The Mexican Ridges Fold Belt, Gulf of Mexico: Deformation, Dynamics of Deposition of Growth Strata, and the Delay of Sedimentary Response to Tectonic Forcing
Abstract:Deformation of the Mexican Ridges fold belt (MRFB), western Gulf of Mexico, initiated in the Late Neogene in response to normal faulting along the Quetzalcoatl Extensional System offshore Veracruz. Previous authors analyzed stacking patterns of growth strata concluding deformation occurred in two stages: the western section of the fold belt developed during the Upper Miocene whereas the eastern part became active during the Lower Pliocene. Here we analyze a regional seismic line to determine whether deformation migrated progressively eastward. In contrast to previous studies we do not use stacking patterns but excess area. This parameter provides direct information of both linear shortening, and superficial mass transport. We construct excess-area plots for each of the folds comprising the MRFB; from them we estimate the shortening and the degradation path of the seafloor deformed by folding. Moreover, by assuming denudation is in steady state, we are able to differentiate sediments derived locally from sediments transported from distant sources.
Results show tectonic transport in the MRFB is 11.8 km; shortening of individual folds ranges 3-16%, with an average strain for the entire MRFB of ~10%; structures grew at a mean uplift rate of 0.2 mm/yr. We estimate the constant of mass diffusivity, which controls the rate of degradation, has a mean value of 0.27 m2/yr. This value is characteristic of rapid, episodic mass movements. Finally, the sedimentation rate is ~0.2 mm/yr. Those parameters, however, are not constant; they decrease toward the deepwater portion of the fold belt. The structures proximal to the continental shelf are rising rapidly and are being degraded more intensely than those in the distal part of the fold belt.
Our results reveal that deformation started synchronously throughout the MRFB during the Late Miocene and not in two episodes as previously reported. The reason for the seeming discrepancy is due to the copious sedimentation in the eastern section of the fold belt with respect to tectonic uplift. This hindered the development of onlap and thinning upward patterns that commonly signal the beginning of fold growth. The delay, in turn, introduces a bias toward younger ages in the identification of the pre-growth/growth strata limit.