Initiation of Subduction Zones as a Consequence of Lateral Compositional Buoyancy Contrast within the Lithosphere: a Petrological Perspective

Tonga and Mariana fore-arc peridotites, inferred to representtheir respective sub-arc mantle lithospheres, are compositionallyhighly depleted (low Fe/Mg) and thus physically buoyant relativeto abyssal peridotites representing normal oceanic lithosphere(high Fe/Mg) formed at ocean ridges. The observation that thedepletion of these fore-arc lithospheres is unrelated to, andpre-dates, the inception of present-day western Pacific subductionzones demonstrates the pre-existence of compositional buoyancycontrast at the sites of these subduction zones. These observationsallow us to suggest that lateral compositional buoyancy contrastwithin the oceanic lithosphere creates the favoured and necessarycondition for subduction initiation. Edges of buoyant oceanicplateaux, for example, mark a compositional buoyancy contrastwithin the oceanic lithosphere. These edges under deviatoriccompression (e.g. ridge push) could develop reverse faults withcombined forces in excess of the oceanic lithosphere strength,allowing the dense normal oceanic lithosphere to sink into theasthenosphere beneath the buoyant overriding oceanic plateaux,i.e. the initiation of subduction zones. We term this conceptthe ‘oceanic plateau model’. This model explainsmany other observations and offers testable hypotheses on importantgeodynamic problems on a global scale. These include (1) theorigin of the 43 Ma bend along the Hawaii–Emperor SeamountChain in the Pacific, (2) mechanisms of ophiolite emplacement,(3) continental accretion, etc. Subduction initiation is notunique to oceanic plateaux, but the plateau model well illustratesthe importance of the compositional buoyancy contrast withinthe lithosphere for subduction initiation. Most portions ofpassive continental margins, such as in the Atlantic where largecompositional buoyancy contrast exists, are the loci of futuresubduction zones. KEY WORDS: subduction initiation; compositional buoyancy contrast; oceanic lithosphere; plate tectonics; mantle plumes; hotspots; oceanic plateaux; passive continental margins; continental accretion; mantle peridotites; ophiolites