Abstract.We apply a three-dimensional stacking method to receiver functions from the Tanzania
Upper mantle seismic velocity variations beneath northern Tanzania coupled with the structure of the 410 and 660 km discontinuities reveal a 200-400-km-wide thermal anomaly extending into but not necessarily through the transition zone beneath the eastern branch of the East African rift system. This finding is not easily explained by small-scale mantle convection induced by passive stretching of the lithosphere or by a broad thermal upwelling extending from the lower mantle into the upper mantle, but it can be attributed to a mantle plume, provided that a plume head is present under the lithospheric keel of the Tanzania craton. A plume interpretation for the deep thermal anomaly is supported by evidence for mantle having the geochemical characteristics of a plume at >150 km depth beneath northern Tanzania.
S U M M A R YReceiver functions are produced by deconvolving the horizontal components of a seismogram by the vertical component. Typically this is performed by spectral division of recordings of single events. Receiver functions from events with similar backazimuth and ray parameter are then stacked to improve the signal-to-noise ratio. To avoid the subjective and time-consuming method of prewhitening typically employed with spectral division, we have cast the deconvolution in the time domain. By performing the time-domain deconvolution as a regularized simultaneous inversion for a group of events that would normally have been stacked after deconvolution, we find that side lobes are reduced and resolution is improved. Furthermore, the regularized inversion allows the user t o choose among a variety of objective model norms. In this paper, we present results from inversions employing the L2 norm and lower-bounded least squares.
The seismologically defined boundary between the transition zone in the Earth's mantle (410-660 km depth) and the underlying lower mantle is generally interpreted to result from the breakdown of the gamma-spinel phase of olivine to magnesium-perovskite and magnesiowustite. Laboratory measurements of these transformations of olivine have determined that the phase boundary has a negative Clapeyron slope and does indeed occur near pressures corresponding to the base of the transition zone. But a computational study has indicated that, because of the presence of garnet minerals, multiple seismic discontinuities might exist near a depth of 660 km (ref. 4), which would alter the simple negative correlation of changes in temperature with changes in the depth of the phase boundary. In particular, garnet minerals undergo exothermic transformations near this depth, acting to complicate the phase relations and possibly effecting mantle convection processes in some regions. Here we present seismic evidence that supports the existence of such multiple transitions near a depth of 660 km beneath southern California. The observations are consistent with having been generated by garnet transformations coupling with the dissociation of the gamma-spinel phase of olivine. Temperature anomalies calculated from the imaged discontinuity depths--using Clapeyron slopes determined for the various transformations--generally match those predicted from an independent P-wave velocity model of the region.
S U M M A R YIn order to improve the signal-to-noise ratio of receiver function data, it is typical to stack receiver functions calculated from events at similar distances and back azimuths. We have adapted the velocity spectrum stacking (VSS) technique, used extensively in reflection seismology, to the receiver function method in order to stack data with different ray parameters, thereby improving further the signal-tonoise ratio. Perhaps more importantly, by producing the velocity spectrum stacks we take advantage of the differences in the shapes of the moveout curves of converted phases and reverberations to identify and separate the various phases and to infer velocity structure. By comparison of velocity spectrum stacks produced from the observed data at the IRIS/IDA broad-band station at Obninsk, Russia (OBN) with those produced from PREM synthetics we have identified Ps phases from the 400 and 670km discontinuities. The P-to-S ( P s ) converted phase from the 400km discontinuity observed at OBN is much larger, compared with that of the 670 km discontinuity, than is predicted by PREM. This suggests a higher velocity contrast at 400 km than in PREM. By adapting a bootstrap method to assess the depth and velocity estimates determined by VSS, we find that the Ps phase from the 670 km discontinuity is best imaged with higher than PREM upper mantle P and S velocities and at a depth of 663 * 3 km. We find no evidence of a 220 km discontinuity beneath OBN in these data.
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