S U M M A R Y Compared to clear crustal P-and S-waves as well as a strong refracted P,,-wave propagating in the topmost mantle (i.e., 10 to 15 km below the Moho), an anomalously weak appearance of the corresponding refracted shear wave S,, is observed on refraction seismic profiles in SW Germany, France, coastal Maine (NE United States) and Fennoscandia. It is possible that the observation of Pn/PMP ratios close to unity combined with small S,/SMS ratios (i.e., small S, , amplitudes) is typical for the continental uppermost mantle since this was observed for quite different types of continental crust. It is shown that the observed phenomenon is not a source effect. Several possible causes to explain the observations are studied, also with the aid of synthetic seismograms. The effects of temperature, attentuation and anisotropy have been investigated. Their influence cannot explain the observations. The most likely candidate is a depth-increasing vp/vs ratio (an increase in the order of 0.027 over a depth range of lOkm), forcing P-and S-waves to propagate on different paths in the topmost mantle. The observed phenomenon provides new constraints on the change of composition with depth for the crust mantle transition zone with basalt depletion as the most likely explanation.
A recent delay time tomography experiment has revealed the still ongoing processes that formed the East African Rift, the world's largest continental rift system. These new data have been used to produce tomographic images of the crust and upper mantle beneath the Chyulu Range (Figure 1), a Quaternary volcanic field on the rift's eastern shoulder.
The images show a prominent low‐velocity zone where the youngest volcanism occurred directly beneath the Chyulu Range. Velocity contrasts as large as 4% have been found in 40–90 km depth (Figure 1). The reduced seismic velocity beneath the volcanic range probably suggests the presence of small active magma chambers in the crust and upper mantle. A large‐scale asthenospheric upwelling underneath the rift shoulder is excluded as cause of the volcanism because the anomaly is too small in size as well as in percentage of velocity reduction.
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