Abstract
Abstract Traditional structural analysis in fold and thrust belts has focused 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 1.8–1.9 km) was followed by flexural subsidence in the Late Cretaceous and Cenozoic (tectonic subsidence >0.6 km) in response to the closure of the Neo-Tethys Ocean. The main phase of continental collision during the Neogene resulted in the 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 the 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.
Highlights
Permian rifting along the northern margin of the Arabian Plate was followed by the opening of the Neo-Tethys Ocean starting in the late Early Permian (Stampfli & Borel 2002; Aqrawi et al 2010)
Thermal subsidence of the adjacent passive margin is indicated by the stratigraphic record in Kurdistan, but additional subsidence due to local rifting events is known to have continued during the Mesozoic (Frizon de Lamotte et al 2011)
The stratigraphic record for the three wells can be explained by thermal subsidence alone from the Middle Permian until the Late Cretaceous (Campanian– Maastrichtian); no additional rifting events are necessary during this time to explain the stratigraphic record
Summary
Permian rifting along the northern margin of the Arabian Plate was followed by the opening of the Neo-Tethys Ocean starting in the late Early Permian (Stampfli & Borel 2002; Aqrawi et al 2010). Displacement along the southern crustal ramp resulted in uplift of these anticlines and the development of the Shaikan Anticline and structures of the Foreland Folded Zone to the south The timing of this event can be dated with greater confidence based on the presence of a Pliocene–Quaternary growth sequence that is present on the south side of the Shaikan Anticline (seismic profile of Gulf Keystone 2013 tied to surface geological mapping by CGG NPA 2011). The elevation of the same horizon at the crest of the anticline measured from this datum provides an estimate of the apparent uplift This method can only be used in the Foreland Folded Zone due to the additional basement-involved uplift to the north of the Mountain Front. Individual structures are not resolved by the sparse compilation of points, with the exception of Sangaw Mountain in SE Kurdistan where erosion estimates have been made for both wells drilled on this structure
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