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

地壳对构造和重力的响应方式取决于地壳岩石的强度分布。人们普遍认为，在某些环境中，地壳下半部分的岩石可能足够弱，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都有重要的影响，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的测试。这些PI将在爱达荷州的先锋山脉集中研究，暴露出可能代表坚硬岩石和底层较弱流动岩石之间的分界线的岩石，如果是这样的话，将提供一个极好的机会来测试涉及较弱下地壳岩石流动的模型。对岩石强度垂直变化的更好的野外记录将为我们提供对大陆变形的更全面的看法。许多最近的地壳形变模型都以中到下地壳为特征，其足够弱，以至于它解耦并响应侧向压力梯度而流动。这种流动对地壳形变的几个方面具有深远的影响。然而，目前的许多弱解耦中下地壳流动模型都是通过间接观测得出的，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出去耦合边界的地壳中段，这为测试弱和去耦合流动模型提供了机会。在这个项目中，将通过野外构造和AMS分析，结合U-Pb地质年代学和Ar/Ar热年代学，通过记录地壳中层岩石的应变历史来测试具有变量解耦测试模型的PI。结合这些数据，我们将能够详细比较不同构造层次的运动学和应变史，并研究中地壳流变分层、穹隆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.