The Cadomian (∼550 ma) orogen in North Africa and Arabia

Abstract

The Cadomian orogeny reflects S-directed subduction on the northern margin of Greater Gondwana in Ediacaran – Cambrian time (∼600-500 Ma). It is well known from the eastern seaboard of North America, western and southern Europe, Turkey and Iran but is rarely identified in what was its backarc in N. Africa and Arabia. Here I summarize evidence for the Cadomian across ∼6500 km of Arabia and N. Africa from Oman to Morocco. The beginning of the Cadomian in Afro-Arabia cuts across and overlaps in time with earlier Neoproterozoic orogenies leading to the formation of Greater Gondwana, so emphasis is on rocks <570 Ma. Three Cadomian backarc crustal tracts or segments are identified. The eastern segment (Oman, Persian Gulf, eastern Saudi Arabia) mostly consists of buried transtensional basins with little igneous activity. The central segment was characterized by lower crust magma underplating and strong uplift along a N–S axis (Jordan) accompanied by rifting leading to deposition of thick sediments reminiscent of a passive continental margin, and strong evidence for regional metasomatism (Israel). The Zenifim passive margin is 580–610 Ma, older than our arbitrary 570 Ma limit for identifying the Cadomian but clearly post-dates terminal continental collision to form the East African Orogen and so must be related to Cadomian back-arc rifting. Along with related NE-SW trending dike swarms in NE Egypt and Jordan, the orientation of the Zenifim rifted margin, orthogonal to the Cadomian convergent margin, is another indication of strong Cadomian transtension. In contrast, the western segment (Libya, Algeria, Morocco) experienced abundant igneous activity, with evidence for this found especially far south in Libya. Recognition of the Cadomian orogeny in northern Africa provides a useful first step in breaking out distinct tectonic episodes in the ∼300 m.y. long Pan-African orogeny, at least in the north. Finally, the orthogonal relationship between older N–S orogens in the interior of Greater Gondwana and the E-W Cadomian orogen illustrates how the Neoproterozoic plate mosaic grew by subduction initiation exploiting lithospheric weaknesses along its continental margin. Ten recommendations for future research are also suggested.