Molasse Basin - Tectonic History

Tectonic History

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, resulting in the formation of a deep marine foredeep. In the Eocene epoch (55 to 34 million years ago) the basin became deeper until it formed a small oceanic trench north of the developing orogen, in which flysch sediments were deposited.

Due to the huge amounts of sediments that were eroded from the forming mountain chain the basin was filled up and got shallower. During the Oligocene and Miocene epochs (more exactly between 10 and 30 million years ago), undeep marine to continental molasse was deposited in the basin. Around 10 to 5 million years ago, tectonic uplift had raised the basin to such a height that netto 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 located in between the Alps and the Jura mountains, as a large piggy-back 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 at 350 to 400 meters above sea level at its northern rim, slowly rises southwards and can attain more than 1,000 m at its contact with the Alps.