Abstract
The geometry, kinematics and dynamics of fold‐and‐thrust belts are strongly influenced by the mechanical behaviour of the basal décollement. However, many fold‐and‐thrust belts also include mechanically weak lithologies such as evaporites and marls or mudstones at different levels within the shortened stratigraphy. The kinematics and dynamic evolution of these thrust belts are controlled by the mechanical behaviour both of the basal décollement and of the weak units embedded within the overlying stratigraphic succession. In the Zagros fold‐and‐thrust belt (ZFTB), the shortened sedimentary cover is between 7 and 12 km thick and mechanically weak lithologies compartmentalize the stratigraphic column at shallow and intermediate levels. In this paper, satellite, field and seismic data from the Kurdistan Region of Iraq are used to identify structures of different sizes and surface traces. The observations are used to underline the role of mechanically weak horizons within the Zagros stratigraphy and the decoupling of deformation both laterally and with depth in the belt.The decoupling between shallow and deeper structures observed in seismic profiles from the Kurdistan Region of Iraq is also reported from field observations from the Iranian part of the Zagros fold‐and‐thrust belt, where folds with different surface traces occur. Decoupling between shallow and deep layers by incompetent lithologies at intermediate depths (e.g. marls, mudstones and evaporites) results in the formation of disharmonic folds. The geometry, size and location of such folds may differ between outcropping and subsurface structures. Decoupling may have a significant impact on hydrocarbon exploration in different parts of the Zagros fold‐and‐thrust belt due to potential offsets between outcropping and subsurface structures and their associated traps.
Highlights
The geometry, kinematics and dynamics of fold-andthrust belts are strongly influenced by the mechanical behaviour of the basal décollement (Davis and Engelder, 1986)
For example in the sector of the Zagros foldbelt in SW Iran, the Middle Miocene Gachsaran evaporites acted as a shallow décollement and decoupled the middle Miocene and overlying layers from the underlying Mesozoic and Palaeozoic units (O’Brien, 1957, Stöcklin, 1968; Kent, 1970; Murris, 1980)
In the examples from the Zagros fold-andthrust belt shown above, even though the spacing between the competent layers is in some cases less than the “dominant” wavelength, the stratigraphy comprising both competent and incompetent lithologies does not always fold as a single multilayer
Summary
The geometry, kinematics and dynamics of fold-andthrust belts are strongly influenced by the mechanical behaviour of the basal décollement Regional stratigraphy of the Zagros: presence of incompetent lithologies The Phanerozoic stratigraphic column of the Zagros basin contains evaporites and other mechanically weak lithologies (e.g. mudstones, argillaceous carbonates and marls) at different levels (Fig. 2) These weak horizons include the Infracambrian Hormuz Salt located at the base of the sedimentary cover, together with Middle Miocene evaporites which are referred to as the Gachsaran Formation in Iran and the Lower Fars Formation in Iraq (O’Brien, 1957; James and Wynd, 1965; Stocklin, 1968; Beydoun, 1991; Bahroudi and Koyi, 2004; Aqrawi et al, 2010; Najafi et al, 2014). Potential detachment levels in the Triassic succession in the Kurdistan part of the Zagros include the anhydrite-dominated lower part of the Kurra Chine Formation (Carnian-Norian), the marl-rich Lower Triassic Beduh Formation, and the
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