The use of stress trajectory analysis in the elucidation of part of the Najd Fault System, Saudi Arabia

The Ad Dawadmi Batholith in central Saudi Arabia is cut by three strike-slip faults which form part of the Najd transcurrent fault system. Two of the three faults end in the area studied; the third, which now extends beyond the area, is shown to have formed in two stages. Two of the faults are associated with second order fractures. One of these fracture systems, together with a dyke swarm, occupying extension fractures and shown to be synkinematic, has been used to construct trajectories of principal compressive stress and maximum shearing stress as they existed during one episode of second order faulting. The sequence of formation of the three master faults, their second order fracture systems and the dyke intrusion has been established. It is shown that the faults, during their formation, propagated into the area from the south-east and one subsequently extended north-westward. Comparison shows that the second order fault pattern conforms closely with a theoretical model of fractures associated with the tip of a master fault under ‘uniaxial compressive stress’ conditions. The construction of stress trajectories permitted the forecasting of potential second order fault orientations and sense of displacement throughout the area. This is economically important because many of the second order faults and associated pinnate extension fractures contain mineral veins.