Elisa Fitz-Díaz

The role of folding in the development of the Mexican fold-and-thrust belt

The Mexican fold-and-thrust belt in central Mexico has overall characteristics that fit the critical tectonic wedge model. It is thin-skinned, forward propagating, tapers toward the toe (the east), and displays an overall decrease in deformation toward the toe. The internal structures and heterogeneity of deformation are not typical of fold-and-thrust belts, however, due to the presence of two large carbonate platforms, flanked by more thinly bedded basinal carbonates. Kilometer-scale thrusts dominate deformation in the platform carbonates (a more brittle behavior), and mesoscopic buckle folds and associated cleavage dominate deformation in the basinal carbonates (a more ductile behavior). Total shortening across the belt, including both platforms and basins, is ∼55%–65%, with higher values in the basins than in the platforms and a concentration of deformation near the platform borders. The dominant mechanism of folding in the basinal rocks is buckling, with thin chert horizons behaving as single layers and limestone and shaly limestone interbeds buckling as multilayers, with a dominant chevron style. A significant shear component of the deformation is indicated by monoclinic fold symmetry, with a consistent sense of vergence of top toward the foreland. We estimated strain and strain history from mesoscopic analysis of fold geometry and internal strain distribution at several locations across the basin and used this information used to assess the overall kinematics and progressive deformation in the basins, which involve both shortening and shear components. The implications of this for the kinematics of the fold-and-thrust belt are discussed.

Comparison of tectonic styles in the Mexican and Canadian Rocky Mountain Fold-Thrust Belt

Abstract Despite the fact that most fold–thrust belts around the world share many features, successfully explained by the critical wedge model, the details of their geometric evolution and tectonic style development are poorly understood. In the classic section of the southern Canadian Rocky Mountains the dominant tectonic style consists of imbricate thrust sheets with relatively little internal deformation of the individual slices. In the Mexican fold–thrust Belt (Central Mexico), the age of deformation, the overall structural pattern and the total amount of shortening are similar, but the individual thrust sheets exhibit much more internal deformation as manifest by metre-scale buckle folds. One of the differences between these localities is the lateral variation of facies resulting in massive platform limestone separated by thinly-bedded basinal limestone in the Central Mexico section. Strain is concentrated toward the margins between platforms and basins. In Canada, thick platform carbonates form continuous resistant units across the Front Range. Possible reasons for the differences in tectonic style between the two sections include the dominant lithology, distribution of lithologies, taper angle of the tectonic wedges, amount of friction along the basal detachment and the degree of anisotropy of the basin facies rocks.

Progressive, episodic deformation in the Mexican Fold-Thrust Belt (central Mexico): evidence from isotopic dating of folds and faults

We used illite Ar/Ar dating to obtain absolute ages of folds and shear zones formed within the Mexican Fold–Thrust Belt (MFTB). The methodology takes advantage of illite dating in folded, clay-bearing layers and the ability to obtain accurate ages from small-size fractions of illite using encapsulated Ar analysis. We applied our approach to a cross-section that involves folded Aptian–Cenomanian shale-bentonitic layers interbedded with carbonates of the Zimapán (ZB) and Tampico–Misantla (TMB) Cretaceous basins in central-eastern Mexico. Basinal carbonates were buried by syn-tectonic turbidites and inverted during the formation of the MFTB in the Late Cretaceous. Results from folds and shear zones record different pulses of deformation within this thin-skinned orogenic wedge. Mineralogical compositions, variations in illite polytypes, illite crystallite size (CS), and Ar/Ar ages were obtained from several size fractions in limbs and hinges of the folds and in the shear zones. 1Md-illite polytype (with CS of 6–9 nm) dominates in two folds in the TMB while 2M1-illlite (with CS of 14–30 nm) dominates in the third fold, in the ZB, and in the fold/shear zone. From west (higher grade) to east (lower grade): Ar retention ages indicate shearing occurred at ~84 Ma in the westernmost shear zone, folding at ~82 Ma in the ZB with subsequent localized shearing at ~77 Ma, and Ar total gas ages constrain the time of folding at ~64 Ma on the west side of the TMB and ~44 Ma on the eastern edge. These results are consistent with the age and distribution of syn-tectonic turbidites and indicate episodic progression of deformation from west to east.

Dating synfolding remagnetization: Approach and field application (central Sierra Madre Oriental, Mexico)

Growth of magnetite has been variably linked to fluid-bearing events or clay diagenesis, and the development of a chemical remagnetization as a result of such events. In this study we examine remagnetized carbonate rocks from the central Sierra Madre Oriental (the Mexican fold-thrust belt) in order to develop a method for dating synfolding remagnetizations. By combining 40 Ar/ 39 Ar deformation ages with new paleomagnetic results, we present a quantitative method for absolute dating of synfolding remagnetization. We find that the history of the central Sierra Madre Oriental involved two separate remagnetization events in our study area; synfolding remanence acquisition ca. 77 Ma (Late Cretaceous) in the Zimapan Basin and a younger synfolding remagnetization event ca. 44 Ma (mid-Eocene) in the Tampico-Misantla Basin. The growth of magnetite leading to chemical remagnetization detected in these limestones is interpreted as the result of rock interactions with an Fe-bearing fluid.