Seismic evidence for a deeply rooted low-velocity anomaly in the upper mantle beneath the northeastern Afro/Arabian continent

We present seismic results that support the presence of a small, low shear velocity anomaly deeply rooted in the upper mantle transition zone beneath southern Arabia and the Red Sea. The low shear velocity anomaly persists down to the 660 kin discontinuity. It is found from the waveform inversion of 2741 Rayleigh wave seismograms taking into account several higher modes. We use records from the permanent IRIS and GEOSCOPE stations completed with data collected after various field deployments of portable stations in the Horn of Africa (INSU experiment), Tanzania, Saudi Arabia and Tibet (PASSCAL experiments). The complete dataset provides a dense ray coverage of the Afro/Arabian continent and allows shear-wave heterogeneities to be resolved with wavelengths of a few hundred kilometers. To achieve a good vertical resolution in the whole upper mantle, we analyze up to the fourth Rayleigh mode in the period range 50-80 s, in addition to the fundamental Rayleigh mode in the period range 50-160 s. We discuss whether the pattern of upper mantle shear velocity anomaly could be related to local causes or to one or several plume conduits in the region. Our lateral resolution may intuitively not be sufficient to resolve a narrow plume conduit at transition zone depths. However, we show that when a dense coverage is available, a narrow low-velocity anomaly will affect the path-average measurements for a large number of individual seismograms crossing the anomaly. In this case, the low-velocity perturbation is mapped in the tomographic model, even though smoothed by the lateral resolution. We conclude that our observation is difficult to attribute to a shallow origin or to reconcile with a single narrow plume conduit in the region. It can be explained either by several close narrow plume tails or by a broad region of upwelling. (C) 2001 Elsevier Science B.V. All rights reserved.