Download Ocean-Ocean Subduction Zones System

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 intraoceanic arcs is strongly variable.
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