A serpentinite sample on the Po River bank, near Monte Viso, Italy

In a field trip to the western Alps in 2017 a small team including UWA Adjunct Researcher Dr Huaiyu Yuan, set to explore a “textbook” tectonic subduction zone using novel Bayesian Transdimensional Tomographic Inversion modelling.

Subduction, the movement of one continental plate beneath another and into the mantle, is a geological process fundamental to Plate Tectonics and the evolution of the surface of the Earth. Subduction initiation and its internal mechanisms are strongly controlled by the deformation and flow (rheology) of the deep mantle. Serpentinite, a rock whose name comes from the similarity of the rock’s texture with the look of the skin of a snake, has orders of magnitude lower rheological strength than that of the normal mantle rocks. Therefore serpentinization in subduction zones is critical to facilitate the subduction process.

“Seismologists came on board, contributing data and helping with calculations. The geologists stressed the importance of serpentinites after seeing the new model. The whole team then started integrating other models (gravity, more seismic, as well as petrologic data) and manuscript writing, which I remember started on a bullet-train going to fieldwork in China in late 2018."

At Earth’s surface, serpentine can be found in many young and ancient subduction zones. A ~10 km thick belt of serpentinized rocks are exhumed to the surface in the Viso unit in the western European Alps. The western Alps have world-class (ultra-)high-pressure (UHP) metamorphic rocks that were carried down by subduction and then returned to the surface. The buoyant European continental plate is supposedly resistant to subducting beneath the smaller Adriatic plate. The presence of serpentines is therefore expected to facilitate this subduction process.

Conventional seismic imaging techniques so far have not detected any signs of serpentine existing in this subduction system. Under the “Tethys Dynamics” Project, the group of international scientists from China, Italy, France, Ireland, and Australia, carried out a high-resolution earthquake seismic imaging project in the region.

"Challenges remain in the complexity of regional tectonics itself and the long haul in international collaboration.” The international team led by Prof Liang Zhao of the CAS (Chinese Academy of Sciences) has worked on the topic for nearly a decade and published many peer-reviewed research papers; yet new observations are still being made from field work conducted in 2017.

The team found 15–25 km thick slow seismic velocity (3.7 km/s) layer is present between 50 and 70 km depths along the western Alps subduction channel. Laboratory experiments show that the best candidate responsible for the slow velocity is stacked remnants of serpentinites, formed in different stages at the ocean bottom, and in the mantle wedge underneath the Adriatic plate. Such a massive volume (estimated as ~300,000 km2) “lubricates” the continental plate boundary. Potentially, these layers act as reservoirs where magmas and hot fluids originate, which flux the metals we rely on every day, into the Earth’s crust.

You can read their groundbreaking work in the open access journal Nature Communications.

Media references

Dr Huaiyu Yuan: huaiyu.yuan@gmail.com