SEISMIC EVIDENCE FOR A LOW-VISCOSITY LOWER CRUST BENEATH THE CONTINENTAL DEEP DRILLING LOCATION, OBERPFALZ, GERMANY

A network of six deep seismic reflection profiles was recorded in the Oberpfalz as part of the pre-site investigations for the German deep drilling programme. In addition, this network is crossed by a longer regional profile. A striking observation is the highly variable reflectivity of the lower crystalline crust. Based on the relationship between (temperature-dependent) viscosity and crustal reflectivity it seems possible to relate the changes encountered here to the tectono-thermal regime. We propose that processes forming the Eger Graben also thermally influcnce the crust beneath the Oberpfalz. These mechanisms should have a considerable impact on the maximum depth of the deep borehole. The results of the pilot hole give strong support to our interpretation: the temperatures are higher than the maximum estimates from near-surface heat flow determinations and the maximum depth for the borehole has been reduced from 14 to 10 km.