CHARACTERISTICS AND EVOLUTION OF FOLDS FROM INTEGRATED FIELD DATA AND LAND SURFACE FEATURES: ZAGROS FOLD-THRUST BELT, KURDISTAN REGION-IRAQ

This study concentrates on the deformation style of a number of folds within the NW part of the Zagros fold-thrust belt in Kurdistan Region-Iraq. This fold belt results from convergence of the Iranian and Arabian plates, which started in late Cretaceous. The Kurdistan segment of the fold-thrust belt exhibits recess-type orogenic curvature. The deformation style has been studied using cross-sections drawn from field data, the mechanical stratigraphy of the sedimentary cover units, and the geometry of the folds, fold indices and other land surface features, which are obtained from satellite images.

In the area, a NE-SW shortening was accommodated by an echelon folding style of open to gentle, box shaped folds with associated thrusts in their southwest-dipping forelimbs. The folds have hinge lengths of 23.5-51.4 km, widths of 4-6.5 km and aspect ratios of 4:1-11:1. Based on the relationship of the length of hinge line to fold aspect ratio and fold symmetry index, folds are categorized to detachment folds and fault-bent folds.

The sedimentary cover consists of c. 8 km thick rock units of rigid carbonate interbedded with weak units. The c. 0.7 km thick Cretaceous carbonates control the growth and wavelengths (about 10 km as average) of the folds; while the shale and evaporite units that inter-bedded within Lower Triassic and Lower Jurassic units act as possible detachments. The thicker sedimentary cover in the eastern side of recess led foreland-ward deformation to propagate further than on its western side.

The shortening across the belts was determined to be 14% in the eastern part of the study area and 23% in its western part. The changing orientation of fold hinge lines indicates that the compression direction was changed through time. Changing of fold trend direction from NW-SE direction (Zagroside) in eastern part to nearly E-W direction (Tauruside) tends to be a primary arc from pre-existing Arabian plate edge shape rather than an orocline bending due to rotation of the Arabian plate.