| Abstract: | The physical processes that govern the grain size of rocks in the upper mantle are examined. The analysis is based on the experimental data on creep, recrystallization, and grain growth in dunites and on a theoretical model for the thermomechanical structure of the cooling moving lithosphere. The grain size of rocks is shown to be determined by the in situ stress only at the deeper part where the temperature is high enough to allow significant strain rate. Above this depth, the microstructures record the thermomechanical history of rocks rather than the in situ stress.In the case of the oceanic lithosphere where the thermomechanical history is best known, the following features of grain-size distribution are found. At the uppermost mantle, where the amount of grain growth is limited, the grain size is determined by the initial value and the growth rate, and, where the effect of grain growth dominates, it increases with depth. When the amount of grain growth becomes large and the grain size reaches the steady state size corresponding to the ambient stress while the rock is hot enough to deform, the grain size is then determined by the applied stress. This grain size is, however, frozen, when the rock gets cool and the strain rate becomes too small to induce any further dynamic recrystallization. Thus, at the intermediate depth region, the grain size records the fossil (frozen) stress at which the microstructures of rock have been frozen. Since the frozen stress increases with age, the grain size in this depth interval decreases with depth. Finally, the grain size below this level reflects the in situ stress, and increases with depth, its extent being dependent on the nature of return flow in the deep mantle.Thus the grain size versus depth relation may show a sigmoid curve. The qualitative features of this curve may be similar also in the case of the continental lithosphere, if a similar thermal event (i.e., the intrusion of hot material and subsequent cooling) occurs. The results are quite consistent with the observed depth variation of olivine grain size in peridotite nodules (Avé Lallemant et al., 1980). The present model suggests that the depth of minimum grain size (65 and 150 km at the continental rift zone and the shield region respectively) corresponds to that where the mechanical properties of the upper mantle change from elastic to ductile at tectonic stress levels (~ 1 MPa) and in the geological time scale. This result leads to a new definition of the thickness of lithosphere in terms of its rheological properties. This thickness is about twice as large as that inferred from the flexure of lithosphere but approximately equal to seismic thickness. The model suggests the importance of grain growth as well as dynamic recrystallization and plastic flow in determining the texture of upper mantle rocks and therefore seismic anisotropy. |
