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 sil tstones in the Bristol Channel basin, U.K. Two different types of brit tle damage zone have been recognized: (a) fractures branching directly from the fault tip; and (b) fractures forming an en Echelon array, wh ich are disconnected from the fault tip, Similar damage zones are repe ated at regular intervals along the faults and they are interpreted to represent paleo-tip lines or sticking points along fault planes. Betw een these zones there is little visible damage outside the fault plane s, which are typically 25-1000 mu m thick along small displacement (<0 .1 m) normal faults. Hence, faults propagated within their own plane a t the scale of observation (25 mu m), but dilational out-of-plane frac tures are preserved at their arrested tips. Strike-slip and thrust fau lts were observed to produce more variable damage zone geometries comp ared to those at normal fault tips. Damage zone geometries around late ral tips of normal and strike-slip faults indicate that they can form by a different mechanism than up- and down-dip tip zones. Subsidiary f racture patterns can be used to recognize the direction of fault propa gation.