January 18, 2017

Hotspot created when Russia and China collided

Geophysicists solve mystery of the Perm Anomaly’s formation 

Deep below the city of Perm in Russia, about 2500 km down, lies an unusual geophysical structure known as the Perm Anomaly, a region of the Earth’s mantle that is significantly hotter than the material surrounding it.

Its discovery in 2012 took scientists’ understanding of the deep mantle zone of the Earth’s interior to an unprecedented level of detail. (The mantle is the section of the Earth’s interior between the base of the crust and the top of the core at the centre. About 2850 km thick, it accounts for about 84 per cent of the Earth’s volume.) Now, new discoveries about the formation of the anomaly are further challenging our understanding of the mantle and its relationship with the process of plate tectonics.

In a research paper published on Wednesday, 18 January, 2017, in Nature Communications, a team of geophysicists from University of Wollongong, University of Sydney, California Institute of Technology and ETH Zürich, have solved the mystery of how the Perm Anomaly was formed. They reveal it was linked to the death of the ancient Mongol-Okhotsk Ocean between what is now Russia and China. The research also shows the deep mantle to be more dynamic – and hot structures within it more mobile – than previously thought. Since the collision between Russia and China, the anomaly has moved 1500 km to the west at a rate of 1 cm per year, and continues to do so.

Team leader Dr Nicolas Flament, from the University of Wollongong’s School of Earth and Environmental Sciences, said a better understanding of how the mantle has changed over time will increase our knowledge of how the process of tectonics has shaped the surface of the Earth, and influenced the evolution of climate and life.

“Understanding how the deeper, solid Earth works is important for understanding how life has evolved in the past, and then forecasting what may happen in the future. To some extent, the past is the key to the future,” Dr Flament said.

The researchers used high-performance Virtual Earth simulations to uncover the origins of the anomaly.

“One of the constraints we use is tomography models, which are seismic images of the Earth’s interior [created by studying energy waves from earthquakes as they move through the Earth]. We have known for the past 30 years that very deep within the Earth – between 2000 and almost 3000 km down or nearly halfway to the centre of the Earth - there are two large piles of material that are seismically slower than the surrounding mantle. That suggests these piles are hotter - because temperature is one of the main parameters controlling seismic velocities deep within the Earth.

“About five years ago, a group of researchers in the US used tomography models to show that in addition to these two large piles there was a smaller structure they called the Perm Anomaly, which instead of being 15,000 km across is only about 1000 km across, and instead of being 1000 km high it might be only half that.”

The piles are formed through the processes of plate tectonics and mantle convection. When tectonic plates converge, the ocean floor between them is pushed down through a process known as subduction, and slowly sinks deeper and deeper into the mantle. As these relatively cold slabs sink, they push aside hot rock, forming large piles of hotter material, some of which gives rise to narrower mantle plumes that trigger large volcanic eruptions and lava flows at Earth’s surface.

The two larger piles, which lie under the Pacific Ocean and Africa, are associated with volcanic activity in those areas. Flament says the creation of the Perm Anomaly may have led to the formation of the Emeishan volcanics - multiple layers of igneous rock laid down by massive volcanic eruptions in south-western China during the Triassic Period.

To understand how the Perm Anomaly was formed, Dr Flament and his colleagues used computer modelling to reconstruct the flow of material in the Earth’s mantle deep into the geological past, attempting to create a model that matched up what we know of the past from the rock record with what we know of the present-day mantle from seismic imaging.

“The good thing about the rock record is that it goes back in time. That’s how we’re able to put the tectonic puzzle back together in time,” Dr Flament said.

"We started our models deep in the past, 230 million years ago when dinosaurs roamed about, and then simulated the flow within the solid Earth forward in time to predict the present-day structure of the mantle, and its temperature in particular. We can then compare that present-day structure to tomography models.

“What we found in our models was that we had two big piles akin to the observed ones- and many models had that before - but we also retrieved the smaller pile, and that’s when we got excited.”

The models showed the anomaly was formed in a similar way to the two larger piles, being shaped by a surrounding network of subduction zones, with a key event in its evolution being the death of the Mongol-Okhotsk Ocean. As the ocean floor between Russia and China subducted it closed the network of subduction zones that formed the Perm Anomaly. When the Russian and Chinese plates collided 150 million years ago, ocean floor stopped subducting between them and the Perm Anomaly began being pushed westward by ongoing subduction east of Asia, migrating at the rate of 1cm per year to its current location below Perm (about 1200 km east of Moscow).

Dr Flament said it was an exciting time to be working on the mapping of the underworld as improving techniques and a growing database meant more discoveries were likely in the coming years.

“What’s exciting is that we are inferring the structure of the deeper Earth to unprecedented levels of detail,” he said.

“The team that identified the Perm Anomaly has identified a couple more anomalies in the mantle, so it will be interesting work to try to explain each of these different anomalies.”

The research paper, "Origin and evolution of the deep thermochemical structure beneath Eurasia" by N. Flament, S. Williams, R.D. Müller, M. Gurnis and D.J. Bower, is published in Nature Communications (January 18, 2017).