Upper Mantle Structure Beneath the Eifel from Receiver Functions

The average Moho depth in the Eifel is approximately 30 km, thinning to ca. 28 km under the Eifel volcanic fields. Receiver function (RF) images suggest the existence of a low velocity zone at about 60 to 90 km depth underneath the West Eifel. This observation is supported by P- and S-wave tomographic results and absorption (Ritter this volume). Indications for a zone of increased velocity near 200 km depth, again agree with S-wave and absorption tomographic results. This anomaly, surprisingly not visible in P-wave tomography, could be due to an area of S-wave anisotropy that compensates for elevated plume temperatures. All three RF anomalies — at the Moho, at 60 to 90 km and near 200 km depth — have a lateral extent of about 100 km. The aperture of the Eifel network limits the resolution of tomographic methods to the upper 400 km. The RF method does not suffer from this limitation and can resolve deeper structures. The 410 km discontinuity under the Eifel is depressed by 15 to 25 km. Lowering of the 410 km discontinuity could be explained by a maximum temperature increase of +200 to +300 °C. The second surprising feature in the 3-D RF image of the Eifel Plume is the occurrence of two additional, currently unexplained conversions between 410 and 550 km depth. They could represent remnants of previous subduction or anomalies due to delayed phase changes. The lateral extent of the two additional conversions and the depression of the 410 km discontinuity is about 200 km. The 660 km discontinuity, in contrast to the 410 km discontinuity, does not show any depth deviation from its expected value, a scenario also encountered in the western US. Based on these observations we present the following scenario for the Eifel plume. The Eifel plume is a plume with temperature excess relative to the surrounding mantle of about +200 to +300 °C. The plume is imaged in the upper mantle and might be fed by regions imaged as low velocity anomalies in the lower mantle under Central Europe. Seismological methods provide only a blurred present day snap-shot. Thus we can not exclude the possibility that ascent of plume material, possibly coming even from the lower mantle, is intermittent and we see only the present day effects and configuration of the plume.