THE ROLE OF MULTIPLE WEAK LITHOLOGIES IN THE DEFORMATION OF COVER UNITS IN THE NORTHWESTERN SEGMENT OF THE ZAGROS FOLD-AND-THRUST BELT

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.