Early development of a transform fault in a young ocean basin: insights from the Zabargad Fracture Zone, Northern Red Sea

The Red Sea hosts a young (< 13 Ma) ultra-slow spreading ridge organized into right-stepping segments. The largest ridge offset, about 100 km in the N–S direction, occurs at the transition between the northern and central Red Sea and is known as the Zabargad Fracture Zone (ZFZ). However, its precise geometry and tectonic structure remain poorly determined owing to widespread Miocene evaporites that obscure basement structures. This limited knowledge prevents addressing first-order questions such as: What can we learn about the early development of large ridge offsets from the structure of the ZFZ and its relationships with inherited continental fabrics? What is the seismic hazard posed by this structure for coastal communities, and how does it relate to a reported Mw ~6.5 historical earthquake? To address these questions, we acquired new high-resolution bathymetric data of the ZFZ seafloor, performed detailed mapping of a range of different seafloor structures, and analyzed the outcome in combination with existing geophysical and geological studies. Our results indicate that the ZFZ is composed of one 50-km-long and seismically active transform fault along with smaller non-transform offsets that gradually connect to the Mabahiss Deep spreading center in the northern Red Sea. Moreover, the transform fault runs in continuity with a seismically inactive shear zone that is marked by highly deformed seafloor and extends towards the Saudi coastline before apparently connecting with an inherited Proterozoic shear zone onshore. We propose that this inactive shear zone acted as a transfer zone during the continental rifting phase of the Red Sea. We further reconstructed the early development of the ZFZ by sequentially restoring the oceanic basin to specific time periods. This reconstruction suggests that the initiation of the transform fault was delayed by a few million years relative to the onset of oceanic spreading along nearby ridge segments, an interval during which plate motion was accommodated through an evolving ridge-offset geometry initiated from the transfer zone.