When obduction meets accretion: new geochronological and metamorphic constraints on an Albian-Campanian transient subduction-obduction system in NE Oman

Late Cretaceous convergence between the Afro-Arabian and Eurasian plates triggered a short-lived yet complex subduction–obduction cycle along the Arabian margin, culminating in the Campanian obduction of Neo-Tethyan oceanic lithosphere onto the Arabian continent. Although the Oman Mountains preserve an exceptional record of this evolution, the timing and tectonic significance of deformation and metamorphism along the Arabian margin remain poorly constrained. This is particularly true for the Jabal Akhdar Window (JAW), a tectonic feature traditionally regarded as exposing the most external, non-metamorphic and little deformed portions of the margin.

We present new geochronological constraints integrated with structural and metamorphic data that call for a revision of the tectonic history of the Cryogenian–Cretaceous JAW succession. Raman spectroscopy of carbonaceous material (RSCM) and chlorite–white mica–quartz–water multiequilibrium thermobarometry constrain synkinematic temperatures and pressures for two distinct regional contractional deformation phases (D₁ and D₂), while U–Pb dating of structurally constrained calc-mylonites provides much-needed absolute age control on deformation.

D₁ records pervasive brittle–ductile to ductile deformation affecting both the pre-Permian basement and the overlying Permian passive-margin carbonates, with an overall top-to-the-NE transport direction. U–Pb geochronology of top-to-the-NE calc-mylonites yields an age of 106 ± 13 Ma, consistent with field-based constraints and regional correlations. Synkinematic RSCM and thermobarometric data indicate upper greenschist- to lower blueschist-facies conditions (~370 °C, minimum ~0.7 GPa), consistent with an Albian HP–LT accretionary event related to early convergence along the Arabian margin. D₁ is interpreted as due to progressive nappe stacking within a NE-verging accretionary prism formed above an immature and short-lived intracontinental SW-directed subduction zone.

D₂ represents a younger, lower-grade, top-to-the-S/SW-verging deformation event localized within Upper Cretaceous carbonates, mainly in the northern JAW. RSCM analyses from top-to-the S/SW calc-mylonites constrain synkinematic temperatures to ~330 °C, while U–Pb carbonate ages from the same rocks cluster between ~80 and 75 Ma. These data link D₂ deformation to southwestward obduction, coeval with the emplacement of the Semail Ophiolite and Hawasina nappes.

We propose a two-stage evolution for the northeastern Oman Mountains, involving a transient, SW-directed Albian subduction associated with HP–LT accretion followed by Late Cretaceous obduction driven by a newly established NE-dipping intraoceanic subduction zone.

These results demonstrate that HP–LT metamorphism in the JAW is regionally developed and temporally resolved, highlighting the critical role of geochronology of structurally well-constrained fabrics in deciphering transient subduction–obduction processes during the Late Cretaceous Wilson Cycle evolution of the Arabian margin.