The Pacific/North America plate boundary system is being transformed from an Andean-type convergent margin that has existed since the Triassic and produced the North American Cordillera with large subduction-zone earthquakes and pervasive magmatism, to the predominantly strike-slip boundary that we observe today. This unfinished transition has produced several fascinating plate boundary structures in addition to standard subduction of the Pacific Plate beneath Alaska. These include subduction of small young remnants of the Farallon Plate beneath Cascadia and southern Mexico, extension in the Basin and Range, deformation of the western North America Cordillera, and, of course, initiation of the San Andreas fault system.
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Visualizations
Many research facilities are using EarthScope instrument data to produce scientific models and visual representations.
SIO Visualization Center
EarthScope Voyager, Jr.
UC Davis KeckCAVES
Convergent Margin Processes
Convergent margins are the most dynamic tectonic environments on Earth. Subduction of oceanic lithosphere leads to arc volcanism, an essential component in the creation and development of continental crust. Volcanic arcs are found above subduction zones, and represent one of the most common forms of global volcanism. Arc volcanoes are often explosive, and eruptions pose significant hazards to local populations and air traffic. Volcanism builds new continental crust either in situ, or through the development of island arcs on oceanic crust that may later be accreted onto a continent. The output of these systems offers a window into the lower crust and upper mantle, through melt chemistry and xenoliths.
Convergent margins provide an excellent setting for resolving transient deformation on a variety of time scales. Typically deformation rates are high, and the free surface on which we observe deformation lies above a large and gently dipping fault, providing a geometry that is favorable to detecting and characterizing time-dependent deformation. Subduction zones are the source of the world's largest earthquakes, tsunamis, and volcanic eruptions, all of which impose severe hazards to populations nearby (e.g., Seattle, Portland, and Anchorage in the United States).
The largest earthquakes in the world all have occurred on megathrusts at subduction zones. Great earthquakes at subduction margins pose hazards not only to nearby regions, but also across the ocean basins via destructive tsunamis. Recurrence studies of great earthquakes shows complex behavior, just as is found on strike-slip margins. It is important to compare and contrast aspects of subduction earthquakes with those at strike-slip margins and extensional regimes to understand whether any features of earthquake occurrence are universal to faults and whether any are specific to particular types of faulting. Although subduction related faults are generally less accessible at the surface than continental transforms such as the San Andreas system, the magnitude of the earthquakes and deformation signals found at subduction zones are substantially greater, providing ample signals for in-depth studies. In addition, paleo-earthquakes at subduction zones are recorded in sediments over a broad area instead of only at the fault trace.
Research Questions
- What is the geometry of the plate boundary megathrust and how does it relate to spatial and temporal variations in convergence, strain rate, seismicity, and volcanism along the convergent margin?
- What is the deeper slab and upper mantle structure and how does it relate to intermediate-depth subduction zone seismicity?
- How is strain partitioned?
- What controls the lithospheric architecture?
- What controls the locus of volcanism?
- How do convergent margin processes contribute to growth of the continent through time?
The EarthScope Contribution
Many scientific questions are being addressed by the measurements obtained from the EarthScope instruments summarized below:
