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Structural Architecture of the NW Kurdistan Thrust Belt: Elements of the ‘Dohuk Passive Roof’

Abstract

The Kurdistan fold and thrust belt is characterized by a segmentation in structural style that is caused by the interplay of the level of exposure of this very youthful orogen and variations in active detachment levels along strike. The stratigraphy is dominated by thick carbonate units interbedded with discrete intervals of weak shaly silici-clastic rocks often bearing evaporate horizons, although not salt as such. The main detachment levels occur within these silici-clastic units, notably in the lower Fars (e.g. at Kirkuk), the Gercus-Kolosh, the Jurassic, the lower Triassic-upper Permian, and presumably at one or more levels within the Paleozoic (e.g. Ora Shale) including near the base of the section. This stratigraphic architecture sets up a structural style composed of ramps cutting across the strong carbonates and flats (stratigraphically parallel faults) running in the silici-clastic sections. Where ramps group together they form duplexes composed of narrowed repeated fault slices that build structural relief. The lower detachment into which these gather is called the floor thrust, and the upper detachment isolates the duplex from an overlying carapace of less deformed rocks, the ‘passive roof’ of the duplex. The major salient in the thrust front in the Dohuk region is just such a structure with Triassic through Neogene section uplifted above a duplex within the Paleozoic, the floor thrust running near basement and the roof running in the Mirga Mir-Beduh section. Similar passive roofs occur at the topographic front in the area of Pirmam, Bina Bawi, and Miran. Traps in this productive petroleum province are anticlines in the passive roof above the fault slices in the duplex, such as Shaikan, Behr Bar, Sheikh Adi, Atrush, and Swara Tika. The roof itself contains an older, folded thrust system with flats running within the Paleozoic on the north at the latitude of Gara anticline, and ramps branching upward to several intermediate detachments until it tips out within the Gercus near Sheikh Adi and Shaikan anticlines. The Dohuk Passive Roof also features a reactivated Neogene-and-older normal fault system with locally thickened intervals in the column, notably the Paleogene. These reactivated faults form not only frontal ramps of the folds at Atrush, Sheikh Adi, and possibly Behr Bar, but also some obliquely slipped faults, such as at Betnaar and Simrit.