Ductile strain in the footwall of a metamorphic core complex: A field example to test models for dynamics, timescales, and controls of mid-crustal flow.

与许多山脉带一样，北美西部的科迪勒拉经历了水平拉伸，使地球变薄--S地壳。在太平洋西北部，大约5000万至5500万年前造山结束后开始变薄，并高度局限于断层和剪切带，这些断层和剪切带通常记录了数十公里的伸展。地壳更深层次对这种局部伸展和减薄的反应方式目前尚有争议。目前流行的一个模型是，由于减薄区域的重力应力减弱，深部地壳足够弱，以至于它从未延伸的区域流入局部减薄区域。这项研究将测试这一模型，并解决决定这种弱重力诱导的流动是否发生的控制措施。在这项研究中进行的研究将通过支持一个重要的STEM(科学、技术、工程和数学)学科的研究生和本科生的培训，从而促进具有全球竞争力的劳动力的发展，从而为重要的社会成果做出贡献。该项目将为本科生提供参与研究的机会，包括完成基于他们研究的论文。该项目还包括重要的外展和教育活动，通过佛罗里达大学为高中生提供的“佛罗里达未来科学家”暑期计划，研究生将通过在大学分析实验室协助这些“未来科学家”获得指导经验。因此，该项目有助于提高科学素养和公众对STEM的兴趣。该项目将使用美国国家科学基金会资助的佛罗里达大学新的和现有的分析设施。因此，该项目的资金将确保这些设施被积极用于地质研究，并为未来的NSF项目，包括佛罗里达大学以外的合作项目进行充分校准和运作。这些仪器的持续研究和教育使用对于维持研究基础设施至关重要。在本研究期间获得的研究技术成果将通过在专业地球科学会议上的陈述和同行评议的科学文献广泛传播。变质核杂岩的高度局部化伸展被认为是由于局部折返带来的地形压力梯度而导致弱下地壳侧向流入伸展区域。虽然间接观测对这种均衡诱导的流动有很大的支持，但在岩心复杂的底部，这种过程的直接现场例子很少。在已经推断出这一过程的例子中，地质年代学通常不会明确地证明不同水平上应变的同步性。此外，一些经常引用的例子缺乏根据流入预测的汇聚流的证据。均衡诱导流的数值模型预测了特定的收敛结构，这些结构与少量的场例非常相似。然而，很可能在自然环境中有一些控制不能在数值模型中得到充分解决。这些不同的特征可能导致了一系列的结构，而这些结构的全部范围还没有在该领域得到确认。在爱达荷州的先锋岩心杂岩中，我们发现了一个在线理方向上暴露良好的突然垂直转变，这可能被解释为一个分离的均衡诱导流区。重要的是，变形和未变形的岩脉在线理转换的上方和下方都暴露了大量的岩套。铀铅年代学应用于这些岩脉，提供了一个难得的机会来测试这些组构是同步形成的还是顺序形成的。这一地质年代学与详细的运动学分析、变质压力-温度-时间分析相结合，所有这些都与详细的制图相结合，提供了一个独特的机会，不仅可以测试均衡诱导流动的想法，而且可以充分描述所产生的构造的详细几何形状，以及流动的时间尺度和控制。

As is the case of many mountain belts, the western North American Cordillera underwent horizontal stretching that thinned the Earth?s crust. In the Pacific Northwest thinning began around 50 to 55 million years ago immediately following the end of mountain building, and was highly localized along faults and shear zones that commonly record tens of kilometers of extension. The manner in which the deeper levels of the crust respond to this localized stretching and thinning is presently debated. A currently popular model is that the deep crust is weak enough that it flows from unextended areas into the region of localized thinning due to the lessened gravitational stresses in the areas of thinning. This study will test this model and address the controls that determine whether or not this type of weak gravitationally induced flow occurs. The research conducted during this study will contribute to important societal outcomes by providing support for the training of graduate and undergraduate students in an important STEM (science, technology, engineering, and mathematics) discipline, thus contributing to development of a globally competitive workforce. The project will provide opportunity for undergraduate students to be involved in the research, including the completion of theses based on their research. The project also contains important outreach and educational activities through the University of Florida's "Florida Future Scientists" summer program for high school students, the graduate student will gain mentoring experience by assisting these "future scientists" in the universities analytical laboratories. Thus, the project contributes to increased scientific literacy and public engament with STEM. This project will use new and existing NSF-funded analytical facilities at the University of Florida. Funding for the project will therefore insure that these facilities are actively used for geological research and fully calibrated and operational for future NSF projects, including collaborations outside of the University of Florida. Continuous research and educational use of these instruments is critical for maintaining research infrastructure. Technical results of the research obtained during this study will be widely disseminated through presentations at professional geoscience meetings and the peer-reviewed scientific literature. The highly localized extension that characterizes metamorphic core complexes is thought to lead to lateral flow of the weak lower crust into the extending region in response to topographically induced pressure gradients brought about by localized exhumation. While there is significant support from indirect observations for this isostatically induced flow, direct field examples of this process in core complex footwalls are few. In the examples where this process has been inferred, the geochronology does not typically unambiguously demonstrate synchroneity of strain at the different levels. Furthermore, some of the often cited examples lack evidence for convergent flow, which is predicted from inflow. Numerical models of isostatically induced flow predict specific convergent structures that strongly resemble a small number of field examples. However, it is likely that there are controls in natural settings that are not able to be adequately addressed in numerical models. These differing characteristics likely lead to a spectrum of structures, the full range of which is yet to be identified in the field. In the Pioneer core complex (Idaho), we have identified a well-exposed abrupt vertical transition in lineation directions that may be interpreted as a zone of decoupled isostatically induced flow. Importantly, extensive suites of both deformed and undeformed dikes are exposed above and below the lineation transition. Uranium-lead geochronology applied to these dikes offers a rare opportunity to test whether these fabrics formed synchronously or sequentially. This geochronology integrated with detailed kinematic analysis, metamorphic Pressure-Temperature-time analysis, all tied to detailed mapping offers a unique opportunity to not only test the idea of isostatically induced flow, but to fully characterize the detailed geometries of the resulting structures, as well as the timescales and controls on flow.