The Jura Mountains — an active foreland fold-and-thrust belt?

Abstract

Recently completed in situ stress measurements using the borehole slotter at 33 new test sites within the Swiss and French Jura Mountains, combined with previously published stress data, allow a detailed description of the contemporary state of stress in this fold-and-thrust belt and the adjacent foreland. Five stress provinces can be recognized, with two different general orientations of maximum horizontal stress S H: (1) the Central and Southwest Provinces with a NNW–SSE to NNE–SSW orientation; (2) the East, Northwest and Southeast Provinces with an E–W to NW–SE orientation. Stress magnitudes are in general higher in the southern than in the northern Jura Mountains. Characteristic of the northern Jura is an E–W-trending zone of low stress magnitudes which can be traced from the Bresse Depression south of Besançon to the eastern end of the Jura northwest of Zurich. Boundaries of stress provinces show poor agreement with boundaries of tectonic units in the Jura. In addition, stress provinces extend beyond the boundaries of the Jura Mountains into their foreland. Furthermore, palaeo-stress orientations show large discrepancies with respect to the contemporary stress orientation. These findings are considered to indicate a termination of Jura thin-skinned foreland tectonics, which probably took place some time between 9 and 4 Ma according to palaeontological and tectonic evidence. It is suggested that a new style of tectonics has commenced in the Jura Mountains, deforming both the basement directly below the Jura fold-and-thrust belt and the cover rocks in a similar mode.