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

新西兰西部一座被严重侵蚀的山脉提供了一个非常不寻常的机会，可以直接研究整个50公里厚的大陆地壳柱，因为它在3500万年的时间里进化了。这一发现特别重要，因为实地观测和数值模型使地球科学家强调下地壳在控制大陆岩石圈变形行为方面的作用。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。该项目旨在确定古造山带在经历从地壳增厚收缩到地壳变薄扩展的转变过程中，应变是如何在垂直方向上进行划分的。在这个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构造将白垩纪造山带（25-50公里深）异常深的地壳与代表同一造山带（8-27公里深）中上层地壳的岩石放在一起。该研究包括利用该带的原始中生代构造来测试伸展变形可能通过流变演化的地壳柱垂直转移的不同方式。具体而言，该项目包括测量1.08 - 1.9亿年前伸展过程中形成的中、上、下地壳构造的三维特征和运动学，确定不同地壳深度同时演化的伸展构造之间是否存在运动学相容性，确定下部变形和岩浆活动的绝对和相对时间。利用铀铅锆石年代学，确定了伸展期形成的高磷麻粒岩和上角闪岩相组合的压力和温度条件。初步结果表明，与岩浆活动和厚下地壳部分熔融有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为不均匀和短暂。

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.