The President (Professor P. Allen) asked Mr. Moore if he had been able to determine the form of the master faults in the vertical dimension. Could they have extended further (e.g. at a higher level) than seen at the present depth of erosion? If this were so, might it provide an alternative explanation of the structures now seen at the termination of the master faults?

In reply, the Author stated that: The subdued topography of the outcropping Najd Fault System makes direct observation of fault geometry in the vertical dimension very difficult. Interpretation of aeromagnetic maps indicates that the outcropping faults are underlain by more continuous structures up to several hundreds of km in length. Extensional structures are very common at high structural levels but give way downward to shear fractures. Vertical geometry around the terminations of major faults is complex, and many arrays of secondary faults are not vertical and indicate that principal axes in the secondary stress field were commonly not oriented orthogonally with respect to the master fault or the present land surface.

It appears from combined field and geophysical evidence that outcropping major faults merge downward into more continuous, perhaps partially ductile, shear zones. Increasing complexity of secondary structure associated with fault terminations appear to be related to decreasing depth of formation (reduction of lithostatic pressure and increasing importance of pore fluid pressure).

Dr M. K. Wells congratulated the Author on the presentation of his paper. For the purpose of analysing the fault patterns, emphasis had

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