Interplay between bed-parallel slip and fault linkage in breached relay ramps: a case study in NW Red Sea margin, Egypt

Abstract

The Umm Karabah Breached Relay Ramp (UKBRR), developed within the El Nakheil Master Fault system along the northwestern Red Sea margin of Egypt, provides an exceptional natural laboratory for investigating the internal deformation and evolution of breached relay ramps. This study integrates detailed structural mapping, fault-slip analysis, fracture-intensity measurements, and paleostress inversion to characterize the geometry, kinematics, and deformation mechanisms responsible for the development of the UKBRR. The relay zone contains a complex network of synthetic and antithetic normal faults that accommodated displacement transfer between overlapping segments of the El Nakheil Master Fault. Bed-parallel slip (BPS) is widespread and occurs at multiple stratigraphic levels, with individual slip surfaces accommodating displacements from approximately 10 cm to several meters. Cross-cutting relationships demonstrate that BPS developed at different stages relative to normal faulting, indicating repeated reactivation and progressive partitioning of deformation within the relay zone. Fracture-intensity measurements further demonstrate a strong influence of mechanical stratigraphy on deformation localization, with competent chert and dolomitic limestone layers recording fracture frequencies of up to 90 fractures per meter, whereas weaker marl and chalky limestone intervals preferentially accommodated deformation through folding and distributed shearing. Fault-slip analysis indicates predominantly dip-slip normal faulting with localized oblique-slip components, reflecting kinematic heterogeneity within the relay zone. The structural observations support a three-stage evolutionary model involving (1) reactivation of inherited Precambrian basement structures during the initiation of Red Sea extension, (2) propagation and overlap of normal-fault segments, and (3) progressive fault linkage and breaching accompanied by repeated BPS activity and strain redistribution. The results demonstrate that the evolution of breached relay ramps in mechanically layered carbonate successions is controlled by the interaction of fault segmentation, structural inheritance, mechanical stratigraphy, and bed-parallel deformation. These findings provide new insights into the mechanisms governing fault linkage, deformation partitioning, and structural connectivity in rift-related carbonate systems.