WHAT CONSTITUTES 'EMPLACEMENT' OF AN OPHIOLITE?: MECHANISMS AND RELATIONSHIP TO SUBDUCTION INITIATION AND FORMATION OF METAMORPHIC SOLES

Abstract

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 world's 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.