|Sev Kender at his microscope|
Subduction zones, like the deep Mariana Trench off the south coast of Japan, are where one plate is pushed under another as they move towards each other. The underlying plate is consumed into the Earth’s mantle, and creates hot magma that erupts from volcanoes on the surface of the overlying plate (e.g. the Northern Mariana Islands). It is quite problematic to explain how a piece of passive ocean crust should suddenly break apart and start to form a trench, and there are two leading models that exist to explain how a subduction zone may start: ‘spontaneous’ (one side sinks because it is more dense) or ‘induced’ (forced by pressure from another, distant, source). But it is difficult to test these ideas, because the process cannot be observed happening today. Subduction zones persist for many millions of years, and the initiation period happened millions of years ago in most cases.
|The location of the research into the crust|
of the Izu-Bonin-Mariana arc
Myself and 30 other scientists travelled to the Philippine Sea in summer 2014 on the drillship JOIDES Resolution, operated by the International Ocean Discovery Program, to drill into the crust of the Izu-Bonin-Mariana arc. This is an extinct ocean trench zone south of Japan, where the modern-day Mariana Trench initiated. In our article in Nature Geoscience we report how we successfully collected 1.5 km of borehole through the overlying sediments and into the crust itself, dating the rocks with microfossils and magnetic field reversal ‘magnetochrons’ (known past reversals that have been dated by other techniques in other records).
We found the crust to be much younger than expected (Eocene, about 50 million years old), a stunning discovery indicating that we needed to readjust our ideas of how the subduction zone formed. The crust has chemical characteristics indicating it was formed at the time the subduction zone started, rather than much earlier. The crust may have formed in an extensional setting through seafloor spreading, in some ways similar to that formed at mid-ocean ridges today, although in this case near the newly-formed subduction zone.
Mid-ocean ridges are where fresh new oceanic crust is formed and are the opposite of subduction zones. There are numerous 'transform faults' near ridges today, enormous fractures through the crust that form due to the spreading plates interaction with the curvature of the earth.
|A thin section through the young crust|
By Sev Kender (Research Fellow within the Centre for Environmental Geochemistry, BGS-University of Nottingham).
Follow Sev on twitter @SevKender
Sev Kender at his microscope
The location of the research into the crust of the Izu-Bonin-Mariana arc
A thin section through the young crust