Studies on the S-velocity structure of the North American upper mantle.

摘要

Several surface wave tomographic studies of the upper mantle structure beneath North America are presented, which take advantage of high quality seismic data, primarily recorded in the last decade, to create better resolved S-wave tomographic models. The first study presents a regional 3D model, IL05, of the upper mantle beneath the mid-continent United States, which focuses on a low-velocity heterogeneity beneath the Illinois basin. This anomalous structure is best explained by an unusual crust-mantle mixture, where either (1) oceanic crust became incorporated in the mantle of an over-riding plate during a Proterozoic episode of flat-slab subduction, or (2) a relatively cool mantle wedge is preserved with hydrous minerals. In both of these cases, a flat dip-angle is necessary to avoid slab dehydration and the associated partial melting, which would deplete the mantle, and result in a seismically fast lithosphere.;A continental model of North America is also presented, which investigates how S-velocities in the upper mantle vary spatially, with varying data coverage and quantity, as well as with varying model parameters. By inverting for a suite of good-fit models, a mean model N A07 is created which shows that the southern and eastern edges of the North American craton appear to be defined by the Paleozoic Ouachita and Appalachian orogens rather than the Proterozoic Grenville orogeny. In addition, N A 07 models the average Archean portion of the craton to be ∼200 km thick, while the Paleozoic part is on average ∼175 km thick with an ∼80 m/s lower S-velocity.;Using additional seismic data from Earthscope's USArray project from the years 2007-2008, a higher-resolution model of the western United States is created. This model, WUS08, has resolving capabilities of ∼300 km in the uppermost mantle. In addition to modeling smaller-scale anomalies than previously possible in regional surface-wave models, WUS08 demonstrates the improvements to tomographic modeling that will be possible across the continental United States as USArray's Transportable Array continues its traverse across the continent.