into layer 2A & layer 2B.
Layer 2A is believed to comprise pillow basalt and sheet lavas, and Layer 2B is believed to comprise vertical sheet dykes emplaced in the shallow, brittle extensional environment at the ridge axes. Many dykes have only one chill margin, implying that later dikes split and intruded earlier ones.
The upper portion of the layer 2A has seismic velocities lower than predicted for basalt in laboratory, interpreted this as resulting from fractures and cavities (13-41% porosity) in the upper part of the basalts. Below the porous zone layer 2B has seismic velocities commensurate with laboratory measurement.
Some investigators distinguish the porous zone as layer 2A and non porous layer as layer 2B they call the sheet dikes as layer 2C.
Layer 3 :
It is more complex and and a bit more controversial but is generally believed to comprise mostly gabbro crystallized from a shallow axial magma chamber that fed the dykes and basalt. Layer 3A again by analogy with ophiolites, represents uppermost isotropic and lower, somewhat foliated gabbro where Layer 3B is more layered, typically exhibiting cumulate textures.
The layering may be horizontal but more commonly dips (towards the ridge axis) at angles locally up to 900. Both layers 3A and 3B are well foliated and lineated in the semali ophiolite.
Layer 4 :
Layer 4 has seismic velocities that correlate well with ultramafic rocks. In ophiolites, the base of the gabbro grades into layered cumulate wehrlite and gabbro. Diapir like bodies of wehrlite also appear to have moved upward into the layered gabbro. Cumulate dunite with harzburgite xenoliths and chromite lenses is usually found below the wehrlite layer.
Below this is a tectonic harzburgite and dunite interpreted to be the unmelted refractory residuum of the source mantle left behind after basaltic magma was extracted. A few gabbroic dikes may also occur in this layer.
- The boundary between Layers 3 and 4 is, broadly the moho.
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