High-resolution estimates of lithospheric thickness from Missouri to Massachusetts, USA

This paper presents a new three-dimensional (3-D) model, NA00, of the S-velocity of the upper mantle beneath North America. The model differs from its predecessor NA95 in that it exploits seismograms recorded by a recent dense, broadband array, MOMA, and from independent measurements of North American crustal thickness. Model NA00 is derived by fitting the waveforms of broadband seismic S and surface waves recorded by the MOMA array and inverting them together with the database of waveform fits used for NA95 and the crustal thickness estimates. It is demonstrated that including data from the dense, broadband MOMA array yields a resolving power beneath the array that is of unprecedented quality and relatively constant over a large depth range. This improved resolution provides a unique opportunity for quantifying the structure of the upper mantle in and below the lower, thick Precambrian lithosphere. The high-resolution seismic structure of the imaged high-velocity lithosphere is compared with the thermal structure (estimated from heat flow), compositional structure (estimated from xenoliths and electrical conductivity) and the elastic structure (estimated from gravity and topography). There is a remarkable agreement between the seismic, thermal, and compositional estimates. The seismic lithosphere is 180 km thick below Missouri and Illinois, 200 km thick below Indiana, Ohio and Pennsylvania, practically undefined below New York, and 80 km below Massachusetts and the Atlantic continental shelf. The thick lithosphere is underlain by a layer with lower S-velocities that could represent a relatively low-viscosity channel. However, the S-velocities in this layer are much higher than those of typical oceanic asthenosphere.