Obduction

Upwedging in subduction zones This process is operative beneath and behind the inner walls of oceanic trenches (subduction zone) where slices of oceanic crust and mantle are ripped from the upper part of the descending plate and wedged and packed in high pressure assemblages against the leading edge of the other plate. Weakening and cracking of oceanic crust and upper mantle is likely to occur in the tensional regime. This results in the incorporation of ophiolite slabs into the overriding plate. for the emplacement of the Semail Ophiolite complex in Oman and argued by Church and Church and Stevens for the emplacement of the Bay of Islands sheet in western Newfoundland. This concept has subsequently been replaced by hypotheses that advocate subduction of the continental margin beneath oceanic lithosphere. Transformation of a spreading ridge to a subduction zone Many ophiolite complexes were emplaced as thin, hot obducted sheets of oceanic lithosphere shortly after their generation by plate accretion. suggest that a common form of ophiolite obduction is related to the closure of rear-arc marginal basins and that, during such closure by subduction, slices of oceanic crust and mantle may be expelled onto adjacent continental forelands and emplaced as ophiolite sheets. In the high heat-flow region of a volcanic arc and rear-arc basin the lithosphere is particularly thin. This thin lithosphere may preferentially fail along gently dipping thrust surface if a compressional stress is applied to the region. Under these circumstances, a thin sheet of lithosphere may become detached and begin to ride over adjacent lithosphere to finally become emplaced as a thin ophiolite sheet on the adjacent continental foreland. This mechanism is a form of plate convergence where a thin, hot layer of oceanic lithosphere is obducted over cooler and thicker lithosphere. Continental collision As an ocean is progressively trapped in between two colliding continental lithospheres, the rising wedges of oceanic crust and mantle rise are caught between the continental plates and detach and begin to move up the advancing continental rise. Continued convergence may lead to the overthrusting of the arc-trench gap and eventually overthrusting of the metamorphic plutonic and volcanic rocks of the volcanic arc. Following total subduction of an oceanic tract, continuing convergence may lead to a further sequence of intra-continental mechanisms of crustal shortening. This mechanism is thought to be responsible for the various ocean basins of the Mediterranean region. The Alpide belt is believed to register a complex history of plate interactions during the general convergence of the Eurasian and African plates. ==Examples==

There are few continental plates being obducted under an oceanic plate known today, but in the past it appears to have happened a number of times. Thus there are examples of oceanic crustal rocks and deeper mantle rocks that have been obducted and are now exposed at the surface, worldwide. New Caledonia is one example of recent obduction. The Klamath Mountains of northern California contain several obducted oceanic slabs, most famously the Coast Range Ophiolite. Obducted fragments also are found, for example, in the Hajar Mountains of Oman, the Troodos Mountains of Cyprus, Newfoundland, New Zealand, the Alps of Europe, the Shetland islands of Unst and Fetlar, Leka island in Norway, and the Blue Ridge Ophiolite in the Appalachian Mountains of eastern North America. ==See also==