NEOTECTONIC ACTIVITY IN AND AROUND THE SOUTHWESTERN RHENISH SHIELD (WEST-GERMANY) - INDICATIONS OF A LEVELING COMPARISON

Precise levelling data are re-examined in the southwestern Rhenish shield and its foreland by comparing them analytically along levelling profiles. Instead of generalizing regional vertical movement trends, this method emphasizes the activity of individual structural elements, which can be located with a high accuracy. It is shown that present-day vertical motions concentrate on fractures which cut the massif into a number of tectonic blocks. These undergo more or less independent movements. Significant measured displacements range from 1 to 3.5 cm over an average 20-year period and correspond mostly to aseismic slip. High displacements are found near pre-existing faults, sometimes also zones of joint concentration which are favourably oriented with respect to the current regional stress field. In the Mosel area, SW-NE-trending faults are predominantly reactivated as reverse faults. The motion inferred for the Hunsruck border fault is also consistent with the compressive regime presently observed in that area, with sigma(1) oriented to the northwest. The direction of vertical motions along the western border fault of the upper Rhine graben changes from south to north in relation to the different azimuths of the central and northern segments of the graben, inducing a S-N-oriented transition from compressional to extensional shear. Within the graben itself, some N160 degrees E-trending normal faults are identified, one of them having probably ruptured in a swarm of microearthquakes not long before the second survey was performed. The western Saar-Nahe trough is characterized by N-S-oriented fractures which cannot be related to mapped faults but show a close connection with photolineaments.