27 August 2018

Why subduction zones form

Subduction zone
A subduction zone.

An international team of earth scientists from Canada, the Netherlands, Norway, and the UK now showed for the first time that subduction zones form because plate tectonic changes elsewhere start pushing one plate below another. Previously, it was widely assumed that subduction zones form spontaneously because of gravitational instability. They published their results in Nature Geoscience on 27 August.

The outer, rigid shell of the Earth is broken into 60-200 km thick tectonic plates that move along each other along large fault zones, causing most of Earth’s earthquakes. The most violent of plate boundaries are subduction zones, where one plate dives below another, back into earth’s interior. Once active, subduction zones can exist for tens or even hundreds of millions of years, but how do they form?

The widely accepted idea of how subduction zones form suggested that a piece of plate started to spontaneously sink into the mantle. This, then, creates a gap at the Earth surface that becomes filled with magma producing new, young crust. Only after millions of years would the sinking plate become heavy enough to start pulling the plates together, creating horizontal plate motion and regular subduction.

The study’s lead author Dr Carl Guilmette of Laval University, Quebec, Canada explains: 'there are rocks on earth that we think formed in the early stages of a subduction zone. Such rocks are for instance found in Oman, where most rocks formed by filling a gap with new crust next to a new subduction zone, and some formed right at the subduction interface. Previously, we could only date when the rocks of the new crust formed and assumed that this occurred simultaneously with the first formation of a subduction zone’.

Dr Matthijs Smit, a dating specialist at the University of British Columbia, Canada, and co-author of the study, now managed to also date the formation of the rocks at the subduction interface. “We show that they are at least 8 million years older than the age of the crust that filled the gap. This means that horizontal plate motion came first, and the formation of the gap came later as a result, and not as a cause, of subduction.”

Prof Douwe van Hinsbergen of Utrecht University, the Netherlands, explains that the new result has major implications for our understanding of plate tectonics. “For the first time, we unequivocally demonstrates that subduction zones form because of changes in plate motion elsewhere. This means that changes in motion between two plates may have unexpected effects across the planet, including the formation of new subduction zones. Focus of follow-up research is now on which processes exactly drove the formation of the subduction zone that existed in Oman, but we may be searching across the planet.”