UPPER MANTLE SEISMIC STRUCTURE BENEATH THE CORDILLERA

Global shear wave tomography shows that beneath the Cordillera, from Patagonia to Alaska, the upper 150 to 200 km of the mantle has slow shear velocity relative to most continental regions. The slow upper mantle corresponds almost exactly to the eastern extent of Cenozoic tectonic uplift with a broad region of slow shear velocity beneath the Western United States and Mexico, and a more narrow region beneath the Andes in South America. On a global scale, Eastern China and Eastern Australia also have slow upper mantle relative to most continental areas. Both these regions bordered subduction zones in the Mesozoic. In the case of eastern China, the slow upper mantle lies beneath Archean crust and in fact, the Yangtze and Sino-Korean cratons are the only large Archean blocks on Earth that do not have fast seismic roots beneath them. It is clear that continental lithosphere is substantially modified when a plate is subducted beneath it. It is not clear from seismology alone, however, what modifications occur. Possibilities include hydration or metasomatism of the overriding plate, heating due to melt migration, or physical removal of the overriding lithosphere. Xenolith and petrologic data indicate all processes occur to some extent.

High resolution regional seismic studies find significant heterogeneity within the Western United States and Eastern China. The seismic heterogeneity may be the manifestation of shallow convection occurring beneath these regions and may be the cause of present day intraplate magmatism. Recent detailed seismic waveform modeling has found a low velocity zone above the 410 km discontinuity beneath the Western United States, Eastern Mexico, and Northeast China. The low velocity zones lie above regions where subducted plate has stagnated within the transition zone of the mantle and have been interpreted as partial melt zones caused by the addition of water from deep slabs (Revenaugh and Sipkin, 1994). The subduction process may not only modify the shallow mantle, but also modify the deeper upper mantle with consequences for the overriding plate long after shallow subduction has ceased.