Summary The Red Sea is one of the youngest ultraslow-spreading ridges on our planet and an ideal place to investigate the transition from continental rifting to oceanic spreading. Within this context, the Zabargad Fracture Zone (ZFZ) stands out as a particularly intriguing region. The ZFZ hosts notable features, including an offset in the Red Sea spreading axis, the Mabahiss Mons submarine volcano, and the Kebrit Deep brine pool. Additionally, this region is seismically active, posing a hazard to nearby coastal communities. Despite previous geophysical studies, few seismic velocity models image shallow subsurface structures in the ZFZ. In this work, we use approximately one year of seismic ambient noise recorded by five broadband stations and 14 ocean-bottom seismometers to estimate the shear-wave velocity structure of the ZFZ. For this, we compute vertical-vertical, radial-radial, and transverse-transverse noise correlations, obtain group- and phase-velocity dispersion curves of Rayleigh and Love waves in the 3 to 12 s period band, and employ transdimensional tomography to estimate 1-D and 3-D isotropic shear-wave velocity models of the ZFZ. Our 1-D velocity model suggests that, on average, the ZFZ crustal structure comprises a 1.5 km thick layer including hemipelagic sediments and evaporites, a 2.8 km thick oceanic basement, and a crust-mantle transition extending from 6.5 to 8 km below sea level. Meanwhile, our 3-D model agrees with previous geological and geophysical observations and reveals new subsurface structures. In particular, it shows low velocity areas to the east and south of Mabahiss Deep that correlate with known sedimentary basins. Moreover, our 3-D model contains a low-velocity area near Kebrit Deep that correlates with a region of low seismic activity and a recently inferred spreading-axis segment. Based on previous evidence of inactive hydrothermal vents, we infer that this low-velocity area indicates higher basement temperatures near Kebrit Deep compared to other areas. Lastly, our 3-D model displays a velocity contrast in the southern ZFZ that correlates with a contrast in free-air gravity anomalies and a gradient in evaporite topography. Based on these observations, we interpret this velocity contrast as a lineament related to folded evaporites. Our findings present new constraints on the crustal structure of the ZFZ and serve as a reference for other young ultraslow-spreading ridges.
Cano et al. (Fri,) studied this question.