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

近十年来，世界各国对高应变断裂带中伊利石的变形进行了大量研究。在折叠过程中，允许这种应用于断层泥的反应也会发生。拟议项目的核心是分析由弯曲滑动/流动折叠的富含粘土层中的伊利石。弯曲滑动是一种通常在多层中观察到的折叠机制，在多层中，称职层（如石灰岩或石英岩）和无能层（如页岩和膨润土）交替发生。在弯曲褶皱中，能力层的褶皱分支沿褶皱铰线向相反方向旋转，在能力层中涉及大量平行于顺层的剪切；在对称褶皱中，这种剪切集中在分支上，在铰链处接近于零。仔细分析弯曲褶皱的变形，伊利石在褶皱分支和铰链上的转化，以及沿褶皱层确定伊利石年龄，为局部和区域褶皱演化提供了宝贵的时空信息。在这个项目中，研究人员建议开发并应用一种方法，通过对粘土大小的分数进行Ar定年，来获得在低变质条件下形成的褶皱的绝对年龄。在伊利石定年的应用中，研究人员利用了这样的假设，即伊利石定年是一个由低温变形引起的过程，伊利石结构中含有的钾可以通过包裹Ar定年（作为钾的衰变产物）来确定伊利石的准确年龄。该方法将应用于墨西哥中部墨西哥褶皱冲断带（MFTB）的一个记录良好的剖面，该剖面涉及白垩纪盆地碳酸盐岩单元，其特征是薄层石灰岩、页岩、膨润土和燧石的交替。一项针对代表性折叠的初步研究证明了成功的可能性。在整个褶皱冲断带的横截面上也观察到类似的弯曲褶皱，表现出多种样式，形成于一系列温度条件下（100-250℃）。通过分析沿剖面不同位置的褶皱，研究人员将在一个合适的温度窗口和不同的褶皱几何形状下确定变形的年代，并可能获得单个结构的速率日期。这种方法很可能为地质学家提供一种强大的新工具，用于研究世界各地的浅层褶皱冲断带。褶皱定年也可以应用于其他构造环境，例如侧向断裂系统中的抑制弯曲，以及伸展系统中与正断层相关的变形。此外，该项目将支持一名年轻的研究人员开发可能在构造地质学中有广泛应用的新研究技能，并将为高年级本科生提供实践研究经验。

EAR-1216750The (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.