The lithosphere asthenosphere boundary zone (LABZ) has been of great interest in recent decades for the simple reason that understanding the LABZ—how it is defined, how it forms, how it evolves with time—is crucial to understanding mantle dynamics. In the framework of plate tectonics, the LABZ represents a boundary layer having a certain thickness (McKenzie and Bickle 1988; see thermal boundary layer in their Fig. 3), where heat, momentum, and materials are exchanged between the conductive mantle (lithosphere) and the underlying convective mantle (asthenosphere and below). Our knowledge of the composition and material properties of the LABZ is...

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