Geometry and Development of Relay Ramps in Normal Fault Systems

ABSTRACT Relay ramps occur between normal fault segments that overstep in map view. The geometry and evolution of exposure-scale relay ramps are described from the Somerset coast, England, and are compared with larger scale ramps from elsewhere. Relay ramps can be classified into four groups based on the degree of interaction and linkage between the overstepping segments; these groups are interpreted as being evolutionary stages. In stage 1, the segments do not interact. Stage 2 involves the reorientation of bedding between two interacting faults to produce a relay ramp. In stage 3, connecting fractures start to break the relay ramp. Stage 4 is when the relay ramp is destroyed to produce a single fault that has an along-strike bend. These evolutionary stages can develop through time, but they can also be seen spatially. A branch line between normal faults or an along-strike bend may represent a stage 4 relay, with progressively earlier stages occurring updip or downdip. Characteristic variability in displacement-distance profiles for fault segments and linked faults accompanies the interaction and linkage processes. Displacement transfer by relay ramps is accompanied by steep displacement gradients along fault segments at oversteps. Relay ramps often contribute to a minimum in total fault displacement at a linkage point.