LOWER CRUSTAL THINNING IN THE RHINEGRABEN - IMPLICATIONS FOR RECENT RIFTING

Seismic near-vertical and undershooting experiments carried out in the southern upper Rhinegraben area between 1984 and 1991 show pronounced lateral variations of deep crustal properties. Significant difference s in the apparent thickness of the reflective lower crust and the tran sparent upper crust appear to be related to different structural setti ngs. A 12- to 14-km-thick reflective lower crust beneath a 15-km-thick transparent upper crust of the eastern graben shoulder (Black Forest) probably originated during Permo-Carboniferous reequilibration of thi ckened Variscan orgenic crust. Thinning of this lower crust by about 5 km beneath the graben and rifted domains transitional between graben and shoulder (the Dinkelberg block) is interpreted to be related to Ce nozoic extensional faulting. A discrepancy between moderate extension of upper crust and lower crustal geometry indicates mechanical decoupl ing at depth during extension. Congruent modification of other physica l properties is suggested by strong single reflective elements in the topmost parts of the thinned lower crust. In the transitional Dinkelbe rg block such an anomalously strong reflector occurs at a depth of 20 km below a pronounced local maximum of earthquake activity; it is inte rpreted to be the presently active zone of decoupling which in time sh ifted from the rift axis to the eastern transition area.