INVESTIGATION OF THE LITHOSPHERE BENEATH THE VOGELSBERG VOLCANIC COMPLEX WITH P-WAVE TRAVEL-TIME RESIDUALS

With the aim of investigating the P-wave velocity structure below the Tertiary volcano Vogelsberg, a network of 10 mobile short period seism ograph stations was installed in May 1987 for a period of 20 months. P -Wave travel time residuals relative to the station Kleiner Feldberg/T aunus (TNS) were determined for 168 seismic events using the Jeffreys- Bullen travel time tables. At all stations the relative residuals show ed a positive sign, indicating a low velocity zone beneath the Vogelsb erg. Maxima were found in the northern part of the Vogelsberg (station VAD +0.5 s) and in the region of the Amoneburger Basin (station RAU 0.28 s). The travel time residuals were inverted using the tomographic inversion method of Aki et al. (1977). The slowness perturbations of the single blocks were calculated relative to a crustal and upper mant le model of the Rhenish Massif. The results show an intracrustal low v elocity body (about -9%) striking in a Variscan direction and underlyi ng the north-eastern part of the Vogelsberg, and another velocity mini mum (about -6%) in the region of the Amoneburger Basin. In the lower c rust and the upper mantle the velocities are reduced by about 4% relat ive to the starting model. The Variscan alignment of the low velocity zone under the Vogelsberg correlates with results of other geological studies. It can be assumed that during the rifting phase of the Upper Rhinegraben Variscan lineations have been reactivated, favouring upris ing of magma along these old structures. The position and extension of the low velocity zone correlate with the assumed sediment distributio ns in the area of investigation. This may account for about one-half o f the observed anomaly. The reason for the velocity reduction of about 4% in the entire underground region of the Vogelsberg down to a depth of about 70 km can be explained by the intensive fracturing of the li thosphere, caused by thermal and pressure gradients during the magma e ruption process.