Fundamental controls on mid-crustal 'escape' flow in orogenic systems

在大陆-大陆碰撞形成的大型山地带中，必须通过各种机制将形成高山地形的巨大岩体堆积物从系统中移除。在地壳浅层，这主要是由河流和冰川的侵蚀作用完成的。在更深的地方，中地壳和下地壳可能由于部分熔融而变弱，然后可能在一个称为通道或逃逸流的过程中流出碰撞区。在喜马拉雅-西藏系，有人提出地壳逃逸流主导了西藏东部边缘下地壳的变形，甚至可能导致印度东北部、缅甸北方和中国南部沿沿着上地壳走滑断层的滑动和地震活动。了解这些地壳逃逸流是否真的存在，以及它们如何在大型山脉带系统中运作，仍然是构造研究的一个基本挑战，特别是因为它们无法在深度直接观察到。对于我们理解逃逸流如何运作的关键是指导这种流动的边界的性质以及它们如何与通道系统动态地相互作用，因此理想情况下，这种类型的研究应该通过检查已经去除了上地壳“盖子”的地壳通道来进行。该项目正在研究一个可能的地壳逃逸流系统，该系统暴露在北卡罗来纳州、南卡罗来纳州和格鲁吉亚的阿巴拉契亚内皮埃蒙特南部的地表。此外，这项工作正在为一个典型的美国东部油田地区产生一套重要的地质年代学和岩石构造分析，该地区缺乏大部分基本数据。该项目通过支持重要科学，技术，工程和数学（STEM学科）研究生的先进现场和实验室培训，促进了预期社会成果的进步，并为早期职业调查员提供支持。最重要的是，这个项目还将在主要研究者的指导下支持至少六名本科研究人员的研究培训和协作经验。尽管经典的通道流模型相当优雅地解释了喜马拉雅系统中许多看似无关的特征，可能导致失活和/或在青藏高原南部前缘的通道或改向仍然是这个模型的一个持久的谜。具体而言，有必要了解：（1）什么样的条件或机制变化可能导致从造山带正常河道流为主的系统转换为适应造山带前缘沿沿着主要逆冲断层的临界楔形滑动缩短的系统，（2）在经历这种转换的大型热造山带中，相同的条件和机制是否可能导致转换为平行于造山带的逃逸流，以及（3）引导这种流动的边界的性质是什么，它们如何与造山带平行通道系统动态地相互作用？要做到这一点，有必要确定一个潜在的通道系统，现在暴露在表面。在阿巴拉契亚山脉南部的内皮埃蒙特，以前的研究提出了一个新阿卡第（泥盆纪-密西西比纪）地壳逃逸流系统的基础上观察到的地图规模弯曲线理模式和保存广泛的地区普遍混合岩化，夕线石I-II变质，和众多的深熔侵入岩。本研究利用综合变质和构造岩石学、化学扩散模型，和geo-独居石和钛石激光烧蚀分流技术和热年代学（40 Ar/39 Ar角闪石和白云母）结合古压力和石英组构衍生的变形温度，以建立压力-温度-温度-该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

In large mountain belts that result from continent-continent collision, the tremendous accumulation of rock mass that creates the high mountainous topography must be removed from the system by a variety of mechanisms. In the shallow crust, this is mostly accomplished by the erosive action of rivers and glaciers. At greater depths the middle and lower crust can be weakened by partial melting and then may flow out of the collision zone in a process known as channel or escape flow. In the Himalayan-Tibetan system, it has been proposed that crustal escape flow dominates deformation in the lower crust beneath the eastern Tibetan margin and may even contribute to slip and seismicity along upper crustal strike-slip faults in northeastern India, northern Myanmar, and southern China. Understanding if these crustal escape flows are truly present and how they operate in large mountain belt systems remains as a fundamental challenge for tectonics research, particularly because they cannot be directly observed at depth. Critical to our understanding of how escape flow may operate is the nature of the boundaries that direct such flow and how they dynamically interact with the channel system, so ideally a study of this type should be carried out by examining a crustal channel that has had its upper crustal 'lid' removed. This project is examining a possible crustal escape flow system that is exposed at the surface in the southern Appalachian Inner Piedmont of North Carolina, South Carolina, and Georgia. In addition, this work is producing a critical suite of geochronologic and petrotectonic analyses for a classic eastern U.S. field area that lacks much of this basic data. This project contributes to the advancement of desired societal outcomes by supporting advanced field and laboratory training for a graduate student in an important science, technology, engineering and mathematics (STEM discipline), and is providing support for an early career investigator. Most importantly, this project will also support research training and collaboration experience for at least six undergraduate researchers under the guidance of the principal investigator.Although the classic channel flow model quite elegantly explains a number of seemingly unrelated features in the Himalayan system, the condition(s) that may have led to deactivation and/or redirection of the channel at the southern front of the Tibetan Plateau remains as a persistent enigma for this model. Specifically, it is necessary to understand: (1) what conditions or mechanistic changes might lead to a switch from an orogen-normal channel flow dominated system to one that accommodates shortening by critical wedge-style slip along major thrusts at the front of an orogen, (2) could the same conditions(s) and mechanisms be responsible for a switch to orogen-parallel escape flow in large hot orogens that undergo this transition, and (3) what is the nature of the boundaries that direct such flow and how do they dynamically interact with the orogen-parallel channel system? To do this, it was necessary to identify a potential channel system now exposed at the surface. In the southern Appalachian Inner Piedmont, previous studies proposed the presence of a Neoacadian (Devonian-Mississippian) crustal escape flow system based on the observation of a map scale curved lineation pattern and preservation of an extensive area of pervasive migmatization, sillimanite I-II metamorphism, and numerous anatectic plutons. This study is addressing the questions above in the Inner Piedmont using integrated metamorphic and structural petrology, chemical diffusion modeling, and geo- (monazite and titanite laser ablation split stream technique) and thermochronology (40Ar/39Ar hornblende and muscovite) combined with paleopiezometry and quartz fabric-derived deformation temperatures to establish Pressure-Temperature-temperature-stress paths and system residence times for representative structural levels across the interpreted channel system.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.