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.

We analyse a regional 2D seismic section of the Mexican Ridges foldbelt (MRFB), western Gulf of Mexico, and construct excess-area diagrams for each of the structures comprising the foldbelt to estimate shortening, the onset of folding and the degradation of the folded seafloor. From the chronostratigraphy, we derive rates of tectonic and superficial mass transport and illustrate how they change across the MRFB. The resulting tectonic transport in the MRFB is 11.8 km forming a train of twelve buckle folds above a detachment at a depth of ca. 6 s of two-way travel time, with an average strain of ca. 10%. The fold train grew at a mean uplift rate of ca. 0.21 mm year À1 . Cross-sectional balancing demonstrates that shortening balances the downslip motion of the Quetzalcoatl extensional system (QES), suggesting that horizontal compaction, volume loss and other penetrative deformation mechanisms are negligible. By assuming steadystate denudation, we are able to distinguish sediments derived locally from sediments transported from distant sources. The constant of mass diffusivity, a parameter controlling the degradation rate, is ca. 0.42 m 2 year À1 , which is characteristic of rapid, episodic, superficial mass movements. The combined sedimentation rate from both, local and distal sources is ca. 0.23 mm year À1 . Those values are not constant; structures proximal to the continental shelf are rising rapidly and are being degraded more intensely than those in the distal part of the MRFB, where sedimentation outweighs tectonic uplift. Our results indicate deformation initiated up to 3 Myr earlier than estimated from stacking patterns. Moreover, we find deformation started synchronously during the Late Miocene throughout the MRFB and not in two episodes as the stacking relations suggest. The discrepancy can be explained by a delay in the sedimentary response to folding. During early fold growth, nearly constant thickness strata are deposited before a progressive unconformity and other converging geometries develop. The development of growth strata is fast in the folds near the QES, which are being uplifted rapidly and degraded vigorously. Under these conditions, the stratigraphic relations give only a broad estimate of the pretectonic/syntectonic limit when compared to the excess-area method. On the other hand, the development of growth strata took twice as much time for folds near the abyssal plain, which are being uplifted at a slower rate and where degradation is less intense. Consequently, the delay takes more time, and the use of stratigraphic relations introduces an even more pronounced bias towards younger ages in the identification of the onset of folding.Correspondence: Ismael Yarbuh,

Los remolinos son estructuras transitorias que influyen en gran medida en la circulación promedio del océano. Modifican la distribución de masa y propiedades como calor, sal, clorofila y partículas inertes. La capacidad que tienen los remolinos para transportar propiedades o partículas depende de su capacidad de retención. En este estudio se identificaron y caracterizaron los remolinos de mesoescala del noroeste del golfo de México (NOGM) a través de un método lagrangiano que permite evaluar el tiempo de retención y la fracción de masa que pueden retener y transportar. Para el análisis, se utilizaron datos diarios de altimetría de 1993 a 2016. En el periodo de estudio se detectaron un total de 254 remolinos, 73 anticiclones y 181 ciclones. Se identificó una región (94.75º W, 26.75º N) donde ocurren ~30% del total de los remolinos ciclónicos detectados entre las isóbatas de 1,000 y 2,500 m. En promedio, el radio de los remolinos fue de ~40 km para la isobata <1,000 m y ~70 km para la isobata >2,500 m. Los remolinos de mesoescala del NOGM pueden trasportar ~60% de la masa que contenían al momento de ser detectados. En promedio, el transporte de masa ocurrió por 33 d para los ciclones y por 26 d para los anticiclones. Rara vez ocurrió por 60 d o más.

En este trabajo realizamos un análisis cinemático del Sistema Extensional de Cerralvo (SEC), al sur del Golfo de California (GC) y proponemos un modelo de extensión cortical a partir del procesado e interpretación de 57.4 km de datos de sísmica de reflexión multicanal en dos dimensiones. La imagen sísmica muestra que en el SEC las fallas principales buzan hacia el este con ángulos entre 10 y 25° formando un rift asimétrico. La magnitud de extensión horizontal calculada es de 28.0 km, lo que corresponde a un factor de adelgazamiento cortical de 2.0. La tasa de subsidencia promedio en el SEC es de 0.3 mm/año, lo que resulta en un transporte tectónico de hasta tres veces mayor en magnitud respecto a la sedimentación marina que alcanza valores de 0.1 mm/año. A lo largo del SEC el basamento acústico está delimitado por una capa con reflectores semicontinuos, lo cual puede representar fabricas cataclásticas y/o miloníticas asociadas con las fallas normales de bajo ángulo. La deformación penetrativa, el desarrollo de cuencas delimitadas por fallas normales de bajo ángulo y la magnitud de extensión horizontal a través del SEC, sugieren que la extensión cortical debe estar acomodada por un mecanismo de deformación por cizalla simple desde el Mioceno tardío hasta el reciente. Se infiere, además, que el SEC es una prolongación oriental del sistema estructural del Bloque Los Cabos (BLC). Así, nuestro modelo regional propone que en fases avanzadas del proceso de ruptura continental y apertura del GC, el sistema de fallas del BLC y del SEC convergen a profundidad formando una superficie de despegue regional que atraviesa a lo largo de la placa superior, en donde el bloque de techo rota hacia bajos ángulos de buzamiento formando cuencas asimétricas con transporte tectónico hacia el este. Nuestros resultados fueron contrastados con la cartografía del fondo marino al oriente del SEC. Aquí se infiere que en el límite occidental de la cuenca pull-apart Pescadero Norte, la superficie de despegue regional debe adquirir una geometría antilístrica, atravesando la placa inferior formando una zona de cizalla de alto ángulo. Este estilo de deformación podría resultar en la formación de complejos de núcleo metamórfico asociados con estructuras regionales tipo rolling hinge.

We present a dynamical model that links through multiple length and time scales the formation of detachment folds, degradation of topography, and deposition of sheet‐like sedimentary bodies in deepwater foldbelt environments based on a nonlinear reaction–diffusion equation. We represent the stratigraphic response predicted by our model in a two‐dimensional parametric space whose axes are given by the Peclet number (Pe), the ratio of the mass transported by tectonic uplift and the mass transferred by diffusive hillslope processes, and the sediment delivery number (Sd), the ratio of sediments transported by density currents to those produced by intrabasinal processes. Notably, we identify three well‐defined regions of sedimentary behavior in that parametric space: a central domain (Pe > 2,Sd > 2) of cyclic stratigraphy produced by self‐sustained oscillations in the sedimentary system and two regions near the graph axes of stable sedimentation in which a continuous stratigraphy is produced. In the central domain, all the mass fluxes are equally important. This creates the conditions for a series of complex interaction between hillslope sediment transport, fold growth, and sedimentation. The other two regions correspond to end‐members in which either the Pe or Sd is small. For small Pe values, topographic degradation and sediment dispersal by hillslope transport dominate over tectonic uplift. Thus, any tectonic perturbation appearing in the system dissipates readily. For small Sd values, sediment transport is dominated by creep, and the system rapidly evolves toward a steady state.

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&lt;p&gt;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&amp;#176;&amp;#8211;45&amp;#176;, 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.&lt;/p&gt;