Orogenic wedge formation during obduction: insights and perspectives from the Oman Mountains

Obduction causes overthrusting of dense oceanic rocks on top of lighter continental units at convergent margins. Despite many conceptual models addressing both its initiation and the counterintuitive significant horizontal displacements of large and heavy rafts of oceanic lithosphere, obduction is only partially understood and remains quite an enigmatic process. Uncertainty remains on the triggering mechanisms and the emplacement modes under mechanically unfavourable frameworks, with recent contributions stressing the role of far-field boundary conditions, such as the impact of bursts of “plate acceleration”. The processes governing convergent margin deformation and the structuring of an orogenic wedge in association with obduction and ophiolite emplacement also remain mostly unexplored. In that setting, complex orogenic architectures may form during the imbrication of mobile and deformable continental crust slivers underneath advancing, and possibly several kilometre-thick, ophiolitic successions. 

The northeastern Oman Mountains allow studying one such orogenic wedge in the Jabal Akhdar Dome (JAD), an Arabian Plate related domain that is now exhumed to the surface from beneath the allochthonous and far-travelled Semail Ophiolite. At odds with the general view, recent and ongoing studies indicate that parts of the Arabian Plate therefrom experienced a complete cycle of subduction-exhumation broadly concurrent with the Semail Ophiolite obduction in the Late Cretaceous, thus recording high pressure-low temperature (HP-LT) blueschist facies conditions of ≥ 0.9 GPa (based on the presence of aragonite in carbonates) and ≃ 350 °C. Preservation of such a metamorphic signature in the relatively undeformed external portion of the Arabian Plate calls for a re-evaluation (i) of the regional picture framing HP-LT metamorphism formation in the absence of obvious links with long-lived subduction or major continental collision and (ii) of the mechanisms capable to exhume the HP-LT rocks and accrete them beneath the Semail ophiolitic sequence. 

Our on-going structural, stratigraphic and metamorphic investigations within the JAD document a twofold history sequentially encompassing: 1) Cenomanian top-to-the NE imbrication and accretion under HP-LT conditions in the subduction channel of a SW-dipping Arabian Plate-directed subduction zone nucleating on transitional passive margin crust; 2) Late-Cretaceous top-to-the SW lower-grade shearing during SW-ward thrusting and imbrication of the Hawasina nappes and the obduction of the Semail Ophiolite. This would have been triggered by an embryonic NE-ward intraoceanic subduction close to the Semail spreading centre, which set in motion the ophiolite basal thrust that, through >400 km of SW-ward transport, overrode the by-then failed subduction zone of (1); 3) Finally, the current NE-ward Makran subduction zone initiated farther outboard in the Paleogene.