A model for the Semail ophiolite thrust duplex and a late out-of-sequence thrust, northern Oman/UAE Mountains

The Cenomanian Semail ophiolite was obducted onto the Mesozoic shelf and into the foreland basin mostly as a single thrust sheet along the serpentinized Semail thrust, except for a ca. 65 km long thrust duplex. At least six, ca. 5 km thick, foreland-facing thrust sheets were thrust from east to west along intensely serpentinized mantle-crust transition zone rocks. The late out-of-sequence Fizh and Hilti Block thrust sheets concealed the hinterland. Nevertheless, the conditions that formed the duplex can be postulated. A fault system, like the one mapped in this study, cut the Hawasina and Haybi Complexes and overlying ophiolite duplex during obduction, forming fault scarps that interrupted obduction of the ophiolite and triggered deformation and uplift in the hinterland. When the taper angle of the wedge became supercritical, breakback thrusting created the first thrust sheet. Repetition of thrusting is explained by a continuous increase in the width of the wedge as the height was reduced by erosion and, possibly removal of ca. 5 km of crustal rocks during each deformation-erosion/thrusting cycle. The cycles continued until the wedge's width was too great for uplift to raise the taper angle to a supercritical value. A deeper décollement then formed in the upper Hawasina and lower Haybi, creating the late out-of-sequence Hawasina-Haybi thrust sheet emplaced over the eroded(?) thrust duplex. Serpentinization played a key role in thrusting of the ophiolite and in the formation of the duplex and may be a prerequisite for thrusting of ophiolites.