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

Abstract

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. 1M d -illite polytype (with CS of 6-9 nm) dominates in two folds in the TMB while 2M 1 -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)

“…The studied clay samples in central Mexico were collected along the same Aptian-Albian shale horizon and in all cases these samples provided welldefined Ar-Ar illite ages that were younger than deposition. This is in good agreement with textural observation showing only one generation of authigenic illite in these rocks (Fitz-Díaz et al, 2014).…”

“…Based on microthermometry of fluid inclusions in syntectonic veins and vitrinite reflectance, temperatures within the Tampico-Misantla Basin ranged from 80 to 180 °C, and in the Zimapán Basin ranged from 220 to 250 °C during deformation (Gray et al, 2001;Fitz-Díaz et al, 2014). Since the basins did not reach temperatures necessary for a TVRM, we conclude that the remanent magnetization in the Tampico-Misantla Basin is a chemical remagnetization.…”

“…The Zimapán Basin and Tolimán Sequences, farthest to the west in the study area, have deformation ages ranging from 83.5 ± 1.5 to 76.5 ± 1.0 Ma (determined by Fitz-Díaz et al, 2014). The coeval remagnetization is of normal polarity and its directions are concentrated to the northwest and down.…”

“…The absolute age of folds was determined by clay grain-size separation, illite polytype characterization, and 40 Ar/ 39 Ar dating of multiple size fractions (Fitz-Díaz and van der Pluijm, 2013; Haines and van der Pluijm, 2008). Ages of thrusting were determined for the Tolimán Sequences (83.5 ± 1.5 Ma) on the western edge of the study area, the western and eastern Zimapán Basin (82 ± 0.5 Ma and 76.5 ± 1.0 Ma, respectively), and the western and eastern Tampico-Misantla Basin (64 ± 2.0 Ma and 43.5 ± 0.5 Ma, respectively); see Figure 2 for details (Fitz-Díaz et al, 2014). The ability to successfully date mesoscopic folds made this an ideal area to test the feasibility of absolute dating of synfolding remagnetizations.…”

“…Illitization associated with flexural folding of carbonate-shale successions in the study area (central Sierra Madre Oriental, Mexico) was examined (Fitz-Díaz and van der Pluijm, 2013;Fitz-Díaz et al, (2014). Scanning electron microscopy, X-ray diffraction, stable isotope, and geochronological analyses of samples from several folds showed that illite grew along shear-related horizons during folding.…”