Definition of tectonostratigraphic terranes in the Mauritanide, Bassaride, and Rokelide orogens, West Africa

Abstract

The West African orogens (Mauritanides, Bassarides, Rokelides) record a locally complex, polyphase, tectonothermal evolution. The earliest event corresponded to westward rifting of a continental fragment from the West African craton at ca. 700 Ma. This led to development of a rift-facies lithotectonic succession that included sedimentary units and intracontinental igneous sequences. Rifting must have been of limited extent because a western ensialic arc began to develop as early as 680 Ma. The associated convergence culminated in an episode of folding and metamorphism at ca. 650 Ma (Pan-African I orogenesis) in the Mauritanides and Bassarides.Following widespread deposition of Late Proterozoic glacial and flyschoid sediments, a second tectonothermal event (most penetratively developed in the Rokelides) occurred between ca. 575 and 550 Ma (Pan-African II orogenesis). This may have resulted, in part, from collision of the Guyana craton (South America) and southern portions of the West African craton.The West African orogens were marked by general tectonic quiescence throughout most of early and middle Paleozoic time. Late Paleozoic (ca. 300 Ma) collision of Gondwanaland and Laurentia resulted in eastward translation of previously tectonized Mauritanide units over their foreland, and emplacement at highest structural levels of previously imbricated nappes, which include sequences with poorly defined palinspastic origins. The late Paleozoic (Hercynian) transport was largely intracontinental, and only westernmost portions of the exposed Mauritanides record a penetrative tectonothermal overprint.The polyphase tectonothermal evolution of the West African orogens complicates definition of tectonostratigraphic terranes. Three preliminary and likely composite terranes are outlined. These include: (1) sequences related to the initial Late Proterozoic rifting; (2) ensialic arc sequences and their continental hosts (with uncertain affinity to the West African craton); and, (3) westernmost, variably retrogressed and complexly imbricated continental units with poorly defined palinspastic affinities.131 132

The relations between Gondwana and the adjacent peripheral Cadomian domain—constrains on the origin, history, and paleogeography of the peripheral domain

“…Based on Ar/Ar dating, Lécorché et al (1989) and Villeneuve et al (2010) concluded that a 575-550 Ma or somewhat younger tectonothermal event occurred all along the west margin of the WAC, best expressed south of ca. 10°S (Rockelide orogen).…”

“…lat. 13°N it was reworked by the Variscan Mauritanide orogeny (Lécorché et al, 1989;Lytwyn et al, 2006;Villeneuve, 2008;Caby and Kienast, 2009;Villeneuve et al, 2010). The eastern units, thrust over the WAC, consist of low grade metasediments (with a Variscan overprint) that resemble the Neoproterozoic cover of the WAC, but with intercalated volcanics.…”

The nature and location of the suture zone in the Rokelide orogen, Sierra Leone: Geochemical evidence

“…In Sierra Leone, a suture may outcrop somewhere between the western boundary of the Rokel River Group and eastern boundary of the Kasila Group (Burke and Dewey, 1972;Burke et al, 1977) in a region partially bordered by mylonites and westerly-dipping thrust faults (Williams and Culver, 1982). Lécorché et al (1989) placed the suture along ductile shear zones between the Kasila Group and ''Kenema Assemblage''. The suture is cryptic south of the Rokel River Group outcrop where it separates the Kasila gneisses from the West African craton (Burke and Dewey, 1972).…”

“…Earlier collision between the Kasila Group and this microcontinent/arc would then be followed by later Pan African collision between the Kasila-arc collage and West African Craton. , Ponsard et al (1988) and Lécorché et al (1989) devised similar tectonic models for evolution of the Mauritanides. According to their models, westward rifting of a continental fragment (Western Block Craton) from West Africa occurred around 700 Ma and was accompanied by emplacement of alkaline to tholeiitic basalts.…”

“…If the Kasila Terrane was a piece of the western West African Craton, it may have rifted away from the latter as part of the Western Block Craton at 700 Ma. Continental rifting was followed by opening of a small intervening ocean basin (Ponsard et al, 1988;Lécorché et al, 1989). The Terméssé and Guingan volcanic-sedimentary sequences were deposited along the margins of the separating West African and Western Block cratons, respectively (Ponsard et al, 1988).…”