Their rationale is a postulated low-velocity zone near and above the 650 km discontinuity with additional properties including local low melting temperature, active chemical migration and fractionation, and low-viscosity.
[2] W. Jason Morgan (1972), originally suggested that hotspots (inferred by J. Tuzo Wilson) beneath such active volcanic regions as Hawaii and Iceland form a fixed "absolute" frame of reference for the motion of the overlying plates.
However, the existence of a globally fixed reference frame for island-seamount chains and aseismic ridges ("traces") that are inferred to have originated from hotspots was quickly discounted by the primitive plate reconstructions available in the mid-1970s (Molnar and Atwater, 1973).
As plate reconstructions have improved over the succeeding three decades since Morgan's original contribution, it is become apparent that the hotspots beneath the central North and South Atlantic and Indian Oceans may form one, distinct frame of reference, while those underlying the plates beneath the Pacific Ocean form a separate reference frame.
Since the hotspots of the Hawaiian set appear to form a frame of reference (like points on a lithospheric plate, they don't appear to be moving at a very great rate relative to one another), the hotspots and that part of the upper mantle in which they are embedded is termed the "Hawaiian mesoplate".
Additional evidence for mesoplates comes from observations that intraplate stresses in stable continental interiors of North America and Africa are consistent with plate motions in the Tristan hotspot frame.
This observation implies that the sublithospheric mantle over which the plates are moving comprises the same reference frame in which the hotspots are embedded.
The origin of hotspots, whether from deep mantle plumes, mid-mantle melting anomalies, or intraplate fractures, is constrained somewhat by the mesoplate hypothesis.
The principal alternative models for the origin of hotspot traces, propagating fractures, are still actively advocated by many workers (see mantleplumes.org).
However, it would need to be modified to recognize that the lack of motion between hotspots represents a kind of "embedding" of the "plume" in the upper mantle (shallow mesosphere) of the Earth.
In the pre-plate tectonics era, Daly (1940) inferred three spherical layers comprise the outer Earth: lithosphere (including the crust), asthenosphere, and mesospheric shell.
In their conception, the base of the asthenosphere extended as deep as the deepest (650–700 km) earthquakes in the inclined seismic zones where descending lithospheric plates penetrate the upper mantle.