Strains at the intersections of synchronous conjugate normal faults

Processes accommodating the mutual offsetting of intersecting conjugate faults are investigated by outcrop examination of three pairs of conjugate normal faults with maximum throws from 5 min to 10 cm. In two cases resolved fault throws decrease sharply towards the intersection zones which, on the scale of observation, are zones of ductile thinning of inter-fault layers that accommodate the throw reductions. Strain within the intersection zone of conjugate faults in fluvio-glacial sands is accommodated by inter- rain slip. and layer thinning is complemented by layer-parallel extension at constant volume. In a second example, strain within the intersection zone of conjugate faults in inter-layered chalk and marl is accommodated by volume reduction, without layer extension. In a third example, also in fluvio-glacial sands, strain in the intersection zone is accommodated by small faults (throws greater than or equal to 1 mm): when throws on all resolvable faults are aggregated, there is no decrease in throw towards the conjugate intersection. The presence or absence of a decrease in resolved throw towards conjugate intersections and of a complementary ductile zone accommodating the change in throw. is a function not only of the resolution limit of the observations but also of the scale of the deformation process within the conjugate zone. (C) 1998 Elsevier Science Ltd. All rights reserved.