New Research Reveals Grain Boundary Sliding Drives Serpentinite Deformation in Subduction Zones
Earth's surface is composed of over a dozen tectonic plates. In subduction zones globally, including the Japanese Islands, these plates converge, causing dense oceanic plates to sink into the Earth's interior. These regions, particularly plate boundaries, are characterized by frequent seismic activity. Scientists have increasingly recognized the crucial role of water in earthquake phenomena within subduction zones and are actively researching its influence on processes in earthquake source regions.
The Transformative Role of Water and Serpentinite
When water is present, peridotite, the primary component of the upper mantle, can transform into serpentinite. This process is believed to occur extensively in the mantle wedge, which is the mantle region on the overriding-plate side above a subducting oceanic plate. The conversion of peridotite to serpentinite involves chemical reactions that alter the rock's mineral assemblage.
Since minerals possess distinct physical properties like deformability, serpentinization is expected to significantly change the overall rock's physical characteristics. While peridotite deformation mechanisms have been studied for a long time, those of serpentinite are still under investigation, making serpentinite a key research focus for understanding the physical properties of subduction zone plate boundaries.
"Serpentinite is a key research focus for understanding the physical properties of subduction zone plate boundaries."
Unraveling Antigorite's Deformation Mysteries
Previously, dislocation creep was proposed as the dominant deformation mechanism in antigorite, the main mineral of serpentinite. As deformation progresses, a crystallographic preferred orientation (CPO) develops, aligning crystal orientations. Dislocation creep in antigorite typically produces an "A-type" CPO pattern, where crystallographic a-axes are aligned parallel to the shear direction.
However, various antigorite CPO patterns exist in nature besides A-type, and their formation mechanisms are not fully understood. This suggests that antigorite in the Earth's interior may deform through mechanisms other than dislocation creep.
Breakthrough: Grain Boundary Sliding Forms Common B-Type CPO
A research team, led by Associate Professor Takayoshi Nagaya from Waseda University and including Professor Simon R. Wallis from The University of Tokyo, has demonstrated that grain boundary sliding (GBS) can form the most common natural CPO pattern, known as the "B-type." In this pattern, the crystallographic b-axes of antigorite are preferentially aligned parallel to the shear direction.
Their groundbreaking findings were published in Volume 13, Issue 4 of the Progress in Earth and Planetary Science journal on January 21, 2026.
The team has demonstrated that grain boundary sliding (GBS) can form the most common natural CPO pattern, known as the "B-type."
Implications for Aseismic Slip and Earthquake Understanding
The team utilized natural serpentinite samples from the Besshi and Shiraga areas in Shikoku, Japan, to investigate serpentinite deformation mechanisms at plate boundaries. Their discovery that antigorite deforms via GBS suggests that serpentinite deformation at plate boundaries is associated with aseismic slip, which generates minimal to no seismic waves and does not produce felt earthquakes.
Nagaya and Wallis stated that the study improves methods for inferring how rocks deform to form shear zones. This enables a more advanced understanding of rock deformation mechanisms and contributes to insights into deformation processes and earthquake generation in the Earth's interior, particularly in subduction zones. Nagaya added that these findings may also help understand the relationship between slow earthquakes, which have attracted attention for their potential links to great megathrust earthquakes, and large earthquakes from a materials science perspective.