Subduction zone formation finally explained
7 Oct 2015 by Evoluted New Media
Geologists have explained how subduction zones – zones where one tectonic plate moves under another and sinks into the mantle – form.
Geologists have explained how subduction zones – zones where one tectonic plate moves under another and sinks into the mantle – form.
A research team at the University of Nottingham used samples from the Amami Sankaku Basin in the northwest Philippine Sea and found that the crust was much younger than expected. This indicated spontaneous formation of the subduction zone where one tectonic plate sinks because it is denser, rather than the plates being forced together by pressure.
Dr Sev Kender from the University of Nottingham said: “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 today. Before it there was no single accepted theory of why oceans and mountains formed, and why continents look like they used to be linked together. This latest discovery addresses one of the last links in the theory that explains how geology works – which is a really significant find.”
In the study, published in the journal Nature Geoscience, the team drilled into the crust of Izu-Bonin-Mariana (IBM) volcanic arc and collected borehole data. They analysed the age of the crust to see how it behaved before subduction started.
“During our expedition to the IBM Arc, we successfully collected 1.5km of borehole through the overlying sediments and into the crust itself, dating the rocks with microscopic fossils and magnetic field reversals that took place throughout Earth’s history,” said Dr Kender.
The geologists believe that the chemistry of the crust suggests it was formed at the same time as the IBM subduction zone, 52–48 million years ago, in a mode consistent with the spontaneous initiation of subduction. They suggest the crust formed in a similar manner to the way mid-ocean ridges form as convection currents rise in the mantle beneath the oceanic crust and create magma where the two tectonic plates – the Pacific Sea Plate and Philippine Sea Plate – meet.
Dr Kender said: “We found the crust to be much younger than expected, 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.
“One idea is that the subduction zone formed along a previous line of weakness in one of these fracture zones, although it is not proven. Our new records 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 finding really takes us one step closer to discovering how plate tectonics really work.”
Paper: http://www.nature.com/ngeo/journal/v8/n9/full/ngeo2515.html
