Collaborative Research: Mid-crustal Strain during Extension: A Field-based Investigation of Rheological Transitions, Doming, and Vertical Coupling

地壳对构造力和重力的反应方式取决于地壳岩石的强度分布。人们广泛认为，在某些情况下，地壳下半部的岩石可能足够脆弱，以至于它们会因地形梯度引起的压力差而横向流动。这种行为对大陆地壳的许多过程具有重要意义，例如山地高原的生长、盆地和大陆架的形成以及大陆内的地震活动。 然而，这些模型需要通过更好的实地调查进行严格测试。这些 PI 将集中研究爱达荷州的先锋山脉，那里暴露的岩石可能代表坚固岩石和下面较弱流动岩石之间的边界区域，如果是这样，这将提供一个绝佳的机会来测试涉及弱下地壳岩石流动的模型。更好地记录岩石强度的垂直变化将为我们提供关于大陆变形的更全面的观点。最近的许多地壳变形模型都以中下地壳为特征，该地壳足够弱，可以响应侧向压力梯度而解耦和流动。这种流动对地壳变形的几个方面具有深远的影响。然而，当前许多弱解耦的中下地壳流模型都是从间接观测中得出的，模型流变层在岩石记录中的表示程度尚不清楚。初步工作表明，爱达荷州的先锋核心复合体包含一个暴露解耦边界的中地壳部分，这为测试弱流和解耦流模型提供了机会。在该项目中，将通过基于现场的结构和 AMS 分析，结合 U-Pb 地质年代学和 Ar/Ar 热年代学记录中地壳岩石的应变历史，对具有变量解耦测试模型的 PI 进行测试。这些数据将使我们能够详细比较不同结构层面的运动学和应变历史，并研究中地壳流变层、穹顶和上地壳伸展之间的关系。

The manner in which the Earths crust responds to tectonic and gravitational forces depends on the strength distribution of crustal rocks. It has been widely proposed that in certain settings, rocks in the lower half of the crust may be sufficiently weak that they flow laterally in response to pressure differences induced by topographic gradients. Such behavior has important implications for many processes in the continental crust, for example, the growth of mountain plateaus, formation of basins and continental shelves, and seismicity within the continents. These models, however, need to be rigorously tested through better field-based investigations. The Pioneer Mountains of Idaho, where these PIs will focus their study, expose rocks that may represent a boundary zone between strong rocks and underlying weaker flowing rocks, and if so will provide an excellent opportunity to test models that involve flow of weak lower crustal rocks. Better field documentation of the vertical changes in rocks strength will provide us with a more comprehensive view of continental deformation.Many recent models of crustal deformation feature a middle to lower crust that is sufficiently weak that it decouples and flows in response to lateral pressure gradients. Such flow has profound implications for several aspects of crustal deformation. Many of the current models for weak decoupled middle to lower crustal flow, however, are derived from indirect observation and the extent to which model rheological layers are represented in the rock record is unclear. Preliminary work shows that the Pioneer core complex in Idaho contains a midcrustal section exposing a decoupling boundary, which presents an opportunity to test models of weak and decoupled flow. In this project the PIs with test models of variable decoupling will be tested through documentation of the strain history in the midcrustal rocks through field-based structural and AMS analysis, combined with U-Pb geochronology, and Ar/Ar thermochronology. Together these data will allow us to compare in detail, the kinematic and strain histories at the various structural levels and investigate the relationships between the mid-crustal rheological layering, domes, and upper crustal extension.