Effect of water on rock deformation and rheological structures of continental and oceanic plates

Abstract

Rheological structures for continental and oceanic plates were calculated using the laboratory-based frictional and flow laws. Our results show that Peierls creep becomes the dominant mechanism for plastic deformation at low temperatures and high stresses under both dry and wet conditions. When Peierls creep, rather than dislo-cation-accommodated power-law creep, is the dominant mechanism, there is a lower maximum mechanical strength, and a shallower depth to the brittle-plastic transition in our model. The rheology of continental lithosphere is complex, but may feature a weak lower crust sandwiched between a strong upper crust and mantle. Extensive deformation of this weak zone may explain crustal duplication in continental collision zone. The thick continental lithosphere beneath craton is stable for billions of years. This is partly because partial melting has depleted these regions of water, causing an increase in the mechanical strength of the continental lithosphere. The depth of brittle-plastic transition at island arcs, which we inferred from the maximum depth of seismicity, suggests that the fore-arc regions are enriched in water, but that the back-arc regions are depleted. The rheological structure of oceanic plate is dependent on the age of the plate, and the thickness of oceanic lithosphere is largely consistent with a dry rheology. A relatively thin elastic thickness found for the oceanic lithosphere in the Mariana and Bonin arcs may be explained by local weakening as a result of hydration along outer-rise faults.