IMPORTANCE OF INHERITED RIFT MARGIN STRUCTURES IN THE EARLY NORTH ALPINE FORELAND BASIN, SWITZERLAND

The earliest evolution of the North Alpine Foreland Basin in Switzerla nd was characterized by deposition in small, structurally partitioned sub-basins during the Late Cretaceous and Early Tertiary, rather than in a single, large foredeep. These sub-basins, which were probably loc ated between old rift margin fault-blocks reactivated during Alpine co mpression, were incorporated into the thrust wedge during thin-skinned deformation. In eastern Switzerland, the most external sub-basins wit h respect to the orogenic wedge (North Helvetic Flysch and Blattengrat units) have at their base an unconformity attributed to flexural fore bulge erosion. More internal sub-basins (Sardona and Prattigau units) contain a conformable succession from the underlying passive margin st age and are dominated by deep-water sedimentation. In western Switzerl and, both external sub-basins, now found in the Helvetic Diablerets an d Wildhorn nappes, and deep-water internal sub-basins (Hochst-Meillere t Flysch, Neisen Flysch, Tarentaise Flysch) preserve a well-developed basal unconformity. Comparison of the eastern and western Swiss transe cts shows important intrabasinal lateral variations to be present. The western Swiss area was a topographic high for much of the Late Cretac eous and Early Tertiary; this is demonstrated by the increased chronos tratigraphic gap at the karstified basal unconformity surface in weste rn Switzerland. The strata onlapping this unconformity young to the we st, suggesting that drowning of the emergent area was delayed compared with the east. In addition, reactivation and uplift of the rift margi n structures occurred earlier in western Switzerland compared with eas tern Switzerland. There is therefore strong evidence for lateral topog raphic gradients in the early foreland basin caused by differential am ounts of tectonic reactivation of rift margin structures. In the early foreland basin-fill, these lateral variations are as important in det ermining depositional patterns as strike-normal changes across the bas in.