Dating Transpression and Extrusion at Mid-Crustal Depths

自Sanderson和Marchini（1984）的原始模型以来，大陆碰撞带的挤压和挤压理论模型已经有了显著的发展，主要是由对真实挤压带的建模和分析推动的。中地壳挤压和挤压带展示了一系列结构，这些结构记录了明显不同的有限应变状态，暗示了收缩、扭曲和伸展，这是一个天然的实验室，可以测试挤压和挤压模型的基本方面：通过精确的地质年代学来测试整个系统是否发生了同步或顺序变形。在一定尺度上的制图和构造分析表明，阿巴拉契亚造山带北部的逆挤压/挤压系统包含经典逆挤压模式的元素，为更复杂的三斜对称系统提出的元素，以及模型未预测的元素。沿边界的联系运动学和缺乏套印关系与一些元素是同时划分的右旋变形带的组成部分相一致，而其他元素似乎是在变形历史中较晚发展或在其他区域的变形中持续时间较长。地质年代学方法应用于有策略选择的样品，将用于确定挤压/挤压系统内变形的绝对时间。将完成伪剖面分析和常规矿物学地温测量，以获得解释地质年代学的温度约束条件和前中地壳背景目前暴露水平的深度约束条件。将对系统的每个元件进行应变和涡度分析，以评估同轴与简单剪切的贡献。像阿巴拉契亚山脉这样的山脉带是大陆板块碰撞的结果。由于碰撞带的挤压应力，地壳可能垂直隆起和/或横向被推到一边（挤压）。充分了解山带形成的碰撞历史和机制需要：(1)检查岩石的结构和矿物学特征，这些特征在地图上（公里到数百公里）、露头（米到数百米）、手工样品和微观尺度上表示；(2)利用放射性测年法确定地质事件的绝对年龄。我们之前在马萨诸塞州中部的阿巴拉契亚山脉北部进行的研究发现了倾斜（侧击）的特征，而不是正交（正面）碰撞，这是普遍的范式。拟议的研究将涉及由一组地球科学家进行实地和实验室研究，包括肯塔基大学的一名教员、博士后研究助理、研究生和本科生，与其他三家机构的教员和当地学校系统的一名中学地球科学教育者合作，收集、测量和解释必要的数据，以区分两种碰撞替代方案。为了进行地质年代学研究，需要精确地确定地壳变形发生的时间。项目活动将促进初级地球科学家的专业发展，研究结果将在专业会议和同行评议的期刊上传播。

Theoretical models for transpression and extrusion in continental collisional zones have evolved markedly since the original model of Sanderson and Marchini (1984), driven largely by modeling and analysis of real zones of transpression. A zone of mid-crustal transpression and extrusion exhibiting a range of structures that record markedly different states of finite strain suggestive of contraction, wrenching, and extension serves as a natural laboratory to test a fundamental aspect of transpression and extrusion models: testing via precise geochronology whether simultaneous vs. sequential deformation occurred across the system. Mapping and structural analysis at a range of scales demonstrate that the transpression/extrusion system in the northern Appalachian orogen contains elements of classical transpression models, elements proposed for more complex systems of triclinic symmetry, and elements not predicted by models. Linked kinematics along boundaries and lack of overprinting relationships are consistent with some elements being components of a simultaneous partitioned dextral transpression zone, whereas other elements appear to have developed later in the deformation history or outlasted deformation in the other zones. Geochronologic methods applied to strategically selected samples will be used to determine absolute timing of deformation within the transpression/extrusion system. Pseudosection analysis and conventional mineralogic geothermobarometry will be completed in order to obtain temperature constraints for interpreting geochronology and depth constraints for the present level of exposure of the former mid-crustal setting. Strain and vorticity analysis in each element of the system will be performed in order to assess contributions of coaxial vs. simple shear.Mountain belts such as the Appalachians are the expression of colliding continental plates. As a result of the compressional stresses in collision zones, the crust may be uplifted vertically and/or pushed aside laterally (extruded). Full understanding of the collision history and mechanism of mountain belt formation requires (1) examination of structural and mineralogical features in rocks that are expressed at the map (kilometers to hundreds of kilometers), outcrop (meters to hundreds of meters), hand sample, and the microscopic scale; and (2) determination of the absolute age of geologic events via radiometric dating methods. Our previous research in the northern Appalachians of central Massachusetts identified features suggestive of oblique (glancing blow), rather than orthogonal (head-on) collision, which is the prevailing paradigm. The proposed research will involve field and laboratory studies by a team of geoscientists, including a faculty member, post-doctoral research associate, graduate student, and undergraduate student from the University of Kentucky, working in collaboration with faculty at three other institutions and a secondary earth science educator from the local school system, to collect, measure, and interpret the data necessary to distinguish between the two collision alternatives, and to carry out the geochronology needed to determine precisely when the deformation of the crust occurred. The project activities will further the professional development of the junior geoscientists and the research results will be disseminated at professional meetings and in peer-reviewed journals.