Special Seismo Lab Seminar

The Lithosphere-Asthenosphere Boundary (LAB) is fundamental to our understanding of how plate tectonics works. For oceanic plates, the LAB is generally imaged as a seismic discontinuity and is commonly interpreted to reflect the presence of partial melt. However, the fate of a melt-rich LAB is unclear after these plates descend into the mantle at subduction zones. Some recent geophysical studies suggest the melt-rich LAB immediately underlying some old and cold subducting slabs. However, whether melt-rich LAB is commonly present remains debated, especially for young and warm slabs such as in the Cascadia subduction zone. In this study, we provide evidence for its presence at Cascadia in the form of strong (~9.8% dVs) and sharp (<3 km) seismic velocity reduction. Similar degrees of velocity reduction at the bottom of both old and young slabs, as well as along the base of oceanic plates before subduction, suggest widespread presence of melt-rich LAB. The melt-rich layer has low strength and therefore may strongly influence subduction dynamics and earthquake cycles by mechanically decoupling the slabs from the underlying asthenosphere.