Ocean-Ocean Subduction Zones System Jia Zhang

intra-oceanic subduction brings oceanic slabs under the overriding plates of oceanic origin  oceanic magmatic arcs comprise nearly 40% of the subduction margins of the Earth

A comprehensive review on tectonic and magmatic processes: Convergence rates are in the range 5–13 cm/yr; Ages of subducting slabs range from 150 Ma to 0; Sediment thicknesses are notably variable (from 70m to >6 km, typically 150–650 m); Most modern intraoceanic arcs are non-accreting; Most of the arcs have closely associated back-arc rifts; Arc thicknesses depend on arc maturity; Pre-arc basements of the arcs are very variable;

Issues • Initiation of intra-oceanic subduction • Internal structure and composition of arcs • Subduction channel processes • Dynamics of crustal growth • Geochemistry of intra-oceanic arcs

Initiation of Intra-oceanic Subduction gravitational instability & ridge push  the bending and shear resistance of the lithosphere what forces can trigger subduction ? (12 hypotheses) Transference initiation & Polarity reversal initiation

transform/fracture zone

passive continental/arc margin

Internal Structure of Intra-oceanic Arcs

Subduction Channel Processes Burial and exhumation of high-pressure metamorphic rocks in subduction zones are likely affected by progressive hydration (serpentinization) of the fore-arc mantle lithosphere; The shape of the P–T path, and the maximum P–T conditions achieved by an individual high-pressure metamorphic rock, depend on the specific trajectory of circulation in the subduction channel; An array of diverse, though interrelated, P–T paths rather than a single P–T trajectory is expected to be characteristic for subduction-related metamorphic complexes.

Magmatic Crust Growth and Thermal-Chemical Convection in the Mantle Wedge dehydration of subducting slabs and hydration of the overlying mantle wedges are key processes controlling magmatic activity and consequently crustal growth above subduction zones detailed thermal structure and melt production patterns above slabs?

petrological-thermomechanical models including water transport and melting Development of plumes by 2D model

thermal-chemical mantle wedge convection by 3D model

Geochemistry of Intra-oceanic Arcs Analyses of geochemical data sets for the input and output signals of rock-members from several arc systems Melting of hydrated mantle and subducted tectonic melanges in respectively unmixed and mixed thermal-chemical plumes rising from the top of the slab

Conclusions Modern intra-oceanic subduction zones comprise around 40%, of the convergent margins of the Earth and most of them are not accreting sediments and have back-arc extension. two major types of subduction zone nucleation scenarios are proposed: induced and spontaneous Internal structure and compositions of intra- oceanic arcs is strongly variable.

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