Deep mantle upwelling may play an important role in driving arc rifting and back-arc spreading in subduction zones, according to a new study of the Izu-Bonin-Mariana (IBM) subduction zones.

The study, conducted by a research team from the Institute of Oceanology of the Chinese Academy of Sciences (IOCAS), revealed contrasting patterns of deep mantle upwelling beneath the two zones.

This study was published in Journal of Geophysical Research: Solid Earth on August 10.

Subduction zones are a key site for the circulation of material between the Earth's surface and its interior. Arc rupture and back-arc expansion reflect the complex interactions between deep processes and surface structures, playing an important role in our understanding of oceanic crust formation, material cycling and the habitability of Earth.

However, the role of deep mantle upwelling in arc rifting and back-arc spreading in subduction zones has remained unclear due to the lack of robust tomographic constraints.

To address this issue, the researchers made a three-dimensional (3D) P-wave tomographic study of the Izu-Bonin-Mariana (IBM) subduction zone. By integrating seismic data recorded at both land-based seismic stations and ocean bottom seismometers, they imaged the 3D P-wave velocity structure at depths of 20 to 700 km. 

They discovered distinct low-velocity anomalies beneath the Izu-Bonin and Mariana subduction zones. These anomalies may be indicative of deep mantle upwelling, which could be caused by processes such as the dehydration of deeply subducted slabs, the melting of oceanic crust and the migration of volatiles. In the Izu-Bonin zone, the upwelling appears to originate obliquely from the base of the upper mantle (at a depth of around 400 km) and tilt away from the arc, suggesting that it does not directly impact the arc region.

In contrast, the upwelling beneath Mariana rises nearly vertically from the mantle transition zone (at a depth of ~550 km) to the Mariana Trough. This provides materials and dynamic support for arc rifting and back-arc spreading in this region. These contrasting upwelling patterns may explain why the two regions are at different evolutionary stages of arc rifting and back-arc spreading.

The results further suggest that deep mantle upwelling and its associated material circulation may be important dynamic factors driving arc rifting and back-arc spreading in subduction zones, similar to continental rifting and the formation of new oceanic crust, in addition to shallow tectonic stresses.

"Our study provides new seismic evidence that deep mantle processes are closely linked to surface tectonic evolution in subduction zones," said Prof. FAN Jianke, corresponding author of the study. "This helps us better understand how material circulation within Earth's interior contributes to the formation of oceanic crust and the evolution of subduction systems."

The study sheds new light on the dynamic coupling between deep mantle processes and shallow tectonic deformation in the IBM subduction zone and emphasises the significant impact of deep mantle upwelling on the internal and surface evolution of the Earth.

A schematic diagram of deep mantle upwelling, material circulation, arc rifting, and back-arc spreading in the IBM region. (Image by IOCAS)