Abstract

The Najd Belt is a major transcurrent (strike-slip) fault system of Proterozoic age in the Arabian Shield. The belt is a braided complex of parallel and curved, en echelon faults. Complex arrays of secondary structures including strike-slip, oblique-slip, thrust and normal faults, together with folds and dyke swarms are associated with some major faults, particularly near their terminations. The secondary structures indicate that compressional and extensional/dilational conditions existed synchronously in different parts of the fault zone. The outcrop traces of faults and syn tectonic dykes are used to interpret the configuration of principal compressive stresses during formation of parts of the secondary fracture systems. Second order deformation was a series of separate events in a complex episodic faulting history. Comparison with model studies indicates that master faults extended in length in stages and periodically developed arrays of secondary structures. Propagation of the major faults took place along splay trajectories which inter-connected to form a sub-parallel sheeted and braided zone. Interpretation of the aeromagnetic maps indicates that the Najd Belt is broader at depth than the outcropping fault complex, and that more continuous structures underlie arrays of faults at surface. The fault pattern is mechanically explicable in terms of simple shear between rigid blocks beneath the exposed structures.

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