Cenozoic Faulting in the Rabigh Area, Central-West Saudi Arabia (Including the Sites of King Abdullah Economic City and King Abdullah University of Science and Technology)

SGS-TR-2008-6 M John Roobol and Khalid A. Kadi
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ABSTRACT


A coastal block measuring 110 km N-S by 95 km E-W centered on the site of the King Abdullah Economic City and King Abdullah University of Science and Technology and including the towns of Rabigh and Tuwwal (Thuwal), has been mapped on a scale of 1:50,000 (sixteen quadrangles) with the objective of identifying Cenozoic faults for consideration in seismic hazard assessment.  The area proved to demonstrate a very straightforward system of two generations of Cenozoic faults of different types and ages.  As the two systems have different orientations, the area provides a textbook example of how to distinguish the two sets of faults, which in other areas are found to be parallel and less easily distinguished.  Immediately alongside the Red Sea is the 40 km wide coastal gravel plain, which is underlain by the youngest fault system that is poorly exposed.  These are here named the RED SEA COASTAL PLAIN FAULTS and are believed to be a listric set of normal rotational faults that formed after the opening of the Red Sea.  It is inferred that they formed by gravity collapse of the margin of the Arabian Tectonic Plate into the newly formed Red Sea graben.  The Red Sea Coastal Plain Faults are therefore SYN-RIFT in age.  The faults are very poorly exposed as they cut unconsolidated Tertiary and Cenozoic sediments of the coastal plain, so that fault scarps are often eroded or at best represented by low gravel banks.   The fact that they are curved normal rotational listric faults has been demonstrated by drilling as at Jabal Dhaylan near Umm Lujj, and by unpublished deep seismic reflection sections by ARAMCO when exploring for hydrocarbons within the Red Sea coastal plain.  Immediately inland of the coastal plain are the foothills of the Precambrian shield that rise steadily inland to the top of the Red Sea Escarpment.  These hills are partially draped in Cenozoic basaltic lava flows of Harrat Rahat.  The foothills are abruptly bounded against the gravel plains of the coastal plain by what is named here as the MASTER LISTRIC FAULT, which in the mapped area runs north-south in the south and curves around to the NNW in the north.  This fault is one of the Red Sea Coastal Plain Faults and parallels the Red Sea coast, which is also controlled by the same set of faults.  Immediately inland of the Master Listric Fault, the Precambrian foothills are cut by an intense swarm of parallel faults that are oriented NNW and intersect or are cut across by the Master Listric Fault.  Clearly this system of faults is older than the Red Sea Coastal Plain Faults.  In the mapped area, the aeromagnetic map of the Arabian Shield clearly shows that many of these faults are occupied and followed by major gabbro dikes that are not exposed at the surface in the mapped area.  The presence of these dikes is a well-known aeromagnetic phenomenon along the western margin of the Arabian Tectonic Plate.  Although poorly exposed, the dikes have been K/Ar dated at between 25 and 20 Ma and as such give a minimum age for these faults.  The system is here named the FOOTHILLS FAULT SYSTEM and is PRE-RIFT in age.  The system comprises normal faults.  They are well exposed in some ridges and dip both eastwards and to the west.  They control the topography of the foothills, as there are horst blocks and graben structures that still have topographic expression today.  Nowhere in the field has the dip of these faults been seen to be less than 80 degrees.  These faults have been previously interpreted as rotational faults.  However, the presence of the very sharply defined aeromagnetic anomalies (representing gabbro dikes in them) cannot be explained if the fault is rotational for the dike angle will decrease with depth.  It is concluded here that the sharp linear aeromagnetic anomalies can best be explained if the dikes and their host faults are near vertically inclined.  Therefore in this report the faults and dikes are shown to be planar and dipping at 80 to 90 degrees.  This prerift fault system is believed to represent the crustal extension that occurred along the site of the Red Sea before the opening of the Red Sea rift.  It therefore records a period of crustal extension and thinning.  The initial opening was not simple and a number of small en-echelon grabens such as Wadi Azlam occurred in the north and an early ophiolite complex – the Tihama Asir Complex representing a rift zone with freshwater lakes with freshwater fossil fish - formed in the south.  A sketch cross section at latitude 22o15’N. has been drawn showing the pre-rift fault system with its gabbro dikes and the later Syn-Rift normal rotational faults of the coast plain.  In terms of hazard to King Abdullah city, a single normal rotational fault appears to underlie the city site.  The fault can be traced for 25 km north-south where it is represented on the surface by a line of Quaternary sabkhah deposits and a low bank.  The fault is here named THE QADIMAH FAULT.  It is recommended that this fault be verified by geophysical methods and its latest movement dated by paleoseismicity studies of the micro-flora and fauna of the overlying sabkhah deposits as well as direct dating of sediments cut by the fault.

 

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Roobol, M.J. and Kadi, K.A., 2008,  Cenozoic faulting in the Rabigh area, central-west Saudi Arabia (including the sites of King Abdullah Economic City and King Abdullah University of Science and Technology): Saudi Geological Survey Technical Report SGS-TR-2008-6, 12 p., 1 fig. 2 pls.