Rhine Rift Valley

The Upper Rhine Plain,Rhine Rift Valley or Upper Rhine Graben (German: Oberrheinische Tiefebene, Oberrheinisches Tiefland or Oberrheingraben, French: Vallée du Rhin) is a major rift, about 350-kilometre-long (220 mi) and on average 50-kilometre-wide (31 mi), between Basel in the south and the cities of Frankfurt/Wiesbaden in the north. Its southern section straddles the border between France and Germany. It forms part of the European Cenozoic Rift System, which extends across central Europe. The Upper Rhine Graben formed during the Oligocene as a response to the evolution of the Alps to the south and remains active to the present day. Today, the Rhine Rift Valley forms a downfaulted trough through which the River Rhine flows.

The Upper Rhine Plain was formed during the Early Cenozoic era, during the Late Eocene epoch. At this time, the Alpine Orogeny, the major mountain building event that was to produce the Alps, was in its early stages. The Alps were formed because the continents of Europe and Africa collided. It is thought that because the collision was irregular, the initial contact between the two continents resulted in the formation of dilational (extensional) structures in the foreland basin to the north of the Alps. The result was substantial crustal thinning, forming a major extensional graben and causing isolated volcanic activity. The stretch factor is estimated to be ~2.

To both the east and west of the Rhine Plain, two major hill ranges have formed that run the length of the basin. To the west, in France, these hills are known as the Vosges mountain range and in the east, in Germany, the hills comprise the Black Forest. These ranges exhume the same types of rocks in their cores, including deep crustal gneiss. Both ranges correspond to uplifts of more than 2,500 metres, much of which has since been eroded. This uplift has occurred because of the isostatic response associated with the formation of an extensional basin. As a consequence, the highest mountains exist immediately adjacent to the margin of the basin, and become increasingly low outwards. The boundaries between the hill ranges and the Rhine Graben are defined by major, normal fault zones.

...
Wikipedia