Absolute Plate Motions

The relative motion between the major plates, averaged over the past few million years, can be determined with remarkable precision, as described in the preceding section. It would be of considerable interest, particularly in relation to the driving mechanism for plate motions, if the motion of plates, and indeed plate boundaries, across the face of the Earth could also be determined. If the motion of any one plate or plate boundary across the surface of the Earth is known, then the motion of all other plates and plate boundaries can be determined because the relative motions are known. In general, within the framework of plate tectonics, all plates and plate boundaries must move across the face of the Earth. If one or more plates and/or plate boundaries are stationary, then this is fortuitous. A particular point on a plate, or, less likely, on a plate boundary, will be stationary if the Euler vector of the motion of that plate or plate boundary passes through that point (Fig. 5.6).

The absolute motion of plates is much more difficult to define than the relative motion between plates at plate boundaries, not least because the whole solid Earth is in a dynamic state. It is generally agreed that absolute plate motions should specify the motion of the lithosphere relative to the lower mantle as this accounts for 70% of the mass of the solid Earth and deforms more slowly than the asthenosphere above and the outer core below. In theory if the lithosphere and asthenosphere were everywhere of the same thickness and effective viscosity, there would be no net torque on the plates and hence no net rotation of the

Figure 5.6 The absolute velocities of plates, assuming the hotspot reference frame. The arrows indicate the displacement of points within the plates if the plates were to maintain their current angular velocities, relative to the hotspots, for 40 Ma. Filled circles indicate the pole (or antipole) of rotation for the plate if this occurs within the plate. The medium solid lines are approximate plate boundaries; where barbed, they indicate subduction zones with the barb on the overriding plate. Note that, because of the Mercator projection, arrows at high latitudes are disproportionately long compared to those at low latitudes (modified and redrawn from Gripp & Gordon, 2002 with permission from Blackwell Publishing).

Figure 5.6 The absolute velocities of plates, assuming the hotspot reference frame. The arrows indicate the displacement of points within the plates if the plates were to maintain their current angular velocities, relative to the hotspots, for 40 Ma. Filled circles indicate the pole (or antipole) of rotation for the plate if this occurs within the plate. The medium solid lines are approximate plate boundaries; where barbed, they indicate subduction zones with the barb on the overriding plate. Note that, because of the Mercator projection, arrows at high latitudes are disproportionately long compared to those at low latitudes (modified and redrawn from Gripp & Gordon, 2002 with permission from Blackwell Publishing).

lithosphere relative to the Earth's deep interior. If plate velocities are specified in the no net rotation (NNR) reference frame, the integration of the vector product of the velocity and position vectors for the whole Earth's surface will equal zero. By convention, space geodesists specify absolute plate motions in terms of the NNR criterion (Prawirodirdjo & Bock, 2004).

An alternative model for the determination of absolute motions utilizes the information provided by volcanic hotspots on the Earth's surface. Wilson (1963) suggested that the volcanic ridges and chains of volcanoes associated with certain major centers of igneous activity such as Hawaii, Iceland, Tristan da Cunha in the South Atlantic, and Reunion Island in the Indian Ocean, might be the result of the passage of the Earth's crust over a hotspot in the mantle beneath. Morgan (1971) elaborated on this idea by suggesting that these hotspots are located over plumes of hot material rising from the lower mantle, and hence provide a fixed reference frame with respect to the lower mantle. This hypothesis is considered further in the next section, and in Chapter 12. The hotspot model is attractive to many geologists and geophysicists in that the tracks of hotspots across the face of the Earth offer the possibility of determining the absolute motion of plates throughout the past 200 Ma (Morgan, 1981, 1983).

The model of Gripp & Gordon (2002) for the current absolute motion of plates, based on the trends and rates of propagation of active hotspot tracks, is illustrated in Fig. 5.6. It averages plate motions over the past 5.8 Ma, approximately twice the length of time over which relative velocities are averaged. Two propagation rates and 11 segment trends from four plates were used in deriving this model.

Several other frames of reference for absolute motions have been suggested, but not pursued. One of these proposed that the African plate has remained stationary during the past 25 Ma. Following a long period of quiescence, in terms of tectonic and volcanic activity, large parts of Africa have been subjected to uplift and/or igneous activity during the late Cenozoic. This was considered to be a result of the plate becoming stationary over hot spots in the upper mantle. Another proposal was that the Caribbean plate is likely to be stationary as it has subduction zones of opposite polarity along its eastern and western margins. Subducting plates would appear to extend through the asthenosphere and would be expected to inhibit lateral motion of the overlying plate boundary. Similar reasoning led Kaula (1975) to suggest a model in which the lateral motion of plate boundaries in general is minimized.

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