The (trans)formation of illite in fault zones at high strain has been addressed in a number of studies around the world in the last decade. The reactions allowing this application to fault gouge also happen during folding. Central to the proposed project is to analyze illite in clay rich layers that were folded by flexural slip/flow. Flexural slip is a folding mechanism commonly observed in multilayers where competent (e.g. limestone or quartzite) and incompetent layers (e.g. shale and bentonite) alternate. In flexural folding the fold limbs of competent layers rotate in opposite directions about the fold hinge line, involving considerable amounts of shear parallel to bedding in the incompetent layers; in symmetric folds this shear is concentrated in the limbs and is close to zero in the hinge. Careful analysis of deformation in flexural folds, of illite transformation on the limbs and hinges, and determination of illite ages along the folded layer provide valuable spatio-temporal information on fold evolution, both locally and regionally.
In this project, the researchers propose to develop and apply a method for obtaining absolute ages of folds that formed under low grade metamorphic conditions using Ar dating of clay-size fractions. In this application of illite dating, the researchers utilize the assumptions that illitization is a process induced by deformation at low temperature and that the potassium hosted by illite in its structure will allow the identification of accurate ages of illite with encapsulated Ar dating (as a decay product of potassium). The method will be applied in a well-documented section of the Mexican Fold-Thrust Belt (MFTB) in Central Mexico, which involves Cretaceous basinal carbonate units that are characterized by alternations of thinly bedded limestone, shale, bentonite and chert. A pilot study on a representative fold demonstrated the likelihood for success. Similar flexural folds have been observed along a cross section of the entire fold-thrust belt, exhibiting a variety of styles, and having formed under a range of temperature conditions (100-250°C). By analyzing folds from different locations along the section the researchers will be dating deformation within a suitable temperature window and with different fold geometries, as well as potentially obtaining rate dates for individual structures.
The approach is likely to provide a powerful new tool for geologists to apply to shallow fold-thrust belts around the world. Fold-dating can also be applied to other tectonic settings, such as in restraining bends in lateral fault systems, and deformation associated with normal faults in extensional systems. In addition, the project will support a young researcher by developing new research skills that may have broad application in structural geology, and will offer hands-on research experiences for senior undergraduates.
We developed an isotopic method for fold dating that combines XRD quantification and Ar chronology of small illite minerals to determine absolute ages of deformation. Folds in the frontal segment of the Mexican Fold-Thrust Belt (MFTB) were used to successfully apply the method and demonstrate its future potential. In addition to fundamental insights in crustal evolution, understanding foreland fold-thrust belts is critical to the mineral and petroleum industries that support today’s economies and the daily needs of the planet's citizens. Moreover, the project was key to the professional development of a young, Latina scientist, toward a more diverse presence in STEM research and education. She has since moved from a post-doctoral fellow position at the University of Michigan to a junior faculty position at the Universidad Nacional Autónoma de México. Variations in mineral composition, illite-polytype, crystallite-size and Ar total gas ages were analyzed in the limbs and hinge of two mesoscopic folds the frontal segment of the Mexican Fold-Thrust Belt (MFTB). This examines potential effects of strain variation on illitization and the Ar isotopic system along folded layers, and possible regional thermal overprints. The Ar total gas ages for 9 samples in Fold 1 vary between 48.4 and 43.9 Ma. The % of 2M1 (detrital) illite vs. Ar total-gas ages tightly constrains the age of folding at 43.5±0.3 Ma. Nine ages from three samples in Fold 2 range from 76.2 to 62.7 Ma, which results in a folding age of 63.9±2.2 Ma. Both ages are in excellent agreement with more broadly constrained stratigraphic timing. Subsequently, we expanded our approach to a cross-section that involves folded Aptian-Cenomanian shale-bentonitic layers interbedded with carbonates of the Zimapán (ZB) and the Tampico-Misantla (TMB) Cretaceous basins in central-eastern Mexico. Basinal carbonates were buried by syntectonic turbidites and inverted during the formation of the MFTB in the Late Cretaceous. Results from three chevron folds and two 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 dominates in two folds in the TMB while 2M1-illlite dominates in the third fold, in the ZB and in one shear zone. From west (higher grade) to east (lower grade) the results are: Ar retention ages indicate shearing occurred at ~83.5 Ma in the westernmost shear zone; folding at ~82 Ma in the ZB with subsequent localized shearing at ~77 Ma; Ar total gas ages constrain the time of folding at ~64 Ma on the west side of the TMB and ~43.5 Ma on the eastern edge. These results are in excellent agreement with the age and distribution of syntectonic turbidites, and indicate episodic progression of deformation from west to east, further demonstrating the validity of our approach. Our method (outlined in Figure 1) is a novel approach to radiometric dating of deformed, clay-bearing units, with excellent potential to constrain dates and rates of regional and local deformation, both along and across foreland orogenic belts worldwide. To date, the outcomes are presented in two peer-reviewed papers and in multiple presentations at major scientific meetings and workshops. Future contributions are exploring correction methods for Ar dating and application elsewhere in Mexico’s MFTB.
