Mesoplates - Development of The Concept

Development of The Concept

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. For convenience, the hotspots beneath the Pacific Ocean are referred to as the "Hawaiian set" after Hawaii, while those beneath much of the Atlantic and Indian Ocean are called the "Tristan set" after the island of Tristan da Cunha (the Tristan hotspot), one of the principal inferred hotspots of the set. Within a single hotspot set, the traces tied to their originating hotspot can be fit by plate reconstructions which imply only minor relative motion among the hotspots for perhaps the past 130 m.y. (million years) for the Tristan set and 80 m.y. for the Hawaiian set. However, the two hotspot sets are inconsistent with the hypothesis of a single hotspot reference frame; distinct motion between the two sets is apparent between 80 to 30 Ma (m.y. before Present; e.g., Raymond, et al., 2000).

It is important to acknowledge that radiometric dating of volcanism along hotspot traces may or may not accurately and precisely constrain the position of the plate above the underlying hotspot at the analytically produced age. However, reconstruction models for the Hawaiian set are constrained in age by the hotspot beneath Easter Island and its traces on the Pacific and Nazca plates between approximately 50 and 30 Ma, as the hotspot was beneath the spreading center during that time interval, and resulting relative plate reconstructions constrain motion of the plates relative to the hotspot. Prior to 50 Ma and since 30 Ma, reconstructions can be determined that fit virtually all existing Hawaiian set traces; the actual ages have the greatest uncertainty. Similarly, plate reconstructions relative to the Tristan set are best constrained in age by relative plate reconstructions, a fortuitous consequence of spherical plate tectonics of three or more plates.

Lithospheric plates are recognized in terms of their lack of internal deformation. Thus two points on the same plate will not move relative to one another, even if the plate moves relative to another plate (or relative to the Earth's rotational poles). Plates are not explicitly defined in terms of their mechanical properties. In a sense, then, "plates" are a heuristic—rather like fitting a straight line through a set of points without a clear functional relationship. Analogously, the term "mesoplate" was introduced. 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". The "Tristan mesoplate" is similarly defined. A third mesoplate, "Icelandic", is inferred to underlie the northernmost Atlantic Ocean, the Arctic Ocean, much of Eurasia to the north of the Alps and Himalayas; since the Iceland hotspot trace is not consistent with either the Hawaiian or Tristan set.

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 was first made for contemporary stresses (the maximum horizontal principal compressive stress – sigma-hx); and also appears to hold for paleostress indicators between approximately 100 and 20 Ma (Pilger, 2003). 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 mesoplate heuristic is very much a hypothetical construct. Several observations could discount it. It is conceivable that a missing plate boundary between the plates beneath the Pacific and those beneath the Atlantic and Indian Oceans might be hidden and responsible for the discrepancy between the two hotspot sets. However, progressive study of the most likely region for such a boundary has failed to find it.

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). Such a model does not explicitly recognize sublithospheric reference frames. However, it cannot completely explain all of the features of the most familiar hotspot traces (Pilger, 2007).

The mantle plume hypothesis for the origin of hotspots need not be inconsistent with mesoplates. 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. One of Morgan's rationales for plumes was the existence of an "absolute motion" reference frame. Numerical modeling now indicates that such a reference frame would be unlikely in the context of plume convection.

If continued research were to demonstrate the continued applicability of the mesoplate hypothesis, it would have important implications for the nature of convection in the upper mantle: Convective motion beneath plates is almost entirely vertical within individual mesoplates; lateral motion in the mantle would be confined to mesoplate boundaries and to greater depths.