As much of what we see and access is found in the North Oman Mountains, the concept of “ophiolite” is important. Most will understand the basics of tectonic plates wandering around the earths surface where they collide and form oceanic trenches and mountain ranges. If we consider this process in a little more detail using the diagram below we can imagine red continental crust moving towards blue continental crust with green oceanic crust in between. The green oceanic crust is shown to have a couple of options, it can, as in the right side of the diagram, be pushed under the blue continental crust in which case it is referred to as having been “subducted”. Or as to the left of the diagram it may be pushed ontop of the red continental crust, in which case we say it has been “obducted“. In extreme cases as in the lower part of the diagram, we may see oceanic crust thrust ontop of red continental crust and then blue continental crust thrust ontop of both. Welcome to North Oman.
So what is Ophiolite?
The term ophiolite is frequently misused as a rock type when in reality it refers to a sequece of rocks or rock types. We often hear people saying “where can i see some ophiolite” or “where can i get a sample of ophiolite“. Ophiolite is actually a section or sequence of the earth’s oceanic crust and upper mantle that has been uplifted and typically is exposed on top of other continental crust. The emplacement of oceanic crust on top of continental crust is as noted above, a geological process called obduction.
This is shown in the adjacent diagram where (top) two elements of continental lithosphere converge forcing oceanic lithosphere to be subducted under the continental lithosphere. Eventually oceanic lithosphere is pushed onto continental lithosphere (centre left). Finally the two elements of continental lithosphere collide and ride over the obducted ophiolite sequence.
In the begining, red, green and blue crust sit ontop of the “mantle”. After obduction we may see mantle overlain by red then green and finally by blue crust.
This process produces a characteristic sequence of rock types collectivel referred to a an ophiolite sequence. This is expanded and shown in more detail below.
The adjacent diagram shows a schematic ophiolite sequence. The ‘moho’ (or mohorovicic discontinuity) horizons are important as they divide the upper mantle sequence from the oceanic crust sequence. The seismic moho is the horizon physically detected from seismic data while the petrologic moho represents a chemical change in the rock.
Mantle sequence:
In Oman, we see the uppermost mantle comprised of peridotite and a sequence of ultramafic cumulates immediately prior to the seismic moho. This is seen in Wadi Abyad.
Oceanic crust sequence:
In Wadi Abyad, the lowermost/ deepest oceanic crust sequence consists of gabbro and layered gabbro.
While not seen in Wadi Abyad, elsewhere in north Oman we see shallower parts of the sequence comprised of sheeted dykes, pillloow lavas and deep ocean sediments.
Oceanic crust is most commonly seen being created at ocean spreading centres such as the mid Atlantic ridge or the Pacific ring of fire. Magma forces its way up from the mantle as dikes. When these cool and provide a line of weakness for further intrusions, we see more dikes forming adjacent to the earlier ones as they push the new crust apart. The dikes are numerous and are referred to as sheeted dikes. When the dikes extrude into ocean water they cool rapidly and form unique lava types known as pillow lavas.
If the spreading process caused by the dikes is halted and reversed by continental collision, the whole sequence of upper mantle – sheeted dikes – pillow laveas – and overlying oceanic sediment may be pushed/ thrust/ obducted onto the adjacent continental mass. This obducted mass is known as ophiolite and it slides on a line of weakness known as the Moho (short for Mohorovicic Discontinuity after the geologist who discovered its existence).
Why do I labour the point? Quite simply because Oman has possibly the most ophiolite, and the best ophiolite exposed any where on earth. It has upper mantle peridotites, lower crust gabbro, it has the fossilised moho as well as sheeted dikes and pillow lavas as well as oceanic sediments in the obducted sequence. The pillow lavas alone are recognised as being world class.
So lets take a look, remember this is not a training course its a photo gallery.
Upper Mantle/ Lower Crust and the Moho
Where? Wadi Abyad
Obduction
Ophiolite Structure
The above section, not based on Oman, is remarkably similar to the top photo of the Wadi Abyad section whch shows part of the ophiolite sequence from peridotite through into gabbro.
Sheeted Dikes
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Pillow Lavas
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Deep Ocean Sediments
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