Analysis of mesoscopic fractures in the Dhruma-Nisah segment of the central Arabian graben system

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Abstract
The central Arabian graben and trough system is a 560 km arcuate intraplate fault zone cutting gently tilted Mesozoic-Cenozoic sedimentary rocks in eastern Saudi Arabia. The mesoscopic fracture pattern in the 150 km Dhruma-Nisah segment provides an excellent example of extensile brittle deformation, and is also of interest because palaeostress trajectories inferred from it suggest that the segment evolved as a result of early Cenozoic stretching of the Mesozoic cover above a major fault zone in the crystalline basement. The ten sets of vertical or high-angle joints, veins, and small faults are arranged symmetrically about the en echelon graben. During the development of the five sets comprising System 1 there was elongation perpendicular to graben and the formation of a single set of extension fractures, and two subsystems of small dihedral angle sets in the shear-extension fracture transition. System 2 resulted in elongation parallel to graben, and consists of a single set of extension fractures plus a pair of small dihedral angle sets. Conjugate shear sets enclosing a large dihedral angle form System 3. All sets developed during several episodes of fracturing. The range of conjugate angles is attributed to variations in rock strength, differential stress and fluid pressure.

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