Magmatic Fingerprints of Subduction Initiation and Mature Subduction of the Izu-Bonin-Mariana Subduction Zone: Numerical Modelling and Observations

Subduction of oceanic lithosphere has been proposed as the main driving mechanism for plate tectonics for decades and it represents a key process for the geochemical cycles on Earth. However, the physical processes and melting that occur as the subduction zone began foundering and evolved to reach a mature stage is still debated. The Izu-Bonin-Mariana (IBM) intra-oceanic subduction zone, that represents the boundary between the Pacific Plate and the Philippine Sea, is an ideal natural laboratory to study subduction zone processes from their inception to their stabilization. The rock record produced in IBM reveals a rapid compositional variability in slab-fluid tracers as well as in mantle depletion-enrichment over a short timescale (within 1 to 5 Ma of subduction inception). Despite this geochemical evolution, it is still highly debated whether IBM initiated as a forced or spontaneous subduction zone, i.e. induced by or in the absence of horizontal forcing, respectively.

Here, we conducted 2D high-resolution petrological-thermomechanical subduction models that include spontaneous deformation, erosion, sedimentation and slab dehydration processes, as well as melting, assuming a visco-plastic rheology using the i2VIS code. We aimed to model the initiation and the early stage of IBM with ultra-low horizontal forcing and inception triggered by transform collapse. Our new numerical model proposes a viable scenario for the transition from juvenile to mature subduction zone. This evolution includes initiation by gravitational collapse of the slab and the development of a near-trench spreading, the gradual build-up of a return flow of asthenospheric mantle and the progressive maturation of the volcanic arc. Our numerical results of mantle depletion within the mantle wedge and the overall subduction history of IBM are compared further with seismological and geochemical evidences.