Possible Quaternary growth of a hidden anticline at the front of the Jura fold-and-thrust belt; geomorphological constraints from the Foret de Chaux area, France

This study presents new constraints for Plio-Quaternary (post-2.4 Ma to present-day) anticline growth along the frontal zone of the Jura fold-and-thrust belt, in the Foret de Chaux area, located 30 km SW of Besancon. The Foret de Chaux area consists of a N080 degrees E-elongated depression bordered by the Doubs and Loue Rivers to the north and south respectively, and filled with sundgau-type Pliocene alluvial deposits. The upper surface of the Pliocene deposits between the Loue and Doubs Rivers is marked by a N065 degrees E-trending ridge crossing the depression in a median position. A differential uplift along this ridge, post-dating the deposition of the gravels (2.4 Ma), is suggested by several geomorphological observations such as the opposite river migration on each side of the ridge as well as variations of drainage geometry and incision intensity. Geological and geophysical subsurface data indicate that the ridge roughly coincides with the axis of an anticline hidden beneath the Pliocene deposits. The observed uplift is presumably related to a post-2.4 Ma anticline growth. The fact that the azimuth of the hidden anticline axis is parallel to the strike of deep-seated Late Paleozoic basement faults and not to the local strike of the thin-skinned Jura structures indicates that the inferred post-Pliocene deformation could possibly be an expression of a recent thick-skinned deformation of the basement of the northern Alpine foreland. The focal depth (15 km) of the February 24 (super th) , 2004, Besancon earthquake supports the hypothesis of a basement fault reactivation.