Initiation of subduction zones along transform and accreting plate boundaries, triple-junction evolution, and forearc spreading centres—implications for ophiolitic geology and obduction

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Abstract
Summary: Most large ophiolite complexes have been interpreted as representing the obducted remnants of oceanic basement of forearc regions. A knowledge of the tectonic setting of ophiolite formation as well as the mechanism of their entrapment within forearc regions is essential to achieve a more complete understanding of the often complicated geology of ophiolite complexes and the tectonic processes that result in their evolution and ultimate obduction. The mechanism of entrapment of an ophiolite complex within a forearc region is intimately related to the mechanisms involved in trench initiation. The most easily documented and apparently likely mechanisms of trench initiation and isolation of oceanic basement within forearcs are: (i) polarity flips along weakened back-arc/island-arc interfaces, and (ii) plate boundary evolution involving the conversion of active transform and accreting plate boundaries into subduction zones. These latter mechanisms of trench initiation are predicted by plate theory to occur by the evolution of transform and accreting plate boundaries upon (usually) gradual and (less often) abrupt changes in their position with respect to the instantaneous pole of relative motion, and by the evolution of certain types of triple junctions that leads to rapid conversion of transform and accreting plate boundaries to subduction zones. These plate boundary evolutionary schemes involve spatially and temporally complex overlap of subduction and plate accretion processes in forearc regions. The overlapping processes occurring during these conversions may result in ‘atypical’ oceanic-crustal magmas that are produced during or soon after sea-floor spreading within the forearc region. These magmas may not resemble typical mid-ocean-ridge basaltic (MORB) compositions. Such ‘abnormalities’ in the petrology of some ophiolite complexes need not necessarily imply that a process other than sea-floor spreading was the dominant plate-boundary process responsible for the generation of these ophiolite complexes. The term ‘island-arc ophiolite’ attached to several ophiolite complexes on the basis of their petrology may not appropriately reflect the dominant plate-boundary process (i.e. sea-floor spreading) and should be used cautiously and only if supported by non-petrologic data (e.g. field observations and structural relationships).