Lithological Controls on 3D Fold Geometry in Mechanically Layered Rocks

Folding and thrusting are key processes in accommodating shortening in evolving orogens. In the outer parts of mountain belts, the combination of tectonism and sedimentation often leads to viable petroleum systems where folds trap migrating hydrocarbons. A key aspect of the success of these traps is the 3D fold geometry, which must prevent up-plunge hydrocarbon escape. Fold shapes in a multilayered system are governed by the fold mechanism, and the rheology of the layers. In sedimentary environments where laterally heterogeneous thicknesses of interbedded carbonates, clastics, and evaporates may be present, predicting the sub-surface geometry of structures is challenging.

We present field data combined with satellite and seismic interpretation from the Zagros fold and thrust belt of SE Turkey where Neogene shortening has affected an Ordovician to Miocene aged sedimentary pile comprising shales, sandstones, carbonates and minor evaporites. Miocene carbonates capping the sequence allow 3D fold geometries to be ascertained at surface with a high degree of confidence, whilst the underlying ~1km of shales and mudstones allows folds to develop that depart substantially from standard text-book geometries. Beneath these incompetent units a further 1.7km of carbonates and clastics overlie Ordovician shales up to 1.5km thick. In relatively low strain sections asymmetric, angular, kink-like folds form long wavelength structures and thrust faults rarely reach the surface. With increasing strain, the wavelength/amplitude ratio decreases and thrusts cut through the fold limbs.

Folds are interpreted to detach above a thick sequence of Ordovician shales. They originate by buckling of the competent units within the two weaker horizons, with initial perturbations probably provided by sedimentary heterogeneities. As the folds amplify, thrusts form in the more competent units above the shale. These then propagate upwards with fold amplification dominated by fault-tip tri-shear. The mechanical heterogeneity between the near surface shales and overlying carbonate leads to the kink-like geometries seen at surface. In this model, thrust formation is governed by the location of the initial buckle folding. Since the thrusts mostly originate from the top of the Ordovician shales, they form a linked system detaching into the Ordovician so that displacement can transfer from one structure to the next along strike of the orogen. It is the interaction of folds with varying amplitude which is essential in the identification of viable hydrocarbon traps. Incorporation of numerical models which allow ductile folding and brittle failure with these field and remote sensing based studies will inform further exploration in similar areas and allow investigation of the effects of parameters such as lithological thickness on the location and geometry of structures in collisional orogens.

KEYWORDS: Folds and folding, Rheology: crust and lithosphere, Continental tectonics: compressional, Continental structures.