Oceanic Crust & Ophiolites
The Semail Ophiolite Belt of the South Batinah Coast of the Sultanate of Oman represents what was once a section of the Earth's sea floor -- or Oceanic Crust. Most of the Earth's oceans are underlain by Oceanic Crust, which is typically 5-7 kilometres thick and sits directly on top of the Upper Mantle. In contrast the Earth's land surface is almost exclusively made up of Continental Crust, forming a series of Plates or Continental Plates. Continent Crust is much thicker than Oceanic Crust and can reach up to 70 kilometres thick under present day or ancient mountain chains.
The geological term "Ophiolite" refers to a specific sequence of ultramafic, mafic and sedimentary rocks which form as Oceanic Crust on the sea floor of our planet. Oceanic Crust forms between Continental Plates as they "drift" across the planet's surface. As Continent's drift apart, new Oceanic Crust is formed at what is called the "Mid-Oceanic Ridge". For example, as Africa and South America have drifted apart over the last 100-200 million years, new Oceanic Crust has been forming continuously at the Mid-Atlantic Ridge.
Schematic stratigraphy through Oceanic Crust & the Semail Ophiolite
(From Eastern Mediterranean Resources (EMED) "AIM Admission Document" -- May 2005)
The other important characteristic of Ophiolites is that they have been structurally emplaced on top of Continental Crust -- they have been "Obducted," or pushed, or thrusted into place. Thus a portion of Oceanic Crust is emplaced on the land surface.
The Semail Ophiolite in Oman and the Troodos Massif in Cyprus are the best preserved and most academically studied ophiolite complexes in the world. It is largely from these two locations that the full sequence of rocks within the Oceanic Crust and within Ophiolites has been determined:
The Semail Ophiolite - Oman
||Harzburgites -- strongly banded harzburgite-lherzolite-dunite
(in Oman these are "tectonised" and therefore thought to represent the residual component of partially melted spinel lherzolite -- ie residual material which originally gave rise to the basaltic melts seen higher up in the sequence)
Dunites -- as pod-like or lensoid intrusive (with associated chromite)
(representing intrusive complexes which are the feed for the magmas which form the spreading ridge basalts)
||Gabbro/Peridotites -- layered mafic and ultramafic cumulates and intrusive rocks which formed as linear magma chambers
(presence of plagioclase feldspar represents the boundary between Upper Mantle and Lower Crustal sequence)
||Gabbros/Diorites/Trondhjemites -- formed at the top of the magma chambers in vertical, sheeted-dyke complexes. The attitudes of the dykes parallel the ridge axis and confirm the 100% crustal extension which takes place at the spreading ridge margin|
|Extrusive Pillow Basalts
||Basalts -- pillowed and massive basalts which overly the sheeted dyke complex and were extruded on to the sea floor, predominantly of tholeiitic composition. Volcanogenic massive sulphide (VMS) deposits develop in these basalts, often related to metalliferous sediments
(in Oman this layer is some two kilometres thick with five distinct types of pillow lavas mapped)
||Sediments -- deep sea sediments, characteristically metalliferous (umbers), siliceous (radiolarian cherts) or very fine grained cherts (typically chocolate-brown or purple in colour)
(these sediments represent quieter periods as gaps between the more frequent volcanic activity associated with the spreading ridge)
The Semail Ophiolite has been radiometrically dated as between 97.9 and 93.5 million years (Mid-Cretaceous) and is therefore part of the wider Tethyan Orogen. It is believed to have formed in a "Proto-Ocean" located approximately where The Gulf and The Gulf of Oman are today. This Proto-Ocean formed in the Mid-Cretaceous but closed again in the Late Cretaceous (approximately 70-80 million years ago). As the Proto-Ocean closed, the Oceanic Crust which had formed as the Continental Crust parted, rather than being subducted beneath, was at least partly pushed up onto of the Arabian Plate to the west, to form the Semail Ophiolite. This pushing onto the Continental Plate was achieved in a number of discrete but related thrusting episodes.
The Semail Ophiolitic Belt is approximately 600 kilometres long, between 100 to 150 kilometres wide and 5 to 10 kilometres thick. The modern desert terrain in Oman has resulted in the unique exposure of most of the ophiolite belt. Six tectonic regions have been identified, relating to six major episodes of ophiolite emplacement. Within each of these tectonic regions, individual thrusts and emplacement faults can be mapped and identified.
Tectonically defined geological blocks in the Semail Ophiolite Belt in the Sultanate of Oman
Each block consists of a structurally distinct complete ophiolite section, from the mantle to volcanics
(From Kenex Ltd -- "Block 5 -- Prospectivity Study and Exploration Targeting Review for Volcanogenic Massive Sulphide Mineralisation" - March 2009)