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

地壳对构造力和引力的反应方式取决于地壳岩石的强度分布。人们普遍认为，在某些情况下，地壳下半部分的岩石可能足够脆弱，因此它们会根据地形梯度引起的压力差向横向流动。这种行为对大陆地壳的许多过程具有重要意义，例如，高山高原的生长，盆地和大陆架的形成，以及大陆内部的地震活动。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将把研究重点放在爱达荷州的先锋山脉，那里暴露的岩石可能代表着强岩石和底层弱流动岩石之间的边界地带，如果是这样，将提供一个极好的机会来测试涉及弱下地壳岩石流动的模型。更好的岩石强度垂直变化的现场记录将使我们对大陆变形有一个更全面的认识。许多最近的地壳变形模型都有一个中下地壳的特征，它足够弱，可以解耦并随侧向压力梯度流动。这种流动对地壳形变的几个方面具有深远的影响。然而，目前许多弱解耦的中下地壳流动模型都是间接观测得来的，而且模型流变层在岩石记录中的表现程度尚不清楚。初步工作表明，爱达荷州的Pioneer岩心复合体包含一个暴露出解耦边界的地壳中部剖面，这为测试弱流和解耦流模型提供了机会。在本项目中，将结合U-Pb地质年代学和Ar/Ar热年代学，通过对地壳中部岩石应变历史的记录，结合野外构造和AMS分析，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地比较不同结构水平上的运动和应变历史，并研究地壳中部流变分层、穹窿和上部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.