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

新西兰西部的一条被严重侵蚀的山脉提供了一个非常不寻常的机会，可以直接观察整个50公里厚的大陆地壳柱，因为它在3500万年的时间里演化。这一发现具有特别重要的意义，因为实地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造成了对下大陆地壳流动的特征和后果的高度不确定性。该项目正在确定古造山带从地壳增厚和收缩到地壳减薄和伸展的转变过程中应变是如何垂直分配的。在这个项目中使用直接观测方法是可能的，因为新西兰西部的前新生代构型将在白垩纪造山带(25-50公里深)异常深部形成的地壳置于代表该造山带中地壳和上地壳水平(8-27公里深)的岩石旁边。这项研究涉及使用该带原始的中生代建筑来测试不同的伸展变形可能通过流变演化的地壳柱垂直转移的方式。具体地说，该项目涉及测量1.08亿至1.9亿年前伸展期间形成的中、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构造之间是否存在运动学兼容性，利用铀-铅锆石年代学确定地壳下部、上部和中部的变形和岩浆作用的绝对和相对时间，以及确定在伸展过程中形成的高P麻粒岩和上部角闪岩相组合的压力和温度条件。初步结果表明，与岩浆作用和厚地壳部分熔融有关的流变学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。

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.