INITIATION OF SUBDUCTION ZONES AS A CONSEQUENCE OF LATERAL COMPOSITIONAL BOUYANCY CONTRAST WITHIN THE LITHOSPHERE: A PETROLOGICAL PERSPECTIVE

Abstract

Tonga and Mariana forearc peridotites, inferred to represent their respective subarc lithospheres, are compositionally highly depleted (low Fe/Mg) and thus physically buoyant relative to abyssal peridotites representing normal oceanic lithosphere (high Fe/Mg) formed at ocean ridges. The observation that the depletion of these forearc lithospheres is unrelated to, and predates, the inception of present-day western Pacific subduction zones demonstrates the pre-existence of compositional buoyancy contrast at the sites of these subduction zones. These observations allow us to suggest that lateral compositional buoyancy contrast within the oceanic lithosphere creates the favored and necessary condition for subduction initiation. Edges of buoyant oceanic plateaus, for example, mark a compositional buoyancy contrast within the oceanic lithosphere. These edges under deviatoric compression (e.g., ridge push) could develop reverse faults with combined forces in excess of the oceanic lithosphere strength, allowing the dense normal oceanic lithosphere to sink into the asthenosphere beneath the buoyant overriding oceanic plateaus, i.e., the initiation of subduction zones. We term this concept the Oceanic Plateau Model. This model explains many other observations and offers testable hypotheses on important geodynamic problems on a global scale. These include (1) the origin of the 43 Ma bend along the Hawaii-Emperor seamount chain in the Pacific, (2) mechanisms of ophiolite emplacement, (3) continental accretion etc. Subduction initiation is not unique to oceanic plateaus, but the plateau model well illustrates the importance of the compositional buoyancy contrast within the lithosphere for subduction initiation. Most portions of passive continental margins, such as in the Atlantic where large compositional buoyancy contrast exists, are the loci of future subduction zones.