Structure of the upper mantle in the north-western and central United States from USArray S-receiver functions

摘要

We used more than 40 000 S-receiver functions recorded by the USArrayproject to study the structure of the upper mantle between the Moho and the410 km discontinuity from the Phanerozoic western United States to thecratonic central US. In the western United States we observed thelithosphere–asthenosphere boundary (LAB), and in the cratonic United States we observedboth the mid-lithospheric discontinuity (MLD) and the LAB of the craton. Inthe northern and southern United States the western LAB almost reaches themid-continental rift system. In between these two regions the cratonic MLDis surprisingly plunging towards the west from the Rocky Mountain Front toabout 200 km depth near the Sevier thrust belt. We interpret these complexstructures of the seismic discontinuities in the mantle lithosphere as anindication of interfingering of the colliding Farallon and Laurentia plates.Unfiltered S-receiver function data reveal that the LAB and MLD are notsingle discontinuities but consist of many small-scale laminateddiscontinuities, which only appear as single discontinuities after longerperiod filtering. We also observe the Lehmann discontinuity below the LABand a velocity reduction about 30 km above the 410 km discontinuity.