Cenozoic Faulting in the Jizan Area, Southwest Saudi Arabia
M. John Roobol and Khalid Kadi
A 200-km-long section from the Yemen border, past Jizan, to Ad Darb along the foothills of the Red Sea coastal plain has been remapped on a scale of 1:50,000 with the objective of identifying Cenozoic faults for consideration in seismic hazard assessment. New tools previously unavailable in this area are the aeromagnetic map of Saudi Arabia and Google Earth images. These have led to the recognition of a 35-km-wide zone of steep normal faulting where the aeromagnetic signatures of many thick gabbro/diorite dikes are present showing mainly normal polarity but some with reverse. A similar fault zone was recognized in the Rabigh area north of Jeddah and named the Foothills Fault System. The fault zone appears to run for the entire length of the Red Sea coast of Saudi Arabia. The fault zone is believed to record the first thinning of the Arabian-Nubian shield before the opening of the Red Sea. The Saudi Arabian National Center for Earthquakes and Volcanoes has produced a wealth of new seismic data, which shows a concentration of activity along the margins of the Red Sea. In the Jizan area, epicenters recorded since 2007 to 2014 correspond to some of the Cenozoic faults identified in this report. The presence of three hot springs in the Jizan area situated on some of the Red Sea Foothills Faults suggests some Quaternary reactivation. The newly identified section of the Foothills Fault System with its steeply dipping faults with aeromagnetic signatures of unexposed mafic dikes contained within some of them is incompatible with previous models. The latter require east-dipping low-angle normal faults overlying a major east-dipping detachment fault. The latter were proposed by previous workers because of the presence of west-dipping stratification in faulted blocks of Wajid Sandstone present in the foothills and lying some 2,000 meters below their counterparts above the Red Sea escarpment. The Wajid Sandstone blocks were previously mapped in detail and sit on low-angle slip surfaces. The Wajid Sandstone occurrences are reinterpreted here as mega-blocks in a mega-breccia sitting on paleosols. The mega-blocks appear to have slipped down into a depression due to crustal extension before the opening of the Red Sea. This interpretation does not require east-dipping normal faults overlying a detachment fault. On Jabal Makan, a block of 50° west-dipping Wajid Sandstone overlies a low-angle slip surface on top of Precambrian basement cut by thin Tertiary mafic dikes that do not extend into the sandstone above. Drag of the dikes beneath the sandstone is towards the east suggesting that some of the blocks slipped down from the African shoulder of a pre-rift extensional depression. The coastal plain adjacent to the foothills is occupied by a belt of bimodal alkaline volcanic rocks (the Jizan group) with interstratified fresh water fossils and intruded by sheeted dikes, gabbros and granophyres which together comprise the Tihamat-Asir Complex. The Jizan group belt is 7 km wide at the Yemen border and narrows northward for 700 km almost to the outskirts of Jeddah. These rocks are interpreted as flooring a proto-Red Sea rift valley similar to those in present-day East Africa with freshwater lakes. This first opening of the Red Sea rift was later propagated northwards. Seaward of these belts, the new aeromagnetic map reveals what are probably strips of oceanic crust, some of which partly underlie the Tertiary gravel fans and the Quaternary loess deposits of the Red Sea coast plain. The Red Sea opened in two stages. During the first stage of opening from 27 to 14 Ma, it opened passively with spreading away from the central axis. By 14 Ma the sea had opened between 65 and 85 km (including the Gulf of Suez). At 14 Ma the system changed with the initiation of the Gulf of Aqaba-Levant transform fault system and the Owen's fault system in the Gulf of Aden. The African and Arabian plates then moved apart due to sinistral displacement along the Gulf of Aqaba-Levant fault system. The Gulf of Suez was bypassed. During the opening of the Red Sea, transform faults formed and offset the magnetic stripes of the Red Sea floor. Some of the transforms can be traced onto the coastal areas of Saudi Arabia in the Jizan area where they cross the coastal plain to die out in the Red Sea foothills. Their signatures are well seen on the aeromagnetic map as they also contain mafic dikes. They can also be detected by offsets of the contact between the Tihamat-Asir Complex and the Precambrian shield. There is also a scatter of Tertiary scoria cones and basalt lavas on the coastal plain. The western side of Yemen and a small area of Saudi Arabia adjacent to the border are characterized by many Cenozoic faults, numerous hot springs and frequent seismic activity related to the present episode of opening of the Red Sea. In Yemen there is a strong correlation between the distribution of the hot springs and the positions of historic earthquake epicenters. The hot springs of the Jizan area are thought to identify Foothills Faults (that formed before the opening of the Red Sea) and have been reactivated recently. The present-day seismic data now being collected together with mapping young faults provides the basis for better understanding seismic hazard in different parts of Saudi Arabia.
Roobol, M.J. and Kadi, K.A., 2014, Cenozoic faulting in the Jizan area, southwest Saudi Arabia: Saudi Geological Survey Technical Report SGS-TR-2013-14, 32 p., 18 figs., 3 plates.