WHAT CONSTITUTES "EMPLACEMENT" OF AN OPHIOLITE?

Ophiolites have come to be recognized as on-land fragments of oceanic crust. Oceanic crust becomes an ophiolite as a result of a process known as "emplacement". For Tethyan ophiolites, emplacement has been defined as the thrusting of an ophiolite over a continental margin, an event that took place within the first several million years after intraoceanic thrusting or subduction initiated beneath the ophiolite. This emplacement process is driven by the collision of a leading edge of a continental mass with a subduction zone, above which the future ophiolite is situated. In contrast, emplacement for other types of ophiolites is either poorly defined, or differs markedly from the definition of emplacement for Tethyan ophiolites. For example, the Macquarie Island ophiolite has been emplaced over oceanic crust (instead of a continental margin) due to transpression as a result of oblique convergence along the plate boundary between the Australian and Pacific plates. Many ophiolites in the North American Cordillera were tectonically emplaced onto a subduction complex, rather than onto a continental mass. The process of subduction and accretion beneath many of the Cordilleran ophiolites was progressive with no uniquely defined emplacement episode, except for the initiation of subduction. For example, the Franciscan subduction complex formed beneath the Coast Range ophiolite of California as a result of over 140 million years of continuous subduction. No specific accretionary event in the history of the Franciscan complex can be singled out as the emplacement of the Coast Range ophiolite, with the exception of the inception of the Franciscan subduction. Thus, for the Macquarie Island and many Cordilleran ophiolites, emplacement may be best defined as the inception of intraoceanic thrusting or subduction and subsequent accretionary uplift with continued subduction, whereas for Tethyan ophiolites emplacement can be construed as collision-driven thrusting of fossil oceanic crust over the leading edge of a continental margin, following the initiation of intraoceanic thrusting.