Collaborative Research: An Exhumed Field Example of Heterogeneous Lower Crustal Flow, Athabasca Granulite terrane, Canada

地球下部地壳(深度25-30公里)在大陆动态演化中所起的作用是地球科学的一个基本目标。特别是，在世界各地的许多构造活动区提出了普遍存在的下地壳韧性流动，以使地形和其他地表地质和气候过程与岩石圈规模的地球动力学和地球物理数据集相一致。虽然通常在二维中进行均匀建模，但自然观察表明，这一过程基本上是不均匀的(例如，在流场中具有横向边界和内部梯度)。在这个项目中，我们试图在一个暴露的下地壳流实例中记录这种非均质性的空间变化和物理特征，以便在活动构造区的地壳深部更容易远程(即地震)识别这一过程。两名研究生将以这项工作为基础撰写论文，本科生将以现场助理和研究人员的身份参与样本分析和建模。位于加拿大地盾西部的阿萨巴斯卡麻粒岩地体可以说是北美最大的下陆壳裸露。在该地体中，一种缓倾的局部普遍存在的麻粒岩相变形组构记录了古深度30-40公里处的亚水平韧性流动。科拉湖剪切带是一条明显的千米级次垂直走滑变形带，被认为与相邻的次水平流动组构在运动学上是相容和共生的。我们假设：1)科拉湖剪切带的早期历史类似于某些现代下地壳流体制中的非均质性和应变分配；2)浅层和陡峭组构区域的组构强度和规模足以进行地震观测。我们将用三个目标来验证这些假设：1)详细的野外作图，目的是构建下地壳流动特别是流动非均质性的地图；2)压力-温度-时间-变形模拟，以约束非均质流的深度和时间演化；3)岩石物理分析和合成地震模拟，以约束地壳深部的流变演化，并阐明如何通过地震各向异性方法识别和解释相似的构造。最终目标是建立下地壳中这些构造发展的模型，以及它们的流变性和地震性质的演化。

The role played by Earths lower crust (25-30 km depth) in the dynamic evolution of continents is a fundamental target in Earth sciences. In particular, pervasive ductile flow of the lower crust has been proposed in many tectonically active regions around the world in order to reconcile topography and other surface geologic and climatic processes with lithospheric-scale geodynamic and geophysical data sets. Although commonly modeled homogeneously in two-dimensions, natural observations suggest that this process is fundamentally heterogeneous (e.g., with lateral boundaries and internal gradients in the flow field). In this project, we seek to document the spatial variation and physical characteristics of this heterogeneity in an exposed example of lower crustal flow so that this process may be more easily recognized remotely (i.e., seismically) in the deep crust of active tectonic regions. Two graduate students will base their dissertations on this work, and undergraduate students will participate as field assistants and as researchers in sample analysis and modeling. The Athabasca Granulite Terrane in the western Canadian Shield is arguably the largest exposure of lower continental crust in North America. Within this terrane, a gently dipping and locally pervasive granulite-facies deformation fabric records sub-horizontal ductile flow at 30-40 km paleodepth. The Cora Lake shear zone is a distinct kilometer-scale sub-vertical zone of high-grade strike-slip deformation that is thought to be kinematically compatible and co-eval with the adjacent sub-horizontal flow fabrics. We hypothesize that 1) the early history of the Cora Lake shear zone is an analog for heterogeneity and strain partitioning in some modern lower crustal flow regimes and 2) that the fabric intensity and scale of the shallow and steep fabric domains are sufficient for seismic observation. We will test these hypotheses with three objectives: 1) detailed field mapping with the goal of constructing a map of lower crustal flow and especially flow heterogeneity, 2) Pressure-Temperature-time-Deformation modeling in order to constrain the depth and temporal evolution of heterogeneous flow, and 3) petrophysical analysis and synthetic seismic modeling in order to constrain the rheological evolution of the deep crust and illuminate how similar structures may be recognized and interpreted through seismic anisotropy methods. The ultimate goal is a model for the development of these structures in lower crust and for the evolution of their rheological and seismic properties.