Three-Dimensional Fold Geometries in N Iraq and SE Turkey: Oblique Collision between Arabia and Eurasia

The Zagros-Taurus fold and thrust belt developed in response to late Mesozoic ophiolite obduction and Cenozoic oblique continental collision between Arabia and Eurasia. Many of the resultant folds are large elongate four-way closing structures that are easily delineated in regional satellite data. However, the relatively simple first-order fold geometry seen at regional scales often masks considerable structural complexity when analysed in more detail, including scales that are important in predicting the performance of potential hydrocarbons systems. Typical complexity involves very rapid along-strike variation of a number of geometrical fold parameters, including: the dip of fold limbs (from shallow, to upright, to overturned); the number of anticlinal fold hinges (i.e. parts of some folds have box-like geometries, with multiple straight-limb segments and narrow hinge zones; others have more rounded profiles); and fold tightness (open, to tight, isoclinal, and elasticas). Apparent along-strike differences in stratigraphic thickness may not necessarily be caused by primary depositional variations, but in places may be due to high mobility of incompetent mud-prone units, which have high competency contrast relative to the regionally extensive carbonates which appear to control the mechanical stratigraphy. Further structural complexity arises due to the difficulty of accommodating shortening when folds die out laterally, and also due to interactions between adjacent, overlapping periclinal folds. This emphasises that fold geometries seen in 2D are not a reliable indicator of trap potential or effective fold amplitude. Although appraisal methods based on analysis of satellite data are extremely powerful, in general, three-dimensional fold structures cannot be adequately understood without field work to constrain interpretations.
Keywords: Structural traps; Fold closure; Kurdistan; Northern Zagros