Enigmatic earthquake-generated large-scale clastic dyke in the Biberach area (SW Germany)

Abstract

We discovered and describe a large-scale (exposed dimensions ~17 m in lateral and ~15 m in vertical extension, and up to 10 cm in width) clastic dyke in the Middle Miocene (between ~17.1 and ~14.5 Ma) sandy deposits (‘Fluviatile Untere Serie’) of the North Alpine Foreland Basin, close to Biberach an der Riß in SW Germany. The dyke appears to be of sedimentary–tectonic origin. It formed by liquefaction and injection of clastic material that occurred simultaneously with fracture opening and propagation in sandy deposits, presumably as a result of strong seismic shaking triggered by an intense earthquake. We, therefore, interpret the dyke as a seismite and a useful palaeoseismic indicator. The clastic dyke cuts through a layer of distal Ries ejecta (‘Brockhorizont’) and, thus, postdates the Ries impact. The age of the Ries event, in combination with other stratigraphic constraints, defines a relatively narrow time frame of ~14.8 to 14.5 Ma for the formation of the clastic dyke. The source of the strong earthquake responsible for dyke formation might be seismicity related to the Alpine orogeny and tectonism, seismic activity in one of the volcanic fields of the Paleogene to Quaternary European Volcanic Province, or seismicity induced by a Miocene impact event in southern Germany. We suggest the relatively strong earthquake in question may have been triggered by a large phreatomagmatic explosion in the Urach-Kirchheim volcanic field or, perhaps more likely, the Steinheim impact event on the eastern Swabian Alb plateau.