Abstract - The geometry, formation and breaching of relay zones on segmented thrusts have been studied using faults with maximum displacements of 0.64-8m which outcrop in well bedded sequences. Two and near-three dimensional data permit characterisation of thrust geometries and displacements at contractional, extensional and neutral relays. Thrusts at contractional relays are separated by asymmetric folds, with steep to moderate bed dips (ca 30-75º) between fault segments and shallow bed dips outside the relay. Folds verge in the direction of relative hangingwall motion, with the steeper limb of the fold often defining a parallelogram-shaped zone between thrusts. Displacement-distance diagrams show rapid decreases in fault displacements approaching relays which are mirrored by an increase in fold amplitude; beds with the greatest amount of folding record the lowest fault displacements. High displacement gradients are mainly accommodated by bed rotations within the relay which allow aggregate displacement to be maintained across the segment boundary and reflect the transfer of fault slip to folding.
Extensional and neutral relays differ from contractional examples in that bed rotations are lower, while in the former case they are also more distributed along the faults. Fault segments form after minimal displacement and folds grow between segments as displacement accrues on the thrusts. We propose that thrust tips remain fixed relative to stratigraphy and therefore moved relative to each other parallel to the direction of slip with increasing finite displacement and bed rotations. As is the case with strike-slip and normal faults, thrust relays become breached with increasing fault displacement. Contractional relays remain intact at higher displacements than extensional relays, while increases in the bedding-thrust angle and fault separation also promote relay longevity.
Abstract of talk given to:
Tectonic Studies Group Annual Meeting, Tectonic Studies Group, University of Manchester, January 2000