The Zagros fold-and-thrust belt in the Fars province (Iran): II. Thermal evolution

Abstract

Abstract Temperature-dependent clay minerals and vitrinite reflectance data, surface and subsurface geological constraints were used to unravel the burial evolution of the Ordovician-Quaternary sedimentary successions from the inner to the outer zones of the Zagros fold-and-thrust belt in the Fars province (Iran). These sedimentary successions were buried to their thermal maxima during early to late diagenesis, achieving temperatures corresponding to the immature to early mature stages of hydrocarbon generation. They experienced low levels of thermal maturity in the Interior Fars, corresponding to vitrinite reflectance values between 0.38 and 0.66%, to mixed layers illite-smectite (I-S) with an illite content between 30 and 75% and to KI values between 0.97 and 1.18°Δ2θ. In the Central and Coastal Fars, vitrinite reflectance ranges between 0.35 and 0.51%, the illite content in I-S displays values between 20 and 85% and KI data are between 0.71 and 1.30°Δ2θ. In individual anticlines, mixed layers I-S show an increase of the illite content as a function of stratigraphic age (depth), suggesting that levels of thermal maturity are controlled by sedimentary burial. One dimensional thermal history models allowed us: (i) to estimate the maximum burial experienced by the sedimentary successions and the amount of the sedimentary pile currently removed by erosion, (ii) to determine the thickness of the ophiolite units obducted during Late Cretaceous time in the High Zagros, and (iii) to define the onset of oil generation for the Albian source rocks throughout the Zagros belt. Paoleothermal data were used to constrain the geometry of eroded structures in a 253 km long cross-section extending from the High Zagros to the Coastal Fars. Along the cross-section, lithostatic load slightly decreases towards the foreland (e.g., from 3.65 km to 3.2 km for the Bangestan Group) and the amount of the eroded material varies between ∼6 km (above anticlines in the Central and Interior Fars) and ∼200 m (above synclines in the external part of the belt).