Evidence for symmetric extension and exhumation of the Yaounde nappe (Pan-African fold belt, Cameroon)

Abstract

It is commonly assumed that the Yaounde nappe was deformed and emplaced onto the Congo craton during two compressional Pan-African deformation phases ( D 1 and D 2). A detailed field and microstructural study of the mica schists, gneisses and pyriclasites of the Yaounde nappe in the Yaounde area was carried out and it appeared, however, that the D 2 deformation phase more likely corresponds to a major N–S and moderate E–W ductile pure shear extension, rather than compression. This was inferred from the study of various strain indicators, including the geometry of shear zones, the shape of boudins, the shape of pressure shadows, the orientation of deformed veins, of fold axes, the mineral lineations, as well as the preferred orientation pattern of quartz c-axes. The P– T conditions pertaining during this extensional phase were variably recorded by the mica schists (675 °C and 9 kbar), gneisses (800 °C and 12 kbar) and pyriclasites (750 °C and 10 kbar). Pure shear extension turned into minor top-to-the-south simple shear at lower temperature during retrogression into the greenschist facies. We therefore suggest that the syn- D 2 extension associated with subsequent uplift and erosion led to (Pan-African) exhumation of the Yaounde nappe. The D 1 phase of deformation is poorly known due to strong D 2 phase overprinting. However, the high P– T conditions indicated by the metasedimentary rocks in the area suggest that D 1 was compressional. The peak conditions of the related prograde metamorphism correspond to D 1– D 2 transition at the amphibolite–granulite interface. We propose that D 1 and D 2 respectively represent the compressional and extensional phases of a Pan-African tectono-metamorphic cycle within the Yaounde nappe.