The outcrop can be divided into three parts or zones: a central core of Ordovician
orthogneiss, called the
Streifengneis ("striped gneiss"); flanked on both sides by zones of
mica schist or paragneiss. The northern paragneiss zone contains lenses of various
lithologies, such as
calcareous schists,
amphibolite,
eclogite,
peridotite (mostly thoroughly
serpentinized) and
gabbro. The southern paragneiss zone contains characteristic
hornblende-
garnet schists called
Garbenschiefer, part of the
Tremola series. The crystalline rocks of the Gotthard nappe carry strong traces of at least two older
orogenies before the
Alpine orogeny. The oldest phase, sometimes (erroneously) called "
Caledonian", took place in the
Ordovician. The paragneiss of the Gotthard nappe was once oceanic sediment, which was deformed into an
accretionary wedge during this orogeny. The gabbros and peridotites, now lenses in the paragneiss, probably represent pieces of
oceanic crust underneath these sediments, that were incorporated into the Ordovician accretionary wedge. The eclogite metamorphism has been dated at about 470
Ma (million years ago). In the late stages of the Ordovician orogenic phase,
granitoid magma bodies intruded, especially
granite. During the
Hercynian orogeny, about 320 Ma, all of these units were strongly deformed. Today, the Ordovician intrusives form the
Streifengneis of the central zone. The Hercynian orogeny ended with another phase of granitoid
intrusions. Four Hercynian intrusive bodies are found in the current outcrop of the Gotthard nappe: the
Rotondo granite north of the
Val Bedretto, the
Fibbia and
Gamsboden granites close to the
Gotthard Pass and the
Cristallina granite around the
Lukmanier Pass. These intrusions have ages between 305 and 290 Ma. ==See also==