Tectonic transition from thrusting to polyphase non-confining deformation within the Semail Ophiolite along the sinistral, transtensional Issmaiya Fault Zone (Sultanate of Oman)

Abstract

• The Oman Mountains record the transition between shortening during the obduction of the Semail ophiolite . • At least three extensional events: affected the autochthonous and allochthonous units after obduction: two between the latest Cretaceous and early Eocene and one after the mid-Eocene. • Allochthonous units underwent major extensional shearing immediately after the emplacement of the ophiolite. • The allochthonous units underwent two further extensional events: One between the latest Cretaceous and early Eocene, and the other one after mid-Eocene. Obduction of the Semail Ophiolite and deep-sea basin-derived rocks represent a major tectonic event during Late Cretaceous evolution of the Oman Mountains. These allochthonous units were thrust SW-ward onto the passive Arabian margin and platform, followed by the main uplift of the Saih Hatat Dome and minor uplift of the Jabal Akhdar Dome. We present new structural data that indicate major extensional and transtensional deformation affecting the ophiolite unit after its emplacement. The studied area is in the southeastern Oman Mountains and hosts the brittle “Issmaiya Fault Zone” (IFZ; ∼3 km in wide and ∼ 30 km long). Field work and satellite image analysis show that most faults affected the Semail Ophiolite and only a few faults cut through latest Cretaceous to Paleogene sedimentary rocks of the Ibra Basin. These constraints allow us to date most faulting to immediately after the Late Cretaceous emplacement of the Semail Ophiolite, lasting to the post-Paleogene. The IFZ strikes ∼NW-SE at an acute angle of ∼30° to the WNW-ESE oriented southern margin of the Saih Hatat Dome. Northwestward, the IFZ terminates at the southern boundary fault of the Saih Hatat Dome. Fault kinematics based on shear-fracture slip-indicators reveal vertical to steep NW/SE-striking fault planes with occurrence of mainly sinistral strike-slip and some extensional dip-slip faults. This highlights a latest phase in a constrictional strain field with N/S-extension, perpendicular to E/W-oriented and vertical shortening axes of a sinistral transtensional fault zone. We suggest that SW-ward thrusting of the Semail Ophiolite was followed by mainly sinistral transtension.