A sharp lithosphere-asthenosphere boundary imaged beneath eastern North America

Area of Science:

• Geophysics
• Seismology
• Tectonics

Background:

• Plate tectonics relies on lithosphere movement over the asthenosphere.
• The lithosphere-asthenosphere boundary's nature is crucial for understanding mechanical strength differences.
• Seismic velocity gradients at this boundary provide insights into physical and chemical properties.

Purpose of the Study:

• To investigate the sharpness of the lithosphere-asthenosphere boundary using seismic data.
• To constrain the physical and chemical properties responsible for the boundary's characteristics.
• To differentiate between thermal and compositional/melt-driven weakening mechanisms.

Main Methods:

• Analysis of converted seismic phases in eastern North America.
• Imaging seismic velocity gradients at the base of the lithosphere.
• Quantifying shear-wave velocity drop over specific depth ranges.

Main Results:

• A sharp seismic velocity gradient was observed at the lithosphere-asthenosphere boundary (90-110 km depth).
• Shear-wave velocity dropped 3-11% over 11 km or less.
• This sharp boundary is inconsistent with a purely thermal gradient.

Conclusions:

• The asthenosphere likely contains a few percent partial melt or is volatile-enriched.
• These factors, rather than solely temperature, explain the asthenosphere's weakness.
• The findings refine models of mantle flow and plate tectonics.