Towards a Lithospheric History of North America

地球岩石圈代表了地球深部对流过程和地壳构造过程之间的边界层，构造过程定义了地壳的历史。要理解地表板块构造和深层地幔动力学之间的关系，以及这种关系是如何随时间变化的，就需要对岩石圈结构的了解达到与主要地壳构造块相当的详细程度。我的计划是通过地震方法绘制大陆岩石圈图，将岩石圈特征与已知年龄的地壳体联系起来，并发展贯穿整个板块厚度的岩石圈过程的历史。

The Earth's lithosphere represents a boundary layer between the convective processes of the deep Earth and the tectonic processes that define the history of the crust. Understanding the relationship between plate tectonics at the surface and mantle dynamics at depth, and how this relationship has changed over time, requires knowledge of the structure of the lithosphere at a level of detail comparable to the major crustal tectonic blocks. My program is to map the continental lithosphere through seismic methods, relate lithospheric features to crustal bodies of known age, and develop a history of lithospheric processes that extends through the full thickness of the plate. The largest-scale aspect of this program is production of a new tomographic model of the Canadian lithosphere using two-station analysis of surface waves. In this approach, the change in waveform between two instruments aligned along a common path to the source is examined, producing a dispersion curve related to the laterally-averaged structure along the path between the instruments. Given a large set of instruments, we can build a grid of intersecting paths on which the average structure is known; combining the paths via tomographic inversion yields 3-D images of structure. I propose to combine existing and new dispersion-curve data to form a 3-D image of the lithosphere across Canada. Existing North American models are largely based on single-station methods which are poorly adapted to low-seismicity settings like the Canadian Shield; the model produced by this technique will thus provide a novel constraints on the structure and dynamics of the North American lithosphere and the relationship between crustal geology and upper-mantle structures. I further propose to instrument the Trans-Hudson Orogen, west of the Superior, to fill in the largest gap in Canadian instrument coverage. More focused studies are also proposed here, based on data from the Mid-Continent Rift, a billion-year-old rift structure curving through the Great Lakes. A new data set from this region has become available as of 2013, via the international SPREE project. I propose two student projects to examine crustal and lithospheric fabric across the rift axis and in the adjacent Superior Province, to explore the effect of the rifting process on the lithosphere, the role of lithospheric factors in the rifting process, and the imprint of the Superior accretionary process on lithospheric fabric; new techniques for measuring lithospheric fabric in detail will be explored. A related study will examine a similar data set from a contemporary rift -- the Main Ethiopian Rift -- in order to compare fossil and active rift structures.   Better constraints on Canadian lithospheric structure will have large-scale impact on Canadian geology, through a better understanding of lithospheric participation in tectonic events, and will assist in large-scale mineral exploration.