ABSTRACT Relay ramps occur between normal fault seg- ments 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 inter- preted as being evolutionary stages. In stage 1, the segments do not interact. Stage 2 involves the reori- entation of bedding between two interacting faults to produce a relay ramp. In stage 3, connecting frac- tures 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–dis- tance profiles for fault segments and linked faults accompanies the interaction and linkage processes. Displacement transfer by relay ramps is accompa- nied by steep displacement gradients along fault seg- ments at oversteps. Relay ramps often contribute to a minimum in total fault displacement at a linkage point. INTRODUCTION Normal fault zones play a major role in the devel- opment of basins and in the migration and trapping of hydrocarbons. The mapping of normal fault sys- tems using seismic data requires careful correlation of faults on adjacent sections, a procedure that often leads to the interpretation of faults as having long, continuous, sinuous traces. Much recent work involving detailed mapping of fault traces (e.g., Zhang et al., 1991), first by using land exposures but more recently using three-dimensional seismics, has demonstrated that faults are usually made up of many overstepping segments, linked by areas of complex deformation, termed transfer zones or relay ramps. These areas are difficult to interpret from limited subsurface data, yet are often sites of hydrocarbon traps (Morley et al., 1990; Nelson et al., 1992) and their evolution is important to the under- standing of the formation of many oil and gas fields. This paper concentrates on small-scale fault zones (with displacements of less than 0.5 m) in Lower Jurassic sedimentary rocks on the Somerset coast in southwestern England. Small-scale examples have been analyzed because they are well exposed and easy to map and their geometries may provide useful guidance in the interpretation of larger fault zones. The small-scale examples are compared with larger fault zones, including those from West Chevington opencast coal mine, northeastern England, and from oil and gas fields in the North Sea. RELAY RAMPS Normal fault zones are generally composed of arrays of overstepping (or en echelon) and linked seg- ments, with transfer zones transferring displacement between fault segments that overstep in map view (e.g., Morley et al., 1990). Transfer zones can develop between overstepping normal fault segments that have opposite dip directions (the conjugate transfer zones of Figure 1 of Morley et al., 1990), but small- scale examples have not been observed on the Somer- set coast, which is the main field area described in this 147 ©Copyright 1994. The American Association of Petroleum Geologists. All rights reserved. 1 Manuscript received, March 22, 1993; revised manuscript received, August 12, 1993; final acceptance, October 25, 1993. 2 Department of Geology, University of Southampton, Southampton, SO9 5NH, United Kingdom. Current address: Department of Geological Sciences, University of Plymouth, Drake Circus, Plymouth, Devon PL4 8AA, United Kingdom. 3 Department of Geology, University of Southampton, Southampton, SO9 5NH, United Kingdom. This work was funded by Amoco. The reviews by Kevin Biddle, William Brown, David Klepacki, and John Lorenz greatly improved this paper. Tom Patton and Randall Marrett are thanked for their help, and Rob Gawthorpe is thanked for providing a preprint of his paper. Permission by British Coal to use the West Chevington mine plans is appreciated. Geometry and Development of Relay Ramps in Normal Fault Systems 1 D. C. P. Peacock 2 and D. J. Sanderson 3 AAPG Bulletin, V. 78, No. 2 (February 1994), P. 147–165.