Fault relays, bends and branch-lines

Branch-lines between normal faults and their sub-parallel splays mapped fro m 3D seismic reflection data show a range of forms from straight lines to c losed loops. The different geometries are interpreted as representing stage s in the failure of relay zones and in the progressive replacement of fault tip-lines with fault branch-lines. The geometries of these normal fault br anchlines are similar to those for thrusts previously inferred from limited two-dimensional data. The orientation of the axis of a relay and its assoc iated bends relative to a fault slip direction is identified as an importan t control on the structures developed within the relay. Neutral restraining and releasing bends call each occur on any fault type (normal, reverse and strike-slip), but data bias is a major factor in determining which bend ge ometry is most often observed with each fault type. On normal faults the initial relay zone geometry controls the dominant bran ch-line orientation and the same control is likely on branch-lines associat ed with the other modes of faulting. A review of the relay geometries and s trains occurring with all three modes of faulting highlights the role of th e orientation of the mechanical anisotropy of a bedded sequence relative to the orientations of fault surface and slip directions. This relative orien tation determines how the relay strain is accommodated and hence the degree of hard-linkage and development of branch-lines. (C) 1999 Elsevier Science Ltd. All rights reserved.