STRUCTURAL HISTORY OF THE SEMAIL OPHIOLITE, NORTHERN JEBEL AKHDAR CULMINATION, OMAN

At over 10,000 square kilometers, Oman’s Semail ophiolite is the world’s largest and best-exposed, however the processes associated with both the detachment and emplacement of the Semail ophiolite onto upper crustal strata remain enigmatic. In this study we examine the mantle part of the ophiolite, its metamorphic sole, and the peri-autochthonous strata beneath the allochthonous rocks in a ~30 km² area between Taww and Buwah at the northern end of the Jebel Akhdar culmination. Our purpose is to understand the structural history of ophiolite emplacement and resolve the deformation kinematics with qualitative and quantitative analyses of 3-D strain and vorticity within the metamorphic sole.

The ophiolite is characterized by both massive and banded peridotite. Banded peridotite becomes more prominent within ~0.5 km of the metamorphic sole and is penetratively serpentinized. Banding strikes E-W and typically dips gently to moderately north. The metamorphic sole is up to a few hundred meters thick and includes well-foliated phyllite, quartzite, metachert, and amphibolite. Foliation typically dips to the north, but rolls over to more southerly dips within ~100 m of the contact with the underlying Jurassic limestone. Petrographic observations indicate that flattening strains are common and symmetric microstructures are consistent with general to pure shear dominated conditions.

Two generations of E-W striking normal faults separate the metamorphic sole from the underlying Jurassic limestone. The older faults dip 25 to 35˚ N and are characterized by semi-ductile deformation. At some locations the metamorphic sole is cut out by these faults and mantle peridotite forms a serpentinized mélange with abundant top-to-the north sense-of-shear indicators. High-angle (~60 to 70˚ N) younger brittle faults cut all other structures. Post-emplacement normal faulting and crustal extension played a significant role in generating the structural geometry of the Jebel Akhdar culmination.