Inside a fault

In September, Klaus and me lead a MSc student excursion to southwestern Germany. We not only went through the entire stratigraphy of that area (mainly Triassic and Jura), but also visited the salt mine in Stetten, quarries in Dotternhausen (opalinus clay, posidonia schist, Malm), the Kaiserstuhl carbonatite volcano complex and the Upper Rhine Graben area. One of my personal highlights was the Freudenstädter Graben, a small tectonic graben striking NW-SE, whose NE main fault is exposed in an old mine in Hallwangen.

A (very) rough sketch of the Freudenstädter Graben with the Hallwangen mine being situated at the NW tip of the NE boundary fault.

 

The Freudenstädter Graben boundary faults have a vertical displacement of up to 150 m. Its hanging wall forms a roll-over anticline with secondary structures like syn- and antithetic faults (Asprion et al., 1997; Reicherter et al., 2009). The mine “Himmlisch Heer” is open to the public. Here, copper, fluorite and baryte have been mined most likely since the 13th century. Ore mining activity stopped in 1921. The copper ores and the baryte were enriched locally along the NE graben boundary fault that dips to the SW. The old miners did a great job in preparing the fault plane and also managed to preserve the slickensides. It’s just a great experience to see a fault plane exposed for more than 100 m some 40 m below surface! At some places, the tunnels cross the fault plane so that you can simply walk through the fault.

The SW dipping fault plane.

 

The plane is exposed for more than 100 m and beautiful slickensides are perfectly preserved.

 

Almost the entire mine follows the fault.

 

Don’t miss to visit the mine once you are there and also don’t miss this virtual tour.

Literature:

Asprion, U., Reicherter, K. & Meschede, M. 1997. Das Bodenradar und seine Anwendung zur Erkennung tektonischer Strukturen: ein Beispiel aus dem Freudenstädter Graben (Schwarzwald, Südwestdeutschland). Jber. Mitt. Oberrhein. Geol. Ver., N.F., 79, 111-124.

Reicherter K., Froitzheim, N., Jarosinski, M., Badura, J., Franzke, H.-J. Hansen, M., Hübscher, C., Müller, R., Poprawa, P. Reinecker, J., Stackebrandt, W., Voigt, T., von Eynatten, H. & Zuchieweicz, W. 2009. Alpine tectonics north of the Alps. In: T. McCann (ed.) The Geology of Central Europe. The Geological Society (London), p.1233-1285.

Where is it?

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Who was it?

Christoph Grützner
works at the Neotectonics and Natural Hazards Group, RWTH Aachen University, Germany. He likes the Mediterranean and uses geophysics to search for ancient earthquakes.