Building a continent: Integration of surface geology, rock physics, and seismic observations to investigate the tectonic history of the contiguous United States

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The North American continent formed over billions of years via the collision of continental fragments and the rifting apart of larger continents. Geologists can read some of this history by looking at rocks exposed at the Earth's surface that were deformed in the past. The aim of this project is to extend this ability below the Earth's surface. While buried rocks cannot be seen and mapped directly, they are constantly traversed by seismic waves from earthquakes around the world. These waves are imprinted with information on the rocks they travel through before reaching the Earth's surface. We combine mapped rock deformation at the surface with seismic information from the subsurface in order to find out how to extract the imprinted information from the seismic data. The information can then be used to map how the Earth's crust was deformed. The project advances knowledge of Earth science by allowing us to read the record of past geological processes beneath the surface. Because current geological processes also deform rocks, the method can also be used to map the roots of earthquake-generating faults, which is useful for estimates of seismic hazard. The rock characteristics investigated in this process are also relevant to mapping geological resources. Additionally, accurate knowledge of the imprint of rock fabrics on the seismic signal enables us to avoid biases in other uses of the seismic data, such as for location and characterization of natural and human-made seismic sources.A stated goal of the EarthScope program is to derive constraints on the deformation history of the continent. This project aims to bridge the scales from microstructural rock fabrics measured in the laboratory, to deformation structures mapped on outcrop exposures, and to rock fabrics causing seismic anisotropy at km-scale seismic wavelengths. Project components are 1) integration of seismic anisotropy observations from multiple techniques and seismic networks, 2) contribution of existing digital structural data sets containing foliations, lineations, and mapped major ductile and brittle faults to existing EarthCube-funded and U. S. Geological Survey-held databases as well as calculation of bulk elasticity tensors from ~1:100,000 scale maps to hand samples and 3) synthesis and joint interpretation of seismic observations and structural data extracted from EarthCube-funded databases using straightforward correlations as well as an explicit scaling from elastic tensors representative of rock types to geological foliation maps to seismic wavelength-scale anisotropy.The ability to relate crustal seismic anisotropy to the strain history of the crust requires 1) a representative catalog of elastic properties of crustal rocks and how they average at scales of seismic wavelengths and 2) a compilation of major structural parameters such as orientations of dominant rock fabrics (foliation and lineation) in metamorphic and igneous rocks and their internal coherence in unique domains, as well as orientations of more discrete features such as fault zones, in order to enable quantitative comparisons to anisotropy measured seismically. The proposed work will centralize existing compilations and make them widely available online.