The Semail ophiolite, a thick thrust sheet of Late Cretaceous oceanic crust and upper mantle, was obducted onto the previously rifted Arabian continental margin in the Late Cretaceous, and now forms part of the United Arab Emirates (UAE)-Oman mountain belt. A deep foreland basin along the west and SW margin of the mountains developed during the obduction process, as a result of flexure due to loading of the ophiolite and underlying thrust sheets. The nature of the crust beneath the deep sedimentary basins that flank the mountain belt, and the extent to which the Arabian continental crust has thickened due to the obduction process are outstanding questions. We use a combination of active- and passive-source seismic data to constrain the stratigraphy, velocity structure and crustal thickness beneath the UAE-Oman mountains and its bounding basins. Depth-migrated multichannel seismic-reflection profile data are integrated in the modelling of traveltimes from long offset reflections and refractions, which are used to resolve the crustal thickness and velocity structure along two E-W onshore/offshore transects in the UAE. Additionally, we apply the virtual deep seismic sounding method to distant earthquake data recorded along the two transects to image crustal thickness variations. Active seismic methods define the Semail ophiolite as a high-velocity body dipping to the east at 40-45˚. The new crustal thickness model presented in this work provides evidence that a crustal root is present beneath the Semail ophiolite, suggesting that folding and thrusting during the obduction process may have thickened the crust by 16 km.

A common deviation from typical subduction models occurs when thrust sheets of oceanic-crust and upper-mantle rocks are emplaced over more buoyant continental lithosphere. The archetypal example of ophiolite obduction is the Semail ophiolite in the United Arab Emirates (UAE)-Oman orogenic belt, formed and obducted onto the Arabian continental margin during the Late Cretaceous. The Strait of Hormuz syntaxis, the northern extent of the UAE-Oman mountains, marks the transition from ocean-continent convergence in the Gulf of Oman to continental collision along the Zagros Mountains. Based on new seismic data from a focused recording network, we infer crustal and mantle deformation in the northeastern corner of the Arabian plate (including the southern Zagros and the UAE-Oman mountains), using observations from anisotropic tomography and shear-wave splitting measurements. We recover a change of ~90˚ in the axis of fast-anisotropic orientations in the crust from the Zagros to the UAE-Oman mountain belt, consistent with the dominant strike of the orogenic belts. We also find evidence for localized fossil deformation in the lithospheric mantle underlying the UAE-Oman mountain range, possibly related to stress-induced tectonism triggered by underthrusting of the proto-Arabian continental margin beneath the overriding Semail ophiolite. These orientations, averaging 15˚ anticlockwise, provide the first geophysical verification of geological

Studies of the high-pressure (HP) As Sifah eclogites in the NE part of the Saih Hatat window, Oman, have used different radiometric dating results (Ar/Ar, Sm-Nd vs. U-Pb, Rb-Sr) to interpret disparate tectonic models for the timing, geometry, and cause of continental subduction – including its association with the Samail Ophiolite. To determine the absolute timing of continental subduction, we coupled geochronological and geochemical analyses of major (garnet) and accessory phases (zircon, rutile) from the highest-grade metamorphic rocks in the Saih Hatat (As Sifah eclogites). Early Permian (283.8+/-0.7 Ma) tuffaceous zircon cores are consistent with earlier interpretations that the As Sifah rocks were sourced from a distal portion of the Arabian continental margin. Data from a range of bulk compositions, metamorphic assemblages, and rock textures consistently suggest a single metamorphic event, with garnet growth starting by ~81 Ma and ceasing by ~77 Ma, with slight but consistent offsets in the timing of metamorphic (re)crystallization between different lithologies. These new data confirm previous structural, metamorphic, and geochronological interpretations for continental HP metamorphism in a single NE-dipping subduction zone beneath the already obducted Samail Ophiolite; there is no robust evidence for a ~110 Ma event or a continental-ward dipping subduction zone. Combined with literature constraints, our data suggest that the As Sifah unit was subducted and exhumed relatively slowly (~5 mm/yr) compared to other continental high-pressure settings - likely associated with the dragging to mantle depths by a mafic root, followed by long residence in the lower to middle crust.