AAPG Middle East Region Geoscience Technology Workshop

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The Semail Gap Fault Zone (Oman Mountains) - Influences of an Inherited Basement Structure on Cenozoic Deformation?


The eastern margin of the Jabal Akhdar Dome is flanked by a remarkable straight, ~70 km long and NNE-oriented lineament, known as the “Semail Gap”. On its western side (footwall), this geomorphological element features steeply ESE-dipping Mesozoic sedimentary rocks of the Hajar Supergroup (carbonates of the Arabian Platform) and the Aruma Group (shaley foreland basin fill). On its eastern side (hanging wall), rolling hills are composed of mainly ultramafic rocks of the Semail Ophiolite. Sandwiched between the carbonates and the ophiolite is a 1-3 km narrow zone of ESE-dipping Hawasina nappes (mostly shale). Beneath the Semail Gap is a dextral transtensional fault zone (the Semail Gap Fault Zone, SGFZ; Scharf et al., 2019). This fault zone with a throw of up to 5.5 km and a horizontal displacement of up to 2 km, was active during the exhumation of the Jabal Akhdar Dome, probably during the Late Eocene to Miocene in the course of orogenic collapse (Scharf et al., 2019). The hanging-wall block appears to be a relatively thick but thin-skinned gravity unit. Further to the east of the fault zone is the Saih Hatat Dome. Satellite image, map analyses and field observations reveal that lateral shearing parallel to the Semail Gap Fault Zone caused formation of Riedel shears in the carbonates of the Arabian Platform in the footwall. However, no information is available regarding the vertical extent of the fault zone (i.e., is Precambrian crystalline basement affected as well?). At the site of the Semail Gap Fault Zone, a preexisting fault was located already during the Neoproterozoic (“Proto-SGFZ”). This is indicated by different Neoproterozoic to Paleozoic lithologies, thicknesses and formations on either side of the fault zone in the Jabal Akhdar and Saih Hatat domes (e.g., Mattern and Scharf, 2019). An origin of the initial fault zone may be considered during accretion of different terranes during the Cryogenian (Romine et al., 2004). Based on the straight nature and the different lithologies on both sides of the SGFZ, we suggest that a major primary NNE-oriented structure/fault zone is located beneath it. Furthermore, the above mentioned Riedel shears indicate that the SGFZ must vertically extend for at least several kilometers into the footwall. Based on the paleogeographic reconstruction of Stampfli and Borel (2002), Mattern and Scharf (2018) concluded that the Proto-SGFZ functioned as a sinistral transform fault or transform zone during the Pangea rifting. This major fault zone was (re)activated during different tectonic stress fields ranging from the Neoproterozoic to present. The Eocene to Miocene reactivation is evident at the surface by the SGFZ. Perpendicularly oriented to the transform fault and within the Arabian basement, major extensional faults may have formed in the course of the Pangea rifting (compare Stampfli and Borel, 2002). The surface expression of such WNW-ESE-striking basement faults may be the southern margins of the Jabal Akhdar and Saih Hatat domes as well as at the northeasterner most limit of the Jabal Akhdar Dome (Nakhl Subdome). Reactivation of major basement structures may explain the present shape of the Jabal Akhdar and Saih Hatat domes.