NEW DATA AND VIEWS ON THE BORDER ZONE TAB ULAR JURA - FOLDED JURA (ZIEFEN-REIGOLDSWIL AREA, BASEL JURA)

The Basel region is distinguished by the close vicinitiy or superposit ion of a number of important tectonic features. It combines the struct ures of the southeastern corner of the Paleogene Rhinegraben with thos e of the Early to Late Miocene front of the Alps. This in turn is comp osed of both the shallow decollement fold and thrust belt at the lip o f the obducted orogenic wedge and the forebulge of the subducted part of the lithosphere. The recently remapped small area of Ziefen-Reigold swil SSE of Basel contains a wealth of information about the developme nt of the wider Basel region. The Tabular Jura is a marginal field of Paleogene normal faults from the border of the Rhinegraben. It was sub ject to erosion and peneplanation at the end of the Early Miocene- an event that is contemporaneous with the development of the Helvetic nap pes in the Alps. At the same time the stress field associated with the Rhinegraben ceased to exist and was replaced by a stress field compat ible with the are of the Western Alps. In the Middle Miocene the penep lain was first covered by a shallow sea and subsequently uptilted to t he Vosges-Black Forest high at the southern end of the former Rhinegra ben. This event marks the onset of the Jura phase of Alpine compressio n and documents the forming of its forebulge. It also heralds the adve nt of the decollement fold and thrust belt of the Jura at the front of the associated orogenic wedge. In the Ziefen-Reigoldswil area there a re two thrust sheets stacked on top of Middle Miocene sediments. The t hrust plates consist of a mosaic of Paleogene normal fault blocks that were cut by a thrust-plane in lower Jurassic to upper Triassic beds. In order to do so, this nearly bedding-parellel composite decollement thrust had to propagate through a number of fault barriers, without ap parently being much affected by them. This is surprising in view of th e situation in neighboring parts of the Jura where such pre-existing f aults were reactivated as sinistral transfer faults or as stress conce ntrators for the activation of new ramps.