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

地球下地壳（25-30公里深）在大陆动态演化中所发挥的作用是地球科学的一个基本目标。特别是，为了将地形和其他表面地质和气候过程与岩石圈尺度的地球动力学和地球物理数据集相协调，世界各地许多构造活动区域都提出了下地壳普遍的延性流。尽管通常在二维中均匀建模，但自然观察表明该过程本质上是异质的（例如，流场中具有横向边界和内部梯度）。在这个项目中，我们试图在下地壳流动的暴露例子中记录这种异质性的空间变化和物理特征，以便在活动构造区域的深地壳中更容易地远程（即通过地震）识别这一过程。两名研究生将根据这项工作撰写论文，本科生将作为现场助理和研究人员参与样本分析和建模。加拿大地盾西部的阿萨巴斯卡麻粒岩地体可以说是北美下大陆地壳最大的暴露地体。在该地体内，轻微倾斜且局部普遍存在的麻粒岩相变形组构记录了 30-40 公里古深度的次水平韧性流。科拉湖剪切带是一个独特的千米级的高级走滑变形的亚垂直带，被认为与邻近的亚水平流结构在运动学上是相容和共同评估的。我们假设：1）科拉湖剪切带的早期历史是一些现代下地壳流态中的非均质性和应变分配的类比；2）浅层和陡峭结构域的结构强度和规模足以进行地震观测。我们将通过三个目标来测试这些假设：1）详细的现场绘图，目的是构建下地壳流动图，特别是流动非均质性图，2）压力-温度-时间-变形建模，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建模，以约束深部地壳的流变演化，并阐明如何通过地震各向异性方法识别和解释相似的结构。最终目标是为下地壳中这些结构的发育及其流变和地震特性的演化建立一个模型。

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.