Inversion of P-wave traveltime residuals of teleseismic events recorded by a dense network of short-period stations yielded a detailed image of the 3-D velocity structure of the lithosphere-asthenosphere system beneath the southern Rhine Graben area. The upper part of the lithosphere, marked by velocity anomalies orientated along the graben strike, can be related to the development of the graben. These upper 50 km display stronger lateral heterogeneities than the weak anomalies observed at deeper levels which show roughtly an E-W orientation. The teleseismic inversion models yield no indications for significantly reduced seismic velocities in the upper mantle as could be expected from an actively upwelling mantle diapir located beneath the southern Rhine Graben area. The observation of generally weak velocity variations at greater depth indicates a rather smooth transition from the lower lithosphere to the upper asthenosphere. Deeper parts of the lithosphere were probably altered by metasomatic processes as a consequence of the intrusion of melts during middle and late Miocene times. These melts cooled and solidified from the end of the Miocene when rifting and thermal doming ceased in the southern Rhine Graben area.

The lithosphere beneath a continental rift should be significantly modified due to extension. To image the lithosphere beneath the Rio Grande rift (RGR), we analyzed teleseismic travel time delays of both P and S wave arrivals and solved for the attenuation of P and S waves for four seismic experiments spanning the Rio Grande rift. Two tomographic inversions of the P wave travel time data are given: an Aki‐Christofferson‐Husebye (ACH) block model inversion and a downward projection inversion. The tomographic inversions reveal a NE‐SW to NNE‐SSW trending feature at depths of 35 to 145 km with a velocity reduction of 7 to 8% relative to mantle velocities beneath the Great Plains. This region correlates with the transition zone between the Colorado Plateau and the Rio Grande rift and is bounded on the NW by the Jemez lineament, a N52°E trending zone of late Miocene to Holocene volcanism. S wave delays plotted against P wave delays are fit with a straight line giving a slope of 3.0 ± 0.4. This correlation and the absolute velocity reduction imply that temperatures in the lithosphere are close to the solidus, consistent with, but not requiring, the presence of partial melt in the mantle beneath the Rio Grande rift. The attenuation data could imply the presence of partial melt. We compare our results with other geophysical and geologic data. We propose that any north‐south trending thermal (velocity) anomaly that may have existed in the upper mantle during earlier (Oligocene to late Miocene) phases of rifting and that may have correlated with the axis of the rift has diminished with time and has been overprinted with more recent structure. The anomalously low‐velocity body presently underlying the transition zone between the core of the Colorado Plateau and the rift may reflect processes resulting from the modern (Pliocene to present) regional stress field (oriented WNW‐ESE), possibly heralding future extension across the Jemez lineament and transition zone.

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