Abstract - Faults propagate as arrays of kinematically related segments. Displacement is transferred between adjacent fault segments via zones of high strain referred to here as relay zones. The character of relay zones is well documented from outcrop studies of, for example, relay ramps between normal faults, and extensional and contractional oversteps between strike-slip faults. However, outcrop studies are generally restricted to a single plane of inspection so that the 3D structure of relay zones cannot be determined. High quality seismic reflection data provide views of map view relay zones on normal faults, but only very rarely views of cross-sectional relay zones. Here we describe the 3D geometries of relay zones and failed, or breached, relay zones recorded from serial sections of small (mm-cm displacement) normal faults offsetting soft-sediments.
The 3D geometry of small faults was mapped from photographs of serial sections (spaced ~1cm) carved through small-scale faults in soft, finely laminated sediments of Miocene age along Danish coastal outcrops. The fault pattern is complicated by complex cross-cutting relationships and interactions between faults but here we concentrate on relatively simple structures related to fault segmentation and relay zone growth. The geometries and displacement distributions of the structures analysed can be related to the different stages of relay zone growth, and examples of each of the following have been mapped i) fault surface bifurcation during propagation, ii) pairs of overlapping fault lobes with intervening relay zones, (iii) relay zone breaching and hard linkage of fault segments and iv) full linkage between overlapping segments preserved as fault bonded lenses. These stages in geometrical evolution have been observed both for extensional and contractional relay zones seen in cross-section, and for map view relay zones. While largely consistent with existing models of relay zone evolution, the observed 3D structures suggest some refinements to these models. For example, cross-sectional relay zone geometries give way along strike to breached relay zone geometries demonstrating that relay zones may be both intact and breached at different points along their length. In such structures relay breaching occurs by propagation of the parent fault through the relay zone from the point of bifurcation, rather than by the development of a new linking fault.
Abstract of talk given to:
Tectonic Studies Group Annual Meeting , Manchester, January 2006.