Cadomian Orogeny

The Cadomian orogeny occurred in the Late Neoproterozoic-Early Cambrian, 700-425 Ma. There was a continuum between its post-orogenic processes and the opening of the Rheic Ocean and the concomitant rifting of Avalonia.

In 1921 L. Bertrand named this orogeny after Cadomus, the Latin version of the Gaulish name for Caen in Normandy. He defined it as the late Precambrian orogeny which in the Northern Armorican Massif resulted in the folding and uplift of the Brioverian succession (see below) prior to the deposition of the Early Palaeozoic red bed sequences.

This orogeny occurred as a result of one or more collision between various terranes and the northern margin of Gondwana. These terranes have since drifted from Gondwana and have been accreted to Baltica (the proto European tectonic plate). Therefore, their palaeogeographical position is uncertain. The investigations of this orogeny started with examinations of its remnants in various part of Europe. More recently there also have been studies of this orogeny in locations in North Africa and also the Arabian Peninsula, Jordan, Israel and the Persian Gulf as these areas were also affected by it.

This collection of terranes is collectively referred to as peri-Gondwanan terranes because they were along the northern margin of Gondwana. Those which were accreted to Baltica in the Variscan orogeny (Late Devonian- Carboniferous) when Africa was approaching Laurussia are: Franconia and Thuringia (now in Germany) in the Saxothuringian Zone; Bohemia, which forms the Teplá-Barrandian and Brunovistulian zones of the Bohemian Massif (Czech Republic, northern Austria, eastern Germany, southern and Poland); Armorica (in France); the Ossa-Morena and Central-Iberian zones of the Iberian Massif in the Iberian Peninsula.

The Cadomian orogeny started with a southward oblique subduction beneath the northern margin of Gondwana that began ca. 760 Ma. It involved the development of an Andean-type volcanic arc ca. 650-600 Ma and the formation of a back-arc basin behind this arc which opened ca. 590 to 560 Ma and was closed ca. 545-540 Ma. A short-lived retro-arc basin formed c. 543 Ma. The slab of the subducting plate broke off. Rift basins formed in the Early to Middle Cambrian (c. 530–500 Ma). This continued until the opening of the opening of the Rheic Ocean.

The collision was an island arc-continent type and was a two-phase one due to the subduction being oblique. It first started in the eastern part of Peri-Gondwana at c. 560–570 Ma. There was a continuum between the opening of the rift basins and the opening of the Rheic Ocean. This was a two-stage process, too. It started from the west of Peri-Gondwana and expanded eastward.