Andersonian theory for infinitesimal strains predicts that normal faults will form at high angles to the surface in extensional terranes. In contrast, low-angle normal faults are sometimes observed on which tens of kilometres of slip has occurred, and moderate-angle faults with slip of 5 to 15 km are common. A simple extension of the theory to account for the stresses required to drive finite deformation shows that low angles are preferred for large extension on individual faults. The formation of low-angle faults may require inhomogeneities in stress or material properties, but once slip begins on a low-angle fault it is likely to continue, whereas slip on high-angle faults is likely to end after modest slip, to be replaced by the formation of new high-angle faults.

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