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Kosuke2, Ueda, University of Bergen (Switzerland)
Taras Gerya, ETH Zürich (Switzerland)
Stephan Sobolev, GeoForschungsZentrum Postdam (Germany)
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The initiation of subduction has been studied to some extent for compressional tectonic regimes. Initiation appears to be hard to trigger and to occur close to criticality, since both the location of a plate-boundary and downwards forcing of slabs are required. In this study, we have investigated the evolution of a plume-lithosphere configuration and its potential development of intra-plate subduction zones. Numerical experiments were performed with a marker-in-cell conservative finite-difference technique (Gerya and Yuen, 2003), applying visco-plastic rheology and accounting for phase changes. No external forcing was implemented.
Test series have lead to different tectonic results. A failure mode shows sub-lithospheric underplating of the plume material; on the other hand, large, strongly buoyant plumes might segment the overlying lithosphere (fragmentation mode). Self-sustaining and retreating subduction is observed for the interaction of thermal-chemical plumes with young oceanic lithosphere under the presence of fluids. Sensitivity studies revealed that 1) chemical buoyancy is more important than thermal buoyancy; 2) release and migration of fluids from the slab is essential for the stabilization of subduction; 3) the plastic strength of the lithosphere exerts a large influence on the initiation. However, weakening of the lithosphere is observed locally above the plume material and is related to melt and/or fluid propagation where the plastic strength is decreased to minimal values.
Local weakening seems to be a key characteristic of plume-triggered subduction initiation. The intensity of weakening, and the decoupling of lithosphere portions, depends on plume volume, plume buoyancy (composition), lithosphere thickness and on the presence of high-pressure fluids. Initial downward forcing of slabs is provided by the plume head which crosses the lithosphere and spreads at the surface, thus overriding the slabs. When slabs reach the necessary depth, they undergo transition of gabbroic portions to denser eclogite, resulting in a downwards feedback that might make subduction effectively self-sustaining. Possible analogues might be Archean tectonics with already present oceans, or corona structures on Venus which could match fluid-free, instable and circular subduction responses to mantle plumes.
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