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

该项目提供了关于造山带增厚的下地壳在造山塌陷期间如何伸展和变薄的新信息。 野外研究和数值模型表明，下陆壳的强度、厚度和力学性质是影响造山作用期间整个岩石圈行为的关键因素，包括应变在剪切带中的局部化。 然而，这些性质在造山旋回中如何变化以及它们如何影响变形模式仍然是不确定的。新西兰峡湾为解决这些问题提供了一个天然实验室，因为它暴露了超过3000平方公里的古代中生代下地壳。该项目正在确定时间尺度、物理尺寸以及镁铁质-中质岩浆作用、地壳熔融、变质和变形对这一下地壳部分演化的影响。 这项工作将野外制图与三维变形分析、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结合起来。这些工具正在帮助解决峡湾下地壳2500万年来、深度35-50公里、从缩短和地壳增厚到伸展和地壳变薄过渡期间的热结构和力学结构。它们还揭示了应变局部化过程的变化性质，包括形成一种对称样式的伸展断层，这些断层与高级变质岩的穹丘相邻。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，形成了一个由相对未变形的大区域（1000平方公里）和弱变形区域分隔开的拼凑体。在最年轻的岩体就位后不久，发展出抵抗变形的强区域。最初，伸展广泛分布在扩散剪切带，反映了大规模的薄弱地区。后来，火成岩的快速冷却和熔体的移除或结晶强化了大部分，改变了薄弱区域的位置和大小。在冷却过程中，干燥的岩芯变得比含水的、富含石英的寄主岩石更坚固，导致变形从岩体内部迁移到较软的寄主岩石中。这里支持的研究是佛蒙特大学和亚拉巴马塔斯卡卢萨大学科学家合作项目的一部分。除了研究目标外，该项目还支持这两个机构的STEM学科的研究生和本科生培训。学生们参与了该项目的各个方面，并从使用各种来源的最先进的分析设施中获得经验，包括他们的主办机构，北卡罗来纳州查佩尔山大学和美国地质调查局。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强有力的伙伴关系，提供了未发表的数据和样本档案，这些数据和样本档案是最近一项以1：250，000比例绘制峡湾地图的计划的结果。学生参与这些合作有助于推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。研究成果正被纳入课堂课程，通过在专业科学会议上的介绍和经同行审查的地质文献传播，所产生的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.