Unraveling the enigma of low-velocity zones beneath tectonic plates

The research team, including Jianfeng Yang, Liang Zhao, Ling Chen, Xin Wang, and Hao Shen from the Institute of Geology and Geophysics, Chinese Academy of Sciences, and Manuele Faccenda and Brandon VanderBeek from the University of Padua, has decoded the origin of enigmatic seismic low-velocity zones beneath subducting tectonic plates. Using advanced geodynamic simulations and global seismic observations, they demonstrate that these zones arise from partial melts generated by upwelling water-rich mantle material, overturning long-standing theories about Earth’s deep-water cycle.

Thin layers with anomalously slow seismic waves – termed low-velocity zones (LVZs) – extend from shallow depths (~100 km) down to 660 km beneath subducting plates. These LVZs, exhibiting sharp boundaries and high electrical conductivity, have puzzled geoscientists for decades. While dehydration of subducting slabs explains LVZs above plates, what causes and sustains them beneath plates remains unknown. "These zones persist far deeper than expected," notes Prof. Liang Zhao, co-corresponding author, "hinting at a deeper source of melting."

The mantle transition zone (MTZ) acts as a massive water reservoir, storing oceans’ worth of H₂O in minerals like wadsleyite and ringwoodite. The study shows that subducting slabs "scrape" against these hydrous regions, dragging water-rich material upward. As this material rises, pressure drops trigger dehydration melting, creating melt pockets of 1–3% volume. Crucially, the MTZ must contain >0.2% water over areas >120 km wide to generate sufficient melt conditions confirmed by diamond inclusion studies and electromagnetic surveys.

The melt’s journey is a planetary conveyor belt that accumulates beneath the tectonic plates to be sheared to 10–25-km-thick, and subsequently is dragged back into the MTZ by the subducting slab, completing a global water-recycling loop. This process reduces mantle viscosity, lubricates plate motion, and enables chemical mixing between Earth’s layers.

The study’s mechanism explains LVZs globally – from Pacific subduction zones to continental collisions like the Alps. "Subduction isn’t just a graveyard for plates," says co-corresponding author Prof. Manuele Faccenda. "It’s a recycling plant for Earth’s deep water." Future work will map global MTZ water content using joint seismic-electromagnetic surveys and assess how melt-mediated viscosity influences earthquake generation.