Abstract

The regional stress field in and around the Rhinegraben rift system and in the adjacent parts of the Alps has been investigated by a series of “doorstopper” in situ stress determinations. This is a stress-relief method that measures rock strain by destressing the rock with an overcoring operation. The maximum component of horizontal compression was found to trend approximately northwest to north-northwest. The excess horizontal stress culminates in the central Alps, decreases abruptly at the northern margin of this mountain range, and has relatively low values in the foreland area of the Rhinegraben.

Alpine folding and thrusting ceased in Pliocene time, but strong epeirogenic uplift continues today. While plate-tectonics compression relaxed, stresses caused by topographic effects and unloading increased within the rising mountain body. The directions of maximum horizontal compression are about normally oriented to the isobases of Holocene uplift. The measured excess stresses obviously are not sufficient for thrust progression, since the Alps are considerably consolidated.

The Rhinegraben was formed as an extensional rift valley in mid-Eocene to early Miocene time. The remaining zone of weakness trends about parallel to the sinistral shear of the active regional stress field. Because of this, the rift belt has been remodeled into a shear zone; its slip rates are related to the lateral extension of the Alpine mountain body farther south. Deflected by two changes in the graben's axial trend, the neotectonic deformations of the individual graben segments demonstrate a varying interaction of shear with compression in one case or shear with extension in the others. Where active shearing ceases at the northern end of the graben, the shift of the South German block is transmitted directly to the northward adjacent Rhenish massif. This massif reacts by secondary movements along pre-existing discontinuities, by horizontal flexuring, and by seismic activity. Farther north, in the Lower Rhine embayment, the Holocene peri-Alpine stress regime causes extensional rifting and subsidence.

Tectonic effects to be ascribed to the Holocene regional stress pattern may be recognized back to mid-Pliocene time. Before that, in early Pliocene to mid-Miocene time, tectonic deformations were controlled by different stress regimes. Plate-tectonics processes in the rigid sphere were complemented by plastic mass compensations within the asthenosphere. Mantle rise was an additional factor controlling the Tertiary process of extensional rifting.

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