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 shi eld and its foreland by comparing them analytically along levelling pr ofiles. Instead of generalizing regional vertical movement trends, thi s method emphasizes the activity of individual structural elements, wh ich 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 move ments. 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 zone s 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 inf erred for the Hunsruck border fault is also consistent with the compre ssive regime presently observed in that area, with sigma(1) oriented t o the northwest. The direction of vertical motions along the western b order fault of the upper Rhine graben changes from south to north in r elation 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-tr ending normal faults are identified, one of them having probably ruptu red in a swarm of microearthquakes not long before the second survey w as performed. The western Saar-Nahe trough is characterized by N-S-ori ented fractures which cannot be related to mapped faults but show a cl ose connection with photolineaments.