Tectono-thermal evolution of Oman's Mesozoic passive continental margin under the obducting Semail Ophiolite: a case study of Jebel Akhdar, Oman

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.

Gondwana accretion tectonics and implications for the geodynamic evolution of eastern Arabia: first structural evidence of the existence of the Cadomian Orogen in Oman (Jabal Akhdar Dome, Central Oman Mountains)

“…The final doming of the JAD occurred from Eocene to Miocene (Hansman et al, 2017(Hansman et al, , 2018Grobe et al, 2018Grobe et al, , 2019. This process was accompanied by extensional shearing at the northern and eastern margin of the dome (Mattern and Scharf, 2018;Scharf et al, 2019), and tilting of the Arabian rocks along a WNW-ENE trending rotation axis at the northern and southern margin of the dome by ∼30° and ∼20°, respectively (compare Beurrier et al, 1986;Rabu et al, 1986).…”

“…Obduction was followed by extension (e.g., Fournier et al, 2006;Searle, 2007;Mattern and Scharf, 2018;Grobe et al, 2018Grobe et al, , 2019 as indicated by extensional faults, which displaced the ophiolite Searle, 2007). It is worth mentioning that both SHD and JAD display large-scale listric normal faults bounding their flanks (Searle, 2007).…”

Late-stage tectonic evolution of the Al-Hajar Mountains, Oman: new constraints from Palaeogene sedimentary units and low-temperature thermochronometry

“…A second shortening phase occurred during the Cenozoic and it is responsible for the deformation of the Late Cretaceous to Cenozoic post-nappe package and for the final uplift and doming of the culmination areas of the central Al-Hajar Mountains (Mount et al 1998;Breton et al 2004;Csontos et al 2010;Ali et al 2014;Cooper et al 2014;Hansman et al 2017). This uplift event was constrained by several thermochronological data (Mount et al 1998;Poupeau et al 1998;Saddiqi et al 2006;Wübbeler et al 2015;Hansman et al 2017;Grobe et al 2019) and absolute radiometric dating of veins (Grobe et al 2018;) from the Jabal Akhdar and Saih Hatat tectonic windows and surrounding areas ( Fig. 1a).…”

“…Despite this large amount of data, the timing and mode of exhumation of the main tectonic windows is still debated. Nevertheless, most recent interpretations point toward a rapid exhumation of the Jabal Akhdar Dome (Hansman et al 2017;Grobe et al 2019) starting from the early Eocene.…”

The Role of Chemistry in Fracture Pattern Development and Opportunities to Advance Interpretations of Geological Materials

“…Image is a modified part of a rectified highresolution photograph created by Wüstefeld (2010). For the tectonic, geomechanical and pressure-temperature evolution of the area, see Grobe et al (2019). 10.1029/2019RG000671…”