A three-dimensional Moho depth model beneath the Yemeni highlands and rifted volcanic margins of the Red Sea and Gulf of Aden, Southwest Arabia

Knowing Moho discontinuity undulation is fundamental to understanding mechanisms of lithosphereasthenosphere interaction, extensional tectonism and crustal deformation in volcanic passive margins such as the study area, which is located in the southwestern corner of the Arabian Peninsula bounded by the Red Sea and the Gulf of Aden. In this work, a 3D Moho depth model of the study area is constructed for the first time by inverting gravity data from the Earth Gravitational Model(EGM2008) using the ParkerOldenburg algorithm. This model indicates the shallow zone is situated at depths of 20 km to 24 km beneath coastal plains, whereas the deep zone is located below the plateau at depths of 30 km to 35 km and its deepest part coincides mainly with the Dhamar-Rada ’a Quaternary volcanic field. The results also indicate two channels of hot magmatic materials joining both the Sana’a-Amran Quaternary volcanic field and the Late Miocene Jabal An Nar volcanic area with the Dhamar-Rada’a volcanic field. This conclusion is supported by the widespread geothermal activity(of mantle origin) distributed along these channels,isotopic data, and the upper mantle low velocity zones indicated by earlier studies.

Tidal drag and westward drift of the lithosphere

Tidal forces are generally neglected in the discussion about the mechanisms driving plate tectonics despite a worldwide geodynamic asymmetry also observed at subduction and rift zones.The tidal drag could theoretically explain the westerly shift of the lithosphere relative to the underlying mantle.Notwithstanding,viscosity in the asthenosphere is apparently too high to allow mechanical decoupling produced by tidal forces.Here,we propose a model for global scale geodynamics accompanied by numerical simulations of the tidal interaction of the Earth with the Moon and the Sun.We provide for the first time a theoretical proof that the tidal drag can produce a westerly motion of the lithosphere,also compatible with the slowing of the Earth’s rotational spin.Our results suggest a westerly rotation of the lithosphere with a lower bound ofω≈(0.1-0.2)°/Myr in the presence of a basal effective shear viscosityη≈10^(16)Pa-s,but it may rise toω>1°/Myr with a viscosity ofη≈≤3×10^(14)Pa-s within the Low-Velocity Zone(LVZ)atop the asthenosphere.This faster velocity would be more compatible with the mainstream of plate motion and the global asymmetry at plate boundaries.Based on these computations,we suggest that the super-adiabatic asthenosphere,being vigorously convecting,may further reduce the viscous coupling within the LVZ Therefore,the combination of solid Earth tides,ultra-low viscosity LVZ and asthenospheric polarized small-scale convection may mechanically satisfy the large-scale decoupling of the lithosphere relative to the underlying mantle.Relative plate motions are explained because of lateral viscosity heterogeneities at the base of the lithosphere,which determine variable lithosphere-asthenosphere decoupling and plate interactions,hence plate tectonics.