Subduction and Obduction Processes

Fragments of ancient oceanic lithosphere preserved in mountain belts, though volumetrically subordinate, provide essential insights into past geodynamics and formation and destruction of oceanic lithosphere. This contribution shows how the two types of oceanic fragments, blueschists and eclogites, on one hand, and ophiolites on the other, preserve crucial information on the dynamics of oceanic convergence, that is, subduction and obduction. Their mutual relationships, as well as the similarities and differences in the mechanisms leading to their preservation, allow tracking the evolution of the subduction process through time, from the onset of intraoceanic subduction to the cessation of continental subduction, and, in some cases, to the obduction of ophiolites. Fragments located at the base and immediately below unmetamorphosed (true) ophiolites represent witnesses of intraoceanic subduction initiation and reveal, in particular, initial mechanical resistance to subduction, subsequent cooling, and gradual strain localization. Subducted fragments of oceanic lithosphere metamorphosed as blueschists and eclogites, scraped off the downgoing slab episodically, at shallow or great depths, provide direct access to the composition, structure, and rheology of rocks at the plate interface. Both types reflect the mechanical behavior and "hiccups" of the subduction plate boundary, during subduction initiation and mature subduction, respectively.