Strain state and kinematic evolution of a fold-nappe beneath the Samail Ophiolite, Oman

The Saih Hatat domal culmination of the Oman Mountains, Arabian Peninsula contains a major, refolded, NE-facing, recumbent, anticlinal fold-nappe within Pre-Ordovician sedimentary cover (autochthon) and Mesozoic platform carbonates that underlie the Samail and Hawasina nappes. Tracts of overturned stratigraphy covering over a 1000 km2 are associated with a marked strain gradient towards sheath-like, a-type folds that define the fold-nappe lower limb transitional into an elongated SW-facing recumbent syncline and the underlying upper plate–lower plate shear zone. Directions of stretching recorded by fibrous, pressure shadow fringes on pyrite show a 20–30° counterclockwise rotation in the foliation plane. Strain and kinematic constraints require the fold-nappe to form progressively during top-to-the-NE shearing of the Arabian margin with (1) development of a high-strain zone, and (2) subsequent folding of the strongly deformed to transposed stratigraphy in the hanging wall to the shear zone. Rolling of the autochthonous stratigraphic layering of the upper limb through the hinge accompanied by differential shear strain produced the regional scale, highly attenuated a-type folds at the structurally lowest levels. This occurred by simple underthrusting of the footwall (lower plate), as the upper limb of the fold-nappe has to be pinned to the autochthon.