Lower Crustal Deformation and Vertical Coupling and Decoupling in the Continental Lithosphere During Late Orogenic Extension

造山运动后期下地壳变形与大陆岩石圈垂直耦合与解耦合

• 批准号: 0337111
• 负责人: Keith Klepeis
• 金额: $ 22.43万
• 依托单位: University of Vermont & State Agricultural College
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2004
• 资助国家: 美国
• 起止时间: 2004-01-15 至 2007-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0337111&HistoricalAwards=false
• 关键词: Lower; Crustal; Deformation; Vertical; Coupling

A deeply eroded mountain range in western New Zealand provides the highly unusual opportunity for direct examination of a full 50 km thick column of continental crust as it evolved over a 35 million year period. This discovery is especially significant because field observations and numerical models have led geoscientists to emphasize the role of the lower crust in controlling the behavior of deforming continental lithosphere. However, the large number of unconstrained variables in most experiments and in most exposures creates a high degree of uncertainty about the characteristics and consequences of flow in the lower continental crust. This project is determining how strain was partitioned vertically within an ancient orogen as it underwent a transition from crustal thickening and contraction to crustal thinning and extension. The direct observational approach used in this project is possible because the pre-Cenozoic configuration of western New Zealand places crust that formed at unusually deep levels of a Cretaceous orogen (25-50 km depth) next to rocks that represent the middle and upper crustal levels of this same orogen (8-27 km depth). The research involves using the original Mesozoic architecture of the belt to test the contrasting ways in which extensional deformation may be transferred vertically through a rheologically evolving crustal column. Specifically, the project involves measurement of three-dimensional characteristics and kinematics of middle, upper and lower crustal structures that formed during extension between 108 and 190 million years ago, determination if there was kinematic compatibility between simultaneously evolving extensional structures at different crustal depths, establishment of absolute and relative timing of deformation and magmatism at lower, upper and middle crustal levels using uranium-lead zircon geochronology, and detremination of pressure and temperature conditions of high-P granulite and upper amphibolite facies assemblages formed during extension. The initial results suggest that rheological transitions linked to magmatism and the partial melting of thick lower crust and their effects on the mechanical behavior of continental lithosphere are much more heterogeneous and short-lived than previously believed.

新西兰西部的一座深受侵蚀的山脉为直接检查长达 50 公里厚的大陆地壳柱提供了极不寻常的机会，因为它在 3500 万年的时间里不断演化。这一发现尤其重要，因为现场观测和数值模型使地球科学家强调了下地壳在控制大陆岩石圈变形行为中的作用。 然而，在大多数实验和大多数暴露中，大量不受约束的变量对下大陆地壳流动的特征和后果造成了高度的不确定性。该项目正在确定古代造山带经历从地壳增厚和收缩到地壳变薄和扩张的转变时应变如何在垂直方向上分配。该项目中使用的直接观测方法是可能的，因为新西兰西部的前新生代结构将在白垩纪造山带异常深层（25-50公里深度）形成的地壳置于代表中地壳和上地壳水平的岩石旁边位于同一造山带（8-27 公里深度）。 该研究涉及利用该带的原始中生代结构来测试拉伸变形可能通过流变演化的地壳柱垂直转移的对比方式。具体来说，该项目涉及测量108至1.9亿年前伸展过程中形成的中、上、下地壳结构的三维特征和运动学，确定不同地壳深度同时演化的伸展结构之间是否存在运动学兼容性，建立利用铀铅锆石地质年代学确定下、上、中地壳层次变形和岩浆作用的绝对和相对时间，并确定高磷麻粒岩的压力和温度条件以及伸展过程中形成的上部角闪岩相组合。初步结果表明，与岩浆作用和厚下地壳部分熔融相关的流变转变及其对大陆岩石圈力学行为的影响比以前认为的更加不均匀和短暂。