LINKING THE ACTIVATION OF INHERITED-CRUSTAL WEAKNESS AND CENOZOIC VOLCANIC PLUMPING SYSTEM DURING RIFTING OF THE RED SEA, WESTERN SAUDI ARABIAN MARGIN

Reactivation of basement structures and geometric inheritance has been widely recognised in controlling the location, distribution, and nature of rift-faults and volcanic plumbing systems. Studies have shown that the degree of the obliquity of pre-existing anisotropies is critical to the style of overprinting. The western margin of Saudi Arabia offers an excellent field site to understand the nature of interactions between tectonics and rift-related volcanism. The margin exposes a major Neoproterozoic terrane boundary – the Ad Damm Shear Zone (ADSZ) – and a N-S trend in the strike of foliations and faults, which are oriented obliquely to the NW Cenozoic Red Sea axis. The ADSZ is a NE-striking shear zone that separates the lowlands of the Jeddah terrane in the NW, from the highlands of the Asir terrane in the SE. Recent seismicity and the observed physiographic differences across the ADSZ, suggest that the ADSZ may have been reactivated during Red Sea rift evolution. Our remote sensing and field reconnaissance shows that rift-related faults and intrusions have inherited their geometry from pre-existing anisotropy. Fault-kinematic analyses combined with cross-cutting relationships, reveal three main structural trends of Cenozoic-age faults and dykes: (i) Red Sea-oblique (N-S ±10°) recording E-W extension, (ii) Red Sea-normal (NE-SW) recording NW-SE extension, (iii) Red Sea-parallel (NW-SE) recording NE-SW extension. Our data support non-coaxial strain involving transfer faults (i.e. rift-oblique and rift-normal) that accommodates a combined ENE-WSW transtension tectonic regime, followed by Red Sea-parallel faults associated with regional Red Sea rift extension. Whole-rock geochemistry of Cenozoic sheet intrusion shows a wide range of chemical composition. However, current results indicate an orientation-controlled evolution in magma chemistry: (i) calc-alkaline to alkaline basaltic to rhyolitic Red Sea- transfer dykes, and (ii) tholeiitic Red Sea-parallel dykes. A two-stage extension-reorientation model is supported further according to tectonic-dyke geochemistry associations. Transfer dykes, which were emplaced during transtension, favour the fractional crystallisation and magma mixing processes, causing along-strike variation in the geochemistry, whereas Red Sea- parallel dykes, emplaced during rift-related extension, are significantly less evolved. The observed oblique ENE-WSW extension to the main Red Sea rift axis in the ADSZ suggests ancient structures primarily controlled the rift evolution.