The continents record at least four billion years of Earth history. Continental crust is compositionally distinct from that of the crust of all other planets and satellites, implying that the processes that lead to the formation of Earth's continents are unique in our solar system. The continents and their margins also hold most of Earth's recoverable natural resources. And, we live on the surface of the continents. Despite these special attributes, the continents and the forces that have shaped them remain poorly understood.
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Many research facilities are using EarthScope instrument data to produce scientific models and visual representations.
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Continental Structure, Deformation and Evolution
Along North America's western margin, continental evolution driven by plate interactions is occurring through similar, but currently active tectonic processes. These geologic environments, and those representing the time in between, provide the targets for EarthScope's focus on deformation of the North American continent.
Comparison of predicted and observed fast shear wave polarization directions for the sublithospheric mantle beneath eastern North America based on a mantle flow model for the keel divot caused by a rigid, deep mantle root to cratonic North America, denoted by thick solid line. Background velocity model and map projection are from van der Lee and Nolet (1997). With the exception of region A and portions of regions C and F, fast directions mimic regional fast direction patterns. Figure from Fouch, M.J., K.M. Fischer, M.E. Wysession, and T.J. Clark, 2000, Shear wave splitting, continental keels, and patterns of mantle flow, J. Geophys. Res., 105, 6255-6276.
In contrast, along North America's western margin, continental evolution driven by plate interactions is occurring through similar, but currently active tectonic processes. These geologic environments, and those representing the time in between, provide the targets for EarthScope's focus on deformation of the North American continent.
Estimated sub-lithospheric mantle flow velocity in a hotspot frame (black arrows) along with lithospheric motion (including lithospheric deformation; red arrows), also in a hotspot frame. Ninety-five percent confidence ellipses in sub-lithospheric velocity incorporate formal uncertainty from inversion plus uncertainty in the hotspot frame. Mantle flow is approximately eastward at 5.5 cm/yr. From Silver, P. G. and W. E. Holt, 2002, The mantle flow field beneath western North America, Science, 295, 1054-1057.
The North American active margin provides one of the most diverse plate-boundary regions on Earth. It includes a continental transform system, with extensional, strike-slip and contractional regimes, and continental arc and oceanic arc settings.
It records the long-lived interaction of a continent with the great Pacific and Farallon plates, and their influence on continental accretion and deformation. The diversity of tectonic regimes within the continent violates rigid-plate precepts and requires development of a new paradigm to explain global continental tectonics. Any robust paradigm must be rooted in detailed and comprehensive views of deformation and crustal evolution. North America is being viewed through the unprecedented EarthScope, which can be focused on a spectrum of spatial and temporal scales across a large, complex, and globally important plate boundary. Complex patterns of deformation and mountain building belie the heterogeneity and varied rheology of continental lithosphere, and its complex interactions with the underlying mantle.
Research Questions
- What is a continent?
- How is the lithosphere formed?
- By what mechanisms are continents dispersed and reassembled?
- How are the crust and lithospheric mantle related?
- How are continental structure and deformation related?
- How does magmatism modify, enlarge, and deform continental lithosphere?
- What is the role of extension, orogenic collapse, and rifting in constructing the continents?
- What are the fundamental controls on deformation of the continent?
- What is the strength profile(s) of the lithosphere?
- What defines tectonic regimes within the continent?
- How does convective and advective flow effect plate motions and transfer stresses to the lithosphere?
- What role do fluids play in lithospheric deformation?
- How is deformation distributed throughout the continent?
- How has topography evolved through time?
The Impact of EarthScope
EarthScope data sets are contriubiting to a comprehensive image of the deformation field and material properties of North America and its temporal variation and evolution. This image is sharply focused at a variety of temporal and spatial scales and will form the basis for a new, physics-based description of the dynamics of the whole Earth (Figure 11). We strive to articulate a time-dependent geodynamical model of continental evolution that assimilates and integrates geologic, geodetic, and seismological data sets. We strive to create a geodynamic model that parallels plate tectonics in its power to integrate our understanding
The EarthScope Contribution
EarthScope is providing important components of the answers to these questions. By inversion of a variety of data sets to be collected by USArray and associated denser subnetworks of seismometers, we shall learn the mean seismic structure of the continental crust, associated mantle, and crust-mantle transition as well as the variability in that structure about the mean properties. EarthScope may one day aid in defining continent structure, continental lithosphere formation, and how continental structure and deformation are related.