Diapiric origin of the Chessel-Noville hills of the Rhone Valley interpreted from georadar mapping

In an attempt to understand the origin of the enigmatic Chessel-Noville hil ls in the Rhone River plain of western Switzerland, we have recorded two an d three-dimensional ground-penetrating radar (georadar) data and drilled si x shallow boreholes. Cross-sections and maps of horizons extracted from the georadar data reveal normal fault zones under the hill flanks and minor gr abens and domes below the hill crests. Comparable graben-like structures wi th normal step faults converging with depth are observed at a nearby outcro p. Fine sand and silt within some of the exposed faults may have been dragg ed into the fault planes or injected as elastic dikes under high pore press ures. A silt unit encountered in two of the boreholes correlates with a str ong georadar reflection, Although angular calcareous boulders are scattered across the surface, the early suggestion that the hills are remnants of a large historic rockfall is not compatible with the subsurface lithologies a nd structures. Neither is the frequently referenced hypothesis that they ar e moraines. Moreover, the absence of significant thrust faulting and lack o f strong structural trends in the georadar data and at outcrops are inconsi stent with the more recent interpretation of the hills as glaciotectonic ri dges. Instead, our results indicate that the hills and their internal struc tures represent vertically uplifted and deformed fluvio-deltaic sediments. Diapirism offers a plausible explanation for the observed uplift and deform ation, Saturated fine-grained sediments underlying the former Rhone Delta m ay have been compacted and overpressured during burial by pro-grading delta ic sands and thickening sequences of fluvial sands and gravels. Under these conditions, mud diapirs may develop, Actual diapiric uplift and deformatio n may have been triggered by a large historic earthquake (e.g., Tauredunum event of 563 A,D,) that would have induced additional over-pressurization, liquefaction and upward mobilization of the fine-grained sediments, Such an earthquake may also have triggered the rockfall responsible for the scatte red calcareous boulders.