Abstract
Abstract. We used more than 40 000 S-receiver functions recorded by the USArray project to study the structure of the upper mantle between the Moho and the 410 km discontinuity from the Phanerozoic western United States to the cratonic central US. In the western United States we observed the lithosphere–asthenosphere boundary (LAB), and in the cratonic United States we observed both the mid-lithospheric discontinuity (MLD) and the LAB of the craton. In the northern and southern United States the western LAB almost reaches the mid-continental rift system. In between these two regions the cratonic MLD is surprisingly plunging towards the west from the Rocky Mountain Front to about 200 km depth near the Sevier thrust belt. We interpret these complex structures of the seismic discontinuities in the mantle lithosphere as an indication of interfingering of the colliding Farallon and Laurentia plates. Unfiltered S-receiver function data reveal that the LAB and MLD are not single discontinuities but consist of many small-scale laminated discontinuities, which only appear as single discontinuities after longer period filtering. We also observe the Lehmann discontinuity below the LAB and a velocity reduction about 30 km above the 410 km discontinuity.
Highlights
Lithospheric plates, including thick old cratons, translate over thousands of kilometers over the viscous mantle
We are using the name lithosphere–asthenosphere boundary (LAB) here for seismic velocity reductions observed near 200 km depth in cratons and near 100 km depth in oceans and Phanerozoic regions by tomography and receiver functions (e.g., Yuan and Romanowicz, 2010)
The east-dipping LAB interferes with the west-plunging midlithospheric discontinuity (MLD) in a complicated manner
Summary
Lithospheric plates, including thick old cratons, translate over thousands of kilometers over the viscous mantle. Asthenosphere boundary (LAB) below cratons is still thought to be “elusive” (Eaton et al, 2009) and an additional velocity drop frequently observed in seismic data in the shallow cratonic lithosphere (mid-lithospheric discontinuity, or MLD) is referred to as “enigmatic” (Karato, 2012). We are using the name LAB here for seismic velocity reductions observed near 200 km depth in cratons and near 100 km depth in oceans and Phanerozoic regions by tomography and receiver functions (e.g., Yuan and Romanowicz, 2010). A few tomography studies observed in cratons a shallow lowvelocity zone near 100 km depth, which could be related to the MLD (e.g., Lekic and Romanowicz, 2011). The structure of the mantle lithosphere in western North America was formed by the collision of the Farallon plate with the Laurentia craton and was first resolved by tomographic studies. The boundary of the craton follows approximately the Rocky Mountain Front (see for example Yuan et al, 2011, and Zheng and Romanowicz, 2012)
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