Browsing by Subject "Rock deformation--Mexico, Gulf of"

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Evolution of Neogene contractional growth structures, southern Gulf of Mexico
(1999) Aranda García, Mario; Marrett, Randall
Structural analysis of four contractional growth structures in the Southern Neogene Belt of Mexico was undertaken to test conceptual models of folding mechanisms for contractional deformation and to obtain structural interpretations of important oil prospects and one oil field. Based on seismic data, application of these models provides coherent explanations of structural development incorporating the geometry of the structures as well as the geometry and age of the growth strata, as shown in viable balanced cross-sections, and demonstrate a complex evolution of folding mechanisms in a 3D framework. Folding and faulting are important mechanisms of deformation in the brittle upper crust, and in particular are the main processes of fold-thrust belt development. Excellent examples for the geometries developed in these belts have been documented around the world. However most of them lack growth strata, due mainly to their deep levels of erosion, and for this reason their development between undeformed and final geometry is weakly supported with data. Growth strata are those units deposited during deformation, and ideally they record the entire evolution of a contractional terrain. In modem thrust belts preservation of growth strata is uncommon; marine growth sediments have better potential for preservation of internal and external features than do continental growth sediments. Some excellent examples of contractional growth structures from the southern Gulf of Mexico are used here to test fault-related folding and buckle folding models and to obtain detailed interpretations of one discovered oil field and some important prospects for oil exploration. The structures analyzed are part of the Southern Neogene Belt of Mexico a thrust belt that represents upper crustal shortening in the southern portion of the North America Plate. The Sardinero, Jujo, Tunich and Catemaco anticlines are located in the Veracruz and Campeche-Tabasco-Chiapas basins. These basins were dominantly filled by upper Eocene-Neogene marine deposits. The regional trend of the structures is NW-SE but this orientation is modified in part where the Jurassic salt is present. The basal detachments for the Sardinero and Catemaco structures are located in Paleogene mudstone units and the detachment for the Tunich and Jujo structures is in Jurassic salt. The structures analyzed were selected because they are good examples of contractional growth structures with good seismic data sets and chronostratigraphic well control. The pre-kinematic strata of Sardinero anticline have geometric similarities with a fault-bend fold structure, but growth strata on the backlimb show a more complex evolution. The growth strata were deposited during an early buckle-folding stage between 15.5-3.5 Ma which was-followed by the development of and translation over a fault ramp after 3.5 Ma. The Jujo anticline prekinematic strata has a good fit with predictions for a fault-bend fold and the geometries of growth strata agree with the model. Structural timing from growth strata indicate folding during the mid-Miocene and Pliocene. The Tunich and Catemaco anticlines correspond to curved detachment folds. They have apparent fault-related folding geometries at pre-kinematic levels but they were initiated by buckle folding as the geometries of their growth strata demonstrate. The age of deformation for both anticlines was between 21 to 2.4 Ma and 16.5 to 5.5 Ma respectively.
Seismic structural analysis of deformation in the southern Mexican Ridges
(1982-05) Pew, Elliott; Muehlberger, William R.
The southernmost region of the Mexican Ridges extends from Bryant's gap near 22.5 N latitude to the Campeche Knolls near 19.0 N latitude. Analysis of 23,030 kilometers of sparker and CDP seismic data from six surveys reveals the existence of two separate areas of folding, Zones 4a and 4b. In the Zone 4a foldbelt symmetrical folds form a gentle salient which parallels the curved outline of Isla de Tuxpan. Structural relief often in excess of 500 meters is reflected by similar bathymetric relief. Fold wavelengths average 10-12 kilometers. A detachment or decollement is interpreted in a thick Upper Cretaceous to Lower Tertiary pelagic shale sequence by the existence of relatively undeformed reflectors below this interval. The 3 to 3.5 kilometer thick allochthonous sheet has experienced approximately 1% shortening and a maximum displacement of 1 to 2 kilometers. The Zone 4a foldbelt appears to be a massive gravity slide. Folded Plio-Pleistocene strata establish the youth of these folds. A large deep-rooted structure of questionable origin is observed on GLG 22. This structure, exhibiting roughly 1500 meters of bathymetric relief, acts as a foreland buttress against which the gliding allochthonous mass deforms. The tightly appressed thrust-faulted folds up dip from the buttress exhibit anomalously short wavelengths. While no folding is observed directly down dip from the buttress, folding is observed 30 to 50 kilometers basinward of this structure just a few kilometers to the south. The boundary separating Zones 4a and 4b is a linear feature oriented transversely to regional strike and may be a tear fault. Reflections at depth are not continuous across this feature. The Zone 4b foldbelt lies directly down dip from the Veracruz Basin. Structural relief commonly doubling that observed in Zone 4a is rarely expressed as bathymetric relief. Individual folds are asymmetric, having gently dipping landward flanks and either steeply dipping or growth-faulted seaward flanks. Fold cores appear to contain diapiric material. Fold growth due to gravity sliding began in the Middle Miocene. Subsequent loading by a thick Middle-Upper Miocene section gradually halted downslope movement and initiated flowage of plastic substrata from beneath loaded synclinal troughs into anticlinal cores. This deformation has continued to the present in some folds.