Dating of Folds by Combining Fold Kinematics, Illite Characterization, and Ar-Ar Illite Dating

Regents Of The University Of Michigan - Ann Arbor, Ann Arbor MI

Investigators

Abstract

EAR-1216750 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.