Cenozoic faults and seismicity in Northwest Saudi Arabia and the gulf of aqaba region

Abstract

Following the November 1995 earthquake in the Gulf of Aqaba that affected countries in the area including northwest Saudi Arabia, a study was made to identify the youngest geological faults in the area. These faults have been termed Cenozoic faults, although most are Neogene in age, being younger than 23 Ma. A number of previously unknown structures have been identified. These include a zone of shattered and intensely faulted rocks 25 km wide that extends inland into Saudi Arabia from the coast of the Gulf of Aqaba. In this zone, shattered granite intrusions are eroded in a similar manner to sandstone and produce deep, vertical walled, narrow gorges, such as Tayyib al Ism. Faults parallel to the Gulf of Aqaba occur as deep wadis where the geological contacts are offset by sinistral displacements of up to 9 km, and show systematic shoulder uplift away from the gulf. Another major Cenozoic structure contained within the Saudi Arabian side of the Gulf of Aqaba is a major fold that extends 50 km inland from the coast. In this fold the Precambrian lithologic units, dikes and faults are rotated 90 degrees anticlockwise with the appearance of a drag fold due to the approximately 115 km sinistral offset of the Gulf of Aqaba. This structure is not repeated in Sinai and occurs only on the Arabian side of the Gulf. It is interpreted not as a drag fold but as a fold that probably resulted from the initial displacement along the Arabian Plate from Sinai. Later faults isolated the structure within Saudi Arabia. Reactivated Precambrian faults within the structure are also curved. Aeromagnetic anomalies show that the traces of major Tertiary gabbro dikes that parallel the Red Sea coast of Saudi Arabia are also curved within the fold, but their curvature of about 45 degrees anticlockwise is less than that of the Precambrian rocks and dikes, which suggests dike emplacement occurred after folding commenced. Maps that cover an area 550 km east-west by 360 km north-south and include the Gulf of Aqaba and adjacent areas show the whole area to be cut by thousands of Cenozoic faults (about two and a half thousand of the larger faults are shown here). The oldest faults occur in a 250-km wide belt of pre-rift structures that include graben structures that lie to the east of the Gulf of Aqaba fault system. These faults are oriented north-northwest in the north and curve to the northwest in the south of the study area, and are named Ribbon faults because they divide the country rocks into long thin strips from 100 to 500 m wide. These faults mainly occur within Paleozoic sandstone where they are preserved as walls cemented by iron oxides/hydroxides. Where the Ribbon faults occur within Precambrian Shield and Tertiary sediments the oxide/hydroxide fills are absent. The Ribbon faults result from crustal extension as they form horst-graben structures with topography of a few metres. In some areas open fissures occur without apparent displacement. The aeromagnetic maps show that some of the major graben structures are underlain by large Tertiary gabbro dikes. The system is mainly seismically inactive today with a few local exceptions, such as a cluster of earthquake activity about 75 km southeast of Tabuk city. Most active are the north-south to north-northeast-trending Gulf of Aqaba system of faults that include those of the floor of the Gulf of Aqaba. Other notable fault systems in the area are a prominent set of pre-rift east-west faults, some of which contain traces of Tertiary gabbro dikes and some of which become curved toward the Gulf of Aqaba like the reactivated Precambrian faults. The map area preserves a record of about 25 million years of faulting that commenced in pre-Red Sea rift time, and also records the initiation of separation of the Arabian Plate from Sinai by fold deformation prior to faulting. Some minor folding occurs from drag and compression of a belt of Tertiary sedimentary rocks originally continuous with the Gulf of Suez which were transported 115 km into the Gulf of Aqaba. The Gulf of Aqaba region remains the most active seismic area in Saudi Arabia and is the site of three large earthquakes in 1983, 1993 and 1995.