Obduction

Seafloor spreading eventually pushes oceanic crust into subduction zones where the hydrothermal sediments and rock are recycled back into the mantle. A small fraction of these deposits is uplifted, or obducted, onto land. These rescued deposits are termed ophiolites. Because of their metal enrichments, they serve as major ore bodies and have been mined for various precious metals, such as copper, for thousands of years. 

A U-Pb zircon age of 480 1 Ma from layered gabbros at Scalp (Fig 1b - SC) provides the only direct age for the ophiolitic Tyrone Plutonic Group so far. This age date negates the need for the obduction of contemporaneous ophiolite and an active volcanic arc and simplifies the tectonomagmatic evolution. 

The difference in age between the ophiolitic Tyrone Plutonic Group (c. 480 Ma) and Tyrone Volcanic Group (c. 473 -464 Ma) allows time for northwards directed obduction (c. 475 Ma) of the Tyrone Plutonic Group related to southwards directed subduction (Fig. 4c), followed by the establishment of northwards directed subduction and the development of the Tyrone Volcanic Group arc (Fig. 4d). 

Crystal field theory predicts that Ni2+, because of its high CFSE in octahedral coordination in minerals, should remain behind in refractory phases resulting from partial fusion of the oceanic lithosphere and become enriched in the Upper Mantle. It is possible, however, that the nickel reappears in alpine peri-dotites or obducted ophiolite suites. 

Plate tectonic activity, which is responsible on Earth for subduction zones, spreading centres and obducted ophiolites, as well as associated ore deposits of Cu, Cr and Ni described in 8.6, appears to have been less significant on other terrestrial planets. As a result, local enrichments of these and other transition elements (apart from Fe and Ti) are probably absent on the Moon, Mercury, Venus, Mars and the asteroids. Since Fe and Ti minerals are predominant on terrestrial planets, electronic spectra of Fe2+ and Fe3+ in silicates and oxides influenced by Ti4+ and Ti3+ are expected to dominate remote-sensed spectra of their surfaces. 

These two end-members provide a simple frame for the classification of the ophiolites in two main groups (e.g., Boudier and Nicolas, 1985 Ishiwatari, 1985 Nicolas and Boudier, in press). They are briefly examined in the following, together with the rare subarc mantle sections exposed at the base of obducted island arcs (e.g., Burg et al., 1998). 

The Semail (=Oman) ophiolite is probably the largest piece of oceanic lithosphere (2 X 10" km ) exposed at the surface of the Earth. It is considered to represent part of the Neo-Tethys ocean floor obducted onto the Arabian plate at the end of the Cretaceous. It has been the focus of detailed studies by several research groups and is, therefore, one of the best documented ophiolites (e.g., Glennie et al., 1974 Coleman and Hopson, 1981 Lippard et al., 1986 Boudier and Nicolas, 1988 Peters et al., 1991b Boudier and Juteau, 2000). 

Bard J.-P. (1983) Metamorphism of an obducted island arc example of the Kohistan sequence (Pakistan) in the Himalayan collided range. Earth Planet. Set Lett. 65, 133-144. 

Bard J.-P., MalusM H., Matte P., and Proust F. (1980) The Kohistan sequence crust and mantle of an obducted island arc. Geol. Bull. Univ. Peshawar 11, 87-94. 

Our knowledge of the architecture of the oceanic cmst derives from four main sources studies of portions of the oceanic cmst that have been obducted onto land (ophiolite complexes) drilling of the ocean cmst exposures of the deeper cmst at fracmre zones and rare tectonic windows and geophysical smdies of the seismic properties of the ocean cmst. These smdies have revealed that the general structure of the igneous ocean cmst, from top to bottom, consists of a carapace of basaltic lava flows and pillows... 

Kerr A. C., Tarney J., Nivia A., Marriner G. F., and Saunders A. D. (1998) The internal structure of oceanic plateaus Inferences from obducted Cretaceous terranes in western Colombia and the Caribbean. Tectonophysics 292, 173-188. 

One special type of orogeny that can happen during a continental collision is the rise of ophiolite mountains. On rare occasions the crast beneath the ocean floor fractures along the tectonically active coast of a continent, and oceanic crust is thrast up over the shore and forms mountains. This spectacular form of plate-tectonic backfire is not supposed to happen, yet it does often enough to have its own name obduction, meaning over (ob-) leading (-duction). A piece of oceanic crast, and the... 

Obduction— A geological "accident" wherein a piece of the oceanic crust gets put on top of the continental crust, as opposed to beneath it as usual. 

Searle, M. P. Cox, J. 1999. Tectonic setting, origin, and obduction of the Oman Ophiolite. Geological Society of America Bulletin, 111, 104-122. 

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