Microstructural and Kinematic Characterization of Brittle-Ductile Flow in Late Pan-African Shear Zones: Implications for the Ediacaran Assembly of Gondwanaland

Abstract:

Inquiries regarding the nucleation and assembly of the present-day landmass of Africa are central issues in the recent geological research. East African orogen is characterized by a network of shear zones that initiated in response to deformation processes accompanying the amalgamation of shields and cratons of Africa. Atalla Shear Zone (ASZ) is a conspicuous, NW-SE crustal-scale deformational belt in the Central Eastern Desert of Egypt that corresponds to East and West Gondwanaland’s collisional stage. The shear zone is defined by a 6-8 Km wide, highly deformed belt of mylonitic metavolcanics and metasediments. A non-coaxial sinistral shear is indicated by the widely common kinematic indicators. Crystal-plastic deformation and dynamic recrystallization (bulging and sub-grain rotation) are the dominant deformation mechanisms in quartz porphyroclasts, while feldspars act as rigid clasts with brittle deformation features. These microstructures reflect a frictional-viscous rheological behavior and indicate a deformation near the brittle-ductile transition for feldspar minerals. The observed fabrics are compatible with a vertical vorticity axis with an anticlockwise rotational sense. The mean vorticity number (Wm) values range from 0.5 to 0.83, which implies general shear transpression with a nearly equal contribution of pure and simple shear. Estimates from 3D strain analysis reveal a prolate strain ellipsoid with a mean K value of about 1.5, and major axis oriented NW-SE. Integrating the results allowed the establishment of a new quantitative model that numerically depicts the shear zone kinematics. The model predicts about 50% shortening normal to the shear zone boundary, and shear strain values between 1.3- 1.9.

The value of our findings resides on three main contributions: (1­) creation of a new model of ASZ that characterize quantitatively the kinematics and behavior of brittle-ductile flow in transpressional shear zones associated with Africa’s assembly; (2) the results introduced supportive evidence for the oblique convergence model between East and West Gondwanaland and enabled the constraining of the paleo-convergence angle to about 45°, with convergence vector trending nearly E-W; and (3) the model sheds light on the kinematics of strain partitioning coeval with the Pan-African orogeny.