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.
Overview of Cordilleran oceanic terranes and their significance for the tectonic evolution of the northern Cordillera