The young continental margin of the western Arabian Peninsula is uplifted 3.5 to 4 km and is well exposed. Rift-related extensional deformation is confined to a zone 150 km wide inland of the present coastline at 17 to 18° N and its intensity increases gradually from east to west. Extension is negligible near the crest of the Arabian escarpment, but it reaches a value of 8 to 10 percent in the western Asir, a highly dissected mountainous region west of the escarpment. There is an abrupt increase in extensional deformation in the foothills and pediment west of the Asir (about 40 km inland of the shoreline) where rocks in the upper plate of a system of low-angle normal faults with west dips are extended by 60 to 110 percent. The faults were active 23 to 29 Ma and the uplift occurred after 25 Ma. Tertiary mafic dike swarms and plutons of gabbro and granophyre 20 to 23 m.y. old are concentrated in the foothills and pediment as well. The chemistry of the dikes suggests 1) fractionation at 10 to 20 kbar, 2) a rapid rise through the upper mantle and lower crust, and 3) differentiation and cooling at 1 Atm to 5 kbar. Structural relations between dikes, faults and dipping beds indicate that the mechanical extension and intrusional expansion were partly coeval, but that most of the extension preceded the expansion.
A tectonic reconstruction of pre-Red Sea Afro/Arabia suggests that the early rift was narrow with intense extension confined to an axial belt 20 to 40 km wide. Steep Moho slopes probably developed during rift formation as indicated by published gravity data, two published seismic interpretations and the surface geology.
