QMIII – Project Outcomes (as reported to the NSF)

This project was conducted as a part of the national research program Earthscope that aimed to image the North American continent in unprecedented detail, and to use the new facts about deep Earth interior beneath it to understand how it was formed, how it has evolved, and what happens with it now. Our project addressed the questions of continental formation and evolution by developing a new set of geophysical observations along a nearly 1300 km long line that stretched from the Hudson Bay to the Atlantic. We chose the line (see MAP) so that it would a) sample the geologically oldest (Quebec) and youngest (Atlantic coast) parts of the region; b) run as orthogonal as possible to the boundaries separating regions of different geological history; and c) have enough observations along it to see “small” (20-50 km) objects and changes.  In positioning the observing line as we did, we expanded the reach of the original Earthscope project into the core of the oldest part of the North American continent.

The project was conducted collaboratively by researchers and students of Columbia University, Rutgers University, McGill University (Montreal), and the University of Quebec in Montreal. Additionally, a research group from the Imperial College (London) placed instruments close to our line, and we collaborated on the analysis and interpretation of data. Collectively, the work involved five faculty members and at least 20 students at different stages in their studies. We operated our seismographic observatory array from 2012 to 2015. The resulting data set is presently archived and open to anyone via the Incorporated Research Institutions for Seismology (IRIS) data center.

At Rutgers, the project served as the basis for MS theses of Benjamin Dunham and Yiran Li, the PhD thesis of Xiaoran Chen, and a number of undergraduate independent research efforts and honors theses. PI Levin and his students published four papers (as of 2020) and participated in the development of three more.

We addressed three specific research goals in our research.

Firstly, we attempted to detect and characterize the lower boundary of the North American tectonic plate. Known as the Lithosphere-Asthenosphere Boundary (LAB), and necessarily present as the tectonic plates move relative to the deeper parts of our planet, it proved very elusive to detailed study to date. We confirmed the difficulty of detecting a clear signal from the boundary beneath the older (1 – 3 billion years) parts of our study region. This implies that any changes in density associated with this boundary under the old continent are very gradual, and thus cannot be seen by relatively short (20-40 km) seismic waves we used in our attempts to probe the LAB. Intriguingly, the depth range immediately above the LAB (~150 – 200 km ) contains changes in directional properties (anisotropy) of the rock that most likely reflect regionally coherent deformation. Our initial attempts to probe these more subtle seismological signals ran into technical difficulties, and to overcome them we developed a new computational tool to predict seismic wave propagation in layered anisotropic medium. We intend to apply this tool to our data and data from other continents in the follow-up comparative study of the continental deep interior structure.

Secondly, we explored the depth extent of major tectonic boundaries that are well documented on the surface of the North American continent. We focused on the Grenville Front – a line marking the collision of two continents ~1 billion years ago, the Appalachian Front – a line marking the western extent of deformed rocks younger that 500 million years, and the Norumbega Fault Zone, a shear zone akin to the modern San Andreas Fault, with most recent motion happening ~100 million years ago. As all structures we targeted are quite old, the additional question we asked was whether continents “forget”, that is – whether changes in physical properties representative of the process of assembling different continental fragments together will persist over geologic time.

To address the above questions, we mapped horizontal boundaries in physical properties down to the depth of ~100 km and noted changes in their presence and nature with regards to the major tectonic boundaries.  We found that the Grenville Front and the Norumbega Fault Zone had clear changes in the main such boundary (the crust-mantle transition) beneath them. Interestingly, the Appalachian Front does not have a clear difference in the shallow Earth layering associated with it.

We also mapped the directional dependence (anisotropy) of seismic properties within the Earth’s outer ~300 km and once again looked for changes in the observed patterns in relation to the major tectonic boundaries. These measurements reflect deeper structure within and beneath the North American tectonic plate. We found clear changes in anisotropic signatures, however their association with surface geology was not clear.

Taken together, our findings suggest that the North American continent retains the differences in its interior structure inherited from its assembly. In some cases (e.g. the Grenville Front) the boundaries between distinct constituent elements are near-vertical, and reach 10s of km into the Earth, while in others (e.g. the Appalachian Front) the boundaries are likely inclined at a very shallow angle, causing misalignment of deep and shallow structures. Near-surface (upper 50 km or so) of the continent appears to have a long memory, keeping intact signatures of deformation for 100s of million (Norumbega Fault) or even billions (Grenville Front) years.

Thirdly, we researched the nature of the deep-seated deformation related to the present-day motion of the North American continent relative to the deeper more viscous part of the Earth’s mantle. We showed that the motion of the tectonic plate can explain a part, but not all of the seismological signature that forms when seismic waves go through rocks with systematic fabric. We concluded that additional systematic fabrics, different at different locations in our study region, are likely present within the continental lithosphere and must reflect past deformation episodes. This finding reinforces the notion that continents have long “memory” and preserve a record of past events that shaped them.