Spatial relationships within fault damage zones in sandstone

Orientation analyses of deformation bands in sandstones from the Gullfaks r egion, northern North Sea, show that different sets of bands are a common f eature within damage zones associated with faults with displacement in the order of several tens of metres or more. The different sets may coexist wit hin the same depth intervals, or may occur in cluster zones located at diff erent depths. The strike of the deformation bands is generally subparallel to the main faults, but may deviate from this in complex areas where larger -scale faults with different orientations coalesce. Analyses of smaller-sca le faults (less than a few tens of metres displacement) do not display the same clear "conjugate" sets of deformation bands. A possible explanation is that deformation bands in the early stages of fault development are subpar allel to the main faults. As the displacement along the smaller faults incr eases, the bands may intersect and form a single slip surface with a more c omplex geometry. At this stage, a second set of deformation bands may devel op in order to accommodate the different stresses that exist along and near the fault plane. During slip along a main fault, smaller splay faults may develop in the footwall or hanging wall. The splay faults will be associate d with abundant deformation bands with orientations that may differ from th at of the "conjugate" sets, depending on the orientation of the splay fault s with respect to the main faults. (C) 2000 Elsevier Science Ltd. All right s reserved.