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Ophiolite Emplacement: Mechanisms and Processes |
1 , 1329 Sheridan Lane, Hayward, CA 94544, USA wako{at}tdl.com
2 Department of Geology, Miami University, Oxford, OH 45056, USA
Ophiolites have long been recognized as on-land fragments of fossil oceanic lithosphere, which becomes an ophiolite when incorporated into continental margins through a complex process known as emplacement. A fundamental problem of ophiolite emplacement is how dense oceanic crust becomes emplaced over less dense material(s) of continental margins or subduction-accretion systems. Subduction of less dense material beneath a future ophiolite is necessary to overcome the adverse density contrast. The relationship of subduction to ophiolite emplacement is a critical link between ophiolites and their role in the development of orogenic belts. Although ophiolite emplacement mechanisms are clearly varied, most existing models and definitions of emplacement concern a specific type of ophiolite (i.e. Oman or Troodos) and do not apply to many of the worlds ophiolites. We have defined four prototype ophiolites based on different emplacement mechanisms: (1) Tethyan ophiolites, emplaced over passive continental margins or microcontinents as a result of collisional events; (2) Cordilleran ophiolites progressively emplaced over subduction complexes through accretionary processes; (3) ridge-trench intersection (RTI) ophiolites emplaced through complex processes resulting from the interaction between a spreading ridge and a subduction zone; (4) the unique Macquarie Island ophiolite, which has been subaerially exposed as a result of a change in plate boundary configuration along a mid-ocean ridge system. Protracted evolutionary history of some ocean basins, and variation along the strike of subduction zones may result in more complicated scenarios in ophiolite emplacement mechanisms. No single definition of emplacement is free of drawbacks; however, we can consider the inception of subduction, thrusting over a continental margin or subduction complex, and subaerial exposure as critical individual stages in ophiolite emplacement.
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