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

地球下地壳（25-30公里深度）在大陆动态演化中的作用是地球科学的一个基本目标。特别是，在世界各地许多构造活动区，提出了下地壳普遍存在的韧性流动，以便将地形和其他地表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。虽然通常在二维中均匀建模，但自然观察表明该过程从根本上是异质的（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动的例子中记录这种非均匀性的空间变化和物理特征，以便这个过程可以更容易地远程识别（即，地震学上）在地壳深处的活动构造区。两名研究生将根据这项工作的论文，和本科生将参加现场助理和样品分析和建模的研究人员。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暴露。在这一构造带内，一个缓倾且局部普遍存在的麻粒岩相变形组构记录了30-40 km古深度处的近水平韧性流。科拉湖剪切带是一个独特的大规模的高品位走滑变形，被认为是运动学兼容和co-eval与相邻的子水平流面料的亚垂直区。我们假设：1）科拉湖剪切带的早期历史是一个模拟的不均匀性和应变分配在一些现代下地壳流动制度和2），结构强度和规模的浅，陡组构域是足够的地震观测。我们将通过三个目标来检验这些假设：1）详细的野外测绘，其目的是构建下地壳流动，特别是流动不均匀性的地图，2）压力-温度-时间-变形建模，以约束不均匀流动的深度和时间演化，和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，并阐明类似的结构如何可以通过地震各向异性方法进行识别和解释。最终的目标是为这些结构在下地壳的发展及其流变和地震性质的演变提供一个模型。

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.