Strain partitioning along the Neoproterozoic Central Africa Shear Zone system: structures and magnetic fabrics (AMS) from the Meiganga area, Cameroon

Field and microstructural work supported by anisotropy of magnetic susceptibility (AMS, magnetic fabric) studies were applied to identify the sequence and character of Pan-African structures in gneissic rocks and late-tectonic granitoids in a part of the Central Africa Shear Zone (CASZ) system (Meiganga area, central Cameroon). The area is bounded by two steeply dipping, NE-SW trending Pan-African shear zones, the Mbere-Djerem (MDSZ, in the NW) and the Meiganga (MSZ, in the SE) shear zones, respectively, enclosing a central area, composed mostly of high-grade gneisses and orthogneisses, including reworked Palaeoproterozoic gneisses in the southwestern part (domain I), and mainly late-tectonic granitoid intrusions of variable size in the northeastern part (domain II). In the crystalline basement rocks of domain I, the early planar fabric (S1) was folded during D2 (folding and foliation) and D3 (shearing, wrenching, folding) deformational events. Oblate AMS (strain) ellipsoids characterize the dominant orthogneisses. Magnetic foliations curve from NE-SW to E-W with variable dips. Magnetic lineations trend mostly NE-SW with shallow to medium plunges. This fabric defines both small-scale and km-scale folds with WSW plunging axes sub-parallel with the marginal shear zones. Around the MSZ, AMS ellipsoids are typically prolate, whereas both prolate and oblate shapes are developed around the MDSZ. Magnetic foliations trend ENE-WSW with mostly steep dips. Magnetic lineations trend also ENE-WSW with shallow plunges. This fabric, with dominantly steep foliations and subhorizontal lineations implies that the MSZ and the MDSZ are Pan-African strike-slip shear zones with subordinate components of compression. The spatial separation of coeval kinematic domains of prolate and oblate shape dominated fabrics is interpreted as the product of partitioning between coeval simple shear-dominated strain in the MDSZ and MSZ, and pure shear-dominated strain in domain I. In granites of domain II, variably oriented magnetic foliations and subhorizontal lineations are interpreted as indicating emplacement during strike-slip deformation at the end of Pan-African orogeny. At a regional scale, the present results imply that central and eastern Cameroon with the CASZ, which represent the internal domains of the Pan-African orogen in central Africa, are dominated by high angle faults and partitioned compression and strike-slip tectonics. These structures are also comparable with those of NE Brazil, along strike prior to Atlantic opening.