Scientists discover slippery base on underside of tectonic plate - 05/02/2015

Scientists using underground explosions have discovered a previously unknown slippery layer of rock under the Pacific tectonic plate in research which is helping to understand the way tectonic plates work.

Vaughan Stagpoole installing a seismic device to record the blast waves from the explosions detonated as part of this science project.

The finding comes from a New Zealand-led international study that used 1200 portable seismographs placed in a long line between the Wairarapa and Kapiti coasts.

By setting off explosive charges in the ground and recording the echo with seismic instruments, the scientists were able to build a three-dimensional picture of geological layers to a depth of about 100km below the lower North Island.

The project involved scientists from New Zealand, Japan, and the United States. It was led by GNS Science and Victoria University of Wellington.

The initial focus was the top 50km of subsurface to see how the Pacific and Australian plates interact. However, the exceptional quality of the data enabled the scientists to resolve structures to twice that depth.

“The discovery of this 10km-thick layer of slippery soft rock immediately below the subducting Pacific Plate is purely serendipitous and a product of the excellent data,” said project leader Stuart Henrys of GNS Science.

" To fully understand this requires data on what happens at the bottom of a tectonic plate. It is a real challenge for conventional geophysical methods to obtain quality data from such great depths Dr Stuart Henrys "

The research is reported in a paper entitled ‘A seismic reflection for the base of a tectonic plate’, published in the latest issue of the prestigious science journal Nature . The lead author of this paper is Professor Tim Stern of Victoria University.

The recorded data was much higher resolution than had been achieved previously in this type of research. It shows the bottom of the 73km-thick Pacific Plate is ‘gliding’ on distinct layer of soft rock that is weak enough to allow movement of many centimetres-a-year in any direction.

The idea that the Earth’s surface is made up of at least 16 tectonic plates, moving at different rates, is well established. But the mechanisms that control the movement have remained unclear, Dr Henrys said.

“To fully understand this requires data on what happens at the bottom of a tectonic plate. It is a real challenge for conventional geophysical methods to obtain quality data from such great depths,” Dr Henrys said.

“Through a combination of detonating explosions in steel-cased holes below the ground and using more than 1000 seismometers, we were able to image seismic waves down to 100km below the surface.”

“This, combined with advanced techniques in seismic processing, enabled us to obtain the most detailed picture yet of the bottom of an oceanic tectonic plate.”

The data enabled the scientists to deduce that the Pacific Plate is 73km thick beneath the lower North Island, thinner than some earlier predictions. They then identified a distinct 10km thick layer of soft rock beneath the Pacific Plate.

They call this a ‘decoupling channel’ as it isolates some of the movement of the rigid lithosphere of the Pacific Plate from the flowing asthenosphere below.

Such a channel, sandwiched between the lithosphere and asthenosphere, helped to explain some of the elusive nature of this deep part of the tectonic plate system.

“The data makes a compelling case that the material in the channel is rich in either water or molten rock, which provides a slippery base for a moving tectonic plate.

“Our data is not able to distinguish the sandwich filling, but we were able to tell that the base of the lithosphere is a sharp and distinctive boundary. This means the plates can move around without much resistance at their base. It also explains why tectonic plates can sometimes change direction abruptly.”

The authors of the Nature paper say gaining insights into the boundary between the base of a rigid oceanic plate and the underlying hot convecting mantle was an important breakthrough in understanding plate tectonics and the evolution of planet Earth.

The authors are unsure if what they have discovered beneath New Zealand will hold true for all the tectonic plates on Earth. Answering this question will require further investigations incorporating dense seismometer networks.

Dr Henrys added that the project was a good demonstration of New Zealand’s ability to build international science consortia and attract international funding to help advance the knowledge and understanding of earth sciences.