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Seismic Evidence for a Melt-Rich Lithosphere-Asthenosphere Boundary Along the Base of Young Slab at Cascadia

Description: 

Knowing the nature of the Lithosphere-Asthenosphere Boundary (LAB), which separates the stronger lithosphere from the underlying weaker asthenosphere, is fundamental to understanding how plate tectonics works. For the oceanic lithosphere, the LAB is generally imaged as a negative seismic discontinuity and is widely thought to indicate the presence of partial melt. A melt-rich LAB would mechanically decouple lithospheric plates from the asthenosphere and thus facilitate plate motion. However, the fate of a melt-rich LAB is unclear after these plates descend into the mantle at subduction zones. To date, the LAB beneath subducting slab (hereinafter referred to as slab-LAB) has been reported only in subduction zones in Japan and New Zealand, and in both cases the age of the subducting plate is very old (~120-130 Ma). In contrast, no slab-LAB has ever been reported where young plates are subducting. Here, we provide strong evidence for the presence of slab-LAB at Cascadia, where the subducting plate is as young as ~10 Ma. Using teleseismic waveforms recorded by dense arrays and a coherent receiver function imaging technique, here we image the slab-LAB at Cascadia in the form of a large (9.81.5) decrease in shear-wave velocity over a very small (<3 km) depth interval. We consider the imaged slab-LAB to be compelling evidence for the continuation of the melt-rich plate LAB after subduction. Similarly large and sharp seismic velocity reduction at the bottom of both old and young slabs, as well as along the base of oceanic plates before subduction, possibly represents widespread presence of melt. The melt-rich slab-LAB may strongly influence subduction dynamics and viscoelastic earthquake cycles.

Session: Earth’s Structure from the Crust to the Core - II

Type: Oral

Date: 5/1/2024

Presentation Time: 10:45 AM (local time)

Presenting Author: Qi-Fu

Student Presenter: No

Invited Presentation: 

Authors