Geochemistry, Geophysics, Geosystems

Mantle provinces under North America from multifrequency P wave tomography

Authors


Abstract

This is a survey of mantle provinces (large-scale seismic anomalies) under North America, from the surface down to 1500–1800 km depth. The underlying P velocity model was obtained by multifrequency tomography, a waveform-based method that systematically measures and models the frequency-dependence of teleseismic body waves. A novel kind of three-dimensional rendering technique is used to make the considerable structural complexities under North America accessible. In the transition zone and below, the North American mantle is dominated by seismically fast provinces, which represent distinct subduction episodes of the Farallon plate. I attempt to date and interpret the various slab fragments by reconciling their present positions with paleotrench locations from plate tectonic reconstructions and with major geologic surface episodes. Differences in vertical sinking velocity have led to large vertical offsets across adjacent, coeval slabs. Some of the mantle provinces have not been discussed much previously, including (1) a seismically slow blanket overlying the oldest Farallon subduction along the eastern continental margin, (2) a transition zone slab coeval with the Laramide orogeny (ca. 80–60 Myr), which I discuss in analogy to the “stagnant slab” subduction style commonly found in the western Pacific today, (3) the lower mantle root of present-day Cascadia subduction, which may have started out as intraoceanic subduction,(4) a lower mantle slab under Arizona and New Mexico, the last material to subduct before strike-slip motion developed along the San Andreas boundary, and (5) two narrow plate tears thousands of kilometers long, one of which is the subducted conjugate of the Mendocino Fracture Zone.

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