Dating Transpression and Extrusion at Mid-Crustal Depths

自Sanderson和Marchini（1984）的原始模型以来，大陆碰撞带的平移挤压和挤压理论模型有了显著的发展，这主要是由对真实的平移挤压带的建模和分析所驱动的。一个中地壳的平移挤压和挤压带，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，暗示着收缩、扭曲和伸展，这是一个天然的实验室，可以测试平移挤压和挤压模型的一个基本方面：通过精确的地质年代学测试整个系统是否发生了同步变形和顺序变形。在一系列尺度的映射和结构分析表明，在北方阿巴拉契亚造山带的transpressure/挤压系统包含经典transpressure模型的元素，元素提出了更复杂的系统的三斜对称，和元素没有预测的模型。链接运动学沿着边界和叠印关系的缺乏是一致的，一些元素是一个同时分区的右旋平移挤压带的组成部分，而其他元素似乎已经开发出后来的变形历史或在其他地区的变形。地质年代学方法适用于战略选择的样品将被用来确定绝对的时间内的变形挤压/挤压系统。将完成假剖面分析和常规矿物地质温压测定法，以便获得解释地质年代学的温度限制和前中地壳背景目前暴露水平的深度限制。将对系统的每个要素进行应变和涡度分析，以评估同轴剪切与简单剪切的作用。阿巴拉契亚山脉等山脉是大陆板块碰撞的表现。由于碰撞区的挤压应力，地壳可能被垂直抬升和（或）被横向推开（挤压）。要全面了解碰撞历史和造山带形成机制，需要（1）检查岩石中的结构和矿物学特征，这些特征在地图（公里到数百公里）、露头（米到数百米）、手样和显微镜下表现出来;（2）通过放射性测年方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北方阿巴拉契亚山脉的研究发现了暗示倾斜（掠射）的特征，而不是正交（正面）碰撞，这是流行的范例。拟议的研究将涉及一个地球科学家团队的实地和实验室研究，其中包括来自肯塔基州大学的一名教员、博士后研究助理、研究生和本科生，与其他三个机构的教员和当地学校系统的一名中学地球科学教育工作者合作，收集、测量、并解释必要的数据，以区分两种碰撞选择，并进行所需的地质年代学，以准确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，研究成果将在专业会议和同行评审的期刊上传播。

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.