page_title: Molasse basin
section_title: Tectonic history
hierarchical_section_title: Molasse basin / Tectonic history
caption_attribution_description: English: Sedimentary layers at Wallenried, canton Fribourg, Switzerland. Age: Aquitanian (~22 Ma). Alternation of grey fluviatile sandstone, deposited by meandering rivers and red silty marls, belonging to the Untere Süsswassermolasse (Lower Freshwater Molasse) formation.
context_page_description: The Molasse basin is a foreland basin north of the Alps which formed during the Oligocene and Miocene epochs. The basin formed as a result of the flexure of the European plate under the weight of the orogenic wedge of the Alps that was forming to the south.
  In geology, the name "molasse basin" is sometimes also used in a general sense for a synorogenic foreland basin of the type north of the Alps. The basin is the type locality of molasse, a sedimentary sequence of conglomerates and sandstones, material that was removed from the developing mountain chain by erosion and denudation, that is typical for foreland basins.
context_section_description: Before the development of the Molasse basin, in the Mesozoic era, the region was covered by a shallow sea that was the northern margin of the Alpine part of the Tethys Ocean (Valais Ocean in the west, Tethys Ocean proper in the east). The Molasse basin was formed when the orogenic wedge of the forming Alps was pushed north over the European continental margin, due to the convergent movement of the European and Apulian plates during the Paleogene period. The weight of the orogenic wedge made the European plate bend downward, forming a deep marine foredeep. In the Eocene epoch (55 to 34 million years ago) the basin became deeper until north of the developing orogen it formed a small oceanic trench, in which flysch sediments were deposited.
  The huge amounts of sediments eroded from the forming mountain chain filled the basin and made it shallower. During the Oligocene and Miocene epochs (more exactly, between 10 and 30 million years ago), shallow marine to continental molasse was deposited in the basin. Around 10 to 5 million years ago, tectonic uplift had raised the basin so high that net sedimentation stopped. From the south, the molasse deposits were overthrust about 10 kilometers by the Helvetic nappes, which caused the deformation in the Subalpine Molasse zone. The Jura mountains, a fold and thrust belt along the present Swiss-French border, also originated from this tectonic phase. In some places in the Jura mountains, molasse deposits were folded together with older Mesozoic limestones. In present-day central Switzerland, however, the molasse formed a thick competent mass that was thrust northward in one piece over a decollement horizon at the base of the Mesozoic, in Triassic evaporites. Deformation instead localized further north, thus forming the relatively flat Swiss Mittelland between the Alps and the Jura Mountains. The Swiss part of the Molasse basin is now between the Alps and the Jura mountains, as a large piggyback basin. In the Eastern Alps an external mountain range such as the Jura Mountains never developed. Due to the last phase of tectonic uplift around 5 million years ago, the molasse in the Swiss Plateau, the South Bavarian plain and Eastern Austria is now 350 to 400 meters above sea level at its northern rim, slowly rises southwards and can reach more than 1,000 m at its contact with the Alps.
caption: Alternating grey fluviatile sands and red silty marls of the Lower Freshwater Molasse at Wallenried, Switzerland.
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