TheUpper Rhine Plain,[1]Rhine Rift Valley[2] orUpper Rhine Graben[3] (German:Oberrheinische Tiefebene,Oberrheinisches Tiefland orOberrheingraben,French:Vallée du Rhin) is a majorrift, about 350-kilometre-long (220 mi) and on average 50-kilometre-wide (31 mi), betweenBasel in the south and the cities ofFrankfurt/Wiesbaden in the north. Its southern section straddles theFrance–Germany border. It forms part of theEuropean Cenozoic Rift System, which extends acrossCentral Europe. The Upper Rhine Graben formed during theOligocene, as a response to the evolution of theAlps to the south. It remains active to the present day. Today, the Rhine Rift Valley forms a downfaulted trough through which the riverRhine flows.
The Upper Rhine Plain was formed during the EarlyCenozoic era, during the LateEocene epoch. At this time, theAlpine Orogeny, the major mountain building event that was to produce theAlps, was in its early stages. The Alps were formed because the continents of Europe and Africa collided. It is thought that because the collision wasirregular, the initial contact between the two continents resulted in the formation of dilational (extensional) structures in theforeland basin to the north of the Alps.[4] The result was substantial crustal thinning, forming a major extensionalgraben and causing isolated volcanic activity. Thestretch 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 theVosges mountain range and in the east, in Germany, the hills comprise theBlack Forest. These ranges exhume the same types of rocks in their cores, including deep crustalgneiss. 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.
The northern section of the Rhine Plain is equally framed by somewhat lower mountain ranges, thePalatinate Forest on the western and theOdenwald on the eastern side.
The extension induced by the formation of the Alps was sufficient to thin the crust and provide suitable dilational conduits for magmatic and volcanic activity to occur. This resulted in the emplacement ofmaficdykes, which follow the general structural trend of the extensional faults. In addition, isolated volcanoes such as the Kaiserstuhl were formed.
TheKaiserstuhl (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 theTotenkopf (557 metres). Volcanic activity was most prevalent in theMiocene epoch, some 15 million years ago.[5] Today, the Kaiserstuhl volcano is extinct.
In 1356, theBasel earthquake occurred in the Rhine Plain. It was perhaps the most destructive earthquake ever innorthwest Europe, destroying the city ofBasel and flattening buildings as far as 200 km away. It was the most significant historic seismological event to have occurred in Central Europe. Its epicenter was betweenWaldkirch andSt. Peter inBreisgau-Hochschwarzwald. However, it remains disputed whether the fault that ruptured to cause this earthquake was part of the Rhine Valley extensional system, or simply one of the manythrust faults that make up theAlps to the south. Doubts have been raised over the adequacy of the seismic evaluation and design of theFessenheim Nuclear Power Plant, built in the Rhine Plain close to the faults.[6]
Located below the plain, the Upper Rhineaquifer – one of the largest in Europe – holds an estimated 45,000 km3 (11,000 cu mi) of fresh water and supplies some 3 million people in France and Germany, supplying 75% of their drinking water and 50% of the water used by industry.[7] Since the 1970s it has suffered increasingly serious pollution fromnitrates,pesticides,chloride andVOCs.[7]
A collaboration of 25 universities and government agencies have been researching seismic hazards, neotectonics, and water management of the Quaternary Graben fill. The research focuses on four themes:[9]
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48°57′54″N8°14′02″E / 48.9650°N 8.2340°E /48.9650; 8.2340
