Abstract

The Upper Rhine graben is a north-northeast–trending, small-displacement, crustal-scale rift of Tertiary age. Retrodeformation of its southeastern part demonstrates that it is a product of sinistral oblique rifting. Early extension was toward 80°. Later, the major stretching axis changed to a 60° direction. The modeling results suggest that the eastern Main Border fault developed first, and that faulting later propagated into the evolving graben interior. Considerable along-strike variations in heave, throw, and displacement are evident. Displacement partitioning causes warping of the rift floor with a 30–35-km wavelength. We consider this to be a characteristic of oblique rifting. Contact deformation of the wall rocks to the major faults may have caused widespread smaller scale faulting and brecciation and may be the location of later movements. Close spatial coincidence of the depth projections of some of the faults studied and the hypocenters of recent small earthquakes indicates continuing activity of the fault system. Apparently, three fault segments in the Freiburg area are currently active and may be an increased earthquake risk.

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