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.
The role played by Earth’s 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. In this 4 year project (original 2 years with 2 years of no-cost extensions), we sought to document the spatial variation, age, kinematics, and tectonic significance of a now extinct lower crustal high strain zone exposed in northernmost Saskatchewan, Canada. Such old and deeply exhumed exposures offer unparalleled opportunities for learning how lower crust responds during mountain building processes. Four graduate students (three from the University of Colorado and one from the University of Massachusetts) based all or parts of their theses on this work, and several undergraduate students participated as field assistants and as researchers in sample analysis and modeling. Three of these graduate students have successfully defended their theses and have either moved on to successful careers in the industrial sector or are continuing additional graduate work. The research work involved field-based mapping and rock sampling, laboratory-based rock chemistry and microstructural and petrological analysis, and modeling of synthetic seismic properties to better understand how deformation zones like this might be imaged remotely in modern lower crust. In addition, the research has been used as a major test case for new techniques for dating deformation and metamorphic processes using in-situ mapping and analysis of monazite. Three more journal articles that focus on the timing and tectonic significance of this deformation zone or the physical properties of minerals undergoing chemical change during deformation are currently undergoing the peer-review process (May 2014). Three scientific papers that focus on seismic anisotropy properties of deep crustal deformation zones such as this one, and that in part use data from this study have been published in peer-reviewed international journals. In addition, several publications presenting aspects of monazite geochronology have used results from this study as examples.
