Evidence of blueschist facies shortening in the Jabal Akhdar tectonic window, northern Oman

The Jabal Akhdar and Saih Hatat tectonic windows in the Oman Mountains are key geological features where to investigate the geological record of the Late Cretaceous subduction, obduction and exhumation cycle that affected the northeastern margin of the Arabian Plate. Indeed, the metasedimentary Autochthon A (pre-Permian) and B (post-Permian) Units exposed therein and separated by the so-called Hercynian Unconformity display well-preserved evidence of many of those processes, as they were spared by the obliterating effects of continental collision once subduction terminated. Research on the structural and metamorphic framework of the Jabal Akhdar tectonic window has been scanty until now. This stems from the fact that, unlike Saih Hatat, which records greenschist to eclogite facies metamorphism, Jabal Akhdar has been traditionally considered a portion of the Arabian Plate only recording anchizone metamorphism (only one site has recently been reported as preserving evidence of high pressure-low temperature (HP-LT) metamorphism dated to the Late Cretaceous). Here, we present new field and thermobarometric data from Jabal Akhdar aimed at better constraining its structural and metamorphic framework on a regional scale and possibly correlating it to the greater subduction-obduction cycle of Oman.

In western Jabal Akhdar, field constraints from newly documented top-to-the ENE shearing indicate the local pervasive structural reactivation of the Hercynian Unconformity as a post-Permian ENE-verging thrust. Multiple Raman spectroscopy findings of metamorphic aragonite infilling Mode I veins support a HP-LT metamorphic imprint associated with such shortening. In the eastern sector of the tectonic window, on the other hand, folding and S-C tectonites in Ediacaran slates indicate NW-verging shear. Our new structural data thus suggest NE- to NW-verging shortening and stacking within and between the Autochthon A and B. Further new structural evidence from the structurally higher Autochthon B in northeastern Jabal Akhdar indicates that inherited structures with top-to-the NE kinematics were later overprinted by lower-grade, localised SW-ward thrusting through interlayer slip recorded in Cretaceous marbles. Finally, extensional reactivation followed, associated with Neogene doming and final exhumation.

We employed a multidisciplinary approach to constrain the P-T conditions associated with the identified structures. Chlorite–white mica–quartz–water multiequilibrium analyses integrated with Raman spectroscopy on carbonaceous material indicate T = 370 °C and P = 0.50 GPa in western Jabal Akhdar, associated with NE-verging S-C tectonites, and T = 330 °C and P = 0.66 GPa, in the eastern domain of the tectonic window, related to NW-verging folds. These thermobarometric results place the investigated deformation in the greenschist and lower blueschist facies metamorphic fields, respectively.

These new structural data and the recorded metamorphic signature call for a re-evaluation of the commonly held notion of a mostly undeformed and non-metamorphic geological record within Jabal Akhdar, questions its correlation (or lack thereof) with the Saih Hatat tectonic window and, ultimately, Jabal Akhdar’s role within the framework of the Oman Late Cretaceous subduction and obduction phases.