Friday, 2 October 2015

How do deep ocean trenches form?...by Sev Kender

Sev Kender at his microscope 
One of the biggest questions remaining to be answered in plate tectonics is how subduction zones start, or ‘initiate’. Plate tectonics and seafloor spreading was a ground-breaking theory discovered in the mid-20th-Century that explained much of geology, and started our modern discipline. Before it there was no single accepted theory of why oceans and mountains formed, why continents look like they used to be linked together, and why animals of different continents appeared to have long-lost common ancestors. Here Sev Kender tells us about some recent advances in the science… 

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
One way to understand subduction zone initiation is to drill a long borehole into the ocean crust on the overlying plate, to test the composition and age of the crust, and to see how it behaved (in terms of sea level changes), before the subduction started. The problem is that millions of years of time since the initiation has allowed kilometres of sediment to pile up on top and obscure the crust.

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 
One idea is that the subduction zone formed along a previous line of weakness in one of these fracture zones, but our records do not prove this. They do, however, show that the initiation was probably 'spontaneous' rather than 'induced', as the crust was formed in an extensional setting and did not become uplifted before formation. This has allowed us to begin understanding the process of subduction initiation, and further analyses over the coming years of the rocks collected will help us refine this new model, and understand the evolution of the Izu-Bonin-Mariana arc since its inception.
By Sev Kender (Research Fellow within the Centre for Environmental Geochemistry, BGS-University of Nottingham). 

Follow Sev on twitter @SevKender 



Images

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

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