The structure of the mid-continent rift of North America from combined surface wave and ambient noise tomography

We present results from surface wave tomographic imaging of the crust and upper mantle beneath the northern Mid-Continent Rift Zone (MCRZ) using seismic data from the NSF-funded SPREE (Superior Province Rifting EarthScope Experiment) broadband seismometer deployment. The co-deployment of SPREE seismometers (in Minnesota, Wisconsin, and Ontario, Canada) with the EarthScope Transportable Array seismometers in Minnesota and Wisconsin provides a large-scale array of a total of 260 broadband sensors in the area of study, which includes a significant portion of the buried mid-continental rift. The close spacing of the stations in the resulting geometry allows for the use of a variety of array-processing techniques. We use a combination of ambient noise tomography and two-plane wave tomography. This combination will allow the joint inversion of phase velocities from 5 to 180 s to generate high-resolution 3-D shear wave velocity model from surface to a depth of ~200 km beneath the MCRZ and surrounding regions. The ambient noise tomography will constraint the shallow features along the rift, while the two-plane-wave method will constraint the poorly understood deep-seated sub-crustal features in the lithosphere and asthenosphere, which should still retain signatures of the 1.2-billion-year-old rifting. The ambient noise technique allows extraction of Green's functions between pairs of stations, which are used to generate surface wave group and phase velocities. Applying the two-plane-wave tomography method to teleseismic fundamental-mode Rayleigh waves provides phase velocity dispersion measurements at intermediate and long periods. The basis of the two-plane wave tomography [Forsyth and Li, 2005] is that the arriving surface wave wave-field is a result of two plane waves of six unknowns: initial amplitudes, initial phases, and back azimuths. Preliminary results show that for periods of 5 s down to 90 s the phase velocities continue to vary laterally, implying that the source of rift magmas are deep-seated within the upper mantle beneath the MCRZ.