SOUTHERN RHINE GRABEN - SMALL-WAVELENGTH TOMOGRAPHIC STUDY AND IMPLICATIONS FOR THE DYNAMIC EVOLUTION OF THE GRABEN

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.