Collaborative Research: Time Scales and Dimensions of Rheological Heterogeneity and Fabric Evolution in the Lower Continental Crust during Extensional Orogenic Collapse

这个项目提供了新的信息，说明在造山带中增厚的下地壳是如何在造山带崩塌过程中扩展和变薄的。野外研究和数值模型表明，下大陆地壳的强度、厚度和力学性质是影响整个岩石圈造山过程行为的关键因素，包括应变局部化到剪切带。然而，这些性质在造山旋回期间如何变化，以及它们如何影响变形模式，仍然存在不确定性。新西兰峡湾为解决这些问题提供了一个天然的实验室，因为它暴露了3000多平方公里的古代中生代下地壳。本项目确定了基中岩浆作用、地壳熔融作用、变质作用和变形作用对下地壳剖面演化的时间尺度、物理尺度和影响。这项工作将野外测绘与三维变形分析、岩石的压力-温度-时间-变形路径以及铀-铅和钐-钕同位素地质年代学相结合。这些工具有助于解决峡湾下地壳在2500万年、35-50公里深度从缩短和地壳增厚到伸展和地壳变薄过渡期间的热力学结构。它们还揭示了应变局部化过程的变化性质，包括对称型伸展断层的形成，该断层位于高品位变质岩圆顶的边缘。这项研究的中心假设是，岩浆作用和变质作用导致的温度、成分、强度和地壳厚度的变化，造成了大片（1000平方公里）相对未变形区域的拼凑，这些区域被弱变形区域隔开。在最年轻的岩体侵位后不久，强抗变形区域就形成了。最初，伸展广泛分布在弥漫性剪切带中，反映出弱区规模较大。后来，火成岩的快速冷却和熔体的去除或结晶使大剖面加强，改变了薄弱区域的位置和大小。在冷却过程中，岩体的干核变得比含水、富含石英的寄主岩石更强，导致变形从岩体内部迁移到更软的寄主岩石中。这里支持的研究是佛蒙特大学和阿拉巴马大学塔斯卡卢萨分校科学家合作项目的一部分。除了研究目标之外，该项目还支持两所大学的研究生和本科生在STEM学科方面的培训。学生们参与了项目的各个方面，并从使用各种来源的最先进的分析设备中获得经验，包括他们的主办机构，北卡罗来纳大学教堂山分校和美国地质调查局。该项目正在加强美国和新西兰科学机构之间的合作。与新西兰科学家建立了牢固的伙伴关系，提供了访问未发表的数据和样本档案的机会，这些数据和样本档案来自最近一个以1:25万比例尺绘制峡湾地图的项目。学生参与这些合作有助于促进发现和加强这些伙伴关系。学生们从文化和科学交流中受益，并在一些最好的研究设施中获得重要的经验。研究成果正在被纳入课堂课程，通过专业科学会议的报告和同行评议的地质文献传播，结果数据正在各种社区数据库中存档。

This project is providing new information on how thickened lower crust in mountain belts extends and thins during orogenic collapse. Field studies and numerical models demonstrate that the strength, thickness, and mechanical properties of the lower continental crust are key factors that influence the behavior of the entire lithosphere during orogenesis, including the localization of strain into shear zones. However, there still is uncertainty about how these properties change during orogenic cycles and how they influence patterns of deformation. Fiordland, New Zealand provides a natural laboratory for resolving these questions because it exposes over 3000 square kilometers of ancient Mesozoic lower crust. This project is determining the time scales, physical dimensions, and effects of mafic-intermediate magmatism, crustal melting, metamorphism, and deformation on the evolution of this lower crustal section. The work integrates field mapping with three-dimensional analyses of deformation, pressure-temperature-time-deformation paths for rocks, and uranium-lead and samarium-neodymium isotope geochronology. These tools are helping to resolve the thermal and mechanical structure of Fiordland's lower crust over 25 million years and at depths of 35-50 kilometers during a transition from shortening and crustal thickening to extension and crustal thinning. They also are revealing the changing nature of strain localization processes, including the formation of a symmetric style of extensional faults that border domes of high grade metamorphic rocks.The central hypothesis of this research is that variations in temperature, composition, strength, and crustal thickness, resulting from magmatism and metamorphism, created a patchwork of large (1000 square kilometers) relatively undeformed regions separated by weak deforming regions. Strong regions resisting deformation develop shortly after the emplacement of the youngest plutons. Initially, extension was broadly distributed in diffuse shear zones that reflected the large size of weak areas. Later, rapid cooling of the igneous rocks and removal or crystallization of melt strengthened large sections, changing the location and size of weak areas. During cooling, the dry cores of plutons became stronger than the hydrous, quartz-rich host rocks, resulting in the migration of deformation out of pluton interiors and into softer host rock. The research supported here is part of a collaborative project between scientists at the University of Vermont and the University of Alabama Tuscaloosa. In addition to the research objectives, the project is supporting graduate and undergraduate student training in a STEM discipline at both institutions. The students are involved in all aspects of the project, and are gaining experience from using state-of-the-art analytical facilities from a variety of sources, including those of their host institutions, the University of North Carolina Chapel Hill, and the U.S. Geological Survey. The project is enhancing collaborations among U.S. and New Zealand scientific institutions. A strong partnership with New Zealand scientists provides access to unpublished data and sample archives resulting from a recent program to map Fiordland at a 1:250,000 scale. Student participation in these collaborations is helping to advance discovery and strengthen these partnerships. Students are benefiting from cultural and scientific exchanges and are gaining important experience in some of the best research facilities. Research results are being integrated into classroom curricula, disseminated via presentations at professional science meetings and the peer-reviewed geologic literature, and resulting data is being archived in a variety of community databases.