SEISMOLOGICAL INDICATION AND GEOMORPHIC EVIDENCE FOR QUATERNARY TO RECENT SHORTENING ALONG THE NORTHWESTERN ALPINE FRONT (EASTERN FRANCE)

Along the northwestern Alpine front in eastern France the Late Miocene to Early Pliocene thin-skinned Jura fold-and-thrust belt encroached onto the Eo-Oligocene Rhine-Bresse Transfer Zone, a central segment of the European Rift system that developed along a Late Paleozoic trough system. Until recently this region was characterized by remarkably low seismic activity, in particular when compared to the neighbouring southern Upper Rhine Graben. In February 2004 the area was shook by a M_L 4.8 earthquake. The epicenter of this comparatively strong seismic event was located near the city of Besançon at approx. 15 km depth and revealed the first reliable transpressional focal mechanism in the wider region otherwise predominated by strike-slip and normal faulting mechanisms.

These new seismological indications prompted multidisciplinary field investigations, including subsurface analyses and paleotopographic reconstructions. The results indicate that along this part of the Alpine front, contractional deformation did not cease after the Early Pliocene but continued during the Quaternary and is most probably still ongoing. Most remarkably, paleo-meanders of the Doubs River were found to be folded along an anticline axis within the city center of Besançon recording Late Quaternary fold growth that is associated with a minimum rock uplift of 13 m. Local rock uplift rates related to Quaternary folding estimated from OSL-dating of oxbow lake deposits measure 0.17 ± 0.05 mm a^-1 .

Considering the regional seismicity depth distribution it is apparent that active deformation along the front of the Jura fold-and-thrust belt is no longer entirely thin-skinned but involves a thick-skinned component, possibly the inversion of Paleozoic crustal discontinuities that appear to have governed the tectonic evolution of the region since Early Cenozoic times. Indeed, the earthquake of Besançon may represent a first seismological indication for ongoing tectonic underplating in northern Alpine foreland (Madritsch et al. 2010 and references therein).

Reference:

Madritsch, H., Preusser, F., Fabbri, O., Bichet, V., Schlunegger, F., Schmid, S.M. (2010): Late Quaternary folding in the Jura Mountains: Evidence from synerosional folding of fluvial meanders. Terra Nova 22, 147-154.

.