Geochemistry, Geophysics, Geosystems
© American Geophysical Union
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Online ISSN: 1525-2027
Lithospheric mantle thickness gradient focuses seismic activity
Using an array of 556 seismic sensors, Levander and Miller (2012) charted two key features of the subsurface structure of the western continental United States, with implications for explaining the locations of seismic and volcanic activity. Diving down into the Earth, the solid crust and the solid mantle combine to form the lithosphere, which together overlay the plastic mantle of the asthenosphere. Deeper still are the upper and lower mantles and the liquid and solid cores. The boundary between the crust and the solid mantle is known as the Mohorovičic (Moho), and the one between the solid and plastic mantle is the lithosphere-asthenosphere boundary (LAB). Measuring seismic waves produced by 163 earthquakes from 2005 to 2009, the authors charted the depth of Moho and LAB in the western United States and thus the thickness of the solid lithospheric mantle. Though the Moho depth varied across the study area in agreement with previous research, the authors found regions with sharp transitions in LAB depth. In particular, for the Cordilleran hinge line, a region that cuts through Wyoming, Utah, and Arizona that marks the Precambrian (more than 550 million years ago) western shoreline of Laurasia (the northern part of the Pangean supercontinent), LAB is 50% deeper east of the hinge line than west of it. They suggest that such a sharp gradient in lithospheric mantle thickness would localize and concentrate seismic stress. The authors compared their measurements with records of earthquakes over the recent period and volcanic eruptions over the past 5 million years. They found that across the western U.S. interior volcanism and seismicity are clustered in regions with a gradient in lithospheric mantle thickness, such as the Cordilleran hinge line.