Transpressional deformation of a fold in the Zagros Mountains of Kurdistan, Iraq: Data, speculation, and implications for both local and regional prospectivity.

Results of a recent Hess/Petroceltic/GRL field excursion have dramatically altered our geologic maps, our subsurface interpretation, and our working hypothesis for the structure and evolution of the Bradost anticline. In the core of the structure, a nearly vertical fault juxtaposes moderately dipping Lower and Middle Jurassic strata against near vertical middle Cretaceous Carbonates. The resulting geometry, at the surface, is a plunging, three-way dip closed nose that terminates up-plunge against a steep fault: very different than the four-way dip closed anticlines and thrusts that comprise its surroundings. We interpret the steep fault to have predominately strike slip offset. It cuts across and deforms a pre-existing fold (paleo-Badost) with right-lateral displacement. Related thrusts suggest both a transpressional nature and a flower structure fault geometry at depth. Additionally, the sense of stratigraphic separation changes laterally along the surface trace of the fault, consistent with both strike-slip and transpression. Additionally nearby thrusts and en-echelon normal faults are consistent in both orientation and location with a strike slip interpretation.

In an alternate interpretation, dip-slip offset of an overturned fold on multiple faults might have generated similar results, but that would have required an unlikely, along trend, combination of thrusts and normal faults. A key exposure for our interpretation is located within a natural amphitheatre carved by erosion of a bend in the fault near the surface. We speculate that this bend occurs where strike-slip motion was transferred from a steep fault onto a pre-existing, less steeply dipping thrust. Our hypothesis, if true, implies that the structural evolution of the northern Zagros has had a multi-phase history with early formed compressional structures modified and/or dissected by subsequent transpression. While some folds may have initiated during the early phase of shortening, certain structures may have either formed or have been strongly modified during the second transpressional phase of deformation, leading to differences in both structural styles and petroleum systems timing.