Abstract:
Neotectonic in the karst north of Lake Thoune (Switzerland) - The karstic area north of Lake Thoune is part of the "Border Chain" of the Swiss Alps (Cretaceous, Helvetic). It comprises large caves coming from two catchments. The first one pours out at the Beatushöhle; more than 15 km of galleries are known in this area. The second one contains the Sieben Hengste - Hohgant - Hohlaub - Schrattenfluh region, it pours out at the Bätterich and Gelberbrunnen springs, by Lake Thoune. It includes the very large "Sieben Hengste - Hohgant cave System" (length: 115 km; depth: 1050 m); the Bärenschacht (length: >10 km; depth: 950 m), as well as several other important caves more than 1 km length. Recent shifts along faults were mainly measured in the Sieben Hengste Cave System. Neotectonic indication were of the following types: gallery sections displaced by the fault shifts, displaced pillar structures or shifted, inclined or broken speleothems. The fault movements were placed on a time scale according to the genetic evolution of the region. It indicates that there were three phases of movement, which greatly affected the underground flows and karstification. The geometric and dynamic analysis of the measured shifts and slikken-slides also indicates three phases of movements. The strain direction, causing these movements, was determined. Thus, three plio-quaternary tectonic phases were found: an alpine compressive SSE-NNW phase, followed by an extensive SSE-NNW phase and then again by a compressive one.