Ocean-Ocean Subduction Zones System

1. Ocean-Ocean Subduction Zones System Jia Zhang

2. 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

3. 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 Mato 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;

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

5. 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

6. transform/fracture zone

7. passive continental/arc margin

8. Internal Structure of Intra-oceanicArcs

9. 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.

11. Magmatic Crust Growth andThermal-Chemical Convectionin 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?

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

13. thermal-chemical mantle wedge convection by 3D model

14. 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

16. 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.

17. Thanks