FOLD DEVELOPMENT IN THE MEXICAN RIDGES FOLDBELT, WESTERN GULF OF MEXICO BASIN: INSIGHTS FROM AREA-DEPTH-STRAIN VARIATIONS AND FOURIER ANALYSIS

Examples of natural folds growing in a homogenous mechanical stratigraphy of alternating competent and incompetent thin layers of fine-and coarse-grained sediments are examined, and the fold growth process is quantified. We have made a detailed study of the deformation of the members and structures of the Mexican Ridges Foldbelt in the western Gulf of Mexico Basin by means of area‐depth‐strain and spectrum diagrams. These are mathematical tools useful to estimate the degree of distortion of folds and energy of deformation modes of various wavelengths caused by compressional tectonic forces. Our analysis reveals the overall response to loading of siliciclastic sequences corresponds to that of flexural flow and parallel-to-bedding heterogeneous pure shear. Folds start out as low-amplitude sinusoidal disturbances that rapidly become finite-amplitude folds of heterogeneous strain. We also derive the following scaling relations: i) degree of amplification scales with both the height above the detachment and strain; ii) wavelength selectivity broadens with increasing strain; and iii) deposition of syn-sedimentary geometries is function of strain. These relations are a natural consequence of idealized area-preserving laws of fold growth. From these results we devise a method to estimate fold strain by means of an amplitude vs. depth diagram. We are also able to define a progression of fold shape change as a function of the fundamental parameter strain. Initially, structures grow by limb rotation and the selective amplification of a single dominant wavelength giving rise to sinusoidal folds. When strain reaches ca. 8%, softening/plastic yielding around hinges result in the development of sharp fold profiles, activating higher harmonics of the fundamental folding mode. Limbs lock their dips at 35°-45° suggesting growth in this stage is permitted by hinge mobility along ramps and blind faults. Folds rapidly acquire an angular, chevron‐like geometry and undergo accelerated uplift without the need of incrementing external tectonic forces. These findings suggest that conclusions relating periods of accelerated erosion/uplift in contractional structures to tectonic processes should be treated with caution.DOI: 10.1029/2017TC004948