Abstract
We present a brief review on recent studies of upper mantle structure with surface wave inversions, and waveform and travel time inversions of body waves, focussing on the regional variation of low velocity zone (LVZ) structure. Distinct features summarized are 1) increase of plate thickness with age, 2) increase of S wave velocity in LVZ with age, 3) less distinct LVZ in the ocean older than 130 Ma, and 4) no LVZ in the continental upper mantle.
These seismological observations can be interpreted in terms of the thermal/mechanical structures of the upper mantle once the factors that affect elastic and non-elastic preperties of rocks are known. Although current materials science studies in this field are very limited, they suggest the solid-state mechanisms of inelasticity in the upper mantle, wich are consistent with the seismological observations. Thus, partial melting is limited to young ocean if it does exist in the oceanic upper mantle.
In these mechanisms, the elastic and non-elastic properties vary strongly with temperature and presumably with a magnitude of regional tectonic stresses due to plate motion. In this vein, age-dependent variation of LVZ structure under the same plate can be attributed to temperature decrease with age in LVZ. The difference of LVZ structure between plates with different absolute plate motion velocities may reflect the difference in tectonic differential stresses in LVZ generated by the plate motion.
