Development of Detachment Folds in the Mexican Ridges Foldbelt, Western Gulf of Mexico Basin

Abstract

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. Our analysis reveals that 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 versus 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 ~8%, softening/plastic yielding around hinges results in the development of sharp fold profiles. Limbs lock their dips at 35°–45°, suggesting that growth in this stage is permitted by hinge mobility along ramps and blind faults. Moreover, hinge migration causes fold development to accelerate spontaneously. These findings suggest that conclusions relating periods of accelerated erosion/uplift in contractional structures to tectonic processes should be treated with caution.

Autogenic Organization of Syn‐Tectonic Sedimentary Patterns in Deepwater Foldbelts: A Simple Dynamic Model

“…This process created a foldbelt that is ~110‐km wide and accommodates ~11.8 km of tectonic transport along the subhorizontal detachment. The foldbelt consists of twelve buckle folds which vary from very low strain detachment anticlines ( ε ~3%) to moderate strain detachment folds ( ε ~16%; Figure ; Yarbuh & Contreras, ; Yarbuh et al, ). Moreover, these authors observed that fold shape, amplification rate, and deposition of growth strata vary in a predictable manner with the degree of distortion of the folds.…”

“…In our model both deformation and superficial mass transport are governed by the conservation of mass law rather than by conservation of momentum or sediment transportation mechanics. We use a semi‐empirical model of continuous flexural flow folding that satisfies a series of scaling laws observed in the MRFB that relate the degree of fold amplification and wavelength selectivity to sedimentary thickness and strain (Yarbuh et al, ). We further constrain the folding velocity field v ( x , y ) through the continuity equation governing the deformation of incompressible flows …”

“…Equations and state that flow is the result of the superposition of several deformation modes (Yarbuh et al, ) whose activation and rate of amplification are a function of strain ε (equation ). In those equations, v x and v y are the horizontal and vertical components of the folding velocity field, respectively.…”

“…Only when deformation reaches 8%, the higher deformation modes become active. A simple model that captures this behavior is the following: where H is the Heaviside (step) function, ε * ≈ 8% is the critical strain at which higher deformation modes rapidly expand, and α k represents a series of amplification constants that satisfy the relation ∑ α k =1 (Yarbuh et al, ).…”

“…To reduce the number of free parameters to a handful of unknowns, we only recognize the dominant deformation mode ( k = 1) and its first higher harmonic ( k = 2). The reason is these two deformation modes jointly explain 85% of the geometric variability of the structures of the MRFB (Yarbuh et al, ). Thus, the number of free parameters reduces to α 1 , α 2 , and the critical slope S c , which we calibrated by trial and error.…”