Abstract
Abstract. We present a study of pressure and temperature evolution in the passive continental margin under the Oman Ophiolite using numerical basin models calibrated with thermal maturity data, fluid-inclusion thermometry, and low-temperature thermochronometry and building on the results of recent work on the tectonic evolution. Because the Oman mountains experienced only weak post-obduction overprint, they offer a unique natural laboratory for this study. Thermal maturity data from the Adam Foothills constrain burial in the basin in front of the advancing nappes to at least 4 km. Peak temperature evolution in the carbonate platform under the ophiolite depends on the burial depth and only weakly on the temperature of the overriding nappes, which have cooled during transport from the oceanic subduction zone to emplacement. Fluid-inclusion thermometry yields pressure-corrected homogenization temperatures of 225 to 266 ∘C for veins formed during progressive burial, 296–364 ∘C for veins related to peak burial, and 184 to 213 ∘C for veins associated with late-stage strike-slip faulting. In contrast, the overlying Hawasina nappes have not been heated above 130–170 ∘C, as witnessed by only partial resetting of the zircon (U-Th)/He thermochronometer. In combination with independently determined temperatures from solid bitumen reflectance, we infer that the fluid inclusions of peak-burial-related veins formed at minimum pressures of 225–285 MPa. This implies that the rocks of the future Jebel Akhdar Dome were buried under 8–10 km of ophiolite on top of 2 km of sedimentary nappes, in agreement with thermal maturity data from solid bitumen reflectance and Raman spectroscopy. Rapid burial of the passive margin under the ophiolite results in sub-lithostatic pore pressures, as indicated by veins formed in dilatant fractures in the carbonates. We infer that overpressure is induced by rapid burial under the ophiolite. Tilting of the carbonate platform in combination with overpressure in the passive margin caused fluid migration towards the south in front of the advancing nappes. Exhumation of the Jebel Akhdar, as indicated by our zircon (U-Th)/He data and in agreement with existing work on the tectonic evolution, started as early as the Late Cretaceous to early Cenozoic, linked with extension above a major listric shear zone with top-to-NNE shear sense. In a second exhumation phase the carbonate platform and obducted nappes of the Jebel Akhdar Dome cooled together below ca. 170 ∘C between 50 and 40 Ma before the final stage of anticline formation.
Highlights
The Permian–Mesozoic platform sediments of north Oman (Fig. 1; e.g., Beurrier et al, 1986; Glennie et al, 1974; Lippard et al, 1982) with hydrocarbon accumulations in the southern foreland of the Jebel Akhdar Dome (Figs. 1 and 2) are overlain by the Semail Ophiolite nappe complex, the largest and best-preserved ophiolite on Earth
In this paper we present new thermal maturity, thermochronology, and fluid-inclusion data and integrate them into a numerical basin model of the pressure–temperature evolution along a transect extending from the undeformed passive margin sequence in the south to the Batinah coast in the north (Fig. 2)
Temperatures are generally higher on the northern flank of the Jebel Akhdar and slightly increase with stratigraphy in the autochthons
Summary
The Permian–Mesozoic platform sediments of north Oman (Fig. 1; e.g., Beurrier et al, 1986; Glennie et al, 1974; Lippard et al, 1982) with hydrocarbon accumulations in the southern foreland of the Jebel Akhdar Dome (Figs. 1 and 2) are overlain by the Semail Ophiolite nappe complex, the largest and best-preserved ophiolite on Earth. Less well known is the temperature and pressure evolution of the sub-ophiolite passive margin units and the subsequent cooling history of the Jebel Akhdar (Aldega et al, 2017; Grobe et al, 2018; Hansman et al, 2017; Poupeau et al, 1998; Saddiqi et al, 2006). This information is vital for our understanding of the time– temperature history of overthrusted margins and would allow us to further constrain obduction dynamics and forebulge migration. Combining peak temperature evolution with cooling ages links the burial history with phases of orogeny
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