AAPG Middle East Region Geoscience Technology Workshop

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Quantifying Vertical Movements in Fold and Thrust Belts: Subsidence, Uplift and Erosion in Kurdistan, Northern Iraq

Abstract

Traditional structural analysis in fold and thrust belts has focussed on quantifying horizontal movements. In this paper the importance of quantifying vertical movements is illustrated using a case study from Kurdistan, Northern Iraq. The subsidence history of this area can be determined by analysis of the stratigraphic record from deep exploration wells. A phase of thermal subsidence from Middle Permian to Late Cretaceous (tectonic subsidence 1.8 to 1.9 km) was followed by flexural subsidence in the Late Cretaceous and Cenozoic (tectonic subsidence >0.6 km) in response to closure of the Neo-Tethys Ocean. The main phase of continental collision during the Neogene resulted in development of the Zagros fold and thrust belt; the amount of uplift at individual anticlines can be estimated from their amplitude (up to 3 km), but regional cross-sections indicate that approximately 1 km of additional basement-involved uplift is present NE of the Mountain Front. The timing of basement-involved uplift is interpreted to be coeval with deposition of a Pliocene-Quaternary growth sequence adjacent to the Mountain Front. The amount of erosion resulting from the uplift can be estimated from vitrinite reflectance and cross-sections; these estimates show a similar pattern with maximum erosion in the mountains NE of the Mountain Front (>1.5 km) and lesser erosion in the adjacent foreland basin (generally <0.8 km). The results provide a quantitative understanding of subsidence, uplift and erosion, and have been used to define prospective and high-risk areas for petroleum exploration. The apparent uplift at the crest of the Gara Anticline is 3.0 km measured from the local structural datum. The gross uplift, including additional basement involved uplift, determined from the structural datum at the base of the syncline to the south of the Mountain Front (MF) is 5.1 km. This additional uplift has resulted in failure of the seals within the Triassic Kurra Chine Formation at the Gara-1 well. The domain of severe uplift can be extrapolated along strike using a line connecting the SW margins of anticlines in which Jurassic or older stratigraphy is exposed.