Damage zone geometry around fault tips

Damage zones are described around small scale normal, strike-slip, and reverse faults cutting horizontally-bedded carbonates, shales and siltstones in the Bristol Channel basin, U.K. Two different types of brittle damage zone have been recognized: (a) fractures branching directly from the fault tip; and (b) fractures forming an en échelon array, which are disconnected from the fault tip. Similar damage zones are repeated at regular intervals along the faults and they are interpreted to represent paleo-tip lines or sticking points along fault planes. Between these zones there is little visible damage outside the fault planes, which are typically 25–1000 μm thick along small displacement (<0.1 m) normal faults. Hence, faults propagated within their own plane at the scale of observation (25 μm), but dilational out-of-plane fractures are preserved at their arrested tips. Strike-slip and thrust faults were observed to produce more variable damage zone geometries compared to those at normal fault tips. Damage zone geometries around lateral tips of normal and strike-slip faults indicate that they can form by a different mechanism than up- and down-dip tip zones. Subsidiary fracture patterns can be used to recognize the direction of fault propagation.