Branching shear zones in the Arabian-Nubian Shield
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Abstract
Ductile strike-slip shear zones show a wide variation in their characteristics (length, width and strain-rate). These structures can form large branching or anastomosing shear zone networks and play an essential role in the geodynamic processes of continental collision zones. This thesis reports the nucleation and interconnection of strike-slip shear zones with associated kinematic adjustments. The research is based on detailed structural field mapping on two interacting strike-slip shear zones in contact with a metamorphic core complex, and on paired nucleated shear zones in the Arabian-Nubian Shield. To analyse the rheological effect of the material on the evolution of the shear zone networks and to gain insight into the kinematics during the interaction of the shear strands, two-dimensional numerical models were developed.
Microstructural analysis was performed on thin-sections of the shear zones and the core complex of the Qazaz Core Complex in Saudi Arabia to reconstruct the metamorphic condition of the deformation. The analysis of the structural data together with mineral exchange geothermobarometers provides comprehension of the mechanics of two interacting strike-slip shear zones linked to a detachment structure. The simultaneously operating vertical and lateral shear components is an efficient way to exhume a lower crust, accommodated by a crustal shortening. This structure may be an explanation for isolated core complexes along crustal-scale strike-slip fault systems.
Numerical two-dimensional experiments were performed by MILAMIN_VEP with a visco-elastic-plastic code to reproduce complex anastomosing or branched shear zone networks, improving our understanding of the rheology of the material during shear zone evolution. The simulations represent a large-scale model on a geological time-scale with constant strain-rate boundary conditions, initiating shear localization under Mohr Coulomb plasticity or power-law rheology. Systematic changes to the material’s rheological parameters show that the progression of strain-softening during deformation has an important effect on the geometries of shear zone networks. The context of the strain-softening and its consequences to the development of the shear zones is presented via numerical models. Furthermore, the interaction of anti- and synthetic shear faults in brittle as well as ductile regimes and the simultaneous activity of the shear strands led to a more complex internal kinematic pattern. The more complex shear zone geometry caused by the interconnection of the shear strands locally changed the orientation of the maximum compressive stress (σ1). This is shown by the varying orientation of the new localized shear zones towards the bulk stress. In addition, the numerical experiments indicate that shear zones nucleate along a heterogeneous contrast of the material, induced in the simulations by a difference in the rheology of the material.
To expand the current knowledge of possible types of nucleation of shear zones, parallel propagated shear zones in Jordan were studied by structural and microstructural analysis (Fabric Analyser). The investigation shows an association of the development of paired shear zones with a lithological effect of the material by reactivation and overprinting. Differences in grain-size lead to a strict limitation of the deformed zone by high strain, exhibiting a sharp boundary with the undeformed surrounding rocks at the millimeter-scale. This leads to the conclusion that the nucleation and interaction of shear strands can form complex geodynamic structures in the crust, which lead locally to a change in the kinematic pattern. The similarity of the results of numerical experiments with natural examples adds support to the proposed explanations discussed in this thesis for variations in the orientation of shear zones in networks, and the evolution of the kinematics within the shear zone.