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The Upper Rhine Plain, view from west to east. Front: a vineyard near Neustadt an der Weinstraße, background: Mannheim (right: Mannheim Power Station, far north: the Odenwald

The Upper Rhine Plain^[1] or Rhine Rift Valley^[2] (German: Oberrheinische Tiefebene or Oberrheingraben) is a major extensional rift system in Central Europe, straddling the border between France and Germany. It formed during the Cenozoic as a response to the evolution of the Alps to the south. Today, the Rhine Rift Valley forms a downfaulted trough through which the River Rhine flows. It is bounded to the west by the Vosges mountain range in France, and to the east by the Black Forest in western Germany.

[edit] Formation

Satellite image showing the geography of the Upper Rhine Plain and the rift flanks of Vosges, France, and the Black Forest, Germany.

Schematic cross-section of the Rhine Rift Valley.

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^[3]. The result was substantial crustal thinning, forming a major extensional graben and causing isolated volcanic activity. The stretch factor is estimated to be ~2.

[edit] Rift flank uplift

The Vosges mountain range, France, formed by rift-flank uplift on the margins of the Rhine Graben.

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.

[edit] Volcanic activity

The extension induced by the formation of the Alps was sufficient enough to thin the crust and provide suitable dilational conduits for magmatic and volcanic activity to occur. This resulted in the emplacement of mafic dykes, which follow the general structural trend of the extensional faults. In addition, isolated volcanoes such as the Kaiserstuhl were formed.

[edit] The Kaiserstuhl volcano

The Kaiserstuhl (literally "Emperor's Chair") is a cluster of volcanic hills to the northwest of Freiburg, within the Rhine Graben. The highest point of this small, isolated volcanic centre is the Totenkopf (557 metres). Volcanic activity was most prevalent in the Miocene epoch, some 15 million years ago^[4]. Today, the Kaiserstuhl volcano is extinct.

[edit] Seismic activity

In 1356, the Basel earthquake occurred in the Rhine Plain. It was perhaps the most destructive earthquake ever to have occurred in northwest Europe, destroying the city of Basel and flattening buildings as far as 200 km away. However, it remains disputed whether the fault that ruptured to cause this earthquake was indeed part of the Rhine Valley extensional system, or simply one of the many thrust faults that make up the Alps to the south.

[edit] References