Convergent Margin Processes

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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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.