Giessen nappe

The Giessen nappe (German: Gießener Decke) is a tectonic nappe in the southeastern part of the Rhenish Massif in western Germany. The nappe is an "alien" (allochthonous) unit in the Rhenohercynian zone of the Hercynian orogeny, it was thrust over the usual slightly metamorphosed Devonian and Carboniferous sedimentary rocks in this zone.^[1]

Geologic map of the southeastern part of the Rhenish Massif, showing outcrops of the Giessen nappe in orange. The Giessen nappe has the same colour as the Hörre zone, which is on the other, northwestern side of the Lahn syncline.

The Giessen nappe is often correlated with the Harz nappe in the Harz^[2] and similar units in the Bohemian Massif in the Czech Republic.^[3] These units form the rare erosional leftovers of small microcontinents that were during the Hercynian orogeny squeezed between the paleocontinents Gondwana to the south and Laurussia to the north.

Outcrop area

Giessen nappe outcrops cover about 300 km² from the western edge of the Vogelsberg over Braunfels, Wetzlar and Gießen, almost to Marburg. The river Lahn flows along the northern part of the nappe until it crosses the nappe between Gießen and Wetzlar to continue flowing westwards to the Rhine.