Relay zone geometry and displacement transfer between normal faults recorded in coal-mine plans



P. Huggins, J. Watterson, J. J. Walsh & C. Childs

Abstract - Overlap lengths, separations and throw gradients were measured on 132 relay zones recorded on coal-mine plans. Throws on the relay-bounding fault traces are usually ² 2m and individual structures are recorded on only one seam. Throw gradients associated with relay zones are not always higher than on single faults but asymmetry of throw profiles is diagnostic of relay zones. Bed geometries around larger faults in opencast mines are used to assess the displacement accommodated by shear in the vertical plane normal to the faults and displacement transfer accommodated by shear in the fault-parallel plane. 3-D structure is defined for two relay zones, each recorded on 5 seam-plans. These relay zones are effectively holes through the fault surfaces and overlap occurs between salients or lobes of the parent fault surfaces. Lobes initially terminated at tip-lines but, as the faults grew, gradually re-joined the main fault surfaces along branch-lines. This type of relay zone originates by bifurcation of a single fault surface at a locally retarded tip-line and is an almost inevitable result of a tip-line irregularity.


Journal of Structural Geology 17, 1741-1755, 1995.