ON THE STRUCTURE OF THE LOWERMOST MANTLE BENEATH THE SOUTHWEST PACIFIC, SOUTHEAST-ASIA AND AUSTRALASIA

The region of the lowermost mantle beneath the southwest Pacific, Australasia and southeast Asia (50 degrees S-50 degrees N and 80-190 degrees E) has been studied using a wide variety of seismic techniques. We complement these studies with results obtained from long-period Global Digital Seismograph Network (GDSN) data using a recently developed phase-stripping technique that permits the isolation of D'' reflections from the stronger neighbouring S and ScS signals. We identify patches with D'' reflections and areas where we cannot confidently determine the presence or absence of a D'' reflection. A synthesis of our results with other studies suggests that D'' varies dramatically through the region, generally thickening from 100-150 km in a central zone to about 300 km at the eastern and western margins. However, there are irregularities in this overall pattern, including areas where there seems to be little evidence for a D'' discontinuity. Inspection of waveform amplitudes shows considerable scatter in not only the D'' reflected phases, but also core-mantle boundary reflected phases. Experiments with synthetic seismograms for a variety of D'' models and the observed lateral variability through the region suggest that this is to be expected. Furthermore, ray-theory calculations for large-scale 3D Earth models predict significant variations (+/-30%) in S-wave amplitudes of lowermost mantle turning rays. Finally, we investigate possible correlations between lower-mantle velocity, flow and D'' thickness. We find some correlation between D'' thickness and lower-mantle velocities obtained from tomographic inversions, with a thin D'' layer in high-velocity regions and a thickening of the layer toward slower regions. The relationship between predicted lower-mantle flow and D'' thickness is less clear. These results are qualitatively consistent with thermal boundary layer predictions for D'', but do not preclude the possibility of compositionally distinct material in the layer.