Abstract

Polygonal networks of normal faults in layer-bound sequences of fine-grained mudstones have formed without regional tectonic extension. The two leading hypotheses concerning the generic mechanism responsible for their development are: (1) horizontal stresses are reduced by syneresis; (2) coefficients of residual friction are very low. To discriminate between these hypotheses, the ratios between the minimum horizontal and the vertical effective stresses have been estimated in four North Sea wells penetrating Oligocene and Miocene sequences that contain polygonal fault networks. The effective stress ratios are c. 0.8, consistent with very low coefficients of friction but not with syneresis.